Symposium Organizers
Linyou Cao, North Carolina State University
Bruce Claflin, Air Force Research Laboratory
Thomas Mueller, Vienna University of Technology
Hua Zhang, Nanyang Technological University
Symposium Support
Aldrich Materials Science
APLMaterials|AIP Publishing
HORIBA Scientific
2D Materials and Materials Research Express | IOP Publishing
NT4.1: Excitonic Properties of 2D Materials and Heterostructures
Session Chairs
Linyou Cao
Thomas Mueller
Tuesday PM, March 29, 2016
PCC North, 100 Level, Room 129 B
2:30 PM - *NT4.1.01
The Valley Hall Effect in MoS2 Transistors
Paul McEuen 1,Kin Fai Mak 2,Kathryn McGill 1,Jiwoong Park 1
1 Cornell Univ Ithaca United States,2 Physics Penn State State College United States
Show AbstractElectrons in two-dimensional crystals with a honeycomb lattice structure possess a valley degree of freedom (DOF) in addition to charge and spin. These systems are predicted to exhibit an anomalous Hall effect whose sign depends on the valley index. Here, we report the observation of this so-called valley Hall effect (VHE). Monolayer MoS2 transistors are illuminated with circularly polarized light, which preferentially excites electrons into a specific valley, causing a finite anomalous Hall voltage whose sign is controlled by the helicity of the light. No anomalous Hall effect is observed in bilayer devices, which have crystal inversion symmetry. Our observation of the VHE opens up new possibilities for using the valley DOF as an information carrier in next-generation electronics and optoelectronics. Time permitting, I will also discuss the properties of other novel 2D materials under study in our laboratory.
3:00 PM - NT4.1.02
Engineering Substrate Interactions for High Luminescence Efficiency of Transition Metal Dichalcogenide Monolayers
Yifei Yu 1,Yiling Yu 1,Linyou Cao 1
1 North Carolina State Univ Raleigh United States,
Show AbstractTwo-dimensional (2D) transition metal dichalcogenides (TMDCs) such as monolayer MoS2 and WS2 are promising for developing truly atomic-scale light emission devices. Despite perfect surface passivation and strong exciton binding energy, the luminescence efficiencies of substrate-supported monolayers, however, are grossly poor compared to their suspended counterparts. Here we demonstrate that the luminescence efficiency of these monolayers is strongly dependent on the type of substrates and can be improved by orders of magnitude through substrate engineering. The origins for the substrate effects mainly lie in doping the monolayers as well as affecting their exciton dynamics, including facilitating the non-radiative recombination and slowing down the radiative decay. Using proper substrates like mica and Teflon can minimize the adverse effect of doping, and their luminescence efficiency can be largely tuned at room temperatures. These results provide useful guidance for the rational design of high-performance 2D TMDC light emission devices.
3:15 PM - *NT4.1.03
Exciton Decay Dynamics in Monolayer Semiconductors and their Heterostructures
Goki Eda 1
1 National Univ of Singapore Singapore Singapore,
Show AbstractEnergy transfer between excitonic systems separated by a small barrier layer plays an important role in determining the exciton decay dynamics in such systems. The ability to manage the direction of energy transfer in an intelligently engineered structure is of fundamental interest in semiconductor optoelectrnic devices. A vertical stack of semiconducting 2D crystals is effectively a coupled quantum well system that should allow exciton energy transfer across a sub-nanometer van der Waals gap via near field interaction. Previous studies on hetero-bilayers of group 6 transition metal dichalcogenides (TMDs) suggest that the interlayer charge transfer processes dominate exciton decay dynamics over energy transfer. In this talk, I will discuss our experimental observation of fast interlayer energy transfer in MoSe2/WS2 hetero-bilayer using photoluminescence excitation (PLE) spectroscopy. We find that the energy transfer is Förster-type involving excitons in the WS2 layer resonantly exciting free electron-hole pairs in the MoSe2 layer. Our results indicate that energy transfer competes with interlayer charge transfer with efficiencies exceeding 80% despite the type-II band alignment.
3:45 PM - NT4.1.04
Probing Bandgap Renormalization, Excitonic Effects, and Interlayer Coupling in 2D Transition Metal Dichalcogenide Semiconductors
Miguel Ugeda 5,Aaron Bradley 1,Sufei Shi 1,Felipe da Jornada 1,Yi Zhang 3,Diana Qiu 1,Wei Ruan 1,Sung-Kwan Mo 2,Zahid Hussain 2,Zhi-Xun Shen 4,Feng Wang 5,Steven Louie 5,Michael Crommie 5
1 Physics Department University of California at Berkeley Berkeley United States,5 Materials Science Division Lawrence Berkeley National Laboratory Berkeley United States,1 Physics Department University of California at Berkeley Berkeley United States2 Advanced Light Source Lawrence Berkeley National Laboratory Berkeley United States,3 Stanford Institute for Materials SLAC National Accelerator Laboratory Menlo Park United States2 Advanced Light Source Lawrence Berkeley National Laboratory Berkeley United States3 Stanford Institute for Materials SLAC National Accelerator Laboratory Menlo Park United States,4 Geballe Laboratory for Advanced Materials Stanford University Stanford United States
Show AbstractAtomically-thin transition metal dichalcogenide (TMD) semiconductors have generated great interest recently due to their remarkable physical properties. In particular, reduced screening in 2D has been predicted to result in dramatically enhanced Coulomb interactions that should cause giant bandgap renormalization and excitonic effects in single-layer TMD semiconductors. Here we present direct experimental observation of extraordinarily high exciton binding energy and band structure renormalization in a single-layer of semiconducting TMD [1]. We have determined the binding energy of correlated electron-hole excitations in monolayer MoSe2 grown via molecular beam epitaxy [2] on bilayer graphene (BLG) by using a combination of high-resolution scanning tunneling spectroscopy and photoluminescence spectroscopy. We have measured both the quasiparticle electronic bandgap and the optical transition energy of monolayer MoSe2/BLG, thus enabling us to obtain an exciton binding energy of 0.55 eV for this system, a value that is orders of magnitude larger than what is seen in conventional 3D semiconductors. In a more highly screened environment (on top of graphite), we find that single layers of MoSe2 show a strong 51% reduction in the exciton binding energy and an 11% reduction in the quasiparticle electronic gap, without significantly changing the optical gap. We have corroborated these experimental findings through ab-initio GW and Bethe-Salpeter equation calculations, which show that the large exciton binding energy arises from enhanced Coulomb interactions that lead to blue-shifting of the quasiparticle bandgap. We have also studied the role of interlayer coupling and layer-dependent carrier screening on the electronic structure [3] of few layer MoSe2. We find that the electronic quasiparticle bandgap decreases by nearly 1 eV when going from one layer to three. Our findings paint a clear picture of the evolution of the electronic wave function hybridization in the valleys of both the valence and conduction bands as the number of layers is changed. These results are of fundamental importance for the design and evaluation of room-temperature electronic and optoelectronic nanodevices involving single layers of semiconducting TMDs as well as more complex layered heterostructures.
References:
[1] M. M. Ugeda, A. J. Bradley, et al., Nature Materials 13, 1091 (2014).
[2] Y. Zhang, M. M. Ugeda, et al., Submitted (2015).
[3] A. J. Bradley, M. M. Ugeda, et al., Nano Letters 15, 2594 (2015).
4:30 PM - *NT4.1.05
Probing Intrinsic and Extrinsic Light Emission in Two-Dimensional Transition Metal Dichalcogenides
Ting Yu 3
1 Division of Physics and Applied Physics, School of Physical and Mathematical Sciences Nanyang Technological University Nanyang Singapore,2 Department of Physics, Faculty of Science National University of Singapore Singapore Singapore,3 Graphene Research Centre National University of Singapore Singapore Singapore,
Show AbstractTwo-dimensional (2D) semiconductors, such as transitional-metal-dichalcogenide (TMD) monolayers, have aroused great attention because of the underlying quasiparticle physics and the promising optoelectronic applications such as atomically-thin light-emitting diodes and photodetectors. Even though considerable progress has been achieved recently, intrinsic and extrinsic excitonic states of TMD monolayers are still under discussion and require direct experimental exploration. Here, we report observations of unconventional excitonic emission states of monolayer WS2 and MoS2 under electrical doping and photoexcitation at low temperature. Through electrical and optical injection of charge carriers, tunable excitonic emission has been realized due to interplay of various excitonic states, and the crucial binding energies of trions, biexcitons and bound excitons have been obtained. Particularly, the superlinear emission and the unusual linear polarization behaviour of its photon energy opposite to that of exciton emission have been clearly observed, which is assigned to the presence of biexciton states, evidencing the strong Coulomb interactions in such 2D semiconductors. Extrinsic emission bands in monolayer WS2 and MoS2 have also been revealed and attributed to specific localized states resulting from impurities and structural defects, where the exact origin of such states has been discussed according to our experimental observations and theoretical calculations. In view of applications, we have developed a highly practical strategy to electrically manipulate the diverse excitonic emission in atomically thin semiconductors, which opens up many possibilities for design and improvement of 2D TMD-based optoelectronic devices.
5:00 PM - NT4.1.06
Resonant Dipole-Dipole Energy Transfer in van der Waals Heterostructures
Daichi Kozawa 2,Alexandra Carvalho 3,Ivan Verzhbitskiy 3,Francesco Giustiniano 3,Yuhei Miyauchi 4,Shinichiro Mouri 1,Antonio Castro Neto 3,Kazunari Matsuda 1,Goki Eda 3
1 Kyoto University Kyoto Japan,2 Waseda University Tokyo Japan,3 National University of Singapore Singapore Singapore1 Kyoto University Kyoto Japan,4 PRESTO, Japan Science and Technology Agency Saitama Japan1 Kyoto University Kyoto Japan
Show AbstractEnergy transfer is a well-known phenomenon commonly observed in molecular complexes, biological systems, and semiconductor heterostructures. In van der Waals heterostructures of 2D semiconductors, Förster-type energy transfer (ET) is expected to be efficient due to the parallel orientation and sub-nanometer separation of excitons in the neighboring layers. Here, we report an experimental observation of fast interlayer ET in MoSe2/WS2 heterostructures using photoluminescence excitation spectroscopy. We find that the ET is Förster-type involving excitons in the WS2 layer resonantly exciting higher-order excitons in the MoSe2 layer. Our results indicate that ET competes with interlayer charge transfer with efficiency exceeding 60% despite the type-II band alignment. Efficient ET in these systems offers prospects for optical amplification and energy harvesting through intelligent layer engineering. We anticipate that the ability to manage energy via ET at the sub-nanometer length scales in carefully designed heterostructures will allow realization of novel optical and optoelectronic device concepts.
5:15 PM - NT4.1.07
Modulating Optoelectronic Properties of Two-Dimensional Transition Metal Dichalcogenide Semiconductors by Photoinduced Charge Transfer
Jong Hyun Choi 1,Jungwook Choi 1
1 Purdue Univ West Lafayette United States,
Show AbstractAtomically thin transition metal dichalcogenides (TMDCs) have attracted great interests as a new class of two-dimensional (2D) direct bandgap semiconducting materials. The controllable modulation of optical and electrical properties of TMDCs is of fundamental importance to enable a wide range of future optoelectronic devices. Here we investigate the modulation of optoelectronic properties of 2D TMDCs, including MoS2, MoSe2, and WSe2, by interfacing them with two metal-centered phthalocyanine (MPc) molecules, nickel Pc (NiPc) and magnesium Pc (MgPc). Through energetically favorable electron transfer from the photo-excited TMDCs to the MPc, the photoluminescence (PL) emission characteristics can be selectively and reversibly engineered. NiPc molecules, whose reduction potential is positioned below the conduction band minimum (CBM) of MoSe2 and WSe2 monolayers, but higher than that of MoS2, quench the PL signatures of MoSe2 and WSe2, but not MoS2. Similarly, MgPc quenches only WSe2 as its reduction potential is situated below the CBM of WSe2, but above that of MoS2 and MoSe2. The quenched PL emission can be fully recovered when MPc molecules are removed from the TMDC surfaces. We also find that photocurrents from TMDCs, probed by photoconductive atomic force microscopy (PC-AFM), are promoted when MPcs quench the PL, further supporting the photoinduced charge transfer mechanism. Our results will benefit design strategies for 2D inorganic-organic optoelectronic devices and systems with tunable properties and improved performances.
5:30 PM - *NT4.1.08
Beyond Ground State Excitonics: Anomalous High Energy Excitation, Quasiparticle Bandgap Determination, and Robust Thermalization of Excitons in Monolayer MoS2
P. James Schuck 1
1 Molecular Foundry, Lawrence Berkeley National Lab Berkeley United States,
Show AbstractTwo dimensional (2D) monolayer transition metal dichalcogenide (ML-TMDC) semiconductors are ideal building blocks for atomically thin, flexible optoelectronic and catalytic devices. Although the tightly bound exciton states lie at the heart of the optoelectronic properties of ML-TMDCs, surprising voids in the understanding of the manifold of these states persists. Here, we move beyond ground state excitonics, unifying somewhat contradictory observations by combining PLE spectroscopy, hyperspectral imaging, and DFT calculations of ML-MoS2, revealing anomalous optoelectronic behavior. Signatures of the 1S and 2S states of the A and B excitons are resolved and a reduction in PL quantum yield with increasing excitation energy is observed, confirming previous PLE studies. In contrast to the previous works however, we also observe subtle features in the PLE spectrum that are assigned to the quasiparticle bandgap and resulting onset of interband transitions, which is supported by complimentary density functional theory (DFT) calculations. Additionally, an “anomalous” increase in the experimental PLE spectrum at energies >2.6 eV diverges from previous results and expectations for a 2D quantum well with substantial many-body effects. This feature corresponds nicely to the step-like feature observed in the previous PCE measurements and likely arises from efficient coupling of excitations created in the band-nesting region of the Brillouin zone to the photoemissive direct gap excitons at the K-point. We find that these high-energy excitations can enhance emission from lower-energy states, such as trions or biexcitons. From hyperspectral PLE imaging of the single flakes of ML-MoS2, we find that the Stokes shift between excitation and emission resonances, a common metric for disorder in conventional 2D quantum wells, is significantly more uniform than the exciton energies, implying internal relaxation processes remain substantially more ordered than the exciton energy. Finally, the spatially resolved PLE spectroscopy reveals that spatial dispersion of the anomalous excitation resonances is correlated to the A and B exciton states, suggesting that these high-energy states are similarly excitonic in nature.
NT4.2: Poster Session I
Session Chairs
Linyou Cao
Bruce Claflin
Thomas Mueller
Hua Zhang
Wednesday AM, March 30, 2016
Sheraton, Third Level, Phoenix Ballroom
9:00 PM - NT4.2.01
Preparation of Layer-Controlled Two-Dimensional Black Phosphorus with Environmental Stability
Jingyuan Jia 1,Su Min Jeon 1,Jaeho Jeon 1,Sungjoo Lee 1
1 Sungkyunkwan Univ Suwon Korea (the Republic of),
Show AbstractBlack phosphorous (BP) is known as a most stable allotrope of phosphorous with its unique orthorhombic structure. Recent studies have shown that BP is an elemental layered material with strong in-plane bonds and a weak van der Waals interlayer interaction that make it possible to obtain layered structures through mechanical cleavage. The energy band gap of BP is reported to be 0.33 eV in the bulk and around 1.8 eV for a monolayer, and unlike transition metal dichalcogenides (TMD) such as MoS2 and WSe2, the energy band gap of BP retains its direct band gap character when the layer thickness varies. Moreover, the carrier mobility of BP can theoretically be as high as 20000 cm2/Vs. These fascinating properties mean that layer-structured BP is a promising semiconducting channel material for future nanoscale electronic devices. However, there are several technological obstacles that must be overcome to enable the actual implementation and integration of BP devices. The mechanical cleavage with tape of BP from the bulk is not as easily realized as for graphene and TMD materials making it difficult to control the layer thickness of the exfoliated film. Furthermore BP exfoliated from the bulk is not environmentally stable because it reacts with oxygen and water molecules and forms a rough surface containing impurities which act as charge trapping centers and carrier mobility scattering centers. As a result, considerable degradation of transistor performances, including decreases in mobility and threshold voltage shifts have been observed.
In an attempt to resolve these two issues, we tested in this study a plasma etching process in which the thickness of the layered BP film can be controlled by modulating the plasma etching time at the thinning rate of 2 layers /min. Not only does the plasma treatment control the thickness of the BP film, it also removes the chemical degradation of the exposed oxidized BP surface and that the remaining few-layer BP films exhibit excellent morphology and crystalline structure, which results in enhanced field-effect transistor (FET) performance. Our fabricated BP FETs were passivated with polymethyl methacrylate (PMMA) immediately after the plasma etching process to protect the prepared BP films from water and oxygen molecules in air. With these techniques, a high field-effect mobility was achieved, 1150 cm2/Vs, with an Ion/Ioff ratio of ~105 at room temperature. Furthermore, a fabricated FET with plasma-treated few-layer BP that was passivated with PMMA was found to retain its I-V characteristics and thus to exhibit excellent environmental stability over several weeks.
9:00 PM - NT4.2.02
Studying the Exciton Complexes in 2-Dimensional Gallium Chalcogenides
Hui Cai 1,Robert Kudrawiec 2,Can Ataca 3,Jeffrey Grossman 3,Sefaattin Tongay 1
1 School for Engineering of Matter, Transport and Energy Arizona State University Tempe United States,2 Institute of Physics Wroclaw University of Technology Wroclaw Poland3 Department of Materials Science and Engineering Massachusetts Institute of Technology Cambridge United States
Show AbstractGallium chalcogenides are new class of 2D materials with the chemical formula GaX where X are chalcogen atoms. In the bulk form, GaSe and GaTe are direct bandgap semiconductors displaying strong photoluminescence (PL) but become indirect gap semiconductors in the monolayer form. The direct to indirect transition happens because the valence band renormalizes itself to a “W” shape around the Γ point so that the valence band maximum (VBM) shifts from the Γ point. However, our recent experiments show that for few-layered GaX, the PL comes back at low temperatures and increases by many orders of magnitude as down to 4 K. This finding indicates the 2D GaX can still behave like a direct bandgap semiconductor at low temperatures due to dramatically weakened exciton-phonon interactions. To further study the exciton complexes in these materials, power dependent PL is measured at 5 K. The result shows that PL intensity is well fitted according to I=APα, where α varies in terms of different excitonic processes. Interestingly, in 2D GaX α is found to be different from its bulk form as well as 2D MoS2, indicating a novel exciton recombination process. Based on the abnormal thermal quenching and power dependence of the PL, a three particle recombination process involving one electron and two holes is proposed for 2D GaX.
9:00 PM - NT4.2.03
Current Crowding and Specific Contact Resistance in Two-Dimensional Black Phosphorus Field-Effect Transistors
Qiaoming Wang 1,Yi Gu 1
1 Washington State University Pullman United States,
Show AbstractBy combining electrical measurements, scanning Kelvin probe microscopy, and numerical electrical simulations, we find significant current crowding effect on two dimensional black phosphorus field-effect transistors. This current crowding can lead to localized Joule heating close to the metal contacts, and it is consistent with the features of the device failure observed in this study. Importantly, we find that the commonly used transmission-line model, in general, significantly underestimates the extent of the current crowding by 1 – 2 orders of magnitude, compared to our results. In addition, our approach allows us to determine the black phosphorus layer resistance and the specific contact resistance, an important parameter for evaluating electrical contacts, whereas the simple relation for calculating the specific contact resistance is invalid due to the current crowding. We find that the layer resistance increases significantly when the layer thickness decreases towards about 5 – 6 nm and remains relatively constant for thicker layers. It turns out that the specific contact resistance of Titanium contacts on black phosphorus field-effect transistors is about 0.3 Ωcm2 in average when the layer thickness is larger than 7 nm, and significantly increases when the layer thickness is below 7 nm. The specific contact resistance also increases when the back gate voltage sweeps from -9 V to 9 V. These findings, which are likely to be relevant in other 2D materials, suggest the need to take into account the current crowding effect in designing novel 2D devices.
9:00 PM - NT4.2.04
Dynamic Structural Response and Deformations of Monolayer MoS2 Visualized by Femtosecond Electron Diffraction
Ehren Mannebach 1,Renaki Li 2,Karel-Alexander Duerloo 1,Clara Nyby 1,Peter Zalden 3,Theodore Vecchione 2,Friederike Ernst 4,Alexander Reid 3,Tyler Chase 3,Xiaozhe Shen 2,Stephen Weathersby 2,Carsten Hast 2,Robert Hettel 2,Ryan Coffee 2,Nick Hartmann 2,Alan Fry 2,Yifei Yu 5,Linyou Cao 5,Tony Heinz 4,Evan Reed 1,Hermann Durr 3,Xijie Wang 2,Aaron Lindenberg 4
1 Stanford Univ Stanford United States,2 SLAC National Accelerator Laboratory Menlo Park United States3 Stanford Institute for Materials and Energy Sciences Menlo Park United States1 Stanford Univ Stanford United States,3 Stanford Institute for Materials and Energy Sciences Menlo Park United States,4 PULSE Institute Menlo Park United States1 Stanford Univ Stanford United States,3 Stanford Institute for Materials and Energy Sciences Menlo Park United States5 North Carolina State University Raleigh United States
Show AbstractTwo-dimensional materials are subject to intrinsic and dynamic rippling that modulates their optoelectronic and electromechanical properties. Here, we directly visualize the dynamics of these processes within monolayer transition metal dichalcogenide MoS2 using femtosecond electron scattering techniques as a real-time probe with atomic-scale resolution. We show that optical excitation induces large-amplitude in-plane displacements and ultrafast wrinkling of the monolayer on nanometer length-scales, developing on picosecond time-scales. These deformations are associated with several percent peak strains that are fully reversible over tens of millions of cycles. Direct measurements of electron-phonon coupling times and the subsequent interfacial thermal heat flow between the monolayer and substrate are also obtained. These measurements, coupled with first principles modeling, provide a new understanding of the dynamic structural processes that underlie the functionality of two-dimensional materials and open up new opportunities for ultrafast strain engineering using all-optical methods. The pump-probe techniques described here also allow for studies of small, thermally-induced structural changes under quasi-equilibrium conditions, difficult to resolve with static heating approaches, including direct measurements of thermal boundary resistances across buried interfaces.
9:00 PM - NT4.2.05
Scalable Monolayer MoS2 Bio and Vapor Sensor
Carl Naylor 1,Nicholas Kybert 1,A.T. Johnson 1
1 Univ of Pennsylvania Philadelphia United States,
Show AbstractAtom-thick 2D materials such as Graphene and monolayer Molybdenum Disulphide (MoS2) make very sensitive biosensors and detectors of gaseous organic analytes. Every atom on the surface is exposed, which maximizes the material’s sensitivity to molecules in the external environment. Through an efficient reproducible growth process that yields a high density of monolayer single crystal MoS2 flakes, we are able to manufacture thousands of FET devices with a yield of 95% and average mobility of 36cm2V-1s-1 at room temperature in vacuum. With this method we were able to successfully perform bio and vapor sensing on our samples.
For biosensing, atomic length nickel-mediated linker chemistry is used to enable target binding events close to the MoS2 surface to maximize sensitivity. A computationally redesigned μ-opioid receptor (MOR) is linked to the surface of the MoS2, from which we were able to read out the concentrations of a synthetic opioid peptide with high MOR specificity. The data indicates binding affinities that match the values determined using traditional techniques.
For vapor sensing, we exposed an array of MoS2 FETs to various analytes. Huge responses, up to 94% increase in the electrical conductivity, were observed upon exposure to 33vol% saturated dimethyl methylphosphonate (DMMP) vapor. These large responses were caused by the analyte molecules interacting with either the MoS2 flake or the contacts between gold electrodes and the MoS2 flake. Using Transmission Line Measurements (TLM), we studied the resistance of the MoS2 flake and the resistance of the gold-MoS2 contact as we exposed the sample to different vapor concentrations. This study strives to determine whether the responses to the analytes are driven by the MoS2 flake or gold-MoS2 contacts.
The combination of scalable array fabrication and rapid, precise readout enabled by the MoS2 transistor offers the prospect of a solid-state drug testing platform for rapid readout of the interactions between novel drugs and their intended protein targets. Additionally, the large responses exhibited by MoS2 devices upon exposure to various vapor analytes indicate a highly sensitive surface, which could potentially enable detection of small traces of dangerous compounds in the environment.
9:00 PM - NT4.2.06
Biofunctionalization and Reaction Kinetics for Direct Covalent Modification of Pristine MoS2
Ximo Chu 1,Ahmed Yousaf 1,Duo Li 1,Anli Tang 1,Abhishek Debnath 1,Duo Ma 1,Alexander Green 1,Qing Hua Wang 1
1 Arizona State Univ Tempe United States,
Show AbstractTwo-dimensional (2D) transition metal dichalcogenides (TMDCs) have generated significant research excitement due to their unique thickness-dependent electronic and optical properties, particularly their semiconducting direct bandgap that leads to photoluminescence, making them attractive for applications in next-generation electronics, optics, and chemical and biological sensing. Chemical functionalization is crucial for altering and tuning the properties of nanostructured materials such as graphene and carbon nanotubes in these types of applications, but is in its relative infancy for the TMDCs. In this work, we demonstrate robust biofunctionalization of pristine MoS2 monolayers and bilayers with the fluorescent proteins GFP and mCherry. The fluorescent properties of both proteins are maintained and are simultaneously localized to the MoS2 surface as shown by confocal fluorescence microscopy. Atomic force microscopy shows a uniform coverage of protein molecules, and Raman and PL mapping show that MoS2 also retains its optical properties. The biofunctionalization process here relies on Ni-NTA tethering of polyhistidine-tagged proteins, as well as direct covalent functionalization of the pristine MoS2 basal plane via aryl diazonium salts, which we also show for the first time. Previous methods of covalent modification of MoS2 require harsh treatments such as lithium intercalation, defect engineering, or phase engineering, which disrupt many of the useful semiconducting and photoluminescent properties of the TMDCs. Instead, we demonstrate that diazonium functionalization of the pristine MoS2 surface maintains and even enhances the MoS2 photoluminescence (PL) two-fold. We confirm C–S bond formation by x-ray photoelectron spectroscopy (XPS) and we conduct detailed spatial mapping of the optical emission and surface coverage of reacted sites of the MoS2 at time points ranging from 2 s to 6 h to derive a kinetic model of the covalent functionalization, which we find can be modeled by surface-limited second order reaction kinetics.
9:00 PM - NT4.2.07
Modulation of Photoluminescence of Tungsten Disulfide via Electrical and Optical Method
Zhengyu He 1,Jamie Warner 1
1 Department of Materials University of Oxford Oxford United Kingdom,
Show AbstractAtomically thin transition metal dichalcogenides (TMDCs) are two-dimension (2D) materials. Their unique electrical and optical properties make them an ideal platform for both fundamental studies as well as practical applications. In many cases, the ability to manipulate the optical properties of TMDCs via external modulation is highly desirable.
We have studied the photoluminescence (PL) response of both monolayer and bilayer tungsten disulfide (WS2) under lateral electric field.1 We demonstrate that monolayer and bilayer WS2 responded oppositely towards the lateral electric field. PL of monolayer WS2 can be substantially quenched under lateral electric fields while the emission of bilayer WS2 was enhanced. In both cases, the spectral shifts were negligible. Temperature dependent measurements showed distinctly different behavior from that of the electric field, indicating that joule heating was not the dominant mechanism. Direct ionization and impact ionization were ruled out by comparing the theoretical threshold electric field strength with the experimental value. Considering the multi-valley band structure of tungsten disulfide, we attribute the field induced PL variations to photo-excited electron transfer from one conduction band extremum to another, resulting in the modification of recombination pathways. Such a physical process can only be observed in 2D TMDCs due to their exceptionally large exciton binding energy and the small energy difference between two conduction band extrema.
Laser treatment can also tune the emission spectrum of monolayer tungsten disulfide. We show that strong laser irradiation (>30 kW/cm2) can introduce additional luminescent features at 77 K. Before laser irradiation, the PL spectrum of monolayer WS2 under low excitation power (~3 W/cm2)was dominated by exciton, trion and localized states (LS). However, after laser treatment, the emission intensity of LS was quenched substantially while a new PL peak with emission energy between LS and trion emerged under low excitation power (~3 W/cm2). However, this behavior can only be observed in monolayer WS2 aged in ambient conditions, and thus is associated with defects. We found that the resultant PL shape evolved with laser irradiation time. It was a reversible process where PL spectrum can reverse back to original shape if thermal cycle was applied.
[1] Z. He, Y. Sheng, Y. Rong, G. Lee, J. Li, J. Warner. ACS Nano 2015, 9, 2740-2748.
9:00 PM - NT4.2.08
Mixed Multilayered Vertical Heterostructures Utilizing Strained Monolayer WS2
Yuewen Sheng 1,Jamie Warner 1
1 University of Oxford Oxford United Kingdom,
Show AbstractMonolayer WS2 is a direct band gap semiconductor in the visible spectral region and as part of the monolayer transition metal dichalcogenides (TMDs) family it is opening up new avenues for electronic and optoelectronic devices. Creating alternating layers of two-dimensional (2D) materials forms vertical heterostructures with diverse electronic and optoelectronic properties that could revolutionize future ultrathin devices. These 2D heterostructures can be realized by either transfer or direct growth.
One important aspect for atomically thin TMDs is its flexibility, which is better suited for strain engineering compared with other bulk semiconductors. Monolayer WS2 grown by chemical vapor deposition (CVD) can have inbuilt strain due to interactions with the substrate. The strain modifies the band structure and properties of monolayer WS2 and can be exploited in a wide range of applications. It has been predicted that tunable photonic devices and solar cells with better energy band matching can be realized by building multilayer 2D heterostructures based on stained TMDs. However, building layer-by-layer vertical heterostructures containing a strained TMD layer has been challenging due to the standard aqueous based transfer processes washing off the TMDs from the growth substrate. It is important to develop a non-aqueous transfer approach that allows other 2D crystals to be assembled on top of the strained TMDs to build multi-layered vertical heterostructures.
We demonstrate a non-aqueous transfer method for creating vertical stacks of mixed 2D layers containing a strained monolayer of WS2, with Boron Nitride and Graphene. The 2D materials are all grown by CVD enabling large area vertical heterostructures to be formed. WS2 monolayers grown by CVD directly on Si substrates with SiO2 surface are easily washed off by water and this makes aqueous based transfer methods challenging for creating vertical stacks on the growth substrate. 2D hexagonal Boron Nitride films are used to provide an insulating layer that limits interactions with a top graphene layer and preserve the strong photoluminescence from the WS2. We show that stronger interactions between graphene and monolayer WS2 occur when using non-aqueous based transfer, associated with sharper atomic interfaces from less contamination. This transfer method is suitable for layer-by layer control of 2D material vertical stacks and is shown to be possible for all CVD grown samples, which opens up pathways for the rapid large scale fabrication of vertical heterostructure systems with atomic thickness depth control and large area coverage.
9:00 PM - NT4.2.09
Chemical Vapor Deposition of Single-Crystal, Few-Layered Hexagonal Boron Nitride
Hongseok Oh 1,Joon Young Park 1,Youngbin Tchoe 1,Janghyun Jo 1,Sung-Soo Kim 1,Yong-jin Kim 2,Miyoung Kim 1,Byeong-Hyeok Sohn 1,Gyu-Chul Yi 1
1 Seoul National Univ Seoul Korea (the Republic of),2 University of Manchester Manchester United Kingdom
Show AbstractHexagonal boron nitride (hBN) is a dielectric insulator with a two-dimensional layered structure, having a wide band gap energy of 5.6–6.0 eV. Due to its novel properties, which include a high mechanical strength, electric resistivity, thermal conductivity, and chemical inertness, as well as flexibility and transparency, hBN can provide many advantages to next-generation electronics and optoelectronics such as flexible and transparent transistors or transferrable light-emitting devices. While hBN layers can be prepared by mechanical cleavage methods from bulk crystals, their limited sizes and random thicknesses and orientations may not be suitable for many device applications. On the other hand, large-area hBN layers can be prepared by chemical vapor deposition (CVD); however, they usually exhibit polycrystalline structures with a typical average grain size of several microns. Grain boundaries or dislocations in hBN can affect its electronic or mechanical properties; several theoretical calculations have discussed the lower band gap energy and poorer mechanical strength of polycrystalline hBN. Accordingly, large-area single-crystal hBN layers are required to fully realize the potential merits of hBN. Here, we report the synthesis of single-crystal hBN (SC-hBN) layers by CVD.
