Mauricio Terrones, The Pennsylvania State University
Pulickel M. Ajayan, Rice University
Reshef Tenne, Weizmann Institute of Science
Anupama Kaul, California Institute of Technology
Symposium Support Army Research Office
Materials Research Institute (Penn State) Center for 2-Dimensional and Layered Materials (Penn State)
National Science Foundation (NSF)
Pennsylvania State University
O2: Synthesis of 2D Layered Materials
Pulickel M. Ajayan
Wednesday PM, April 03, 2013
Moscone West, Level 2, Room 2009
2:30 AM - *O2.01
Synthesis, Characterization and Application of Two-Dimensional MoS2 Atomic Layers
Sina Najmaei 1 Zheng Liu 1 Pulickel Ajayan 1 Jun Lou 1
1Rice University Houston USAShow Abstract
Monolayer Molybdenum disulfide (MoS2), a two-dimensional crystal with a direct bandgap, is a promising candidate for 2D nanoelectronic devices complementing graphene. Unlike conductive graphene and insulating h-BN, atomic layered MoS2 is a semiconductor material with a direct bandgap, offering possibilities of fabricating high performance devices with low power consumption in a more straight-forward manner.
In this talk, we will discuss our recent efforts on the large area growth of MoS2 atomic layers by a scalable chemical vapor deposition (CVD) method. The as-prepared samples can either be readily utilized for further device fabrication or be easily released from the growth substrate and transferred to arbitrary substrates. High resolution transmission electron microscopy and Raman spectroscopy on the as grown films of MoS2 indicate that the number of layers range from single layer to a few layers. Our results on the direct growth of MoS2 layers on dielectric leading to facile device fabrication possibilities show the expanding set of useful 2D atomic layers, on the heels of graphene, which can be controllably synthesized and manipulated for many applications.
3:00 AM - O2.02
First-principles Calculations of Molecular Scale Graphane Analogues
Oscar D Restrepo 1 Rohan Mishra 1 Joshua E. Goldberger 2 Wolfgang Windl 1
1The Ohio State University Columbus USA2The Ohio State University Columbus USAShow Abstract
Graphene&’s success has shown that it is not only possible to create stable, single-atom thick sheets of a crystalline material, but that these materials can have electronic structures that are fundamentally different than the parent 3-D material, and that are significantly influenced by the environment. Recent work at The Ohio State University has shown that unique single-layer 2D materials based on silicon and germanium can be synthesized, stabilized by appropriate ligands. In this talk, we will show theoretical calculations of the electronic properties of two-dimensional silicon and germanium sheets, which are strongly dependent on the stabilizing molecules. For our calculations, we will use density-functional theory with hybrid functionals that predict the band gaps in excellent agreement with experiment. We will examine the dependence between orbital structure and calculated bands and discuss the origin of the observed changes in the band structure with changing ligands. We will also discuss the relationship between the observed large-gap band structures for silicon and germanium and the Dirac-cone type band structure case of graphene, and the dependence of the observed properties on the geometric parameters of the atomic structure.
3:15 AM - O2.03
Super-stochiometric Intercalation of Zero-valent Metals into 2D Layered Chalcogenide Nanoribbons
Kristie J Koski 1 Janina Pearl Motter 1 Judy J Cha 1 Desheng Kong 1 Yi Cui 1
1Stanford University Stanford USAShow Abstract
We have developed a novel chemical method to intercalate extremely high densities of metal atoms into 2D layered nanoribbon chalcogenides and oxides. Whereas most traditional methods of intercalation move ions into the layered host, this method moves zero-valent atoms and is specific to ultrathin 2D nanocrystalline samples. The zero-valent nature of the intercalant species allows super-stoichiometric intercalation of Ag, Au, Co, Cu, Fe, In, Ni, and Sn. We show that this method is general and can be used for a variety of 2D layered nanomaterials. This work opens new ground for accessing novel states of quantum matter such as superconducting topological insulators, unique opto-electronic & structural properties, and new energy storage applications.
3:30 AM - O2.04
Synthesis and Properties of WS2 Nano-sheets
Charina Choi 1 Yanguang Li 1 Ju Feng 1 Alla Zak 3 2 Reshef Tenne 2 Hongjie Dai 1
1Stanford University Stanford USA2Weizmann Institute of Science Rehovot Israel3Holon Institute of Technology Holon IsraelShow Abstract
Layered structures including MoS2 and WS2 are promising materials with shape, size, layer number, and interlayer distance all dictating the optoelectronic behavior. In this work, we demonstrate the synthesis of WS2 nanosheet structures through sonochemical unzipping of nanotubes. Nanosheets of different shapes and layer numbers are achieved by a chemical separation technique, and characterized using transmission electron microscopy, x-ray and electron diffraction, and Raman spectroscopy. Our separation technique isolates WS2 particles of various layer numbers, exhibiting unique and tunable optical transitions. These novel layered materials are further explored towards optoelectronic and catalytic applications.
3:45 AM - O2.05
CVD-grown Single Crystal Layered MoS2
Masihhur R Laskar 1 Lu Ma 2 K. Santhakumar 3 Pil Sung Park 1 Sriram Krishnamoorthy 1 Digbijoy N. Nath 1 Edwin Lee II 1 Ye Shao 1 Won-Jin Moon 4 Wu Lu 1 Yiying Wu 2 Siddharth Rajan 1
1The Ohio State Univ Columbus USA2The Ohio State University Columbus USA3Gwangju Institute of Science and Technology Gwangju Republic of Korea4Korea Basic Science Institute Gwangju Center Gwangju Republic of KoreaShow Abstract
We report CVD growth and characterization of large area crystalline Molybdenum disulfide (MoS2) layered semiconductors using sulfurization of thin molybdenum layers on different substrates. Characterization of the films indicates that the films are highly crystalline with orientation defined by the substrate. This is the first report of single crystal large area MoS2 using CVD. MoS2 has received widespread interest recently due to its promise for electronics [1-4], phototransistors  and sensing , but uniform MoS2 sheet/thin films over a large area remain a major challenge. Several groups have reported large area MoS2 by chemical synthesis [7-9], but the films were non-crystalline. MoS2 epitaxy  and mechanical exfoliation lead to flake-shape morphology that limits large-scale device fabrication applications. Different substrates (sapphire, GaN, Si/SiO2) were coated with Mo using e-beam evaporation, and were heated together with sulfur in a sealed quartz tube. Sulfur vapor reacted with the Mo, forming MoS2. The crystallinity and surface morphology of the films were significantly influenced by process conditions and substrates. Raman spectroscopy peaks E21g and A1g were found to be identical in intensity and ratio to mechanically exfoliated MoS2, selective area diffraction TEM and STEM image, XRD measurements indicats the high quality of the films and (002) oriented. Atomic force microscopy indicates smooth monolayer step morphology over the entire surface of the sample. In addition, MoS2 growth on several other substrates sapphire (GaN, SiO2/Si, AlGaN/GaN) at varying temperatures ranging from 500oC to 1100oC was carried out, and details of film properties will be reported. Transmission-line-measurement structures on the MoS2/sapphire showed clear linear and quadratic regions in the I-V curve, indicating ohmic and space charge transport and low background carrier density. The electron mobility, extracted using a modified Mott-Gurney equation for thin films , was 10.72 cm2/V.s. In conclusion, we have reported single crystal large area MoS2 using a simple CVD method that could have applications for high-quality growth of several crystalline layered 2D semiconductors for electronic and sensor applications.
4:30 AM - *O2.06
Liquid Exfoliation of Layered Compounds
Jonathan N. Coleman 1
1Trinity College Dublin Dublin IrelandShow Abstract
Layered materials represent a diverse and largely untapped source of 2-dimensional systems with exotic electronic properties and high specific surface areas that are crucially important for applications including sensing, catalysis and energy storage. While graphene is the most well-known layered material, transition metal dichalcogenides (TMDs), transition metal oxides (TMOs) and other 2-dimensional (2D) compounds such as BN, FeTe, Bi2Te3 and Bi2Se3 are also important. If they could be easily exfoliated in large quantities, such layered materials would become an important source of 2-dimensional crystals. Here we show that layered compounds such as MoS2, WS2, MoSe2, MoTe2, TaSe2, NbSe2, NiTe2, BN, MnO2, MoO3 and Bi2Te3, can be efficiently dispersed and exfoliated in both in common solvents and in aqueous surfactant solutions. Electron microscopy shows these materials may be exfoliated down to individual layers. These dispersions can be deposited as individual flakes or formed into films. By blending with suspensions of other nano-materials or polymer solutions, we can prepare hybrid dispersions or composites which can be cast into films. Such materials demonstrate huge potential for a range of applications. For example, we show that WS2 and MoS2 effectively reinforce polymers, WS2/carbon nanotube hybrid films display promising thermoelectric properties, MoS2 films are viable candidates for battery electrodes while layered oxides are exciting for electrochemical applications. Exfoliated layered compounds are also useful as fillers in polymer based composites for applications such as mechanical reinforcement or gas barrier. We will demonstrate the utility of these materials for such applications. We note that such applications will, eventually, require large quantities of exfoliated layered materials. We will demonstrate a scalable exfoliation method for producing nanosheets in very large quantities.
5:00 AM - O2.07
Large Area Growth and Characterization of WS2 Atomic Nano-sheet on SiO2 Substrate
Jusang Park 1 Hyungjun Kim 1 Jeong-Gyu Song 1
1Yonsei Univ Seoul Republic of KoreaShow Abstract
Recently, Transition metal dichalcogenides (TMD), MX2 (M=Mo, W; X=S, Se, Te), have attracted considerable attention for beyond graphene. Multi-layer WS2 is an indirect gap semiconductor which is larger than that of graphene. Moreover, WS2 becomes a direct gap semiconductor same as MoS2 single layer.
Here we demonstrate the large area growth of WS2 atomic nano sheet on SiO2 substrates by a scalable chemical vapor deposition (CVD) method. WS2 nano sheet was formed in various thickness and substrate temperature. The films were confirmed with Raman spectroscopy. We fabricated bottom-gate WS2 transistors, and characterized the device performances by varying the gate insulator and the various thickness of WS2 nano sheet. Electrical properties of MoS2 films were measured by using probe station.
5:15 AM - O2.08
Intercalation of Hexagonal Boron Nitride with Acids
Nina Kovtyukhova 1 Thomas Mallouk 1
1Penn State University University Park USAShow Abstract
The fundamental and practical interest in intercalated compounds (IC) of crystalline layered solids has been stimulated by their remarkable mechanical, physical, chemical and catalytic properties as well as a possibility to use them as precursors for the exfoliation to single atomic layers. The latter was demonstrated by preparing the monolayers of graphene oxide, some metal chalcogenides and oxides, and graphene cones.
The intercalation chemistry of hexagonal boron nitride (h-BN), however, is much more challenging than that of graphite, despite h-BN and graphite are isoelectronic and have the same crystal structure with very close cell parameters. This difficulty results from the stronger interlayer interaction (due to the different interlayer registry in h-BN with eclipsing B and N atoms in the neighboring layers and polar character of the B-N bonds) and very different electronic structure. h-BN is a wide gap insulator, while graphite is a semimetal. The BN redox potential is ~ 2V more positive relative to graphite and its oxidative intercalation is much harder to achieve. Only a few ICs of h-BN have been reported.
This work demonstrates a possibility of the non-oxidative h-BN intercalation with mineral acids, such as H2SO4 and H3PO4. The h-BN- H2SO4 and h-BN-H3PO4 compounds have the interlayer distances of 6.9-7.4#8491; that implies the formation of the 1st-stage ICs of h-BN. The structure and guest-host interactions have been studied by XRD, TEM, SEM, FTIR, Raman, XPS and thermal analysis. We assume that the acid molecules interact with the h-BN layers by formation of hydrogen bonds of the type Bδ+Nδ-hellip;H-OP(S)< (or even protonation of N atoms), and dative bonds of the type
5:30 AM - O2.09
The Application of Graphene and Boron Nitride 2D NEMS Devices to Non-linear Vibration Energy Harvesting
Riccardo Rurali 1 Miquel Lopez-Suarez 2 Jose Miguel Pruneda 3 Gabriel Abadal 2
1Institut de Ciamp;#232;ncia de Materials de Barcelona (ICMAB-CSIC) Bellaterra (Barcelona) Spain2Universitat Autonoma de Barcelona Bellaterra (Barcelona) Spain3Centre d'Investigaciamp;#243; en Nanociamp;#233;ncia i Nanotecnologamp;#237;a (CSIC-ICN), Campus de la UAB Bellaterra SpainShow Abstract
Recent developments on 2D layered materials engineering have demonstrated that chemical or physical local modifications of the graphene atomic structure can turn its electromechanical properties into piezoelectric , . Nanoelectromechanical (NEMS) energy harvesters based on suspended graphene nanoribons can benefit from those new transducing properties, to convert energy from ambient vibrations into useful energy on the electrical domain . In this work, we present the design of non-linear NEMS energy harvesters based on bistable suspended graphene or boron nitride bridge-like nanoribons. The bistability conditions induced by a controlled compressive stress of the nanomechanical structures are analyzed by ab-initio calculations of the potential energy. From the numerical solution of the NEMS devices movement equation, mechanical and electrical power harvested from the ambient vibrations are calculated, when noise force excitations are considered.
 S. Chandratre and P. Sharma, "Coaxing graphene to be piezoelectric." Appl.Phys. Lett. 100, 023114 (2012).
 M. T. Ong, and E. J. Reed, "Engineered Piezoelectricity in Graphene." ACS Nano 6, 1387 (2012)
 M. Lopez-Suarez, R. Rurali, L. Gammaitoni, and G. Abadal, "Nanostructured graphene for energy harvesting." Phys. Rev. B 84, 161401 (2011).
O3: Poster Session: Synthesis and Transport of 2D Layered Materials
Wednesday PM, April 03, 2013
Marriott Marquis, Yerba Buena Level, Salons 7-8-9
9:00 AM - O3.01
Capacitive Performance of Titanium Carbide Based MXenes
Maria R. Lukatskaya 1 Yohan Dallamp;#8217;Agnese 2 1 Michael Naguib 1 Majid Beidaghi 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 Al from a MAX phase Ti3AlC2 and labeled it MXene, to emphasize its similarity to graphene. MXenes represents a large family of transition metal carbides and carbonitrides, not just one phase. Unlike graphene, whose chemistry is restricted to carbon, 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 the rich surface chemistry makes them attractive for electrical energy storage. However while potential of MXenes as anode materials for Li-ion batteries has already been shown, their use in electrochemical capacitors has not been explored.
Here, we report on our latest findings regarding the capacitive properties of Ti-containing MXenes. Electrodes were fabricated by vacuum filtration of MXene sheets or by conventional film rolling with binder and conductive additives. In some cases prior to electrode fabrication MXene was delaminated in order to achieve better separation of the 2D sheets. Several different electrochemical techniques were employed to understand the mechanism of charge storage in both aqueous and organic electrolytes. 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. Moreover, a 3-5 times increase in capacitance was observed for delaminated MXenes as compared to as-produced material.
