The large-area monolayer of MoS₂, MoSe₂, WS₂ and WSe₂ can be synthesized directly on arbitrary insulating substrates with CVD method using corresponding metal trioxides and S or Se powders as the reactants. To realize the high efficiency solar cells or other optoelectronic devices based on the TMD monolayers, it is important to develop a strategy to tune the optical band gap of these TMD monolayers. I would like to discuss several synthetic methods of obtaining monolayer alloys such as MoS_xSe_1-x, WS_x,Se_1-x, WS_xTe_1-x and so on. The band gap energy of these alloys is controllable depending on their composition.

Two dimensional (2D) materials with a monolayer of atoms represent an ultimate control of material dimension in the vertical direction. Molybdenum sulfide (MoS2) monolayers, with a direct bandgap of 1.8 eV, offer an unprecedented prospect of miniaturizing semiconductor science and technology down to a truly atomic scale. Recent studies have indeed demonstrated the promise of 2D MoS2 in fields including field effect transistors, low power switches, optoelectronics, and spintronics. However, device development with 2D MoS2 has been delayed by the lack of capabilities to produce large-area, uniform, and high-quality MoS2 monolayers. Here we present a self-limiting approach that can grow high quality monolayer and few-layer MoS2 films over an area of centimeters with unprecedented uniformity and controllability. This approach is compatible with the standard fabrication process in semiconductor industry. It paves the way for the development of practical devices with 2D MoS2 and opens up new avenues for fundamental research.

2D materials beyond graphene such as hexagonal boron nitride (h-BN) have recently attracted a lot of research interest.[1] While, chemical vapour deposition (CVD) of h-BN on transition metal catalysts has emerged as a preferred route for synthesis, the mechanism underlying the growth of h-BN are still very much unclear.
Here, using a combination of high-pressure time and depth resolved in-situ X-ray photoelectron spectroscopy (XPS)[2] and in-situ X-ray diffraction (XRD)[2] at realistic CVD conditions of pressure (~0.001 - 1 mbar) and extreme temperatures (700-1000oC) we analyse the behaviour of some of the most popular poly-crystalline metallic catalyst films and foils (eg: Cu, Ni, Co) for h-BN growth during exposure to precursors (both gaseous and liquid precursors).
These measurements allow for a clear understanding of the B and N incorporation in the nanostructure as it happens by identifying the catalyst state at any point of time during CVD. These coupled with ex-situ experiments [3,4] allow for development of a comprehensive growth mechanism to rationally engineer CVD processes to produce high quality material for device applications.
1. Kidambi et al. (manuscript in preparation)
2. Kidambi et al. Nano Letters 13 (10), 4769-4778 (2013).
3. Kidambi et al. J. Phys.Chem. C. 116, 42, 22492-22501 (2012).
4. Kidambi et al. PSS RRL. 5, 9, 341-343 (2011).

Owing to their unusual electronic properties, one atom-thick layers of hybridized hexagonal boron nitride (h-BN) and graphene have recently attracted a great deal of attention [1]. However, many important issues still remain open. Among them, addressing at the atomic scale the in-plane continuity as well as the interfacial electronic properties between h-BN and graphene domains is important for application in nanolectronics.
Here we demonstrate the growth of h-BN/graphene heterostructures that comprise graphene nanoribbons (GNRs) with width of ~ 1nm via a two-step process. Firstly, we produce high-quality h-BN flakes on a Au(111) substrate. Due to the chemical inertness of bulk Au, it is extremely difficult to synthesize either BN or graphene on such a substrate via the conventional chemical vapor deposition process. We overcome this challenge by using a recently developed alternative growth process - reactive magnetron-sputtering of B in N2/Ar gas mixtures - for the h-BN&’s synthesis [2]. Using this method, we are able to obtain high-quality sub-micron size h-BN flakes. Subsequently, we produce GNRs by employing the bottom-up surface-assisted method introduced in Ref. [3]. We use in-situ scanning tunneling microscopy (STM) to investigate the different steps of this process toward graphene/h-BN nano-heterostructures. Tunneling spectroscopy is applied to probe the local electronic properties, in particular near the interface of GNRs stitched to surrounding BN. X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) are used to assess the quality of the synthesized nanostructures.
Detaching the h-BN/GNR nanohybrids from the Au substrate after the growth, through the nondestructive bubbling transfer method [4], allows their transfer to other substrates of technological interest.
References
[1] Ci, L. J.; et al. Nat. Mater. 2010, 9, 430
[2] Sutter, P.; Nano Lett. 2013, 13, 276
[3] Cai, J.; et al. Nature 2010, 466, 470
[4] Gao, L.; et al. Nat. Comm. 2012, 3, 699

Topological insulators (TI), Bi2Se3 in particular, exhibit robust 2D surface states in which spin is strongly coupled to the orbital motion creating the prospect for the realization of electronic devices with novel switching mechanism and novel functionalities. An important requirement is that Bi2Se3 can be grown epitaxially in the form of very thin films, so that the surface properties dominate over the unwanted bulk conduction. Unfortunately in most cases, in order to observe the surface states, the films must be at least 6 quintuple (QL) layer thick (~6 nm) [1, 2], which limits their applicability in functional devices. Here in this paper we show by ARPES that well defined 2D surface metallic states are formed in ultrathin (only about 3 QL thick) Bi2Se3 grown epitaxially on AlN(0001)/Si(111) substrates by Molecular Beam Epitaxy (MBE). The use of AlN, not reported before, opens the possibility of employing insulating substrates first to improve electrical conduction measurements, second to integrate Bi2Se3 with gate dielectrics in order to exploit the quantum capacitance of TIs. The Bi2Se3/AlN will be compared with our control Bi2Se3/Si (111) samples more typically obtained in the literature [3, 4] in order to assess the advantages of growth on AlN. Employing RHEED, XRD/XRR and HRTEM we demonstrate a high epitaxial quality of Bi2Se3 on AlN. Using HRTEM, we identify interfacial misfit dislocations (MDs) that accommodate the high lattice mismatch between Bi2Se3 and AlN as well as rotation and lamellar twins. Strain mapping is performed to reveal the distribution of the strain field of the MDs at the atomic scale. Using in-situ XPS we show that, Bi2Se3 does not react with AlN allowing the growth of high quality ultrathin films at an optimum temperature of 300 oC without the need to grow low temperature buffer layers, as in the case of Si substrates. HRTEM observations reveal a crystalline, albeit distorted, interface between the two materials. Using ARPES we perform a systematic investigation of the dependence of the 2D surface states as a function of the Bi2Se3 thickness. On both substrates we observe a shift of the Dirac point closer to the Fermi level as thickness increases, but remarkably, in the case of Bi2Se3/AlN, the shift is much larger, bringing the Dirac point at about 0.15 eV below EF for a 12 QL sample, which is highly desirable for several envisaged applications. Finally, we will show that Bi2Se3 on AlN may be suitable substrate for the growth of a few layer MoSe2 dichalcogenide, yielding good quality epitaxial growth with well-defined valence bandstructure as evidenced from ARPES.
A. Dimoulas acknowledges an ERC Advanced Grant through project SMARTGATE-291260
[1] Y. Zhang, et al., Nature Physics, 6, 584-588 (2010); [2] Y. Sakamoto, et al., Phys Rev. B 81, 165432 (2010); [3] G. H. Zhang et al., Appl. Phys. Lett. 95, 053114 (2009); [4] A. A. Taskin, et al., Adv. Mater. 24, 5581 (2012).

The isolation of graphene constituted a new paradigm in next generation electronic technologies, and even though graphene is considered transformational, it is only the “tip of the iceberg.” Transition metal dichalcogenides (TMDs), could have an even greater impact on next generation technologies. Similar to graphene, TMDs are composed of vertically stacked, weakly interacting layers that are scalable down to sub-nanometer thicknesses. This, in addition to reported carrier mobilities up to 1000 cm2/Vs, and subtreshold slopes nearing the theoretical limit of 60mV/dec, make them an impressive material candidate for analog and digital electronics. Molybdenum disulfide (MoS2) is currently a leading TMD for scientific exploration, but there are a variety of other suitable, less explored, TMDs and TMD heterostructures that exhibit very attractive bandgaps, charge carrier effective masses, and mobilities for electronic applications. Transition-metal dichalcogenides (TMDs) in the form of MeX2 (where Me = a transition metal such as Mo, W, Ti, Nb, etc. and X = S, Se, or Te) also exhibit extreme flexibility, possession of tunable band gaps, modest electron mobilities, and wide variety of band-offsets. The ability to grow high quality, large area TMDs is the first critical step in the realization of electronics requiring atomic layers with tailored electronic band alignments. In this talk I will discuss the applications of TMDs and TMD heterostructures, discuss our research in the synthesis and characterization of WSe2, WTe2, and TMD/graphene van der Waal solids, and show that heterostructures could be key to advanced electronic applications.