The SC-hBN was grown on a 20 × 20 mm2 Ni (111) substrate from a single ammonia-borane precursor by atmospheric pressure chemical vapor deposition (APCVD). A grown layer was transferred to arbitrary substrates by an electrochemical delamination technique, and the remaining Ni (111) substrate was repeatedly re-used. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and UV-Vis-IR spectroscopy confirmed that the grown films exhibited the characteristics of hexagonal boron nitride layers. Microstructural analysis by transmission electron microscopy (TEM) revealed that the grown layers were single-crystalline over their entire areas, presumably due to heteroepitaxial growth on the Ni (111) substrate. Atomic force microscopy (AFM) and electron backscatter diffraction (EBSD) studies on the Ni (111) substrate proved that there was no degradation after repeated growth and transfer cycles. The effect of growth parameters on the crystal structure of the grown hBN layers were investigated using reflection high-energy electron diffraction (RHEED).
9:00 PM - NT4.2.10
Flexible Transparent MoS2 Field-Effect Transistors for Gate-Tunable Piezoresistive Strain Sensors
Meng-Yen Tsai 1,Alexey Tarasov 1,Zohreh Hesabi 1,Hossein Taghinejad 1,Philip Campbell 1,Corey Joiner 1,Ali Adibi 1,Eric Vogel 1
1 Georgia Institute of Technology Atlanta United States,
Show AbstractThe emerging two-dimensional (2D) layered materials are promising candidates for high-performance and energy efficient, flexible transparent electronics. Graphene is widely studied for transistors, sensors, and optoelectronic applications but is limited because of its lack of band gap. Beyond graphene, transition metal dichalcogenides (TMDCs) such as MoS2 and WSe2 are attractive because alloying, thickness, or strain engineering can be used to tune their band gap. Here, piezoresistive strain sensing with transparent and flexible atomically thin molybdenum disulfide (MoS2) field effect transistors (FETs) made from highly uniform large-area synthesized films is demonstrated. The change in the MoS2 current-voltage characteristics with mechanical strain is observed to be consistent with a change in band gap as measured using optical reflection spectroscopy. The magnitude of the band gap change agrees with previous theoretical and spectroscopic studies. In addition, the gauge factor or the sensitivity of the MoS2 strain sensor is found to be adjustable via gate biasing, which changes the Fermi level of the MoS2. The gate-tunable piezoresistivity is useful to adjust the sensitivity in practical sensing applications. The results show that MoS2 can lead to a new generation of transparent and flexible strain and force sensors.
9:00 PM - NT4.2.11
Effect of Al2O3 Passivation Layers on Multilayer MoSe2 Field-Effect Transistors
Hyun Ah Lee 1,Seonyoung Park 1,Woong Choi 1
1 School of Advanced Materials Engineering Kookmin University Seoul Korea (the Republic of),
Show AbstractTransition metal dichalcogenides such as MoSe2 have attracted great attention due to their desirable properties for electronic and optoelectronic applications. MoSe2 field-effect transistors (FETs) showed promising device characteristics such as high on/off ratio (106) and field-effect mobility (50-200 cm2/Vs). However, strategies to optimize device performance have not been established yet. In this presentation, we explore the possibility of improving MoSe2 FET performance using atomic-layer-deposited Al2O3 passivation layers on MoSe2 channels. By comparing the effect of Al2O3 layers on the device characteristics of bottom-gate multilayer MoSe2 FETs, we observe the consistent enhancement of field-effect mobility and current-voltage hysteresis. These results suggest that the deposition of Al2O3 passivation layers on top of MoSe2 channel can be an efficient method of optimizing the device performance of MoSe2 FETs.
9:00 PM - NT4.2.12
Functionalization of TMDs for Deposition of NanoScale ALD Dielectrics for FETs with Low SubThreshold Swing
Iljo Kwak 1,Jun Park 1,Hao Lu 2,Kasra Sardashti 1,Christopher Ahles 1,Suresh Vishwanath 3,Huili Xing 3,Alan Seabaugh 2,Susan Fullerton 4,Andrew Kummel 1
1 Univ of California-San Diego La Jolla United States,2 Electrical Engineering Department University of Notre Dame South Bend United States3 School of Electrical and Computer Engineering, Cornell University Ithaca United States4 Department of Chemical and Petroleum Engineering University of Pittsburgh Pittsburgh United States
Show AbstractThe fabrication of a top-gated transition metal dichalcogenide (TMD) FET requires a uniform and pinhole free gate dielectric. However the direct deposition of gate dielectrics on TMD materials is challenging due to the absence of surface dangling bonds. To deposit dielectrics with low leakage and low equivalent oxide thickness (EOT) on WSe2, a flat-lying titanyl phthalocyanine (TiOPc) monolayer, deposited via molecular beam epitaxy (MBE), was used as a seed layer for atomic layer deposition (ALD) on the WSe2 surface. ALD pulses of trimethyl aluminum (TMA) and H2O resulted in the uniform deposition of Al2O3 on the TiOPc/WSe2, confirmed by AFM and cross-sectional TEM. After depositing 50 cycles of Al2O3 onto the TiOPc/multilayer WSe2, a top-gated FET was fabricated. The top gate leakage current was measured as 0.046 pA/µm2 at 1 V gate bias with 2.2 nm EOT, which is lower leakage current than all previous results on TMD with similar EOT. A subthreshold swing of 80 mV/dec was measured. While the TiOPc/WSe2 was deposited by MBE, other phthalocyanines can be readily deposited from solution, making the functionalization technique more accessible. For example, to make a Cobalt Crown ether Phthalocyanine (CoCrPc) monolayer on WSe2, 3 uM of CoCrPc/benzene-ethanol (9:1 v/v) solution was prepared. Two droplets of the solution were deposited onto a freshly cleaved, bulk WSe2 substrate. After the solvent was completely evaporated, the sample was annealed at 200 C for 30 min in a N2 filled glove box. An Al2O3 layer was deposited by 50 cycles of ALD using TMA and water at 120 C. The surface of the Al2O3 film was analyzed with ambient Atomic Force Microscopy (AFM), showing that the film is dense and uniform and has no pinholes detectable by AFM.
9:00 PM - NT4.2.13
Black Phosphorus Field-Effect Transistors with High On/Off Ratio via Thickness Control
Suhyun Kim 1,Younghun Jung 1,Jihyun Kim 1
1 Korea University Seoul Korea (the Republic of),
Show AbstractTwo-dimensional (2D) materials have attracted a great attention since the rise of graphene. Recently, black phosphorus (BP), its monolayer known as phosphorene, has been highlighted as a promising 2D material that has properties comparable to that of graphene and other 2D materials. Beside its excellent electronic properties, BP has an advantage over graphene in the presence of direct bandgap. As each layer of phosphorene is held to each other by van der Waals interaction, BP flakes of high crystallinity can be easily transferred using mechanical exfoliation method. However, preparing BP film with desired thickness by mechanical exfoliation is still an issue. Fine control of BP thickness is essential for the fabrication of BP-based electronic devices because the characteristics of BP, such as bandgap, is highly sensitive to its thickness. In our experiments, we employed UV-ozone treatment on exfoliated BP flake to control its thickness by etching each BP layer. BP flakes were prepared on SiO2/Si substrates using mechanical exfoliation method. The in situ optical characteristics of BP flakes were observed with Raman spectroscopy as UV-illumination time increased. Likewise, the thickness-dependent electrical properties of BP-based field-effect transistors were studied in situ. We investigated the changes in various electrical properties, including mobility and on/off ratio, as the BP thickness varied. The obtained results would be reproducible and controllable using our suggested method. Further results and discussion will be presented in detail.
9:00 PM - NT4.2.14
Characterization of 2D MoS2 and WS2 Dispersed in Organic Solvents for Composite Applications
Alberto Delgado 1,Anupama Kaul 2,Hisato Yamaguchi 3,Aditya Mohite 3
1 Metallurgical, Materials and Biomedical Engineering The University of Texas at El Paso El Paso United States,1 Metallurgical, Materials and Biomedical Engineering The University of Texas at El Paso El Paso United States,2 Electrical and Computer Engineering The University of Texas at El Paso El Paso United States3 MPA-11 Materials Synthesis amp; Integrated Devices, Materials Physics and Applications Division Los Alamos National Laboratory Los Alamos United States
Show AbstractComposites offer a facile means to tailor the properties of hybrid, dissimilar material systems for applications ranging from optoelectronics to strain sensors. Two-dimensional (2D) layered materials have attracted significant interest in recent years with the isolation of graphene through the mechanical exfoliation of graphite more than a decade ago. The 2D materials family is also rich with diverse compositions that offer semiconducting characteristics. In this work, we have explored the prospects of MoS2 and WS2, both of which are semiconducting 2D materials, for potential composite applications. In order to form 2D materials composites we have to first disperse them chemically in solution. MoS2 and WS2 powders were oversaturated in N-Methyl-2-pyrrolidone (NMP) solution at 37.5 mg/mL and sonicated at room temperature (RT) for sonication times ranging from 30 minutes to close to 24 hours. After solution processing, the samples with the 2D flakes were transferred to an Isopropyl Alcohol (IPA) bath for particle size distribution analysis.
We have observed significant changes in particle size distribution spanning two orders of magnitude as a function of the sonication conditions. Specifically, the observed changes in particle size distribution for MoS2 and WS2 powders ranged from 44 microns down to 0.409 microns, and 148 microns down to 0.409, respectively, as compared to the untreated materials. Structural analysis was conducted using the SEM and X-Ray diffraction. The structural analysis using the SEM revealed morphological signatures between the two materials, where the MoS2 flakes had a randomly oriented distribution with occasional triangular flakes. In the case of the WS2, regardless of the sonication conditions, the flakes seemed to have a characteristic 120° angular distribution at the vertices, representing a rhombus with concave edges. The XRD analysis showed a minute shift in the characteristic peaks that maybe due to strain-induced effects as a result of the solution processing. Optical characterization of the materials was also conducted using Raman Spectroscopy to validate the average layer number resulting from the solution dispersions and the spatial and compositional uniformity of the two material samples. After the material characterization studies on the parent MoS2 and WS2 were conducted, we integrated these materials with a polymer matrix to form a hybrid composite system. The optical, mechanical and electrical properties of the 2D-polymer composites were investigated for their potential utility in optoelectronic and strain sensing devices.
9:00 PM - NT4.2.15
Atomic Migration and Reactivity on WSe2 and Graphene
Baoming Wang 1,Aman Haque 1,Alexander Mag-isa 2,Jae-Hyun Kim 2,Hak Joo Lee 2,Sarah Eichfield 1,Joshua Robinson 1
1 Pennsylvania State Univ University Park United States,2 Korea Institute of Machinery and Materials Daejeon Korea (the Republic of)
Show AbstractSurface migration of foreign atoms is expected to be promoted for 2D materials, which are virtually all-surface materials. An accompanying issue is chemical reactivity of these migrating atoms, which can dramatically alter the atomic structure and properties of 2D materials. Unfortunately, very little is known about these phenomena and the role of temperature and current density. In this study, we examine freestanding WSe2-graphene heterostructure as well as multi-layer graphene for the effect of heat and charge flow by performing insitu experiments inside the transmission electron microscope. We observe selenium decompose and then silicon atoms migrate from the silicon micromachined test structure to the freestanding specimen. At high temperatures, this initiate rapid degradation through the formation of tungsten disilicide and silicon carbide formation. The role of current flow is to enhance the migration of silicon from the sample holder and to knock-out the selenium atoms. Experimental results show that purely thermal loading requires about 150 C higher temperature than that under combined electrical and thermal loading. We also present evidence of moderate current density, when accompanied with high temperature, promoting migration of foreign atoms on the surface of multi-layer graphene. Our in situ transmission electron microscope experiments show migration of silicon atoms at temperatures above 800 C and current density around 4.2x107 A/cm2. The silicon atoms are observed to react with the carbon atoms in the multi-layer graphene to produce silicon carbide at 900-1000 C temperature. In absence of electrical current, there is no migration of silicon and only pyrolysis
9:00 PM - NT4.2.16
Observation of the Effect of Exposure to Ambient Air on MBE Grown WSe2
Christopher Ahles 1,Jun Park 1,Suresh Vishwanath 2,Xinyu Liu 4,Randall Feenstra 3,Jacek Furdyna 4,Debdeep Jena 2,Huili Xing 2,Andrew Kummel 1
1 University of California, San Diego La Jolla United States,2 Cornell University Ithaca United States4 Notre Dame University Notre Dame United States3 Carnegie Mellon University Pittsburgh United States
Show AbstractIn order to realize the potential applications of layered TMDs, TMD materials have to be exposed to ambient air for device fabrication and characterization. Therefore, it is critical to understand the effect of air exposure on layered TMD materials. The effect of air exposure on MBE 2H-WSe2/HOPG was elucidated on the atomic scale via scanning tunneling microscopy and spectroscopy. The 2H-WSe2 was grown by molecular beam epitaxy (MBE) on HOPG. The WSe2 nucleation is initiated at both the step edges and the terraces of HOPG, consistent with mixing of step flow growth mode and Volmer-Weber growth mode. High resolution STM images revealed nearly zero vacancy defects or dislocations in WSe2 and the band gap of a WSe2 monolayer was determined to be 2.18 ± 0.03 eV by STS. After exposure of air to WSe2/HOPG, only the edges of the WSe2 layer were oxidized, while the terraces of the WSe2 layer were nearly intact. ; Iin the air exposed WSe2 terraces, zero air induced defects were observed and the band gap was nearly identical to that of as-decapped WSe2 ML (2.07 ± 0.05 eV by STS). Conversely, exposure of air to WSe2/HOPG induced topological transitions from smooth edges of WSe2 to bright protrusions along the edges of WSe2 and a significant decrease in tunneling current at the edges of WSe2 ML compared to the edge of as-decapped WSe2 ML. It is hypothesized that the oxidation of the WSe2 edges originates from the dangling bonds on the atoms at the WSe2 edges. ; Iin high resolution STM images of as-decapped WSe2, a bright brim was observed along the WSe2 edge, originating from excess charge states of dangling bonds at the WSe2 edge. In the STS of the as-decapped WSe2 edge, a very narrow band gap was observed, as well as a shifted Fermi level towards the valence bands. However, this electronic edge state is passivated by the oxidation of edges in ambient air. The data is consistent with oxidation passivating defect states at the edges of WSe2 which will be beneficial for device fabricating.Consequently, the present study can serve as a milestone to develop processes for device fabrication based on TMD materials, and elucidate the effects of the presence of electronic edges states on device performance.
9:00 PM - NT4.2.17
Halide-Assisted Atmospheric Pressure Growth of Large WSe2 and WS2 Monolayer Crystals
Shisheng Li 1,Shunfeng Wang 1,Dai-Ming Tang 2,Weijie Zhao 1,Huilong Xu 1,Leiqiang Chu 1,Yoshio Bando 2,Dmitri Golberg 2,Goki Eda 1
1 National University of Singapore Singapore Singapore,2 National Institute for Materials Science Tsukuba Japan
Show AbstractOne of the key challenges in the controlled chemical vapor deposition (CVD) of two-dimensional (2D) tungsten dichalcogenide crystals is achieving a steady flow of tungsten source in the vapor phase. Due to the high sublimation point of tungsten oxide precursors, CVD of tungsten dichalcogenides can be achieved only at high temperatures (>1000 oC) or at low pressures. We demonstrate atmospheric pressure CVD of WSe2 and WS2 monolayers at moderate temperatures (700 ~ 850 oC) using alkali metal halides (MX where M= Na or K and X=Cl, Br or I) as the growth promoters. Chemical analysis of the precursor after a growth run reveals that alkali metal halides react with tungsten oxide at elevated temperatures to form volatile tungsten oxyhalide species during growth, which leads to efficient delivery of the precursor to the growth substrates. The monolayer crystals obtained in this method were found to be highly symmetric, large in size (up to 200 μm), and free of unintentional doping due to alkali metal and halogen atoms. The monolayer crystals also exhibited excellent field-effect transistor (FET) performances with tunable polarity, large current on/off ratios (~107), and high field-effect mobilities (electron and hole mobilities of 102 and 26 cm2 V-1 s-1 for WSe2 and electron mobility of ~14 cm2 V-1 s-1 for WS2 devices), further indicating their high quality.
Reference:Li, S. et al., Applied Materials Today 1 (2015) 60–66
9:00 PM - NT4.2.18
Ultra-Fast Carrier Dynamics in Large Area, CVD Deposited, Monolayer MoS2
Paul Cunningham 1,Kathleen McCreary 1,Aubrey Hanbicki 1,Marc Currie 1,Berend Jonker 1,L. Michael Hayden 2
1 US Naval Research Laboratory Washington United States,2 University of Maryland Baltimore County Baltimore United States
Show AbstractTwo-dimensional transition metal dichalcogenides (TMDC) have recently been under intense investigation due to their direct bandgaps and potential uses in high speed electronic devices. Despite being the most thoroughly studied TMDC, the mechanisms dominating the carrier dynamics in monolayer MoS2 remain poorly understood. Much work to date has been performed on multilayer films, which have an indirect bandgap and reduced Coulombic interactions as compared to monolayer MoS2. Other studies have focused on monolayer exfoliated flakes, which are not scalable for industrial applications.
In this work, we present transient optical absorption and time-resolved terahertz spectroscopy (TRTS) measurements of large area (~ 1 cm2) monolayer films of MoS2 deposited on dielectric substrates via chemical vapor deposition. We report excited state dynamics that are surprisingly similar to those previously reported for multilayer MoS2 in that they can be described by two decay components: one operative on sub-picosecond times scales and the other in 10's of picoseconds. Combining these two techniques allow for the contributions to the two-component excited state dynamics to be identified.
Terahertz conductivity measurements reveal that charge carriers, and not excitons, are responsible for the sub-picosecond dynamics. The frequency-dependent dielectric function is characteristic of localized charges in a polycrystalline material with sub-micron grains. Sub-picosecond lifetime photo-induced conductivity suggests that charges are rapidly trapped. This process complicates the excited state spectrum, which shows changes in line-width and spectral shifts associated with bandgap renormalization in addition to simple state-filling effects. We also report conductivity originating from sub-optical gap excitation, which reveals the existence of mid-gap trap states that may originate from surface defects or grain boundaries. These dynamics are largely insensitive to excitation density, wavelength, temperature and choice of substrate. We assign the slow excited state dynamics to the interband exciton lifetime, which lengthens from 50ps in monolayer films to 150ps in multilayer films due to the transition from direct to indirect bandgap. We see no signature from trions, suggesting that surface adsorbates neutralize the intrinsic n-type behavior.
Our results help add clarity to several conflicting reports of the optoelectronic properties of MoS2, providing explanations for the sub-picosecond excited state dynamics as well as the absence of trions. Our findings suggest that trapping dominates the transient optical properties of monolayer MoS2. They also suggest that CVD grown films hold potential for high-speed optoelectronic applications.
9:00 PM - NT4.2.19
Black Phosphorus Films of Stacked Flakes for Stable and Selective Humidity Detection
Poya Yasaei 1,Amirhossein Behranginia 1,Tara Foroozan 1,Mohammad Asadi 1,Kibum Kim 1,Fatemeh Khalili-Araghi 1,Amin Salehi-Khojin 1
1 Univ of Illinois-Chicago Chicago United States,
Show AbstractBlack phosphorus (BP) is the most thermodynamically stable allotrope of phosphorus with several unique properties that has recently been exfoliated to atomically thin flakes. On the downside, BP atomic layers are observed to irreversibly degrade into phosphorus oxoacids upon exposure to humid air. However, in more robust configurations such as films, composites, and embedded structures, BP can potentially be utilized in many applications. In this study, we fabricated films of stacked black phosphorus nanoflakes made by liquid exfoliation and explored their chemical sensing characteristics upon exposure to different families of chemicals such as ketons, benzenes, alcohols, and water vapor. Interestingly, an ultra-sensitive and selective response was observed toward water vapor which provided for a trace-level moisture detection capability. The drain current of the sensors is observed to modulate by nearly 4 orders of magnitude when the humidity level changes from 10% to 85%. Evidenced by our spectroscopic and control experiments, we propose that the operation principle of the BP film sensors is based on a change in the leakage ionic current caused by auto-ionization of water molecules and ionic solvation of the phosphorus oxoacids produced on moist BP surfaces. Concurrently, our stability tests reveal that the response of the BP film sensors remain quite stable after prolonged exposures to ambient conditions (tested up to 3 months). This study opens up the route for utilizing films of BP stacked flakes in many potential sensing applications as well as energy conversion devices.
9:00 PM - NT4.2.20
Heterojunction of Black Phosphorous on Single Layer Graphene
Vijayarangamuthu Kalimuthu 1,Seungbae Ahn 1,Cheol-Min Park 2,Ki-Joon Jeon 1
1 Department of Environmental Engineering Inha University Incheon Korea (the Republic of),2 School of Materials Science and Engineering Kumoh National Institute of Technology Gyeongbuk Korea (the Republic of)
Show AbstractAfter discovery of graphene, there have been increasing interest in other two-dimensional (2D) materials that possess unique electrical and optical properties. The characteristics of 2D materials include both layered structures and heterostructures of transition metal dichalcogenides, silicone, germanene, and black phosphorous (BP). Among them BP is considered to be an interesting material due to its intrinsic band gap nature, strong in-plane anisotropy, and high electron mobility. These properties make BP as a promising material for applications in electronics, optoelectronics and clean energy. However, there are two main issue with BP, i.e., method of synthesis and degradation under moisture. In the present study, we report the facile synthesis method of few layer BP flakes by treating red phosphorous and further utilized BP flakes to make hetero-structure on single layer graphene. The structural and chemical properties of BP-graphene hetero-structure were evaluated using Raman spectroscopy. The electrical performance of heterojunctions will be discussed.
9:00 PM - NT4.2.21
A Density Functional Theory Study of Electronic and Magnetic Properties of Rare Earth Doped Monolayer Molybdinum Disulphide
Abdul Majid 2,Anum Imtiaz 1,Masato Yoshiya 2
1 Physics University of Gujrat Gujrat Pakistan,2 Department of Adaptive Machine Systems Osaka University Osaka Japan,1 Physics University of Gujrat Gujrat Pakistan2 Department of Adaptive Machine Systems Osaka University Osaka Japan
Show AbstractThe structural, electronic and magnetic properties of MoS2 single layer doped with 4f rare earth metal (RE) atoms Sm, Eu, Gd, Tb and Dy in the form of Mo11S24RE supercells were investigated using Density Functional Theory implemented within ADF-BAND. Our results with Pedrew Burke Ernzerhof (PBE) exchange correlational functional used within generalized gradient approximation (GGA) showed that variation of bond length is maximum around impurity atom in 2D Eu:MoS2 and they are minimum near Sm impurity. The results indicated the appearance of impurity levels in the band gap of MoS2 that lead to band gap narrowing. Analyzing the difference of spin charge densities, the magnetic moment of 3.3 µB, 8.1 µB, 8.5 µB, 6.8 µB and 6.4 µB was seen for Sm-, Eu-, Gd-, Tb- and Dy-doped structures, respectively. The findings of the work point out that the band gap and magnetic properties of 2D MoS2 structures can be tuned by RE 4f doping to make it p-type degenerate semiconductor for device grade applications.
9:00 PM - NT4.2.22
Modeling of the Transfer Characteristics in Graphene/MoS2 Heterostructures
Khoe Nguyen 1,Shih-Yen Lin 4,Yia-Chung Chang 5
2 Molecular Science and Technology Program, Taiwan International Graduate Program, Academia Sinica Taipei 115 Taiwan,3 Department of Physics National Central University Jhongli Taiwan,1 Research Center for Applied Sciences, Academia Sinica Taipei Taiwan,1 Research Center for Applied Sciences, Academia Sinica Taipei Taiwan,4 Graduate Institute of Electronics Engineering National Taiwan University Taipei Taiwan1 Research Center for Applied Sciences, Academia Sinica Taipei Taiwan,5 Department of Physics National Cheng-Kung University Tainan Taiwan
Show AbstractGraphene being a zero band-gap semimetal [1] intrinsically possesses very high mobility [1,2] but with very low on/off ratio [2], whereas two-dimensional MoS2 being a direct band-gap semiconductor of around 1.9 eV [3] has low mobility [4,5], but can reach much higher on/off ratio [5]. Therefore, they are expected to complement each other in graphene/MoS2 heterostructures with high enough mobility and reasonable on/off ratio [6]. Here we present a simple device model based on the Dirac equation for carriers in graphene and effective-mass model for carriers in MoS2 to explain the experimental results for IV curves of graphene/MoS2 heterostructures theoretically. The carrier transfer from graphene to MoS2 is described by the relaxation process involving electron-phonon scattering. The IV curves are simulated according to the transport equations in semiconductors with the use of quasi-equilibrium distribution of carriers in leads. The dependence of transport characteristics on temperature, bias, and gate-voltage with and without optical pumping is simulated. The theoretical results are compared with available experimental data and good agreement is obtained with the use of a few physical parameters, which all fall into reasonable range.
[1] M. I. Katsnelson and K. S. Novoselov, Graphene: new bridge between condensed matter physics and quantum electrodynamics, Solid State Commun. 143, 3 (2007).
[2] J.-H. Chen et al., Intrinsic and extrinsic performance limits of graphene devices on SiO2, Nat. Nanotechnol. 3, 206 (2008); F. Chen et al., Dielectric Screening Enhanced Performance in Graphene FET, Nano Lett. 9, 2571 (2009).
[3] K. F. Mak et al., Atomically thin MoS2: a new direct-gap semiconductor, Phys. Rev. Lett. 105, 136805 (2010); A. Splendiani et al., Emerging photoluminescence in monolayer MoS2, Nano Lett. 10, 1271 (2010).
[4] B. Radisavljevic et al., Single-layer MoS2 transistors, Nat. Nanotechnol, 6, 147 (2011); Y. Zhang et al., Ambipolar MoS2 Thin Flake Transistors, Nano Lett. 12 (3), 1136 (2012).
[5] B. Radisavljevic and A. Kis, Mobility engineering and a metal-insulator transition in monolayer MoS2, Nat. Mater. 12, 815 (2013); K.-K. Liu et al., Growth of Large-Area and Highly Crystalline MoS2 Thin Layers on Insulating Substrates, Nano Lett. 12, 1538 (2012); C. R. Wu et al., Multilayer MoS2 Prepared by One-time and Repeated Chemical Vapor Depositions: Anomalous Raman Shifts and Transistors with High ON/OFF Ratio, J. of Phys. D: Appl. Phy. 48 (43) 435101 (2015).
[6] H. Tian et al., Novel field-effect Schottky barrier transistors based on graphene-MoS2 heterojunctions, Sci. Rep., 4, 5951 (2014); M. Y. Lin et al., Toward Epitaxially Grown Two-Dimensional Crystal Hetero-Structures: Single and Double MoS2/Graphene Hetero-Structures by Chemical Vapor Depositions, Appl. Phys. Lett. 105 (29), 073501 (2014).
9:00 PM - NT4.2.23
Nonlinear Piezoelectric Coefficients of TMX2 (TM=Mo,W, X=S,Se,Te) Monolayers Studied by First-Principles Calculation
Yousong Gu 1,Yue Zhang 1
1 University of Science and Technology Beijing Beijing China,
Show AbstractPiezoelectric properties of transition metal dichalcogenide monolayers are important properties for the application of this kind of 2D material in various fields and nonlinear piezoelectric coefficients are vital since large strains are often observed for 2D materials. In this works, first principle’s calculation in the frameworks of density functional theory were performed to investigate the piezoelectric properties of TMX2 (TM=Mo,W, X=S,Se,Te) monolayers. Polarization of the 1H phases of TMX2 were obtained for a series of strains from -5% to 5% and a nonlinear relationship was found. Piezoelectric coefficients were obtained by fitting the simulated data and nonlinear terms of piezoelectric coefficients must be included to fit the simulated polarization results. The piezoelectric coefficients e11 are 370.5, 410.8 and 537.0 pC/m for MoS2, MoSe2 and MoTe2, and 267.7, 309.7 and 407.9 pC/m for WS2, WSe2 and WTe2, respectively. The nonlinear terms e111 and e122 are -817 and -509 pC/m for MoS2, -813 and -367 pC/m for MoSe2, -1206 and -540 pC/m for MoTe2, respectively. The coefficient e11 and e12 have almost the same value but opposite sign, but e111 and e122 are independent.
9:00 PM - NT4.2.24
Transport Properties across Misoriented Bilayer MoS2 Using ab initio Calculations and Non-Equilibrium Greens Function
Kuan Zhou 1,Darshana Wickramaratne 2,Supeng Ge 1,Roger Lake 1
1 UC Riverside Riverside United States,1 UC Riverside Riverside United States,2 UC Santa Barbara Santa Barbara United States
Show AbstractFabrication of electrical and opto-electronic devices with vertically stacked transition metal dichalcogenides (TMDCs), leads to interfaces that are misoriented. Prior experimental and theoretical studies of misorientation in graphene bilayers demonstrated that a few degrees of misorientation is sufficient to decouple the low energy states of the individual layers. Experimental and ab-initio calculations have shown the bandgap of misoriented bilayer MoS2 remains indirect. Some specific trends of the interlayer distance and band gaps have also been researched. However, the transport properties across the misoriented interface of the bilayer TMDCs are currently unknown. The coherent interlayer transmission across two stacks of MoS2 are calculated for unrotated and rotated MoS2 bilayers using ab-initio calculations and non-equilibrium Greens functions. The energy dependence of the interlayer transmission is analyzed. The effect of rotation angle on the electron and hole interlayer resistance is determined.
9:00 PM - NT4.2.25
Two-Dimensional GaSe/MoSe2 Misfit Bilayer Heterojunctions by vdW Epitaxy
Xufan Li 2,Ming-Wei Lin 2,Junhao Lin 1,Bing Huang 4,Alexander Puretzky 2,Cheng Ma 2,Kai Wang 2,Wu Zhou 1,Sokrates Pantelides 3,Miaofang Chi 2,Ivan Kravchenko 2,Jason Fowlkes 2,Chris Rouleau 2,David Geohegan 2,Kai Xiao 2
2 Center for Nanophase Materials Science Oak Ridge National Laboratory Oak Ridge United States,3 Department of Physics and Astronomy Vanderbilt University Nashville United States,1 Materials Science and Technology Division Oak Ridge National Laboratory Oak Ridge United States4 Department of Materials Science and Engineering University of Utah Salt Lake City United States1 Materials Science and Technology Division Oak Ridge National Laboratory Oak Ridge United States3 Department of Physics and Astronomy Vanderbilt University Nashville United States
Show AbstractTwo-dimensional (2D) heterostructures hold the promise for future atomically-thin electronics and optoelectronics due to their diverse functionalities. While heterostructures consisting of different transition metal dichalcogenide monolayers with well-matched lattices and novel physical properties have been successfully fabricated via van der Waals (vdW) or edge epitaxy, constructing heterostructures from monolayers of layered semiconductors with large lattice misfits still remains challenging. Here we report a two-step chemical vapor deposition (CVD) synthesis to grow vertically stacked GaSe/MoSe2 misfit bilayer heterostructures with well-aligned lattice orientation in an incommensurate superlattice exhibiting Moiré patterns in atomically-resolved scanning transmission electron microscopy images. In addition, we show that the growth of lateral heterostructures results in interesting laterally-striped, vertically-stacked bilayer heterojunctions where GaSe and MoSe2 domains meet. We report that these misfit GaSe/MoSe2 heterobilayers form p−n junctions which exhibit effective transport and separation of photo-generated charge carriers between layers, resulting in a gate-tunable photovoltaic response. These new 2D misfit layered heterostructures, characterized by their Moiré superlattices, open the door to new two-dimensional “building blocks” for not only optoelectronic applications, but a wide range of other physical phenomena ranging from interfacial magnetism, superconductivity, and Hofstadter’s butterfly effect.
Synthesis science sponsored by the Materials Science and Engineering Division, Office of Basic Energy Sciences, U.S. Department of Energy. Materials characterization conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. X.L. and M.L. acknowledge support from ORNL Laboratory Directed Research and Development. J.L. and S.T.P. were supported by Department of Energy grant DE-FG02-09ER46554. W.Z. was supported by the U.S. Department of Energy, Office of Science, Basic Energy Science, Materials Sciences and Engineering Division.