9:00 AM - O3.02
High-performance Thin-film Transistors Based on Multilayer MoS2 Crystals
Woong Choi 1 Sunkook Kim 2 Debdeep Jena 3 Aniruddha Konar 3
1Kookmin University Seoul Republic of Korea2Kyung Hee University Yongin Republic of Korea3University of Notre Dame Notre Dame USAShow Abstract
Unlike graphene, the existence of bandgaps (1-2 eV) in the layered semiconductor molybdenum disulfide (MoS2), combined with mobility enhancement by dielectric engineering, offers an attractive possibility of using single layer MoS2 field-effect transistors (FETs) in low-power switching devices. However, the complicated process of fabricating single layer MoS2 with an additional high-k dielectric layer may significantly limit its compatibility with commercial fabrication. Here we show the first comprehensive investigation of process-friendly multilayer MoS2 FETs to demonstrate a compelling case for their applications in thin-film transistors. Our multilayer MoS2 FETs exhibited high mobilities (> 100 cm2/V s), near-ideal subthreshold swings (~70 mV/decade), and robust current saturation over a large voltage window. With simulations based on Shockley&’s long-channel transistor model and calculations of scattering mechanisms, these results provide potentially important implications in the fabrication of high-resolution large-area displays and further scientific investigation of various physical properties expected in other layered semiconductors.
9:00 AM - O3.04
Large Area Synthesis of WS2 Crystalline Sheets Directly on SiO2 and Their Transfer to Other Substrates
Ana Laura Elias 1 Nestor Perea-Lopez 1 Andres Castro-Beltran 2 1 Ayse Berkdemir 1 Simin Feng 1 Ruitao Lv 1 Aaron Long 1 Takuya Hayashi 3 Yoong Ahm Kim 3 Morinobu Endo 3 Humberto R. Gutierrez 4 Sujoy Ghosh 5 Nihar R. Pradhan 6 Luis Balicas 6 Saikat Talapatra 5 Florentino Lopez-Urias 1 7 Humberto Terrones 1 Mauricio Terrones 1 8 9
1The Pennsylvania State University University Park USA2Universidad Autonoma de Nuevo Leon San Nicolas de los Garza Mexico3Shinshu University Nagano-city Japan4University of Louisville Louisville USA5Southern Illinois University Carbondale USA6Florida State University Tallahassee USA7IPICyT San Luis Potosi Mexico8The Pennsylvania State University State College USA9Shinshu University Nagano-city JapanShow Abstract
Metal dichalcogenides (e.g. MoS2, WS2, NbS2, etc.) have recently attracted the attention of numerous scientists because they are layered materials that could exhibit either semiconducting or metallic properties. However, these properties could be significantly modified when these layered materials become monolayers. To the best of our knowledge, the isolation of few-layered WS2 has only been performed by mechanical and chemical exfoliation with very low yields. Recently, a CVD method has been reported to yield micron size WS2 triangular islands with unprecedented optical properties. In this work, we report for the first time the synthesis of large area (< 1 cm2) few-layer WS2 by a two step method. WOx thin films were first grown on a Si/SiO2 substrate and these films were sulfurized in a second step at high temperatures (750-950 °C). Furthermore, we have developed an efficient route to transfer these WS2 films onto different substrates, such as quartz and TEM grids. WS2 films of different thicknesses have been analyzed by Raman spectroscopy, TEM and AFM techniques. Transport properties of the films have also been measured at different temperatures. Characterization techniques demonstrate the presence of mono-, bi- and few-layered WS2 in the as-grown samples. The novel photoluminescence properties of the films will also be discussed along with high resolution TEM images exhibiting various types of stacking.
9:00 AM - O3.05
Synthesis of Large-scale Atomic-thin MoS2 Nanosheets
Yifei Yu 1 Linyou Cao 1
1North Carolina State University Raleigh USAShow Abstract
The nanosheet of layered transition metal chalcogenide--molybdenum disulfide (MoS2) has attracted great interest due to its potential applications in catalysis, optoelectronics, energy harvesting. But, studies of the nanosheet have been delayed by the lack of capabilities to prepare the nanosheet with control of physical features in atomic-scale precision. Here, we report synthesis of atomic-thin MoS2 by vapor deposition techniques on different substrates. The atomic thickness and high quality of resulting nanosheets are revealed by spectroscope and microscope (Raman, Photoluminance, XRD, XPS, TEM). This synthetic approach is facile, scalable and may generally apply to the growth of nanosheets of other layered transition chalcogenides.
9:00 AM - O3.06
Atomic-layer-deposition of Al2O3 and HfO2 Dielectric Films on Oxygen Plasma-treated Multilayered MoS2 Crystal
Jaehyun Yang 1 Sunkook Kim 2 Woong Choi 4 Sang Han Park 3 Mann-Ho Cho 3 Hyoungsub Kim 1
1Sungkyunkwan Univ. Suwon Republic of Korea2Kyung Hee Univ. Yongin Republic of Korea3Kookmin Univ. Seoul Republic of Korea4Yonsei Univ. Seoul Republic of KoreaShow Abstract
As an alternative to graphene, two-dimensional layer-structured transition metal dichalcogenides (TMDs) have been expected to have a great potential in nanoscale and flexible electronic application areas. Among many available TMD materials, molybdenum disulfide (MoS2) has attracted much research interest for thin film transistor (TFT) application due to its high electron mobility and large bandgap . In realizing high-performance MoS2-based TFT devices, there have been several attempts to adopt an atomic layer deposition (ALD) technique in forming high-k gate dielectrics on top of the MoS2 channel layer [1, 2]. Because the ALD process is strongly dependent on the surface identity and MoS2 has a chemically inert surface condition, it is necessary to significantly decrease the deposition temperature to obtain a uniform high-k dielectric film deposition [2, 3], which may deteriorate the dielectric properties.
In this presentation, first, we will discuss the different growth behavior of the ALD-Al2O3 and HfO2 films on mechanically-exfoliated MoS2 multilayers using trimethylaluminum and tetrakis-(ethylmethylamino)hafnium metal precursors, respectively, along with H2O oxidant. In addition, as an alternative way to obtain a uniform deposition of ALD-high-k films at a high temperature, we propose a surface treatment of the MoS2 surface using an oxygen plasma process, which resulted in a uniform growth of the Al2O3 and HfO2 films even at a deposition temperature of 250 °C. According to the XPS analysis, the MoS2 surface was partially oxidized to form an ultra-thin Mo-oxide layer, which improved the adsorption the ALD metal precursors.
 B. Radisavljevic et. al., Nature Nanotech. 6, 147, (2011).
 H. Liu et. al., Appl. Phys. Lett. 100, 152115, (2012).
 H. Liu et. al., IEEE Electron Dev. Lett. 33, 4, (2012).
9:00 AM - O3.09
Solvents for Low Dimensional Nanostructures - The Role of Solubility Parameters
Shane Bergin 1 2 Milo SP Shaffer 2 Jonathan N Coleman 1
1Trinity College Dublin Dublin Ireland2Imperial College London London United KingdomShow Abstract
The effective dispersion of nanomaterials is universally acknowledged to be critical when one wishes to harness their full potential in devices or applications. Over the past number of years much work has been carried out on overcoming the inherent difficulties associated with producing high-quality dispersions of single-walled carbon nanotubes (SWNTs), graphene and other 2D nanomaterials (such as transition metal dichalcogenides) in the liquid phase.
Crucial to solvent choice for each of these low dimensional nanomaterials has been matching the surface energy of the nanomaterial with the surface energy of the solvent. Such information for organic solvents is normally contained within their solubility parameters. Hildebrand and Hansen solubility parameters have been primarily used to-date to predict which solvents will exfoliate low dimensional nanomaterials. Whilst these have been quite successful, they are unable fully describe the requirements of a solvent for such nanomaterials. In this work, we show that Kamlet Taft parameters dramatically improve our ability to predict solvents for intractable nanomaterials. We will demonstrate that specific solvent-solute interactions can be accounted for using this scale (interactions that are not possible using only Hildebrand or Hansen parameters) - interactions that may explain the ability of certain solvents to produce stable high-quality dispersions of nanomaterials.
9:00 AM - O3.10
Liquid Phase Exfoliation of Two Dimensional Crystals
Arlene C O'Neill 1 Jonathan N Coleman 1
1Trinity College Dublin Dublin 2 IrelandShow Abstract
There is little doubt that two dimensional materials are an intriguing class of materials. Some boast multifunctional properties, such as transparency, high conductivity, high thermal conductivity and high strengths simultaneously, for example. However, their integration into products and technologies will depend upon scalable and reliable ways to prepare the nanosheets with high quality. Liquid phase exfoliation of graphite and other layered inorganic crystals in solvents offers a way to produce billions of 2D nanosheets in a small volume of solvent. It also offers an avenue to prepare dispersions of 2D nanosheets with specific properties. This research aims to optimise the exfoliation of layered crystals in solvents, so that a highly concentrated dispersion of thin and large flakes can be achieved. Taking MoS2 as the model material, we recently showed that highly concentrated dispersions (~40 mg/ml) of flake size separated MoS2 nanosheets could be prepared. However, although the dispersions were highly concentrated and contained laterally large nanosheets ( ~ 2 mu;m), the flakes tended to be thick, as indicated by transmission electron microscopy. Further processing optimisation to reduce the thickness of these nanosheets, by various dilution and agitation methods were performed, with promising results. The knowledge gained throughout this research, was also successfully extended to other layered materials. Thus demonstrating the universality of this optimised preparation method.
9:00 AM - O3.13
Graphene-like Chemical Vapor Deposition of Patternable Large-scale Atomic Layers of MoS2
Intek Song 1 Hee Cheul Choi 1
1Pohang University of Science and Technology (POSTECH) Pohang Republic of KoreaShow Abstract
Lack of band gap in pristine graphene as well as difficulty in its engineering has let atomic layers of MoS2 an attractive candidate of two-dimensional materials especially in nanoelectronics because of their intrinsic direct band gap of 1.8 eV and capability of valley polarization. Nonetheless, there have been limited synthetic methods allowing formation of large-scale and patternable MoS2 layers that is essential to avoid post-synthesis etching steps required during device fabrication due to chemical inertness of MoS2. Herein, we report a novel way to prepare atomic layers of MoS2 in desired geometry and dimension by CVD under mild condition: atmospheric pressure and the lowest temperature reported so far. We used Raman spectroscopy, Raman spectrum mapping, and atomic force microscopy (AFM) to check both the chemical composition and the number of layers of the product. High-resolution transmission electron microscopy (HRTEM) was also employed to check its crystal structure, grain size and the number of layers. X-ray photoelectron spectroscopy (XPS) was carried to check the oxidation state to conclude that the product is chemically MoS2. XPS study on non-sulfidated sample provided mechanistic evidence for surface alloying of Mo on Au film as well. Lastly, we have fabricated an electronic device to measure its electronic properties as a form of field-effect transistor.
9:00 AM - O3.14
Investigation of Rhenium Doped MoS2 Nanoparticles with Fullerene-like Structure
Lena Yadgarov 1 Rita Rosentsveig 1 Reshef Tenne 1
1Weizmann Institute of Science Rehovot IsraelShow Abstract
Hollow-closed fullerene-like nanoparticles of MoS2 thereof were shown to exhibit very favourable tribological properties and are already in commercial use. However, being semiconductors, the effect of the doping of these semiconducting nanoparticles has been overlooked so far. Here the doping of these nanoparticles with rhenium atoms is studied and is shown to have a remarkable influence on their physio-chemical properties.
The Re atoms go to substitutional Mo sites in the nanoparticles lattice as shown by extended fine structure X-ray absorption (EXAFS) and high angle annular dark field (HAADF) studies. Theoretical quantum-chemical calculations show that the Re level is some 180 meV below the conduction band.
The doping leads to marked increase of the doped nanoparticles conductivity and reduced agglomeration in various suspensions, which is attributed to their negative surface charge. Furthermore, due to a robustness, flexibility and high conductivity of these nanoparticles, a precipitous reduction in friction and wear is observed.
9:00 AM - O3.15
Controlled Growth of 2D Layered Chalcogenide Nanomaterials via Vapor Transport Deposition
Chun Li 1 2 Huang Liang 1 Gayatri Pongur Snigdha 1 Yifei Yu 1 Linyou Cao 1
1North Carolina State University Raleigh USA2University of Electronic Science and Technology of China Chengdu ChinaShow Abstract
Originally inspired by the spectacular success of graphene, interest in the 2D nanomaterials is extended well beyond graphene. Unlike graphene whose constituents are limited to be carbon atoms, layered chalcogenide materials (LCMs) comprise cations, which can be semiconductors (like Ge and Sn) or transition metals (such as Mo and W), and anions that can be -S, -Se, or -Te. This richer variety in compositions can provide the 2D LCM nanostructure with new functionality that cannot be obtained from graphene. Developing sophisticated synthetic capabilities is important for exploring the full potential of the 2D nanomaterials. Vapor deposition, as a versatile synthetic strategy, has a proven track record in enabling control of the physical features of nanomaterials with atomic precision. However, the development of controlled vapor phase synthesis of 2D LCM nanostructures necessitates a better understanding of the fundamentals involved. Knowledge on the vapor growth of nanomaterials such as nanowires, nanotubes, and graphene may not simply apply to the 2D LCM nanostructure. In this talk, we report controlled vapor growth of two kinds of LCM nanosheets. In the growth of GeS-type quasi-LCM nanosheets with high growth rate, we find that the nanosheet growth is subject to strong influences of the diffusion of source materials through the boundary layer of gas flows. This boundary layer diffusion is found to be the rate-determining step of the growth under typical experimental conditions, evidenced by a substantial dependence of the nanosheet's size on diffusion fluxes.1 In contrast, in the growth of SnSe2-type LCM nanosheets (nanoplates) whose growth rate is relatively slow, we find that their growth is reaction-limited. The dynamics and orientation of the nanoplate growth show substantial dependences on the strength of the interaction between the SnSe2 materials and the substrate. A weak interaction can enable the facile migration of SnSe2 adatoms on the substrate and facilitate the in-plane growth of the nanoplate along the substrate surface. This is evidenced by the morphological differences in the nanoplates grown on mica and on silicon. The nanoplate grown on mica, whose interaction with SnSe2 materials is weaker, exhibits a larger size and an irregular shape compared to the nanoplates on silicon, which are smaller and hexagonal. Strong interactions, however, may induce strain energies in the nanoplate, which can subsequently suppress the in-plane growth.2 These fundamental understanding of the key factors that influence the chalcogenide nanosheet growth can provide useful guidance for the development of general paradigms to control their synthesis.
1. C. Li, et al. , Role of Boundary Layer Diffusion in Vapor Deposition Growth of Chalcogenide Nanosheets: The Case of GeS, ACS Nano 6 (2012) 8868.
2. L. Cao, et al., Substrate Mediation in the Growth of Layered Chalcogenide Nanoplates: A Case Study of SnSe2, Submitted to ACS Nano.
9:00 AM - O3.17
Electrochemical Characterization of Liquid Phase Exfoliated Layers of MoS2
Andrew Winchester 1 Sujoy Ghosh 1 Ana Laura Elias 2 Mauricio Terrones 2 3 Saikat Talapatra 1
1Southern Illinois University Carbondale Carbondale USA2Penn State University University Park USA3Shinshu University Nagano-city JapanShow Abstract
Resounding from the recent discovery of graphene, there has been a significant increase involving research on layered materials. In particular, Molybdenum disulfide (MoS2) has gathered much attention for its desirable properties and varied applications. Its semiconducting behavior has led to much work on transistor and solar cell applications for example, yet there are other areas where this material&’s properties can be useful. One area of basic research that is of interest is in energy storage. Increasing the capabilities of current energy storage technologies is crucial to support the rapid growth in the needs of various devices, ranging from cell phones to electric and hybrid cars to maintaining power grids. One particular type of storage device that has seen much attention in recent years is the electrochemical double layer capacitor (EDLC). Material for the electrodes in EDLCs has traditionally been carbon or metal-oxide based, but recently it has been indicated that MoS2 can give EDLC performance comparable to that of carbon nanotube based electrodes. In order to gauge the potential of these layered materials as suitable candidates for EDLC electrodes it is necessary to understand their fundamental electrochemical properties. Here, we will present detail electrochemical characterization of thin films consisting of few-layer MoS2 flakes (synthesized via a liquid phase exfoliation). The capacitive charge storage behavior (in presence of various electrolytes) analyzed using cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy techniques will be discussed.
