Peide D. Ye, Purdue University
Debdeep Jena, University of Notre Dame
Andras Kis, Ecole Polytechnique Federale de Lausanne
Jun Lou, Rice University
Symposium Support 2D Semiconductors Inc.
NN2: 2D Materials beyond Graphene: Synthesis II
Shanee D. Pacley
Tuesday PM, April 22, 2014
Moscone West, Level 2, Room 2007
2:30 AM - *NN2.01
Synthesis of Transition Metal Dichalcogenide Monolayer and Alloy
Lance Li 1
1Academia Sinica Taipei TaiwanShow Abstract
The large-area monolayer of MoS2, MoSe2, WS2 and WSe2 can be synthesized directly on arbitrary insulating substrates with CVD method using corresponding metal trioxides and S or Se powders as the reactants. To realize the high efficiency solar cells or other optoelectronic devices based on the TMD monolayers, it is important to develop a strategy to tune the optical band gap of these TMD monolayers. I would like to discuss several synthetic methods of obtaining monolayer alloys such as MoSxSe1-x, WSx,Se1-x, WSxTe1-x and so on. The band gap energy of these alloys is controllable depending on their composition.
3:00 AM - NN2.02
Controlled Scalable Synthesis of Uniform, High-Quality Monolayer and Few-Layer MoS2 Films
Linyou Cao 1
1North Carolina State Universit Raleigh USAShow Abstract
Two dimensional (2D) materials with a monolayer of atoms represent an ultimate control of material dimension in the vertical direction. Molybdenum sulfide (MoS2) monolayers, with a direct bandgap of 1.8 eV, offer an unprecedented prospect of miniaturizing semiconductor science and technology down to a truly atomic scale. Recent studies have indeed demonstrated the promise of 2D MoS2 in fields including field effect transistors, low power switches, optoelectronics, and spintronics. However, device development with 2D MoS2 has been delayed by the lack of capabilities to produce large-area, uniform, and high-quality MoS2 monolayers. Here we present a self-limiting approach that can grow high quality monolayer and few-layer MoS2 films over an area of centimeters with unprecedented uniformity and controllability. This approach is compatible with the standard fabrication process in semiconductor industry. It paves the way for the development of practical devices with 2D MoS2 and opens up new avenues for fundamental research.
3:15 AM - NN2.03
In-Situ Observations of Hexagonal Boron Nitride Growth During CVD
Piran Ravichandran Kidambi 1 Raoul Blume 2 Bernhard C Bayer 1 Carsten Baehtz 3 Robert S Weatherup 1 Robert Schloegl 2 Stephan Hofmann 1
1University of Cambridge Cambridge United Kingdom2Fritz Haber Institute Berlin Germany3Forschungszentrum Dresden-Rossendorf Dresden GermanyShow Abstract
2D materials beyond graphene such as hexagonal boron nitride (h-BN) have recently attracted a lot of research interest. While, chemical vapour deposition (CVD) of h-BN on transition metal catalysts has emerged as a preferred route for synthesis, the mechanism underlying the growth of h-BN are still very much unclear.
Here, using a combination of high-pressure time and depth resolved in-situ X-ray photoelectron spectroscopy (XPS) and in-situ X-ray diffraction (XRD) at realistic CVD conditions of pressure (~0.001 - 1 mbar) and extreme temperatures (700-1000oC) we analyse the behaviour of some of the most popular poly-crystalline metallic catalyst films and foils (eg: Cu, Ni, Co) for h-BN growth during exposure to precursors (both gaseous and liquid precursors).
These measurements allow for a clear understanding of the B and N incorporation in the nanostructure as it happens by identifying the catalyst state at any point of time during CVD. These coupled with ex-situ experiments [3,4] allow for development of a comprehensive growth mechanism to rationally engineer CVD processes to produce high quality material for device applications.
1. Kidambi et al. (manuscript in preparation)
2. Kidambi et al. Nano Letters 13 (10), 4769-4778 (2013).
3. Kidambi et al. J. Phys.Chem. C. 116, 42, 22492-22501 (2012).
4. Kidambi et al. PSS RRL. 5, 9, 341-343 (2011).
3:30 AM - NN2.04
Toward Single-Layer Nano-Heterostructures of Graphene and Hexagonal Boron Nitride
Luca Camilli 1 Eli Sutter 1 Peter Sutter 1
1Brookhaven National Laboratory Upton USAShow Abstract
Owing to their unusual electronic properties, one atom-thick layers of hybridized hexagonal boron nitride (h-BN) and graphene have recently attracted a great deal of attention . However, many important issues still remain open. Among them, addressing at the atomic scale the in-plane continuity as well as the interfacial electronic properties between h-BN and graphene domains is important for application in nanolectronics.
Here we demonstrate the growth of h-BN/graphene heterostructures that comprise graphene nanoribbons (GNRs) with width of ~ 1nm via a two-step process. Firstly, we produce high-quality h-BN flakes on a Au(111) substrate. Due to the chemical inertness of bulk Au, it is extremely difficult to synthesize either BN or graphene on such a substrate via the conventional chemical vapor deposition process. We overcome this challenge by using a recently developed alternative growth process - reactive magnetron-sputtering of B in N2/Ar gas mixtures - for the h-BN&’s synthesis . Using this method, we are able to obtain high-quality sub-micron size h-BN flakes. Subsequently, we produce GNRs by employing the bottom-up surface-assisted method introduced in Ref. . We use in-situ scanning tunneling microscopy (STM) to investigate the different steps of this process toward graphene/h-BN nano-heterostructures. Tunneling spectroscopy is applied to probe the local electronic properties, in particular near the interface of GNRs stitched to surrounding BN. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) are used to assess the quality of the synthesized nanostructures.
Detaching the h-BN/GNR nanohybrids from the Au substrate after the growth, through the nondestructive bubbling transfer method , allows their transfer to other substrates of technological interest.
 Ci, L. J.; et al. Nat. Mater. 2010, 9, 430
 Sutter, P.; Nano Lett. 2013, 13, 276
 Cai, J.; et al. Nature 2010, 466, 470
 Gao, L.; et al. Nat. Comm. 2012, 3, 699
3:45 AM - NN2.05
Observation of Surface Metallic States in Ultrathin Bi2Se3 Topological Insulator Epitaxially Grown on AlN
Athanasios Dimoulas 1 Polychronis Tsipas 1 Spyridon Kassavetis 1 Evangelia Xenogiannopoulou 1 Dimitra Tsoutsou 1 Evangelos Golias 1 Calliope Bazioti 2 George P Dimitrakopoulos 2 Philomela Komninou 2 Matty Caymax 3 Hu Liang 3
1NCSR DEMOKRITOS Athens Greece2Aristotle University of Thessaloniki Thessaloniki Greece3IMEC Leuven BelgiumShow Abstract
Topological insulators (TI), Bi2Se3 in particular, exhibit robust 2D surface states in which spin is strongly coupled to the orbital motion creating the prospect for the realization of electronic devices with novel switching mechanism and novel functionalities. An important requirement is that Bi2Se3 can be grown epitaxially in the form of very thin films, so that the surface properties dominate over the unwanted bulk conduction. Unfortunately in most cases, in order to observe the surface states, the films must be at least 6 quintuple (QL) layer thick (~6 nm) [1, 2], which limits their applicability in functional devices. Here in this paper we show by ARPES that well defined 2D surface metallic states are formed in ultrathin (only about 3 QL thick) Bi2Se3 grown epitaxially on AlN(0001)/Si(111) substrates by Molecular Beam Epitaxy (MBE). The use of AlN, not reported before, opens the possibility of employing insulating substrates first to improve electrical conduction measurements, second to integrate Bi2Se3 with gate dielectrics in order to exploit the quantum capacitance of TIs. The Bi2Se3/AlN will be compared with our control Bi2Se3/Si (111) samples more typically obtained in the literature [3, 4] in order to assess the advantages of growth on AlN. Employing RHEED, XRD/XRR and HRTEM we demonstrate a high epitaxial quality of Bi2Se3 on AlN. Using HRTEM, we identify interfacial misfit dislocations (MDs) that accommodate the high lattice mismatch between Bi2Se3 and AlN as well as rotation and lamellar twins. Strain mapping is performed to reveal the distribution of the strain field of the MDs at the atomic scale. Using in-situ XPS we show that, Bi2Se3 does not react with AlN allowing the growth of high quality ultrathin films at an optimum temperature of 300 oC without the need to grow low temperature buffer layers, as in the case of Si substrates. HRTEM observations reveal a crystalline, albeit distorted, interface between the two materials. Using ARPES we perform a systematic investigation of the dependence of the 2D surface states as a function of the Bi2Se3 thickness. On both substrates we observe a shift of the Dirac point closer to the Fermi level as thickness increases, but remarkably, in the case of Bi2Se3/AlN, the shift is much larger, bringing the Dirac point at about 0.15 eV below EF for a 12 QL sample, which is highly desirable for several envisaged applications. Finally, we will show that Bi2Se3 on AlN may be suitable substrate for the growth of a few layer MoSe2 dichalcogenide, yielding good quality epitaxial growth with well-defined valence bandstructure as evidenced from ARPES.
A. Dimoulas acknowledges an ERC Advanced Grant through project SMARTGATE-291260
 Y. Zhang, et al., Nature Physics, 6, 584-588 (2010);  Y. Sakamoto, et al., Phys Rev. B 81, 165432 (2010);  G. H. Zhang et al., Appl. Phys. Lett. 95, 053114 (2009);  A. A. Taskin, et al., Adv. Mater. 24, 5581 (2012).