Synthesis capability for uniform growth of 2D materials over large areas at lower temperatures without sacrificing their unique properties is a critical pre-requisite for seamless integration of next-generation van der Waals heterostructures into novel devices. We have demonstrated, for the first time, vapor phase growth techniques for precisely controlled synthesis of continuous, uniform molecular layers of all MoX₂ and WX₂ transition metal dichalcogenide (TMD) compounds on diverse substrates, including graphene, hexagonal boron nitride, highly oriented pyrolitic graphite (HOPG), SiO₂, and metal substrates over several square centimeters. Preliminary results show MoX₂ and WX₂ transition metal dichalcogenide materials grown in a novel ultra-high vacuum (UHV) physical vapor deposition (PVD) process demonstrate properties identical or even superior (e.g., electron mobilities >500 cm² V^-1 s^-1) to exfoliated layers. This plasma-based physical vapor deposition (PVD) process for growth of high-quality mono-, bi- and few-layer TMDs is characterized by incident constituent atoms with maximum kinetic energies that are > 10 times higher than those produced in a thermal chemical vapor deposition process, yet below the threshold known to produce defects in the growing film materials. The kinetic energy and arrival rate of atoms is selected by modulating the pulse characteristics of power to the plasma cathode, while the flux of ions is tunable with an external magnetic field. The energetic atoms comprising the TMD film possess high surface mobility upon first contact with the substrate, reducing the processing temperatures necessary for continuous film coverage of the substrate. Further reduction of processing temperatures for application on flexible substrates are expected as computational modeling of growth modes guides experiments to reduce growth temperature without compromising properties via high defect densities. Examples such MoS₂/WS₂ heterostructures and bi-layer MoS₂ on few-layer graphene with a 30% lattice mismatch and TMD/TMD heterostructures are shown to demonstrate how natural accommodation of stresses at 2D van der Waals interfaces has the remarkable potential to transform the way materials selection is considered for synthetic heterostructures, as concerns regarding lattice constant matching can be abandoned with preference given to desired properties and performance. Further development of synthetic heterostructures comprised of TMDs and previously unexplored materials known to have layered structures in bulk form for new 2D sensors and devices with tailored functionality is anticipated.

Two-dimensional (2D) nanomaterials with single or few atomic layers show many unique physical and chemical properties compared with their bulk counterparts. As the most widely and intensively studied 2D materials, graphene shows exotic electrical properties and great potential for next-generation electronic devices. However, the zero band-gap energy of graphene limits its applications in logic electronics and field-effect transistors (FETs). In order to overcome the limitation of graphene, 2D semiconducting materials with band-gaps are highly desired. Gallium selenide (GaSe) is a layered semiconductor with an indirect band-gap of ~2 eV. Bulk GaSe has been widely used in the field of optoelectronics, nonlinear optics, and terahertz experiments. Recently, few-layer GaSe nanosheets have been fabricated through mechanical exfoliation and vapor-phase deposition. These nanosheets show good performance as photodetectors and transistors, revealing great potential of this 2D nanomaterial as an alternative to graphene for electronic devices. However, the growth of large area, single crystal, uniform, single-layer GaSe flakes directly on insulating substrates, which is highly desired for device fabrication, is still a challenge. In this research, we synthesized large (up to ~50 mu;m in lateral size) 2D GaSe flakes on SiO2/Si substrates through a vapor phase deposition method. The thickness (from single to few atomic layers), shape, size, and density of the flakes were well controlled by tuning growth conditions. Atomic resolution images of the as-synthesized GaSe flakes were obtained to investigate the crystal structure, boundaries, and defects. The Raman spectra and photoluminescence spectra of the GaSe flakes change with their thickness in both peaks positions and intensities. These 2D GaSe flakes show dramatic photoresponse when illuminated with visible light. Moreover, they also exhibit typical FET characteristics. The results reveal the potential of our 2D GaSe flakes for photodetector and FET applications.
Synthesis science sponsored by the Laboratory Directed Research and Development program at Oak Ridge National Laboratory. Materials and device characterization conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.

Recent efforts on graphene-like atomic layer materials, aiming at novel electronic properties and quantum phenomena beyond graphene, have attracted much attention for potential electronics and spintronics applications. Compared to the weak spin-orbit-interaction (SOI) in graphene, layered transition-metal chalcogenides (MX2, M = Mo, W; X = S, Se, Te) having heavy 4d/5d elements with strong atomic SOI, not only are able to have an indirect to direct band-gap transition with dimension decreasing to monolayer but also provides a unique way for generating spin polarization based on valleytronics physics. Indeed, such a spin-polarized band structure has been demonstrated theoretically and supported by optical investigations. However, despite these exciting developments, the quantitative band structure of MX2 family compounds (e.g. the band maxima/minima loci in the BZ, the direct and indirect band gap sizes, etc.) have not yet been directly observed - which is particularly valuable to resolve conflicting theoretical investigations (e.g. the conduction band minima are quite important for valley-spin generation but varies with calculations). Especially for those cleaved ultrathin mono- and bi-layer flakes which host most of recently-reported exotic phenomena at their 2D limit, the direct detection for the band dispersion becomes of great importance and urgency.
In this work, we studied the electronic structure of representative transition metal chalcogenides MoS2, WS2 and WSe2 by ARPES and obtained detailed band structure maps as well as their variation between different compounds. We found that although the valence band maximum (VBM) of bulk MX2 compounds always resides at Γ, the conduction band minimum (CBM) loci shift from K (MoS2) towards Γ (WS2 and WSe2). This observation not only addresses the recent controversy in band calculations, but also demonstrates their different gap geometry in mono-layer thin films. In addition, utilizing the newly-developed spatially resolved ARPES technique, we directly studied the band structure evolution of MoS2 and WSe2 flakes from the bulk to the mono-layer limit, which clearly reveals the switching of the VBM apexes (from Γ to K) in both compounds. Such thickness dependent ARPES measurements on ultrathin MX2 flakes not only helps us establish a solid background to understand the underlying physics of these materials and further provides guidance for new materials design and novel device development of MX2 materials, but also demonstrates the possibility to study the band structure of such a group of submicron-size semiconductors used in functional devices.

Transition metal dichalcogenides such as MoS₂ and MoSe₂ have been studied widely for their increase in bandgap and transition to a direct bandgap at the monolayer limit. Here we report on the combination of single-layer MoS₂ with titania, a wide-bandgap semiconductor with numerous (photo-)catalytic applications. Our sample preparation proceeds via a sequence of cleaning of a SiO₂/Si substrate, deposition of titanium, conversion of the titanium to rutile titania by means of oxygen plasma treatment and, finally, CVD growth of single-layer a MoS₂ film. The resultant MoS₂ films exhibit the characteristic photoluminescence feature at 1.87 eV indicative of single-layer material. In my poster I will present a range of further characterization results.

Ultrathin 2-dimensional (2D) nanomaterials (nanosheets, NSs) have been extensively investigated over the past few years due to the unique physical and electronic properties that arise when electrons and holes are confined in a plane that is just a few atomic layers thick (Lle;2.5 nm). Among the most studied species are graphene and transition metal dichalcogenides such as MoS2 and WS2. Typically, these ultrathin 2D nanomaterials are obtained via exfoliation of bulk materials or grown on substrates by vapor deposition techniques. The limitation of these techniques is that they offer little control over size and shape, and are also not suitable to produce free standing NSs. Colloidal preparation methods offer a promising alternative, as they have been very successful in producing nanomaterials with well-defined composition, size, shape, and surface. Additionally, colloidal methods are cheap and easy to upscale. Nevertheless, colloidal ultrathin NSs have only recently been reported, and are limited to CdX NSs (X= S, Se). Although colloidal CdX NSs have shown promising properties, control over their lateral dimensions and shape is still lacking. Moreover, CdX NSs are often entangled, aggregated, or coiled, which limits their applicability in optoelectronic devices. Therefore, to fully exploit the potential of ultrathin colloidal NSs, improved colloidal preparation methods are needed, which should provide control not only over the thickness of the NSs, but also over their shape and lateral dimensions. In this work, we address this need by developing a preparation method for ultrathin (2 nm) 2D colloidal Cu2-xS NSs with well-defined lateral dimensions and shape. This approach yields hexagonal or triangular colloidal NSs with sizes tunable from 100 nm up to several micrometers, and a constant thickness of 2 nm. Moreover, the Cu2-xS NSs produced are not aggregated or coiled, and are thus suitable for application in optoelectronic devices. Colloidal Cu2-xS NSs are promising materials for photovoltaics, photocatalysis, and nanoplasmonics. Furthermore, Cu+ ions in copper chalcogenides have been shown to be easily exchangeable by other cations. This opens up the possibility of using cation exchange reactions to convert Cu2-xS into other metal sulfides, with preservation of the size and shape of the colloidal NSs. In this way, the synthesis method developed here could offer a general preparation strategy to colloidal 2D nanosheets of metal sulfides and other II-VI materials.

Graphene is a promising material which has a very high mobility but strongly influenced by underlying substrate. Therefore a suitable substrate must be used for graphene-based devices. Recently hexagonal boron nitride (hBN) has been adopted as a substrate for graphene-based devices. Graphene on hBN which has the same crystal structure as graphene and atomically smooth surface, shows very high mobility (µCasymp;60,000 cm2 V-1s-1) [1]. Polymer supporting thin films for transferring graphene can leave residue on it. The polymer residue can acts as a scattering site or doping element on graphene layer. In fabricating graphene-based devices, it is very important to avoid residue remaining on graphene so that the quality of the graphene layer is not deteriorated. [2-4] In this study we developed dry transferring techniques to transfer graphene to hBN substrate and to make sandwich structures. Also, we used shadow mask lithography technique to prevent the residue problem caused by e-beam lithography. The graphene layer can be encapsulated between two hBN layers and then is kept intact during fabricating devices. Electrical properties of the sandwich devices also are investigated.

Van der Waals materials that can be mechanically cleaved from bulk crystals with layered atomic structure have recently attracted significant attention. An interesting subgroup of van der Waals materials is the layered transition metal dichalcogenides MX₂, where M=Mo, W, Nb, Ta or Ti and X=S, Se or Te. It was recently demonstrated that the transition temperature to the charge density wave phase can be increased in some of these materials by thinning the exfoliated films [1]. In this presentation, we report results of our investigation of the thermal properties of thin films of tantalum diselenide (2H-TaSe₂) obtained via the “graphene-like” mechanical exfoliation of crystals grown by chemical vapor transport. The ratio of the intensities of the Raman peak from the Si substrate and the E_2g peak of TaSe₂ was used as a convenient metric for quantifying film thickness. The temperature coefficients for two main Raman peaks, A_1g and E_2g, were -0.013 and -0.0097 cm^-1/°C, respectively. The Raman optothermal measurements indicated that the room temperature thermal conductivity in these films decreases from its bulk value of ~16 W/mK to ~9 W/mK in 45-nm thick films. The measurement of electrical resistivity of the field-effect devices with TaSe₂ channels indicates that heat conduction is dominated by acoustic phonons in these van der Waals films. The scaling of thermal conductivity with the film thickness suggests that the phonon scattering from the film boundaries is substantial despite the sharp interfaces of the mechanically cleaved samples. These results are important for understanding lattice dynamics of TaSe₂ films and for device applications of metal dichalcogenide thin films.
The work at UCR was supported in part by the NSF - NRI Nanoelectronics 2020 and Beyond Project.
[1] P. Goli, J. Khan, D. Wickramaratne, R.K. Lake and A.A. Balandin, "Charge density waves in exfoliated films of van der Waals materials: Evolution of Raman spectrum in TiSe2, Nano Letters, 12, 5941 (2012).