9:00 PM - NT4.2.26
MXene Synthesis In Silico
Michael Ashton 1,Kiran Mathew 3,Nicole Trometer 1,Richard Hennig 1,Susan Sinnott 2
1 University of Florida Gainesville United States,3 Materials Science and Engineering Cornell University Ithaca United States2 Materials Science and Engineering Pennsylvania State University State College United States
Show AbstractThe M2AX phases, a family of layered ceramic compounds, have recently been chemically exfoliated to form two-dimensional transition metal carbides, known simply as M2Xenes. In this work, 10,530 bulk compounds belonging to this large class of layered ceramics are screened for their thermodynamic stability using a high throughput first-principles scheme. The objective is to determine which of these bulk compounds are theoretically available as precursors for the formation of new M2Xenes. For the 48 experimentally synthesized M2AX phases we predict energetic stability or small positive (< 30 meV/atom) energetic metastability against energy competing phases. In addition to benchmarking the bulk formation energy of the 48 existing M2AX phases, we predict 267 new bulk compounds with exothermic stability in excess of 100 meV/atom with respect to their most stable competing phases. These stable M2AX precursors can then be chemically exfoliated by aqueous HF to form two-dimensional M2XO2 (M2Xene) nanosheets. We generate and overlay Pourbaix diagrams for the bulk MAX phase precursors and the 2D M2Xenes to determine overlapping solution conditions which will simultaneously result in etching of the M2AX phase and passivation of the M2Xene. These Pourbaix diagrams affirm the solution conditions already used to produce all 5 existing M2Xene nanosheets, and predict that only Al-based M2AX phases can be etched with HF to form M2Xenes. Our predictions offer insight into the experimental conditions that are most likely to result in successful synthesis of both new and existing compounds; for example, the application of a suitable negative electrochemical potential during synthesis appears to be one way to stabilize most M2Xenes against dissolution. These results are expected to serve as a map for experimentalists aiming to design new MXene compositions from scratch.
9:00 PM - NT4.2.27
Local Band Structure of Topological Surface States in Bi1.5Sb0.5Te1.7Se1.3
Wonhee Ko 1,Joonbum Park 2,Insu Jeon 1,Hyo Won Kim 1,Hyeokshin Kwon 1,Youngtek Oh 1,Jun Sung Kim 2,Hwansoo Suh 1,Sungwoo Hwang 1
1 Samsung Advanced Institute of Technology Suwon Korea (the Republic of),2 Pohang University of Science of Technology Pohang Korea (the Republic of)
Show AbstractWe report the observation of the local band structure of topological surface states in Bi1.5Sb0.5Te1.7Se1.3 using scanning tunneling microscopy/spectroscopy (STM/STS). The energy-momentum dispersion relation is locally deduced by extracting the Landau level (LL) energies, which are formed in a high magnetic field, from the STS data. Spatial variation of LLs revealed a shift of the Dirac point energy and a change in Fermi velocity at the nanometer scale. The structure of the potential fluctuation was not correlated with the topography, which indicated the Te/Se substitution did not induce the potential shift because of their same valence. The results show that composition control in Bi2-xSbxTe3-ySey is an efficient method for doping topological surface states without the creation of additional charge states from the substitution.
9:00 PM - NT4.2.28
Fabrication of Devices from Black Phosphorus and 2D SnS Nanosheets Using Inkjet Printing
Pei He 1,Keshav Sharma 1,Jack Brent 1,David Lewis 1,Paul O'Brien 1,Brian Derby 1
1 Univ of Manchester Manchester United Kingdom,
Show AbstractBlack phosphorus (BP) and the analagous isoelectronic IV-VI semiconductors that form the layered herzenbergite crystal structure (e.g. SnS, SnSe, GeS, GeSe) are a relatively unexplored family of 2D materials for device fabrication. These layered materials have been found to display band gaps that are sensitive to the number of atom layers (e.g. 0.3 - 1.0 eV for black phosphorus and 1.1 - 2.0 eV for SnS). They have been identified as candidate materials for a range of applications in electronic devices, sensors and photovoltaics. Both black phosphorus and SnS can be readily exfoliated in NMP and related solvents to form nanosheets with 2 - 5 atomic layers and flake diameters in the range 100 - 1000 nm. If these nanosheets remain stable in dispersion they are potential 2D inks for device fabrication by inkjet printing.
Here we present two studies exploring the use of inkjet printing in fabricating all printed devices from 2D BP (phosphorene) and 2D SnS nanosheets. In order to facilitate drying after printing without the need for heat treatment, we have formulated inks using acetonitrile (CNCH3), which has a boiling point of 82 °C, allowing facile solvent removal after printing at low temperatures. NMP, by comparison, has a boiling point of 202 °C, which requires a high drying temperature (>150 °C) or high vacuum to successfully remove the solvent completely. Inks, which are stable against sedimentation for several days, have been prepared with solid loadings around 1 mgml-1 and with appropriate rheology and surface tension to be deposited by inkjet printing.
All devices were fabricated using an in-house designed and built laboratory scale inkjet printing platform (MPP 1000) equipped with a piezoelectric actuated inkjet print head of internal diameter 60 μm (MJ-ATP-01-60-8MX, Microfab Technologies Inc., Plano, TX, USA). All samples were printed on a silver interdigitated electrode pattern also previously deposited by inkjet printing. After printing the devices are dried in dry N2 at 80 °C. Device performance was characterised electrically using a probe station and its chemical composition and structure studied using Raman spectroscopy and FTIR to determine the integrity of the nanosheets after deposition. The BP device showed extreme sensitivity to moisture and can be used as a highly sensitive humidity detector. Under high humidity conditions the BP shows permanent change in its properties but this can be recovered by a subsequent heat treatment in dry nitrogen. The SnS device shows promising results as a photodetector. These experiments demonstrate the utility of inkjet printing as a technique for the fabrication of electronic devices from a range of 2D materials.
Symposium Organizers
Linyou Cao, North Carolina State University
Bruce Claflin, Air Force Research Laboratory
Thomas Mueller, Vienna University of Technology
Hua Zhang, Nanyang Technological University
Symposium Support
Aldrich Materials Science
APLMaterials|AIP Publishing
HORIBA Scientific
2D Materials and Materials Research Express | IOP Publishing
NT4.3: Controlled Scalable Synthesis of 2D Materials and Heterostructures I
Session Chairs
Wednesday AM, March 30, 2016
PCC North, 100 Level, Room 129 B
9:15 AM - *NT4.3.01
Emerging Frontiers in 2D-Materials Technology
Matthew Chin 1,Barbara Nichols 1,Eugene Zakar 1,Robert Burke 1,Sina Najmaei 1,Alex Mazzoni 1,Amber McCreary 2,Tyler Klarr 3,Daniel Rhodes 4,Daniel Chenet 5,Matin Amani 6, Chakrapani Varanasi 1,Madan Dubey 1
1 U.S. Army Research Laboratory Adelphi United States,2 Pennsylvania State University State College United States3 Oregon State University Corvallis United States4 Florida State University Tallahassee United States5 Columbia University New York United States6 University of California, Berkeley Berkeley United States
Show AbstractRecent progress in the synthesis and characterization of novel 2D materials beyond graphene also known as van der Walls solids has been exceptional. The applicability of these materials for future technologies appear remarkably promising. In addition to single layers, heterostructures of dissimilar 2D materials (stacked layers) also offer unique opportunities. We will discuss current state of the art in the 2D-stacked layers (homogeneous, heterogeneous) and hybrid structures. Synthesis, properties , and possible devices that can be made with these materials with multifunctionality will be emphasized. Basic research issues such as strain engineering in 2D materials and imaging will be discussed in addition to some specific examples of near term applications such as synthetic electronic textiles, power-energy etc.
9:45 AM - *NT4.3.03
Stability, Synthesis and Tailored Modification of 2D MoS2
Jeffrey Grossman 1,Nicola Ferralis 1,Brent Keller 1,Can Ataca 1,Sefaattin Tongay 2,Giacarlo Cicero 3
1 MIT Cambridge United States,2 School of Engineering Arizona State University Tucson United States3 Politecnico di Torino Torino Italy
Show AbstractAmong the 2D transition metal dichalcogenides (TMDs), MoS2 has attracted the most attention due to its structure-dependent electronic properties and potential for a wide range of applications. Yet, in order for 2D MoS2 to be technologically relevant, improved understanding of its properties and phase stability is needed, as well as efficient means to control them. Further, many applications will require high quality large-area films. In this talk, I will discuss our recent computational and experimental work towards understanding and controlling MoS2 phases, through both substitutional doping and chemical functionalization, and tuning junctions between 2 dimensional materials through structural and electronic modulation. In addition, I will show how the use of sulfurization of Atomic Layer Deposited (ALD) oxides could be instrumental to the production of clean, layer controlled, large area two-dimensional materials through control of the ALD nucleation regime.
10:15 AM - NT4.3.04
2D Silicon Telluride (Si2Te3)
Kristie Koski 1
1 Brown Univ Providence United States,
Show AbstractSilicon dominates semiconductor technology not simply because of its properties as a semiconductor, which are actually quite ordinary, but because of its compounds. Silicon processing technology is as much about the mutual processing compatibility of silicon oxide, silicides, and silicon nitride as it is about silicon itself. Thus, silicon telluride seems an obvious choice for bringing a two-dimensional (2D) silicon chalcogenide into the realm of applied electronic materials.
We report the synthesis of high-quality single-crystal layered silicon telluride, Si2Te3, in multiple morphologies that can be exfoliated to form single 2D monolayers. Silicon telluride is a p-type semiconductor with a phololuminescence peak at 641nm. It enjoys chemical and processing compatibility with other silicon-based material including amorphous SiO2 but is chemically sensitive to its environment. We also show the potential for the chemical tunability of this material from a semiconductor to a semi-metal through intercalation of zero-valent metals such as copper.
10:30 AM - *NT4.3.05
Large Area Synthesis of 2D Materials
Eric Vogel 1
1 Georgia Institute of Technology Atlanta United States,
Show AbstractTransition metal dichalcogenides (TMDs) have generated significant interest for numerous applications including sensors, flexible electronics, heterostructures and optoelectronics due to their interesting, thickness-dependent properties. Despite recent progress, the synthesis of high-quality and highly uniform TMDs on a large scale is still a challenge. In this talk, synthesis routes for WSe2 and MoS2 that achieve monolayer thickness uniformity across large area substrates with electrical properties equivalent to geological crystals will be described. Controlled doping of 2D semiconductors is also critically required. However, methods established for conventional semiconductors, such as ion implantation, are not easily applicable to 2D materials because of their atomically thin structure. Redox-active molecular dopants will be demonstrated which provide large changes in carrier density and workfunction through the choice of dopant, treatment time, and the solution concentration. Finally, several applications of these large-area, uniform 2D materials will be described including heterostructures, biosensors and strain sensors.
11:30 AM - *NT4.3.06
2D Crystal Heterostructures and Growth by Molecular Beam Epitaxy
Huili Xing 1
1 Cornell Univ Ithaca United States,
Show AbstractTwo-dimensional (2D) crystals such as transition metal dichalcogenides (TMDs) along with other families of layered materials including graphene, SnSe2, GaSe, BN etc, has attracted intense attention from the scientific community. One monolayer of such materials represent the thinnest “quantum wells”. Numerous novel material properties and device concepts have been discovered, proposed and demonstrated lately. However, the low internal photoluminescence efficiency (IPE, In this talk, I will share our progress and the challenges we face in terms of preparing, characterizing these 2D crystals and their heterostructures using both mechanical exfoliation as well as molecular beam epitaxy.
12:00 PM - NT4.3.07
CVD Grown MoS2 for Flexible Radio-Frequency Electronic Applications
Rudresh Ghosh 1,Harry Chou 1,Atresh Sanne 1,Amritesh Rai 1,Anupam Roy 1,Joon-Seok Kim 1,Deji Akinwande 1,Sanjay Banerjee 1
1 Univ of Texas-Austin Austin United States,
Show AbstractThe extra-ordinary properties of graphene have generated immense interest in the research community. However, the lack of a band-gap makes graphene based digital electronics a difficult challenge. In order to get around this engineers and scientists are exploring other two dimensional (2D) materials. Layered transition metal dichalcogenides (TMDs) which can be metallic, insulating as well semi-conductors are being explored as possible complements of silicon in beyond Moore’s law electronics. Possible applications of these materials include low power, high performance electronics, flexible electronics, novel opto-electronic devices and chemical sensors. Despite initial results showing great promise, lab to consumer electronics transition has been slow. This can be partially blamed on the lack of mature synthesis routes that can provide high yield, high quality material. In this talk we present a brief overview of our efforts in the material synthesis of TMDs using chemical vapor deposition routes. We present novel methods of characterizing defects and impurities using microwave impedance microscopy (MIM) and time of flight-SIMS. Our device results presented show that CVD grown TMDs can achieve similar results as exfoliated samples. We also show that CVD-grown MoS2 can be used for RF applications both on Si-SIO2 substrates and when transferred onto flexible substrates. We conclude our talk with recent developments in CVD synthesis of heterostructures and TMD alloys.
12:15 PM - *NT4.3.08
Stress Driven Synthesis of One- to Two-Dimensional Nanostructures from Nanoparticles
Hongyou Fan 1,Kaifu Bian 1,Leanne Alarid 1,Hattie Schunk 1,Zhongwu Wang 2,Binsong Li 3,Huimeng Wu 4
1 Sandia National Labs Albuquerque United States,2 Cornell High Energy Synchrotron Source Ithaca United States3 Angstrom Thin Film Technology LLC Albuquerque United States4 Olympus Scientific Solution Americas Waltham United States
Show AbstractInspired by the success of graphene, alternative non-graphene 2-dimensional (D) materials have become the focus of intense research due to their unique physical and chemical properties. This presentation highlights our recent progress on development of a new stress driven synthesis of crystalline nanowires and nanosheets from spherical nanoparticles. Starting with ordered spherical nanoparticle arrays, we compress the nanoparticle arrays through controlled pressure fields. In situ high-pressure small-angle synchrotron X-ray scattering studies show that mechanical compression induces oriented ordering of the nanoparticle arrays. When the nanoparticles approach each other along the compression axis, enhanced deviatoric stress drives oriented consolidation of the nanoparticles into crystalline nanorods, nanowires, and nanosheets. Depending on structural dimension of the starting nanoparticle arrays, dimensions such as the diameter of the nanowires and thickness of the nanosheets can be tuned. Applications in materials from metals (Au, Ag, etc) to semiconductors (CdSe, PbS, etc) have been demonstrated. In situ TEM and optical characterizations correlate structural and optical properties of the resulted 1-2D nanostructured materials. Unlike prior methods in materials synthesis processes, such as Ostwald ripening that relies on nucleation and growth, stress driven synthesis involves direct consolidation of nanoparticles into 1-2D materials under mechanical compression offering a facile alternative tool for design and fabrication of advanced functional materials with controlled nanostructures and compositions.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
12:45 PM - NT4.3.09
Artificial Layer-by-Layer Stacking for Wafer Scale Atomically Thin Films
Kibum Kang 1,Kan-Heng Lee 1,Hui Gao 1,Saien Xie 1,Yimo Han 1,David Muller 1,Jiwoong Park 1
1 Cornell Univ Ithaca United States,
Show AbstractLayer-by-layer (LBL) growth allows for novel architectures of thin films such as the ultra-fine tuning of thickness, hetero-stacking of multiple materials, and building of quantum well structures. In order to achieve LBL growth, molecular beam epitaxy and atomic layer deposition have been popularly used under Frank-van der Merwe mode. However, these processes can only be applied to specific materials with limited surface energy and under restricted kinetic conditions. Here, we introduce a novel method for wafer scale artificial LBL stacking with monolayer thick semiconducting films as basis materials. Specifically, our process includes i) wafer scale growth of monolayer MoS2, WS2, MoSe2 and WSe2 using MOCVD, ii) separation of films from growth substrate using dry peel-off process, iii) stacking of multiple layers using van der Waals forces. Low interaction between the MOCVD grown films and substrates allows us to achieve high delamination yield (>99.5%) of monolayer films from substrates. Our LBL stacking process was performed under vacuum without chemical etching or polymer supporter. Therefore, it guarantees an ultra-clean interface without air bubble or chemical contamination. We demonstrated the layer number (N) control by multiple stacking from N=1 to N=20, and vertical heterostructures of MoS2, WS2, MoSe2 and WSe2 monolayer films. We confirm the ultra-clean interface of each layer by photoluminescence and cross sectional scanning transmission electron microscopy.
NT4.4: Controlled Scalable Synthesis of 2D Materials and Heterostructures II
Session Chairs
Bruce Claflin
Thomas Mueller
Wednesday PM, March 30, 2016
PCC North, 100 Level, Room 129 B
2:30 PM - *NT4.4.01
Interaction Effects in Atomically Thin Quasi 2D Materials
Steven Louie 1
1 Physics University of California-Berkeley and Lawrence Berkeley National Lab Berkeley United States,
Show AbstractExperimental and theoretical studies of atomically thin quasi two-dimensional (2D) materials and their nanostructures have revealed that these systems can exhibit highly unusual behaviors. Owing to their reduced dimensionality, quasi-2D materials present opportunities for manifestation of concepts/phenomena that may not be so prominent or have not been seen in bulk materials. Symmetry, many-body interaction, and substrate screening effects often play a critical role in shaping qualitatively and quantitatively their electronic, transport and optical properties, and thus their potential for applications. In this talk, we present theoretical studies on quasi-2D systems such as monolayer and few-layer transition metal dichalcogenides and metal monochalcogenides, as well as other 2D crystals going beyond graphene. Several phenomena are discussed, including novel exciton behaviors, tunable electrical transport and magnetic properties, and the important influence of substrate screening. We investigate their physical origins and compare theoretical predictions with experimental data.
This work was supported in part by the National Science Foundation and the U.S. Department of Energy.
3:00 PM - NT4.4.02
A Comprehensive Understanding of CVD Grown Process for Atomic Layer Transition Metal Dichalcogenides
Bo Li 1,Yongji Gong 2,Zhili Hu 1,Gustavo Brunetto 3,Yingchao Yang 1,Zhuhua Zhang 1,Sidong Lei 1,Jun Lou 1,Douglas Galvao 3,Ming Tang 1,Boris Yakobson 1,Robert Vajtai 1,Pulickel Ajayan 1
1 Department of Materials Science and NanoEngineering Rice University Houston United States,1 Department of Materials Science and NanoEngineering Rice University Houston United States,2 Department of Chemistry Rice University Houston United States1 Department of Materials Science and NanoEngineering Rice University Houston United States,3 Applied Physics State University of Campinas Campinas Brazil3 Applied Physics State University of Campinas Campinas Brazil
Show AbstractThe chemical vapor deposition (CVD) growth of atomic layer transition metal dichalcogenides (TMDs) is a collection of recipes. However, no comprehensive understanding of growth mechanism has been reached due to the challenges from the complexity of reactants and reactions. In current study, we focused on the growth of MoSe2 with MoO3 and Se as the precursors and compared the influence of cooling condition on the growth process. We found the existence of both MoSe2 and nanoparticles on the growth substrate and we were able to build a multi-step growth model of MoSe2 by analyzing the content and distribution of nanoparticles. Our mechanism was unambiguously supported by ab initio molecular dynamics simulations, annealing experiment and feedstock modeling. Our study provides the first comprehensive understanding of CVD process of atomic layer TMDs with a unique growth model fundamentally different from the growth of graphane, nanoparticle or nanowire system.
3:15 PM - NT4.4.03
High Quality Physical Vapor Deposition of Tungsten Disulfide Monolayers
Brian Modtland 1,Xiang Ji 1,Daniela Lopes Mafra 1,Jing Kong 1,Marc A. Baldo 1
1 MIT Cambridge United States,
Show AbstractTransition metal dichalcogenides have been studied for their interesting properties at the monolayer limit. In particular, monolayers of MoS2, MoSe2, WS2, and WSe2 are direct bandgap semiconductors with strong light-matter interactions via tightly-bound excitons. In addition, their hexagonal crystal structure and lack of inversion symmetry leads to degenerate valleys at opposite corners of the reduced Brillouin zone. Combined with large spin-orbit coupling (>150meV), the valley degree of freedom is coupled to carrier spin, creating potential applications in novel optoelectronics, next-generation logic, and quantum information. In most cases, monolayers of transition metal dichalcogenides are created via mechanical exfoliation. With small flake size and randomized deposition, a method of growth is needed to make mass production and commercialization possible. While large-scale growth of TMD monolayers typically makes use of chemical vapor deposition, we show that high quality tungsten disulfide can be grown with a much simpler process that eliminates the need for precise stoichiometry and growth conditions. Using physical vapor deposition, a low-cost tube furnace was used to create monolayers of tungsten disulfide of high quality. The quality of these monolayers is confirmed by optical and electrical measurements, with some device applications discussed.
3:30 PM - *NT4.4.04
Predictive Modeling in 2D Materials: Morphology, Defects, Synthesis
Boris Yakobson 1
1 Rice University Houston United States,
Show AbstractIt is of great interest and importance for materials design to uncover, through computational and theoretical modeling, the following relationships: {basic atomic interactions à structure/morphology à functionality (including electronic)}. We will discuss recent examples from low-dimensional materials, where we seem to achieve satisfactory degree of understanding, mostly focusing on nucleation and islands shapes [1], grain boundaries and dislocations [2], heterojunctions [3], catalysis [4], and even predictive 2D boron synthesis [5].
[1] V. Artyukhov et al. Phys. Rev. Lett. 114, 115502 (2015); V. Artyukhov, Z. Hu et al. unpublished.
[2] X. Zou et al. Nano Lett. 15, 3495 (2015); A. Aziz et al. Nature Comm. 5, 4867 (2014); Y. Liu et al. Nano Lett. 14, 6782 (2014).
[3] Y. Gong et al. Nature Mater. 13, 1135 (2014); A. Kutana, H. Yu et al. unpublished.
[4] Y. Liu et al. Phys. Rev. Lett. 113, 028304 (2014); X. Zou et al. Acc. Chem. Res. 48, 73 (2015).
[5] Y. Liu et al. Angew. Chemie, 52, 3156 (2013); Z. Zhang et al., Angew. Chemie, 127, 13214, DOI: 10.1002/anie.201505425 (2015).
4:30 PM - *NT4.4.05
Synthesis and Characterization of 2D-Layered Materials for Nanodevice Applications
Anupama Kaul 1
1 University of Texas, El Paso El Paso United States,
Show AbstractTwo dimensional (2D) nanomaterials such as graphene and transition-metal dichalcogenides (TMDCs) have attracted tremendous attention over recent years due to their unique properties and potential for numerous applications. In this talk, I will provide an overview of our research efforts in the solution-based exfoliation of 2D layered materials, such as graphene, MoS2 and WS2 where we have chemically exfoliated 2D layered materials for their utility in ink-jet printing. In ink-jet printing, it is necessary to have fluids which possess the right combination of viscosity and surface energy, and a range of organic solvents were explored where the viscosity was engineered through chemical means. The 2D MoS2 and graphite solutions were then patterned onto rigid and flexible substrates and the electrical transport and optical properties of these printed structures were then characterized. The structural characteristics of the solution-dispersed 2D flakes were also analyzed by considering the role of sonication on the particle size distribution which revealed morphological signatures between MoS2 and WSe2 flakes. The 2D solution dispersions were then integrated with organic materials such as polymers for the realization of composite structures for potential sensing applications. In addition, we will also comment on excitonic behavior that we have noted in solution-dispersed 2D materials where measurements on the samples were conducted using a spectrophotometer spanning the ultra violet to near-infra red. Other materials that we have examined include the synthesis of graphene, MoS2 and WSe2 using (CVD), in addition to exploring the intriguing optoelectronic properties of black phosphorus where measurements were made over a range of cryogenic temperatures spanning 350 K to 6 K.
5:00 PM - NT4.4.06
Deterministic Growth of WS2 and WSe2 Heterostructures
Nestor Perea Lopez 2,Victor Carozo 2,Zhong Lin 2,Simin Feng 2,Bruno Carvalho 2,Amber McCreary 2,Ana Laura Elias 2,Sarah Eichfield 2,Joshua Robinson 2,Mauricio Terrones 2
1 Department of Physics Pennsylvania State Univ University Park United States,2 Center for 2D and Layered Materials Pennsylvania State University University Park United States,3 Department of Materials Science and Engineering Pennsylvania State University University Park United States,2 Center for 2D and Layered Materials Pennsylvania State University University Park United States1 Department of Physics Pennsylvania State Univ University Park United States,3 Department of Materials Science and Engineering Pennsylvania State University University Park United States,2 Center for 2D and Layered Materials Pennsylvania State University University Park United States
Show AbstractA novel bottom-up method to create heterostructures of WS2 and WSe2 has been developed. The synthetic method involves the precursors Tungsten Hexacarbonyl (W (CO)6), Hydrogen Disulfide (H2S) and Hydrogen Diselenide (H2Se). Transition metal precursors were pre-deposited lithographically at controlled locations. Selective growth of WS2, WSe2 and their heterostructures was achieved on SiO2, Sapphire and h-BN. Extensive characterization by Raman spectroscopy, atomic force microscopy, scanning and transmission electron microscopies reveled polycrystalline nature of these films which can be grown as thin as mono layer. This method enabled the fabrication of field effect transistors and photo-detectors with performances comparable to those observed on CVD grown and exfoliated samples
5:15 PM - *NT4.4.07
Development of Novel Two-Dimensional Crystals and Their Heterostructures
Zheng Liu 1
1 Nanyang Technological Univ Singapore Singapore,
Show AbstractAlthough the one-atom-think crystal like graphene and h-BN have fantastic properties and attracted tremendous interests in these years, there are a lot of 2D materials beyond graphene to be explored. Using chemical solid reaction and chemical vapour deposition methods, we have successfully synthesized a wide spectrum of 2D materials (both single crystals and few layers), including
1. Binary 2D materials: Borides (h-BN, WB), TMDs (MoS2, WSe2, MoSe2, WSe2, MoTe2, WSe2, ReS2, ReSe2, PtS2, PtSe2, PdS2, PdSe2, NbSe2, SnS2, SnSe, SnSe2, TiS3, HfSe3, HfTe3, TiSe2, TaTe2, TaSe2), and others (InSe, In2Se3, GaSe, SrSi2, Ta3S2, BiI3, PbI2), etc.
2. Ternary and multi-component 2D materials: BxCyNz, MoxW1-xS2, MoWTe4, MoS2xSe2(1-x), WSe2xTe2(1-x), ReS2xSe2(1-x), Ta2NiS5, Ta2NiSe5, Ta2ISe8, TixTa1-xS2, TixNb1-xS2, Ta3Pd3Te14, NiPS3, FePS3, ZnIn2S4, Ta2SeI, V2AlC, W2AlC, CuIP2S4, Tl2Mn2O7
3. Heterostructured 2D materials: Graphene/h-BN, MoS2/WS2, WSe2/MoSe2
4. Organ/Inorganic heterostructures: MoS2/Rubrene, Organic Perovskite/2D
Potential applications of 2D materials have been developed, such as 2D anisotropic electronics (FETs, resonators and photodetectors), energy harvester, lithium ion battery, high performance catalyst and wearable devices etc. These applications pave a promising way to the large scale applications of 2D materials.
References:
1. Lin Niu, Xinfeng Liu, Chunxiao Cong etc. Controlled Synthesis of Organic/Inorganic van der Waals Solid for Tunable Light-matter Interactions, Advanced Materials, 2015
2. Jiadong Zhou, Qingsehng Zeng, Danhui lv etc. Controlled Synthesis of High-quality Monolayered a-In2Se3 via Physical Vapor Deposition, Nano Lett, 2015, 15, 6400
3. Chaoliang Tan, Peng Yu, Yanling Hu etc. High-Yield Exfoliation of Ultrathin Two-Dimensional Ternary Chalcogenide Nanosheets for Highly Sensitive and Selective Fluorescence DNA Sensors, JACS, 2015, 137, 10430
4. Fucai Liu, Wai Leong Chow, Xuexia He, etc. Van der Waals p-n Junction Based on Organic-Inorganic Heterostructure , Advanced Functional Materials, 2015, 25, 5865
5. Zheng Liu, Matin Amani, Sina Najmaei, PM Ajaya, Jun Lou etc. Strain and Structure Heterogeneity in MoS2 Atomic Layers Grown by Chemical Vapor Deposition. Nature Communications 2014, 5, 5246
6. Z. Liu, Y.J. Gong et al., Ultrathin high-temperature oxidation-resistant coatings of hexagonal boron nitride, Nature Communications, 2013, 4, 2541.
7. S. Najmaei,* Z. Liu,* W. Zhou etc. Vapour phase growth and grain boundary structure of molybdenum disulphide atomic layers. Nature Materials 2013, 12, 754-759.
8. Z. Liu, L. Ma, G. Shi, W. Zhou, etc. In-plane heterostructures of graphene and hexagonal boron nitride with controlled domain sizes. Nature Nanotechnology 2013, 8, 119-24.
NT4.5: Poster Session II
Session Chairs
Linyou Cao
Bruce Claflin
Thomas Mueller
Hua Zhang
Thursday AM, March 31, 2016
Sheraton, Third Level, Phoenix Ballroom
9:00 PM - NT4.5.01
Sweep-Dependent Electrical Switching in InSe Thin-Film FETs
Rameez Samnakay 1,Guanxiong Liu 1,Sergey Rumyantsev 2,Antonio Politano 3,Bekir Guerbulak 4,Songuel Duman 4,Alexander Balandin 1
1 Univ of California-Riverside Riverside United States,2 Rensselaer Polytechnic Institute Troy United States3 University of Calabria Rende Italy4 Ataturk University Erzurum Turkey
Show AbstractIndium Selenide (InSe) belongs to the III-VI group of van der Waals material, which are characterized by strong in-plane bonds and weak van der Waals coupling between the layers. Each layer consists of four sub-layers in the form of Se-In-In-Se. The weak coupling allows for mechanical exfoliation of quasi-2D films and investigation of their properties. We fabricated two types of few-layer InSe field-effect transistors (FETs): with protective Boron Nitride (BN) capping and without it. The capped and uncapped devices demonstrated a reproducible hysteresis effect, which allowed for changing the transport type from p- to n- and vice versa depending on the direction of the gate voltage (VG) sweep. The drain current (ID) was dependent on the sweeping speed of the applied gate voltage (VG). The low temperature measurements indicated that the observed effects can be due to deep level traps that capture and emit electrons at different rates depending on temperature. Obtained results can be useful for proposed applications of III-VI materials in electronics and photo-detectors.
The work at UC Riverside was supported, in part, by the Emerging Frontiers of Research Initiative (EFRI) 2-DARE project: Novel Switching Phenomena in Atomic MX2 Heterostructures for Multifunctional Applications (NSF 005400) and by the Semiconductor Research Corporation (SRC) and Defense Advanced Research Project Agency (DARPA) through STARnet Center for Function Accelerated nanoMaterial Engineering (FAME).
9:00 PM - NT4.5.02
Voltage-Induced Charge-Density-Wave Phase Transitions in 1T-TaS2 Thin Films Monitored in situ with Raman Spectroscopy
Guanxiong Liu 1,Timothy Pope 2,Tina Salguero 2,Alexander Balandin 1
1 Univ of California-Riverside Riverside United States,2 University of Georgia Athens United States
Show AbstractSeveral two-dimensional (2D) layered transition metal dichalcogenides (TMD) exhibit interesting charge density wave (CDW) effects at relatively high temperature. Thinning down these materials to their atomic limit opens opportunity to explore physical phenomena that are not present in bulk crystals. It has been demonstrated that the phase transition temperature between different CDW phases can be dramatically changed as the thickness reduces from bulk to atomically thin [1-2]. In this work we study CDW formation in the thin films of 1T-TaS2 with the thickness of 6-9 nm using Raman spectroscopy. To avoid oxidation and degradation of TaS2 films we protect them by capping with a layer of h-BN. The thin films show clear CDW signature peaks at the non-commensurate (NC) – incommensurate (IC) transition temperature of 350 K. We used in-situ Raman spectroscopy to demonstrate that the CDW phase transitions can be triggered electrically, exhibiting a threshold switching phenomenon. The obtained results are important for understanding the electrically- induced phase transition in this CDW system. The threshold switching in 1T-TaS2 is a potentially useful function in various electronic applications, e.g. dynamic random access memory.