4:30 AM - *NN2.06
Synthesis and Properties Atomic Layered Transition-Metal Dichalcogenides
Joshua Robinson 1 2
1The Pennsylvania State University University Park USA2The Pennsylvania State University University Park USAShow Abstract
The 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), could have an even greater impact on next generation technologies. Similar to graphene, TMDs are composed of vertically stacked, weakly interacting layers that are scalable down to sub-nanometer thicknesses. This, in addition to reported carrier mobilities up to 1000 cm2/Vs, and subtreshold slopes nearing the theoretical limit of 60mV/dec, make them an impressive material candidate for analog and digital electronics. Molybdenum disulfide (MoS2) is currently a leading TMD for scientific exploration, but there are a variety of other suitable, less explored, TMDs and TMD 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. The ability to grow high quality, large area TMDs is the first critical step in the realization of electronics requiring atomic layers with tailored electronic band alignments. In this talk I will discuss the applications of TMDs and TMD heterostructures, discuss our research in the synthesis and characterization of WSe2, WTe2, and TMD/graphene van der Waal solids, and show that heterostructures could be key to advanced electronic applications.
5:00 AM - NN2.07
Low Temperature Growth of Uniform Transition Metal Dichalcogenide Layers and Superlattices Over Large Areas
Christopher Muratore 1 2 Jianjun J. Hu 2 3 Md. Aman Haque 4 John E. Bultman 3 Michael L. Jespersen 3 Andrey A. Voevodin 2
1University of Dayton Dayton USA2Air Force Research Laboratory Wright-Patterson Air Force Base USA3Univeristy of Dayton Research Institute Dayton USA4Penn State University University Park USAShow Abstract
Synthesis capability for uniform growth of 2D materials over large areas at lower temperatures without sacrificing their unique properties is a critical pre-requisite for seamless integration of next-generation van der Waals heterostructures into novel devices. We have demonstrated, for the first time, vapor phase growth techniques for precisely controlled synthesis of continuous, uniform molecular layers of all MoX2 and WX2 transition metal dichalcogenide (TMD) compounds on diverse substrates, including graphene, hexagonal boron nitride, highly oriented pyrolitic graphite (HOPG), SiO2, and metal substrates over several square centimeters. Preliminary results show MoX2 and WX2 transition metal dichalcogenide materials grown in a novel ultra-high vacuum (UHV) physical vapor deposition (PVD) process demonstrate properties identical or even superior (e.g., electron mobilities >500 cm2 V-1 s-1) to exfoliated layers. This plasma-based physical vapor deposition (PVD) process for growth of high-quality mono-, bi- and few-layer TMDs is characterized by incident constituent atoms with maximum kinetic energies that are > 10 times higher than those produced in a thermal chemical vapor deposition process, yet below the threshold known to produce defects in the growing film materials. The kinetic energy and arrival rate of atoms is selected by modulating the pulse characteristics of power to the plasma cathode, while the flux of ions is tunable with an external magnetic field. The energetic atoms comprising the TMD film possess high surface mobility upon first contact with the substrate, reducing the processing temperatures necessary for continuous film coverage of the substrate. Further reduction of processing temperatures for application on flexible substrates are expected as computational modeling of growth modes guides experiments to reduce growth temperature without compromising properties via high defect densities. Examples such MoS2/WS2 heterostructures and bi-layer MoS2 on few-layer graphene with a 30% lattice mismatch and TMD/TMD heterostructures are shown to demonstrate how natural accommodation of stresses at 2D van der Waals interfaces has the remarkable potential to transform the way materials selection is considered for synthetic heterostructures, as concerns regarding lattice constant matching can be abandoned with preference given to desired properties and performance. Further development of synthetic heterostructures comprised of TMDs and previously unexplored materials known to have layered structures in bulk form for new 2D sensors and devices with tailored functionality is anticipated.
5:15 AM - NN2.08
Controlled Vapor-Phase Growth of Large, Photoresponsive Monolayer 2D GaSe Crystals
Xufan Li 1 Ming-Wei Lin 1 Alexander A. Puretzky 1 Christopher M. Rouleau 1 Juan C. Idrobo 1 David B. Geohegan 1 Kai Xiao 1
1Oak Ridge National Lab Oak Ridge USAShow Abstract
Two-dimensional (2D) nanomaterials with single or few atomic layers show many unique physical and chemical properties compared with their bulk counterparts. As the most widely and intensively studied 2D materials, graphene shows exotic electrical properties and great potential for next-generation electronic devices. However, the zero band-gap energy of graphene limits its applications in logic electronics and field-effect transistors (FETs). In order to overcome the limitation of graphene, 2D semiconducting materials with band-gaps are highly desired. Gallium selenide (GaSe) is a layered semiconductor with an indirect band-gap of ~2 eV. Bulk GaSe has been widely used in the field of optoelectronics, nonlinear optics, and terahertz experiments. Recently, few-layer GaSe nanosheets have been fabricated through mechanical exfoliation and vapor-phase deposition. These nanosheets show good performance as photodetectors and transistors, revealing great potential of this 2D nanomaterial as an alternative to graphene for electronic devices. However, the growth of large area, single crystal, uniform, single-layer GaSe flakes directly on insulating substrates, which is highly desired for device fabrication, is still a challenge. In this research, we synthesized large (up to ~50 mu;m in lateral size) 2D GaSe flakes on SiO2/Si substrates through a vapor phase deposition method. The thickness (from single to few atomic layers), shape, size, and density of the flakes were well controlled by tuning growth conditions. Atomic resolution images of the as-synthesized GaSe flakes were obtained to investigate the crystal structure, boundaries, and defects. The Raman spectra and photoluminescence spectra of the GaSe flakes change with their thickness in both peaks positions and intensities. These 2D GaSe flakes show dramatic photoresponse when illuminated with visible light. Moreover, they also exhibit typical FET characteristics. The results reveal the potential of our 2D GaSe flakes for photodetector and FET applications.
Synthesis science sponsored by the Laboratory Directed Research and Development program at Oak Ridge National Laboratory. Materials and device 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.
5:30 AM - NN2.09
Electronic Structure Evolution of Layered Chalcogenides: From Bulk to the Mono-Layer Limit
Hongtao Yuan 1 2 Bo Zhou 3 4 Gang Xu 1 Sanfeng Wu 5 Dumitru Dumcenco 6 Yingsheng Huang 6 Xiaodong Xu 5 Shoucheng Zhang 1 2 Zhixun Shen 1 2 Harold Y. Hwang 1 2 Yi Cui 1 2 7 Yulin Chen 3 4 2
1Stanford University Stanford USA2SLAC National Accelerator Laboratory Menlo Park USA3University of Oxford Oxford United Kingdom4Lawrence Berkeley National Laboratory Berkeley USA5University of Washington Seattle USA6National Taiwan University of Science and Technology Taipei Taiwan7Stanford University Stanford USAShow Abstract
Recent efforts on graphene-like atomic layer materials, aiming at novel electronic properties and quantum phenomena beyond graphene, have attracted much attention for potential electronics and spintronics applications. Compared to the weak spin-orbit-interaction (SOI) in graphene, layered transition-metal chalcogenides (MX2, M = Mo, W; X = S, Se, Te) having heavy 4d/5d elements with strong atomic SOI, not only are able to have an indirect to direct band-gap transition with dimension decreasing to monolayer but also provides a unique way for generating spin polarization based on valleytronics physics. Indeed, such a spin-polarized band structure has been demonstrated theoretically and supported by optical investigations. However, despite these exciting developments, the quantitative band structure of MX2 family compounds (e.g. the band maxima/minima loci in the BZ, the direct and indirect band gap sizes, etc.) have not yet been directly observed - which is particularly valuable to resolve conflicting theoretical investigations (e.g. the conduction band minima are quite important for valley-spin generation but varies with calculations). Especially for those cleaved ultrathin mono- and bi-layer flakes which host most of recently-reported exotic phenomena at their 2D limit, the direct detection for the band dispersion becomes of great importance and urgency.
In this work, we studied the electronic structure of representative transition metal chalcogenides MoS2, WS2 and WSe2 by ARPES and obtained detailed band structure maps as well as their variation between different compounds. We found that although the valence band maximum (VBM) of bulk MX2 compounds always resides at Γ, the conduction band minimum (CBM) loci shift from K (MoS2) towards Γ (WS2 and WSe2). This observation not only addresses the recent controversy in band calculations, but also demonstrates their different gap geometry in mono-layer thin films. In addition, utilizing the newly-developed spatially resolved ARPES technique, we directly studied the band structure evolution of MoS2 and WSe2 flakes from the bulk to the mono-layer limit, which clearly reveals the switching of the VBM apexes (from Γ to K) in both compounds. Such thickness dependent ARPES measurements on ultrathin MX2 flakes not only helps us establish a solid background to understand the underlying physics of these materials and further provides guidance for new materials design and novel device development of MX2 materials, but also demonstrates the possibility to study the band structure of such a group of submicron-size semiconductors used in functional devices.
NN3: Poster Session I: 2D Materials beyond Graphene
Tuesday PM, April 22, 2014
Marriott Marquis, Yerba Buena Level, Salons 8-9
9:00 AM - NN3.01
Single-Layer MoS2 on Titania: A Wide-Bandgap Narrow-Bandgap Stacked Material
Sarah Bobek 1 John Mann 1 Quan Ma 1 Miguel Isarraraz 1 Ariana Nguyen 1 David Barroso 1 Dezheng Sun 2 Velveth Klee 1 Edwin Preciado 1 Mark Micklich 1 Koichi Yamaguchi 1 Ludwig Bartels 1
1University of California, Riverside Riverside USA2Columbia Ney York USAShow Abstract
Transition metal dichalcogenides such as MoS2 and MoSe2 have been studied widely for their increase in bandgap and transition to a direct bandgap at the monolayer limit. Here we report on the combination of single-layer MoS2 with titania, a wide-bandgap semiconductor with numerous (photo-)catalytic applications. Our sample preparation proceeds via a sequence of cleaning of a SiO2/Si substrate, deposition of titanium, conversion of the titanium to rutile titania by means of oxygen plasma treatment and, finally, CVD growth of single-layer a MoS2 film. The resultant MoS2 films exhibit the characteristic photoluminescence feature at 1.87 eV indicative of single-layer material. In my poster I will present a range of further characterization results.