A facile way to fabricate functionalized and reduced graphene oxide (RGO) was reported by introducing the dual-function reagent N1-(3-trimethoxysilylpropyl) diethylenetriamine (TSPD), in which (i) the amine functional groups of the organosilane can effectively restore the p bonds of the GO; and (ii) the silanol parts react with the hydroxyl groups of GO and subsequently cross-link to form a monolayer on the graphene surface. The exposed amine groups of the functionalized GO could effectively template the Au nanoparticles (AuNPs) assembly on the graphene surface with high density and good dispersity. TEM images indicate that AuNPs retained their original size and were well-anchored on the graphene surfaces with an average inter-particle distance of 15 nm. The resultant graphene sheets decorated by AuNPs has the capability of sensing single molecules on its surface, e.g., Rhodamine 6G (Rh6G), through the surface-enhanced Raman spectroscopy (SERS).

MoS2 is a unique two-dimensional (2D) electronic material with impressive potential in applications such as ultrathin transistors with high on/off ratio and electron mobility and high sensitive molecular sensing with high selectivity. The recent work in MoS2 2D material evaluation has relied primarily upon materials exfoliated from bulk crystal MoS2. However, such processes are accompanied by surface contamination and are not scalable to industrial device manufacturing. Progress has been made in vapor phase processing over larger areas. Recently MoS2 single/multiple layers have been synthesized from the reaction of sulfur vapor with molybdenum oxide thin film pre-coated on substrates at high temperatures (>700 °C)[1]. An alternative vapor phase technique for growth of 2D materials is magnetically enhanced physical vapor deposition (PVD) directly from MoS2 solid compound targets. The process promotes growth conditions with enhanced surface atom mobility for continuous growth of 2D materials with minimal defect formation, even at low temperatures (<300 °C). Highly parallel basal plane orientation was obtained for single- and multiple-layer films under conditions minimizing nucleation density and maximizing surface diffusion of deposited atoms before burial by incident material or desorption [2]. In this presentation, details on transmission electron microscopy (TEM) studies, including high-resolution TEM, HAADF-STEM and X-ray energy dispersive spectroscopy, will be provided to allow comparison of ultrathin MoS2 films processed using both techniques. Cross-sectional TEM specimens were coated with a protective metal layer, and prepared by lift-out using a focused-ion beam (FIB) microscope. The films were grown by PVD and CVD methods, respectively, and the thickness was controlled from a single layer MoS2 to a few nanometers. Although the both techniques produced high quality MoS2 films in the preferred parallel orientation, differences in microstructure were measured and correlated to differences in electron mobility, photoluminescence intensity and peak position, and other critical properties for applications in 2D electronic devices. The results outline steps along the path toward clear requirements for the next generation of 2D materials including uniform growth over large areas at low temperature.
[1] S.D. Pacley, et al., MRS Spring Meeting & Exhibit, San Francisco, CA, April 21-25, 2014.
[2] C. Muratore, A.A. Voevodin, Thin Solid Films 517 (2009) 5605.

Two dimensional materials are sparking the realm of nanoscience and nanotechnology. Inorganic oxides nanosheets have attracted much attention owing to their semiconducting nature, high thermal as well as chemical stability along with exceptionally rich structural diversity and electronic properties. In this report we are presenting a facile and low cost method to fabricate nanosheets consisting double layers of Molybdenum trioxide (MoO3 - II), from hydrothermally grown hexagonal MoO3 (h - MoO3) nanorods. Nanosheets of MoO3 can be obtained using mechanical exfoliation technique from the annealed h - MoO3 nanorods. Thickness of the as exfoliated nanosheets was ranging from 2.4 nm to 11.3 nm which is equivalent to 3 to 16 double layers of MoO3 - II respectively taking into account their respective Vander Waals gaps (thickness of a MoO3 - II double layer ~ 0.709 nm) as quantified using AFM. Raman Spectroscopy analyses of the as prepared nanosheets detailed the effect of number of double layers on the Raman signals. The appearance of fringes in the HRTEM evaluation signifies the crystalline nature of the as prepared nanosheets and expose (2, -2, 0) plane of MoO3. The electrochemical response of MoO3 nanosheets were examined by the means of Scanning Electrochemical Microscopy (SECM) in the feedback mode in the environment of cationic [Ru (NH3) 6] +3 as well as anionic [Fe (CN) 6] -3 redox mediator. The as prepared nanosheets showed positive and negative feedback response in the surroundings of [Ru (NH3) 6] +3 and [Fe (CN) 6] -3 respectively. The positive and negative feedback behavior in the mild reducing and oxidizing agent is attributable to the electro active nature of MoO3 nanosheets.

Surface morphology of Cu substrates had been known to have significant impacts towards the nucleation density, domain sizes, thickness, uniformity and quality of the CVD-grown graphene film. h-BN, analogous to graphene, is also expected to have the same correlation towards substrate roughness. In this work, we investigate the influence of Cu surface roughness, through the use of electro-polishing to achieve an extremely smooth Cu surface. With the polished Cu as substrates, h-BN ranging from sporadic flakes to a continuous film is grown using CVD. Here, we will study and compare the effects on its nucleation patterns, grain sizes and the quality of the film.

Transition-metal dichalcogenides (TMDs), one of the beyond-graphene two-dimensional (2D) semiconductor materials, has emerged as promising candidates for the next generation electronics due to their distinctive electrical, optical, and thermal properties. In particular, due to the sizable bandgap, molybdenum disulfide (MoS₂) has gained an increasing attention for post-silicon transistor applications, where metal contacts are inevitable and ubiquitous. While the electronic properties of metal-MoS₂ contacts have been the subject of intensive study recently [1,2], fewer investigations are done regarding the vibrational properties and thermal transport of metal-MoS₂ systems.
In this work, we present a first-principles investigation on the thermal transport properties of metal-MoS₂ contacts. Au, Pd, Ru, and Sc, are selected as the electronic contact materials due to small lattice mismatches or low Schottky barriers. Using the Density Functional Perturbation Theory (DFPT) for the interatomic force constant (IFCs) calculation and Landauer formalism for the phonon transport, we have calculated phonon dispersions and transmission coefficients of the metal-MoS₂ systems. Based on the transmission coefficients, interfacial thermal resistances are extracted to characterize the phonon transport across the interfaces. The study reveals two major interfacial categories, namely chemisorption (Ru and Sc) and physisorption (Au), whereas Pd-MoS₂ shows an intermediate nature. At room temperature, the interfacial thermal resistances of the studied systems range from 2×10^-8 Km²/W to 5.7 ×10^-8 Km²/W. Compared with the physisorption, the chemisorption gives a smaller interfacial thermal resistance, which is beneficial in regards to heat transport. This can be explained by the mismatch in phonon spectrums and the difference of interface environment. Further interatomic force constant analysis shows that MoS₂-metal contacts (2×10^-8 Km²/W-5.7 ×10^-8 Km²/W) have much larger interfacial resistances than those of graphene-metal contacts (3.9×10^-9 Km²/W-1.7 ×10^-8 Km²/W), which can be attributed to weaker bondings between MoS₂ and metal species and the diatomic nature of MoS₂. Our findings clearly illustrate the significance of the interfacial atomic details in determining the thermal boundary resistance and pave the way to the optimum thermal management and device optimization of future 2D material-based electronics.
[1] W. Chen et al., Nano Lett., 2013, 13 (2), pp 509-514.
[2] J. Kang et al., Electron Devices Meeting (IEDM), 2012 IEEE International , vol. 17.4.1, no. 17.4.4, pp. 10-13.

We have made a density functional study of the structural and electronic properties of B or N (individual) doped and BN co-doped graphene. The effect of doping has been studied by incorporating the doping concentration amount varying from 2% (one atom of the dopant in 50 host atoms) to 12 % atomic concentration in case of individual doping and from 4% (2 atoms of the dopant in 50 host atoms) to 24 % in case of co-doping, at the same time, altering different doping sites for the same concentration of substitutional doping. We made use of VASP (Vienna Ab-Initio Simulation Package) software based on density functional theory to perform all calculations. While the resulting geometries do not show much of distortion on doping, the electronic properties show a transition from semimetal to semiconductor with increasing number of dopants. The study shows that the BN doping introduces the band gap at the Fermi level unlike individual B and N doping which causes the shifting of Fermi level above or below the Dirac point. It is observed that not only concentration but position of B and N atoms in the hetero-structure also affect the value of band gap introduced.

Molybdenum sulfide (MoS2) has layered structure where each layer is composed of a plane hexagonal array of molybdenum atoms between two sheets of sulfur atoms. Each Mo atom is covalently bound to six S atoms arranged in a triangular prismatic configuration. Each sulfur, in turn, is bound covalently to three Mo atoms. MoS2 monolayers, with a direct bandgap of 1.8 eV, offer an unprecedented prospect of miniaturizing semiconductor science and technology down to a truly atomic scale. Recent studies have indeed demonstrated the promise of 2D MoS2 in fields including field effect transistors, low power switches, optoelectronics, and spintronics. However, device development with 2D MoS2 has been delayed by the lack of capabilities to transfer large-area, uniform, and high-quality MoS2 monolayers. Here we present a transfer method for obtaining high quality and large area monolayer and few-layer MoS2 films on arbitrary substrates.