9:00 AM - O3.19
Single Layer MoS2 and an MoSx Structure with High Affinity for Adsorbate Interaction
Wenhao Lu 1 Dezheng Sun 1 Daeho Kim 1 Ludwig Bartels 1
1Univeristy of California, Riverisde Riverside USAShow Abstract
MoS2 is a semiconducting material consisting of sulfur-molybenum-sulfur tripledecker layers loosely bound by van der Waals interactions. Single layer MoS2 can be exfoliated mechanically similar to graphene. This poster shows an alternative avenue for the fabrication of MoS2 monolayers at comparatively low temperature and mild conditions through sulfur loading of a copper substrate using thiophenol followed by the evaporation of Mo atoms and annealing. Here we also demonstrate that another MoSx structure can be formed in this fashion, which has a far higher affinity to adsorbate interaction. Using anthraquinone and formic acid as test molecules, we titrate the various MoSx and copper-based structures presented on our substrate in order to determine the relative strength of adsorbate interaction.
9:00 AM - O3.20
Novel Morphologies of Single and Few-layered WS2 and MoS2 2D-islands
Ayse Berkdemir 1 Humberto Rodriguez Gutierrez 1 2 Ana Laura Elias 1 Nestor Perea-Lopez 1 Andres Castro-Beltran 1 Florentino Lopez-Urias 1 3 Humberto Terrones 1 Mauricio Terrones 1 4 5
1Penn State University University Park USA2University of Louisville Louisville USA3IPICyT San Luis Potosi Mexico4The Pennsylvania State University University Park USA5Shinshu University Wakasato JapanShow Abstract
Here we report a two-step synthesis of two-dimensional single- and few-layered transition metal dichalcogenides (WS2 and MoS2) islands with different edge geometries. The process involves the high-vacuum deposition of metal tri-oxides thin films (5-20 Å of WO3 or MoO3) followed by high-temperature sulfurization inside a quartz tube reactor. Different island morphologies (i.e. dendritic, triangular, hexagonal and star-shaped) have been obtained by controlling the synthesis parameters such as sulfurization temperature, carrier gas composition, S evaporation rate (or sulfur temperature), metal tri-oxide thickness and cooling-down rate. These morphologies are single-crystal islands originated from a nucleation center. Coalescence of neighboring islands lead to more complex and irregular shapes, which include grain boundaries. Atomic force microscopy and Raman spectroscopy mapping were used to determine the thickness (number of S-W-S layers) distribution at the island center and at edges. Transmission Electron Microscopy revealed the formation of zigzag edges as the most stable edges in islands with regular shapes (non-dendritic). The optical properties of the different morphologies and edges were studied through Photoluminescence and Raman mapping. For triangular islands, PL enhancement was observed near the edges; while hexagonal island presented an alternated pattern of PL enhancement, suggesting that controlling the shape of the islands as well as the structural termination and chemistry of the edge can be used to tailor the optical properties of these systems.
9:00 AM - O3.21
Two-Dimensional Titanium Carbide Based Material
Michael Naguib 1 2 Olha Mashtalir 1 2 Yohan Dallagnese 1 2 Michel W. Barsoum 1 Yury Gogotsi 1 2
1Drexel University Philadelphia USA2Drexel University Philadelphia USAShow Abstract
Recently, we discovered a new family of two dimensional, 2-D, early transition metal carbides produced by the exfoliation of the MAX phases. The latter is a large family (+60 members) of machinable layered ternary carbides and nitrides, with a Mn+1AXn composition, where “M” is an early transition metal, “A” is a group 13 to 16 element, “X” is C and/or N, and n = 1, 2, or 3. The exfoliation process was carried out by selective etching of the A-group element from Mn+1AXn using hydrofluoric acid (HF) at room temperature, resulting in 2-D Mn+1Xn layers, we labeled MXene to emphasis their similarity to graphene. Herein we focus on the first synthesized and most studied member of the MXenes family: Ti3C2 produced by HF etching of Ti3AlC2. The effect of synthesis parameters (temperature, time, particle size) on etching will be discussed. Not only are individual layers formed after exfoliation, but also multi-layer particles and conical scrolls of radii < 20 nm have been demonstrated. DFT simulations showed that band gap of Ti3C2 can be tuned from metallic to semiconductor by changing its surface termination (O, OH and F), and its elastic properties along the basal planes (c11 asymp; 523 GPa) are higher than that of the binary carbide TiC. The 2-D Ti3C2 can be used as a precursor for other unique materials; for example rapid thermal oxidation of Ti3C2 in air at 1100 °C resulted in formation of nano anatase supported on graphene-like carbon sheets. One of the potential applications for 2-D Ti3C2 is in energy storage systems as anodes for lithium ion batteries. Additive-free Ti3C2 anodes, produced by filtering an aqueous dispersion of delaminated Ti3C2, showed capacity of 410 mAh.g-1 at cycling rate of 1C and 110 mAh.g-1 at 36C after 200 cycles.
9:00 AM - O3.23
Ambipolar Transport in MoS2 Based Electric Double Layer Transistors
Jianting Ye 1 Yinjing Zhang 1 Yoshihiro Iwasa 1
1The University of Tokyo Tokyo JapanShow Abstract
Making field effect transistors (FETs) on thin flake of single crystal isolated from layered materials was pioneered by the success of graphene. To overcome the difficulties of zero band gap in graphene electronics, we report the fabrication of a electric double layer (EDL) transistor, an variation of FET based on another layered material, MoS2. Using strong carrier tunability found in EDL coupled by ion movement, MoS2 transistor displayed an unambiguously ambipolar operation in addition to its commonly observed n-type transport. A high on/off ratio >104, large “ON” state conductivity of ~mS, and high reachable n2D ~ 1×1014 cm-2 confirmed the high performance transistor operation important for application. The high-density carriers of both holes and electrons can drove the MoS2 channel to metallic states indicating that new electronic phases could be accessed using the protocol established in making EDL gated transistors on layered materials.
9:00 AM - O3.24
Enhanced Carrier Mobility in High Dielectric 2D Materials
Sivacarendran Balendhran 1 Jian Zhen Ou 1 Sumeet Walia 1 Jianshi Tang 2 Serge Zhuiykov 3 Sharath Sriram 1 Madhu Bhaskaran 1 Kourosh Kalantar-zadeh 1
1RMIT University Melbourne Australia2UCLA Los Angeles USA3CSIRO Highett AustraliaShow Abstract
The discovery of graphene in 2004 , has drawn the interests of both industries and the scientific community on two dimensional (2D) materials. Although the enhanced carrier mobility observed in graphene is highly desirable for electronic applications, the lack of intrinsic bandgap leads to the exploration of alternative 2D materials with a natural bandgap. Molybdenum disulphide (MoS2) is such a material with a direct bandgap of 1.9 eV in atomically thin layers . Thus far the highest reported carrier mobility in single layer MoS2 is ~217 cm2/Vs , in which Kis et al. employ a high dielectric (high-κ) top gate, to reduce coulomb scattering.
This work explores the capabilities of intrinsic high-κ materials as an alternative for achieving enhanced charge carrier mobilities. Molybdenum trioxide (MoO3) is one of the transition metal oxides that has a relative dielectric constant of ~500 (~ 5 for MoS2) and can be exfoliated to minimum resolvable atomically thin layers. But in its intrinsic nature, MoO3 has a wide bandgap (>3 eV) which is not viable for transistor applications. However the bandgap of MoO3 can be easily manipulated to desirable values by several techniques such as hydrogen ion (H+) intercalation, UV irradiation, electron beam bom-bardment etc. [3, 4]. Such techniques produce partially reduced sub-stoichiometric MoO(3-x) which has increased carrier concentration and a high dielectric value which highly favors the enhancement in charge carrier mobility.
In this work, charge carrier mobility in a 2D high-κ material is investigated. Field effect transistors (FET) based on MoO3 are fabricated. The MoO3 flakes were reduced to sub-stoichiometric MoO(3-x) in order to achieve bandgap values viable for transistor applications. Carrier mobilities > 1100 cm2/Vs were observed in a MoO(3-x) FET. The scattering mechanisms limiting the overall mobility with respect to temperature are explored. Theoretical and experimental mobilities are compared in low and high dielectric materials.
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9:00 AM - O3.25
Synthesis of Dielectric Thin Films on Graphene Using a h-BN Buffer Layer
Sang A Han 1 Kang Hyuck Lee 2 Jinyeoung Lee 1 Brijesh Kumar 2 Sang-Woo Kim 1 2
1Sungkyunkwan University Suwon-si Republic of Korea2Sungkyunkwan University Suwon-si Republic of KoreaShow Abstract
Graphene has a unique combination of electrical, mechanical, and optical properties, is being actively explored for future electronic applications. High intrinsic mobility in graphene combined with the ability to modulate the carrier in graphene based field effect transistors(FET), makes graphene a promising material for nano-electronic device. Especially, in order to make top gated-graphene FET devices, a high quality uniform high-k dielectric material should be deposited on graphene surface to realize a device of high breakdown voltage and low leakage current. Dielectric layer effectively control the charge carrier movement, therefore, it should be thin, uniform without any pinholes. In a few reports, gate dielectric was deposited on graphene by physical vapor deposition such as evaporator or sputtering. And atomic layer deposition (ALD) process also used for dielectric deposition by various treatment on graphene. Hence, in the present work we address the issue of uniform deposition of Al2O3, a high-k dielectric material for top gated-graphene FET device. We deposit high-k dielectric on CVD graphene using plasma treated hexagonal boron nitride layer (h-BN) as a buffer layer for high-k dielectric deposition. Dielectric layer was conformed with Raman Spectroscopy, AFM measurement, XPS, and TEM. And graphene based top gated FET device also fabricated. We expect that h-BN exploits graphene&’s superior electronic properties through eliminating charge traps, impurity and charge carrier inhomogeneity at the graphene/h-BN-Al2O3 interface.
9:00 AM - O3.26
Inorganic Biphenylene Analogous: A New Two-Dmensional Boron Nitride Planar Structure
Gustavo Brunetto 1 Eric Perim 1 Pedro Alves da Silva Autreto 1 Ricardo dos Santos 2 Douglas Galvao 1
1Unicamp Campinas Brazil2UNESP Rio Claro BrazilShow Abstract
Due to its unique and remarkable properties graphene is considered a very promising material for a new electronics. However, in its pristine form graphene is a zero bandgap semiconductor which poses serious limitations in done transistors applications. Because of that there is a renewed interest in other possible non-zero bandgap similar structures.
One of this structures is the biphenylene carbon (BPC)  (also known as graphenylene ), a bidimensional pure carbon structure with a bandgap about 0.8 eV. Recently, it was proposed  that the BPC can be obtained from the selective dehydrogenation of porous graphene .
In this work we propose a new two-dimensional boron nitride structure which we called inorganic BPC. Our ab initio molecular dynamics showed that similarly to the BPC case if can be spontaneously formed from the dehydrogenation of porous boron nitride sheets. It also important to mention that the inorganic BPC is thermally stable even at high temperatures. Its unique electronic and mechanical properties are also addressed.
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O1/P6: Joint Session: Graphene and Beyond Graphene
Joshua A. Robinson
Wednesday AM, April 03, 2013
Moscone West, Level 2, Room 2010-2012
9:30 AM - *O1.01/P6.01
Layered Nanostructures - Electronic and Mechanical Properties
Gotthard Seifert 1 Andrey Enyashin 3 1 Tommy Lorenz 1 Alessandro Pecchia 2
1TU Dresden Dresden Germany2University of Rome Rome Italy3Institute of Solid State Chemistry Ekaterinburg Russian FederationShow Abstract
In addition to Graphene 2D transition metal chalcogenide, as for example MoS2 and WS2, nanostructures are promising materials for applications in electronics and mechanical engineering. Though the structure of these materials results in a highly inert surface with a low defect concentration, defects and edge effects can strongly influence the properties of these nanostructures. Therefore, a basic understanding of the interplay between electronic and mechanical properties and the influence of defects, edge states and doping is needed.
We demonstrate on the basis of atomistic quantum mechanical simulations of several types of MoS2 nanostructures how the topology, the edge structure, local defects, dislocations, doping and mechanical deformation can vary the mechanical behavior, the electronic properties, and also determine the electronic device characteristics of such systems.
10:00 AM - *O1.03/P6.03
Tuning Electronic and Optical Properties of 2D Materials
Kirill Bolotin 1
1Physics Dept, Vanderbilt Univeristy Nashville USAShow Abstract
In this talk, we will discuss the approaches to control electronic and optical properties of two-dimensional materials, graphene and monolayer molybdenum disulfide (MoS2).
First, we address the possibility of controlling the effective dielectric constant k of 2D materials by fabricating graphene or MoS2 devices that are suspended inside liquids ranging from hexane (k~1.9) to water (k~80). With increasing k, we observe robust signatures of screening of electrostatic interactions between charge carriers in both graphene and MoS2. For graphene, we observe a rapid increase of carrier mobility with k, with room temperature mobility reaching 60,000 cm2/Vs for high-k liquids. For MoS2, increasing k causes an upshift in photoluminescence energy, consistent with decreasing binding energy of excitons in this material.
Second, we investigate the influence of uniform mechanical strain fields on both monolayer MoS2 and graphene. In suspended graphene, the presence of strain quenches flexural phonons, a strong scatterer of charge carriers. We observe an experimental signature of this quenching -- room temperature resistance of graphene decreasing with strain. In monolayer MoS2, we observe large changes in intensity and positions of photoluminescence peaks. These changes are consistent with predicted transition from direct to indirect band gap character in this material at strain levels of ~2%.
10:30 AM - *O1.04/P6.04
Graphene and The Advent of Other Layered 2D Materials for Nanoelectronics, Photonics and Related Applications
Anupama Kaul 1
1National Science Foundation Arlington USAShow Abstract
It is well known that carbon-based nanomaterials such as graphene and carbon nanotubes exhibit remarkable mechanical, electrical, thermal and optical properties which has stirred a great deal of excitement for considering them for a wide variety of applications ranging from nanoscale transistors, interconnects, field-emission displays, photo-voltaics and nano-electro-mechanical-systems (NEMS). The investigation of graphene as a model 2D system has impacted a diverse array of fields spanning physics, chemistry, materials science, and engineering. While great strides have been made recently for applications that have stemmed from graphene&’s unique properties, in electronics applications, specifically digital electronics, the absence of a band-gap in graphene poses concerns for its attractiveness to enable high ON/OFF ratios. Although a band-gap in graphene is induced through quantum confinement by creating graphene nanoribbons, the band gaps nonetheless are small (few hundred meV) and it is challenging to maintain pristine edge chirality due to defects that are induced during nanofabrication of the ribbons. Recently, layered 2D crystals of other materials similar to graphene have been realized which include insulating hexagonal-BN (band gap ~5.5 eV) and transition metal di-chalcogenides which display properties ranging from superconducting NbS2 to semiconducting MoS2. In addition, it has been shown that bulk MoS2 films transform from an indirect band-gap semiconductor with a band gap of ~1.2 eV to a direct band gap semiconductor with a gap of ~1.8 eV for single atomic layers. The device applications of such systems show promising characteristics; for example transistors derived from 2D monolayers of MoS2 show ON/OFF ratios many orders of magnitude larger than the best graphene transistors at room temperature, with comparable mobilities. In this talk, I will provide an overview of the Electronics, Photonics and Magnetic Devices (EPMD) program in the Electronics, Communications and Cyber Systems (ECCS) division where graphene, as well as other layered 2D nanomaterials, are playing an important role for enabling innovative device applications in electronics, photonics and sensing.