This work was supported in part by NSF EFRI 2-DARE project: Novel Switching Phenomena in Atomic MX2 Heterostructures for Multifunctional Applications (NSF 005400).
[1] P. Goli, J. Khan, D. Wickramaratne, R.K. Lake and A.A. Balandin, Nano Letters, 12, 5941 (2012).
[2] R. Samnakay, D. Wickramaratne, T. R. Pope, R. K. Lake, T. T. Salguero and A. A. Balandin, Nano Letters, 15, 2965 (2015).
9:00 PM - NT4.5.03
Wafer-Scale Vapor-Solid MoS2 Synthesis with Precise Layer Control for Optoelectronic Applications
John Robertson 1,Xue Liu 1,Jiang Wei 1,Matthew Escarra 1
1 Tulane University New Orleans United States,
Show AbstractTransition metal dichalcogenides (TMDC’s), in their 2-D forms, have been shown to possess incredible electrical and optical properties. In particular, monolayer TMDC’s possess direct bandgaps in the visible spectrum, making them candidates for light harvesting and emitting, as well as hydrogen catalysis. It is predicted that MoS2 solar cells could have a power density 3 orders of magnitude higher than existing solar cells [1]. The promise of large-area heterostructure devices has motivated a global research effort to produce wafer-scale and high quality monolayer films with highly controlled synthesis. To achieve this growth, several variations of Chemical Vapor Deposition (CVD) have emerged, which offer scalable production of monolayer MoS2, although at a generally lower crystal quality than exfoliated samples [2]. A wafer-scale CVD method that can reliably produce TMDC crystals with high-quality photoluminescence (PL) and transport properties will allow for fabrication, and eventually commercial-scale production, of TMDC light collectors and emitters. We now present our results on a CVD format that modifies the approach of Wu et al [3] and others and demonstrates precise layer control, allowing for a systematic study of the layer-by-layer transformation of MoS2 using Raman, luminescence, and electronic measurements. Our growth uses MoS2 powder as the sulfur source in a CVD growth chamber under the influence of Ar carrier gas, enabling control over vapor transport. MoS2 growth substrates were pre-deposited with Molybdenum films via electron beam evaporation in one-angstrom increments ranging from 0.1nm to 1.1nm. We have shown that approximately 5Å of starting Mo yields a monolayer of MoS2, 10Å Mo yields a bilayer, and the intermediate thicknesses show the transition phases from monolayer to bilayer. The growths show a PL peak at 677nm, which is characteristic of monolayer MoS2, that is strongest for the 5 angstrom Mo precursor sample. The PL magnitude attenuates as the Mo thickness increases past 0.5nm, showing a transition from direct to indirect bandgap as a bilayer expands over the monolayer. Raman spectroscopy also indicates monolayer MoS2 on the 0.5nm sample, with characteristic Raman peaks of E2g at 382cm-1 and A1g at 404cm-1. The bilayer sample shows Raman peaks at 383cm-1 and 406cm-1, as expected. Electronic characterization was done by fabricating bottom-gated transistors; first measurements of the carrier mobility of the monolayer sample show semiconducting behavior, as desired, with mobility increasing by an order of magnitude from monolayer to bilayer samples. The grown MoS2 using this CVD technique shows potential for highly controlled wafer-scale growth that can be optimized for use in heterostructures as light-emitters and solar cells.
[1] Grossman et Al, Nano Lett 13, 3664-3670 (2013)
[2] Choi et al, Appl. Phys. Lett. 012104 (2015)
[3] Wu et al, Appl. Phys. Lett. 105, 072105 (2014)
9:00 PM - NT4.5.04
Effects of Synthesis Parameters on CVD Molybdenum Disulfide Growth
Gustavo Lara Saenz 1,Chandan Biswas 1,Dalal Fadil 1,Anupama Kaul 2,Aditya Mohite 3,Goran Karapetrov 4
1 Electrical and Computer Engineering Department University of Texas El Paso United States,1 Electrical and Computer Engineering Department University of Texas El Paso United States,2 Metallurgical, Materials and Biomedical Engineering Department University of Texas El Paso United States3 Los Alamos National Laboratory Los Alamos United States4 Department of Physics Drexel University Philadelphia United States
Show AbstractSince the isolation of graphene, a monolayer of sp2-bonded carbon atoms arranged in a hexagonal lattice, two-dimensional (2D) layered materials have attracted a great deal of attention due to their outstanding mechanical, optical and electronic properties. The research areas of interest for these new materials include exploring their novel properties, developing scalable approaches to synthesize these materials, and integrating them into a new generation of nanodevices. The utilization of 2D materials in devices has many advantages, which includes scaled materials to the limit of atomic-scale membranes, and the potential to form device structures on flexible and transparent substrates, among others. Transition metal dichalcogenides (TMDs) monolayers in particular have received increasing attention in recent years, especially molybdenum disulfide (MoS2), which is one of the most well explored compound in these family of two dimensional materials. Both of these materials are intrinsic semiconductors with direct band gap characteristics, making them attractive for optoelectronic devices. In this work we present the synthesis of MoS2 using chemical vapor deposition, where we have varied the synthesis parameters by looking at the role of the transport carrier gas, growth temperature and pressure; we have compared the structure and quality of the CVD synthesized MoS2 and compared the characteristics with those obtained for mechanically exfoliated flakes from the bulk crystal. The MoS2 quality has been analyzed using Raman spectroscopy, scanning electron microscopy and atomic force microscopy. In addition, the electronic characterization was done at cryogenic temperatures using a state-of-the-art vacuum probe stage, and current-voltage characteristics were captured using an ultra low-noise semiconductor parameter analyzer (Keysight B1500A). In addition, in our work we have also conducted stability analysis of MoS2 by heating the samples in air in an oxidizing environment, and measured the structural characteristics as well as the photoelectric response of the aged materials. Finally, we will comment and compare our work with previous results reported in the literature.
9:00 PM - NT4.5.05
Device Applications of Black Phosphorus: Ultrascaled Transistors, Schottky Diodes, and Photodetectors
Jinshui Miao 1,Suoming Zhang 1,Le Cai 1,Chuan Wang 1
1 Michigan State University East Lansing United States,
Show AbstractHere, we report high-performance black phosphorus (BP) field-effect transistors with channel lengths down to 20 nm fabricated using a facile angle evaporation process. By controlling the evaporation angle, the channel length of the transistors can be reproducibly controlled to be anywhere between 20 to 70 nm. The as-fabricated 20 nm top-gated BP transistors exhibit respectable on-state current (174 µA/µm) and transconductance (70 µS/µm) at a VDS of 0.1 V. Owing to the use of two-dimensional BP as the channel material, the transistors exhibit relatively small short channel effects, preserving a decent on-off current ratio of 102 even at an extremely small channel length of 20 nm. Additionally, BP Schottky diodes with asymmetric metal contacts have been demonstrated using gold and aluminium electrodes. The device exhibits rectifying characteristics with current rectification ratio up to 1.5×103. The effect of channel length on the electrical characteristics of BP Schottky diodes has also been studied and the results reveal that the device loses its rectifying behavior (rectification ratio = 1.37) at ultrashort channel length of around 30 nm. The transition from rectifying to nonrectifying characteristics at extremely small channel length is attributed to the electric-field-induced barrier thinning, which results in significantly increased tunneling current under reverse bias. Using the BP Schottky diodes with relatively long channel length (~1 µm), photodetectors with fast response time of less than 2 ms have been demonstrated.
9:00 PM - NT4.5.06
Amplitude and Phase Resolved Infrared Imaging on 2D Materials
Honghua Yang 1,Eoghan Dillon 1,Kevin Kjoller 1,Craig Prater 1
1 Anasys Instruments Santa Barbara United States,
Show AbstractSurface plasmon polaritons (SPPs) and surface phonon polaritons (SPhPs) in 2D materials, with their high spatial confinement, can open up new opportunities for enhanced light-matter interaction, super lenses, subwavelength metamaterial, and novel photonic devices. In situ characterization of these polaritonic excitations in different applications requires a versatile optical imaging and spectroscopy tool with nanometer spatial resolution.
Through a non-invasive near-field light-matter interaction, scattering-type scanning near-field optical microscopy (sSNOM) provides a unique way to selectively excite and locally detect electronic and vibrational resonances in real space. By collecting scattered light from the area underneath the atomic force microscope (AFM) tip, local optical properties of the sample can be measured with spatial resolution limited by the tip radius of ~20nm. The full near-field optical response of the plaritonic resonances can be measured with phase resolved detection, providing both amplitude and phase information.
Here we demonstrate the technique by imaging both SPhPs on hexagonal boron nitride (hBN) and SPPs on graphene with tunable QCL and CO2 laser sources. Amplitude and phase near-field optical images provide complementary information for characterizing the complete polaritonic resonances. >90 deg phase shift of SPhPs are observed on hBN, indicating strong light-matter coupling.
By integrating with broadband and ultrafast light sources, sSNOM can be further used to study nonlinear properties associated with these polaritonic resonances.
9:00 PM - NT4.5.07
Compliant Substrate Epitaxy: Au on MoS2
Yuzhi Zhou 2,Daisuke Kiriya 2,Eugene Haller 2,Joel Ager 2,Ali Javey 2,Daryl Chrzan 2
1 Materials Science and Engineering Univ of California-Berkeley Berkeley United States,2 Materials Science Division Lawrence Berkeley National Laboratory Berkeley United States,3 Electrical Engineering and Computer Sciences University of California, Berkeley Berkeley United States,2 Materials Science Division Lawrence Berkeley National Laboratory Berkeley United States
Show AbstractThe heteroepitaxial growth of Au on MoS2, a layered van der Waals bonded dichalcongenide, is analyzed. It is argued that the weak coupling between the layers in the dichalcogenides enables the first substrate layer to deform elastically almost independently from the substrate layers below, and hence enables epitaxial growth for a larger mismatch than might otherwise be expected. Linear, continuum elasticity theory and density functional theory are used to show that a {111} oriented Au film is the preferred over an {001} and a second {111} Au film that is rotated relative to the MoS2 (rotated {111}), despite the fact that the {111} orientation leads to a much higher elastic strain. During the initial stages of growth, the rotated {111} orientation is favored over the {111} and {001} orientation. As the Au film grows thicker (above 10 layer of Au grown), the elastic relaxation of the first layer of the substrate leads to a reduction in the elastic energy of the growing film. This reduces the elastic energy difference between these three orientations enabling the {111} orientation to remain the most stable one for all film thicknesses above 10 layer of Au. This work is supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
9:00 PM - NT4.5.08
A Contamination-Free Large Area 2D Materials Transfer Using Water Soluble Polyvinyl Alcohol for Device Applications
Dae Joon Kang 1,Ngoc Huynh Van 1
1 Sungkyunkwan Univ Suwon Korea (the Republic of),
Show AbstractThe realization of 2D layer based, next generation electronic applications essentially depends on a reproducible, large-scale production of 2D layers via chemical vapor deposition (CVD). To fabricate electronic devices, it is necessary to transfer 2D layers onto different substrates with high efficiency, large area and high quality. Currently, the most commonly used transfer methods rely on polymethyl-methacrylate (PMMA) to support the 2D films and to prevent folding while the growth substrate is etched.1 However, PMMA is not suitable for the transfer of high quality 2D films, because it is easy to introduce contamination due to residues of PMMA thus resulting in degradation of the intrinsic properties and the reliability of devices based on 2D materials as well as some difficulties in constructing multiple layer heterostructure of 2D materials. Thus, further study of heterostructure of 2D materials using layer-by-layer transferring methods has been greatly hampered. Other transferring methods which do not involve PMMA have been also demonstrated.2,3 However, so far they are still limited on the size, uniformity, degree of freedom and quality of transferred 2D materials. In this work, we report a development of a facile transfer technique for 2D materials by adding a water-soluble polyvinyl alcohol (PVA) layer in-between PMMA and 2D materials grown on the rigid substrate. This technique allows not only effective transfer to a target substrate with a high degree of freedom but also etching-free PMMA-assisted transfer while minimizing the effects of contamination on the overall device performance. We applied our universal technique to transfer different 2D materials grown on various rigid substrates, i.e. graphene on copper foil, h-BN on platinum and MoS2 on SiO2 substrates. We then investigated the quality of 2D materials transferred by different transfer techniques such as substrate etching, bubbling transfer and oxide layer etching as reported elsewhere in the past. FET made using graphene transferred by our transfer technique exhibited a negative shift of charge neutrality point close to zero. Furthermore, both graphene and MoS2 FETs showed higher mobility and current modulation than those FETs prepared by conventional transfer technique, mainly due to the elimination of PMMA contaminants. Our results demonstrated the transferred 2D materials are of high quality, and that the developed transfer method is so versatile that even multilayer stacking of heterostructure of 2D materials can be reliably performed over a wafer-scale.
References
Garcia et al. Nano Lett. 12, 4449-4454, 2012.
Yang et al. Small 11, 175-181, 2015.
Banszerus et al. Sci. Adv. 1-6, 2015.
9:00 PM - NT4.5.09
Strain Engineering of Phosphorene via Bending
Deepti Verma 2,Traian Dumitrica 1
2 Department of Chemical Engineering and Materials Science University of Minnesota Minneapolis United States,1 Department of Mechanical Engineering University of Minnesota Minneapolis United States
Show AbstractPhosphorene (PE) - the newly discovered 2D derivative of Phosphorus - has an inherent band gap and a high current on/off ratio. Manipulating strain in PE films - strain engineering (SE) – will offer the opportunity to further tailor their electronic properties. In particular, their atomically thin nature should give these materials a small but nonzero bending modulus, allowing them to conform to nearly any substrates, and to form folds with no sharp edges. Using objective boundary conditions [1] (OBC) coupled [2] with self-consistent charge density functional tight binding (DFTB), we calculated the bending rigidity of PE and its 2D allotropes (β, γ and δ), by modeling bent PE as large diameter nanotubes (PNTs). OBCs not only allow for drastic reductions in the number of atoms in simulations but also enable simulations of chiral PNTs, which is impossible with periodic boundary conditions. At the same time, the method describes how bending influences the electronic structure. We establish a robust platform for achieving SE for anisotropic 2D films. From in-plane stretching we find that PE along with γ- and δ- PE allotropes are orthotropic in nature, whereas β-PE is isotropic. Using results from our calculations and orthotropic thin shell model we develop equivalent continuum structure (ECS) for PE and its allotropes upon bending, having a definite finite thickness and elastic moduli. As expected for orthotropic phases, the elastic stiffnesses and elastic moduli of ECS for PNTs/bent PE depend on the direction of bending but interestingly, the thickness does too. The developed ECS can be used for performing finite element simulations of PE films on substrates.
[1] Dumitrica and James, “Objective molecular dynamics”, J. Mech. and Phys. of Solids 55 (10), 2206-2236 (2007).
[2] Nikiforov et al., “Ewald summation on a helix: A route to self-consistent charge density-functional based tight-binding objective molecular dynamics”, J. Chem. Phys. 139 (9), 094110 (2013).
9:00 PM - NT4.5.10
ALD of 2D MoS2 on 150 mm Quartz and SiO2/Si Substrates
Arturo Valdivia 1,Douglas Tweet 2,John Conley 1
1 Oregon State Univ Corvallis United States,2 Sharp Labs of America Camas United States
Show AbstractTwo-dimensional transition metal dichalcogenides (TMDs) have recently come under intense investigation as building blocks for van der Waals heterostructure electronics. One of the most promising TMDs is MoS2, which transitions from an indirect bandgap (1.3 eV) in its bulk state to a direct band gap (1.8 eV) in its single layer state making it suitable for optoelectronic and transistor applications. The synthesis of high quality single layer MoS2 on large substrates, however, remains a challenge. Although capable of producing the highest quality material, mechanical exfoliation is is limited by small surface areas and is not scalable [1]. Chemical vapor deposition (CVD) is also widely utilized but exhibits a lack of thickness control, poor process stability, and requires high deposition temperatures (typically above 650 °C) [2]. Atomic layer deposition (ALD) is natural technique for the synthesis of 2D materials. ALD is a CVD technique in which reactants are introduced to the chamber sequentially rather than simultaneously. Sequential self-limiting surface reactions allow for precise thickness control, high conformality, and scalability to large surface areas. ALD of single layer MoS2 has been reported recently. However, depositions were limited to approximately 2.5 cm x 2.5 cm area sapphire substrates and high temperature (800 °C) post deposition anneals were required to yield highly crystalline MoS2 films [3].
Here we demonstrate low temperature ALD of monolayer to few layer MoS2 using purge separated cycles of MoCl5 and H2S precursors at reactor temperatures of up to 475 °C. ALD is achieved uniformly across 150 mm diameter SiO2/Si and quartz substrates, avoiding the need for expensive sapphire substrates. Raman scattering studies show clearly the in-plane (E12g) and out-of-plane (A1g) modes of MoS2. The separation of the E12g and A1g peaks is shown to be a function of the number of ALD cycles, shifting closer together with fewer layers. Raman polarization indicates that the MoS2 crystals are oriented parallel to the surface. X-ray photoelectron spectroscopy (XPS) shows that the stoichiometry is improved by post deposition annealing in a sulfur ambient and results in the sharpening of the E12g and A1g peaks. Sulfur annealing also results in the appearance of the band edge photoluminescence peak and the spin orbit splitting peaks, both indications of monolayer MoS2. Finally, high resolution transmission microscopy (TEM) confirms the atomic spacing of monolayer MoS2.
[1] Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman, and M. S. Strano, Nature 7, 669-712 (2012).
[2] T. J. Larrabee, T. E. Mallouk and D. L. Allara, Rev. Sci. Instrum., 84, 014102 (2013).
[3] X. Huang, Z. Zeng, H. Zhang, Chem. Soc. Rev. 42,1934 (2014).
9:00 PM - NT4.5.11
A Molecular Dynamics Study of the Thermal Transport Properties of Monolayer MoS2 and BN
Alireza Tabarraei 1,Xiaonan Wang 1
1 University of North Carolina at Charlotte Charlotte United States,
Show AbstractMonolayer molybdenum disulfide (MoS2) and hexagonal boron nitride (h-BN) are graphene-like two-dimensional materials with remarkable physical and mechanical properties which have provided them with a wide range of potential applications in advanced nanodevices and nanomaterials. Understanding the phonon thermal transport properties of monolayer MoS2 and h-BN is of importance in their successful utilization in nanodevices and provides further insights regarding the thermal transport properties of graphene-like two-dimensional materials. Such understanding will also allow us to tailor the thermal conductivity of two-dimensional materials through phonon engineering. We Use reverse nonequilibrium molecular dynamics simulations (RNEMD) to study the thermal conductivity of monolayer hexagonal boron nitride nanoribbons and monolayer molybdenum disulfide as a function of length, width, strain and edge chirality.
Our results show that the thermal conductivity of both MoS2 and h-BN is considerably lower than the thermal conductivity of graphene. Similar to graphene, by increasing the length of the ribbons, their thermal conductivity increases. Thermal conductivity of ribbons is significantly affected by the edge chirality of the ribbons; zigzag ribbons have a higher thermal conductivity than armchair ribbons with similar length and width. The strain impact on the thermal properties of two-dimensional materials is not the same. Longitudinal tensile strain has negligible effects on the thermal conductivity of MoS2 ribbons. However, longitudinal strains have an anomalous impact on the thermal conductivity of h-BN; thermal conductivity of h-BN nanroibbons increases under the application of tensile strain. This is in direct contrast with what is observed in bulk materials and some other two-dimensional materials such as graphene as the application of tensile strain on such materials leads to a reduction of their thermal conductivity. Investigation of the phonon-dispersion curves of two-dimensional boron nitride at different strains indicates that under tensile strain out-of-plane phonon acoustic branch stiffens. The stiffening of the out-of-plane acoustic branch leads to an increase in the thermal conductivity.
9:00 PM - NT4.5.12
Band Offset Measurements of 2-D TFET Interfaces by Internal Photoemission Spectroscopy
Qin Zhang 1,Mingda Li 2,Suresh Vishwanath 2,Rusen Yan 2,Shudong Xiao 2,Huili Xing 3,Nhan Nguyen 1
1 Semiconductor and Dimensional Metrology Division National Institute of Standards and Technology Gaithersburg United States,2 School of Electrical and Computer Engineering Cornell University Ithaca United States2 School of Electrical and Computer Engineering Cornell University Ithaca United States,3 Department of Materials Science and Engineering Cornell University Ithaca United States
Show AbstractCurrently tremendous research focuses on different transistor designs for beyond-CMOS technology. Among the latest designs, the tunnel field-effect transistor (TFET) is considered a promising candidate since it offers a much improved on-off current ratio and low power consumption. The TFET operation is based on the band-to-band tunneling (BTBT), instead of thermionic emission of conventional CMOS operation in the sub-threshold regime, which can provide steeper subthreshold slope. However, only few TFETs have been demonstrated with subthermionic subthreshold slope and most of them have much lower ON-currents than the CMOS. As the discovery of two-dimensional (2-D) materials recently, 2-D semiconductors have been proposed and demonstrated as TFET channel materials with supreme electrostatics and much smaller tunneling width, which can improve the subthreshold slope and the on-state BTBT rate. At the same time, the band offsets of the 2-D materials also need to be considered and different heterojunctions can be engineered to reduce the tunneling barrier height to further increase the ON-current. Currently, the band offsets of most 2-D materials are based on theoretical predictions, and have not been experimentally measured yet.
Internal photoemission (IPE) is a robust and accurate technique to determine barrier height at a solid/solid interface, in particular as a most common application to semiconductor/insulator and metal/insulator interfaces. Conventional IPE on TFET structure has been recently proved possible where IPE process was specifically tailored to enhance sensitivity of electron photoemission from each semiconductor component of the heterojunction over a large band gap insulator. Consequently, the barrier height at the semiconductor heterojunction can be deduced.
This talk is intended to provide a basic introduction to IPE and describe the approach used in an IPE measurement to determine band offsets of various 2-D materials that have been proposed in TFET design, i.e. SnSe2, WSe2, MoS2, etc. In particular, we will demonstrate the method of band offset determination applied to the Au-Al2O3-SnSe2 test structure, where the 40 nm heavily-doped n-type SnSe2 layer is grown on GaAs substrate by molecular beam epitaxy (MBE). The band offset from SnSe2 conduction band minimum to Al2O3 conduction band minimum is determined by the threshold of the cube root of IPE yield and found to be 3.5 eV, which confirms that SnSe2 has a larger electron affinity than most semiconductors and can be combined with other semiconductors to form near broken-gap heterojunctions with low barrier heights. To identify the source of photoemission, imaginary part of the pseudo-dielectric function of the 2-D material is also measured by spectroscopic ellipsometry (SE) and aligned with optical features observed in yield plot. We will also present the results from the measurements and data analysis from other 2-D materials being considered for TFET structure.
9:00 PM - NT4.5.14
Direct Writing of 2D Flexible Electronic Devices
Nicholas Glavin 2,Abigail Juhl 2,Michael McConney 2,John Bultman 2,Jianjun Hu 2,Phillip Hagerty 2,Andrey Voevodin 4,Christopher Muratore 2
2 Materials and Manufacturing Directorate Air Force Research Laboratory Wright-Patterson AFB United States,3 University of Dayton Research Institute Dayton United States,2 Materials and Manufacturing Directorate Air Force Research Laboratory Wright-Patterson AFB United States1 Univ of Dayton Dayton United States,2 Materials and Manufacturing Directorate Air Force Research Laboratory Wright-Patterson AFB United States4 Department of Materials Science and Engineering University of North Texas Denton United States
Show AbstractUltra-thin two-dimensional (2D) semiconducting materials possess a combination of large, tunable electronic bandgaps, optical transparency, and mechanical flexibility, and will likely revolutionize electronic devices such as wearable sensors and flexible displays. A primary step in the development of such devices with integrated 2D materials is the development of scalable, transfer-free synthesis over large areas at low temperatures. Electrically insulating amorphous transition metal dichalcogenide (TMD) films can be deposited via physical vapor deposition on large area flexible substrates at room temperature, and crystallized with subsequent illumination with light. Focused laser light with a power density of ~1 kW cm2 is suitable for writing micron scale features in semiconducting transition metal dichalcogenides on polymer substrates. Broad band illumination from a xenon lamp can also be used over the large substrate areas (> 100 cm2), or passed through a physical mask to print features only in desired locations. The semiconducting properties of 2D MoS2 and WS2 materials synthesized in this way have been characterized using conductive atomic force microscopy, and other techniques to observe the expected temperature dependence on electrical conductivity. Structure and composition of the materials can be controlled by altering the incident fluence as well as by controlling the ambient environment during illumination, as verified by Raman spectroscopy, X-ray photoelectron spectroscopy, cross-sectional and plan view transmission electron spectroscopy, and other techniques. Multiple layers of 2D materials can also be treated in this way. For example, both layers in a MoS2/WS2 heterostructure of 10 nm total thickness on a polymer (PDMS) substrate were crystallized upon laser illumination. Diverse 2D architectures and devices built from illumination-based crystallization techniques will be highlighted.
9:00 PM - NT4.5.15
3D Printing of 2D Circuits with Biomolecules
Abigail Juhl 2,Nicholas Glavin 2,Michael McConney 2,Phillip Hagerty 1,Jessica Dagher 2,Gary Leuty 2,Rajiv Berry 2,Steve Kim 2,Rajesh Naik 2,Michael Durstock 2,Christina Harsch 2,Christopher Muratore 2
2 Materials and Manufacturing Directorate Air Force Research Laboratory Wright-Patterson AFB United States,2 Materials and Manufacturing Directorate Air Force Research Laboratory Wright-Patterson AFB United States,1 Univ of Dayton Dayton United States1 Univ of Dayton Dayton United States,2 Materials and Manufacturing Directorate Air Force Research Laboratory Wright-Patterson AFB United States
Show AbstractAttachment of biomolecules, such as peptides, to 2D materials is an attractive approach for enhancing sensitivity and selectivity in molecular sensing applications. The peptides provide receptor sites allowing detection of specific analytes, while the high surface-to-volume ratio associated with molecularly thin 2D materials increases the sensitivity to very small changes in surface potential associated with adsorption of individual molecules. The critical step toward novel 2D molecular sensors is identification and understanding of the structure and chemistry of peptide-transition metal dichalcogenide interfaces. In this study, phage display techniques were employed to identify peptides that selectively bind to 2D targets such as graphene and MoS2 in the forms of micro scale fine powder, bulk crystals, and ultra-thin films (
9:00 PM - NT4.5.16
Observation of Electronic Structure of Silicene and Its Defects by Scanning Tunneling Microscopy
Youngtek Oh 1,Hyeokshin Kwon 1,Wonhee Ko 1,Hyo Won Kim 1,JiYeon Ku 1,Hwansoo Suh 1,Sungwoo Hwang 1,Insu Jeon 1
1 Samsung Advanced Institute of Technology Suwon-si Korea (the Republic of),
Show AbstractSilicene, an atomic monolayer of silicon atoms, has a hexagonal symmetry and is expected to have Dirac fermions. Recently, silicene has been intensively investigated in various substrates such as Ag(111), ZrB2 (0001), and Ir(111). We grew a monolayer of silicene on the Ag(111) surface by ultrahigh vacuum deposition and annealing of silicon atoms. The geometric and electronic properties of silicene grown on the Ag(111) were investigated by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). The (4×4) structures of silicene were observed in LEED patterns and STM images. We observed that domains were formed inside the silicene. The electronic properties of silicene and its defects were measured by scanning tunneling spectroscopy (STS).
9:00 PM - NT4.5.17
Quantum Oscillations and Capacitance Spectroscopy in High Quality Sandwiched Black Phosphorus
Yingying Wu 1,Ning Wang 1
1 Hong Kong University of Science and Technology Hong Kong China,
Show AbstractThe family of two-dimensional materials was recently joined by atomically thin black phosphorus which has a high theoretical mobility and a tunable bandgap structure. However, degradation of properties under atmospheric conditions and high-density charge traps in black phosphorus have largely limited its actual mobility. Here, we report the fabrication of stable sandwiched heterostructures by encapsulating atomically thin black phosphorus between hexagonal boron nitride layers and quantum oscillations in black phosphorus two-dimensional hole gas are observed at low magnetic fields. Besides, vertical BP heterostructure devices offer great advantages in probing the electron states in monolayer and few-layer phosphorene at temperatures down to 2K through capacitance spectroscopy. We also report the negative compressibility in BP-based heterostructure.
9:00 PM - NT4.5.18
Air Can Passivate Chalcogen Vacancies in Two-Dimensional Semiconductors
Yuanyue Liu 1,Paul Stradins 1,Suhuai Wei 1
1 National Renewable Energy Laboratory Golden United States,
Show AbstractDefects play important roles in semiconductors (SCs). Unlike those in bulk SCs, defects in two-dimensional (2D) SCs are exposed to the surrounding environment, which can potentially modify their properties/functions. Air is a common environment; yet its impact on the defects in 2D SCs still remains elusive. In this work, we unravel the interaction between air and chalcogen vacancies (VX)-the most typical defects in 2D SCs. We find that, although the interaction is weak for most molecules in air, O2 can be chemisorbed at VX with a barrier that correlates with the SC cohesive energy and can be overcame even at room temperature for certain SCs. Importantly, the chemisorbed O2 changes the VX from commonly-believed harmful carrier-traps to electronically benign sites. This unusual behavior originates from the iso-valence between O2 and X when bonded with metal. Based on these findings, we propose a facile approach to improve the performance of 2D SCs by using air to passivate the defects. [1]
[1] Y. Liu, P. Stradins & S. Wei, accepted by Angewandte Chemie International Edition
9:00 PM - NT4.5.19
Thickness-Dependent Band Structure and K-Point Spin-Splitting of WS2 Grown by CVD on SiO2/Si
Mark Micklich 2,Michael Gomez 2,Duy Le 3,William Coley 2,Iori Tanabe 2,David Barroso 2,Ariana Nguyen 2,Alexei Barinov 2,Peter Dowben 2,talat Rahman 3,Ludwig Bartels 2
1 Department of Chemistry University of California. Riverside Riverside United States,2 Materials Science amp; Engineering University of California. Riverside Riverside United States,3 Physics Dept University of Central Florida Orlando United States2 Materials Science amp; Engineering University of California. Riverside Riverside United States
Show AbstractWe report on band-mapping of single and few-layer WS2 utilizing the spectromicroscopy beamline oft he Elettra Synchrotron. A micron-scale beams spot allowed us to target different thickness areas in the same growth region. Chemical vapor deposition growth of WS2 utilized a Si substrate with an oxide layer thickness of ~1nm only. This thickness proved to passivate the substrate sufficiently for WS2 growth while being thin enough to prevent charging during photoelectron spectroscopy. Corresponding density functional theory calculations in the presence and absence of strain are in excellent agreement with experimental data. Focusing on the K-point, we are able to resolve the evolution of the WS2 spin splitting with layer thickness.
9:00 PM - NT4.5.20
Copper-Based Growth of Graphene/h-BN Heterostructure
Fan Yang 1,Eui Sang Song 1,Bin Yu 1
1 SUNY Polytechnic Institute CNSE Albany United States,
Show AbstractTwo-dimensional insulator hexagonal boron nitride (h-BN) is an ideal substrate and passivation layer for graphene-based electronic devices owing to the negligible lattice mismatch between the two materials and the significantly reduced optical photon scattering in h-BN as compared with that in SiO2. In this research, we demonstrated copper-based growth of graphene/h-BN heterostructure through chemical-vapor-deposition (CVD) process. Extensive material characterization, including Raman spectroscopy, XPS, and SEM imaging, were conducted to confirm the growth and the resultant material characteristics. Carrier transport behavior of the as-grown graphene/h-BN heterostructure was examined via electrical measurements.
9:00 PM - NT4.5.21
Two Dimensional Materials as Hole Extraction Layer in Perovskite Solar Cells
Yu Geun Kim 1,Ki Chang Kwon 2,Quyet Van Le 1,Ho Won Jang 2,Soo Young Kim 1
1 Chung-Ang Univ Seoul Korea (the Republic of),2 Seoul National University Seoul Korea (the Republic of)
Show AbstractOrganometallic halide perovskites have attracted much attention as a promising material for high-efficiency absorber in photovoltaic devices. It is based on organic-inorganic light absorbing semiconducting material with a perovskite polycrystalline structure, CH3NH3PbX3, where X is halogenatoms (Cl, Br, I, or combination of some of them). The poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was usually used as a hole extraction layer in planar type perovskite photovoltaic cells. However, PEDOT:PSS showed several disadvantages such as hygroscopic properties and a highly acidic suspension, which result in poor stability in air.