We recently produced a new 2-D material, viz. Ti3C2, by selectively etching aluminium from a MAX phase Ti3AlC2 and labelled it MXene. MXenes represents a large family of transition metal carbides and carbonitrides, not just a single phase. MXenes allow a variety of chemical compositions and are establishing themselves as a new class of two-dimensional materials. MXenes possess good in-plane conductivity, which in combination with their rich surface chemistry makes them attractive for electrical energy storage applications. However their use in electrochemical capacitors was only recently demonstrated (Lukatskaya et.al, Science, 2013).
Here, we report on the intercalation of Li+, Na+, Mg2+, K+, NH4+, and Al3+ ions between the 2D Ti3C2Tx layers. In most cases, the cations intercalated spontaneously. The intercalation of some ions, notably Al3+, can be promoted electrochemically. We also report on intercalation-induced high capacitance of >300 Farads per cubic centimeter (much higher than that of porous carbons) of flexible Ti3C2Tx paper electrodes in aqueous electrolytes.
Electrodes were fabricated by vacuum filtration of MXene sheets or by conventional film rolling with binder and conductive additives. In some cases MXene was delaminated prior to electrode fabrication in order to achieve better separation of the 2D sheets. Several different electrochemical techniques were employed to understand the mechanism of charge storage. Electrochemical impedance spectroscopy confirmed the low resistivity of the tested materials showing characteristics for capacitors close to 90° angle on the Nyquist plot. Cyclic voltammetry measurements showed a high rate handling ability along with impressive volumetric capacitance values for the electrodes from the delaminated MXene. Galvanostatic cycling showed no degradation of the capacitive properties after more than 10,000 cycles.

Layered materials represent a unique field of material science due to their large surface area and wealth of electronic properties which are important for applications such as supercapacitors and sensing. However, many applications are hampered by the intrinsic insolubility of these materials. In order to utilize their full potential, a robust and simple solution phase processing route is sought for. We have deminstrated that similar to graphene a variety of inorganic layered materials can be readily exfoliated in solution using suitable solvents or surfactants.^[1] Specifically transition metal dichalcogenides (TMD&’s) and transition metal oxides (TMO&’s) have shown to be promising in applications such as field-effect transistors,^[2] and in hydrogen evolution catalysis^[3] as well as many more. The expansion of our solution phase processing route to other layered materials will thus be of great impact for a broad variety of applications and provides unique access to their characterization in solution.
Herein we demonstrate the liquid phase exfoliation of molybdenum trioxide to yield few layer nanosheets. The liquid phase exfoliation procedure based on sonication, consists of critical variables which affect the quality and stability of the final dispersion. These parameters were identified and optimised to yield a stable dispersion of MoO₃ nano-sheets in a variety of solvents. These widely defect-free nano-sheets were investigated by absorbance and emission spectroscopy, transmission electron microscopy, Raman spectroscopy and atomic force microscopy to fully characterise the material obtained through our process.

Films of MoO₃ nano-sheets were tested for potential use as supercapacitor electrodes. MoO₃ only electrodes exhibited a low capacitance due to poor electrical conductivity, thus composites were prepared with SWCNT to utilize of MoO₃'s electrochemical behaviour. Percolation behaviour was observed with the treshold ~5 wt% SWCNT. Capacitances of ~500 F/g were achieved with addition of >5% SWCNT.
[1] Jonathan N. Coleman, Mustafa Lotya, Arlene O&’Neill, Shane D. Bergin, Paul J. King, Umar Khan, Karen Young, Alexandre Gaucher, Sukanta De, Ronan J. Smith, Igor V. Shvets, Sunil K. Arora, George Stanton, Hye-Young Kim, Kangho Lee, Gyu Tae Kim, Georg S. Duesberg, Toby Hallam, John J. Boland, Jing Jing Wang, John F. Donegan, Jaime C. Grunlan, Gregory Moriarty, Aleksey Shmeliov, Rebecca J. Nicholls, James M. Perkins, Eleanor M. Grieveson, Koenraad Theuwissen, David W. McComb, Peter D. Nellist, Valeria Nicolosi, Science 4, 568-571,(2011)
[2] B. Radisavljevic, A. Radenovic, J. Brivio1, V. Giacometti and A. Kis, Nature Nanotechnology 6,147-150, (2011)
[3] T. F. Jaramillo, K. P. Joslash;rgensen, J. Bonde, J. H. Nielsen, S. Horch, I. Chorkendorff, Science 2007, 317, 100-102.

Graphene has emerged as a plastic electronic material due to its excellent mechanical strength, flexibility, and outstanding electron mobility. In order to make a graphene film for flexible electronics application, the need for high quality mass production of graphene, the homogeneous deposition and high adhesion to polymeric substrates poses a great challenge. For these reasons, graphene generated by chemical exfoliation has attracted a great deal of interest as a carbon-based material for flexible electronics application owing to the possibility of high-volume production and versatility in terms of being well-suited to chemical functionalization. Furthermore, there are a number of transfer and patterning methods, such as lithography and template-assisted synthesis and chemical vapor deposition, etc. However, these technologies involve complicated, multistep procedures that result in high cost, material waste, and difficulty in forming large-area patterns. Therefore, screen printing has been shown as an effective method for defined deposition of conducting materials. Recently, a few research groups have fabricated screen-printed graphene electrode by introducing poly(3,4-thylenedioxythiophene)/poly(styrene-sulfonic acid) (PEDOT/PSS) into graphene. However, it also had the disadvantages of high cost and limited PSS availability for printing process. As an alternative conducting polymer nanofiller, polyaniline (PANI)/PSS has received considerable attention because of its easy synthesis, low monomer cost, good thermal stability, and adequate electrical conductivity. In addition, large numbers of functional amino groups in the PANI chain and its inherent nano-sized microstructure can give PANI the possibility to form stable colloid solution with carbon-based materials.
In this presentation, we develop a simple approach to fabricate multilayer graphene/PANI/PSS (G/PANI/PSS)-based conducting paste for screen printing ink. To the best of our knowledge, this is the first demonstration of a graphene-based hybrid with uniform surface resistance using conducting polymer nanofiller, which is derived from the introducing a PANI/PSS nanofiller into the multilayer graphene matrix via mechanical blending. Importantly, as a compatibilizer and binder, PSS increased dispersibility, interfacial interactions, and mechanical interlocking between a multilayer graphene matrix and PANI nanofiller, leading to uniform surface resistance of G/PANI/PSS hybrid film. Furthermore, high concentration of PSS binder via ex situ polymerization could make it possible to improve adhesion of the hybrid film to flexible substrate. Due to its good electrical conductivity and structural flexibility, micro-patterned G/PANI/PSS hybrid film using screen printing was applied to the practical dipole tag-antenna detecting nearby objects.

Two-dimensional (2D) α-molybdenum trioxide (α-MoO3) has recently brought great attention due to its unique planar structure. This material offer planet of enhancements in contrast to its bulk counterpart including the high mobility of free electrons and large surface to volume ratio. In this work, a liquid-based organic solvent-assisted grinding and sonication method is adopted for the formation of 2D α-MoO3 nanoflakes suspensions. The flakes thicknesses are in the order of a few nanometers. The drop-casted thin films composed of these 2D nanoflakes show large responses to a wide concentration range of H2 gas (from 600 to 10000 ppm) with response and recovery time in the order of seconds. These results suggest that MoO3 thin films made of exfoliated 2D nanoflakes, are excellent materials for establishing H2 gas sensors.

Two dimensional materials have attracted great attention to substitute silicon semiconductors. Bulk MoS2 is an indirect band gap semiconductor with a band gap of 1.3 eV and is built up of covalently bonded S-Mo-S layers that are bound by van der Waals forces. Large area MoS2 thin films are essential for electronic devices. Dielectric functions of MoS2 thin films have not been measured because of lack of available large area MoS2 thin films.
We investigated the dielectric functions (ε(E) =ε1(E) + iε2 (E)) of MoS2 films using spectroscopic ellipsometry, assisted with Raman and photoluminescence spectroscopy. Large area MoS2 N-layer films with N=2, 4, 12 were grown on SiO2/Si substrate using plasma-enhanced chemical vapor deposition (PECVD). Large linewidths in Raman peaks suggested that the PECVD-grown MoS2 thin films had defects larger than exfoliated thin films. The N=4 film had the largest amplitude of the dielectric functions whereas all the structures in dielectric function spectra were largely independent of layer number N. From the absorption coefficients, we determined the indirect band gap energies by Tauc plot method. We found that the indirect band gap energy decreased from 1.60 eV to 1.47 eV with increasing film thickness. Applying standard critical point model analysis to the second derivative of dielectric function, we determined several critical point energies. The d2ε(E)/dE2 spectra showed doublet structure (C and D) for 3 eV peak as well as 2 eV peak (A and B). The splitting of C and D peaks near 3 eV was attributed to valence band splitting.[1] Dielectric function of bulk MoS2 showed doublet structure for both low energy excitons (A and B) and 3 eV excitons. In MoS2 ultra-thin films, the doublet structure of N=4 and 12 was clearly observed only in d2ε(E)/dE2 spectra.
[1] A. Molina-Sánchez et al., Phys. Rev B 88, 045412 (2013).