9:00 AM - NN3.02
Ultrathin Size- and Shape-Controlled Colloidal Cu2-xS 2D Nanosheets
Ward van der Stam 1 Quinten A. Akkerman 1 Johannes D. Meeldijk 2 Xiaoxing Ke 3 Sara Bals 3 Gustaaf van Tendeloo 3 Celso de Mello Donega 1
1Utrecht University Utrecht Netherlands2Utrecht University Utrecht Netherlands3Antwerp University Antwerp BelgiumShow Abstract
Ultrathin 2-dimensional (2D) nanomaterials (nanosheets, NSs) have been extensively investigated over the past few years due to the unique physical and electronic properties that arise when electrons and holes are confined in a plane that is just a few atomic layers thick (Lle;2.5 nm). Among the most studied species are graphene and transition metal dichalcogenides such as MoS2 and WS2. Typically, these ultrathin 2D nanomaterials are obtained via exfoliation of bulk materials or grown on substrates by vapor deposition techniques. The limitation of these techniques is that they offer little control over size and shape, and are also not suitable to produce free standing NSs. Colloidal preparation methods offer a promising alternative, as they have been very successful in producing nanomaterials with well-defined composition, size, shape, and surface. Additionally, colloidal methods are cheap and easy to upscale. Nevertheless, colloidal ultrathin NSs have only recently been reported, and are limited to CdX NSs (X= S, Se). Although colloidal CdX NSs have shown promising properties, control over their lateral dimensions and shape is still lacking. Moreover, CdX NSs are often entangled, aggregated, or coiled, which limits their applicability in optoelectronic devices. Therefore, to fully exploit the potential of ultrathin colloidal NSs, improved colloidal preparation methods are needed, which should provide control not only over the thickness of the NSs, but also over their shape and lateral dimensions. In this work, we address this need by developing a preparation method for ultrathin (2 nm) 2D colloidal Cu2-xS NSs with well-defined lateral dimensions and shape. This approach yields hexagonal or triangular colloidal NSs with sizes tunable from 100 nm up to several micrometers, and a constant thickness of 2 nm. Moreover, the Cu2-xS NSs produced are not aggregated or coiled, and are thus suitable for application in optoelectronic devices. Colloidal Cu2-xS NSs are promising materials for photovoltaics, photocatalysis, and nanoplasmonics. Furthermore, Cu+ ions in copper chalcogenides have been shown to be easily exchangeable by other cations. This opens up the possibility of using cation exchange reactions to convert Cu2-xS into other metal sulfides, with preservation of the size and shape of the colloidal NSs. In this way, the synthesis method developed here could offer a general preparation strategy to colloidal 2D nanosheets of metal sulfides and other II-VI materials.
9:00 AM - NN3.03
All-Dry Process to Fabricate Graphene-Based 2D Heterostructure
Dung Tien Hoang 1 Chul Soo Kim 1 Woo Seok Choi 1 Yongho Seo 1 Philip Kim 2
1Sejong University Seoul Republic of Korea2Columbia University New York USAShow Abstract
Graphene is a promising material which has a very high mobility but strongly influenced by underlying substrate. Therefore a suitable substrate must be used for graphene-based devices. Recently hexagonal boron nitride (hBN) has been adopted as a substrate for graphene-based devices. Graphene on hBN which has the same crystal structure as graphene and atomically smooth surface, shows very high mobility (µCasymp;60,000 cm2 V-1s-1) . Polymer supporting thin films for transferring graphene can leave residue on it. The polymer residue can acts as a scattering site or doping element on graphene layer. In fabricating graphene-based devices, it is very important to avoid residue remaining on graphene so that the quality of the graphene layer is not deteriorated. [2-4] In this study we developed dry transferring techniques to transfer graphene to hBN substrate and to make sandwich structures. Also, we used shadow mask lithography technique to prevent the residue problem caused by e-beam lithography. The graphene layer can be encapsulated between two hBN layers and then is kept intact during fabricating devices. Electrical properties of the sandwich devices also are investigated.
9:00 AM - NN3.05
Thermal and Phonon Properties of Exfoliated TaSe2 Thin Films
Chenglong Jiang 1 Zhong Yan 1 Timothy R. Pope 2 Chu F. Tsang 2 John L. Stickney 2 Pradyumna Goli 3 Jacqueline Renteria 1 Tina T. Salguero 2 Alexander A. Balandin 1 3
1University of California - Riverside Riverside USA2University of Georgia Athens USA3University of California - Riverside Riverside USAShow Abstract
Van der Waals materials that can be mechanically cleaved from bulk crystals with layered atomic structure have recently attracted significant attention. An interesting subgroup of van der Waals materials is the layered transition metal dichalcogenides MX2, where M=Mo, W, Nb, Ta or Ti and X=S, Se or Te. It was recently demonstrated that the transition temperature to the charge density wave phase can be increased in some of these materials by thinning the exfoliated films . In this presentation, we report results of our investigation of the thermal properties of thin films of tantalum diselenide (2H-TaSe2) obtained via the “graphene-like” mechanical exfoliation of crystals grown by chemical vapor transport. The ratio of the intensities of the Raman peak from the Si substrate and the E2g peak of TaSe2 was used as a convenient metric for quantifying film thickness. The temperature coefficients for two main Raman peaks, A1g and E2g, were -0.013 and -0.0097 cm-1/°C, respectively. The Raman optothermal measurements indicated that the room temperature thermal conductivity in these films decreases from its bulk value of ~16 W/mK to ~9 W/mK in 45-nm thick films. The measurement of electrical resistivity of the field-effect devices with TaSe2 channels indicates that heat conduction is dominated by acoustic phonons in these van der Waals films. The scaling of thermal conductivity with the film thickness suggests that the phonon scattering from the film boundaries is substantial despite the sharp interfaces of the mechanically cleaved samples. These results are important for understanding lattice dynamics of TaSe2 films and for device applications of metal dichalcogenide thin films.
The work at UCR was supported in part by the NSF - NRI Nanoelectronics 2020 and Beyond Project.
 P. Goli, J. Khan, D. Wickramaratne, R.K. Lake and A.A. Balandin, "Charge density waves in exfoliated films of van der Waals materials: Evolution of Raman spectrum in TiSe2, Nano Letters, 12, 5941 (2012).
9:00 AM - NN3.06
Facile Decoration of Au Nanoparticles on Reduced Graphene Oxide Surfaces via a One-Step Chemical Functionalization Approach
Haiqing Yao 1 Lin Jin 2 Hung-Jue Sue 1 2
1Texas Aamp;M University College Station USA2Texas Aamp;M University College Station USAShow Abstract
A facile way to fabricate functionalized and reduced graphene oxide (RGO) was reported by introducing the dual-function reagent N1-(3-trimethoxysilylpropyl) diethylenetriamine (TSPD), in which (i) the amine functional groups of the organosilane can effectively restore the p bonds of the GO; and (ii) the silanol parts react with the hydroxyl groups of GO and subsequently cross-link to form a monolayer on the graphene surface. The exposed amine groups of the functionalized GO could effectively template the Au nanoparticles (AuNPs) assembly on the graphene surface with high density and good dispersity. TEM images indicate that AuNPs retained their original size and were well-anchored on the graphene surfaces with an average inter-particle distance of 15 nm. The resultant graphene sheets decorated by AuNPs has the capability of sensing single molecules on its surface, e.g., Rhodamine 6G (Rh6G), through the surface-enhanced Raman spectroscopy (SERS).
9:00 AM - NN3.07
Transmission Electron Microscopy Studies of PVD and CVD of Mono-, Bi- and Few-Layer MoS2 Films
Jianjun Hu 1 Chris Muratore 2 Shanee Pacley 3 Andrey Voevodin 3
1University of Dayton Research Institute, University of Dayton Dayton USA2Department of Chemical and Materials Engineering, University of Dayton Dayton USA3Materials and Manufacturing Directorate, Air Force Research Laboratory, 2941 Hobson Way Wright-Patterson AFB, Dayton USAShow Abstract
MoS2 is a unique two-dimensional (2D) electronic material with impressive potential in applications such as ultrathin transistors with high on/off ratio and electron mobility and high sensitive molecular sensing with high selectivity. The recent work in MoS2 2D material evaluation has relied primarily upon materials exfoliated from bulk crystal MoS2. However, such processes are accompanied by surface contamination and are not scalable to industrial device manufacturing. Progress has been made in vapor phase processing over larger areas. Recently MoS2 single/multiple layers have been synthesized from the reaction of sulfur vapor with molybdenum oxide thin film pre-coated on substrates at high temperatures (>700 °C). An alternative vapor phase technique for growth of 2D materials is magnetically enhanced physical vapor deposition (PVD) directly from MoS2 solid compound targets. The process promotes growth conditions with enhanced surface atom mobility for continuous growth of 2D materials with minimal defect formation, even at low temperatures (<300 °C). Highly parallel basal plane orientation was obtained for single- and multiple-layer films under conditions minimizing nucleation density and maximizing surface diffusion of deposited atoms before burial by incident material or desorption . In this presentation, details on transmission electron microscopy (TEM) studies, including high-resolution TEM, HAADF-STEM and X-ray energy dispersive spectroscopy, will be provided to allow comparison of ultrathin MoS2 films processed using both techniques. Cross-sectional TEM specimens were coated with a protective metal layer, and prepared by lift-out using a focused-ion beam (FIB) microscope. The films were grown by PVD and CVD methods, respectively, and the thickness was controlled from a single layer MoS2 to a few nanometers. Although the both techniques produced high quality MoS2 films in the preferred parallel orientation, differences in microstructure were measured and correlated to differences in electron mobility, photoluminescence intensity and peak position, and other critical properties for applications in 2D electronic devices. The results outline steps along the path toward clear requirements for the next generation of 2D materials including uniform growth over large areas at low temperature.
 S.D. Pacley, et al., MRS Spring Meeting & Exhibit, San Francisco, CA, April 21-25, 2014.