11:30 AM - *O1.05/P6.05
Two-Dimensional Early Transition Metal Carbides and Carbonitrides (MXenes)
Michael Naguib 1 2 Olha Mashtalir 1 2 Michel W. Barsoum 1 Yury Gogotsi 1 2
1Drexel University Philadelphia USA2Drexel University Philadelphia USAShow Abstract
Atomically thin layers can be formed by carbon, BN, metal chalcogenides, oxides and hydroxides, but no carbides have been reported to exist in 2-D form until recently. Transition metal carbides are known for many outstanding properties, but their strong bonds render their exfoliation into 2-D layers quite difficult. Nevertheless, we have synthesized a new family of early transition metals carbides in 2-D form, starting with the MAX phases. The latter is a large family (+60 members) of layered hexagonal ternary metal carbides and/or nitrides, where “M” stands for an early transition metal, “A” stands for a group 13 to 16 element, and “X” stands for carbon and/or nitrogen. Etching the “A” layer from MAX phase using hydrofluoric acid or other etchants at room temperature results in weekly bonded Mn+1Xn layers that can be separated by sonication yielding 2-D layers. To emphasize their similarity to graphene, we labeled them MXenes. The following compounds have already been synthesized: Ti2C, Ti3C2, Nb2C, Ta4C3, TiNbC, (V0.5Cr0.5)3C2, and Ti3CN. A review of the published information about synthesis and properties of MXenes will be presented. DFT simulations showed that their band gaps can be tuned by changing their surface termination, and that their elastic properties along the basal planes are superior to their 3-D binary counterparts. MXene powders are ductile enough to be cold pressed into free-standing discs with conductivities comparable to graphite. Water contact angle measurements showed hydrophilic behavior due to oxygen or OH termination of MXene surfaces. Among a multitude of possible applications, herein we focus on electrical energy storage systems, such as lithium ion batteries, lithium ion capacitors and electric double layer capacitors. The simplicity of the synthesis process enables large-scale production of MXenes in quantities from grams to kilograms.
12:00 PM - *O1.06/P6.06
Electronic Properties and Synthesis of Twisted Bilayer WS2
Humberto Terrones 1 Ana Laura Elias 1 Nestor Perea-Lopez 1 Humberto Rodriguez-Gutierrez 2 Ayse Berkdemir 1 Andres Castro-Beltran 1 Ruitao Lv 1 Florentino Lopez-Urias 1 3 Takuya Hayashi 4 5 Yoong Ahm Kim 4 5 Morinobu Endo 4 5 Mauricio Terrones 1 5 6
1Pennsylvania State University University Park USA2University of Louisville Louisville USA3IPICYT San Luis Potosi Mexico4Shinshu University Nagano Japan5Shinshu University Nagano Japan6Pennsylvania State University University Park USAShow Abstract
Besides graphene and hexagonal boron nitride, transition metal chalcogenides (TMC) such as MoS2, WS2, NbS2 and WSe2 also exhibit a layered structure in which the layers weakly interact via Van der Waals forces, and for this reason these materials exhibit excellent lubrication properties. For TMC, the layers are formed by the transition metal atom sandwiched by the sulfur atoms. MoS2 and WS2 in bulk are indirect band gap semiconducting materials. However, an isolated sheet of MoS2 or WS2 becomes a direct gap semiconductor . This particular behavior makes them very attractive in terms of optical properties such as spin polarization, in which the lack of center of inversion of one layer plays a crucial role [2,3]. Therefore, it is important to study the properties of different configurations of bi-layer TMC systems, generated by displacing or rotating one layer with respect to the other, thus breaking the inversion symmetry of the bulk stacking. In order to first assess this issue, Density functional theory (DFT) calculations were carried out for different bilayer WS2 geometries considering different rotation angles and different displacements. It was found that for particular geometries of the bilayer systems, the indirect and direct band gaps “compete”, thus exhibiting different electronic and optical properties. As in graphene, Raman spectroscopy provides relevant information about the different layer stackings and twistings in these materials . From the experimental standpoint, we will demonstrate that chemical vapor deposition (CVD) routes could result in the growth of TMC twisted layers, thus making this technique adequate to study the optical properties of new configurations experimentally. We will discuss these issues and also possible applications of these twisted bi-layers.
1.- Fai Mak, K., Lee, C., Hone, J., Shan, J., Heinz, T.F., Physical Review Letters, Vol. 105, 136805 (2010).
2.- Zeng, H., Dai, J., Yao, W., Xiao, D., Cui, X., Nature Nanotechnology, Vol. 7, 490-493 (2012).
3.- Fai Mak, K., He, K., Shan, J., Heinz, T.F., Nature Nanotechnology, Vol. 7, 494-498 (2012).
4.- Sato, K., Saito, R.,Cong,C., Yu, T., Dresselhaus, M.S., Physical Review B, Vol. 86, 125414 (2012).
12:30 PM - O1.07/P6.07
Germanium Graphane Analogues
Josh Goldberger 1 Sheneve Butler 1 Elisabeth Bianco 1
1The Ohio State University Columbus USAShow Abstract
Graphene's success has shown that it is not only possible to create stable, single-atom thick sheets from a crystalline solid, but that these materials have fundamentally different properties than the parent material. In this talk, we will discuss our recent results on the creation of two-dimensional single atom thick hydrogen-terminated and organic-terminated germanium (GeR) analogues of graphane (CH). Contrary to the indirect gap of Ge at 0.67 eV, these materials have direct band gaps centered at 1.5 eV for GeH, and are tunable depending on the nature of the surface ligand. These materials represent a new class of covalently-terminated single atom thick derivatives of Group 14 semiconductors, and have great potential for a wide range of optoelectronic and sensing applications.
12:45 PM - O1.08/P6.08
Synthesis of Copious Amounts of SnS2 and SnS2/SnS Ordered Supertructure Nanotubes
Gal Radovsky 1 Ronit Popovitz-Biro 1 Matthias Staiger 2 Christian Thomsen 2 Konstantin Gartsman 1 Tommy Lorenz 3 Gotthard Seifert 3 Reshef Tenne 1
1Weizmann Institute Rehovot Israel2Technische Universitamp;#228;t Berlin Germany3Technische Universitaet Dresden Dresden GermanyShow Abstract
SnS2 and SnS2/SnS superlattice nanotubes were synthesized in copious amounts using both sealed ampoules and a horizontal/vertical flow systems with Bi and Sb2S3 as catalysts . The outer diameter of the tubes ranged from 13 to 165 nm, and their length varied from 90 nm to 3.2 micrometers. Careful examination in HRTEM revealed that most of the tubular structures consisted of layers showing periodic or almost periodic patterns which can be interpreted as a superstructure of SnS2 and SnS layers, whereas others exhibited evenly spaced fringes of SnS2. This observation was further confirmed by Raman measurements of individual nanotubes after the examination in HRTEM.
Chemical profiling showed a constant profile of the catalysts along the tube axis. It is believed that the high sulfur affinity of Bi causes partial decomposition of the pseudo hexagonal SnS2 precursor producing the sulfur deficient SnS and the misfit layered tubular structures of a type (SnS)n(SnS2)m. The scrolling process was promoted by adding miniscule amounts of Sb2S3 powder to the ampoules.
The tubular morphology of the SnS2/SnS ordered superstructure nanotubes is a result of the lattice mismatch between the two sublattices comprising the host-guest structure. This driving force comes in addition to the already established closure mechanism, i.e., annihilation of dangling bonds at the periphery of the layers of the inorganic nanotubes (INT) nanostructures.
The diversity of the structures manifests itself through different stacking orders of SnS2 and SnS layers along their common c-axis and their relative in-plane orientation. Folding vectors and chiral angles of both subsystems can be determined .
The scaling-up of the nanotubes synthesis was accomplished in a horizontal/vertical flow reactor. These reactors have produced 10-20 mg of either SnS2 or SnS/SnS2 ordered superstructure nanotubes phase in 30% yields.
1. G. Radovsky, R. Popovitz-Biro, M. Staiger, K. Gartsman, C. Thomsen, T. Lorenz, G. Seifert and R.Tenne, Angew. Chem, Intl. Ed. 50 (51), 12316-12320, (2011)
2. G. Radovsky, R. Popovitz-Biro, and R. Tenne, Chem. Mater. 24, 3004-3015, (2012).
Mauricio Terrones, The Pennsylvania State University
Pulickel M. Ajayan, Rice University
Reshef Tenne, Weizmann Institute of Science
Anupama Kaul, California Institute of Technology
Symposium Support Army Research Office
Materials Research Institute (Penn State) Center for 2-Dimensional and Layered Materials (Penn State)
National Science Foundation (NSF)
Pennsylvania State University
O5: Characterization of 2D-Layered Materials
Thursday PM, April 04, 2013
Moscone West, Level 2, Room 2009
2:30 AM - *O5.01
Properties and Applications of CVD-grown Graphene and Molybdenum Disulfide
James Hone 1 Arend van der Zande 1
1Columbia University New York USAShow Abstract
This talk will describe our ongoing work on understanding the properties of CVD-grown films of 2D electronic materials, in particular graphene and molybdenum disulfide, and demonstrating their potential in functional devices. We have reported that CVD-grown graphene can achieve electronic performance equivalent to that of exfoliated material1. In more recent work, we demonstrate that CVD graphene can also achieve mechanical strength equal or close to that of the pristine lattice. Moreover, we directly study the mechanical properties of grain boundaries and point defects to understand their role in determining the mechanical behavior of defective graphene. The superior electrical and mechanical properties of CVD graphene combine to enable graphene FETs with performance in the GHz regime at strains of close to 2 percent. We also have recently achieved CVD growth of highly crystalline molybdenum disulfide. We characterize its structural, optical, and electronic properties by a number of techniques. We are able to correlate the shape of crystallites to their crystal structure, and identify common grain boundary morphologies. These large single crystals allow for careful study of electronic transport, in particular separate treatment of contact effects and bulk mobility. Finally, this talk will briefly discuss new techniques for creating layered heterostructures of these materials, and some of their properties and applications.
1. Nicholas Petrone et al., Nano Letters 12, 2751 (2012).
3:00 AM - *O5.02
Order and Stacking of MoS2 and WS2 Layers by HRTEM
Sagi Appel 1 Lothar Houben 2 Maya Bar-Sadan 1
1Ben Gurion University Be'er Sheba Israel2Juelich Research Center Juelich GermanyShow Abstract
The recent interest in layered compounds and especially in thin films of layered compounds is the focus of extensive research. The properties of single layers are calculated and measured when these layers are embedded within devices, while other applications require the use of thin films of a few layers. The layered structure, by definition, consists of anisotropy when comparing the chemical and physical properties within the layers and across them. In addition, in some devices edge effects produce even more complex systems.
It is therefore extremely important to characterize the basic features of the layers within the structure in order to predict and explain the measured properties and in order to design new applications. Important parameters include: determining the stacking of the layers one relative to the other and the induced disorder; Estimating the rotation of layers relative to each other; Imaging the atomic structure of the edges; Measuring physical properties by spectroscopy and associated them with local structural features.
Today, with the development of the high resolution electron microscope into a versatile experimental set-up, these issues can be addressed with unprecedented resolution. Recent results f MoS2 and WS2 will be presented and discussed.
3:30 AM - O5.03
Atomic Resolution Transmission Electron Microscopy of Interfaces in Chemical Vapor Deposition Hexagonal Boron Nitride
Ashley Gibb 1 2 Kris Erickson 1 Nasim Alem 2 Jim Ciston 3 A. Zettl 2
1UC Berkeley Berkeley USA2UC Berkeley Berkeley USA3Lawrence Berkeley National Lab Berkeley USAShow Abstract
Two-dimensional sp2-bonded hexagonal nanomaterials such as graphene and hexagonal boron nitride (h-BN) have received significant research interest due to their excellent mechanical strength, chemical inertness, stability, high thermal conductivity, and a range of electronic properties ranging from conducting to insulating. Understanding the atomic structure of defects in these materials is essential for their future use in devices and other applications. Prior Transmission Electron Microscopy (TEM) work has determined the structure of grain boundaries in CVD graphene, but the structure of many defects in CVD h-BN, which is molecularly isoelectronic with graphene, are still unknown. We have utilized atomic resolution TEM to image defects in CVD h-BN nanosheets, including vacancy defects as well as the interfaces and grain boundaries between adjacent domains.
3:45 AM - O5.04
Electrical Transport in Large Area CVD Grown MoS2
Sujoy Ghosh 1 Sina Najmaei 2 Zheng Liu 2 Robert Vajtai 2 Pulickel Ajayan 2 Saikat Talapatra 1
1Southern Illinois University Carbondale USA2Rice University Houston USAShow Abstract
Recently layered semiconductors based on metal dichalcogenide have drawn quite intense research focus. Among these mechanically exfoliated small flakes of monolayer and few layers of MoS2 were found to exhibit intriguing electronic properties such as high-performance field-effect transistors, electric field induced superconductivity, luminescence etc. These findings indicates that these materials can be potential candidate for future optoelectronic applications. However, for many applications availability of large area atomic layers of these materials are needed. In this regard the recent success in the growth of large area 2D MoS2 layers through chemical vapor deposition method is very encouraging and opens up a wide range of possibility for a variety of electronic applications. Here, we report on the electrical transport properties of large area few layers of MoS2 synthesized by chemical vapor deposition under the application of a D.C as well as an A.C bias. Our results indicate 2D Mott-type variable range hopping transport under D.C. bias. The frequency-dependent conductivity (<100 KHz) increases linearly with frequency. Detail analysis of the A.C. transport data show an evidence of universal A.C transport where all the measurements collapse into one single master curve with appropriate scaling. These results will be discussed within the framework of existing theoretical models and scaling theories.
This work is supported by the U.S. Army Research Office MURI grant W911NF-11-1-0362
4:30 AM - *O5.05
Atomically-layered Structures: Directed-assembly, Defect Structure and Transport Phenomena
Vinayak P. Dravid 1
1Northwestern University Evanston USAShow Abstract
Following the initial results on graphene, many other 2-D layered structures have been explored by researchers around the world. This has generated genuine excitement for interdisciplinary research given the intriguing geometric form(s), defect structures and potential for technologically useful properties anchored by the 2-D confinement effects in such atomically-layered systems.
The fundamental and technological opportunities for 2-D atomically-layered structures are plentiful but so are the scientific and engineering challenges. These require requisite control over synthesis (especially large-scale), lateral conformation of layers (layer size), site-specific patterning and solution-based assembly as well as unambiguous measurements of their true intrinsic properties - all mediated by unconventional defect structure in atomically-layered systems.
Our efforts in this rapidly evolving field are focused on synthesis, assembly and atomic-scale characterization of nanoscale versions of these layered systems, and exploration of their self-assembly and intrinsic transport properties. On one front, nanoscale version of graphene oxide (nGO) is synthesized that is amenable to functionalization which provides exquisite control over self- and supramolecular assembly. Such assemblies have been shown to exhibit excellent sensing behavior for biomolecular structures, such as protein identification. On another front, have developed surface functionalization strategies of atomically-layered chalcogenides (e.g., MoS2) that allow for colloidal stability of atomic sheets for further processing, patterning and measurements. We have developed and introduced classical optical and Raman characterization methods for rapid identification of atomically-layered chalcogenides that are sensitive to individual sheet number, thereby expanding the structural “palette” for exploration of intrinsic properties of single, double and multiple atomic-layered sheets (and heterostructures) with single-layer discrimination. We have fabricated transistor geometries to assess the intrinsic semiconducting characteristics of several chalcogenides and explored their sensitivity to gas and chemical interactions. The initial findings point to truly unusual behavior of not only single and isolated atomic layers but their multiple layer counterparts. The presentation will cover ongoing research focused on isolation and characterization of single and multiple atomic sheets of nGO and chalcogenides; the unusual defect structures as well as their colloidal assembly and transport properties.