Ultrathin two dimensional metal dichalogenide (MoS2, WS2, so forth) is an emerging class of new materials, which exhibits confinement of carriers, evolution of band structure, high on/off rectification, and high thermal absorption. However their incorporation into other systems requires controlled functionalization and/or interaction with other nanoscale entities. Here we enhance the stable sulfur/nobel metal functionalization via both diffusion limited aggregation and instantaneous reaction arresting (using microwaves). The gold nanoparticles are incorporated selectively on MoS2 crystallographic directions (with 60o displacement). The Raman, electrical and thermal studies indicate a remarkably capacitive interaction between gold and thin MoS2 sheet (CAu-MoS2 = 2.17 mF/cm2), a low Schottky barrier (14.52 meV), a vastly reduced carrier-transport thermal-barrier (253 to 44.18 meV after gold functionalization), and increased thermal conductivity (from 15 W/mK to 23 W/mK post gold deposition). This process provides a route to affiliate MoS2 with potential electronic application, such as electrode-attachment to hetero-structures of graphene and MoS2, where the gold nano structure could be grown to act as an electron-tunneling gate-electrode connected to MoS2.

Monolayer molybdenum disulfide (MoS2), a 2D semiconducting dichalcogenide material with a bandgap of 1.8eV, has demonstrated promise for future use in field effect transistors (FETs) and optoelectronics. Research has shown different growth processes for MoS2, using various molybdenum sources, where chemical vapor deposition (CVD) with sulfur vapor reaction with molybdenum trio-oxide has shown most promising results [1]. We have investigated the use of both molybdenum and MoO3 layers for CVD growth of MoS2. Our transmission electron microscopy results show that magnetron sputtering of MoO3 onto sapphire substrates enabled thin continuous layers of MoS2, while sputtered Mo produce very thick layers of island MoS2 growth. The X-ray photoelectron spectroscopy results show the MoS2 produced from MoO3 to be stoichiometric. Raman spectroscopy and optical results demonstrate crystallinity in the film. Finally, electrical measurements were made showing the MoS2 grown using MoO3 was conductive. Correlations between Mo and MoO3 layers, and resulting 2D MoS2 film chemistry and structure are discussed.
[1] S. Najmaei et al. Nature Materials (2013)

As a two-dimensional (2D) semiconductor, ultrathin molybdenum disulfide (MoS₂) has sparked great interests owning to its substantial band gap (~1.2 -1.8 eV), considerably high room temperature electron mobility (~200 cm²/V.s) and switching capability (current ON/OFF ratio~10⁸). Band gap modulation of MoS₂ is of great importance because the ability to manipulate its optical and electronic properties could lead to additional new functionalities in various fields. Applying strain or external vertical electric field to MoS₂ are two common strategies used to modulate its band gap which are well-established theoretically with few experimental demonstration up to date. In this contribution, we report observation of band gap modulation in ultrathin MoS₂, ranging from a monolayer up to trilayer by a novel method using oxygen plasma. The effect of oxygen-treated ultrathin MoS₂ is analyzed by photoluminescence (PL), micro-Raman and X-ray photoemission spectroscopy (XPS) characterization. We observed a layer dependent reduction in the band gap as a function of plasma treatment duration without suffering from severe surface oxidation while retaining its crystalline structure. Our results here served as a pioneering work towards the possibility of band gap engineering by a simple yet effective method using oxygen plasma, providing alternative to conventional approaches.

We report on uniaxial strain-induced band gap modulation in large-area CVD-grown monolayer tungsten disulfide (WS2) and its dependence of the crystal orientation of strain. A reversible change in the optical band gap was observed in triangular flakes of monolayer WS2 when a tensile strain ranging from 0 to 4% was applied along the direction of one of the triangle edges. This optical band gap modulation, accompanied with variation of integrated photoluminescence (PL) intensity of the WS2 flakes, was caused by strain-induced deformation of crystal structure as supported by theoretic calculation. In contrast, when the tensile strain was applied perpendicular to the triangle edge direction, the band gap variation was not observed. We attribute the orientation dependence of band gap strain modulation to the different deformation potentials of the crystal structure when the tensile strain is applied along different crystal directions. In addition, the full-width-at-half-maximum (FWHM) of PL peak of the WS2 flakes increases with strain. We also observed a red-shift of Raman peaks of the WS2 flakes with tensile strain.
These results may provide new approaches to tunable optical device using monolayer WS2 and other 2D semiconducting transition metal dichalcogenides (TMDs).

Molybdenum disulfide (MoS₂), a layered transition metal dichalcogenide, has been recently reported that in the presence of a high-k environment, the mobility of monolayer MoS₂ can reach 200 cm²/(V s). This coupled with the presence of a significant band gap (~1.8 eV) renders layered MoS₂ as an attractive candidate for electronics applications, unlike graphene where the absence of band gap inhibits its use despite large reported mobilities (200000 cm²/(V s)). Although the chemical synthesis of layered MoS₂ for electronics applications is still in its infancy, promising results have been demonstrated with mobilities up to 4.7 cm²/(V s), which is comparable to that of the mechanically exfoliated MoS₂ (not in a high-k environment). In this work, we study the Raman spectroscopy and AFM morphology on the chemically synthesized MoS₂ based on the synthesis method introduced by Li et al. (L.-J. Li et al., Nano Lett. 12, 1538, 2012). According to the Raman spatial mapping, the sapphire substrates are covered by MoS₂ continuously throughout the wafers. Judging from the peak wavenumber differences from the two characteristic Raman peaks (the E¹_2g mode ~ 382 cm^-1 and the A¹_g mode ~ 405 cm^-1), most of the areas are uniformly covered by 2 to 3 layers of MoS₂. On the other hand, the intensities of the Raman modes vary across the wafers, which may result from the variation of the MoS₂ crystallinity. From AFM topologies of transferred MoS₂ (onto SiO₂/Si substrates), we found that there are areas continuously covered by MoS₂ and there are porous areas with less than 100% MoS₂ coverage. Combined with the Raman study, this implies spatial variation to the MoS₂ crystal quality. More detailed micro-characterization (e.g., electron microscopy) needs to be taken in order to verify the conclusion here.

In this contribution, we present the results of the dynamics of one atom thick h-BN suspended nanoribbons obtained by first performing ab-initio calculations of the deformation potential energy and then solving numerically a Langevine type equation to explore their use as energy harvesting devices. Similarly to our previous proposal for a graphene-based harvester [1], an applied compressive strain is used to drive the clamped-clamped nanoribbon structure into a bistable regime, where quasi-harmonic vibrations are combined with low frequency swings between the minima of a double-well potential. Due to its intrinsic piezoelectric response, the mechanical harvester naturally provides an electrical power that is readily available or can be stored by simply contacting the monolayer at its ends. Engineering the induced non-linearity, the proposed device is predicted to harvest an electrical rms power of more than 150 fW when it is excited by a noisy external force characterized by a white Gaussian frequency distribution with an intensity in the order of Frms=5pN.
[1] Loacute;pez-Suárez, M., R. Rurali, et al. (2011). "Nanostructured graphene for energy harvesting." Physical Review B 84(16): 161401.

Hexagonal boron nitride (h-BN), as a promising dielectric with similar graphite structure, currently attracts considerable attention, both for the fascinating properties of the individual monolayer and its successful incorporation as a complementary two-dimensional (2D) dielectric substrate in graphene based electronics. H-BN is a wide band gap III-V compound with remarkable physical properties and chemical stability such as inertness, high temperature stability, low dielectric constant, large thermal conductivity, and high mechanical strength. Similar with graphite, each layer of h-BN is held together by weak van der Waals forces. Therefore, monolayer h-BN films could be peeled off from bulk BN crystal by micromechanical cleavage. Chemical vapor deposition (CVD) has already been used to demonstrate the growth of single layer h-BN by many groups. However, methods for preparing good quality h-BN layers over large areas would bring new opportunities to investigate the properties and potential applications based on h-BN. Here we report an approach to synthesize uniform and continuous h-BN films on metal substrate by using gas source molecular beam epitaxy (MBE). MBE can provide atomic layer epitaxy accuracy and controllability due to high vacuum, low temperature, excellent shutter control, etc. Elemental boron oxide evaporated by effusion cell and active nitrogen generated by electron cyclotron resonance (ECR) plasma source were used as the group-III and-V sources, respectively. H-BN epitaxial growth on cobalt substrates was demonstrated at a growth temperature of 900oC. Raman and X-ray photoelectron spectroscopy (XPS) characterizations confirm the growth of h-BN, and atomic force microscopy (AFM) images show the resulting h-BN film has atomically smooth surface and few layers. The results of the growth and characterization of h-BN films on other substrates such as nickel and copper will also be presented.

Solution-exfoliated MoS2 nano-platelets were formed into thin films by deposition onto a water
surface followed by transfer to indium tin oxide coated glass. After drying, a gold electrode was
evaporated on top to give a sandwich structure with quasi-Ohmic contacts. Illumination of this
device with broadband light of 1 kW/m2 intensity gave a fourfold increase in conductivity. The
photocurrent increased sub-linearly with intensity and exponentially with time indicating the
presence of traps. The photo-responsivity at low intensity was 10^-4 A/W at 15 V. This work
demonstrates the potential for liquid-exfoliated, inorganic nanosheets to be fabricated into
low-cost optoelectronic devices.

In this presentation, we will report on the synthesis of few atomic-layers BNNSs and fabrication of a Schottky diode. The synthesis process is carried out by irradiating hexagonal boron nitride target using CO2 laser pulses. High resolution transmission electron microscopy (HRTEM) showed the sheets are mostly defect-free and characteristic honeycomb structure of six-membered B3-N3 hexagon can easily be interpreted. The thicknesses of individual BNNSs is measured (1.7 nm) using HRTEM by imaging the edges of nanosheets. Additionally, doping in BNNSs with carbon atoms adjusted the band-gap and made it feasible to fabricate nano-electronic device e.g. Schottky diode. BNNSs-based metal-semiconductor-metal junction has been developed and current versus voltage (IV) characteristics are recorded at different temperatures 25 oC, 50 oC and 75 oC. It has been identified that slight doping carbon into BNNSs would bring a significant change in the output characteristics of diode. A rapid increase in the forward current (35µA) and breakdown voltage lower than -70V is achieved, indicating that breakdown electric field is almost up to 108 V/cm.