 C. Muratore, A.A. Voevodin, Thin Solid Films 517 (2009) 5605.
9:00 AM - NN3.09
Formation of Metastable MoO3 Nanosheets and Their Evaluation of Electrochemical Response Using Scanning Electrochemical Microscopy (SECM)
Vipin Kumar 1 Afriyanti Sumboja 1 Liu Liang 2 Daniel Mandler 2 Pooi See Lee 1
1Nnanyang Technology University Singapore, 639798 Singapore Singapore2Institute of Chemistry The Hebrew University of Jerusalem Jerusalem , 91904 , Israel Jerusalem IsraelShow Abstract
Two dimensional materials are sparking the realm of nanoscience and nanotechnology. Inorganic oxides nanosheets have attracted much attention owing to their semiconducting nature, high thermal as well as chemical stability along with exceptionally rich structural diversity and electronic properties. In this report we are presenting a facile and low cost method to fabricate nanosheets consisting double layers of Molybdenum trioxide (MoO3 - II), from hydrothermally grown hexagonal MoO3 (h - MoO3) nanorods. Nanosheets of MoO3 can be obtained using mechanical exfoliation technique from the annealed h - MoO3 nanorods. Thickness of the as exfoliated nanosheets was ranging from 2.4 nm to 11.3 nm which is equivalent to 3 to 16 double layers of MoO3 - II respectively taking into account their respective Vander Waals gaps (thickness of a MoO3 - II double layer ~ 0.709 nm) as quantified using AFM. Raman Spectroscopy analyses of the as prepared nanosheets detailed the effect of number of double layers on the Raman signals. The appearance of fringes in the HRTEM evaluation signifies the crystalline nature of the as prepared nanosheets and expose (2, -2, 0) plane of MoO3. The electrochemical response of MoO3 nanosheets were examined by the means of Scanning Electrochemical Microscopy (SECM) in the feedback mode in the environment of cationic [Ru (NH3) 6] +3 as well as anionic [Fe (CN) 6] -3 redox mediator. The as prepared nanosheets showed positive and negative feedback response in the surroundings of [Ru (NH3) 6] +3 and [Fe (CN) 6] -3 respectively. The positive and negative feedback behavior in the mild reducing and oxidizing agent is attributable to the electro active nature of MoO3 nanosheets.
9:00 AM - NN3.10
Influence of Copper Surface Morphology on Hexagonal-Boron Nitride Grown by Chemical Vapor Deposition
Roland Yingjie Tay 1 3 Mark H. Griep 4 Siu Hon Tsang 3 Ram Sevak Singh 1 Shashi P. Karna 4 Govind Mallick 3 4 Edwin Hang Tong Teo 1 2
1Nanyang Technological University Singapore Singapore2Nanyang Technological University Singapore Singapore3Temasek Laboratories@NTU Singapore Singapore4U.S. Army Research Laboratory Aberdeen USAShow Abstract
Surface morphology of Cu substrates had been known to have significant impacts towards the nucleation density, domain sizes, thickness, uniformity and quality of the CVD-grown graphene film. h-BN, analogous to graphene, is also expected to have the same correlation towards substrate roughness. In this work, we investigate the influence of Cu surface roughness, through the use of electro-polishing to achieve an extremely smooth Cu surface. With the polished Cu as substrates, h-BN ranging from sporadic flakes to a continuous film is grown using CVD. Here, we will study and compare the effects on its nucleation patterns, grain sizes and the quality of the film.
9:00 AM - NN3.11
First-Principles Calculation of Thermal Transport in the Metal/MoS2 System
Rui Mao 1 ByoungDon Kong 1 Ki Wook Kim 1
1North Carolina State University Raleigh USAShow Abstract
Transition-metal dichalcogenides (TMDs), one of the beyond-graphene two-dimensional (2D) semiconductor materials, has emerged as promising candidates for the next generation electronics due to their distinctive electrical, optical, and thermal properties. In particular, due to the sizable bandgap, molybdenum disulfide (MoS2) has gained an increasing attention for post-silicon transistor applications, where metal contacts are inevitable and ubiquitous. While the electronic properties of metal-MoS2 contacts have been the subject of intensive study recently [1,2], fewer investigations are done regarding the vibrational properties and thermal transport of metal-MoS2 systems.
In this work, we present a first-principles investigation on the thermal transport properties of metal-MoS2 contacts. Au, Pd, Ru, and Sc, are selected as the electronic contact materials due to small lattice mismatches or low Schottky barriers. Using the Density Functional Perturbation Theory (DFPT) for the interatomic force constant (IFCs) calculation and Landauer formalism for the phonon transport, we have calculated phonon dispersions and transmission coefficients of the metal-MoS2 systems. Based on the transmission coefficients, interfacial thermal resistances are extracted to characterize the phonon transport across the interfaces. The study reveals two major interfacial categories, namely chemisorption (Ru and Sc) and physisorption (Au), whereas Pd-MoS2 shows an intermediate nature. At room temperature, the interfacial thermal resistances of the studied systems range from 2×10-8 Km2/W to 5.7 ×10-8 Km2/W. Compared with the physisorption, the chemisorption gives a smaller interfacial thermal resistance, which is beneficial in regards to heat transport. This can be explained by the mismatch in phonon spectrums and the difference of interface environment. Further interatomic force constant analysis shows that MoS2-metal contacts (2×10-8 Km2/W-5.7 ×10-8 Km2/W) have much larger interfacial resistances than those of graphene-metal contacts (3.9×10-9 Km2/W-1.7 ×10-8 Km2/W), which can be attributed to weaker bondings between MoS2 and metal species and the diatomic nature of MoS2. Our findings clearly illustrate the significance of the interfacial atomic details in determining the thermal boundary resistance and pave the way to the optimum thermal management and device optimization of future 2D material-based electronics.
 W. Chen et al., Nano Lett., 2013, 13 (2), pp 509-514.
 J. Kang et al., Electron Devices Meeting (IEDM), 2012 IEEE International , vol. 17.4.1, no. 17.4.4, pp. 10-13.
9:00 AM - NN3.13
A DFT Study of B, N and BN Doped Graphene
Pooja Rani 1 V. K. Jindal 1
1Panjab University Chandigarh IndiaShow Abstract
We have made a density functional study of the structural and electronic properties of B or N (individual) doped and BN co-doped graphene. The effect of doping has been studied by incorporating the doping concentration amount varying from 2% (one atom of the dopant in 50 host atoms) to 12 % atomic concentration in case of individual doping and from 4% (2 atoms of the dopant in 50 host atoms) to 24 % in case of co-doping, at the same time, altering different doping sites for the same concentration of substitutional doping. We made use of VASP (Vienna Ab-Initio Simulation Package) software based on density functional theory to perform all calculations. While the resulting geometries do not show much of distortion on doping, the electronic properties show a transition from semimetal to semiconductor with increasing number of dopants. The study shows that the BN doping introduces the band gap at the Fermi level unlike individual B and N doping which causes the shifting of Fermi level above or below the Dirac point. It is observed that not only concentration but position of B and N atoms in the hetero-structure also affect the value of band gap introduced.
9:00 AM - NN3.14
High Quality Transfer of Large-Area Monolayer and Few-Layer MoS2 Films
Alper Gurarslan 1 2 Yifei Yu 1 Yiling Yu 3 Linyou Cao 1 3
1North Carolina State University Raleigh USA2North Carolina State University Raleigh USA3North Carolina State University Raleigh USAShow Abstract
Molybdenum sulfide (MoS2) has layered structure where each layer is composed of a plane hexagonal array of molybdenum atoms between two sheets of sulfur atoms. Each Mo atom is covalently bound to six S atoms arranged in a triangular prismatic configuration. Each sulfur, in turn, is bound covalently to three Mo atoms. MoS2 monolayers, with a direct bandgap of 1.8 eV, offer an unprecedented prospect of miniaturizing semiconductor science and technology down to a truly atomic scale. Recent studies have indeed demonstrated the promise of 2D MoS2 in fields including field effect transistors, low power switches, optoelectronics, and spintronics. However, device development with 2D MoS2 has been delayed by the lack of capabilities to transfer large-area, uniform, and high-quality MoS2 monolayers. Here we present a transfer method for obtaining high quality and large area monolayer and few-layer MoS2 films on arbitrary substrates.
9:00 AM - NN3.15
Cation Intercalation and High Volumetric Capacitance of Two-Dimensional Titanium Carbide
Maria R. Lukatskaya 1 Chang E. Ren 1 Olha Mashtalir 1 Yohan Dallamp;#8217;Agnese 1 2 Michael Naguib 1 Patrice Simon 2 Michel W. Barsoum 1 Yury Gogotsi 1
1Drexel University Philadelphia USA2Universitamp;#233; Paul Sabatier Toulouse FranceShow Abstract
We recently produced a new 2-D material, viz. Ti3C2, by selectively etching aluminium from a MAX phase Ti3AlC2 and labelled it MXene. MXenes represents a large family of transition metal carbides and carbonitrides, not just a single phase. MXenes allow a variety of chemical compositions and are establishing themselves as a new class of two-dimensional materials. MXenes possess good in-plane conductivity, which in combination with their rich surface chemistry makes them attractive for electrical energy storage applications. However their use in electrochemical capacitors was only recently demonstrated (Lukatskaya et.al, Science, 2013).
Here, we report on the intercalation of Li+, Na+, Mg2+, K+, NH4+, and Al3+ ions between the 2D Ti3C2Tx layers. In most cases, the cations intercalated spontaneously. The intercalation of some ions, notably Al3+, can be promoted electrochemically. We also report on intercalation-induced high capacitance of >300 Farads per cubic centimeter (much higher than that of porous carbons) of flexible Ti3C2Tx paper electrodes in aqueous electrolytes.
Electrodes were fabricated by vacuum filtration of MXene sheets or by conventional film rolling with binder and conductive additives. In some cases MXene was delaminated prior to electrode fabrication in order to achieve better separation of the 2D sheets. Several different electrochemical techniques were employed to understand the mechanism of charge storage. Electrochemical impedance spectroscopy confirmed the low resistivity of the tested materials showing characteristics for capacitors close to 90° angle on the Nyquist plot. Cyclic voltammetry measurements showed a high rate handling ability along with impressive volumetric capacitance values for the electrodes from the delaminated MXene. Galvanostatic cycling showed no degradation of the capacitive properties after more than 10,000 cycles.