5:00 AM - O5.06
Edges, Mirrors, and Grain Boundaries in Monolayer Large-grain CVD MoS2
Pinshane Y Huang 1 Arend M. van der Zande 2 Daniel A. Chenet 3 Timothy C. Berkelbach 4 David R. Reichman 4 James Hone 3 2 David A. Muller 1
1Cornell University Ithaca USA2Columbia University New York USA3Columbia University New York USA4Columbia University New York USAShow Abstract
Characterizing defects and crystal structure is critical for controlling quality and materials properties in the expanding array of 2D materials growths. In particular, while defects such as grain boundaries have dramatic effects on the electrical and mechanical properties of graphene (1), much less is known about the atomic structure of these defects for other 2D materials such as MoS2. MoS2 has received recent attention as a 2D direct band-gap semiconductor suitable for building electronics from layered materials (2). Here, we present an in-depth study of the structure of monolayer MoS2 grown by chemical vapor deposition (CVD) using a combination of atomic-resolution scanning transmission electron microscopy (STEM) and dark-field TEM (DF-TEM).
First, we demonstrate that with CVD growth on SiO2 substrates, (3) we can achieve single crystals of monolayer 2H MoS2 up to 100 microns across. The edges of these CVD MoS2 crystals are strongly faceted along 2 distinct zig-zag [-1010] directions: the S-terminated and Mo-terminated faces. By correlating the 3-fold symmetry of the monolayer MoS2 diffraction pattern (4) to the shape of MoS2 single crystals, we further demonstrate that the orientation and edge termination of MoS2 crystals can frequently be approximately deduced from their shapes. In polycrystalline films, we use DF-TEM (1) to show that both overlapping and atomically abrupt rotational grain boundaries form with grain sizes from 1-100 micron scales, and that the sizes and types of boundaries present depend strongly on growth conditions.
Importantly, we report the discovery of a common line defect in CVD MoS2 systems—mirror twin boundaries. In monolayer MoS2, mirror twins are the intersections of two MoS2 crystals with a relative in-plane rotation of 60 (or 180) degrees, a half-unit cell lattice shift that effectively swaps the positions of Mo and S sites. We show that, while these boundaries can be easily missed in a simple diffraction analysis, mirror twins appear clearly in DF-TEM. Atomic-resolution STEM images of the twin boundary show that the line defect is formed primarily from 4 and 8-membered rings that avoid homo-elemental bonds, but require changes in atomic bonding states. The MoS2 twin boundary thus contrasts with twin (5) and rotational boundaries (1) in graphene, which are more commonly formed by 5- and 7- membered rings without dangling bonds. In summary, our large-grain, high-quality growths and analysis of atomic structure of edges, grain boundaries, and twin boundaries in CVD MoS2 demonstrates the immense utility in applying TEM techniques to further the growth and understanding of CVD materials beyond graphene.
1. P. Y. Huang et al., Nature 469, 389-392 (2011).
2. B. Radisavljevic, et al., Nat. Nanotechnol. 6, 147-150 (2011).
3. Y.-H. Lee et al., Adv. Mater. 24, 2320-2325 (2012).
4. J. Brivio et al., Nano Lett. 11, 5148-5153 (2011).
5. J. Lahiri, Y. et al., Nat. Nanotechnol. 5, 326-329 (2010).
5:15 AM - O5.07
Tunable Metamaterials Based on 2D Layered Chalcogenides
Judy J Cha 1 Kristie Koski 1 Kevin Huang 1 Ken Wang 1 Weidong Luo 1 Desheng Kong 1 Zongfu Yu 1 Shanhui Fan 1 Mark L Brongersma 1 Yi Cui 1 2
1Stanford University Stanford USA2SLAC National Accelerator Laboratory Menlo Park USAShow Abstract
Exploring new plasmonic materials with functionally more diverse and potentially superior properties compared to the popular noble metals such as gold and silver is actively ongoing. Correspondingly, two-dimensional (2D) conductors such as graphene have gained great interest as new plasmonic materials . Here, we demonstrate that 2D layered metal chalcogenides such as Bi2Se3 and Bi2Te3 in the form of ultrathin nanoplates  can be tunable metamaterials that can be engineered at atomic-scale. The anisotropic 2D bonding nature and the van der Waals gap present new ways to tune the plasmonic and photonic modes of the chalcogenide nanoplates.
Monochromated electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) has proven to be a powerful technique to study the local spatial variations of plasmons of nanostructures, as demonstrated by the plasmon mappings of silver triangular nanoplates  and plasmon resonances of silver nanoparticles [4,5]. EELS can also probe photonic density of states [6,7]. Using STEM-EELS, we examine and directly visualize the photonic and plasmonic modes that are supported in the chalcogenide nanoplates. We show that these modes can be widely tuned by various methods such as geometric control and compositional tuning at nano- and atomic-scale. The wide tuning range and the co-existence of the photonic and plasmonic modes afforded in these metal chalcogenide nanoplates will be useful for various applications.
 Jablan, M., Buljan, H., and Solja#269;icacute;, M., Phys. Rev. B 80, 245435 (2009).
 Kong, D. et al., Nano Lett. 10, 2245 (2010).
 Nelayah, J. et al., Nature Phys. 3, 348 (2007).
 Ouyang, F., Batson, P. E., and Isaacson, M., Phys. Rev. B 46, 15421 (1992).
 Scholl, J. A., Koh, A. L., and Dionne, J. A., Nature 483, 421 (2012).
 García de Abajo, F. J., Rev. Mod. Phys. 82, 209 (2010).
 Cha, J. J. et al., Phys. Rev. B 81, 113102 (2010).
5:30 AM - O5.08
Covalent Radical Functionalization of Boron Nitride Nanosheets
Toby Sainsbury 1 Jonathan Coleman 1
1Trinity College Dublin Dublin IrelandShow Abstract
Hexagonal Boron-Nitride (h-BN) has been identified as an extremely attractive material for multiple technological applications due to its exceptional mechanical, thermal and electronic properties. The highly stable and mechanically strong structure of hexagonal boron nitride exhibits extremely high chemically stability and resistance to oxidation until temperatures of over 800 oC thus making h-BN highly attractive for a range of applications in the bulk-phase where chemical stability and structural integrity are demanded. It is therefore particularly challenging to initiate chemical reactions involving atoms in the bulk h-BN lattice whilst retaining the intrinsic properties of the material. It is apparent that for the application of exfoliated h-BN nanosheets, novel chemical strategies will be necessary in order to integrate and utilize BNNSs in applications such as surface coatings and nanocomposites.
Here we present the covalent functionalization of exfoliated h-BN nanosheets (BNNSs) via solution phase radical addition chemistry. We present functionalization approaches which involve the generation of reactive radical species within a solution of exfoliated BNNSs. In one case, we describe the covalent functionalization of BNNSs via the addition of Nitrene radicals to B atoms within the h-BN sp2 hybridized lattice. We demonstrate the modification of the surface chemistry of BNNSs by forming polycarbonate analogue composites with the modified BNNSs. We also extend this functionalization methodology to covalently bind polycarbonate analogue chains directly to the BNNSs.
In a second case, we present the covalent functionalization of BNNSs via oxygen radicals. In a two-step procedure we demonstrate the oxygen radical functionalization of BNNSs via the thermolysis of an organic peroxide molecule in the presence of exfoliated BNNSs. Oxygen radical functionalization results in the grafting of organic butoxy groups grafted to boron atoms in the h-BN sp2 hybridized lattice. The successful grafting of the organic functional groups to the BNNSs is then followed by the hydrolytic de-functionalization of the groups to yield hydroxyl groups covalently bound to the B atoms in the lattice. The hydroxyl functionalized BNNSs (OH-BNNSs) are then demonstrated to exhibit hydrophilic character and are further chemically derivitized via carbamate linkages to form borocarbamate functional groups. Modification of the surface chemistry of the BNNSs based on the hydroxyl functional groups is utilized in order to form BNNS-polymer composites which are mechanically analyzed.
Tuning the chemistry of h-BN in these ways demonstrates the rational design of these 2-D nanosheets in order to enhance the integration and chemical compatibility within polymer matrices. This in turn provides the basis for enhanced pathways for mechanisms of stress transfer to optimize the efficient reinforcement of next generation of polymer nanocomposites.
5:45 AM - O5.09
Growth Studies of Thin h-BN and Stacked h-BN/Few-layer Graphene Films: Towards Large-scale Functional Devices
Ariel Ismach 1 Harry Chou 1 Xiangke Cao 2 Richard Piner 1 Shaul Aloni 3 Jingyu Lin 2 Hongxing Jiang 2 Luigi Colombo 4 Rodney S. Ruoff 1
1The University of Texas at Austin Austin USA2Texas Tech University Lubbock USA3Lawrence Berkeley National Laboratory Berkeley USA4Texas Instruments Dallas USAShow Abstract
We present chemical vapor deposition methodologies for the controlled synthesis of mono- and few- layer h-BN and the sequential growth of stacked layers of h-BN/few-layer graphene. Growth of such thin h-BN films was achieved using solid (ammonia-borane) and gaseous (ammonia and diborane) precursors. The growth rate and its mechanism(s) were studied with the “stop-growth” method in which the reaction is stopped after running under various conditions, and then the resulting films are characterized by Raman spectroscopy including micro-Raman mapping spectroscopy, by scanning and transmission electron microscopy (SEM, TEM), atomic force microscopy (AFM), and photo and cathode-luminescence. Emphasis was given to the growth methods that yielded large h-BN domain size (up to hundreds of microns), as one of the most important parameters that typically has a strong influence on the electronic, mechanical, thermal, chemical, and optical properties of polycrystalline thin films is the grain size.
The direct synthesis of h-BN/few-layer graphene stacked layers was done by a combination of deposition of h-BN on a metal substrate enriched with carbon in its bulk, followed by the controlled segregation of C to the h-BN-metal interface. 13C-enriched methane was used as the carbon source to minimize overlap between the graphene Raman D-band (red-shifted about 50 cm-1 by the use of 13C) and the h-BN Raman band. We expect to be able to present the assembly of mono and few-layer graphene/h-BN stacked films into functional devices at the conference.
Acknowledgements: We appreciate support from the Army Research Office (grant W911NF1010428) and the NRI SWAN. Work at TTU was supported by DOE under Contract No. FG02-09ER46552. H.X.J. and J.Y.L. would like to acknowledge the support of Whitacre endowed chair positions through the ATT foundation.
O4: Transport in 2D-Layered Materials
Thursday AM, April 04, 2013
Moscone West, Level 2, Room 2009
9:00 AM - *O4.01
Quasi-2D Correlated Oxide Membranes: Transistors and Sensor Devices
Shriram Ramanathan 1
1Harvard University Cambridge USAShow Abstract
Freestanding oxide layers approaching two-dimensionality allow us to probe the fundamentals of carrier transport in confined structures and the role of surface proximity in influencing conductance transitions. They also serve as useful test structures to probe emergent phases under high electric fields. The role of space charge boundary zones may also become more pronounced in such layers and in turn may be of interest in design of low power sensors and field effect structures. I will present our on-going efforts in this area with emphasis on two classes of oxide materials: one that displays metal-insulator conductance transition by electric bias such as vanadium dioxide and the other related to ionic-electronic conductance transitions induced by chemical doping. Especially for transition metal oxides that display strong electronic correlations, point defect induced conduction or local stoichiometry changes becomes crucial in decoupling the physics from electrochemically-induced processes. Particular emphasis will be on experimental methods to fabricate quasi-2D oxide membranes and building integrated devices with such layers, including field effect transistors and sensor devices. Given the typical stress relaxation that occurs during phase transitions (especially during structural transitions or lattice distortion during redox processes involving oxygen exchange), one has to fabricate suspended structures with great care to retain mechanical integrity and we will consider these in detail. We will then present experimental efforts to investigate transistor action in such ultra-thin self-supported membranes and the role of surface proximity. Custom instrumentation to study the properties of such ultra-thin membranes will also be briefly discussed.
O6: Poster Session: Characterization and Optical Properties of 2D-Layered Materials
Pulickel M. Ajayan
Thursday PM, April 04, 2013
Marriott Marquis, Yerba Buena Level, Salons 7-8-9
9:00 AM - O6.01
Half-metallic One-Dimensional Electron Gases at Zigzag Interfaces of Two-Dimensional Honeycomb Insulators
Miguel Pruneda 1 Nicholas C. Bristowe 2 3 Massimiliano Stengel 7 8 Peter B. Littlewood 2 6 Emilio Artacho 2 4 5
1CIN2 (CSIC) Bellaterra Spain2Cavendish Laboratory, University of Cambridge Cambridge United Kingdom3Universite de Liege Sart-Tilman Belgium4CIC Nanogune and DIPC San Sebastian Spain5Barque Foundation for Science Ikerbasque Bilbao Spain6Argonne National Laboratory Argonne USA7ICREA - Institutio Catalana de Recerca i Estudias Avanamp;#231;ats Barcelona Spain8ICMAB (CSIC) Barcelona SpainShow Abstract
Insulating compounds that are isostructural to graphene may have a significant degree of ionic character. This brings up the important question of whether "polar discontinuities''
might play a role in these low-dimensional systems. Here, we perform extensive density-functional theory calculations for coplanar zigzag interfaces between II-VI, III-V and IV-IV two-dimensional honeycomb insulators. Our results confirm that these interfaces are polar, with a net charge of excess charge that, according to the interface theorem, is uniquely determined from bulk properties of the participating materials, and is related to the nontrivial topology of their respective polarization lattice. It is shown that, in the limit of increasingly wide domains, the logarithmically divergent electrostatic potential produced by the lines of charge triggers a Zener-like breakdown mechanism not dissimilar to what happens at insulating oxide interfaces. Such mechanism gives rise to spin-polarized one-dimensional electron/hole gases.
9:00 AM - O6.02
Self-focusing and Light Deflection by a Transition Metal Dichacogenides Van Der Waals Solid
Carlos Torres-Torres 1 Nestor Perea-Lopez 2 Ana Laura Elias-Arriaga 2 Humberto Rodriguez-Gutierrez 3 Ayse Berkdemir 2 Florentino Lopez-Urias 4 Humberto Terrones 2 Mauricio Terrones 2 3 5
1Instituto Politamp;#233;cnico Nacional Mexico Mexico2Pennsylvania State University University Park USA3University of Louisville Louisville USA4IPICyT San Luis Potosamp;#237; Mexico5Shinshu University San Luis Potosamp;#237; JapanShow Abstract
Theoretical and experimental results about the nonlinear optical response of a transition metal dichacogenides Van Der Waals solid are presented. A two-wave mixing configuration with nanosecond pulses at 532 nm wavelength was employed in order to study the third order optical nonlinearities of the samples. Strong optical Kerr effects were observed by intense optical excitation with and without the contribution of nonlinear optical absorption. An electronic response together with thermal effects was identified as the dominant physical mechanism responsible for the resulting nonlinear refractive index. Using a vectorial nanosecond technique, we were able to control the modification of the optical phase and the self-focusing effect associated with one of the interacting beams in propagation through the samples. This control of the optical phase and the deflection of the signal beam can be achieved by the separate excitation of the nonlinear refracting phenomena exhibited by the samples with large sensibility to the polarization of the optical pumping in a multi-wave mixing experiment. Our results indicated that the nonlinear refractive index and the optical beam direction can be simultaneously tailored by an electronic nonlinear refractive index, and by a thermal effect, both resulting from optical irradiation excitation. The different participation of the Kerr nonlinearity related to the samples seems to promise potential application for developing laser-induced controlled ultrafast devices.