Transition metal dichalcogenides with layered structure, especially MoS2, have attracted much physical-chemical interest due to their 2D structure analogous to graphene. Using a versatile chemical route, layered MoS2 nanostructures were obtained on RGO. The structure and morphology were characterized by Scanning Electron Microscopy, Transmission Electron Microscopy and Atomic Force Microscopy. By X-ray Photoelectron Spectroscopy and FT-IR, we detected that the first layers of MoS2 are bonded to oxygen of the RGO by chemically covalent bond (Mo-O-C). The electrochemical characterization was made for three MoS2/GO ratios. The specific capacitance measured values at 10 mVs-1 are 128, 265 and 148 Fg-1 for the MoS2/RGO with low, medium and high concentration of MoS2 respectively. Disregarding any synergetic effect between both and considering only MoS2 as an active material, an intrinsic specific capacitance value of 1558 F g-1 and a calculated energy density of 63 Wh kg-1 can be observed for the MoS2 in the low concentration hybrid material. To demonstrate the activity of the layered nanostructures for hydrogen evolution reactions, exchange current density (Jo) and Tafel slope (b) was evaluated. The exchange current density value for the hybrid material with medium (J0 = 120 mu;A/cm2) and high (J0 = 117 mu;A/cm2) MoS2 concentrations shows a value similar to the exchange current density measured for Pt (J0 = 117 mu;A/cm2).

As an atomically thin layered material, hexagonal boron nitride (h-BN) has gained a widespread research interest due to its potential applications in different areas of materials engineering. By contrast to its carbon counterpart graphene, the insulating nature of h-BN makes it versatile for using as a dielectric medium while manipulating/fabricating heterostructures [1,2]. To a great interest, the interface between h-BN and metal substrates opens up ample possibility of developing new class of atomically thin heterostructures.
Extensive research from the past decade has demonstrated that electronic properties of graphene and h-BN can be modulated by doping [3,4]. In heterostructures, such methods have demonstrated ability to modify properties of individual layer as well as that of the complete structure itself [3]. In this contest, response of h-BN in the presence of alkali elements raises interesting questions about its electronic structure.
By using first principles density functional theory (DFT), in conjunction with core and valence band photoemission spectroscopy, we present the effect of Li and K on the electronic structure of h-BN supported by Au/Ni(111). Experimental ARPES scan shows a large down-shift in the pi-bands of the h-BN in the presence of both Li and K. A rigid down-shift in all the h-BN bands calculated from DFT rationalizes the experimental findings.
The calculations attribute the origin of the shift in the pi-bands to an interfacial electric field arising from the dopants. Charge rearrangement within the system results in the generation of a capacitor like field at the Au-(alkali) interface. Efficient utilization of such interfacial electric field generated by the presence of alkali adatoms could lead towards promising applications in several areas such as tunnel junctions, gate insulator, tuning of band alignment etc.
[1] Kenji Watanabe, et al., Nat. Mater., 3, 404, 2004.
[2] Dimitri Golberg, et al., Acs Nano.,4,2979, 2010.
[3] Menno Bodkam, et al., Nano. Lett., 11, 4631, 2011.
[4] Sen Lin, et al., J. Phys. Chem. C, 117, 17319, 2013.
[5] Han Wang, et al.,IEEE Electr. Dev. Lett., 32, 1209, 2011.

Using first-principles calculations and deformation potential theory, we investigate the intrinsic carrier mobility (mu;) of monolayer MoS2 sheet and nanoribbons.In contrast to the dramatic three orders of magnitude of deterioration of mu; in graphene upon forming nanoribbons, the magnitude of mu; in armchair MoS2nanoribbons is comparable to its sheet counterpart, albeit oscillating with ribbon width. Surprisingly, a room-temperature transport polarity reversal is observed with mu; of hole (h) and electron (e) being 200.52 (h) and 72.16 (e) cm2V-1s-1 in sheet, and 49.72 (h) and 190.89 (e) cm2V-1s-1 in 4 nm-wide nanoribbon. The high and robust mu; and its polarity reversal are attributable to the different characteristics of edge states inherent in MoS2nanoribbons. Our study suggests that width-reduction together with edge engineering provide a promising route for improving the transport properties of MoS2 nanostructures. For any potential nanoelectronic applications, the thermal conduction and management in the MoS2 based devices are ultimately important. Through calculating the phonon dispersion and Gruneisen parameters, we analyze the intrinsic relaxation time and mean free path of MoS2 monolayer based on U-scattering process of phonons. The thermal conductivity of MoS2 monolayer is calculated based non-equilibrium Green's function.

Hydrogenated graphene has been emerging as the cynosure of the subject for numerous studies with their conductivity, ferromagnetism, and energy storage as well as drug delivery. However, how to find a decent way to overcome the graphene bending barrier and modify the graphene from planar structures to 3D structures remains to be further explored. By virtue of molecular mechanics/dynamics simulations, here we present the formation of carbon nanohelix from a pristine graphene nanoribbon by doping it with hydrogen atoms in a specific pattern. Meanwhile, we quantitatively investigate the effect of interatomic potential on the process of helical structure formation, thermal stability and mechanical properties of the carbon nanohelix as well as its potential application in molecule packing. Carbon nanohelix portrays an intriguing zigzag strain-stress curve and amazing extensibility under tension as well as relatively limited deformability under compression, which represents its unique signature of mechanical properties to differentiate the carbon nanohelix from the behavior of carbon nanotube and graphene. These findings lends compelling credence to envision that the carbon nanohelix opens up a viable avenue for nanofabrication and is perceived as a novel nanomaterial for a variety of applications such as electronics, sensors, energy storage, drug delivery and nanocomposites.

While there have been many reports on the optical and electronic properties of two-dimensional (2D) semiconductor nanostructures in transition metal compounds such as molybdenum and tunsten chalcogenides, it remains challenging to study 2D nanostructures in other materials which do not readily grow as 2D structures or do not possess layered crystal structures. We demonstrate unusual crystal growth of lead (Pb) chalcogenides, which due to the rock salt crystal structure typically lacks anisotropic growth. However, chemical engineering of colloid growth for Pb chalcgenides can induce selective growth from zero dimensional to one and two-dimensional nanoscale crystals.
Due to the large Bohr radius (PbS 18 nm, PbSe 46 nm) and narrow direct band gap (PbS 0.41 eV, PbSe 0.28 eV) of Pb chalcogenides, the above mentioned 2D Pb chalcogenide nanostructures exhibit the effect of quantum confinement of carriers. We will explore how asymmetric confinement of carrier is manifested in the characteristics of charge carriers, exciton dynamics, and other optoelectric properties for these 2D Pb chalcogenide nanostructures based on theoretical and spectroscopy studies. Furthermore, there are huge opportunities of Pb chalcogenide nanostructures in photovoltaics, sensors, transistors and other optoelectronic devices. We will discuss how the unique properties of 2D nanostructures can be utilized for those applications.

Recently, there has been a growing interest in two-dimensional (2D) materials - termed van der Waals materials - that can be mechanically exfoliated in a “graphene-like” fashion. Tantalum diselenide (TaSe₂) is a transition metal dichalcogenide that belongs to this group of materials. Each individual layer of TaSe₂ crystal consists of a sandwich of Ta atom with two Se atoms on either side of the Ta atom. The single layer thickness is ~0.6359 nm. In this presentation, we report the results of our investigation of the optical response of the exfoliated thin films of TaSe₂. The single-crystal TaSe₂ bulk samples were synthesized by the chemical vapor transport (CVT) method. The thin films were prepared by the “graphene-like” mechanical exfoliation. In order to investigate the photo-response of thin films of TaSe₂, we fabricated devices using electron beam lithography (EBL). The contacts were made with a 10-nm thick layer of Ti followed by a 100-nm thick layer of Au. The thickness of the fabricated device channels ranged from 20 nm to 40 nm. The source-drain current as a function of the source-drain voltage was measured under dark and illuminated conditions. At small bias, the channel was resistive allowing for negligible current only. As a threshold field was reached (corresponding to some threshold voltage), one observed a fast increasing non-linear current. The light illumination shifted the on-set of the high non-linear current regime to larger voltages. There was a substantial difference between the threshold voltages at dark and illuminated conditions allowing for optical switching applications. It is interesting to note that unlike a conventional photo-resistor, the TaSe₂ device switched on in the dark and switched off under illumination. The physical mechanism of the observed optical effects in TaSe₂ thin films was different from that in conventional semiconductors and can be related to collective or excitonic effects. The light sensitivity of TaSe₂ thin films was tested between 645 nm and 340 nm wavelengths. It was found that the threshold voltage of the TaSe₂ devices increases as the wavelength of light decreases.
Work in the Balandin group was supported, in part, by the National Science Foundation (NSF) and Semiconductor Research Corporation (SRC) Nanoelectronic Research Initiative (NRI) project “Charge-Density-Wave Computational Fabric: New State Variables and Alternative Material Implementation” NSF-1124733 and by STARnet Center for Function Accelerated nanoMaterial Engineering (FAME) - Semiconductor Research Corporation (SRC) program sponsored by MARCO and DARPA.

Tuning band energies of semiconductors through strain engineering can significantly enhance their electronic, photonic and spintronic performances. Although low-dimensional nanostructures are relatively flexible, the reported tunability of bandgap is within 100 meV per 1% strain. It is also challenging to control strains in atomically thin semiconductors precisely and monitor the optical and phonon properties simultaneously. Here, we developed a system that can apply strain to MoS2. Photoluminescence and Raman characterizations show that the direct bandgap can be blue-shifted for ~300 meV per 1% strain. First-principles investigations confirm the shift of the direct bandgap and reveal a higher tunability of the indirect bandgap than the direct one. The exceptional high strain tunability of electronic structure in MoS2 promising a wide range of applications in functional nanodevices and the developed methodology should be generally applicable for two-dimensional semiconductors.