9:00 AM - NN3.16
Molybdenum Trioxide Nano-Sheets Produced by Liquid Phase Exfoliation
Damien Hanlon 1 2 Claudia Backes 1 2 Thomas Higgins 1 2 Niall McEvoy 2 3 Georg Duesberg 2 3 Beatriz Mendoza Sanchez 2 3 Henrik Pettersson 2 3 Valeria Nicolosi 2 3 Jonathan N Coleman 1 2
1Trinity College Dublin Dublin Ireland2Trinity College Dubin Dublin Ireland3Trinity College Dubin Dublin IrelandShow Abstract
Layered materials represent a unique field of material science due to their large surface area and wealth of electronic properties which are important for applications such as supercapacitors and sensing. However, many applications are hampered by the intrinsic insolubility of these materials. In order to utilize their full potential, a robust and simple solution phase processing route is sought for. We have deminstrated that similar to graphene a variety of inorganic layered materials can be readily exfoliated in solution using suitable solvents or surfactants. Specifically transition metal dichalcogenides (TMD&’s) and transition metal oxides (TMO&’s) have shown to be promising in applications such as field-effect transistors, and in hydrogen evolution catalysis as well as many more. The expansion of our solution phase processing route to other layered materials will thus be of great impact for a broad variety of applications and provides unique access to their characterization in solution.
Herein we demonstrate the liquid phase exfoliation of molybdenum trioxide to yield few layer nanosheets. The liquid phase exfoliation procedure based on sonication, consists of critical variables which affect the quality and stability of the final dispersion. These parameters were identified and optimised to yield a stable dispersion of MoO3 nano-sheets in a variety of solvents. These widely defect-free nano-sheets were investigated by absorbance and emission spectroscopy, transmission electron microscopy, Raman spectroscopy and atomic force microscopy to fully characterise the material obtained through our process.
Films of MoO3 nano-sheets were tested for potential use as supercapacitor electrodes. MoO3 only electrodes exhibited a low capacitance due to poor electrical conductivity, thus composites were prepared with SWCNT to utilize of MoO3's electrochemical behaviour. Percolation behaviour was observed with the treshold ~5 wt% SWCNT. Capacitances of ~500 F/g were achieved with addition of >5% SWCNT.
 Jonathan N. Coleman, Mustafa Lotya, Arlene O&’Neill, Shane D. Bergin, Paul J. King, Umar Khan, Karen Young, Alexandre Gaucher, Sukanta De, Ronan J. Smith, Igor V. Shvets, Sunil K. Arora, George Stanton, Hye-Young Kim, Kangho Lee, Gyu Tae Kim, Georg S. Duesberg, Toby Hallam, John J. Boland, Jing Jing Wang, John F. Donegan, Jaime C. Grunlan, Gregory Moriarty, Aleksey Shmeliov, Rebecca J. Nicholls, James M. Perkins, Eleanor M. Grieveson, Koenraad Theuwissen, David W. McComb, Peter D. Nellist, Valeria Nicolosi, Science 4, 568-571,(2011)
 B. Radisavljevic, A. Radenovic, J. Brivio1, V. Giacometti and A. Kis, Nature Nanotechnology 6,147-150, (2011)
 T. F. Jaramillo, K. P. Joslash;rgensen, J. Bonde, J. H. Nielsen, S. Horch, I. Chorkendorff, Science 2007, 317, 100-102.
9:00 AM - NN3.17
Graphene/Polyaniline/Poly(4-styrenesulfonate) Hybrid Film with Uniform Electrical Conductivity and High Structural Flexibility for Flexible Dipole Tag-Antenna Application
James Sangmin Lee 1 Keun-Young Shin 1 Jyongsik Jang 1
1Seoul National University Seoul Republic of KoreaShow Abstract
Graphene has emerged as a plastic electronic material due to its excellent mechanical strength, flexibility, and outstanding electron mobility. In order to make a graphene film for flexible electronics application, the need for high quality mass production of graphene, the homogeneous deposition and high adhesion to polymeric substrates poses a great challenge. For these reasons, graphene generated by chemical exfoliation has attracted a great deal of interest as a carbon-based material for flexible electronics application owing to the possibility of high-volume production and versatility in terms of being well-suited to chemical functionalization. Furthermore, there are a number of transfer and patterning methods, such as lithography and template-assisted synthesis and chemical vapor deposition, etc. However, these technologies involve complicated, multistep procedures that result in high cost, material waste, and difficulty in forming large-area patterns. Therefore, screen printing has been shown as an effective method for defined deposition of conducting materials. Recently, a few research groups have fabricated screen-printed graphene electrode by introducing poly(3,4-thylenedioxythiophene)/poly(styrene-sulfonic acid) (PEDOT/PSS) into graphene. However, it also had the disadvantages of high cost and limited PSS availability for printing process. As an alternative conducting polymer nanofiller, polyaniline (PANI)/PSS has received considerable attention because of its easy synthesis, low monomer cost, good thermal stability, and adequate electrical conductivity. In addition, large numbers of functional amino groups in the PANI chain and its inherent nano-sized microstructure can give PANI the possibility to form stable colloid solution with carbon-based materials.
In this presentation, we develop a simple approach to fabricate multilayer graphene/PANI/PSS (G/PANI/PSS)-based conducting paste for screen printing ink. To the best of our knowledge, this is the first demonstration of a graphene-based hybrid with uniform surface resistance using conducting polymer nanofiller, which is derived from the introducing a PANI/PSS nanofiller into the multilayer graphene matrix via mechanical blending. Importantly, as a compatibilizer and binder, PSS increased dispersibility, interfacial interactions, and mechanical interlocking between a multilayer graphene matrix and PANI nanofiller, leading to uniform surface resistance of G/PANI/PSS hybrid film. Furthermore, high concentration of PSS binder via ex situ polymerization could make it possible to improve adhesion of the hybrid film to flexible substrate. Due to its good electrical conductivity and structural flexibility, micro-patterned G/PANI/PSS hybrid film using screen printing was applied to the practical dipole tag-antenna detecting nearby objects.
9:00 AM - NN3.18
Two-Dimensional alpha;-MoO3 Nanoflakes: Application for H2 Gas Sensing
Manal Mohammad Alsaif 1 Sivacarendran Balendhran 1 Matthew Field 2 Wojtek Wlodarski 1 Kay Latham 2 Jian Zhen Ou 1 Kourosh Kalantar-zadeh 1
1RMIT University Melbourne Australia2RMIT University Melbourne AustraliaShow Abstract
Two-dimensional (2D) α-molybdenum trioxide (α-MoO3) has recently brought great attention due to its unique planar structure. This material offer planet of enhancements in contrast to its bulk counterpart including the high mobility of free electrons and large surface to volume ratio. In this work, a liquid-based organic solvent-assisted grinding and sonication method is adopted for the formation of 2D α-MoO3 nanoflakes suspensions. The flakes thicknesses are in the order of a few nanometers. The drop-casted thin films composed of these 2D nanoflakes show large responses to a wide concentration range of H2 gas (from 600 to 10000 ppm) with response and recovery time in the order of seconds. These results suggest that MoO3 thin films made of exfoliated 2D nanoflakes, are excellent materials for establishing H2 gas sensors.
9:00 AM - NN3.21
Dielectric Function and Luminescence of Large Area MoS2 Ultra-Thin Films Grown Using Chemical Vapor Deposition
Jun Woo Park 1 Hyeon Seop So 1 Sung Kim 1 Suk-Ho Choi 1 Jinhwan Lee 2 Changgu Lee 2 Hosun Lee 1
1Kyung Hee University Yong-in Republic of Korea2Sungkyunkwan University Suwon Republic of KoreaShow Abstract
Two dimensional materials have attracted great attention to substitute silicon semiconductors. Bulk MoS2 is an indirect band gap semiconductor with a band gap of 1.3 eV and is built up of covalently bonded S-Mo-S layers that are bound by van der Waals forces. Large area MoS2 thin films are essential for electronic devices. Dielectric functions of MoS2 thin films have not been measured because of lack of available large area MoS2 thin films.
We investigated the dielectric functions (ε(E) =ε1(E) + iε2 (E)) of MoS2 films using spectroscopic ellipsometry, assisted with Raman and photoluminescence spectroscopy. Large area MoS2 N-layer films with N=2, 4, 12 were grown on SiO2/Si substrate using plasma-enhanced chemical vapor deposition (PECVD). Large linewidths in Raman peaks suggested that the PECVD-grown MoS2 thin films had defects larger than exfoliated thin films. The N=4 film had the largest amplitude of the dielectric functions whereas all the structures in dielectric function spectra were largely independent of layer number N. From the absorption coefficients, we determined the indirect band gap energies by Tauc plot method. We found that the indirect band gap energy decreased from 1.60 eV to 1.47 eV with increasing film thickness. Applying standard critical point model analysis to the second derivative of dielectric function, we determined several critical point energies. The d2ε(E)/dE2 spectra showed doublet structure (C and D) for 3 eV peak as well as 2 eV peak (A and B). The splitting of C and D peaks near 3 eV was attributed to valence band splitting. Dielectric function of bulk MoS2 showed doublet structure for both low energy excitons (A and B) and 3 eV excitons. In MoS2 ultra-thin films, the doublet structure of N=4 and 12 was clearly observed only in d2ε(E)/dE2 spectra.