C. Torres-Torres acknowledges the financial support from the Instituto Politécnico Nacional
9:00 AM - O6.04
Selective Polyester Polymer Transfer of Exfoliated h-BN for Graphene Devices
Alice Lay 1 2 Jianhao Chen 1 2 A. Zettl 1 2
1University of California, Berkeley Berkeley USA2Center of Integrated Nanomechanical Systems Berkeley USAShow Abstract
Without an efficient method to prepare thin layer hexagonal boron nitride (h-BN), research of h-BN devices and applications have been limited compared to its sister nanomaterial, graphene. This is unfortunate because h-BN is a promising substrate for graphene due to its flatness and analog hexagonal structure. We investigated a systematic method to select and transfer a thin h-BN flake from one substrate onto another using a polyester polymer. This flexible method not only allows researchers to pick particular flakes and thicknesses of h-BN from exfoliation, but also reduces surrounding tape residue and flakes. Optical microscopy and atomic force microscopy confirmed 2-30nm thick h-BN flakes after tape method exfoliation. The polymer transfer process results in a thin coating with 20-30nm particles over the target flake. Annealing the sample in hydrogen significantly reduces polymer residue. Overall, this new process has the potential of adding user control to the preparation of cleaner thin layered h-BN for use in devices.
9:00 AM - O6.05
Complementary In-situ Probing (ESEM, XPS and XRD) of 2D Nanostructures during CVD
Piran Ravichandran Kidambi 1 Bernhard C Bayer 1 Raoul Blume 2 Zhu-Jun Wang 2 Marc Willinger 2 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
Growth mechanisms of 2D nanostructures (eg: graphene, h-BN) have so far been described by rather simplistic models of elemental solubility of the constituent elements (eg: C, B etc.) in metallic catalysts eg: Ni (high solubility - precipitation from bulk) and Cu (low solubility - surface reaction). These models are speculated based on ex-situ experiments but in-situ experimental evidence remains elusive.
Using a combination of environmental scanning electron microscopy (ESEM), high-pressure depth and time resolved in-situ X-ray photoelectron spectroscopy (XPS)[2,3] and in-situ X-ray diffraction (XRD)[2,3] at realistic CVD conditions, i.e. pressure (~0.001 - 0.5 mbar) and extreme temperatures (800-1000oC) we study the behaviour of polycrystalline transition metal catalyst films during 2D nanostructure growth. These experiments allow us to study the elemental incorporation in the 2D nanostructure on the catalyst surface as it happens by identifying the state of the catalyst and the elemental species at any point of time during CVD. The growth of 2D nanostructures is actually found to be isothermal along with some precipitation for both Ni and Cu, i.e. we observe catalyst behaviour during growth as a complex interplay of isothermal and precipitation based growth mechanisms with kinetic effects playing an important role.
We highlight the use of this approach by proposing a generic framework to study the growth of 2D nanostructures during CVD which allows for the development of a comprehensive understanding of the fundamental growth mechanisms based on direct in-situ evidence avoiding the need for any speculation.
1. Kidambi et al. J. Phys.Chem. C. 116, 42, 22492-22501 (2012).
2. Kidambi et al. (in preparation)
3. Weatherup et al. Nano Letters. 11, 4154 (2011).
9:00 AM - O6.07
Chemical Vapor Deposition of Ultra-thin Hexagonal Boron Nitride Films for Integration with Graphene and Nanomechanical Structures
Michael S. Bresnehan 1 2 3 Matthew J. Hollander 2 3 4 Maxwell Wetherington 1 2 3 David W. Snyder 3 5 Joshua A. Robinson 1 2
1The Pennsylvania State University University Park USA2The Pennsylvania State University University Park USA3The Pennsylvania State University University Park USA4The Pennsylvania State University University Park USA5The Pennsylvania State University University Park USAShow Abstract
Recently, hexagonal boron nitride (h-BN) has attracted increased interest as a dielectric material to graphene electronics. Traditional dielectrics, such as SiO2 or various high-k materials, can introduce scattering from charged surface states, impurities, surface optical phonons, and substrate roughness; significantly degrading the transport properties of graphene. [1, 2] Hexagonal boron nitride boasts several key advantages over SiO2 and high-k dielectrics. Most notably, is an atomically smooth surface that is expected to be free of dangling bonds, leading to an interface that is relatively free of surface charge traps and adsorbed impurities. Additionally, h-BN&’s high energy surface optical phonon modes lead to reduced phonon scattering from the dielectric. Using h-BN (grown via CVD on copper foil) as a gate dielectric to epitaxial graphene devices, we report a >2.5x increase in intrinsic current gain cut-off frequency and a >3x increase in mobility over HfO2 gated devices. However, small domain sizes and surface defects resulting from the h-BN CVD growth process result in a rough film morphology (RMS roughness > 1nm) and significant leakage currents (VBreakdown < 1V) for films <10nm thick.
Here, we present a detailed investigation of the CVD growth mechanisms of h-BN on various transition metals and show that the choice of substrate has a significant effect on the domain size and layer thickness of the h-BN films as determined via TEM. On Cu substrates, we find that after formation of a single monolayer, growth proceeds via nucleation of nano-domains over the original monolayer. This is due to the poor surface reactivity of borazine on h-BN compared to the Cu surface. We show that tailoring of the borazine precursor concentration (sublimed from ammonia borane) and use of a copper overpressure results in an increased domain size and a reduction of 3D islanding for h-BN on Cu substrates. Conductive AFM verifies a reduction of leakage current with increasing h-BN domain size, indicating that control of the domain size is required for scalable 2D h-BN dielectrics. Finally, we discuss the incorporation of these same h-BN films into nanomechanical structures. The vibrational response of nanomechanical resonators reveals the level of internal stress, as well as variations in mechanical properties, of the films as a function of growth conditions.
 Hollander M. J. et al. Enhanced Transport and Transistor Performance with Oxide Seeded High-k Gate Dielectrics on Wafer-Scale Epitaxial Graphene. Nano Lett. 2011, 11, 3601-3607.
 Robinson, J. A. et al. Epitaxial Graphene Materials Integration: Effects of Dielectric Overlayers on Structural and Electronic Properties. ACS Nano 2010, 4, 2667-2672.
 Bresnehan, M. S. et al. Integration of Hexagonal Boron Nitride with Quasi-freestanding Epitaxial Graphene: Toward Wafer-Scale, High-Performance Devices. ACS Nano. 2012, 6, 5234-5241.
9:00 AM - O6.08
Inversion of Electrical and Optical Properties in Acoustically Irradiated h-BN
Avinash Nayak 1 Andrei Dolocan 1 Jongho Lee 1 Li Tao 1 Hsiao-Yu Chang 1 Twinkle Pandhi 1 Milo Holt 1 Deji Akinwande 1
1UT Austin Austin USAShow Abstract
Hexagonal boron nitride (h-BN) is known to have a large optical band-gap and is considered an insulating material. Here, we find that altering pristine h-BN platelets via acoustic irradiation in deionized water changes the material from a white, near-UV bandgap electrical insulator, to a dark, electrically conducting material. A drastic optical band-gap reduction from 5.46eV to 3.97eV and a three order increase in conductivity is observed. We will present on the experimental study of the inversion in the material properties of h-BN, and elucidates the underlying mechanisms that result in fragmentation and partial oxidization of h-BN. This provides a facile avenue for the realization of two-dimensional advanced nanomaterials with tuned physical and chemical properties that depart from the intrinsic behavior of pristine h-BN. We employ ToF-SIMS, SEM-EDX, UV-Vis, Raman, TEM, XPS and XRD to examine the acoustically irradiated h-BN.
9:00 AM - O6.10
Polymer Reinforcement Using Liquid Exfoliated Boron Nitride Nanosheets
Umar Khan 1 Peter May 1 Jonathan N Coleman 1
1Trinity College Dublin Dublin IrelandShow Abstract
We have exfoliated hexagonal Boron Nitride (BN) by sonication in solutions of polyvinylalcohol (PVA) in water. The resultant nanosheets are sterically stabilised by adsorbed polymer chains. Such dispersions can be used to produce PVA-BN composite films. We find both the modulus, Y, and strength, σB, of these composites to increase linearly with volume fraction, Vf, up to Vf ~ 0.1vol% BN before falling off. The rates of increase are extremely high - dY/dVf = 670 GPa and dσB/dVf = 47 GPa. The former value matches shear lag theory modelling while the latter value is consistent with remarkably high polymer-filler interfacial strength. However, because the mechanical properties increase over such a narrow volume fraction range, the maximum values of both modulus and strength are only ~40% higher than the PVA itself. This phenomenon has also been observed for graphene-filled composites and represents a serious challenge in the production of high performance polymer-nanosheet composites.
9:00 AM - O6.11
Heterogeneous Boron Nitride & Graphene Oxide: A New 2D Material System
Maxwell Turner Wetherington 1 2 Ganesh Rahul Bhimanapati 1 Mike Kelly 1 2 Michael Bresnehan 1 2 Mohan Manoharran 3 Michael Lanagan 3 Joshua Robinson 1
1The Pennsylvania State University State College USA2Penn State Electro Optics Center Innovation Park USA3The Pennsylvania State University University Park USAShow Abstract
Providing a simple route towards achieving high quality optical and nano-mechanical material systems would be beneficial for many scientific and industrial research areas. One way to accomplish this is through the manipulation of 2-D material systems (Ref 1). Here we present the first process-property relationship study on the heterogeneous integration of hexagonal boron nitride (h-BN) and graphene oxide (GO), referred to as BoroCarbon OxyNitride (BCON).
Our group has developed a fundamental understanding of the chemical exfoliation process required for the heterogeneous integration of graphene oxide and hBN. We have found that forming a heterogeneous suspension between GO and h-BN is primarily dependent upon the pH of the GO material. Fourier transform infrared spectroscopic (FTIR) data indicated the degradation in the B-N-B bending of the daughter BCON solution as the pH of the parent GO solution decreased. This represents a reduction in particle isolation which results in agglomeration of loosely bonded h-BN sheets in the GO suspension. Optimizing this suspension near a pH of 5 provides a means for forming very stable and uniform GO suspensions. Subsequently this suspension was mixed, sonicated and dried to form a paper material. X-ray photoelectron spectroscopy (XPS) shows an organic amide functional group that is formed during this synthesis process which could be the result of localized heating from the sonication process. These changes in chemistry, along with variations in GO:hBN concentrations, are expected to impact the electronic bandgap (Ref 1). We will present ellipsometry and UV-Vis measurements correlating optical and material properties of BCON.
Imaging techniques, such as scanning electron microscopy (SEM), have shown that the h-BN particles are intercalated between the agglomerated sheets of GO. The intercalation of these particles warp the GO structure and induces high stress points at these sites. When the BCON material is hBN rich (> 50%), it easily tears into micro-ribbon formations (~30um). Electrical measurements have shown that the impedance decreases from 100Omega; to 14Omega; as the material is compressed in-plane, indicating cross-linking between GO nano-platelets in the BCON structure, which is recoverable upon a return to the non-compressed state. We will discuss a quantitative correlation between the compressive force and electrical properties. Additionally, we will show impedance values of BCON as a function of hBN percentage and temperature.
(Ref 1) Gao, Guanhui, Wei Gao, E. Cannuccia, Jaime Taha-Tijerina, Luis Balicas, Akshay Mathkar, T. N. Narayanan, et al. “Artificially Stacked Atomic Layers: Toward New Van Der Waals Solids.” Nano Letters 12, no. 7 (2012): 3518-3525.
9:00 AM - O6.12
Large-scale Synthesis of High-quality Hexagonal Boron Nitride Nanosheets for Large-area Graphene Electronics
Hyeon Jin Shin 1 Kang Hyuck Lee 2 Jinyeong Lee 2 Jae-Young Choi 1 Sang-Woo Kim 2
1Samsung Electronics. Co. Ltd. Youngin-si Republic of Korea2Sungkyunkwan University Suwon-si Republic of KoreaShow Abstract
Hexagonal boron nitride (h-BN) has received a great deal of attention as a substrate material for high-performance graphene electronics because it has an atomically smooth surface, lattice constant similar to that of graphene, large optical phonon modes, and a large electrical band gap. Herein, we report the largescale synthesis of high-quality h-BN nanosheets in a chemical vapor deposition (CVD) process by controlling the surface morphologies of the copper (Cu) catalysts. It was found that morphology control of the Cu foil is much critical for the formation of the pure h-BN nanosheets as well as the improvement of their crystallinity. For the first time, we demonstrate the performance enhancement of CVDbased graphene devices with large-scale h-BN nanosheets. The mobility of the graphene device on the h-BN nanosheets was increased 3 times compared to that without the h-BN nanosheets. The onminus;off ratio of the drain current is 2 times higher than that of the graphene device without h-BN. This work suggests that high-quality h-BN nanosheets based on CVD are very promising for high-performance large-area graphene electronics.
9:00 AM - O6.14
Edge Elastic Properties of Non-stochiometric Edges in Two-Dimensional Crystals
Junkai Deng 1 Iaonna Fampiou 2 J. Z. Liu 1 Ashwin Ramasubramaniam 2 Nikhil Medhekar 1
1Monash University Clayton Australia2University of Massachusetts Amherst USAShow Abstract
The elastic properties of edges are among the most fundamental properties of finite two-dimensional (2D) crystals. Using a combination of the first-principles density functional theory calculations and a continuum elasticity model, we present an efficient technique to determine the edge stresses of non-stoichiometric orientations in multicomponent 2D crystals. Using BN and MoS2 as prototypical examples of 2D binary monolayers with threefold in-plane symmetry, we unambiguously compute unique edge stresses of commonly observed non-stoichiometric edges. Our results show that the edge stresses for these structurally distinct orientations can differ significantly from the average values that have been typically reported to date.
 J. Deng, I. Fampiou, J. Z. Liu, A. Ramasubramaniam and N. V. Medhekar, Applied Physics Letters 100, 251906 (2012).
9:00 AM - O6.15
Microscopic Charge Fluctuations in Hexagonal Boron Nitride
Adela Nicolaev 1 2 Claudia Roedl 1 Giulia Pegolotti 1 Ralf Hambach 1 Stefan Antohe 2 Lucia Reining 1
1Laboratoire des Solides Irradies,UMR,7642,CNRS-CEA, Ecole Polytechnique and European Theoretical Spectroscopy Facility (ETSF) Palaiseau France2University of Bucharest, Faculty of Physics, amp;#8220;Materials and Devices for Electronics and Optoelectronicsamp;#8221; Research Center Magurele RomaniaShow Abstract
We present an ab initio approach to the electron dynamics through the calculation of the
total polarizability matrix, including the off-diagonal elements. The charge density
induced in a system by an external perturbation is computed in real space and time,
following the idea of Abbamonte et al. . The difference between our approach and the
one from Ref.  is that we can calculate not only the diagonal response chi;(q,q,omega;), but
also the off-diagonal elements of the matrix chi;(q,q',omega;). Hence, we have access to the
microscopic charge oscillations which are induced by the local-field effects.
We have studied these charge oscillations at various frequencies comprising interband-
transition and plasmon-excitation energies. The real-space approach allows us to see
which electrons (or orbitals) contribute to which kind of excitation. The final goal is to
offer theoretical support and benchmark to future inelastic x-ray scattering experiments
that may measure also the off-diagonal elements of the polarizability.