In this respect, we have investigated the possibility of two-dimensional materials including MoS2, WS2, and graphene oxide (GO) as a hole extraction layer in perovskite photovoltaic cells. MoS2 and WS2 layers with a polycrystalline structure were synthesized by a chemical deposition method using uniformly spin-coated (NH4)MoS4 and (NH4)WS4 precursor solutions. GO was made by following Hummer’s method. Raman spectra, transmission electron microscope image, and x-ray diffraction data confirmed the well-synthesis of two-dimensional materials. The device structure is indium tin oxide/PEDOT:PSS or MoS2 or WS2 or GO/CH3NH3PbIxCl3-x/bathocurione/LiF:Al. The PCE values of PEDOT:PSS, GO, MoS2 and WS2-based one are measured to be 9.93, 9.62, 9.53 and 8.02 %, respectively. These result indicated that two-dimensional materials could replace the PEDOT:PSS layer for the improvement of device stability.
9:00 PM - NT4.5.22
Controlling the Threshold Voltage of MoS2 Field-Effect Devices by Sulfur-Vacancy Engineering
Wei Sun Leong 1,John Thong 1
1 Electrical and Computer Engineering National University of Singapore Singapore Singapore,
Show AbstractWe report a facile approach to achieve bidirectional threshold voltage tuning of MoS2 field-effect transistors. By increasing and decreasing the amount of sulfur vacancies in the basal plane of MoS2, the threshold voltage of MoS2 transistors can be left- and right-shifted, respectively. In this work, we show that transistors fabricated on sulfur-treated MoS2 flakes, which have few sulfur vacancies, exhibit a large and positive threshold voltage and two-fold mobility enhancement. On the other hand, our elegant hydrogen treatment is able to tune the threshold voltage of such MoS2 transistors to a small value (nearly zero) without any performance degradation by creating a substantial amount of sulfur vacancies in MoS2. We further note that both surface treatments proposed here are compatible with silicon complementary metal-oxide-semiconductor (CMOS) fabrication processes. By performing first-principles calculations, we show that the observed tunability of the threshold voltage arises from the fact that sulfur vacancies create defect states in the band gap of MoS2, which shift in energy as the vacancy density changes. When the energy of these defect states aligns with the Fermi level from the source/drain electrodes, there is a sudden increase in current, thus the transistor switches from off- to on-state.
9:00 PM - NT4.5.23
Exfoliation of Aluminum Oxide Nanosheets from a Glycothermal Precursor
Nelson Bell 1,Stanley Chou 1,Laura Biedermann 1
1 Sandia National Laboratories Albuquerque United States,
Show AbstractCreating novel 2D devices requires integration of the basic material components of conductors, semi-conductors, and insulators. Insulators must be electrically and chemically stable, and alumina is common in energy storage components such as capacitors. Great progress in conductor and semi-conductor 2D materials have been made, but insulator chemistries are limited to exfoliated clays or double hydroxides. Single cation nanosheets are limited in scope.
Using glycothermal synthesis, it was discovered for the first time that aluminum oxide nanosheets can be exfoliated from a precursor phase in select alcohol solvents. The chemical modification of existing layered compounds by the glycothermal environment decreases the interlayer attraction between the (oxy)hydroxide atomic layers, allowing for nanosheet separation. The exfoliation process is characterized by HR-TEM and AFM. Solution processing of nanosheets by Langmuir trough characterized the formation of nanosheets and deposition of monolayers on Si wafer substrates. Glycothermal processing has the potential to provide a route to other single cation nanosheet exfoliation by mitigation of interlayer hydrogen bonding forces.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
9:00 PM - NT4.5.24
Computational Discovery of Novel 2D Materials with a Genetic Algorithm and a High-Throughput Approach
Joshua Paul 1,Benjamin Revard 2,Richard Hennig 2
1 Materials Science and Engineering University of Florida Gainesville United States,2 Materials Science and Engineering Cornell University Ithaca United States1 Materials Science and Engineering University of Florida Gainesville United States,2 Materials Science and Engineering Cornell University Ithaca United States
Show AbstractSingle-layer materials present a new class of materials that have the potential to create innovate solutions to problems in nanoelectronics and energy applications. To accelerate the discovery of these materials, a genetic algorithm and a high-throughput approach are applied to promising material systems to identify novel 2D materials. Our grand-canonical genetic algorithm for structure prediction - GASP - code is modified to enable searches for the structure and composition of 2D materials. The high-throughput approach - based on our Python package MPInterfaces - employs known 2D structures and periodic trends to explore potentially stable 2D materials. Both approaches, when coupled with density-functional methods, identify already known single-layer materials as well as novel ones. Characterization of the electronic, magnetic, chemical, and mechanical properties of these materials reveals their potential for applications.
9:00 PM - NT4.5.25
Monolayer III-VI Chalcogenides for Ultra Low-Power Field Effect Transistors
Protik Das 1,Darshana Wickramaratne 1,Gen Yin 1,Somaia Sylvia 1,Bishwajit Debnath 1,Khairul Alam 2,Roger Lake 1
1 University of California, Riverside Riverside United States,2 East West University Dhaka Bangladesh
Show AbstractIII-VI materials such as GaS, GaSe, InS and InSe are an emerging class of layered materials that have an anisotropic valence band when confined to a few monolayers. Below a thickness of four monolayers ab-initio calculations show that the valence band of these materials exhibit a ring-shaped valence band edge. This feature in the electronic structure results in a singular density of states and a step function density of modes at the valence band edge. This motivates the question of what impact does this ring-shaped valence band have on the hole transport and how does it compare to that in layered semiconductors with a parabolic dispersion. We address this by calculating the momentum relaxation scattering rates with wavevector dependent screening in a Random Phase Approximation (RPA) for ionized impurity scattering. From these rates, we determine the mobility and mean free path lengths. The energy dependent mean free paths are then used in a Landauer model to calculate current in an ultra-scaled FET. The resulting on and off currents are compared to those from a MoS2 channel FET with parabolic dispersion.
Our calculations show a single monolayer GaS device has an ON-current of 318.54μA/μm which is approximately 5 times larger than the ON-current for a MoS2 device of similar dimensions. The GaS device ON-current also exceeds the target ON-current that has been projected for low-power devices by the International Technology Roadmap for Semiconductors by approximately 10%. The improved performance of the GaS device results from the step function density of modes available for transport at the band edge and the ring shaped valence band which, on average, requires larger momentum transfer for backscattering.
Acknowledgement: This work is supported in part by FAME, one of six centers of STARnet, a Semiconductor Research Corporation program sponsored by MARCO and DARPA.
9:00 PM - NT4.5.26
Excitons in Transition Metal Dichalcogenides: MoS2 and MoSe2
Premlata Yadav 1,Subhasis Ghosh 1
1 Jawaharlal Nehru University New Delhi India,
Show AbstractTwo dimensional transition metal dichalcogeniges (TMDs) are of a great interest to the scientific community due to its exciting potential for applications in electronics, optoelectronics and sensing. In particular, sulfides and selenides of molybdenum (Mo) have attracted interest as they possess a band gap, which is important for integration into electronics device structures. Thus, a scalable method is required to fully explore their exceptional optical and electronic properties. In this paper, we report (1) the synthesis of thin films of MoS2 and MoSe2 using chemical vapor deposition and (2) study of exciton in bilayer and trilayer MoS2, MoSe2 samples using absorption spectroscopy. Highly homogeneous TMD films are produced over desired areas, by modifying the thickness of pre-deposited metal layer. The formation of MoS2 and MoSe2 thin films were confirmed by X-ray diffraction, UV-visible absorption spectroscopy, Raman spectroscopy, atomic force microscopy and spectroscopic ellipsometry. Systematic trends are observed for band gaps, transition energies and for exciton binding energies of MoS2 and MoSe2. The thickness dependent band gap for MoS2 /MoSe2 changes from 1.55eV/1.02eV to 1.64eV/1.2eV, as one move from trilayer to bilayer, respectively. Two main features associated with the A and B excitons are noticed in the absorption spectra of MoS2 and MoSe2. The presence of A and B excitons has been attributed to spin-orbit coupling induced valence band splitting, which is higher for MoSe2 (0.19eV) as that of MoS2 (0.13eV), is associated with higher atomic mass of selenium than sulfur. The binding energy for A exciton are estimated to be 0.32eV and 0.22eV for bilayer MoS2 and MoSe2. Same trend was observed for exciton B, where it changes from 0.18eV (MoS2) to 0.09eV (MoSe2) respectively. In a similar fashion, binding energy for A (B) exciton was estimated to be 0.4eV (0.26eV) and 0.36eV (0.16eV) for trilayer MoS2 and MoSe2. This suggests that, as one move from bulk to few layer films of TMDs exciton binding energies increases. These optical transitions herein reported enhance our understanding of TMDs and thus provides informative guidelines for MoS2 and MoSe2 based optical device designs.
9:00 PM - NT4.5.27
Gas Adsorption on MXene Surfaces: Density Functional Theory Calculations
Anchalee Junkaew 1,Raymundo Arroyave 2
1 National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency (NSTDA) Pathum Thani Thailand,2 Department of Materials Science and Engineering Texas Aamp;M University College Station United States
Show AbstractTwo dimensional graphene-liked materials, so-called MXenes, have been discovered recently. MXenes are layers of transition metal carbide and nitride compounds. According to their large surface area and their distinctive properties, MXenes are promising materials for many applications such as energy storage, supercapacitor, gas storage and thermoelectric applications. This work investigated gas adsorption on MXenes (i.e. Ti2C, V2C, Nb2C and Mo2C) and their oxygen-functionalized surfaces (O-MXenes) by using the periodic Density Functional Theory (DFT) calculations. The adsorbates are N2, NO, NO2, NH3, CO, CO2, O2, H2, H2S, SO2 and H2O molecules. Both dissociative and molecular adsorption processes were observed depending on adsorption sites, types of MXene and adsorbate molecules. The functional group on the surface also plays an important role in its gas adsorption ability. On bare surfaces, the chemisorption process with high adsorption energy indicates high reactivity of MXene towards gas molecules. In the functionalized cases, O-MXenes show weaker gas adsorption strength than bare MXenes, but they are more selective to particular gas species. The results are useful for applying these materials in gas separation, gas storage and gas sensor applications. Structural and charge analysis were performed for understanding the interaction between adsorbates and substrates.
9:00 PM - NT4.5.28
Improved Carrier Mobility and Photoresponsivity of Tungsten Disulfide by Molecular Doping
Sang-Soo Chee 1,Chohee Oh 1,Moon-Ho Ham 1
1 Gwangju Institute of Science and Technology (GIST) Gwangju Korea (the Republic of),
Show AbstractTwo-dimensional (2D) transition metal dichalcogenide (TMD) materials have attracted much attention in the past decades because of their unique electrical and optical properties. MoS2 is one of the most studied TMD materials, and has been widely used as a channel in field-effect transistors. More recently, other TMD materials are receiving increasing attention, and WS2 is expected to exhibit higher mobility compared to that of MoS2. In this study, we demonstrate the modulated electrical and photoelectric properties of WS2 via a simple doping process with hydrazine molecules. Doping of hydrazine molecules onto WS2 resulted in n-typed doping effect, and the electrical properties including carrier mobility were modulated with doping time and concentration. This is due to the strong electron-donating nature of the hydrazine molecules. In addition, upon illumination of visible light (λ = 625 nm), the photocurrent was clearly observed. The photoresponsivity increased with hydrazine doping, and showed a maximum value of 1.54 A/W. This high value has been obtainable only in heterostructures based on 2D materials.
9:00 PM - NT4.5.29
Optical Writing by Photo-Induced Defects Passivation on 2D Materials
Sijie Yang 1,Xiuqing Meng 2,Bin Chen 1,Hasan Sahin 3,Toshihiro Aoki 1,Kedi Wu 1,Anupum Pant 1,Jun Kang 3,Francois Peeters 3,Sefaattin Tongay 1
1 Arizona State University Tempe United States,2 Research Center for Light Emitting Diodes Zhejiang Normal University Jinhua China3 Department of Physics University of Antwerp Antwerpen Belgium
Show AbstractDefects engineering as a means to control material properties is central to practical optoelectronic application, and many applications, e.g. LEDs and FETs, are enabled by introducing controlled amount and type of point defects into the conventional material systems. In two-dimension (2D) systems, defects are anticipated to have even greater impacts on the material properties, mostly due to strong quasi-particle interactions and tightly localized electron wave-functions. However, current knowledge regarding effects of defects on the optical response of 2D systems is limited to identification of bound exciton complexes at cryogenic temperatures, and inherent link between structural imperfections and optical performances of 2D materials remains unclear. Here, we report on a new phenomenon: light emission enhancement by photo-induced passivation of surface point defects in 2D transition metal dichalcogenides (TMDCs) produced by chemical vapor deposition. In particular, light emission is enhanced by close to three orders of magnitude, a world-record value, on tungsten disulfide (WS2). Real-time optical spectroscopy measurements show that light emission intensity increases in the course of measurement (laser exposure), with the enhancement factor strongly dependent on the surrounding atmosphere. Environmental and computational modelling studies reveal that physisorption of O2 molecules at the defect sites greatly improves optical properties of 2D TMDCs by minimizing non-radiative recombination processes, such as Auger recombination and TAMM surface state recombination, and stabilizing highly luminescent neutral exciton complexes. Accordingly, we demonstrate ‘laser writing’ on 2D TMDCs for the first time by registering a highly-luminescent ‘state’ onto 2D TMDCs , and offer novel way to boost light emission of 2D semiconducting material with potential applications in LEDs, optosensors, and optics-based information technologies.
9:00 PM - NT4.5.30
Preparation of Wafer-Scale MoS2 Atomic Layers by Chemical Vapor Deposition
Hung Nguyen 1,Feng Zhao 1
1 Washington State Univ Vancouver United States,
Show AbstractAtomically thin molybdenum disulfide (MoS2) has emerged as a desirable material for a new generation of electronics and photonics. In order to obtain MoS2 atomic layer, different approaches have been taken such as exfoliation, chemical synthesis, and physical or chemical vapor deposition (CVD) processes. For device fabrication and large-scale production, wafer-scale MoS2 is very important. In this paper, we report a hydrogen-free and promoter-free CVD growth technique to synthesize large-scale MoS2 atomic layers. The film quality, uniformity, thickness and layer numbers were characterized by a variety of techniques such as optical microscopy, AFM, PL mapping, Raman, XPS, HREM and STEM. The high quality MoS2 atomic layers were demonstrated, forming a foundation to develop wafer-sized material platform for device fabrication and production.
9:00 PM - NT4.5.31
Environmental Excitonic Dynamics Changes in MoTe2
Bin Chen 1,Hasan Sahin 2,Aslihan Suslu 1,Laura Ding 3,Mariana Bertoni 3,Francois Peeters 2,Sefaattin Tongay 1
1 School for Engineering of Matter, Transport and Energy Arizona State University Tempe United States,2 Department of Physics University of Antwerp Antwerp Belgium3 School of Electrical Computer and Energy Engineering Arizona State University Tempe United States
Show AbstractUnlike other monolayers of group VI transition metal dichalcogenides which have direct gaps in the visible range, MoTe2 has a gap located in the infrared region. However, its environmental stability is of particular interest, as tellurium compounds are acutely sensitive to oxygen exposure. Here, our environmental (time-dependent) measurements reveal two distinct effects on MoTe2 monolayers: For weakly luminescent monolayers, photoluminescence signal and optical contrast disappear, as if they are decomposed, but yet remain intact as evidenced by AFM and Raman measurements. In contrast, strongly luminescent monolayers retain their optical contrast for a prolonged amount of time, while their PL peak blue-shifts and PL intensity saturates to slightly lower values. Our X-ray photoelectron spectroscopy measurements and DFT calculations suggest that the presence of defects and functionalization of these defect sites with O2 molecules strongly dictate their material properties and aging response by changing the excitonic dynamics due to deep or shallow states that are created within the optical band gap. Similar unusual environmental changes are also observed in GaTe. Presented results not only shed light on environmental effects on fundamental material properties and excitonic dynamics of MoTe2 monolayers but also highlight striking material transformation for metastable 2D systems such as WTe2, silicone, and phosphorene.
Symposium Organizers
Linyou Cao, North Carolina State University
Bruce Claflin, Air Force Research Laboratory
Thomas Mueller, Vienna University of Technology
Hua Zhang, Nanyang Technological University
Symposium Support
Aldrich Materials Science
APLMaterials|AIP Publishing
HORIBA Scientific
2D Materials and Materials Research Express | IOP Publishing
NT4.6: Photophysics and Electrical Properties of 2D Materials
Session Chairs
Thursday AM, March 31, 2016
PCC North, 100 Level, Room 129 B
9:00 AM - *NT4.6.01
Defect Passivation, Chemical Doping, Heterostructures and Devices of Layered Semiconductors
Ali Javey 1
1 Univ of California-Berkeley Berkeley United States,
Show AbstractTwo-dimensional semiconductors exhibit excellent device characteristics, as well as novel optical, electrical, and optoelectronic characteristics. In this talk, I will present our recent advancements in defect passivation, contact engineering, surface charge transfer doping, and heterostructure devices of layered chalcogenides. We have developed a defect repair/passivation technique that allows for near-unity photoluminescence quantum yield in monolayer MoS2. The work presents the first demonstration of an optoelectronically perfect monolayer. Forming Ohmic contacts for both electrons and holes is necessary in order to exploit the performance limits of enabled devices while shedding light on the intrinsic properties of a material system. In this regard, we have developed different strategies, including the use of surface charge transfer doping at the contacts to thin down the Schottky barriers, thereby, enabling efficient injection of electrons or holes. As a result, we have been able to show high performance n- and p-FETs with various 2D materials. Additionally, I will discuss the use of layered chalcogenides for various heterostructure device applications, exploiting charge transfer at the van der Waals heterointerfaces. I will also present progress towards achieving tunnel transistors using layered semiconductors.
9:30 AM - NT4.6.02
Strong Broadband and Narrowband Atomically Thin MoS2 Film Light Absorber
Lujun Huang 1,Guoqing Li 1,Alper Gurarslan 1,Ronny Kirste 1,Wei Guo 1,Ramon Collazo 1,Junjie Zhao 1,Gregory Parsons 1,Michael Kudenov 1,Linyou Cao 1
1 North Carolina State Univ Raleigh United States,
Show AbstractLayered transition-metal dichalcogenides (TMDC) such as MoS2 and WS2, belonging to the family of two dimensional material, has emerged as new materials because of their distinct physical properties. However, the atomically thin thickness makes MoS2 absorb light inefficiently. Here, we report the theoretical design and experimental demonstration on both strong broadband and narrowband light absorbers with atomically thin MoS2 film. The design strategy builds upon the coupled leaky mode theory, one which we developed and can turn the design for MoS2 absorber to the design for leaky modes.For the narrowband MoS2 absorber, the experiments results show that four layers MoS2 (t=2.48nm) can absorb almost 75% incident light in a single wavelength. Wide tunability on the absorption peak is demonstrated over the visible range via tuning the geometrical parameters, and simultaneously strong light absorption is maintained. For the broadband MoS2 absorber, more than 70% incident wave is absorbed in a very broadband range (400nm-630nm) with only four layer MoS2 (t=2.48nm). The experimental results show excellent agreement with the theoretical calculation. Our results may pave the way for developing atomically scale optoelectronic devices, such as solar cell and photodetector.
9:45 AM - *NT4.6.03
Optoelectronics of 2D Materials Beyond Graphene: Material Physics, Challenges and Opportunities
Farhan Rana 1,Haining Wang 1
1 Cornell University Ithaca United States,
Show AbstractTwo-dimensional atomically thin materials beyond graphene, most notably transition metal dichalcogenides (TMDs), have generated tremendous interest among researchers. TMDs are direct bandgap semiconductors with bandgaps in the visible to near-IR wavelength range. The strong light absorption exhibited by these materials makes them attractive for opto-electronic applications. Despite the recent progress in material synthesis and growth, carrier lifetimes and nonradiative electron-hole recombination mechanisms in TMDs remain poorly understood. Developing a better understanding of the non-radiative electron-hole recombination mechanisms in TMDs is especially important because the reported quantum efficiencies in both TMD light emitters and detectors are extremely poor; typically in the .0001-.01 range. Most of the electrons and holes injected either electrically or optically in TMDs recombine non-radiatively. The mechanisms by which electrons and holes recombine non-radiatively, and the associated time scales, remain to be clarified. We will present our work in studying the dynamics of electrons and phonons in 2D materials and optoelectronic devices using ultrafast optical/terahertz pump-probe and correlation spectroscopy. Our experimental work on metal dichalcogenide materials and devices (such as photodetectors and light emitters) as well as our theoretical results show that defect assisted recombination involving capture of excitons and carriers by Auger scattering is the fastest mechanism for the non-radiative recombination of photoexcited electrons and holes. In particular, the very Coulomb interaction that resulted in the strongly bound excitons in these materials, causes extremely fast capture of the excitons by defects resulting in extremely poor quantum efficiencies in optoelectronic devices. The large sensitivity of device performance to defects is thus fundamental to 2D TMD materials. Our optical pump-probe experiments have shown the time scales and the physical mechanisms associated with the capture and recombination of photoexcited carriers. Our ultrafast two-pulse photovoltage correlation experiments show that the photoresponse of TMD photodetectors can be very fast making them useful for operation at frequencies in the hundreds of gigahertz range. Carrier lifetimes in multilayer 2D material samples are governed by the competition between the fast surface and the slow bulk recombination rates. Our recent experimental work has shown that other than electronics and optoelectronics, 2D materials could be very promising for high frequency MEMs devices. Our work has shown that mechanical oscillations in these atomically thin membranes can reach terahertz frequencies and are tunable from few tens of gigahertz to almost one terahertz. 2D material membranes can therefore enable MEMs resonator structures with record frequency-quality factor products at these high frequencies.
10:15 AM - NT4.6.04
Probing Interface Interaction in 2-Dimensional Layered Materials
Kai Liu 1,Yinghui Sun 2,Sefaattin Tongay 3,Feng Wang 4,Junqiao Wu 5
1 State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering Tsinghua University Beijing China,2 University of Science and Technology Beijing Beijing China3 School for Engineering of Matter, Transport and Energy Arizona State University Tempe United States4 Department of Physics University of California, Berkeley Berkeley United States5 Department of Materials Science and Engineering University of California, Berkeley Berkeley United States
Show Abstract2-dimensional (2D) homo-/hetero-structures built up from layered materials have received growing attention owing to their simple fabrication of straightforward stacking and various types of band alignments. Optical and electrical properties have been investigated intensively to probe the interface coupling in 2D homo-/hetero-structures. The mechanical interface interaction, however, has yet to be well studied. Here I will show two approaches, nanoindentation and surface plasmon enhanced Raman scattering, to probe effective modulus and local strain in 2D homo-/hetero-structures, respectively. Both results reflect the weak interlayer interaction in 2D layered structures. First, nanoindentation experiments indicate that the effective 2D modulus of a bilayer structure is lower than the sum of 2D modulus of each layer due to the interlayer sliding. Second, by coating 2D structures with silver or gold nanoparticles, the local strain existing at the metal-2D layer boundary will split the featured Raman peaks, but this splitting effect weakens as the number of layers increases because of the weak interlayer interaction. Our results not only provide mechanical insight to understanding interface interactions in 2D homo-/hetero-structures, but also potentially allow engineering of their properties as desired.
References:
[1] S. Tongay, H. Sahin, C. Ko, A. Luce, W. Fan, K. Liu, J. Zhou, Y.-S. Huang, C.-H. Ho, J. Yan, et al. Nature Commun. 2014, 5, 3252.
[2] S. Tongay, W. Fan, J. Kang, J. Park, U. Koldemir, J. Suh, D. S. Narang, K. Liu, J. Ji, J. Li, R. Sinclair, J. Wu. Nano Lett. 2014, 14, 3185.
[3] K. Liu, Q. Yan, M. Chen, W. Fan, Y. Sun, J. Suh, D. Fu, S. Lee, J. Zhou, S. Tongay, J. Ji, J. B. Neaton, J. Wu. Nano Lett. 2014, 14, 5097.
[4] Y. Sun*, K. Liu*, X. Hong, M. Chen, J. Kim, S. Shi, J. Wu, A. Zettl, F. Wang. Nano Lett. 2014, 14, 5329.
[5] K. Liu, C.-L. Hsin, D. Fu, J. Suh, S. Tongay, M. Chen, Y. Sun, A. Yan, J. Park, K. M. Yu, W. Guo, H. Zheng, D. Chrzan, J. Wu. Adv. Mater. 2015, in press.
[6] K. Liu, J. Wu. J. Mater. Res. 2016, in press (invited review).
10:30 AM - *NT4.6.05
Chalcogenides and Beyond: Exploring Synthesis, Doping and Properties of Next Generation 2D Materials
Joshua Robinson 1
1 Pennsylvania State Univ University Park United States,
Show AbstractThe isolation of graphene constituted a new paradigm in next generation electronic technologies, and even though graphene is considered transformational, it is only the “tip of the iceberg.” Transition metal dichalcogenides (TMDs) and their heterostructures could have an even greater impact on next generation technologies. Molybdenum disulfide (MoS2) is currently a leading TMD for scientific exploration, but there are a variety of other suitable, less explored, TMDs and heterostructures that exhibit very attractive bandgaps, charge carrier effective masses, and mobilities for electronic applications. Transition-metal dichalcogenides (TMDs) in the form of MeX2 (where Me = a transition metal such as Mo, W, Ti, Nb, etc. and X = S, Se, or Te) also exhibit extreme flexibility, possession of tunable band gaps, modest electron mobilities, and wide variety of band-offsets. Additionally, monochalcogenides (i.e. GaSe) have the potential to provide routes toward alternative wide bandgap 2D materials suitable for high power and UV optoelectronics. Furthermore, synthesizing and heterogeneously combining these atomic layered materials to form van der Waals (vdW) solids, where each layer may be different from the previous, is a powerful way to develop novel nanoscale materials. This talk will elaborate on recent breakthroughs for direct growth of two-dimensional atomic heterostructures (MoS2, WSe2, and hBN) on a graphene template, discuss recent breakthroughs in the synthesis of 2D nitrides beyond hBN, and elucidate mechanism for achieving selective area growth of 2D materials.
11:30 AM - *NT4.6.06
STM/STS Studies of 2D Transition Metal Dichalcogenide Surfaces and Heterointerfaces
Andrew Wee 1
1 National Univ of Singapore Singapore Singapore,
Show AbstractGraphene, an atomically thin layer of carbon, is a semi-metal that can be used in applications such as transparent conducting electrodes in flexible electronics. The electronic and chemical properties of graphene can be engineered through a variety of methods such as by molecular adsorption [1,2], or fabricating graphene nanoribbons [3,4]. Unlike graphene, transition metal dichalcogenides (TMDs) such as MoS2 and WSe2, are semiconductors with tunable direct bandgaps dependent on the number of atomic layers, and have potential electronic and optoelectronic applications. We use high resolution scanning tunneling microscopy/spectroscopy (STM/STS) to study the atomic structure and intrinsic electronic properties of MoS2 layers (mono-, bi-, tri-) directly deposited on HOPG substrates by chemical vapour deposition (CVD) [5]. We report an unexpected bandgap tunability with distance from the grain boundary in single-layer MoS2, which also depends on the grain misorientation angle. We have similarly investigated the atomic scale electronic properties of CVD-grown WSe2 monolayers as well as their interactions with molecules and our latest results will be presented[6].
[1] W. Chen, S. Chen, D.C. Qi, X.Y. Gao, A.T.S. Wee, J. Am. Chem. Soc. 129 (2007) 10418.
[2] H.Y. Mao, Y. Hong, Y.H. Lu, J.D. Lin, S. Zhong, A.T.S. Wee, W. Chen, Prog. Surf. Sci. 88 (2013) 132.
[3] H. Huang, D.C. Wei, J.T. Sun, S.L. Wong, Y.P. Feng, A.H. Castro Neto, A.T.S. Wee, Scientific Reports 2 (2012) 983.
[4] D.C. Wei, L.F. Xie, K.K. Lee, Z.B. Hu, S.H. Tan, W. Chen, C.H. Sow, K.Q. Chen, Y.Q. Liu, A.T.S. Wee, Nature Commun. 4 (2013) 1374.
[5] Y.L. Huang, Y.F. Chen, W.J. Zhang, S.Y. Quek, C.H. Chen, L.J. Li, W.T. Hsu, W.H. Chang, Y.J. Zheng, W. Chen, A.T.S. Wee, , Nature Commun. 6 (2015) 6298.
[6] Y.J. Zheng, in preparation.
12:00 PM - *NT4.6.07
Charge-Induced Second-Harmonic Generation in Bilayer Transitional Metal Dichalcogenides – A Case Study of WSe
Huakang Yu 1,Jacob Khurgin 2,Qihua Xiong 1
1 Nanyang Technological Univ Singapore Singapore,2 John Hopkins University Baltimore United States
Show AbstractTo control nonlinear light-matter interaction is important for both fundamental science and future optoelectronic devices. Over the past decades, extraordinary optical and electronic properties of 2D atomic materials have been intensively investigated, and meanwhile 2D crystals have created opportunities to manipulate nonlinear optical processes electrically. Here, for the first time, we report a strong charge-induced second-harmonic generation (CHISHG) in a 2D WSe2 bilayer crystal caused by a back gate field. Fundamentally different from numerous prior works on electric field- or current-induced SHG, CHISHG takes place only when the gate polarity causes charge accumulation rather than depletion. A bond-charge model has been set up for tracing the origin of SHG, which owns to the non-uniform field distribution within a single monolayer, caused by the accumulated sub-monolayer screening charge in the tungsten plane. Our results demonstrate significance of charge and field distribution on the scale of a single atomic layer, and should serve as an impetus for potential applications in noninvasive probing of charge and current distributions in future low dimensional electronic devices.
References
H.K. Yu, D. Talukdar, W.G. Xu, J.B. Khurgin and Q.H. Xiong*, "Charge-Induced Second-Harmonic Generation in Bilayer WSe2", Nano Lett. 15, 5653-5657 (2015)
12:30 PM - NT4.6.08
Optical and Electrical Properties of Li Intercalation in 2D MoS2 Material
Jiayu Wan 1,Wenzhong Bao 2,Steven Lacey 1,Dennis Drew 1,Michael Fuhrer 3,Liangbing Hu 1
1 Univ of Maryland-College Park College Park United States,2 Department of Microelectronics Fudan University Shanghai China3 School of Physics Monash University Victoria Australia
Show AbstractIntercalation of species in 2D materials has recently attracted broad attention as this method drastically tuned the properties of 2D materials. We developed a novel platform that allows us to measure in situ electrical transport and optical properties of two-dimensional (2D) molybdenum disulfide (MoS2) at the level of individual crystallites, with a few microns in extent along the basal plane, and a few nanometers in thickness. For the first time, we measured the resistance and optical transmittance change at real time with Li-ion intercalation. At a rapid charging rate during the first lithiation cycle, we observed a large conductivity increase in thick MoS2 due to the formation of a percolative Mo nanoparticle network imbedded in Li2S matrix, which is confirmed by in situ transmission electron microscopy. The proposed methodology can be applied to study a broad range of nanomaterials electrodes via electrochemical intercalation, and the excellent properties of ion intercalated 2D materials will inspire a variety of applications such as electrochromic devices, transparent electrodes and superconductors.
12:45 PM - NT4.6.09
Tuning Electrical, Optical and Thermal Properties in MoS2 Nanosheets via Li Intercalation
Feng Xiong 1,Haotian Wang 1,Aditya Sood 1,Xiaoge Liu 1,Jie Sun 1,Mark Brongersma 1,Kenneth Goodson 1,Eric Pop 1,Yi Cui 1
1 Stanford University Stanford United States,
Show AbstractTwo-dimensional layered materials like MoS2 have shown promise for nanoelectronics, energy harvesting and energy storage. The interlayer separation in MoS2 (~0.65 nm) provides perfect sites to accommodate guest species such as alkali metal ions (Li+, Na+ and K+) through a process known as intercalation. The intercalation process alters the interlayer bonding and changes the electronic structure of the host 2D materials. This gives us a rare opportunity to controllably and reversibly modify their properties at the nanoscale.