 A. Molina-Sánchez et al., Phys. Rev B 88, 045412 (2013).
9:00 AM - NN3.22
Crystallographically Guided Metal-Nanoparticle Incorporation on MoS2 via Chemical and Microwave Routes: Electrical, Thermal, and Structural Properties
Vikas Berry 1 Phong Nguyen 1 Sreeprasad Theruvakkattil Sreenivasan 1 Namhoon Kim 1
1Kansas State University Manhattan USAShow Abstract
Ultrathin two dimensional metal dichalogenide (MoS2, WS2, so forth) is an emerging class of new materials, which exhibits confinement of carriers, evolution of band structure, high on/off rectification, and high thermal absorption. However their incorporation into other systems requires controlled functionalization and/or interaction with other nanoscale entities. Here we enhance the stable sulfur/nobel metal functionalization via both diffusion limited aggregation and instantaneous reaction arresting (using microwaves). The gold nanoparticles are incorporated selectively on MoS2 crystallographic directions (with 60o displacement). The Raman, electrical and thermal studies indicate a remarkably capacitive interaction between gold and thin MoS2 sheet (CAu-MoS2 = 2.17 mF/cm2), a low Schottky barrier (14.52 meV), a vastly reduced carrier-transport thermal-barrier (253 to 44.18 meV after gold functionalization), and increased thermal conductivity (from 15 W/mK to 23 W/mK post gold deposition). This process provides a route to affiliate MoS2 with potential electronic application, such as electrode-attachment to hetero-structures of graphene and MoS2, where the gold nano structure could be grown to act as an electron-tunneling gate-electrode connected to MoS2.
9:00 AM - NN3.23
Influence of Mo and MoO3 on Chemical Vapor Deposition Growth of 2D MoS2
Shanee Pacley 1 Jianjun Hu 2 Michael Jespersen 2 Albert Hilton 2 Neil Murphy 2 Sha'Nique Mackey 2 Tasha Adams 2 Emory Beck-Millerton 2 William C Mitchel 2 Andrey A Voevodin 2
1Air Force Research Laboratory Wright Patterson USA2University of Dayton Research Institute Dayton USAShow Abstract
Monolayer molybdenum disulfide (MoS2), a 2D semiconducting dichalcogenide material with a bandgap of 1.8eV, has demonstrated promise for future use in field effect transistors (FETs) and optoelectronics. Research has shown different growth processes for MoS2, using various molybdenum sources, where chemical vapor deposition (CVD) with sulfur vapor reaction with molybdenum trio-oxide has shown most promising results . We have investigated the use of both molybdenum and MoO3 layers for CVD growth of MoS2. Our transmission electron microscopy results show that magnetron sputtering of MoO3 onto sapphire substrates enabled thin continuous layers of MoS2, while sputtered Mo produce very thick layers of island MoS2 growth. The X-ray photoelectron spectroscopy results show the MoS2 produced from MoO3 to be stoichiometric. Raman spectroscopy and optical results demonstrate crystallinity in the film. Finally, electrical measurements were made showing the MoS2 grown using MoO3 was conductive. Correlations between Mo and MoO3 layers, and resulting 2D MoS2 film chemistry and structure are discussed.
 S. Najmaei et al. Nature Materials (2013)
9:00 AM - NN3.24
Band Gap Modulation in Ultrathin Two-Dimensional Molybdenum Disulfide by Oxygen Plasma
Wai Leong Chow 1 Hong Li 2 Linfeng Sun 3 Beng Kang Tay 1 Zexiang Shen 3
1Nanyang Technological University Singapore Singapore2Stanford University Stanford USA3Nanyang Technological University Singapore SingaporeShow Abstract
As a two-dimensional (2D) semiconductor, ultrathin molybdenum disulfide (MoS2) has sparked great interests owning to its substantial band gap (~1.2 -1.8 eV), considerably high room temperature electron mobility (~200 cm2/V.s) and switching capability (current ON/OFF ratio~108). Band gap modulation of MoS2 is of great importance because the ability to manipulate its optical and electronic properties could lead to additional new functionalities in various fields. Applying strain or external vertical electric field to MoS2 are two common strategies used to modulate its band gap which are well-established theoretically with few experimental demonstration up to date. In this contribution, we report observation of band gap modulation in ultrathin MoS2, ranging from a monolayer up to trilayer by a novel method using oxygen plasma. The effect of oxygen-treated ultrathin MoS2 is analyzed by photoluminescence (PL), micro-Raman and X-ray photoemission spectroscopy (XPS) characterization. We observed a layer dependent reduction in the band gap as a function of plasma treatment duration without suffering from severe surface oxidation while retaining its crystalline structure. Our results here served as a pioneering work towards the possibility of band gap engineering by a simple yet effective method using oxygen plasma, providing alternative to conventional approaches.
9:00 AM - NN3.26
Orientation-Dependent Strain Engineering of Band Gap in CVD Grown Monolayer WS2
Wen Fan 1 2 Sefaattin Tongay 1 Kai Liu 1 3 Joonki Suh 1 Jie Ji 2 Junqiao Wu 1 3
1UC Berkeley Berkeley USA2University of Science and Technology of China Hefei China3Lawrence Berkeley National Laboratory Berkeley USAShow Abstract
We report on uniaxial strain-induced band gap modulation in large-area CVD-grown monolayer tungsten disulfide (WS2) and its dependence of the crystal orientation of strain. A reversible change in the optical band gap was observed in triangular flakes of monolayer WS2 when a tensile strain ranging from 0 to 4% was applied along the direction of one of the triangle edges. This optical band gap modulation, accompanied with variation of integrated photoluminescence (PL) intensity of the WS2 flakes, was caused by strain-induced deformation of crystal structure as supported by theoretic calculation. In contrast, when the tensile strain was applied perpendicular to the triangle edge direction, the band gap variation was not observed. We attribute the orientation dependence of band gap strain modulation to the different deformation potentials of the crystal structure when the tensile strain is applied along different crystal directions. In addition, the full-width-at-half-maximum (FWHM) of PL peak of the WS2 flakes increases with strain. We also observed a red-shift of Raman peaks of the WS2 flakes with tensile strain.
These results may provide new approaches to tunable optical device using monolayer WS2 and other 2D semiconducting transition metal dichalcogenides (TMDs).
9:00 AM - NN3.27
Raman and AFM Studies on Chemically Synthesized MoS2
Shin Mou 1 Don Abeysinghe 1 Joshua Myers 1 2 Yan Zhuang 2 Lain-Jong Li 3
1Air Force Research Laboratory Wright-Patterson AFB USA2Wright State University Dayton USA3Academia Sinica Taipei TaiwanShow Abstract
Molybdenum disulfide (MoS2), a layered transition metal dichalcogenide, has been recently reported that in the presence of a high-k environment, the mobility of monolayer MoS2 can reach 200 cm2/(V s). This coupled with the presence of a significant band gap (~1.8 eV) renders layered MoS2 as an attractive candidate for electronics applications, unlike graphene where the absence of band gap inhibits its use despite large reported mobilities (200000 cm2/(V s)). Although the chemical synthesis of layered MoS2 for electronics applications is still in its infancy, promising results have been demonstrated with mobilities up to 4.7 cm2/(V s), which is comparable to that of the mechanically exfoliated MoS2 (not in a high-k environment). In this work, we study the Raman spectroscopy and AFM morphology on the chemically synthesized MoS2 based on the synthesis method introduced by Li et al. (L.-J. Li et al., Nano Lett. 12, 1538, 2012). According to the Raman spatial mapping, the sapphire substrates are covered by MoS2 continuously throughout the wafers. Judging from the peak wavenumber differences from the two characteristic Raman peaks (the E12g mode ~ 382 cm-1 and the A1g mode ~ 405 cm-1), most of the areas are uniformly covered by 2 to 3 layers of MoS2. On the other hand, the intensities of the Raman modes vary across the wafers, which may result from the variation of the MoS2 crystallinity. From AFM topologies of transferred MoS2 (onto SiO2/Si substrates), we found that there are areas continuously covered by MoS2 and there are porous areas with less than 100% MoS2 coverage. Combined with the Raman study, this implies spatial variation to the MoS2 crystal quality. More detailed micro-characterization (e.g., electron microscopy) needs to be taken in order to verify the conclusion here.
9:00 AM - NN3.29
Piezoelectric 2D Materials for Bistable NEMS Energy Harvesters
Miquel Lopez-Suarez 1 Gabriel Abadal 1 Miguel Pruneda 2 Riccardo Rurali 3
1Universitat Autamp;#242;noma de Barcelona Bellaterra Spain2Centre d'Investigaciamp;#243; en Nanociamp;#232;ncia i Nanotecnologia (CSIC-ICN) Bellaterra Spain3Institut de Ciamp;#232;ncia de Materials de Barcelona (ICMAB-CSIC) Bellaterra SpainShow Abstract
In this contribution, we present the results of the dynamics of one atom thick h-BN suspended nanoribbons obtained by first performing ab-initio calculations of the deformation potential energy and then solving numerically a Langevine type equation to explore their use as energy harvesting devices. Similarly to our previous proposal for a graphene-based harvester , an applied compressive strain is used to drive the clamped-clamped nanoribbon structure into a bistable regime, where quasi-harmonic vibrations are combined with low frequency swings between the minima of a double-well potential. Due to its intrinsic piezoelectric response, the mechanical harvester naturally provides an electrical power that is readily available or can be stored by simply contacting the monolayer at its ends. Engineering the induced non-linearity, the proposed device is predicted to harvest an electrical rms power of more than 150 fW when it is excited by a noisy external force characterized by a white Gaussian frequency distribution with an intensity in the order of Frms=5pN.
 Loacute;pez-Suárez, M., R. Rurali, et al. (2011). "Nanostructured graphene for energy harvesting." Physical Review B 84(16): 161401.