The method is applied to hexagonal boron nitride (h-BN) which is the most stable of the
three existing structures (hexagonal, cubic, and wurtzite) at room temperature and
ambient pressure. Because of its thermal stability it is a widely used material in vacuum
technology. The interest in h-BN has been renewed by the possibility of preparing boron-
nitride nanotubes that are far more resistant to oxidation than carbon nanotubes and,
therefore, suitable for high-temperature applications .
Adela Nicolaev acknowledges support from the ESF through the project POSDRU 107/1.5/S/80765.
 P. Abbamonte et al., Phys. Rev. Lett. 92, 237401 (2004)
 B. Arnaud et al., Phys. Rev. Lett. 96, 026402 (2006)
9:00 AM - O6.16
Hydrogen-terminated Germanane: An ir-stable Analogue of Graphene
Sheneve Butler 1
1Ohio State University Columbus USAShow Abstract
Graphene's success has shown that it is not only possible to create stable, single-atom thick sheets from a crystalline solid, but that these materials have fundamentally different properties than the parent material. We have created two-dimensional single atom thick hydrogen-terminated germanium (GeH) analogues of graphane (CH). A combination of absorbance measurements and high-level theory simulations show that the 0.67 eV indirect band gap of bulk germanium is converted into a 1.55 eV direct gap in GeH. These GeH sheets are resilient towards oxidation in air for at least 60 days based on fourier transform infrared spectroscopy (FTIR) measurements. These sheets can be mechanically exfoliated as single layers onto SiO2/Si surfaces. This material represents a new class of covalently-terminated single atom thick graphane analogues and has great potential for a wide range of optoelectronic applications.
9:00 AM - O6.17
Magnetic Behavior and Clustering Effects in Manganese-doped Boron Nitride Sheets
Tudor Luca Mitran 1 Adela Nicolaev 1 George Alexandru Nemnes 1 Lucian Ion 1 Stefan Antohe 1
1University of Bucharest Bucharest RomaniaShow Abstract
Ab initio calculations are performed in the framework of density functional theory on Mn-doped boron nitride sheets, which are candidates for two-dimensional diluted magnetic semiconductors (DMSs). Each type of substitution reveals a qualitatively different magnetic behavior encompassing ferromagnetic, anti-ferromagnetic and spin glass ordering. The ability of formation of these defects is also discussed. We analyze the dependence of the exchange couplings on the distance between impurities and the typical range and distribution are extracted. Multiple-impurity configurations are considered and the results are mapped on an Ising-type Hamiltonian with higher order exchange interactions, revealing deviations from the standard two-spin models. The percolation of interacting magnetic moments is discussed and the critical concentration is determined for the underlying transition from a ferromagnetic to a super-paramagnetic state. We conclude our study by providing the optimal conditions for doping in order to obtain a ferromagnetic DMS. In addition we investigate the changes in the magnetic behavior in the context of finite width BN nanoribbons.
These types of structures can constitute the building blocks of future spintronic devices.
References: T.L. Mitran, Adela Nicolaev, G.A. Nemnes, L. Ion and S. Antohe, J. Phys.: Condens. Matter 24, 326003 (2012)
9:00 AM - O6.19
Transition Metal Single-layer Materials: A Density Functional Theory Study
Pere Miro Ramirez 1 Agnieszka Kuc 1 Thomas Heine 1
1Jacobs University Bremen Bremen GermanyShow Abstract
The development of small electronic components is fundamental in our highly technology dependent society. Currently, the electronic industry is rapidly approaching the limit of silicon-based complementary metal-oxide-semiconductor (CMOS) technology. As a consequence, the development of new technology to replace silicon has rapidly become a hot topic not only in the academic community but also in industry. This new Holy Grail of electronic materials has to perform better than silicon at smaller scales (>10 nm) and if possible add new functionalities for electronic devices such as flexible electronics. In this direction, Prof. Kis has recently showed that single layers of MoS2 exhibit electrical properties and notably field-effect transistor characteristics that are comparable to silicon thin films with the development of the first MoS2-based transistor.
We have studied by means of periodic density functional calculations a wide variety of transition metal-based single-layer materials (including transition metal dichalcogenides). The band structures of the studied materials allowed us to identify the ones that could be used in the development of new technologically oriented materials. Additionally, we investigated doping effects on the electronic structure of these materials.
9:00 AM - O6.20
Optical and Magnetic Properties of WS2: Single Layers, Clusters, and Nanoribbons
Florentino Lopez-Urias 1 2 Humberto R. Gutierrez 1 3 Nestor Perea-Lopez 1 Ana Laura Elias 1 Ayse Berkdemir 1 Andres Castro-Beltran 1 4 Ruitao Lv 1 Humberto Terrones 1 Mauricio Terrones 1 5 6
1The Pennsylvania State University University Park USA2IPICyT San Luis Potosi Mexico3University of Louisville Louisville USA4Universidad Autonoma de Nuevo Leon San Nicolas de los Garza Mexico5The Pennsylvania State University State College USA6Shinshu University Nagano-city JapanShow Abstract
Transition metal chalcogenides are layered materials, similar to graphite. Their layers weakly interact via van der Waals forces and they have been used in hydrogenation catalysis and as high temperature solid lubricants. Inspired in recent experiments on the synthesis and photoluminescence enhancement of single-layer WS2 sheets and triangular islands , in the present work, first-principles density functional theory calculations are carried out on different WS2 nanostructures. In addition, we have studied WS2 clusters with different 2-D morphologies (triangles, hexagons, stars, etc), nanoribbons with zigzag and armchair edges, as well as single- and few-layered WS2. The electronic density of states, scanning tunneling microscopy simulations, structural and magnetic ordering stability, and edge chirality are studied. Bethe-Salpeter equation for the electron-hole two particle Green function has been solved in order to calculate the in-plane polarized optical spectrum and exciton wave functions. In addition, the role of spin-orbit coupling on the electronic properties of single layer WS2 is discussed.
 H. R. Gutierrez, N. Perea-Lopez, A. L. Elias, A. Berkdemir, B. Wang, R. Lv, F. Lopez-Urias, V. H. Crespi, H. Terrones, M. Terrones. Extraordinary room-temperature photoluminescence in WS2 monolayers, Submitted to Nano Lett. (2012).
9:00 AM - O6.21
Extraordinary Room-temperature Photoluminescence in WS2 Monolayers
Humberto R. Gutierrez 1 2 Nestor Perea-Lopez 1 Ana Laura Elias 1 Ayse Berkdemir 1 Bei Wang 1 Ruitao Lv 1 Florentino Lopez-Urias 1 3 Vincent H. Crespi 1 Humberto Terrones 1 Mauricio Terrones 1 4 5
1The Pennsylvania State University University Park USA2University of Louisville Louisville USA3IPICyT San Luis Potosi Mexico4The Pennsylvania State University State College USA5Shinshu University Nagano-city JapanShow Abstract
Individual monolayers of metal dichalcogenides are atomically thin two-dimensional crystals with attractive physical properties different from their bulk layered counterpart. Here we describe the direct synthesis of WS2 monolayers with triangular morphologies and strong room-temperature photoluminescence (PL). The Raman response as well as the luminescence as a function of the number of S-W-S layers is also reported. The PL becomes weaker with the increase of S-W-S layers number due to a transition from direct (in a monolayer) to indirect band gap (in multilayers). The edges of WS2 monolayers exhibit PL signals with extraordinary intensity, around 25 times stronger than the platelets center. The structure and composition of the platelet edges appear to be critical for the PL enhancement effect. Electron diffraction revealed that platelets present zigzag edges, while first-principles calculations indicate that sulfur-rich zigzag WS2 edges possess metallic edge states, which might tailor the optical response reported here. These novel 2D nanoscale light sources could find diverse applications including the fabrication of flexible/transparent/low-energy optoelectronic devices.
9:00 AM - O6.22
Strongly Anisotropic Fracture Behavior in Single-layer WS2
Xiaolong Zou 1 Ana Laura Elias 2 Nestor Perea-Lopez 2 Simin Feng 2 Mauricio Terrones 2 3 4 Boris I. Yakobson 1
1Rice University Houston USA2The Pennsylvania State University University Park USA3The Pennsylvania State University University Park USA4Shinshu University Nagano-city JapanShow Abstract
The synthesis of monolayer WS2 islands has been recently reported. These highly crystalline 2-dimensional islands sometimes exhibit cracks when quenched during synthesis. In this work, we have performed theoretical modeling in order to study and explain those cracked patterns. Our theoretical analysis shows that the unexpected anisotropic crack behavior in 2D WS2 with isotropic elastic properties is attributed to the monotonic dependence of edge energy on edge orientation with respect to the lattice. Monolayer WS2 islands show various crack patterns, depending on different S compositions along the edges of two flakes, with crack energy for armchair (AC) direction about 40% higher than that for the zigzag (ZZ) direction. The islands and the cracks were carefully characterized by SEM, HRTEM, ED and AFM techniques. These experimental results are in agreement with our theoretical calculations.
9:00 AM - O6.23
Low Temperature Synthesis of Novel Boron Nitride Nanosheets for Nano-electronic Devices
Muhammad Sajjad 1 Maxime J-F Guinel 1 Gerardo Morell 1 Peter Feng 1
1University of Puerto Rico Rio Piedras USAShow Abstract
New applications using two-dimensional (2D) sheets obtained from bulk layered materials are very promising as evidenced from graphene. Boron nitride nanosheets (BNNSs) are one example. These nanosheets have an identical crystal structure and similar lattice parameter to those of graphene sheets. However, growing quality BNNSs consisting of only several atomic layers remains a challenge.
In this presentation, we report on the catalyst/solvent free, low temperature (300 oC) synthesis of large scale, high-purity few atomic layers BNNSs onto different metal substrates using CO2-Pulsed laser deposition technique. The obtained BNNSs appear highly flat and transparent. The thickness of a signal sheet is recorded down to 5nm and diameter is even more than 200nm, from which no defects and impurities are detected based on high-resolution TEM measurements. Though we operated the TEM at 200keV, we mitigated the appearance of defects due to knock-on damage by examining the materials as quick as possible with reduced illumination. After functionalizing/doping with carbon, BNNSs-based metal/semiconductor/metal junction diode and sensing device were fabricated. Cycling test of sensing behavior of BNNSs-based gas sensor toward methane (CH4) diluted with dry air at 150 oC indicates that the response time is less than 1s. The signal out of sensor became large following an increase of methane concentration. Repetition is also excellent. In contrast, poor response to oxygen was observed.
In the case of BNNSs-based Schottky diode, current-voltage (IV) characteristics at different temperatures 25 oC, 50 oC and 75 oC indicates that slight doping carbon into BNNSs would bring a significant change in its characteristics. A rapid increase in the forward current (35µA) and breakdown voltage lower than -70V is achieved, indicating the characteristics of BNNSs as PN-junction diode.
9:00 AM - O6.24
Mechanical Properties and Fracture Dynamics of Silicene Membranes
Tiago Botari 2 Eric Perim 2 Pedro Alves da Silva Autreto 2 Ricardo Paupitz Barbosa dos Santos 1 Douglas Soares Galvao 2
1Unesp Rio Claro Brazil2UNICAMP Campinas BrazilShow Abstract
The great variety of possible carbon-based structures can be attributed to the multiple and different carbon chemical hybridizations. A similar versatility can be attributed to another tetravalent element, the silicon, which has been proven to be able to form many different structures. Nevertheless, one reason for the great importance of silicon today resides in the fact that our present electronics is silicon-based.
Amidst the emergence of graphene as one of the most promising materials for technological applications, many have hypothesized the substitution of silicon by carbon in future electronic devices. However, the fact remains that our existing processing plants are designed towards dealing with silicon. Besides that there is the zero bandgap challenge of graphene, which poses serious limitations to its use in some kind of transistors. In this way, there is a renewed interest in novel possible silicon structures that could prove to be the valuable for future electronics. Silicene could be one of these structures.
Silicene, a two-dimensional planar honeycomb silicon sheet, analogous to graphene, was first theoretically predicted  via ab initio calculations and experimentally realized recently by different groups [2-5]. It has been demonstrated that silicon can indeed form a honeycomb sheet, altough not totally planar as graphene, but exhibiting some level of buckling. Silicene has very interesting electronic and magnetic properties, making it a very promising material for electronic and spintronics applications.
In this work we have investigated, using reactive classical molecular dynamics (ReaxFF ) simulations, the mechanical properties and fracture dynamics of silicene membranes under strain. We calculated their Young modulus values along different directions. Our results show that the strained membranes exhibit two different regimes, one at the beginning, in which there is some structural buckling present, and when the strain is increased, a second regime appears in which the structures become planar, until before being fractured. The dynamics of the whole fracturing processes of silicene membranes was also analyzed, revealing unique characteristics, quite distinct from the ones observed for the case of graphenes.
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9:00 AM - O6.25
Large Scale Boron Carbon Nitride Nanosheets with Enhanced Lithium Storage Capabilities
Weiwei Lei 1 Si Qin 1 Dan Liu 1 David Portehault 2 Zongwen Liu 3 Ying Chen 1
1Deakin University Geelong Australia2UPMC Univ Paris 06 Paris France3The University of Sydney Sydney AustraliaShow Abstract
Graphene has outstanding electrical properties, flexibility, large surface area and high chemical tolerance. It has also motivated a renewal of interest in two-dimensional (2-D) nanomaterials, including analogues made of heteroelements. Exfoliated layered boron nitride (hexagonal h-BN) and boron carbon nitrides (BCN) are the flagships of such hetero compounds, with high chemical stability, much higher oxidation resistance than graphene, high thermal conductivity, excellent mechanical properties, and above all, a finite electronic band gap which clearly distinguishes these materials from graphene.
Recent works on graphene-based nanocomposites showed their exciting potential for energy storage and catalysis, which was attributed to the large surface area, chemical stability, and high electrical conductivity of graphene. Few-layers BCN nanosheets, as analogues of graphene, might exhibit complementary properties, especially because of the polarity of B-N bonds. In particular, they could show interesting properties as environmentally friendly, metal-free electrodes in lithium-ion batteries. Up to now, little research has been conducted on the electrochemical behaviour of BCN nanostructures,14 and to our knowledge, no study has reported on the Li insertion properties of nanostructured BCN.
In this work, we have used a simple salt melt approach for the synthesis of few-layers BCN nanosheets. Most of the BCN sheets are composed of 2-6 atomic layers with a large surface area. They show promising storage performance in lithium batteries which is clearly superior to bulk BCNs. Moreover, the nanostructured material exhibits high-rate performance and stable capacity (~100 mAh gminus;1) at 2 A g-1 for 5000 cycles. This work opens the door to the design of innovative light elements anode materials, which could meet the needs of the next generation of rechargeable batteries for light-weight and high power applications.
9:00 AM - O6.26
Atomic and Electronic Structure of Multilayer Graphene on a Monolayer Hexagonal Boron Nitride
Celal Yelgel 1 Gyaneshwar P. Srivastava 1
1University of Exeter Exeter United KingdomShow Abstract
Recently, high-quality exfoliated multilayer graphene devices on single-crystal hexagonal boron nitride (h-BN) substrates have been fabricated [1-4]. In this work, we have performed first-principles calculations based on density functional theory and planewave pseudopotential method to investigate the atomic and electronic structure of monolayer, bilayer, and trilayer graphene on a monolayer h-BN (MLBN). We found that the energetically most stable configuration for monolayer graphene on MLBN substrate is with one carbon atom to lie on top of a boron atom and a neighbouring carbon atom to lie at the centre of the h-BN. We have used this configuration for multilayer graphene on MLBN systems. The graphene sheet is weakly adsorbed on the h-BN substrate with a binding energy of 40 meV per C atom. We show that the electronic band gap is tunable for different multilayer graphene on h-BN substrate. The origin of the gap tuning this gap is a charge transfer from the h-BN to the graphene which leads to the development of a dipole moment across graphene. The resulting dipole moment generates an internal dipole potential across graphene. We calculate LDA band gaps of 57 meV, 287 meV, and 41 meV at the K point for monolayer, bilayer, and trilayer graphene on MLBN, respectively. The dispersion of the lowest unoccupied band near the K point changes from linear to a mixture of linear, quadratic, and cubic for multilayer graphene on MLBN.