In this study, we develop an in-situ platform to electrochemically intercalate Li ions into the interlayer spacing of ultrathin MoS2 nanosheets, controllably tuning their electrical, optical and thermal properties. We adopt a planar nanobattery configuration, where Li metal and exfoliated MoS2 nanosheets (~2-50 nm thick) are used as counter and working electrodes, respectively. The device with electrolyte (1M LiPF6 in 1:1 w-w EC/DEC) is encapsulated inside a transparent electrochemical cell, where the top transparent window allows in-situ optical, Raman and thermal characterizations while we move Li ions in and out of MoS2 interlayer gaps.
With our platform, we capture the first in-situ optical observations of the dynamics of Li intercalation in MoS2 nanosheets. Our Raman measurement shows reversible 2H to 1T phase transitions in MoS2 upon Li intercalation and de-intercalation and reveals possible cycling-induced structural damages. Due to the change in electronic structure (semiconducting 2H to metallic 1T) upon lithiation, we achieve substantial improvement of MoS2 optical transmission (up to 90% for a 4-nm flake) as well as more than two orders of magnitude increase in electrical conductivity for all flakes. This large and tunable enhancement in both optical transmission and electrical conductivity through intercalation are promising and provide new opportunities in optoelectronics and transparent electrodes.
We also study how the cross-plane thermal transport in MoS2 is affected by intercalation. While we perform Li intercalation and de-intercalation, we use time-domain thermoreflectance (TDTR) to monitor changes in the thermal conductivity of MoS2 films (kMoS2) as a function of Li concentration and spatial locations (edge vs. center). We find that the cross-plane thermal conductance of MoS2 nanosheets can be dynamically tuned by a factor of 5-8x, on time scales of the reversible Li intercalation (minutes).
This capability to reversibly engineer the physical and chemical properties of nanomaterials through intercalation is promising and could enable exciting opportunities in optoelectronics, transparent electrodes, energy harvesting and storage.
NT4.7: Optoelectronic and Electronic Devices of 2D Materials
Session Chairs
David Geohegan
Yong Zhang
Thursday PM, March 31, 2016
PCC North, 100 Level, Room 129 B
2:30 PM - *NT4.7.01
Bridging the Gap: Layered Black Phosphorus for Electronics and Optoelectronics
Fengnian Xia 1,Bingchen Deng 1
1 Yale University New Haven United States,
Show AbstractBlack phosphorus recently emerged as a promising new 2D material due to its widely tunable and direct bandgap, high carrier mobility and remarkable in-plane anisotropic electrical, optical and phonon properties. It serendipitously bridges the zero-gap graphene and the relatively large-bandgap transition metal dichalcogenides such as molybdenum disulfide (MoS2). In this talk, I will first cover the basic properties of few-layer and thin-film black phosphorus, followed by a discussion of recent observation of highly anisotropic robust excitons in monolayer black phosphorus. Finally I will present a few potential applications of black phosphorus such as radio-frequency transistors and wideband photodetectors.
3:00 PM - NT4.7.02
Probing Electrical, Optical and Phonon Anisotropy in Black Phosphorus
Michael Snure 1,Dennis Walker 1,Stefan Badescu 1,Michael Buzbee 1,Qing Paduano 1,Shiva Vangala 1
1 AFRL Wright Patterson AFB United States,
Show AbstractBlack phosphorus has emerged as a promising two dimensional (2D) semiconductor, which possess both high p-type mobility and a tunable direct band gap. The combination of these properties makes black phosphorous a very unique 2D material possessing the required properties for high frequency 2D electronic devices and tunable efficient optoelectronic devices. In addition to its amazing electrical and optical properties, black phosphorus has a unique structure, which unlike graphene and MoS2, is anisotropic in-plane as well as out-of plane. This structural anisotropy translates to some very unique orientation dependent properties. Along the arm chair direction the hole mobility is three times higher than in the zigzag direction. The optical and phonon properties are also highly directional as observed by angle dependent extinction spectra, polarization dependent Raman, and linearly polarized emission. This anisotropy can present unique opportunities and challenges. For example polarization dependent Raman can be used as a simple nondestructive method for identification of crystallographic alignment. In this paper we present characterization and analysis of these directional properties in few-layer black phosphorous across a range of thicknesses. Polarization dependent Raman shows a show clear directional dependence. Correlating these results with electrical properties we will demonstrate how optical methods can be used as a nondestructive technique to orient black phosphorous for electrical and optical devices.
3:15 PM - *NT4.7.03
Manipulating Phosphorene for Nanoelectronics and Energy Applications
Yong-Wei Zhang 1,Yongqing Cai 1,Weifeng Li 1,Zhun-Yong Ong 1,Gang Zhang 1,Qingxiang Pei 1,Viacheslav Sorkin 1
1 Institute of High Performance Computing Singapore Singapore,
Show AbstractPhosphorene is a direct bandgap semiconductor with high carrier mobility, suitable for high-performance nanoelectronics and solar energy conversion. In addition, their interesting chemical affinities with various small molecules and atoms render it suitable for high performance molecular sensors and energy storage. Different methods can be used to manipulate the structure of phosphorene, for example, by forming nanoribbons, physical or chemical adsorption, layering, strain engineering et al. These structure manipulations can significantly change its electronic, optical, magnetic and thermal properties, and thus greatly widen the range of its applications. To do so, however, it is both important and necessary to understand and further control these structures and properties.
In this talk, we will present our first-principles studies on phosphorene, focusing on the effects of finite sizes, few-layer, strain engineering, phyiscal and chemical adsoption, on several of its physical properties. We will highlight the role of edge hydrogenation on nanoribbons in tuning the nature and magnidue of the band gap, and the layer-depenent band alignement and work function of few-layer phosphorene, and strain engineering on its thermal transport and phononic ansotropy. In addition, we will show the ultrafast and directional diffusion of lithium in monolayer and few-layer phosphorene, which is suitable for high-performance lithium ion battery. Finally, we will present our study on the energetics, charge transfer and magnetism of small molecules physisorbed on phosphorene, which are important for molecular sensing. The present work demonstrates several practical routes to tune the properties of phosphorene, which may be useful for applications in nanoelectronic devices and energy applications.
3:45 PM - NT4.7.04
Chemistry of Black Phosphorus: Exfoliation, Oxidation and Non-Covalent Functionalization
Gonzalo Abellan 1,Vicent Lloret 1,Udo Mundloch 1,Mario Marcia 1,Christian Neiss 1,Maria Varela 2,Frank Hauke 1,Andreas Goerling 1,Andreas Hirsch 1
1 University of Erlangen-Nurnberg Furth Germany,2 Universidad Complutense de Madrid Madrid Spain
Show AbstractTwo-dimensional (2D) materials have attracted increasing interest in the last few years due to their unique morphology and properties and their use in a variety of applications, ranging from electronics to gas storage or separation, catalysis, high performance sensors or inert protective coatings, among others. Beyond graphene, layered chalcogenides, phosphates, titanates, perovskites and metal oxides or layered double hydroxides (LDH) appear as promising alternatives to this ubiquitous 2D material, displaying complementary physical properties, that open the door for the development of new hybrid multifunctional materials. Black phosphorus (BP) is the newest two-dimensional material, which is attracting tremendous interest due to its applications in electronics, energy storage, ultrafast saturable absorbers, gas sensors or fillers for composite reinforcement. However, its lack of environmental stability severely limits its synthesis and processing.
It has recently demonstrated that high-quality, few-layer BP nanosheets, with controllable size and observable photoluminescence, can be produced in large quantities by liquid phase exfoliation under ambient conditions. The rate and extent of the degradation acid-base disproportionation reaction depends on the water/oxygen content. Beyond this success, a more sophisticated chemical modification is highly desirable, like the molecular doping, that is an effective and flexible method towards modulating the electronic properties of 2D materials, as previously demonstrated for graphene, boron nitrates or MoS2 monolayers, for example. Herein, we have developed a chemical route towards the non-covalent functionalization of black phosphorous with different moieties, including electron-withdrawing organic molecules or tailor-made surfactants. There exist considerable charge transfer and strong non-covalent interaction between these molecules and black phosphorus, provoking its exfoliation into few-layers. It is expected that this chemical reactions will pave the way for the effective modulation of the physical properties of black phosphorus, a matter of utmost importance for expanding its applications.
4:30 PM - *NT4.7.05
2D Tunnel-FETs: Toward Green Electronics
Kaustav Banerjee 1
1 Nanoelectronics Research Lab, Department of Electrical and Computer Engineering University of California, Santa Barbara Santa Barbara United States,
Show AbstractMOSFETs have been the workhorse of the worldwide semiconductor industry and the primary building blocks of most electronic products of everyday use since the 1970’s. However, continuous miniaturization of MOSFETs, well beyond 100 nanometers, to sustain the ever growing need for increased transistor densities has given rise to a daunting power dissipation challenge during the past decade due to increasing leakage power arising from a fundamental limitation of their turn-on characteristics. This talk will examine the genesis of this challenge, and provide an overview of the recently demonstrated ATLAS-TFET, which is a 2D semiconductor channel inter-band tunnel-FET or 2D-TFET, that overcomes this fundamental challenge by employing several innovations. This talk will also bring forward the advantages of 2D-TFETs as next generation ultra-low power and ultra-high sensitivity bio/gas sensors that combine the advantages of a 2D semiconducting channel in terms of superior electrostatics and the unprecedented sensitivity of a band-to-band tunnel-FET (owing to their steep turn-on characteristics), and can be employed for building a revolutionary new class of sensors.
5:00 PM - NT4.7.06
The Hysteresis of MoS2 Based FETs
Qing Chen 1,Jiapei Shu 1,Gongtao Wu 1,Yao Guo 1,Xianlong Wei 1
1 Peking Univ Beijing China,
Show AbstractMolybdenum disulfide (MoS2) nanosheets, a two-dimensional nanomaterial with layered structures, are of considerable interest because of their potential applications in optoelectronics, electronics, valleytronics, transparent and flexible devices, and memory device. Hysteresis is often observed in the transfer curves of the field effect transistors (FETs) based on MoS2 nanosheets. The presence of this hysteresis renders the MoS2 as a candidate for memory devices. However, for the application in the electronic and optoelectronic devices, the hysteresis introduces instability to the state of the devices. The origin of the hysteresis has been studied by several groups, but is still not clear yet. Some reports have suggested the interface between the MoS2 and the SiO2 introduces charge traps and causes the hysteresis. While other evidences have shown that the adsorption/desorption of gases is the origin of the hysteresis. It should be noted that all these studies have been performed on the supported MoS2, so that the effect of the substrate cannot be avoided.
Here, for the first time, we study the hysteresis behavior of the FETs with suspended MoS2 in the vacuum to exclude the effects of the SiO2 substrate and the gases. Through a comparison between suspend and SiO2-supported devices, we find the origin of the hysteresis and threshold instability of MoS2 FET comes from the MoS2 itself.
5:15 PM - *NT4.7.07
Quantum Transport in 2D Membranes
Chun Ning (Jeanie) Lau 1
1 Department of Physics University of California Riverside United States,
Show AbstractTwo dimensional materials constitute an exciting platform for investigation of both fundamental phenomena and electronic applications. Here I will present our results on transport measurements on phosphorene devices. As the only non-carbon elemental layered allotrope, few-layer black phosphorus or phosphorene has emerged as a novel two-dimensional (2D) semiconductor with both high bulk mobility and a band gap. In this talk I will present our fabrication and transport measurements of phosphorene-hexagonal BN (hBN) heterostructures with one-dimensional (1D) edge contacts. These transistors are stable in ambient conditions for >300 hours, and display ambipolar behavior, a gate-tunable metal-insulator transition, and mobility up to 4000 cm2/Vs. At low temperatures, we observe Shubnikov de Haas (SdH) magneto-oscillations and Zeeman splitting in magnetic field with an estimated g-factor ~2. The cyclotron mass of few-layer phosphorene holes is determined to increase from 0.25 to 0.31 me as the Fermi level moves towards the valence band edge. I will also report our latest results on weak localization and ionic liquid gating of few layer phosphorene dvices. Our results underscore the potential of phosphorene as both a platform for novel 2D physics and an electronic material for semiconductor applications.
References
N. Gillgren, D. Wickramaratne, Y. Shi, Tim Espiritu, J. Yang, J. Hu, J. Wei, X. Liu, Z. Mao, K. Watanabe, T. Taniguchi, M. Bockrath, Y. Barlas, R. K. Lake, C. N. Lau, “Gate Tunable Quantum Oscillations in Air-Stable and High Mobility Few-Layer Phosphorene Heterostructures”, 2D Materials, 2, 011001 (2015).
5:45 PM - NT4.7.08
Improved Electrical Properties of Multilayer Molybdenum Disulfide Transistors by Dielectric Passivation
Seong Yeoul Kim 1,Seonyoung Park 1,Hyejoo Lee 1,Woong Choi 1
1 Kookmin Univ Seoul Korea (the Republic of),
Show AbstractTransition metal dichalcogenides such as MoS2 have received great interest as they exhibit interesting electrical, optoelectronic, and chemical properties. Single layer MoS2 thin-film transistors (TFTs) showed significantly improved electrical properties, such as high mobility (~100 cm2/Vs) and on/off ratio (106~108), after a high-k oxide layer was deposited on the top of channel. However, not much interest has been given to the effect of high-k dielectric layers deposited on multilayer MoS2 TFTs. In this presentation, we report improved electrical properties of bottom-gate multilayer MoS2 TFTs by high-k dielectric passivation. After depositing high-k dielectric layers on bottom-gate MoS2 TFTs fabricated with mechanically exfoliated multilayer MoS2 flakes, we observe that field effect mobility increases by two-fold and current-voltage hysteresis almost disappears. We also investigate interface properties to correlate them with the improved device performance. These results suggest that dielectric passivation can be an effective method of improving device performance of multilayer MoS2 TFTs.
NT4.8: Poster Session III
Session Chairs
Linyou Cao
Bruce Claflin
Thomas Mueller
Hua Zhang
Friday AM, April 01, 2016
Sheraton, Third Level, Phoenix Ballroom
9:00 PM - NT4.8.01
3D WS2 Nanosheet-Networks as H2O2 Produced in the Brain Cells for Cell Signaling
Jing Tang 1,Jun Li 1,Biao Kong 1,Gengfeng Zheng 1
1 Fudan Univ Shanghai China,
Show AbstractHydrogen peroxide (H2O2) is gradually becoming a newly accepted messenger in cellular signal transduction. However, the potential for elucidating its manifold roles in complex biological environments is limited, which is lack of effective methods for probing this reactive oxygen metabolite in living systems with specificity. Herein, a general strategy to fabricate three-dimensional (3D) WS2 nanosheet networks with flower-like morphology based on a facile chemical vapour deposition (CVD) approach is proposed. The semiconducting 2H-WS2 into nanosheets with largely exposed edge sites on the three-dimensional substrate results in the excellent electrical transport and proliferation of catalytically active sites. Owing to the unique features of 3D WS2, such as high permeability to biosubstrates and rapid electron transfer with enhanced interlayer coupling of electron orbitals, the 3D WS2-based nano-bio-interface demonstrate high selectivity for H2O2 and are capable of visualizing endogenous H2O2 produced in living cells by growth factor stimulation, including the first direct sensing of peroxide produced for brain cell signaling. The combined features of reactive oxygen species selectivity, sensitivity to signaling levels of H2O2, and live-cell compatibility presage many new opportunities for 3D WS2 for exploring the physiological roles of H2O2 in living systems. It indicates excellent bioprobing performance with a wide linear range and high sensitivity, and rapid response time (∼3 s). The activity of neuronal cells has been enhanced by promoting the interaction between subcellular structures and 3D topographic features, exhibiting higher density and greater cell polarization. Furthermore, the 3D H2O2 bio-interface with excellent electrocatalytic activity, surface-rich microstructure and maxmized exposure of active sites leads to a wide detective range (1 nM to 400 μM), and an ultra sensitive detection limit (as low as 0.02 nM) towards H2O2, the best performance based on biocatalyisis of transition metal dichalcogenides (TMDs). Computational analyses further testify that the enhanced sensitivity of probing H2O2 is associated with the spontaneous adsorption on WS2 nanosheets. The trace amount of H2O2 released from Raw 264.7 cells and neurons is also successfully recorded, which achieves the sensitive and real-time quantitative probing of H2O2 in biological environment. They were active for the electrochemical oxidation of H2O2, with a detection limit reaching 0.02 nM. Due to the large surface and strong electron coupling, excellent electrocatalytic activity has been demonstrated.
9:00 PM - NT4.8.02
Schottky Barrier Heights at Two-Dimensional Metallic and Semiconducting Transition-Metal Dichalcogenide Interfaces
Adiba Zahin 1,Darshana Wickramaratne 1,Roger Lake 1
1 University of California Riverside (UCR) Riverside United States,
Show AbstractTwo dimensional (2D) semiconducting transition-metal dichalcogenide (TMDs) with intrinsic band gaps (1 –2 eV) are considered to be promising candidates as channel materials for next-generation transistors. Low-resistance metal contacts to TMDs are necessary for good device performance. However, high contact resistances with the monolayer TMDs significantly degrade the performance of TMD transistors [1]. Metal deposition on single-layer planar surfaces such as TMDs or graphene may exhibit different degrees of clustering [2], which may induce inefficient electron and phonon transport [3] across the metal TMD interfaces. One approach to alleviate the poor contact resistances with the semiconducting TMDs is to improve the interface between the metal and the TMD semiconductor. To address this we propose and investigate the use of 2D metallic TMDs (MX2; M=Ta, Nb; X= S, Se,Te) as metal contacts instead of noble metals (Au, Pd, Ti, In etc) to 2D semiconducting TMDs (MX2; M=Mo, W; X= S, Se,Te). The interface between the metallic and semiconducting TMDs is governed primarily by vdW forces. We are carrying out a systematic study of the barrier heights and energy band lineups of the 2D semiconducting TMDs such as MoSe2, WSe2 and MoTe2 with the 2D metallic TMDs such as TaS2, NbS2, NbSe2, TaSe2 using ab initio density-functional theory (DFT) calculations with hybrid functionals. Using the calculated energy level alignments, we provide values for the Schottky barrier heights for electron and hole injection for each combination of interfaces.
Acknowledgement: This work is supported in part by FAME, one of six centers of STARnet, a Semiconductor Research Corporation program sponsored by MARCO and DARPA.
Reference
[1] H. Liu, A. T. Neal, and P. D. Ye, Channel Length Scaling of MoS2 MOSFETs, ACS Nano 6, 8563(2012)
[2] Zhang, Y.; Franklin, N. W.; Chen, R. J.; Dai, H. J. Metal Coating on Suspended Carbon Nanotubes and Its Implication to Metal-Tube Interaction. Chem. Phys. Lett. 2000, 331, 35–41.
[3] Mao, R.; Kong, B. D.; Gong, C.; Xu, S.; Jayasekera, T.; Cho, K.; Kim, K. W. First-Principles Calculation of Thermal Transport in Metal/Graphene Systems. Phys. Rev. B 2013, 87, 165410.
9:00 PM - NT4.8.03
Nanometer-Thick Single-Crystalline Nanosheets Grown at the Water-Air Interface
Fei Wang 1,Jung-Hun Seo 1,Guangfu Luo 1,Matthew Starr 1,Zhaodong Li 1,Dalong Geng 1,Xin Yin 1,Dane Morgan 1,Zhenqiang Ma 1,Xudong Wang 1
1 University of Wisconsin - Madison Madison United States,
Show AbstractTwo-dimensional (2D) nanomaterials, particularly when their thickness is just one or a few atomic layers, exhibit physical properties dissimilar to those of their bulk counterparts and other forms of nanostructures. Graphene and transition metal dichalcogenides (TMDs) have epitomized the applications of 2D nanostructures in many electronic, optoelectronic, and electrochemical devices. Nonetheless, real-world 2D nanostructures so far have been largely limited to naturally layered materials, i.e. the van der Waals solids, synthesized either from top-down or bottom-up. A much larger and diverse portfolio of 2D materials including non-layered compounds are desirable to meet the specific requirements of individual components in various devices. We demonstrate that ionic surfactant monolayers could serve as a soft template supporting the nucleation and growth of 2D nanomaterials in large area beyond the limitation of van der Waals solids. Through this approach, 1 to 2 nm thick, single-crystalline, free-standing ZnO nanosheets with sizes up to tens of microns were synthesized at the water-air interface. The formation process was investigated by TEM. We found that an amorphous thin films was initially formed and subsequently crystallizes into small crystals which finally merge into large nanosheets. In this process, an ionic surfactant monolayer is first formed at the water-air interface, under which the aqueous solution contains precursors for the desired materials. Due to the specific bonding between ionic surfactant head groups and the aqueous precursor ions containing the opposite electric charge, the intermolecular spacings between the ionic surfactant at the monolayer can adapt to the lattice spacings of the grown material and guided the 2-D growth of nanosheets. It is thus named adaptive ionic layer epitaxy (AILE). We proposed a double layer model that describes a Zn-concentrated zone near the water-air interface that controlled the thickness of the initial amorphous film as well as the final single-crystalline nanosheet. Thin film transistors (TFTs) were fabricated using ZnO nanosheets and the I-V characteristics showed the rare but much coveted p-type conductivity. We simulated the possible crystal structures of the nanosheets with adsorbed surfactant molecules and their band gap, which supported the p-type conductivity. This AILE technique, with similar attributes in the processes found in biomineralization, shows great promises as a novel and versatile synthesis paradigm for forming nanosheets from a wide range of inorganic materials including and beyond the van der Waals solids.
9:00 PM - NT4.8.04
Bose-Einstein Condensate in Transition Metal Dichalcogenides Electron-Hole Bilayer System
Bishwajit Debnath 1,Yafis Barlas 1,Mahesh Neupane 1,Darshana Wickramaratne 1,Roger Lake 1
1 University of California Riverside Riverside United States,
Show AbstractWe have investigated the possibility of achieving a Bose-Einstein condensate (BEC) of electron-hole pairs in monolayer MoS2, MoSe2, WS2 and WSe2 films separated by a thin hexagonal-BN spacer. The nearly identical electron-hole effective masses in the transition metal dichalcogenides (TMDs) leads to almost equivalent particle-hole symmetry. The large effective masses on the order of 0.5m0 result in large exciton binding energies and small exciton Bohr radii. Depending on the thickness of the intermediate insulator layer, the exciton binding energies vary from 50 to 200 meV, as the BN spacer thickness varies from 3.3 nm to .25 nm. The effective Bohr radius remains near 2~3 nm for BN thicknesses between 1 and 1.5 nm. To investigate the interlayer coherence, a detailed analysis using BCS mean field theory is applied to calculate the order parameter Δ. An unscreened coulomb potential opens up a gap of 15~50 meV for the TMD-BN-TMD system. At high carrier densities on the order of 2x1012 cm-2, screening is a dominant factor. Using the random phase approximation (RPA) to account for the wave-vector dependent same-layer and different-layer static polarizability (Ps/d) in a self-consistent calculation, we find a reduction in the excitonic energy gap.
Acknowledgement: This work was supported in part by NSF EFRI-143395, Novel Switching Phenomena in Atomic Heterostructures for Multifunctional Applications and by FAME, one of six centers of STARnet, a Semiconductor Research Corporation program sponsored by MARCO and DARPA.
9:00 PM - NT4.8.05
Controlled Synthesis of ZnO Nanostructures for Development of Electrochemical Biosensor
Nandhinee Radha Shanmugam 1,Sriram Muthukumar 2,Shalini Prasad 1
1 Univ of Texas-Dallas Richardson United States,2 Enlisense LLC Allen United States
Show AbstractFlexible diagnostics has significant value in the area of sensors. This work demonstrates the fabrication and performance of 1D ZnO nanostructures based flexible electrochemical biosensors for multiplexed detection of cardiac biomarkers, troponin-T (cTnT) and troponin-I (cTnI). Electrode characteristics such as material, dimensions and its surface modifications dictate the performance of electrochemical biosensors. In this regard, we demonstrate the methodology to design ultra-small electrodes using 1D ZnO nanostructures via hydrothermal synthesis with zinc nitrate and hexamethylene tetramine as precursors. Growth characterization using SEM reveal the feasibility of flexible polyimide substrates for aligned and dense growth of ZnO nanostructures. We have established previously that the density of ZnO nanostructure on microelectrode platform can be effectively controlled by varying precursor concentration, which in turn modulates the electrode behavior (micro/ nanoelectrode). Nanoelectrode behavior enables biomolecule detection under steady state condition due to improved electrochemical reaction rates and increased signal to noise ratio. The charge perturbations due to biomolecular binding events on nanostructured sensing sites were recorded using electrochemical impedance spectroscopy (EIS). Results demonstrate amplified signal response and larger dynamic range for cTnT and cTnI detection with limit at femtogram/mL. Concurrent detection of these cardiac biomarkers have greater significance in improving quality of life for those suffering from acute myocardial infarction.
9:00 PM - NT4.8.06
Towards Controlled Defect Generation and Doping in Two-Dimensional MoS2
Dilbagh Singh 1,Frederick Aryeetey 1,Shyam Aravamudhan 1
1 North Carolina Aamp;T Univ Greensboro United States,
Show AbstractIn recent years, two-dimensional Transition Metal Dichalcogenide (TMDC) materials such as MoS2 have drawn considerable interest due to their intriguing electrical, optical, sensing, and catalytic properties [Castellanos-Gomez et al. 2012]. While Graphene-based devices have shown high carrier mobility up to ~ 105 cm2/Vs [Bolotin et al. 2008] and cut-off frequency higher than ~ 100 GHz [Liao et al., 2010], the inherent zero – band gap electronic structure of graphene results in high OFF state current. On the other hand, monolayer MoS2 is a semiconductor with a direct band gap of ~1.9eV [Mak et al. 2010]. MoS2 transistors are a promising alternative for the semiconductor industry due to their large ON/OFF current ratio (> 10e10) [Yoon et al., 2011], immunity to short channel effects, and abrupt switching. However, realization of practical mono/few layer MoS2 devices is limited by the free charge density in intrinsic MoS2 (10e10/cm2) [Rastogi et al. 2014]. The objective of this work is to utilize He-ion beam patterning to write pattern of vacancies (< 10 nm) onto CVD synthesized 2D MoS2. The vacancies thus generated are then doped via molecular doping technique to improve upon the intrinsic free carrier concentration in 2D MoS2. Briefly, the method is as follows. (a) First, highly crystalline 2D MoS2 are synthesized on SiO2/Si substrate using MoO3 and S powder as the precursors in a CVD furnace. (b) Next, He-ion beam patterning is used to generate vacancies in only the very top S layer. The shorter de Broglie wavelength of He-ions allows for milling at higher resolution than other charged particle microscopes. (c) Next, solution-based molecular doping method is used to realize non-degenerate n-type doping in 2D MoS2. This is accomplished by soaking the patterned 2D layers in 1, 2-dichloroethane (DCE) solution at room temperature for about 12 hours. As a result, the patterned S vacancies are occupied by Cl atoms which donate the extra electrons to the MoS2 system. The electronic properties are tuned by varying the MoS2 layer thickness and solution concentration. Post-doping, the electrostatic properties are measured by using the different modes of AFM, including Kelvin force microscopy (KFM) and Scanning microwave microscopy (SMM). An important contribution of this work is the presentation of optimized He-ion beam conditions, including the accelerating voltage and the effects of defect generation on the sub-surface layers. He-ion beam based defect generation creates a shallow penetration depth, which makes them more suitable for single S vacancies. Finally, the changes in the surface potential and a quantitative analysis of the dopant density were measured suing KFM and SMM respectively, indicating that doping is in the non-degenerate regime.
9:00 PM - NT4.8.09
Magnetic Tunnel Junctions with Monolayer h-BN Tunnel Barriers
Maelis Piquemal-Banci 1,Regina Galceran 1,Sabina Caneva 2,Marie-Blandine Martin 2,Robert Weatherup 2,Piran Kidambi 2,Karim Bouzehouane 1,Stephane Xavier 3,Abdelmadjid Anane 1,Frederic Petroff 1,Albert Fert 1,John Robertson 2,Stephan Hofmann 2,Bruno Dlubak 1,Pierre Seneor 1
1 Unité Mixte de Physique, CNRS, Thales, Univ. Paris-Sud, Université Paris-Saclay Palaiseau France,2 Engineering Dept University of Cambridge Cambridge United Kingdom3 Thales Research and Technology Palaiseau France
Show AbstractWhile hexagonal boron nitride (h-BN) has already proved its efficiency in terms of improving the performance of graphene devices, the experimental demonstration of its tunnel properties is scarce. Yet h-BN, a bidimensional insulating isomorph of graphene, has been shown to be a suitable tunnel barrier [1]. In particular, it is a promising candidate for spintronics [2]. However to establish the potential of h-BN tunnel barriers for spintronics, it is important first to characterize its spin transport properties in a simple reference magnetic tunnel junction (MTJ). We will present Co/h-BN/Fe MTJ, where the large area monolayer h-BN tunnel barrier is grown directly by chemical vapor deposition (CVD) on Fe via borazine exposure [3]. Fe is deposited by physical vapor deposition (sputtering) on an oxidized Si wafer support. The characterization of the h-BN layers (homogeneity, resistance topography and tunnel properties) is carried out with Conductive Tip Atomic Force Microscopy (CT-AFM) measurements. This study is done for both an h-BN monolayer grown by CVD and for two layers with an additional layer obtained via a transfer method. The tunnel properties are also characterized for an h-BN monolayer through the integration into complete micronic MTJ devices with Co top ferromagnetic electrode. We finally show that atomically thin directly grown CVD h-BN exhibits tunneling of spin polarized electrons thanks to I(V) and dI/dV(V) measurements and we further report on its magneto-transport properties.
[1] Britnell et al., Nano Lett. 12, 1707 (2012) ;
[2] Kamalakar et al., Scientific Reports 4, 6146, (2014) ;
[3] Caneva et al., Nano Lett. 15, 1867 (2015).
9:00 PM - NT4.8.10
Magnetic and Electrical Properties of MXenes - A Computational Study
Shanshan Su 1,Kuan Zhou 1,Gen Yin 1,Jia Fu 1,Yafis Barlas 1,Roger Lake 1
1 University of California, Riverside Riverside United States,
Show AbstractRecently, a new type of two-dimensional (2D) material, MXene, has been proposed and created with a procedure suitable for large-scale production [1]. The formula for MXene is Mn+1Xn where M is a transition metal element (Sc, Ti, V, Cr, Zr, Nb, Ta, Mo, W) and X is either C or N. Combinations of different terminating functional groups and different metallic elements strongly affect the electronic property, which varies from metallic to semiconducting, indicating promising device applications [3-4]. For the metallic MXenes, the conductivity has been reported to be comparable with graphite [2]. Large Seebeck coefficients have been reported for semiconducting MXenes [1]. Weng et al. [5] recently showed that the strong spin orbit coupling introduced by the heavy metal elements protects a quantum spin Hall state in the material, making the material a 2D topological insulator (TI). When terminated by oxygen atoms, the band gap of the TI is up to 194 meV. In this work, we use density functional theory (DFT) to evaluate the electronic properties of MXenes with different atomic structures and terminating functional groups. The important properties for device performance such as the effective mass and density of states are extracted from DFT results. By substituting the transition metal in MXenes with elements like Fe, the magnetic properties of MXenes are also investigated. By calculating the Berry curvature, the possibility of observing the anomalous Hall effects (AHE) or quantum anomalous Hall effects (QAHE) in MXenes is evaluated.
This work is supported by NSF EFRI-143395, Novel Switching Phenomena in Atomic Heterostructures for Multifunctional Applications. Ab initio calculations of magnetic properties are also supported as part of the Spins and Heat in Nanoscale Electronic Systems (Spins) an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award DE-SC0012670. The Extreme Science and Engineering Discovery Environment (XSEDE) is used, which is supported by the NSF grant OCI-1053575.
[1] M. Khazaei, M. Arai, T. Sasaki, C.-Y. Chung, N. S. Venkataramanan, M. Estili, Y. Sakka and Y. Kawazoe, Adv. Funct. Mater. 23, 2185-2192 (2013).
[2] M. Naguib, O. Mashtalir, J. Carle, V. Presser, J. Lu, L. Hultman, Y. Gogotsi and M. W. Barsoum, ACS Nano 6(2), 1322 (2012).
[3] J. Come, M. Naguib, P. Rozier, M. W. Barsoum, Y. Gogotsi, P. L. Taberna, M. Morcrette and P. Simon, J. Electrochem. Soc. 159(8), A1368 (2012).