9:00 AM - NN3.30
Growth of Hexagonal Boron Nitride Atomic Layers by Molecular Beam Epitaxy
Zhongguang Xu 1 Renjing Zheng 1 Zheng Zuo 1 Jianlin Liu 1
1UC riverside riverside USAShow Abstract
Hexagonal boron nitride (h-BN), as a promising dielectric with similar graphite structure, currently attracts considerable attention, both for the fascinating properties of the individual monolayer and its successful incorporation as a complementary two-dimensional (2D) dielectric substrate in graphene based electronics. H-BN is a wide band gap III-V compound with remarkable physical properties and chemical stability such as inertness, high temperature stability, low dielectric constant, large thermal conductivity, and high mechanical strength. Similar with graphite, each layer of h-BN is held together by weak van der Waals forces. Therefore, monolayer h-BN films could be peeled off from bulk BN crystal by micromechanical cleavage. Chemical vapor deposition (CVD) has already been used to demonstrate the growth of single layer h-BN by many groups. However, methods for preparing good quality h-BN layers over large areas would bring new opportunities to investigate the properties and potential applications based on h-BN. Here we report an approach to synthesize uniform and continuous h-BN films on metal substrate by using gas source molecular beam epitaxy (MBE). MBE can provide atomic layer epitaxy accuracy and controllability due to high vacuum, low temperature, excellent shutter control, etc. Elemental boron oxide evaporated by effusion cell and active nitrogen generated by electron cyclotron resonance (ECR) plasma source were used as the group-III and-V sources, respectively. H-BN epitaxial growth on cobalt substrates was demonstrated at a growth temperature of 900oC. Raman and X-ray photoelectron spectroscopy (XPS) characterizations confirm the growth of h-BN, and atomic force microscopy (AFM) images show the resulting h-BN film has atomically smooth surface and few layers. The results of the growth and characterization of h-BN films on other substrates such as nickel and copper will also be presented.
9:00 AM - NN3.31
Photoconductivity of Solution-Processed MoS2 Films
Graeme Cunningham 1 2 Jonathan N Coleman 1 2 Umar Khan 1 2 Claudia Backes 1 2 John F Donegan 1 2 David McCloskey 1 2 Damien Hanlon 1 2
1Trinity College Dublin Dublin Ireland2CRANN amp; AMBER Dublin IrelandShow Abstract
Solution-exfoliated MoS2 nano-platelets were formed into thin films by deposition onto a water
surface followed by transfer to indium tin oxide coated glass. After drying, a gold electrode was
evaporated on top to give a sandwich structure with quasi-Ohmic contacts. Illumination of this
device with broadband light of 1 kW/m2 intensity gave a fourfold increase in conductivity. The
photocurrent increased sub-linearly with intensity and exponentially with time indicating the
presence of traps. The photo-responsivity at low intensity was 10^-4 A/W at 15 V. This work
demonstrates the potential for liquid-exfoliated, inorganic nanosheets to be fabricated into
low-cost optoelectronic devices.
9:00 AM - NN3.32
2-Dimentional Boron Nitride Nanosheets; Synthesis and Device Application
Muhammad Sajjad 1 Gerardo Morell 1 Peter Feng 1
1University of Puerto Rico San Juan USAShow Abstract
In this presentation, we will report on the synthesis of few atomic-layers BNNSs and fabrication of a Schottky diode. The synthesis process is carried out by irradiating hexagonal boron nitride target using CO2 laser pulses. High resolution transmission electron microscopy (HRTEM) showed the sheets are mostly defect-free and characteristic honeycomb structure of six-membered B3-N3 hexagon can easily be interpreted. The thicknesses of individual BNNSs is measured (1.7 nm) using HRTEM by imaging the edges of nanosheets. Additionally, doping in BNNSs with carbon atoms adjusted the band-gap and made it feasible to fabricate nano-electronic device e.g. Schottky diode. BNNSs-based metal-semiconductor-metal junction has been developed and current versus voltage (IV) characteristics are recorded at different temperatures 25 oC, 50 oC and 75 oC. It has been identified that slight doping carbon into BNNSs would bring a significant change in the output characteristics of diode. A rapid increase in the forward current (35µA) and breakdown voltage lower than -70V is achieved, indicating that breakdown electric field is almost up to 108 V/cm.
9:00 AM - NN3.34
Layered Nanostructured MoS2 on Reduced Graphene Oxide (RGO) as an Advanced Energy Material: As a Supercapacitor and Electrocatalyst to Hydrogen Evolution Reaction (HER)
Edney Silveira Firmiano 1 Adriano Rabelo 1 Antonio Pinheiro 1 Cleocir Dalmaschio 1 Ernesto Pereira 1 Wido Schreiner 2 Edson Leite 1
1Federal University of Samp;#227;o Carlos Samp;#227;o Carlos Brazil2Federal University of Paranamp;#225; paranamp;#225; BrazilShow Abstract
Transition metal dichalcogenides with layered structure, especially MoS2, have attracted much physical-chemical interest due to their 2D structure analogous to graphene. Using a versatile chemical route, layered MoS2 nanostructures were obtained on RGO. The structure and morphology were characterized by Scanning Electron Microscopy, Transmission Electron Microscopy and Atomic Force Microscopy. By X-ray Photoelectron Spectroscopy and FT-IR, we detected that the first layers of MoS2 are bonded to oxygen of the RGO by chemically covalent bond (Mo-O-C). The electrochemical characterization was made for three MoS2/GO ratios. The specific capacitance measured values at 10 mVs-1 are 128, 265 and 148 Fg-1 for the MoS2/RGO with low, medium and high concentration of MoS2 respectively. Disregarding any synergetic effect between both and considering only MoS2 as an active material, an intrinsic specific capacitance value of 1558 F g-1 and a calculated energy density of 63 Wh kg-1 can be observed for the MoS2 in the low concentration hybrid material. To demonstrate the activity of the layered nanostructures for hydrogen evolution reactions, exchange current density (Jo) and Tafel slope (b) was evaluated. The exchange current density value for the hybrid material with medium (J0 = 120 mu;A/cm2) and high (J0 = 117 mu;A/cm2) MoS2 concentrations shows a value similar to the exchange current density measured for Pt (J0 = 117 mu;A/cm2).
9:00 AM - NN3.35
Electric Field Induced Band Structure Modulation in Alkali Doped-Quazi Free Standing Hexagonal Boron Nitride
Praveen Chandramathy Surendran 1 Alexander Fedorov 2 5 Alexander Grueneis 3 2 Simone Piccinin 1 Nikolay I. Verbitskiy 3 4 Stefano Fabris 1
1CNR IOM Democritos Theory at elettra group and Sissa Trieste Italy2IFW Dresden Dresden Germany3University of Vienna Wien Austria4Moscow State University Moscow Russian Federation5St. Petersburg State University St. Petersburg Russian FederationShow Abstract
As an atomically thin layered material, hexagonal boron nitride (h-BN) has gained a widespread research interest due to its potential applications in different areas of materials engineering. By contrast to its carbon counterpart graphene, the insulating nature of h-BN makes it versatile for using as a dielectric medium while manipulating/fabricating heterostructures [1,2]. To a great interest, the interface between h-BN and metal substrates opens up ample possibility of developing new class of atomically thin heterostructures.
Extensive research from the past decade has demonstrated that electronic properties of graphene and h-BN can be modulated by doping [3,4]. In heterostructures, such methods have demonstrated ability to modify properties of individual layer as well as that of the complete structure itself . In this contest, response of h-BN in the presence of alkali elements raises interesting questions about its electronic structure.
By using first principles density functional theory (DFT), in conjunction with core and valence band photoemission spectroscopy, we present the effect of Li and K on the electronic structure of h-BN supported by Au/Ni(111). Experimental ARPES scan shows a large down-shift in the pi-bands of the h-BN in the presence of both Li and K. A rigid down-shift in all the h-BN bands calculated from DFT rationalizes the experimental findings.
The calculations attribute the origin of the shift in the pi-bands to an interfacial electric field arising from the dopants. Charge rearrangement within the system results in the generation of a capacitor like field at the Au-(alkali) interface. Efficient utilization of such interfacial electric field generated by the presence of alkali adatoms could lead towards promising applications in several areas such as tunnel junctions, gate insulator, tuning of band alignment etc.
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 Menno Bodkam, et al., Nano. Lett., 11, 4631, 2011.
 Sen Lin, et al., J. Phys. Chem. C, 117, 17319, 2013.
 Han Wang, et al.,IEEE Electr. Dev. Lett., 32, 1209, 2011.
9:00 AM - NN3.37
Carrier Mobility and Thermal Conduction in MoS2 Monolayer: A First-Principles Study
Yongqing Cai 1
1Institute of High Performance Computing Singapore SingaporeShow Abstract
Using first-principles calculations and deformation potential theory, we investigate the intrinsic carrier mobility (mu;) of monolayer MoS2 sheet and nanoribbons.In contrast to the dramatic three orders of magnitude of deterioration of mu; in graphene upon forming nanoribbons, the magnitude of mu; in armchair MoS2nanoribbons is comparable to its sheet counterpart, albeit oscillating with ribbon width. Surprisingly, a room-temperature transport polarity reversal is observed with mu; of hole (h) and electron (e) being 200.52 (h) and 72.16 (e) cm2V-1s-1 in sheet, and 49.72 (h) and 190.89 (e) cm2V-1s-1 in 4 nm-wide nanoribbon. The high and robust mu; and its polarity reversal are attributable to the different characteristics of edge states inherent in MoS2nanoribbons. Our study suggests that width-reduction together with edge engineering provide a promising route for improving the transport properties of MoS2 nanostructures. For any potential nanoelectronic applications, the thermal conduction and management in the MoS2 based devices are ultimately important. Through calculating the phonon dispersion and Gruneisen parameters, we analyze the intrinsic relaxation time and mean free path of MoS2 monolayer based on U-scattering process of phonons. The thermal conductivity of MoS2 monolayer is calculated based non-equilibrium Green's function.
9:00 AM - NN3.40
Molecular Simulation Study of Nanohelix from Hydrogenated Graphene
Xianqiao Wang 1 Liuyang Zhang 1
1University of Georgia Athens USAShow Abstract
Hydrogenated graphene has been emerging as the cynosure of the subject for numerous studies with their conductivity, ferromagnetism, and energy storage as well as drug delivery. However, how to find a decent way to overcome the graphene bending barrier and modify the graphene from planar structures to 3D structures remains to be further explored. By virtue of molecular mechanics/dynamics simulations, here we present the formation of carbon nanohelix from a pristine graphene nanoribbon by doping it with hydrogen atoms in a specific pattern. Meanwhile, we quantitatively investigate the effect of interatomic potential on the process of helical structure formation, thermal stability and mechanical properties of the carbon nanohelix as well as its potential application in molecule packing. Carbon nanohelix portrays an intriguing zigzag strain-stress curve and amazing extensibility under tension as well as relatively limited deformability under compression, which represents its unique signature of mechanical properties to differentiate the carbon nanohelix from the behavior of carbon nanotube and graphene. These findings lends compelling credence to envision that the carbon nanohelix opens up a viable avenue for nanofabrication and is perceived as a novel nanomaterial for a variety of applications such as electronics, sensors, energy storage, drug delivery and nanocomposites.