An intriguing feature of monolayer transition metal dichalcogenides (TMDs) is that they can exist in more than one structural phase. These different phases have vastly different physical properties. Monolayer polymorphism has important potential applications in the case of group VI TMDs such as MoS2. In these compounds, the naturally occurring semiconducting phase can be complemented by a usually metastable metallic phase in an electronics context, and metallic phases in group VI TMDs show great promise for hydrogen evolution catalysis. While there are known chemical routes to obtaining metastable octahedral metallic group VI TMDs, the thermodynamic phase behavior of metallic monolayer TMD phases are largely yet to be elucidated. One would like to know under what conditions (if any) a metallic monolayer phase is stable instead of metastable. We have discovered that the energetic ordering of structural phases in all six group VI TMDs (MoS2 through WTe2) can be controlled through the application of stress and strain. After developing a two-dimensional thermodynamic framework that includes room temperature thermal effects, we use density functional theory and hybrid density functional/Hartree-Fock calculations to discover that the deformations required to yield a semiconductor-to-metal phase transition range from 0.5 to 15% engineering strain for the studied compounds. These values fall within the experimentally demonstrated range of elastic deformations afforded by the exceptional mechanical strength of TMD monolayers. The potential applications for this discovery range from catalysis to information storage and nanoscale electronics.

Molybdenum disulfide (MoS2) is an attractive material which has good electrical properties for use in field effect transistors (FETs) [1]. MoS2 synthesis methods are mechanical exfoliation, chemical exfoliation, thermolysis etc. However, it is still difficult to synthesize large area and high quality MoS2 thin film with good electrical performance. Generally, solution-processed MoS2 thin film with large-area capability exhibits low crystalline or amorphous structure which can be attributed to the carbon contamination from the organic residues of solvent [2].
In this study, bottom gate MoS2 FETs are produced by solution-process using (NH4)2MoS4 precursor. To fabricate MoS2 FETs with good electrical properties, the two-step annealing was attempted to increase the thermolysis temperature of precursor in forming gas (H2, 5% and N2) and to effectively remove carbon residue. Two-step annealed MoS2 FETs exhibited the high field effect mobility of 26cm2/V-s, the subthreshold swing of 0.92V/dec, Ion-off ratio of 10-7.
[1] B. Radisavlijevic, A. Radenovic, J. Brivio, B. Giacometti, and A. Kis, Nat. Nanotechnology, 6, 147 (2011).
[2] Keng-Ku Liu, Wenjing Zhang, Yi-Hsien Lee, Yu-Chuan Lin, Mu-Tung Chang, Ching-Yuan Su, Chia-Seng Chang, Hai Li, Yumeng Shi, Hua Zhang, Chao-Sung Lai, and Lain-Jong Li, Nano Lett., 12, 1538 (2012).

Epitaxial growth between graphene, hexagonal boron nitride (h-BN) and chalcogenide compounds can be achieved in terms of van der Waals hetero-epitaxy due to their layered nature in crystal structure. Van der Waals hetero-epitaxy enable abrupt heterojunction with minimized defects or dislocations, because it can allows efficient stress relaxation which originated from its lattice mismatch by weakly bound by unoccupied dangling bonds. Here, we report on van Der Waals epitaxy of layer-by-layer growth of Bi₂Te₃-Sb₂Te₃on h-BN using a two-step non-catalytic vapor deposition. Atomic force microscopy shows that as-grown Bi₂Te₃-Sb₂Te₃ 2-D vertical heterostructure confirms that its growth proceeded by quintuple atomic layers as a unit. Then we found that the formation of Bi₂Te₃-Sb₂Te₃ on h-BN flake is spatially confirmed by confocal Raman spectroscopy. Furthermore, cross-sectional transmission electron microscopy shows that Bi₂Te₃-Sb₂Te₃ plates are epitaxially grown on h-BN by forming abrupt interface in spite of its lattice mismatch. Our epitaxial heterostructure of few-layer topological insulator heterostructure and insulating h-BN can be the essential basis for exotic topological insulator physics and provide practical implication for new electronics and photonics.

In its bulk form, the thermoelectric material bismuth telluride (Bi₂Te₃) has a high figure of merit (ZT) at room temperature. When bulk Bi₂Te₃ is exfoliated to smaller dimensions, micro and nanosheets, its thermoelectric effects are magnified thus increasing the ZT. Nevertheless, individual micro or nanosheets are of little use in an industrial application due to the difficulty involved incorporating them with energy harvesting systems. This difficulty motivated the search for a form of Bi₂Te₃ that retains the high ZT of the nanosheets with the capability of being deposited onto device infrastructures. This work showcases an exfoliation of the Bi₂Te₃ to individual nanosheets that are then recombined into a practical thin film that can be distributed across a surface, while still maintaining the benefits of the Bi₂Te₃ nanosheets. These thin films of Bi₂Te₃ were deposited from a paintable paste that was developed by mixing with a polymer. Recent advances in chemical exfoliation in combination with reflux, distillation and sonication of the material resulted in the production of a thermoelectric paste which, when enveloped with a polymer, creates a stable product that can be applied to surfaces using an innovative painting technique. Atomic force microscopy and scanning electron microscopy have shown that this painting technique forms highly reproducible Bi₂Te₃ thin films. Electrical transport studies show that the films have conductive pathways over a range of surfaces and various structural formations. The combination of the facile preparation method and the immense scope for diverse surface configurations bodes well for integration with heat producing devices for energy harvesting applications.

Two-dimensional (2D) layered structures such as Bismuth Telluride (Bi₂Te₃) are materials with exotic electronic properties that are important for sensing, catalysis and are under investigation for energy storage applications. Recent findings focus on solvent exfoliation of bulk layered materials with strong in plane bonds but very weak out of plane bonds to create nanosheets that are micrometres wide, Bi₂Te₃ is one such material. Other literature focuses on Bi₂Te₃ and its thermoelectric properties at nanoscale dimensions; however there is very little information on how to use the desirable properties of Bi₂Te₃ in everyday devices. This work proposes a facile combination of recent advances in chemical exfoliation and the complementary techniques of reflux and distillation to create a thermoelectric paste. Combining this paste with a polymer creates a manoeuvrable paint that can be spread with an innovative painting technique on surfaces to create cohesive and conductive thin films. The paste was prepared using several different solvents and Bi₂Te₃ powder:solvent ratios in order to optimize the formulation of thin films. The two solvents that were found to be the most successful were cyclohexylpyrrolidine (CHP) and n-mythl-2-pyrrolidone (NMP); these were then used in various ratios with Bi ₂Te₃ powder (1:1, 2:1, 4:1, 1:2) throughout the study. In order to create conductive and stable thin films, a nonconductive polymer polyethylene glycol 400 (PEG), was added to the mixture and electrical transport investigations showed they facilitate conductive electrical pathways between the exfoliated flakes of Bi₂Te₃ in the painted films. The marriage of this chemical exfoliation method and film-painting allows an easily scalable and very reproducible way to coat 2D nanosheet dispersions on planar or curved surfaces for functional thin film paintable materials in energy harvesting and energy storage.

Single-layer molybdenum disulfide (SL-MoS₂) has attracted attention as a 2-dimensional material due to its promising electronic band structure. Unlike graphene, single-layer MoS₂ exhibits a direct band gap of ~1.8 eV [1], and may be a more suitable thermoelectric material amongst other applications. Kaasbjerg et al. [2] have derived the relaxation times for electron-phonon interactions in SL-MoS₂ from first principle calculations. A similarly rigorous study of phonon transport is currently missing but remains necessary to evaluate the thermoelectric properties of SL-MoS₂.
Here, we utilize the spectral energy density [3] method to calculate anharmonic phonon relaxation times. Within this approach, the required velocities of atoms in the simulation box at each time step are calculated from ab initio Car-Parrinello molecular dynamics (CPMD) simulations [4]. The conventional cell contains 48 atoms and has the dimensions 8radic;3α × α where α is the lattice constant of the MoS₂ crystal. The longer side of the simulation box is along the Γ-M direction in reciprocal space. This choice of simulation cell allows us to calculate the relaxation times for phonons with smaller q-vectors in the Γ-M direction. Phonon dispersion curves are readily calculated using density functional perturbation theory (DFPT) [5].
We discuss key frequency dependence in phonon relaxation times and their implication for thermal transport. Using relaxation times for electron-phonon scattering from the literature, we calculate electrical conductivity, the Seebeck coefficient and the power factor for thermoelectric energy conversion. While electrical conductivity increases with carrier density (n) up to 1.4×10⁵ S/m for n=10¹²cm^-2, the Seebeck coefficient decreases with n from ~1500 µV/K at n=10⁸ cm^-2 to ~800 mu;V/K at n=10¹² cm^-2. In addition, for n=10¹² cm^-2, both electrical conductivity and Seebeck coefficient decrease with temperature within the temperature range of T=200-500 K. Finally, we discuss the overall prospects for MoS₂ as a thermoelectric material.
1. K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, Physical Review Letters, 105(13), 136805 (2010).
2. K. Kaasbjerg, K. S. Thygesen, and K. W. Jacobsen, Physical Review B, 85(11), 115317 (2012).
3. J. A. Thomas, J. E. Turney, R. M. Iutzi, C. H. Amon, and A. J. McGaughey, Physical Review B, 81(8), 1-4 (2010).
4. R. Car and M. Parrinello, Physical review letters 55(22), 2471 (1985).
5. S. Baroni, S. de Gironcoli, A. Dal Corso, and P. Giannozzi, Review Modern Physics 73, 515 (2001)

The unique properties of novel nano-materials are often hard to assess in particular when conventional models are used to extract the same. This is in part due to the fact that electrical transport can be highly non-uniform in low-dimensional systems. Another reason lies in the confined transport in typical nanomaterials, i.e. current flows in an ultra-thin body that impacts the available states and electrostatic conditions. Two-dimensional layered materials are an example of a class of systems that falls into this category. Bias conditions can alter the flow of the current substantially and result in phenomena that are
typically not observed in bulk type systems. Understanding and evaluating the properties of two-dimensional layered systems is thus key to develop novel device ideas and applications.
In this talk I will first discuss the impact of an ultra-thin body structure on the transport in two- and three-terminal device structures. In particular, I will elucidate the impact of the geometric screening length on the scaling of devices and the injection of carriers in comparison to conventional device structures. A simple temperature dependent Schottky barrier model that is tailored towards low dimensional systems will be discussed and used to extract information about contact related aspects at the interface between a source/drain metal electrode and the layered material that acts as the channel. Based on this discussion, critical information about the dichalcogenide-to-metal interface properties are gained. The last part of the presentation will include the impact of layer thickness and channel length on the gated transport in two-dimensional layered materials.