[4] J. Hu, B. Xu, C. Ouyang, S. A. Yang and Y. Yao, J. Phys. Chem. C 118(42), 24274 (2014).
[5] H. Weng, A. Ranjbar, Y Liang, Z. Song, M. Khazaei, S. Yunoki, M. Arai, Y. Kawazoe, Z. Fang and X. Dai, Phys. Rev. B 92, 075436 (2015).
9:00 PM - NT4.8.11
Oxidation of Ultrathin GaSe
Thomas Beechem 1,Brian Kowalski 2,Michael Brumbach 1,Anthony McDonald 1,Catalin Spataru 1,Stephen Howell 1,Taisuke Ohta 1,Jesse Pask 2,Nikolai Kalugin 2
1 Sandia National Labs Albuquerque United States,2 New Mexico Tech Socorro United States
Show AbstractGallium selenide (GaSe) is a layered binary chalcogenide of practical interest owing to its optoelectronic properties. In two-dimensional form, it is being pursued for use in hyperspectral emitters and photodetectors spanning the spectrum from the ultraviolet to visible. The surface of bulk GaSe is known to oxidize over time, however, leading to questions surrounding the viability of devices based on 2D GaSe. In response, the oxidation of ultrathin GaSe is examined here.
Specifically, the oxidation of ultrathin GaSe is investigated using a combination of Raman, photoluminescence (PL), Auger, and X-Ray photoelectron (XPS) spectroscopies in conjunction with atomic force microscopy (AFM) and density functional theory (DFT)-based bandstructure calculations. Reductions of ultrathin GaSe PL intensity are shown to occur via oxidation resulting in not only Ga2O3 but also by-products of the reaction, namely Ga2Se3 and amorphous Se (a-Se), which exhibit clear Raman signatures. Utilizing these signatures, oxidation is then tracked as it is ``written" using a laser based photo-inducement. Taken together, the results highlight the necessity of surface passivation for retainment of GaSe's intrinsic properties while simultaneously suggesting the possibility of creating 2D heterostructures via photoinduced oxidation of the material.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
9:00 PM - NT4.8.12
Graphene Stabilization of Two-Dimensional Gallium Nitride
Zakaria Al Balushi 2,Ke Wang 3,Ram Krishna Ghosh 4,Rafael Vila 2,Sarah Eichfield 3,Dennis Paul 5,Suman Datta 4,Joan Redwing 3,Joshua Robinson 3
1 Materials Science and Engineering The Pennsylvania State University University Park United States,2 Center for 2-Dimensional and Layered Materials The Pennsylvania State University University Park United States,2 Center for 2-Dimensional and Layered Materials The Pennsylvania State University University Park United States,3 Materials Research Institute The Pennsylvania State University University Park United States2 Center for 2-Dimensional and Layered Materials The Pennsylvania State University University Park United States,4 Electrical Engineering The Pennsylvania State University University Park United States1 Materials Science and Engineering The Pennsylvania State University University Park United States,2 Center for 2-Dimensional and Layered Materials The Pennsylvania State University University Park United States,3 Materials Research Institute The Pennsylvania State University University Park United States5 Physical Electronics Chanhassen United States2 Center for 2-Dimensional and Layered Materials The Pennsylvania State University University Park United States,3 Materials Research Institute The Pennsylvania State University University Park United States,4 Electrical Engineering The Pennsylvania State University University Park United States
Show AbstractThe spectrum of two-dimensional (2D) and layered materials “beyond graphene” has been continually expanding. The realization of wide bandgap 2D materials “beyond hexagonal boron nitride (hBN)”, however, has been limited. Group-III nitride semiconductors such as indium nitride (InN), gallium nitride (GaN), and aluminum nitride (AlN) are proposed to have thickness tunable energy bandgaps ranging from ~0.7 – 7.0 eV as a result of quantum confinement. Despite the extensive computational discovery of 2D materials, the synthesis of 2D III-nitrides “beyond hBN” is still elusive. We demonstrate for the first time that 2D atomic layers of GaN not only can be stabilized, but also exhibit unique structural properties from that of bulk material. Our work utilizes a novel migration enhanced encapsulated growth process that involves the use of graphene. We elucidate the mechanism of 2D GaN formation and discuss graphene’s ability in providing sufficient thermodynamic stabilization of the (direct bandgap) 2D buckled structure both experimentally and theoretically with density functional theory (DFT). Our results provide evidence that during the growth process, a layer of gallium intercalates between graphene and the supporting substrate, and is subsequently converted to bilayer 2D GaN via nitrogen intercalation and ammonolysis. Theoretically we show that current 2D GaN DFT models may not properly describe the layered crystal structure, in which we verify our findings by directly identifying the positions of the nitrogen and gallium atomic columns in the bilayer 2D GaN structure using aberration corrected scanning TEM (STEM) in annular bright field (ABF) mode. Additionally, we correlate theoretical results with experiment and show bandgap tunability in GaN layers as they are thinned to their atomic limits. Our approach provides a novel pathway towards stabilizing new 2D materials (III-nitrides and other compound semiconductors) that are not naturally layered in bulk crystals. Recognizing the impact of 2D III-nitrides, it can be expected that the addition of 2D GaN and related materials will open up new avenues of research in novel electronic and optoelectronic devices, such as single photon emitter for quantum communication and miniaturized deep ultraviolet lasers, composed of new heterostructures of 2D atomic layers of group III-nitride semiconductors.
9:00 PM - NT4.8.13
Properties of 2D PbI2 and Its Potential Application
Kedi Wu 1,Hasan Sahin 2,Bin Chen 1,Emmanuel Soignard 1,Cong Wang 3,Hui Cai 1,Aslihan Suslu 1,Sefaattin Tongay 1
1 Arizona State University Tempe United States,2 Physics University of Antwerp Antwerp Belgium3 National Center for Nanoscience and Technology Beijing China
Show AbstractAtomically thin two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (sTMDs), and boron nitrides have been considered as very promising candidates for a wide range of applications in nanoelectronic devices due to their tremendously novel properties. A main task today is to broaden the 2D material library, and thus to extend its range of applications. Here, we have successfully synthesized single crystal lead (II) iodide (PbI2), and cleaved it down to few- and monolayer nanosheets onto SiO2/Si substrate. Depending on Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), and Raman Spectroscopy measurements, we were able to investigate its crystal and vibrational properties. Photoluminescence (PL) studies were performed at 2D PbI2 sheets of various numbers of layers, and the results show a direct to indirect band transition going from 3D to 2D, which is in consist with the our density function theory (DFT) calculation. Strain induced changes in optical properties of qusi-2D PbI2 flakes have also been explored using PL and micro UV-Visible absorption measurements. The results demonstrate that optical band gap of this material is tunable by strain engineering, predicting great opportunities for flexible devices applications.
9:00 PM - NT4.8.14
Synthesis of Carbon/Sulfur Nanolaminates by Electrochemical Extraction of Titanium from Ti2SC and Other "AX" Structures
Mengqiang Zhao 1,Maria Lukatskaya 1,Michael Ghidiu 1,Boris Dyatkin 1,Darin Tallman 1,Michel Barsoum 1,Yury Gogotsi 1
1 Drexel Univ Philadelphia United States,
Show AbstractThe selective extraction processes to produce 2D materials has been attracted much attention due to its ability to achieve well-controlled morphologies and structures, and even generate new materials that cannot be produced by other methods.[1] Recently, a family of new 2D materials, so called MXenes, were synthesized by selective extraction of ‘A’ layers from the MAX phases (a family of 3D layered, ternary carbides and nitrides).[2] Carbide-derived carbons with tunable size were produced by extracting both the ‘M’ and ‘A’ elements from the MAX phases.[3]
Herein we electrochemically and selectively extract Ti from the MAX phase Ti2SC to form carbon/sulfur, C/S, nanolaminates at room temperature.[4] The products are composed of multi-layers of C/S flakes, with predominantly amorphous and some graphene-like structures. Covalent bonding between C and S is observed in the C/S nanolaminates, which render the latter promising candidates as electrode materials for Li-S batteries. We also show that it is possible to extract Ti from other MAX phases, such as Ti3AlC2, Ti3SnC2, and Ti2GeC, and the corresponding “AX” structures, that is, Al/C, Sn/C and Ge/C nanostructures, were produced. Considering that >70 MAX phases are known to exist,[5] it is not difficult to foresee that the “AX” structures represent a new family of nanostructured materials, much of which will probably be 2D. The various “A” and “X” combinations known render the “AX” structures highly attractive for a number of potential applications, such as electrical energy storage, catalysis, etc.
References:
[1] M. Naguib, Y. Gogotsi, Acc. Chem. Res. 2015, 48, 128 – 135.
[2] M. Naguib, V. N. Mochalin, M.W. Barsoum, Y. Gogotsi, Adv. Mater. 2014, 26, 992 – 1005.
[3] M. R. Lukatskaya, J. Halim, B. Dyatkin, M. Naguib, Y. S. Buranova, M.W. Barsoum, Y. Gogotsi, Angew. Chem. Int. Ed. 2014, 53, 4877 – 4880.
[4] M. Q. Zhao, M. Sedran, Z. Ling, M. R. Lukatskaya, O. Mashtalir, M. Ghidiu, B. Dyatkin, D. J. Tallman, T. Djenizian, M. W. Barsoum, Y. Gogotsi, Angew. Chem. Int. Ed. 2015, 54, 4810 – 4814.
[5] M.W. Barsoum, MAXPhases: Properties of Machinable Ternary Carbides and Nitrides, Wiley, Hoboken, 2013.
9:00 PM - NT4.8.15
Controllable Synthesis of Mono-Layer Hexagonal Boron Nitride Thin Film by Atmospheric Pressure Chemical Vapor Deposition
Yijing Stehle 1,Panos Datskos 1,Sergei Smirnov 2,Ivan Vlassiouk 1
1 Oak Ridge National Lab Oak Ridge United States,2 Department of Chemistry and Biochemistry New Mexico State University Las Cruces United States
Show AbstractTailoring the atmospheric pressure chemical vapor deposition (APCVD) processing parameters allow the control of the nucleation and morphology of single crystal hexagonal boron nitride(hBN), and further facilitated synthesis of high quality large area monolayer hBN. Here we investigate the growth stages of the hBN single crystals, show that hBN crystals change their shape from triangular to truncated triangular and further to hexagonal depending on copper substrate position. We suggest that the observed hBN crystal shape variation is affected by the boron to nitrogen active species ratio on copper surface inside the CVD reactor.
9:00 PM - NT4.8.16
Engineering Electronic Properties of 2D Carbides (MXenes) by Manipulating Their Transition Metal Layers
Babak Anasori 2,Chenyang Shi 3,Eun Ju Moon 1,Yu Xie 4,Cooper Voigt 1,Eric Dooryhee 5,Paul Kent 6,Steven May 1,Simon Billinge 7,Michel Barsoum 1,Yury Gogotsi 2
1 Materials Science amp; Engineering Drexel University Philadelphia United States,2 A.J. Drexel Nanomaterials Institute Drexel University Philadelphia United States,3 Applied Physics amp; Applied Mathematics Columbia University New York United States1 Materials Science amp; Engineering Drexel University Philadelphia United States4 Center for Nanophase Materials Sciences Oak Ridge National Laboratory Oak Ridge United States5 Photon Sciences Directorate Brookhaven National Laboratory Upton United States4 Center for Nanophase Materials Sciences Oak Ridge National Laboratory Oak Ridge United States,6 Computer Science and Mathematics Division Oak Ridge National Laboratory Oak Ridge United States3 Applied Physics amp; Applied Mathematics Columbia University New York United States,7 Condensed Matter Physics amp; Materials Science Brookhaven National Laboratory Upton United States
Show AbstractMXenes are two-dimensional (2D) transition metal (M) carbides and/or nitrides with the formula of Mn+1Xn, where X is carbon or nitrogen and n= 1, 2 or 3. Their structure consists of stacks of n+1 layers of M covering n layers of C or N in an [MX]nM arrangement. Recently, two new families of double transition metal carbides MXenes were discovered, M'2M"C2 and M'2M"2C3 - where M' and M" are two different early transition metals, such as Mo, Cr, Ta, Nb, V, and Ti. M' atoms are only occupying the outer layers and M" filling the central layers, such as Mo-C-Ti-C-Mo in Mo2TiC2 . In this study, we present how the electronic properties can be engineered by changing the M' or M" element. For instance, experimental conductivity and magnetoresistance measurements show that when two outer layers of Ti in the metallic Ti3C2 are replaced with Mo, forming Mo2TiC2, the material no longer exhibits metallic-like conductivity. Density functional theory (DFT) calculations suggest that OH terminated Mo2TiC2 MXenes are semiconductors, with quite small band gaps (
9:00 PM - NT4.8.17
Ultrathin Two-Dimensional Wide Bandgap Ga2O3 for High-Voltage Field Effect Transistor
Xin Yin 1,Layane Vieira 1,Munho Kim 1,Jung-Hun Seo 1,Zhenqiang Ma 1,Xudong Wang 1
1 University of Wisconsin-Madison Madison United States,
Show AbstractMorphology can essentially give rise to the extraordinary physical, chemical, and mechanical properties in nanomaterials. When the dimensions of the materials are reduced to two dimensions (2D), remarkable properties emerge, which do not exist in bulk materials. Benefited from layered crystal structure, graphene has attracted a lot of attention due to the ease of syntheses and the superior properties, like room temperature electron mobility, intrinsic strength, thermal conductivity, and optical absorption, proving the importance of the morphology influence on the properties. Inspired by graphene, layered transitional dichalcogenides, like MoS2, WS2, subsequently attracted much attention due to the semiconductor properties, exhibiting promising electronic and photonic applications. However, many materials with more important properties have nonlayered crystal structure. It is still challenging to synthesize 2D nanostructures of nonlayered materials.
Here, through chemical vapor deposition, large scale ultrathin 2D Ga2O3 plates have been successfully synthesized on silicon substrates. These plates have various shapes, like hexagons, triangles, and rectangles. The plates are ~16 nm thick and ~1.5 mm large. Statistics shows that the deposition temperature has an obvious influence on the thickness: The thickness increases with the deposition temperature from 16.6 nm to 25.6 nm when the deposition temperature increases from 404 K to 420 K. However, at the same deposition temperature zone, the thickness is quite uniform and independent of the plate size, indicating the much larger lateral growth rate than the vertical growth rate. It is proposed that the low supersaturation in the reaction chamber contributes to the plate lateral growth while inhibiting the vertical growth due to the difficult nucleation of new layers at low supersaturation, resulting to the ultrathin 2D nanostructures. Due to the wide band gap of Ga2O3, it has been successfully demonstrated that this ultrathin 2D plates can be used as channels of transistors to switch high voltages, showing the promising applications in power electronics.
This work shows a way to synthesize ultrathin 2D nanostructures with nonlayered crystal structure, which have the potentials to bring new physical and chemical properties and the applications in electronic devices.
9:00 PM - NT4.8.19
Physical Properties of 2D Nanomaterials Heterostructures
Muhammad Sajjad 1,Vladimir Makarov 1,Frank Mendoza 1,Ali Aldalbahi 2,Peter Feng 1,Gerardo Morell 3,Brad Weiner 3
1 Univ of Puerto Rico San Juan United States,2 College of Science, King Saud University Riyadh Saudi Arabia1 Univ of Puerto Rico San Juan United States,3 Institute of Functional Nanomaterials San Juan United States
Show AbstractAlthough graphene (G) and boron nitride nanosheets (BNNSs) show different physical properties, these materials have similar crystal structure and slight lattice mismatch about 1.5%; therefore potentially superb to fabricate heterostructures. In this presentation, we will focus on physical properties of G/BNNS heterostructures. BNNS were synthesized by laser ablation of h-BN target while graphene was synthesized by hot filament chemical vapor deposition method. The heterostructures were fabricated transferring graphene on BNNS through mechanically method (poly-methyl meth acrylate technique). The crystalline quality of the G/BNNS heterostructures was evaluated by Raman spectroscopy mapping and temperature-dependent Raman spectroscopy to analyze the phonon-phonon interactions between the graphene and BNNS layered structures. The physical properties of the heterostructure films were carefully studied by analyzing sheet resistance, thermal conductivity, X-ray diffraction, and electron microscopy, respectively. From G/BNNS heterostructure, it is concluded that the physical properties of graphene improved by much in case of BNNS used as substrate material for graphene.
9:00 PM - NT4.8.20
In Situ Reduction of Gold Nanoparticles on Liquid Exfoliated Tungsten Disulfide Nanosheets
Jeremy Dunklin 1,Damien Hanlon 3,Andrew Harvey 3,Zahra Gholamvand 3,Gregory Forcherio 4,Donald Roper 4,Jonathan Coleman 3,Claudia Backes 5
1 Ralph E. Martin Department of Chemical Engineering University of Arkansas Fayetteville United States,2 Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN) Trinity College Dublin Dublin, Dublin 2 Ireland,3 School of Physics Trinity College Dublin Dublin, Dublin 2 Ireland4 MicroElectronics-Photonics Program University of Arkansas Fayetteville United States1 Ralph E. Martin Department of Chemical Engineering University of Arkansas Fayetteville United States,4 MicroElectronics-Photonics Program University of Arkansas Fayetteville United States5 Institute for Physical Chemistry Ruprecht-Karls-University Heidelberg Heidelberg Germany
Show AbstractTransition metal dichalcogenides (TMDs) such as tungsten disulfide (WS2) are an exciting class of two-dimensional (2D) semiconductor materials. Liquid-phase exfoliation offers a low cost, potentially scalable method of producing 2D TMD nanosheets from bulk crystal; however, the need for improved size selection and optoelectronic tunability limit their overall effectiveness. Functionalization via reduction of plasmonic nanoparticles (NPs) could provide improvement in these areas for use in optoelectronic and catalytic applications. A facile synthesis method to reduce AuNPs on WS2 was developed without the need for harsh chemical treatment or reducing agents. It was shown that at high Au salt concentration, aggregates form preferentially on multi-layer WS2. These multi-layer aggregates readily precipitate out of the solution, leaving only lightly decorated monolayer flakes. The resulting AuNP-WS2 material with AuNP functionalization and reduced number of layers show improved photoluminescent response and hydrogen evolution rates. Preliminary work suggests this technique can be extended to a variety of noble metal and transition metal dichalcogenide materials. This method paves the way for exciting new implementations of these hybrid nanocomposite materials.
9:00 PM - NT4.8.21
The Effect of High-K Dielectric on the Photoluminescence of MoS2 Directly Grown on Transition Metal Oxides
Sinu Mathew 1,Soumya Sarkar 2,Surajit Saha 1,Antony George 3,Mary Scott 4,Brijesh Kumar 1,Michal Dykas 1,Pulickel Ajayan 3,Andrew Minor 4,T. Venkatesan 2
1 NUS Nanoscience and Nanotechnology Institute (NUSNNI-NanoCore) National University of Singapore Singapore Singapore,1 NUS Nanoscience and Nanotechnology Institute (NUSNNI-NanoCore) National University of Singapore Singapore Singapore,2 NUS Graduate School for Integrative Sciences and Engineering National University of Singapore Singapore Singapore3 Department of Materials Science and Nano-Engineering Rice University Houston United States4 National Center for Electron Microscopy Lawrence Berkeley National Laboratory Berkeley United States
Show AbstractTransition Metal di-chalcogenides (TMDCs) in the two-dimensional form have gained renewed interest due to the presence of a direct bandgap in the visible region, existence of excitons at room temperature and van Hove singularities in their density of states. On the other hand, the family of transition metal oxides are extremely rich with remarkable properties like tunability of dielectric constant over a wide range, ferroelectricity, and ferroelasticity in combination with comparable surface smoothness of SiO2, thus making them ideal substrates to explore TMDC growth. Therefore, integration of TMDC with transition metal oxides is an exciting pathway to explore novel multi-functionalities of 2D materials. In the present work, we combined the above systems by direct growth and investigated the interaction of the quasiparticles of MoS2 with high-κ dielectric and various phonons of the transition metal oxide substrates. Direct growth of large area MoS2 on non-silicon based oxide substrates (for e.g. SrTiO3 whose dielectric constant can be varied from 300-10000 as a function of temperature), has been achieved. The reduced dielectric screening and the heavy effective mass from d-orbitals of Mo atoms significantly enhances Coulomb interaction between electrons and holes and form neutral and charged excitons called trions. A temperature dependent study of the Photoluminescence shows an anomalous trend in the behavior of the quasiparticles of MoS2. We show that, the intensity ratio of the trion to the exciton and their linewidths in the PL, which is a measure of the quasiparticle scattering life-time can be selectively tuned by a proper choice of the dielectric background. The quasiparticle dynamics have been further verified by femtosecond pump-probe transient spectroscopy. The role of this dielectric environment, temperature and strain due to the underlying substrate with various crystallographic orientations will be discussed. For the first time, we are able to provide a clear evidence of tuning the quasiparticles in MoS2 by appropriate ‘dielectric engineering’ over a very wide range. Our results would be helpful to better understand the quasiparticle properties in TMDCs and open up possibilities for more fundamental studies of many-body interactions with potential applications in optoelectronics and valleytronics.
9:00 PM - NT4.8.22
Layered Transition Metal Carbides (Ti2CTx, T: -OH, -F and –O): Surface Group Modification and Carrier Transport Property
Shen Lai 1,Yajie Yang 1,Seunghyuk Choi 1,Sungjoo Lee 1,Jaeho Jeon 1
1 Sungkyunkwan Univ Suwon Korea (the Republic of),
Show AbstractA family of two-dimensional (2D) early transition metal carbides and nitrides, labeled MXenes, has recently been experimentally demonstrated. MXenes can be obtained by selective removal of component A from the MAX phase, where M is an early transition metal, A is mainly a group IIIA or IVA element such as one belonging to groups 13 or 14, and X is C and/or N. The unique 2D structures of MXenes, as well as their good electronic conductivities, are of special interest. In particular, tailoring the surface functional groups of the MXene 2D layers is a useful method for tuning their morphologies and properties. However, only a few experimental studies of the use of MXenes in energy storage applications such as lithium ion batteries and electrochemical capacitors have been reported. Further, research into the electronic device applications of MXenes and the modification of their properties has been rather limited.
Of the more than 60 MAX phases reported to date, Ti2AlC is particularly significant for practical applications because it has excellent oxidation resistance and its component materials are readily available and relatively inexpensive. Immersing Ti2AlC in aqueous HF can result in the formation of Ti2C(OH)xFy, which is the MXene material corresponding to Ti2AlC (the MAX phase). By performing the thermal annealing of Ti2C(OH)xFy, its surface groups can be modified. Theoretical studies have shown that by varying the surface group, Tx (-OH, -F, and/or -O in this study), the electronic structure of Ti2CTx can be modulated. Surface groups have significant influence on the Ti2CTx band gap, so the controllable modification of Tx is expected to be an effective approach to the engineering of its electrical properties.
In this study, we demonstrate that the carrier transport behaviour of 2D Ti2CTx, can be tuned by modifying the surface group Tx. Our results show that 2D Ti2C(OH)xFy and Ti2COx films can be obtained via simple chemical treatment, thermal annealing, and mechanical exfoliation processes. For the first time, we experimentally study the carrier transport properties of 2D Ti2CTx field effect transistors (FETs), obtaining the high field effect carrier mobilities around 10000 cm2 V-1s-1 at room temperature. The temperature dependent resistivity of the Ti2COx film exhibits semiconductor like Arrhenius behaviour, from which we estimate the band gap of 80 meV. One interesting feature of the FETs based on transition metal carbides is that the field effect mobility at room temperature is less sensitive to the measured transport gaps. Our results open up the possibility that new 2D materials with high mobilities and the appropriate band gaps can be achieved, and broaden the range of electronic device applications of Ti2CTx films.
9:00 PM - NT4.8.23
Facile Synthesis of Single Crystal Vanadium Disulfide Nanosheets by Chemical Vapor Deposition for Efficient Hydrogen Evolution Reaction
Jiangtan Yuan 1
1 Rice Univ Houston United States,
Show AbstractTransition metal dichalcogenides (TMDs) have received great attention due to their promise in potential electronic and optoelectronic applications. Vanadium disulfide (VS2) is a typical metallic member of TMD family without a band gap in its electronic structure. Metallic 2D TMDs are an indispensible component for building the muchsought in-plane or vertical heterostructures using 2D building blocks. Recent experimental and theoretical progress also suggests that TMD nanoparticles or nanosheets are inexpensive and earth abundant electrocatalysts for hydrogen evolution reaction (HER), a critical reaction to generate hydrogen by electrochemical reduction of water. Metallic 1T-MoS2 and 1T-WS2 have shown better HER performance than their semiconducting counterparts. In this work, a facile CVD method to synthesize VS2 singe crystal nanosheets and the investigation of HER performance is reported. The hexagonal 1T crystalline structures of VS2 were determined using X-ray diffraction and transmission electron microscopy. With the availability of single crystals, the Raman spectrum of VS2 was also obtained for the first time. The metallic nature of VS2 single crystal was confirmed by variable-temperature transport measurements and photoluminescence spectroscopy. The electrocatalytic HER activities of the 1T-VS2 showed extremely low overpotential (-68meV) at the specific 10 mA cm-2, small Tafel slopes (∼34mV/decade) and high stability.
9:00 PM - NT4.8.25
Synthesis of Ultrathin and Thickness-Controlled Cu2−xSe Nanosheets via Cation Exchange Reaction
Yuanxing Wang 1,Maksym Zhukovskyi 1,Pornthip Tongying 1,Masaru Kuno 1
1 Univ of Notre Dame Notre Dame United States,
Show AbstractThe usage of cation exchange reaction to synthesize ultrathin and thickness-controlled Cu2-xSe nanosheets (NSs) beginning with CdSe NSs, a well-established system with recent significant work on their colloidal growth, is demonstrated. In this manner, extremely thin (i.e., 1.6 nm thickness) Cu2−xSe NSs, beyond which can be made directly, have been obtained. The optical properties of both starting CdSe and resulting Cu2-xSe NSs show size dependence. Furthermore, they represent the thinnest NSs produced via cation exchange reactions. Notably, the exchange reaction preserves the starting morphology of the CdSe NSs and also retains their cubic crystal structure, because the parent anion sublattice is maintained during cation exchange. Resulting Cu2−xSe NSs also show the existence of a localized surface plasmon resonance in the infrared range due to the presence of copper vacancies. Efforts to isolate intermediates during the cation exchange reaction show that it occurs via a mechanism where entire sheets are rapidly converted into the final product once the exchange reaction commences, precluding the isolation of alloyed and heterostructure species.
9:00 PM - NT4.8.26
Substrates Affect the Optical Properties of 2D Hexagonal Boron Nitride
Kevin Kahn 2,Daniel Schmidt 2,Catriona McGilvery 1,Andrivo Rusydi 2,Michelle Moram 1
1 Imperial College London London United Kingdom,2 National University of Singapore Singapore Singapore,2 National University of Singapore Singapore Singapore1 Imperial College London London United Kingdom
Show AbstractHexagonal boron nitride (h-BN) is of great current interest for applications in photo- or neutron- detectors and deep ultraviolet light emitters for water treatment in the developing world. However, its properties must be determined accurately to enable successful device design and application. Previous studies relied on photoluminescence spectroscopy, which is challenging to interpret and does not give signal from the thinnest layers of technological interest. Here, we determine the optical properties of technologically relevant 1-3 monolayer h-BN layers using temperature-dependent spectroscopic ellipsometry (SE) and electron energy loss spectroscopy (EELS). We demonstrate that these properties differ from those determined previously, agree directly with theory, and can be influenced by the substrate due to electronic correlations. The interesting new physics demonstrated by few-layers h-BN-based heterostructures opens up possibilities both for solving existing device problems and for developing new types of functional device. Few-layer h-BN is also a model two-dimensional (2D) material system and therefore these data indicate that the influence of the substrate should be considered when creating any 2D device.
HRTEM data are acquired on a FEI TITAN 80-300, EELS spectra collected on a JEOL 2100F with a Gatan GIF Quantum spectrometer, and SE measurements acquired on a vertical Vase Woollam ellipsometer.
9:00 PM - NT4.8.27
Bias-Stress-Induced Instability of Multilayer Molybdenum Disulfide Field-Effect Transistors
Jeongkyun Roh 1,In-Tak Cho 1,Jong-Ho Lee 1,Sung Hun Jin 2,Changhee Lee 1
1 Department of Electrical and Computer Engineering Seoul National University Seoul Korea (the Republic of),2 Department of Electronic Engineering Incheon National University Incheon Korea (the Republic of)
Show AbstractAs an alternative of graphene, molybdenum disulfide (MoS2) has been intensively studied as a great candidate for next-generation electronic material, and a variety of applications have been demonstrated using MoS2 field-effect transistors (FETs). However, the intrinsic nature of instability of MoS2 which is associated with easy adsorption of gaseous molecules limits the performance of MoS2 FETs significantly. Those adsorbed molecules influence on the device stability, thus removing and separating gaseous molecules from the MoS2 surface is the key for achieving highly stable MoS2 FETs. In this study, we investigated the effect of thermal annealing and passivation on the bias-stress-induced instability of multilayer MoS2 FETs. Thermal annealing was introduced to remove present gaseous molecules from the MoS2 surface, and passivation was employed to prevent additional adsorption. For the systematic investigation of effect of thermal annealing and passivation, we evaluated the bias stability of the MoS2 FETs in each step; before and after thermal annealing and after passivation. For the quantitative analysis of bias-stress-induced instability, the stretched hyperbola function was employed to extract the activation energy and the barrier distribution. It was found that the average energy barrier height for charge trapping was increased by 20% after thermal annealing, and the barrier distribution became even narrower after employing passivation.
9:00 PM - NT4.8.28
Two Dimensional Transition Metal Dichalcogenides: 1H and 1T Polymorphs, Structural Transitions, Anomalous Properties and Hydrogen Evolution Reactions
Anjali Singh 1,Sharmila Shirodkar 2,Umesh Waghmare 1
1 Theoretical Sciences Unit Jawaharlal Nehru Centre for Advanced Scientific Research Bangalore India,2 School of Engineering and Applied Sciences Harvard University Cambridge United States
Show AbstractMonolayered transition metal dichalcogenides (MX2, M= Mo, W and X= S, Se) are two-dimensional semiconductors exhibiting interesting physical and chemical properties, and have potential to be used in nano-electronic applications. Among the 1H and 1T structures exhibited by monolayers of transition metal dichalcogenides, the group VI compounds MX2 (M= Mo, W and X= S, Se) largely occur in the 1H form. Recently, transformation of the 1H form to the 1T form with metallic electronic structure at high temperatures was demonstrated in MoS2 with Re-substitution and electron irradiation.[1] Here, we use first-principles calculations to determine the energy landscape associated with the 1H to 1T phase transition, and relate the observed[1] intermediate structures to structural instabilities of the 1T structure of MX2 compounds. We show that the metallic centrosymmetric 1T (c1T ) structure of these compounds is unstable with respect to dimerization or trimerization of metal atoms, leading to competing metallic √3x1 1T form and ferroelectric semiconducting √3x √3 1T form respectively.[2] While the former is a more stable 1T form of MoSe2, WS2 and WSe2, the latter is a more stable 1T form of MoS2 exhibiting rich ferroelectric dipolar domain structure. With vicinity to metal-semiconductor transitions, their semiconducting forms are shown to exhibit anomalous response to electric field. To facilitate experimental verification of these subtle features of the 1T forms of MX2 monolayers, we present comparative analysis of their vibrational properties, and identify their Raman and Infra-Red spectroscopic signatures. We present a comparative first-principles analysis of chemical activity of 1T, 1H-MoS2 and 1T, 1H-MoSe2 towards H2 evolution and make connections with the experiments.[3]
[1] Y. C. Lin, D. O. Dumcenco, Y. S. Huang and K. Suenaga, Nature Nanotechnology, 9, 391 (2014).
[2] Anjali Singh, Sharmila N. Shirdkar and Umesh V. Waghmare, 2D Materials, 2, 035013 (2015).
[3] Uttam Gupta, B. S. Naidu, Urmimala Maitra, Anjali Singh, Sharmila N. Shirodkar, Umesh V. Waghmare and CNR Rao, APL Materials, 2, 092802, (2014).