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9:00 AM - O6.27
The Hydrogenation Dynamics of hBN Sheets
Eric Perim 1 Pedro Alves da Silva Autreto 1 Ricardo Paupitz 2 Douglas Galvao 1
1UNICAMP Campinas Brazil2Unesp Rio Claro BrazilShow Abstract
Hexagonal boron nitride (hBN), also known as inorganic graphite, is the structural analogue of graphite, presenting the same morphology of pilled up honeycomb sheets. Even the interlayer and bond distances are almost identical, the only important structural difference being the stacking order. As a consequence, almost all carbon nanostructures derived from graphite have been found to have a BN analogous. Despite these similarities, the lower symmetry of hBN significantly changes its electronic properties, in a way that while graphene sheets are zero gap semiconductors, hBN sheets are wide bandgap insulators. This is also reflected on the BN nanostructures.
In an effort to produce graphene-based nano electronic devices, many approaches to create a bandgap in graphene have been tried. One of the most interesting is the conversion from graphene to graphane (hydrogenated graphene), via plasma hydrogenation of pristine graphene membranes [1, 2]. It has been observed  the formation of correlated hydrogenated domains leading to the structural shrinkage of the graphene membranes.
A natural question is what is dynamics and consequences of the hydrogenation of the inorganic graphene, i. e., the hydrogenation of boron nitride membranes leading to the formation of the equivalent inorganic graphane.
We simulated the complete process of the hydrogenation of BN membranes using reactive classical molecular dynamics simulations (ReaxFF) ). The simulations were carried out exposing both sides of the BN membranes to atomic hydrogen atmospheres. Our results show that the rate of hydrogen bonded to the membrane atoms is highly dependent on the temperature and that only at low temperatures there is a preferential hydrogen bonding to boron or nitrogen sites. Unlike graphane, hydrogenated BN membranes do not exhibit the formation of correlated domains and become increasingly defective proportional to the number of incorporated hydrogen atoms. After a critical incorporation rate the membranes collapse and completely lost their 2D-like topologies. The diffusion of hydrogen atoms on the sheet surface, as well as, their radial pair distribution and the second-nearest neighbor correlations were also analyzed.
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 A. C. T. van Duin , S. Dasgupta , F. Lorant , and W. A. Goddard III, J. Phys. Chem. A v105, 9396 (2001).
9:00 AM - O6.28
Novel Heterostructures by Layering 2D Materials
Lei Wang 1 Cory R. Dean 1 2 Inanc Meric 2 Philip Kim 3 Ken Shepard 2 James Hone 1
1Columbia University New York USA2Columbia University New York USA3Columbia University New York USAShow Abstract
Since the first isolation of single-layer graphene in 2004, this novel 2D material has continued to receive intense interest. Recently, attention has shifted to an expanded set of exfoliatable materials such as boron nitride and the transition metal dichalcogenides. Boron nitride has been identified as an ideal substrate on which to build graphene devices, improving mobilities close to that of suspended structures. The transition metal dichalcogenides are wide band-gap 2D semiconductors, whose applications are just now being explored. New layered heterostructures, fabricated by alternately stacking different sets of these materials, offers unlimited possibilities for exploring new device functionalities. The fabrication of multi-layered structures, however, has been problematic owing to two issues: i) As devices are built up in a layer-by-layer approach, polymer residues accumulate at interfaces resulting in bubbles, wrinkles and poorly contacted interfaces. ii) metal contact to buried layers requires a multi-step fabrication process. In this talk I describe recent advancements we have made to address both of these issues. First I will describe a new polymer-free transfer process that allows unlimited stacking of multi-layer device structures. Using this technique we report BN-encapsulated graphene devices of unprecedented quality. We also demonstration an extension of this technique to realize high mobility MoS2. Second, I will describe a novel method to electrically contact fully encapsulated graphene devices. The contact resistance is comparable to the lowest value measured among the conventional metal-graphene contact in literature. Our techniques open up the possibility for pursuing new directions in 2D materials.
9:00 AM - O6.30
In situ Characterization of Epitaxial Silicene
Daniele Chiappe 1 Carlo Grazianetti 1 2 Eugenio Cinquanta 1 Grazia Tallarida 1 Marco Fanciulli 1 2 Alessandro Molle 1
1CNR Agrate Brianza Italy2Universitamp;#224; degli Studi di Milano Bicocca Milano ItalyShow Abstract
The existence of silicene, the graphene-like form of silicon, has been recently demonstrated by combining theoretical modeling and experimental evidences. [1,2] Due to the honeycomb lattice, silicene exhibits an electronic dispersion resembling that of relativistic Dirac fermions similarly to graphene.  Unlike graphene, silicene arranges in stable buckled configurations due to the large ionic radius of silicon atoms. The vertical distortion of the honeycomb lattice is expected to alter many physical properties of silicene which, consequently, might exhibit relevant advantages over graphene such as a band gap opening and its tunability.
The present work is intended to rationalize basic concepts of the epitaxial silicene on Ag(111) by means of a systematic in situ scanning tunneling microscopy-spectroscopy investigation. A position dependent density of states is reported  and interpreted as a buckling related electronic band structure which - similarly to the case of graphene  - can derive from the symmetry breaking in the silicene honeycomb lattice. In a more applicative view, silicene encapsulation is here successfully shown in order to prevent easy oxidation and to access ex situ characterization. New hints at the silicene growth on non-metal substrates with proper surface arrangement are also discussed aiming at device-oriented applications.
These outcomes disclose exceptionally novel issues in the physics of the emerging silicene and promote a renewed interest in nanoscaled silicon as active material for electronic devices.
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 D. Chiappe, C.Grazianetti, G. Tallarida, M. Fanciulli and A. Molle, Adv. Mater. 24, 37, 5088 (2012).
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O4: Transport in 2D-Layered Materials
Thursday AM, April 04, 2013
Moscone West, Level 2, Room 2009
9:30 AM - O4.02
Two Dimensional Nature of Liquid Gating Induced Superconductivity in MoS2
Hongtao Yuan 1 Hisashi Inoue 1 Seung Sae Hong 1 Chris Bell 2 Yasuyuki Hikita 2 Desheng Kong 1 Harold Hwang 1 2 Yi Cui 1 2
1Stanford University Stanford USA2SLAC National Accelerator Laboratory Menlo Park USAShow Abstract
One of the new focuses in nano science and technology is the search for graphene-like atomic layered materials, with recent efforts aiming at novel electronic properties and device functionalities beyond graphene. Among the various candidates, the layered transition metal dichalcogenides such as MoS2 have attracted great interest in terms of valleytronics and electric-field induced superconductivity. In this context electric-double-layer transistors (EDLTs) are powerful tools to tune and even create novel electronic states in these fascinating materials. The ability of EDLTs to access high sheet carrier densities (ns) at gated liquid/solid interfaces up to ~ 1015 cm-2, beyond the regime of chemical doping or solid dielectric field effect transistors, has enabled the realization of ionic liquid gated superconductivity in MoS2. However, the character of this superconducting state, as well as its origin - an electrostatic field effect or electrochemical intercalation - remain open questions.
Here we present comprehensive studies on the dimensionality of the superconductivity in ionic liquid gated EDLTs on single crystal MoS2 flakes. By controlling the external bias of the EDLT gate, we succeeded in observing superconductivity at low temperatures following an insulator-to-metal transition. The superconducting transition temperature Tc showed a maximum of Tc = 10 K centered around ns ~ 4 X 1014 cm-2, decreasing systematically as ns deviated from this optimal value. More importantly, the angular dependence of the superconducting upper critical field showed strong anisotropy, with a sharp peak when the magnetic field was applied parallel to the layers. These data could be well described the Ginzburg-Laudau theory for two dimensional (2D) superconductors, indicating that the field effect induced superconductivity in MoS2 has 2D character, not the anisotropic 3D character of the bulk. These findings not only help us to understand the 2D nature of the induced superconducting state in MoS2 bulk crystals, but also demonstrate a strong evidence for its electrostatic nature.
9:45 AM - O4.03
Electronic Structure of Metal-chalcogenide Monolayers
Wolfram Jaegermann 1 Andreas Klein 1
1TU Darmstadt Darmstadt GermanyShow Abstract
The development of 2D electronic semiconductors prepared from two-dimensional layered metal-chalcogenides has renewed the interest in the electronic structure of these materials in dependence of the number of layers, but in particular of those of single layer thiockness. We will present in this contribution a summary of our previous investigations on the electronic structure of layered chalcogenide epitaxial films. The determination of the electronic structure of single layers is often impeded by the interaction of the layers with the substrate material. In the case of metal chalcogenides, the interaction between layers is governed by weak van der Waals-like interactions. When using a substrate material with a dissimilar electronic structure and strong lattice mismatch, the electronic coupling between electronic states of film and substrate can be even weaker.
We have analyzed the electronic structure of metal-chalcogenides using energy dependent and angle-resolved ultraviolet photoelectron spectroscopy. We present results of three different layer/substrate combinations: (i) InSe on highly oriented pyrolytic graphite; (ii) WS2 on single crystalline graphite; (iii) GaSe half-sheets on Si(111). The films have been prepared by using the concept of van der Waals epitaxy in growing well defined layers of different thickness as analysed by STM. In all three cases, for single layers the electronic structure of the films parallel to the surface can well be described in terms of wave function symmetry and energy band dispersion of bulk band structures. The electronic structure of a single layer is hence not influenced by the substrate, indicating an electronic decoupling of the layers from the substrate. A clear transition is observed from single to double layer film thickness and to thicker bulk like films for the layered chalcogenides grown on graphite substrates. Of specific interest is the GaSe half-sheet which forms a chemically stable nearly perfect electronic passivation layer on the Si(111) surface, which provides a straight forward connection between single layer metal-chalcogenide electronics and Si technology.
10:00 AM - O4.04
Two Dimensional Dirac Fermions in Layered Topological Insulator Bi2Se3 Nanoribbons
Seung Sae Hong 1 Judy J. Cha 2 Desheng Kong 2 Yi Cui 2 3
1Stanford University Stanford USA2Stanford University Stanford USA3SLAC National Accelerator Laboratory Menlo Park USAShow Abstract
In the example of graphene and other atomically-thin materials, two-dimensional (2D) electronic states form when the material becomes thin enough to confine the electron movement in plane. In contrast, certain materials known as topological insulators (TI) can accommodate 2D electronic states on the surface of bulk due to their unusual bulk property of strong spin-orbit effect. The exemplar TI material bismuth selenide (Bi2Se3), layered metal chalcogenide, possesses the 2D electronic states on the surface. The gapless surface bands, residing in the bulk insulating gap, have Dirac cone band structure similar to graphene, which has attracted great attention in science and applications. In this presentation, we report detailed transport studies on the topological surface states of Bi2Se3. Vapor-liquid-solid grown Bi2Se3 nanoribbons are protected with Se layer against environmental degradations and fabricated into nanodevices for electron transport at cryogenic temperature. With large magnetic field up to 9T, we observe clear magneto-oscillations known as Shubnikov-de Haas (SdH) oscillations from high mobility carriers in the nanoribbon device. Angle dependent measurements confirm that the high mobility electronic states are of 2D nature and careful analysis of Landau levels manifests the Berry&’s phase from Dirac band structure. Multi-frequency oscillatory features in SdH oscillations and gate-tunability of each surface band will be discussed as well.
10:15 AM - O4.05
Properties of Field-effect Transistors of CVD Grown MoS2 Single Atomic Layers on CVD Grown h-BN
Nihar Ranjan Pradhan 1 Daniel Rhodes 1 Qiu Zhang 1 Ana Laura Elias 2 Zheng Liu 3 Sina Najmei 3 Nestor Perea Lopez 2 Saikat Talapatra 4 Jun Lou 3 Mauricio Terrones 2 5 6 Pulickel Ajayan 3 Luis Balicas 1
1National High Magnetic Field Laboratory, Florida State University Tallahassee USA2The Pennsylvania State University Pennsylvania USA3Rice University Huston USA4Southern Illinois University Carbondale USA5The Pennsylvania State University Pennsylvania USA6Shinshu University Nagano JapanShow Abstract
Two dimensional crystalline layered materials such as MoS2, WS2, have recently become an intense focus of research activities due to their exceptional electronic and optical properties as well as their availability. A single- or a few atomic layers of these materials show quite promising charge conduction characteristics, such as large mobility or fast on/off switch ratios, which lead to a few recent examples of integrated circuits based on these materials. Here, we will present a comparison among the electronic transport properties of, either mechanically exfoliated or CVD grown MoS2 under different substrates, i.e. on SiO2, on exfoliated or on CVD grown h-BN, and suspended. We will also discuss results obtained from back and top gated configurations with different dielectrics.
11:00 AM - *O4.06
Single-layer MoS2 - Electrical Transport Properties, Devices and Circuits
Andras Kis 1
1EPFL Lausanne SwitzerlandShow Abstract
After quantum dots, nanotubes and nanowires, two-dimensional materials in the shape of sheets with atomic-scale thickness represent the newest addition to the diverse family of nanoscale materials. Single-layer molybdenum disulphide (MoS2), a direct-gap semiconductor is a typical example of new graphene-like materials that can be produced using the adhesive-tape based cleavage technique originally developed for graphene. The presence of a band gap in MoS2 allowed us to fabricate transistors that can be turned off and operate with negligible leakage currents. Furthermore, our transistors can be used to build simple integrated circuits capable of performing logic operations and amplifying small signals.
I will report here on high-performance 2D MoS2 transistors with increased currents and transconductance due to enhanced electrostatic control. Our devices also show current saturation for the first time in a 2D semiconductor. Electrical breakdown measurements of our devices show that MoS2 can support very high current densities, exceeding the current carrying capacity of copper by a factor of fifty. Next, I will show optoelectronic devices based on MoS2 that have a sensitivity surpassing that of similar graphene devices by several orders of magnitude. Finally, I will present temperature-dependent electrical transport and mobility measurements that show clear mobility enhancement due to the suppression of the influence of charge impurities with the deposition of an HfO2 capping layer.
11:30 AM - O4.07
Field Induced Superconductivity in MoS2
Yijin Zhang 1 Jianting Ye 1 Ryosuke Akashi 1 Mohammad Saeed Bahramy 2 Ryotaro Arita 1 2 Yoshihiro Iwasa 1 2
1the University of Tokyo Bunkyo-ku, Tokyo Japan2RIKEN Hirosawa, Wako, Saitama JapanShow Abstract
Transition-metal dichalcogenides (TMDs) are ones of the most famous layered materials. In particular, semiconducting TMDs are nowadays attracting great interests after the invention of so-called “Scotch-tape method” established in graphene research. Semiconducting TMDs are front-runners of “post graphene” since they endow finite band gap which is crucial for device applications. Recently high-performance monolayer TMD-FET with high-k gate dielectric is reported using channel material of MoS2, molybdenum disulfides . MoS2 is the most common TMD because of the practical usage as solid lubricant. It has also been a target for material scientist because its alkali or alkaline-earth doped compounds shows superconductivity with highest TC around