9:00 AM - NN3.41
Unconventional Two Dimensional Semiconductor Nanostructures: Quantum Confinement of 2D Lead Chalcogenide Nanoplates
Weon-kyu Koh 1 Andrew Fidler 1 Jeffrey Pietryga 1 Victor Klimov 1
1Los Alamos National Lab Los Alamos USAShow Abstract
While there have been many reports on the optical and electronic properties of two-dimensional (2D) semiconductor nanostructures in transition metal compounds such as molybdenum and tunsten chalcogenides, it remains challenging to study 2D nanostructures in other materials which do not readily grow as 2D structures or do not possess layered crystal structures. We demonstrate unusual crystal growth of lead (Pb) chalcogenides, which due to the rock salt crystal structure typically lacks anisotropic growth. However, chemical engineering of colloid growth for Pb chalcgenides can induce selective growth from zero dimensional to one and two-dimensional nanoscale crystals.
Due to the large Bohr radius (PbS 18 nm, PbSe 46 nm) and narrow direct band gap (PbS 0.41 eV, PbSe 0.28 eV) of Pb chalcogenides, the above mentioned 2D Pb chalcogenide nanostructures exhibit the effect of quantum confinement of carriers. We will explore how asymmetric confinement of carrier is manifested in the characteristics of charge carriers, exciton dynamics, and other optoelectric properties for these 2D Pb chalcogenide nanostructures based on theoretical and spectroscopy studies. Furthermore, there are huge opportunities of Pb chalcogenide nanostructures in photovoltaics, sensors, transistors and other optoelectronic devices. We will discuss how the unique properties of 2D nanostructures can be utilized for those applications.
9:00 AM - NN3.43
Optical Switching Phenomena in Tantalum Diselenide Thin Films
Rameez Rauf Samnakay 1 Jacqueline Renteria 2 Chenglong Jiang 2 Timothy R. Pope 3 Zhong Yan 2 Pradyumna Goli 1 Maxim Stolyarov 2 Tina T Salguero 3 Alexander Balandin 2 1
1University of California - Riverside Riverside USA2University of California - Riverside Riverside USA3University of Georgia Athens USAShow Abstract
Recently, there has been a growing interest in two-dimensional (2D) materials - termed van der Waals materials - that can be mechanically exfoliated in a “graphene-like” fashion. Tantalum diselenide (TaSe2) is a transition metal dichalcogenide that belongs to this group of materials. Each individual layer of TaSe2 crystal consists of a sandwich of Ta atom with two Se atoms on either side of the Ta atom. The single layer thickness is ~0.6359 nm. In this presentation, we report the results of our investigation of the optical response of the exfoliated thin films of TaSe2. The single-crystal TaSe2 bulk samples were synthesized by the chemical vapor transport (CVT) method. The thin films were prepared by the “graphene-like” mechanical exfoliation. In order to investigate the photo-response of thin films of TaSe2, we fabricated devices using electron beam lithography (EBL). The contacts were made with a 10-nm thick layer of Ti followed by a 100-nm thick layer of Au. The thickness of the fabricated device channels ranged from 20 nm to 40 nm. The source-drain current as a function of the source-drain voltage was measured under dark and illuminated conditions. At small bias, the channel was resistive allowing for negligible current only. As a threshold field was reached (corresponding to some threshold voltage), one observed a fast increasing non-linear current. The light illumination shifted the on-set of the high non-linear current regime to larger voltages. There was a substantial difference between the threshold voltages at dark and illuminated conditions allowing for optical switching applications. It is interesting to note that unlike a conventional photo-resistor, the TaSe2 device switched on in the dark and switched off under illumination. The physical mechanism of the observed optical effects in TaSe2 thin films was different from that in conventional semiconductors and can be related to collective or excitonic effects. The light sensitivity of TaSe2 thin films was tested between 645 nm and 340 nm wavelengths. It was found that the threshold voltage of the TaSe2 devices increases as the wavelength of light decreases.
Work in the Balandin group was supported, in part, by the National Science Foundation (NSF) and Semiconductor Research Corporation (SRC) Nanoelectronic Research Initiative (NRI) project “Charge-Density-Wave Computational Fabric: New State Variables and Alternative Material Implementation” NSF-1124733 and by STARnet Center for Function Accelerated nanoMaterial Engineering (FAME) - Semiconductor Research Corporation (SRC) program sponsored by MARCO and DARPA.
9:00 AM - NN3.44
Exceptional Tunability of Band Energy in Strained MoS2 Naonosheets
Yeung Yu Hui 1 Ming-Hui Chiu 2 Chang-Lung Hsu 2 Shu Ping Lau 1 Lain-Jong Li 2
1The hong kong polytechnic university Hong Kong Hong Kong2Institute of Atomic and Molecular Sciences Taipei TaiwanShow Abstract
Tuning band energies of semiconductors through strain engineering can significantly enhance their electronic, photonic and spintronic performances. Although low-dimensional nanostructures are relatively flexible, the reported tunability of bandgap is within 100 meV per 1% strain. It is also challenging to control strains in atomically thin semiconductors precisely and monitor the optical and phonon properties simultaneously. Here, we developed a system that can apply strain to MoS2. Photoluminescence and Raman characterizations show that the direct bandgap can be blue-shifted for ~300 meV per 1% strain. First-principles investigations confirm the shift of the direct bandgap and reveal a higher tunability of the indirect bandgap than the direct one. The exceptional high strain tunability of electronic structure in MoS2 promising a wide range of applications in functional nanodevices and the developed methodology should be generally applicable for two-dimensional semiconductors.
9:00 AM - NN3.45
Mechanical Thermodynamic Stabilization of the Elusive Metallic Phase of Two-Dimensional Mo- and W-Dichalcogenides
Karel-Alexander Duerloo 1 Evan Reed 1
1Stanford University Stanford USAShow Abstract
An intriguing feature of monolayer transition metal dichalcogenides (TMDs) is that they can exist in more than one structural phase. These different phases have vastly different physical properties. Monolayer polymorphism has important potential applications in the case of group VI TMDs such as MoS2. In these compounds, the naturally occurring semiconducting phase can be complemented by a usually metastable metallic phase in an electronics context, and metallic phases in group VI TMDs show great promise for hydrogen evolution catalysis. While there are known chemical routes to obtaining metastable octahedral metallic group VI TMDs, the thermodynamic phase behavior of metallic monolayer TMD phases are largely yet to be elucidated. One would like to know under what conditions (if any) a metallic monolayer phase is stable instead of metastable. We have discovered that the energetic ordering of structural phases in all six group VI TMDs (MoS2 through WTe2) can be controlled through the application of stress and strain. After developing a two-dimensional thermodynamic framework that includes room temperature thermal effects, we use density functional theory and hybrid density functional/Hartree-Fock calculations to discover that the deformations required to yield a semiconductor-to-metal phase transition range from 0.5 to 15% engineering strain for the studied compounds. These values fall within the experimentally demonstrated range of elastic deformations afforded by the exceptional mechanical strength of TMD monolayers. The potential applications for this discovery range from catalysis to information storage and nanoscale electronics.
9:00 AM - NN3.46
High Performance Solution-Processed MoS2 Field Effect Transistor by Two-Step Annealing
Juyeon Won 1 Chul-Kyu Lee 1 Byeong-Geun Son 1 Hyo Jin Kim 1 Soyeon Je 1 Jae-Kyeong Jeong 1
1Inha Univ. Inchon Republic of KoreaShow Abstract
Molybdenum disulfide (MoS2) is an attractive material which has good electrical properties for use in field effect transistors (FETs) . MoS2 synthesis methods are mechanical exfoliation, chemical exfoliation, thermolysis etc. However, it is still difficult to synthesize large area and high quality MoS2 thin film with good electrical performance. Generally, solution-processed MoS2 thin film with large-area capability exhibits low crystalline or amorphous structure which can be attributed to the carbon contamination from the organic residues of solvent .
In this study, bottom gate MoS2 FETs are produced by solution-process using (NH4)2MoS4 precursor. To fabricate MoS2 FETs with good electrical properties, the two-step annealing was attempted to increase the thermolysis temperature of precursor in forming gas (H2, 5% and N2) and to effectively remove carbon residue. Two-step annealed MoS2 FETs exhibited the high field effect mobility of 26cm2/V-s, the subthreshold swing of 0.92V/dec, Ion-off ratio of 10-7.
 B. Radisavlijevic, A. Radenovic, J. Brivio, B. Giacometti, and A. Kis, Nat. Nanotechnology, 6, 147 (2011).
 Keng-Ku Liu, Wenjing Zhang, Yi-Hsien Lee, Yu-Chuan Lin, Mu-Tung Chang, Ching-Yuan Su, Chia-Seng Chang, Hai Li, Yumeng Shi, Hua Zhang, Chao-Sung Lai, and Lain-Jong Li, Nano Lett., 12, 1538 (2012).
9:00 AM - NN3.47
Layer-by-Layer Growth of Bi2Te3-Sb2Te3 on h-BN via van Der Waals Heteroepitaxy
Hoseok Heo 1 2 Moon-Ho Jo 1 3
1Institute for Basic Science (IBS) Pohang Republic of Korea2Pohang University of Science and Technology(POSTECH) Pohang Republic of Korea3Pohang University of Science and Technology(POSTECH) Pohang Republic of KoreaShow Abstract
Epitaxial growth between graphene, hexagonal boron nitride (h-BN) and chalcogenide compounds can be achieved in terms of van der Waals hetero-epitaxy due to their layered nature in crystal structure. Van der Waals hetero-epitaxy enable abrupt heterojunction with minimized defects or dislocations, because it can allows efficient stress relaxation which originated from its lattice mismatch by weakly bound by unoccupied dangling bonds. Here, we