9:00 PM - NT4.8.29
Modification of Electronic and Vibrational Properties of Doped Black-P Films
Sayan Sarkar 1,Prashant Sarswat 1,Michael Free 1
1 Univ of Utah Salt Lake City United States,
Show AbstractAmong all the polymorphs of phosphorus, black phosphorus (black-P) research has been the most absent for 100 years since its date of first synthesis in 1914. However, recently it has been re-explored due to its unique puckered single layer geometry. Few or single atomic layer forms of black-P can be isolated using techniques such as micromechanical or liquid exfoliation. However, the exfoliation techniques limit the use of black-P, hence a method of black-P layer deposition onto a substrate is needed. Few or atomic layer deposition of black-P leads to substrate-material interactions and a possible appearance of a band gap opening at the K point. Hence, a series of experiments were conducted in order to grow black-P on different substrates. Some modifications such as incorporation of doping elements, variation in deposition temperature, and inclination of reaction tubes have been applied in order to improve the film adhesion and the electrical properties. It was observed that sulfur and boron doped films exhibit a reduction in sheet resistance whereas selenium doped films show higher sheet resistance than parent black-p coated film. Effect of doping was also reflected in Raman A1g and A2g modes. The shift in peak position was consistent with molar mass of dopants.
9:00 PM - NT4.8.30
Layer-Controlled Chemical Vapor Deposition Growth of MoS2 Vertical Heterostructure
Leith Samad 1,Sage Bladow 1,Qi Ding 1,Junqiao Zhuo 2,Song Jin 1
1 Univ of Wisconsin-Madison Madison United States,1 Univ of Wisconsin-Madison Madison United States,2 Peking University Beijing China
Show AbstractThe fascinating semiconducting and optical properties of monolayer and few-layer transition metal dichalcogenides, as exemplified by MoS2, have made them promising candidates for optoelectronic applications. As a result, a variety of synthetic methods to produce monolayers have been developed in recent years. However, these methods typically rely on metal oxide precursors and high temperature reactions (>700 oC) and produce individually grown plates on the order of ~10 µm from arbitrary nucleation sites. Here we report a direct chemical vapor deposition (CVD) synthesis of MoS2 monolayer/multilayer vertical heterostructures with controlled layer numbers on a variety of layered material substrates via van der Waals epitaxy. Through precise control of the vapor pressures of the MoCl5 and elemental sulfur precursors, we have successfully refined a low temperature CVD reaction capable of reproducibly growing vertical heterostructures of MoS2 with areas > 1 mm2 on several layered material substrate (SnS2, TaS2, graphene, etc.). Importantly, careful regulation of the precursor temperatures relative to ambient atmospheric conditions permits controlled growth of MoS2 heterostructures with controllable layering from 1 to 6 layers. The monolayer MoS2 heterostructure was independently verified using cross-sectional transmission electron microscopy (TEM) while Raman and photoluminescence spectroscopy confirmed the MoS2 layer-controlled growth and heterostructure interactions. Raman, photoluminescence, and energy dispersive x-ray spectroscopy (EDS) mappings verified uniform coverage of the MoS2 layer, and nanoscale electrical device measurements probed the electronic properties of the resulting heterostructures. This reaction provides an ideal method for the scalable layer-controlled growth of transition metal dichalcogenide heterostructure for a variety of optoelectronic applications.
9:00 PM - NT4.8.31
Aging of Two-Dimensional Layered Transition Metal Dichalcogenides
Bo Li 2,Sidong Lei 1,Yongji Gong 3,Yingchao Yang 2,Jun Lou 2,Robert Vajtai 2,Pulickel Ajayan 2
2 Department of Materials Science and NanoEngineering Rice University Houston United States,1 Rice University Houston United States2 Department of Materials Science and NanoEngineering Rice University Houston United States,3 Department of Chemistry Rice University Houston United States
Show AbstractThe stability of two dimensional (2D) layered transition metal dichalcogenides (TMDs) is one of the major concerns for the measurements and applications of 2D TMDs. With most of the atoms exposed, the 2D TMDs are susceptible for corrosive substances from the environment. In this study, MoSe2 was chosen as the target material and we have performed the time dependence measurement of Raman, PL, TEM and electrical measurement under different aging conditions (desiccator, laboratory atmosphere and humid air with 100 % relative humidity) over several months. Encouragingly, we found that the samples stored in a commercial vacuum desiccator did not show apparent change after 4 month. However, the samples in atmosphere and in humid air experience significant degradation. The humid air is proved to be most harmful condition for 2D TMDs. We were also able to determine the lateral distribution of degradation through AFM study. This is the first comprehensive study of stability of TMDs layers and its degradation mechanism and will shine the light on the application and protection of 2D devices.
9:00 PM - NT4.8.32
The Role of Grain Boundaries in 2D Hexagonal Boron Nitride for Memristive Switching Device
Stefan Tappertzhofen 1,Ruizhi Wang 1,Sabina Caneva 1,Stephan Hofmann 1
1 University of Cambridge Cambridge United Kingdom,
Show AbstractMemristive switches (RRAMs) are promising candidates for future non-volatile memories and memory-in-logic architectures [1]. In these two-terminal devices, logic states are encoded by different resistance levels. The resistance transition is attributed to the growth and rupture of a nanoscale conductive filament driven by the migration of mobile ions. In case cations, such as silver or copper ions, are the mobile ion species ultra-low switching energies below 10 fJ/bit are predicted [2][3]. Despite their advantages including energy-efficiency, scalability and compatibility with standard back-end-of-line fabrication processes, the inferior device stability due to uncontrolled cation diffusion impedes their practical application [4]. Graphene has been recently suggested as ultra-thin diffusion barrier [5]. However, the cation diffusion through 2D materials integrated in RRAMs is unexplored which hinders device optimisation. We report on hexagonal boron nitride (hBN) monolayers integrated in vertical and lateral memristive structures. In particular, we studied the switching behaviour in presence of grain boundaries and discuss if and how these can be tuned to improve the device characteristics. Electrical measurements are complemented by in situ electron microscopy studies and a recently introduced technique, so called plasmon-enhanced spectroscopy [6], to probe morphological changes upon resistive switching. By using an insulator such as hBN, we avoid additional pattern and etching processes which are required if conductive or semi-conductive 2D layers are integrated in RRAMs. Therefore, we believe that 2D hBN is the ideal candidate for ultra-thin diffusion barriers in memristive switches.
[1] R. Waser and M. Aono, Nature Materials, vol. 6, pp. 833 - 840, 2007.
[2] D. Ielmini, “Filamentary-switching model in RRAM for time, energy and scaling projections,” Proc. IEEE IEDM, pp. 17.2.1 - 17.2.4 , 2011.
[3] S. Tappertzhofen, I. Valov and R. Waser, “Quantum conductance and switching kinetics of AgI-based microcrossbar cells,” Nanotechnology, vol. 23, no. 14, p. 145703, 2012.
[4] I. Valov, E. Linn, S. Tappertzhofen, S. Schmelzer, J. v. d. Hurk, F. Lentz and R. Waser, “Nanobatteries in redox-based resistive switches require extension of memristor theory,” Nature Communication, vol. 4, p. 1771, 2013.
[5] M. Lübben, P. Karakoli, V. Sougleridis, P. Normand, P. Dimitrakis and I. Valov, “Graphene-Modified Interface Controls Transition from VCM to ECM Switching Modes in Ta/TaOx Based Memristive Devices,” Advanced Materials, vol. 27, pp. 6202-6207, 2015.
[6] G. D. Martino, S. Tappertzhofen, S. Hofmann and J. Baumberg, submitted, 2015
Symposium Organizers
Linyou Cao, North Carolina State University
Bruce Claflin, Air Force Research Laboratory
Thomas Mueller, Vienna University of Technology
Hua Zhang, Nanyang Technological University
Symposium Support
Aldrich Materials Science
APLMaterials|AIP Publishing
HORIBA Scientific
2D Materials and Materials Research Express | IOP Publishing
NT4.9: New 2D Materials and Heterostructures
Session Chairs
Friday AM, April 01, 2016
PCC North, 200 Level, Room 227 B
9:00 AM - *NT4.9.01
Spin-Orbit Engineering in Transition Metal Dichalcogenide Alloy Monolayers
Cedric Robert 1,Gang Wang 1,Aslihan Suslu 2,Bin Chen 2,Sijie Yang 2,Sarah Alamdari 2,Iann Gerber 1,Thierry Amand 1,Sefaattin Tongay 2,Bernhard Urbaszek 1,Xavier Marie 1
1 University of Toulouse - CNRS Toulouse France,2 Arizona State University Tempe United States
Show AbstractThe spectacular progress in controlling the electronic properties of graphene has triggered research in alternative atomically thin two-dimensional crystals. Monolayers (ML) of transition-metal dichalcogenides (TMDC) such as MoS2 have emerged as very promising nanostructures for optical and electronic applications. Most studies have focused on binary monolayers (MoS2, MoSe2, WS2, WSe2…) that share common properties: direct optical bandgap [1], spin-orbit (SO) splittings of hundreds of meV, light-matter interaction dominated by robust excitons [2-4] and coupled spin-valley states [5].
Studies on alloy-based monolayers are more recent, yet they may not only extend the possibilities for TMDC applications through specific engineering but also help understanding the differences between each binary material. Here, highly crystalline Mo1−xWxSe2 has been synthetized to show engineering of the direct optical bandgap and the SO coupling in ternary alloy monolayers.
We investigate the impact of the tuning of the SO spin splitting on the optical and polarization properties. We show a non-linear increase of the optically generated valley polarization as a function of tungsten concentration, where 40% tungsten incorporation is sufficient to achieve valley polarization as high as in binary WSe2.
We also probe the impact of the tuning of the conduction band SO spin splitting on the bright versus dark exciton state population i.e. photoluminescence (PL) emission intensity. We show that the MoSe2 PL intensity decreases as a function of temperature by an order of magnitude, whereas for WSe2 we measure surprisingly an order of magnitude increase over the same temperature range (T=4-300K) [6]. The ternary material shows a trend between these two extreme behaviors. These results are discussed on the basis of the reversal of the sign of the spin-orbit splitting in the conduction band for MoSe2 and WSe2 leading to different temperature dependences of the emission yield.
[1] K. Mak et al, PRL 105, 136805 (2010)
[2] G. Wang et al, PRL 114, 97403 (2015)
[3] G. Wang et al, PRL 115, 117401 (2015)
[4] D. Lagarde et al, PRL 112, 047401 (2014)
[5] G. Sallen et al, PR B 86, 081301(R) (2012)
[6] G. Wang, C. Robert et al, arXiv 1506.08114 (2015)
9:30 AM - *NT4.9.02
New Materials for Van der Waals Heterostructures
Roman Gorbachev 1
1 University of Manchester Manchester United Kingdom,
Show AbstractProbably the most important benefit of graphene’s discovery is the attention it has brought to many other two-dimensional (2D) layered materials. Using similar strategies to those applied to graphene one can extract single atomic layers from other bulk crystals or achieve large scale growth.
In the last couple of years, a novel field has emerged which deals with structures and devices assembled layer-by-layer from various atomically-thin crystals. These new multilayer structures have proved to be extremely versatile, showing exceptional electronic and optical properties, new physics and new functionality. This is mostly due to the fact that each atomic layer can be chosen among many different materials including metals, semiconductors, superconductors or even topological insulators. For example, graphene’s ‘sister’ material hexagonal boron nitride has similar mechanical properties but on the contrary is a wide gap insulator. Another example is that by using transition metal dichalcogenides we can now overcome the band gap problem by producing tunnelling transistors that consist of two layers of graphene separated by a few atoms thick layer of molybdenum disulphide.
In this talk I will review recent progress in this field and present important milestones in its development. Specific attention will be paid to fabrication of such structures and their chemical stability as well as charge transport and optical experiments demonstrating various proof-of-concept functionalities. I will also discuss new additions to the 2D material family such as black phosphorus and niobium diselenide and present results on their properties down to one atomic layer thickness.
10:00 AM - NT4.9.03
Two-Dimensional Metals: From Low Contact-Resistance Electrodes to High Activity Catalysts
Yuanyue Liu 2,Jingjie Wu 2,Ken Hackenberg 2,Jun Lou 2,Pulickel Ajayan 2,Brandon Wood 3,Boris Yakobson 2,Paul Stradins 1,Suhuai Wei 1
1 National Renewable Energy Laboratory Golden United States,2 Rice University Houston United States,2 Rice University Houston United States3 Lawrence Livermore National Laboratory Livermore United States1 National Renewable Energy Laboratory Golden United States
Show AbstractMost of the current studies on two-dimensional (2D) materials are focused on semiconductors or semi-metal (graphene), while 2D metals (in particular, group-5 metal dichalcogenides: VS2, NbS2, TaS2) remain relatively less explored. Here we demonstrate their interesting properties that are appealing for applications in electronic devices and catalysis.
(1) When stacking on 2D semiconductors, they form van der Waals metal-semiconductor junctions which have weak Fermi level pinning due to the suppression of metal-induced gap states. This property enables the tuning of Schottky barriers by varying the work function of electrodes. Since these 2D metals have high work function, they are promising candidate electrodes for hole transport through the junction.[1]
(2) Their high work function also enhances the H binding on their surfaces, which fall within the optimal binding range for electrochemical hydrogen production, leading to an overall higher activity than edge-active metal dichalcogenides catalysts, as confirmed by our experiments. [2]
References:
[1] Y. Liu, P. Stradins, S. Wei, submitted
[2] Y. Liu, K. Hackenberg, J. Wu, et al, submitted
10:15 AM - NT4.9.04
Ferroelasticity and Domain Physics in Two-Dimensional Transition Metal Dichalcogenide Monolayers
Wenbin Li 1,Ju Li 1
1 MIT Cambridge United States,
Show AbstractMonolayers of group VI transition metal dichalcogenides (TMDs) can exist in several structural polymorphs including 2H, 1T and 1T'. While most of the TMD monolayers adopt 2H structure at ambient conditions, the low-symmetry 1T' phase is the thermodynamically stable phase for WTe2 monolayers and metastable for others. An interesting feature associated with the 1T' phase is that it has three different orientation variants, resulting from the three equivalent directions of Peierls distortion in the parent 1T phase. Using first-principles calculations, we predict that mechanical strain can switch the relative thermodynamic stability between the different orientation variants of the 1T' phase. We find that such strain-induced variant switching only requires a few percent elastic strain, which is eminently achievable experimentally with TMD monolayers. Calculations indicate that the transformation barrier associated such variant switching is small (less than 0.2 eV per chemical formula unit), suggesting that strain-induced variant switching can happen under normal experimental conditions and timescale. Monolayers of TMDs with 1T' structure therefore have the potential to be ferroelastic and shape memory materials with interesting domain physics.
10:30 AM - *NT4.9.05
Metal-Organic Chemical Vapor Deposition of BN on Sapphire and Its Heterostructures with 2D and 3D Materials
Qing Paduano 1,Michael Snure 1
1 Air Force Research Lab Wright Patterson AFB United States,
Show AbstractThere have been tremendous efforts to study two-dimensional (2D) layered materials, for their potential to create a new generation of electronics and opto-electronics. BN as an insulating substrate or gate dielectric is an essential building block for these applications. Recently, Van der Waals epitaxy using MOCVD has drawn considerable attention as a potentially scalable, bottom-up process. 2D epitaxial films grown on sp2-bounded BN represent a new opportunity for large scale fabrication of 2D heterostructures, 3D epitaxial films grown on sp2 bounded BN allow the transfer of films of 3D material onto desirable substrates.
With these motivations in mind, we studied MOCVD processing for direct growth of BN on 2” sapphire substrates. The combined experimental evidence points to three growth modes: self-terminating, 3D random, and layer-by-layer, all of which are dependent on V/III ratio, temperature, pressure, and substrate surface modification via nitridation. At moderate temperature (950-1050oC), BN growth using high V/III ratio is self-terminating, resulting in preferentially c-oriented films aligned in-plan with respect to the orientation of the sapphire substrate. BN films grown under low V/III ratios are 3D, randomly oriented, and nano-crystalline. At higher temperature (1100oC), 3D random-oriented growth subsides, and self-terminating growth transitions to a continuous layer-by-layer growth mode.
When BN growth is self-terminating, films exhibit atomically smooth surface morphology and highly uniform thickness over a 2” sapphire wafer. These BN/sapphire templates are used to study direct growth of 2D on 2D layered material, by which we deposited graphine on BN in a continued process within the same MOCVD system. Furthermore, we explore the growth of planar 3D materials on BN, which is very challenging due to suppressed nucleation on the extremely low-energy surface of sp2-bounded BN. With an AlN or GaN buffer, GaN single crystalline films on BN/sapphire with ~2um thickness can be obtained without self-delaminating.
11:30 AM - *NT4.9.06
Atomic-Scale Derivatives of Group IV Semiconductors
Joshua Goldberger 1
1 Ohio State Univ Columbus United States,
Show AbstractSimilar to how carbon can be sculpted into low-dimensional allotropes such as fullerenes, nanotubes, and graphene, one of the major themes of our research program is that the framework connectivity of atoms for any crystalline solid can be ligand-terminated along specific axes to produce stable, crystalline van der Waals materials comprised of single or few atom thick fragments. These new atomic-scale materials can have completely different and transformative physical properties compared to the original material. Here, we will describe our recent successes in the creation of hydrogen and organic-terminated group IV (Si, Ge, Sn) 2D graphane analogues. We have established numerous synthetic principles for topotactically converting layered Zintl phase precursors into van der Waals materials systems. We will describe how the optical, electronic, and thermal properties of these materials can be systematically controlled by substituting either the surface ligand or via alloying the framework with different elements. These atomic-scale materials represent an intriguing regime in which both surface functionalization and solid-state chemistry can be uniquely exploited to design properties and phenomena.
12:00 PM - NT4.9.07
Ferroelectric Control of Monolayer MoS2 via Direct Single-Layer Growth on LiNbO3
Ariana Nguyen 1,Edwin Preciado 1,Velveth Klee 1,Dezheng Sun 1,Daniel Lu 1,David Barroso 1,Ludwig Bartels 1
1 UC Riverside Riverside United States,
Show AbstractWe present the direct chemical vapor deposition (CVD) growth of monolayer molybdenum disulfide (MoS2) onto periodically poled lithium niobate. Single-layer MoS2 displays a preference for the ferroelectric domains polarized "up" with respect to the surface. This may offer the possibilities of templated growth of transition metal dichlacogenide (TMD) films using the substrate ferroelectric polarization as a pattern. Piezoresponse force microscopy reveals that the MoS2 film maintains the substrate polarization on the "up" domains while partially quenching it on the "down" domains. Nonlinear photoresponse validates the presence of trapped states in the MoS2 film. Surface acoustic spectroscopy allows us to measure the majority carrier types in the TMD material. Electrical transport measurements suggest the ability to invert the single-layer CVD MoS2 majority charge carrier via gating dependence on the domain orientation of the periodically poled lithium niobate substrate.
12:15 PM - NT4.9.08
Polarization and Resistive Switching Behavior of Ferroelectric Thin Films with 2D-Layered Dichalcogenides
Tao Li 1,Alexey Lipatov 1,Pankaj Sharma 2,Hyungwoo Lee 3,Chang-Beom Eom 3,Alexander Sinitskii 1,Alexei Gruverman 1
1 University of Nebraska-Lincoln Lincoln United States,2 University of New South Wales Sydney Australia3 University of Wisconsin-Madison Madison United States
Show AbstractTransition metal dichalcogenides (TMDs) are emerging 2-dimensional (2D) materials of the MX2 type, where M is a transition metal atom (Mo, W, Ti, Sn, Zr, etc.) and X is a chalcogen atom (S, Se, or Te.). Comparing to graphene, TMDs have a sizable band gap and can be metal, half-metal, semiconductor or superconductor. Their band structures can be tuned by external bias, mechanical force, or illumination. Their rich physical properties makes TMDs potential candidates for a variety of applications in nanoelectronics and optoelectronics.
Ferroelectric tunnel junctions (FTJs) are actively studied as a next-generation of non-volatile memory elements. An FTJ comprises a ferroelectric tunnel barrier sandwiched between two electrodes. In this work, we investigated the resistive switching behvaior of MoS2/BaTiO3/SrRuO3 and MoS2/Pb(Zr,Ti)O3/La1-xSrxMnO3 heterostructures using the Piezoresponse Force Microscopy (PFM) technique. The switched area of the ferroelectric barrier can be modulated via electric bias or mechanical force opening a possibility of tunable electroresitance effect. Furthermore, effect of optical illumination on the polarization reversal dynamics has been observed and analyzed based on the polarization-induced modulation of the MoS2 layered electronic properties.
12:30 PM - *NT4.9.09
Solution Processable 2D Materials Based Nanoplates and Heterostructures for High Performance Electronics
Yu Huang 1
1 Department of Materials Science and Engineering Univ of California-Los Angeles Los Angeles United States,
Show AbstractSolution dispersible inorganic nanostructures have emerged as interesting ink materials for low temperature solution processing of electronic thin films on various substrates including flexible substrates. The two-dimensional (2D) colloidal nanoplates, exhibiting few dangling bonds, represent an ideal geometry for the assembly of highly uniform continuous thin films with greatly reduced grain boundaries dictated by large-area conformal plane− plane contact with atomically flat/clean interfaces. It can therefore lead to efficient charge transport across neighboring nanoplates and throughout the entire thin film to enable unprecedented electronic performance. Here, we demonstrate the synthesis of colloidal 2D nanoplates, as well as theheterostructures, of layered materials in solution as a new class of ink materials. We further demonstrate that these new ink materials can be assembled into high-performance electronic thin films. In particular, we show that Bi2Se3 and Bi2Te3 nanoplates can be synthesized with well-controlled thickness (6−15 nm) and lateral dimension (0.5−3 μm) and can be used for the assembly of highly uniform continuous thin films with a full surface coverage and an excellent room temperature carrier mobility >100 cm2●V−1●s−1, approaching that of chemical vapor deposition grown materials. These studies demonstrates the great promise in scaling up the synthesis of various 2D material structures in solution and demonstrates a general strategy of using 2D nanostructure s as a unique building block for future flexible electronics and optoelectronics.
NT4.10: Phonon and Thermal Conductivity of 2D Materials
Session Chairs
Bruce Claflin
Michael Snure
Friday PM, April 01, 2016
PCC North, 200 Level, Room 227 B
2:30 PM - *NT4.10.01
The Effects of Substrate on the Properties of Monolayer Transition Metal Dichalcogenides
Yong Zhang 1
1 UNC Charlotte Charlotte United States,
Show AbstractDespite a large volume of research on monolayer or a-few-layer transition metal dichalcogenides (TMDs), many basic intrinsic material properties are not accurately known so far. For example, the reported values for the fundamental bandgap of monolayer WS2 fluctuate in a range over 30 meV. The situation for TMDs is also largely true for other 2D materials, including graphene and phosphorene. This situation is in stark contrast with that for a conventional semiconductor, for instance, GaAs, where the bandgap is known with an accuracy of ~ 0.1 meV at low temperature and ~ 1 meV at room temperature. Therefore, a shift in bandgap as small as a fraction of meV can be observed with controllable impurity incorporation, and many impurity related states can be identified within a few tens of meV from the bandgap. Because of the impact of the substrate is significant but not well understood, predictions or expectations based on an ideal free-standing material cannot be verified, and vice versa, the experimentally observed results cannot be interpreted using the theoretic results assuming free-standing material. What has led to the large uncertainty of the TMC bandgap value, for instance, is the insufficient understanding and control over samples used in the previous studies, in particular the effects of substrate. We demonstrate that high temperature Raman spectroscopy is an efficient technique to probe the film morphology, bonding condition or epitaxial strain with the substrate, and further the annealing induced changes in these aspects. In conjunction with photoluminescence and reflectance techniques, and comparing samples on different substrates through either epitaxial growth or transfer, we can explain the underlying mechanisms for the substrate dependence of Raman frequency, PL/bandgap energy, and the intensity of the Raman and PL signal, thus, derive the intrinsic values for these quantities as a free-standing monolayer.1,2 Furthermore, the degree of the substrate influence manifests differently for each specific material property. For instance, the impact on the electronic structure diminishes quickly with increasing the monolayer number, but the substrate effect on thermal conductivity may remain for a significantly larger number of the monolayers, as recently found for thin layers of Black Phosphorus (BP).3 Accurate understanding of the substrate effects will allow us to control, manipulate and engineer the material properties for real world applications.
In collaboration with L. Su and J.-W. Wang of UNCC, Y.-F. Yu and L.Y. Cao of NCSU, and Y. Li and J.-B. Li of IOS, CAS.
1 L. Su et al., Nanoscale, 4920 (2014).
2 L. Su et al., Nano Research 8, 2686 (2015).
3 L. Su and Y. Zhang, Appl. Phys. Lett. 107, 071905 (2015).
3:00 PM - NT4.10.02
Twisted Bilayers of Transition Metal Dichalcogenides with Variable Coupling Revealed by Low-Frequency Raman Spectroscopy
Alexander Puretzky 1,Liangbo Liang 1,Xufan Li 1,Kai Xiao 1,Bobby Sumpter 1,Vincent Meunier 2,David Geohegan 1,Masoud Mahjouri-Samani 1
1 Oak Ridge National Lab Oak Ridge United States,2 Rensselaer Polytechnic Institute (RPI), Physics Troy United States
Show AbstractDetailed experimental study and modeling of low-frequency shear and breathing Raman modes of transition metal dichalcogenide (TMD) bilayers composed of twisted monolayers of MoSe2 showed unique bilayers with variable coupling between the monolayers. These stacking configurations were observed in the narrow range of twist angles from 60° to 57° based on appearance of two breathing modes, B1 and B2, corresponding to the mixture of stable commensurate patches of 2H stacking configurations and incommensurate regions of the randomly aligned monolayers. In the wide range of the twist angles between 5° and 57° only one breathing mode, B2, was observed indicating a simple random alignment of MoSe2 monolayers without in-plain restoring forces required to generate a shear mode. The bilayers with variable coupling and spacing may provide a new platform for optoelectronic applications of these materials.
Research sponsored by the U.S. Dept. of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Div. (synthesis science) and Scientific User Facility Div. (characterization science).
3:15 PM - NT4.10.03
Multiscale Modeling of Low Frequency Phonons in New Two-Dimensional Materials for Thermoelectric Applications
Vinod Tewary 1
1 NIST Boulder United States,
Show AbstractSince the advent of graphene, many new atomistically thin two-dimensional (2D) materials have been either already fabricated or theoretically proposed. The new materials represent a revolutionary new development in the science and technology of materials. These materials include homopolar materials like phosphorene, silicene, and germanene and polar materials like transition metal dichalcogenides. All these materials have a strong potential for diverse industrial and defense applications. Compared to the normal 3D solids, the electronic as well as phononic characteristics of these materials are found to be quite unusual that can be exploited in the design of efficient thermal management and thermoelectric energy conversion devices. Phonons, particularly low frequency phonons, play important role in determining the thermal response of materials. Modeling and measurement of phonons in these new materials are therefore research areas of strong topical interest. It is essentially a multiscale problem that requires modeling of a large many-body system at the atomistic scale and over large time scales extending from femto seconds to nano seconds or even microseconds.
Modeling temporal behavior of a many-body system over such an extended time range using conventional molecular dynamics is a formidable numerical task even with modern computers. We have developed a causal Green’s function method that has been shown to be very efficient for modeling the long-time temporal response of materials at the atomistic scales. This method has been further refined and improved by coupling the multiscale space and the causal Green’s functions into a unified formulation. We have found that the modified Green’s function formulation can extend the time scales by several orders of magnitude and is ideally suited for modeling low frequency phonons.
The application of this mathematical technique to simulate phonon transport in the new 2D homopolar materials will be described along with the calculation of the thermoelectric figure of merit. These calculations should be of interest for the design of thermal management and thermoelectric devices. The input parameters for calculating the Green’s function are obtained by using the density functional theory, a first-principle based technique, as available in the literature. Numerical results will be presented for phonons, density of states, and thermal conductivity for phosphorene and silicene.
3:30 PM - NT4.10.04
Size Dependence and Ballistic Limits of Thermal Transport in Anisotropic Layered Two-Dimensional Materials
Zuanyi Li 1,Yizhou Liu 3,Lucas Lindsay 4,Yong Xu 2,Wenhui Duan 3,Eric Pop 1
1 Electrical Engineering Stanford Univ Stanford United States,2 Dept. of Physics and State Key Laboratory of Low-Dimensional Quantum Physics Tsinghua University Beijing China,3 Collaborative Innovation Center of Quantum Matter Beijing China4 Oak Ridge National Laboratory Oak Ridge United States2 Dept. of Physics and State Key Laboratory of Low-Dimensional Quantum Physics Tsinghua University Beijing China
Show AbstractWhen device dimensions are comparable to the energy carrier mean free paths (MFPs), typically 10-100 nm, heat flow can no longer be described using a classical (diffusive) thermal conductivity, becoming quasi-ballistic and size-dependent. How this occurs in anisotropic, layered two-dimensional (2D) materials has received very little attention, particularly “beyond graphene,” which is a major roadblock in understanding such materials in truly nanoscale devices.
In this work, we present a comprehensive study of ballistic and size-dependent thermal transport in monolayer graphene, h-BN, MoS2 and WS2, as well as their bulk counterparts. Based on full phonon dispersions obtained from ab initio simulations, we calculate the in-plane (for monolayer and bulk) and cross-plane (for bulk) ballistic thermal conductance Gball of these materials. Due to their stronger chemical bonding, graphene (graphite) and h-BN generally show higher Gball than MoS2 and WS2 above ~100 K. We then obtain length-dependent thermal conductivity of these materials by using a ballistic-diffusive model, which shows good agreement with available experimental measurements. An overall phonon MFP as a rigorous average of all phonon modes can be condensed to an expression including Gball and the diffusive thermal conductivity kdiff. We find that kinetic theory can reach the same MFP estimate as long as a correctly averaged phonon group velocity is used. Importantly for anisotropic layered materials, 2D and 1D forms of the kinetic theory should be used for in-plane and cross-plane transport, respectively. Based on the ballistic-diffusive model, k converges to >98% of kdiff only around sample lengths ~100 times the MFP, which ranges from ~300 nm for h-BN cross-plane to ~60 µm for suspended graphene in-plane. These results provide a deeper understanding of microscopic thermal transport, revealing that device scales below which thermal size effects should be taken into account are generally larger than previously thought.
3:45 PM - NT4.10.05
Thermal Properties of Two-Dimensional Nanostructures via Temperature-Dependent Raman Spectroscopy
Yuan Li 1,Jeffrey Cain 1,Eve Hanson 1,Fengyuan Shi 1,Xinqi Chen 1,Vinayak Dravid 1
1 Materials Science and Engineering Northwestern University Evanston United States,
Show AbstractAtomically thin two-dimensional (2-D) structures, such as layered transition metal dichalcogendies (TMDCs) and black phosphorus (BP), are being explored as potential candidates for future nanoelectronic devices due to their unique physical/chemical properties and intrinsic band gap. Understanding the thermal properties of these 2-D materials is of particular interest for two reasons. One lies in the significant dependence of their electronic device performance and service life on the heat dissipation and hence their thermal conductivity, meanwhile, the other is regarded to the fundamental understanding of anharnonicity of the temperature-dependent vibrational modes and electron-phono interactions in these 2-D materials. However, to the best of our knowledge, thermal transport in various 2-D materials beyond graphene have not been well-studied and their concrete thermal conductivity was not experimentally valued. In this presentation, we report a series of our research focused on developing a reliable temperature-dependent Raman spectroscopy for understanding the unique thermal conduction performance of various 2-D materials including CVD-grown MoS2 nanosheets, MoS2 nanoribbons and BP nanoribbons. The as-grown MoS2 nanosheets were transferred to a Si3N4-coated Si substrate with uniformly patterned micro-holes, which allows for the direct measurement of the temperature coefficiency of phonon shift by a temperature-dependent Raman spectroscopy and further calculation of their in-plane thermal conductivity. The nanoribbons of MoS2 and BP were fabricated using a mask-controlled photolithography technique. Considering their anisotropic in-plane thermal conduction property, a Raman spectroscopy equipped with polarized laser were used to determine their thermal conductivity. Moreover, regard to the chemical instability of BP during long time exposure, we evaluated the influence of surface oxidation on the thermal conduction of BP nanoribbons, of which the associated mechanism were further co-explained with the assistant of XPS, SIMS and electronic characterizations. Our studies provide fundamental understanding on the unique thermal conduction of 2-D structures and are possible to give inspiration for the real fabrication of their nanoscale electronic devices where thermal management is critical.