Two-dimensional materials beyond graphene have seized considerable attention in recent years, especially, for transition-metal dichalcogenides (TMDs, e.g. MoS2) due to its narrow bandgap [1-5]. One of the fundamentals of semiconductor research is that the concentration and distribution of dopant atoms controls local electronic properties. With TMDs rapidly rising as the semiconductor-of-choice for future flexible nanoelectronics, controlled chemical doping provides extra flexibility to fine-tune the band gap for specific applications. However, in order to make these materials more versatile, further modification of their physical and chemical properties is required. Ternary two-dimensional (2D) dichalcogenide alloys exhibit compositionally modulated electronic structure and hence, control of dopant concentration within each individual layer of these compounds provides a powerful tool to efficiently modify their physical and chemical properties. The main challenge will be the synthesis of ternary atomic layers without phase separation, as well as quantifying and locating the dopant atoms within layers.
Based on our previous success of synthesizing large-scale and monolayer MoS2 crystal [3, 5], molybdenum disulfide substitutionally doped with a broad range of selenium concentrations has been developed via one-step chemical vapor deposition method [6], resulting in over 10% band gap modulations in atomic layers. We show that the bandgap of the material could be fine-tuned between 1.85 and 1.60 eV by changing the Se doping level. The excellent transport properties and control over the band structure of the material paves the way for future investigation of physical properties and applications of Se doped MoS2 ternary systems. Chemical analysis using Z-contrast imaging provides direct maps of the dopant atom distribution in individual MoS2 layers and hence a measure of the local band gaps for both mono- and bi-layered Se doped MoS2.Our work provides new insights into the growth mechanism and alloying behavior in two-dimensional dichalcogenide atomic layers.
References:
[1] Z. Liu et al., Nat. Nano., 8, 119 (2013)
[2] Z. Liu et al., Nat. Comm., 4, 2541 (2013)
[3] S. Najmaei, Z. Liu et al., Nat. Mater., 12, 754 (2013).
[4] W. Zhou et al., Nano Lett., 13, 2651 (2103)
[5] Y. Zhan, Z. Liu et al., Small, 8, 966 (2012)
[6] Y. Gong, Z. Liu et al., Nano Lett., in press (2013).
This work was supported by the Welch Foundation grant C-1716, the NSF grant DMR-0928297, the U.S. Army Research Office MURI grant W911NF-11-1-0362, the U.S. Office of Naval Research MURI grant N000014-09-1-1066, the National Research Foundation Singapore under NRF RF Award No. NRF-RF2013-08. This research was supported in part by a Wigner Fellowship through the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory (ORNL), managed by UT-Battelle, LLC, for the U. S. DOE (WZ).

Recently, atomically thin two dimensional (2D) transition metal disulfides (TMDs) MS2 (M = Mo, W) have attracted great attention to complementing graphene. The bulk TMDs exhibit weak van der Waals force between TMDs layer and layer which is composed of metal atoms sandwiched by sulfur layers with strong sulfur and metal covalent bonds, and indirect band gap (MoS2 = 1.2 eV and WS2 = 1.3 eV). The mono-layer TMDs that can be exfoliated by mechanical force from bulk TMDs show direct bandgap (1.8 eV and 2 eV), high carrier mobility and flexibility which offering possibilities for wide range of applications. Based on this, atomically thin MoS2 film has been synthesized several methods: sulfurization of molybdenum thin film, chemical vapor deposition (CVD) with MoO3 and sulfur powder and annealing (NH4)2MoS4 thin films. Although, reported methods showed high crystallinity and good stoichiometry, there are limitations in control of layer number of WS2 and large scale deposition up to 2 inch. This unsatisfactory uniformity and controllability in the growth of WS2 result in the boundary of applications in 2D material. Here, we investigated the synthesis of WS2 film on the SiO2 substrate through sulfurization of plasma enhanced atomic layer deposition (PE-ALD) WO3-x thin film, which showed all advantages of ALD such as thickness control in atomic scale, very large area uniformity and high conformallity. We systematically controlled layer number of WS2 from mono- to tetra-layer by control of PE-ALD cycle number of WO3-x, and WS2 film showed very large area uniformity over 6 inch wafer scale. The electrical properties of our WS2 film was evaluated by top gate field effect transistor. Furthermore, we developed new process to fabricate WS2 nanotubes utilizing high conformality of ALD process.

The realization of viable electronics technology based on new semiconductors requires low resistance contacts for achieving practical drive currents. Ultrathin molybdenum disulphide (MoS2) has emerged as an interesting new semiconductor because its finite band gap and absence of dangling bonds. Mitigation of large contact resistance is an active area of research in two-dimensional and ultra-thin MoS2 field effect transistors (FETs). Typical contact resistance values between metal and ultra-thin MoS2 range from MOmega; to tens of kOmega;, leading to Schottky behavior limited transport. Decreasing the contact resistance is crucial for obtaining large on-state currents. In this study, we engineer the metallic 1T phase of MoS2 as the source and drain electrodes in FETs. We demonstrate that the 1T phase can be locally induced and patterned on to semiconducting 2H phase MoS2 nanosheets, which decrease the contact resistance to record values (200 - 300 Omega;-µm at zero gate bias compared to ~ 0.7 - 1 kOmega;-µm). FETs with metallic 1T phase electrodes fabricated and tested in air without any annealing exhibit mobility values of ~ 50 cm2/V-s, subthreshold slope values of < 100 mV/decade, on/off ratios of >107, drive currents approaching ~ 100 µA/µm, and excellent current saturation. We also demonstrate that independent of the choice of metals used as electrodes on the metallic 1T phase, there is no influence on the FET performance, suggesting that it is the 1T/2H interface that controls the injection of carriers into the channel. Our results provide a new strategy based on phase engineering for achieving low resistance contacts and reproducible performance of FETs based on ultrathin MoS2.

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. In consequence, the development of new technologies 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, single layer transition metal dichalcogenides (TMDs) have recently emerged in nanoelectronics, although the study of layered TMDs beyond MoX2/WX2 is still incipient.
We studied the electronic structure of Ti, Zr, Hf, Ni, Pd and Pt 2D TMDs and nanoflakes/nanodiscs were investigated via periodic density functional theory and density-functional based tight-binding calculations. For group X TMDs, disulfide monolayers are indirect band gap semiconductors while the diselenides and ditellurides analogues present significant band gap reduction and can even become semimetallic or metallic materials. We also explored the stacking effects of these materials. A mechanical strain of these materials leads rapidly to a metallic electronic structure if compressed, but to quasi-direct band gap semiconductors if stretched. This behavior could be exploited towards the application of these materials in nanoelectronics. On the other hand, diverse group IV TMDs nanoflakes/nanodiscs shapes up to 10nm were explored and their properties (electronic structure, interlayer interaction, edge composition...) were compared with the pristine mono- and multilayer 2D materials.

Two dimensional transition metal di-chalcogenides present an attractive set of semiconductor materials that could lead to new and improved applications for both electronics and optoelectronics. In this presentation, we shall discuss about two specific areas: (i) potential for using these materials for ultra scaled digital switches and (ii) possibility of synthesizing novel heterostructures for optoelectronic applications. For both cases, we shall highlight the unique functionality provided by the di-chalcogenides that are otherwise not available from conventional materials. We shall also discuss difficulties and challenges that exist for technological adoption.

Recently, there has been interest in two-dimensional materials such as transition-metal dichalcogenides (TMDs) (MoS2, WSe2, WSe2, WSe2) for their possible use in electronic devices. Monolayer TMDs have direct band gaps, which allows their FETs to be turned off and have saturated output characteristics [1]. Mooser [2] noted that bulk MoS2 had a room temperature electron mobility of 50-200 cm2/V.s by Hall effect. These materials may be more useful for thin film transistors that for ‘end of road map&’ FETs [3]. On the other hand, there is little known about their ability to be doped, their band line-ups and how to design good contacts.
Here we calculate the band structures of MoS2, their charge neutrality levels (CNLs), and the electronic structure of the S vacancy, the lowest cost defect. We use the screened exchange hybrid functional [4], to correct the band gap errors of standard DFT. We calculated the effect of various substitutional impurities in MoS2 using supercells. We find that Br, Cl are deep donors, and As and P are shallow acceptors. Hence donors would be interstital Br and Cl. We also calculated the CNLs of various semiconducting TMDs from their band structure. This allows their relative band offsets to be given, and also their band offsets against gate insulators such as HfO2 and Al2O3.
The neutral S vacancy was found to have a formation energy of 2.35 eV in the S-rich limit. Thus the S vacancy is the dominant defect [5]. It gives a doubly degenerate state ~ 1.4 - 1.42 eV above the VBM in monolayer MoS2. The defect formation energy has 0/- and -/2- transitions levels at 1.23 eV and 1.28 eV above VBM. The similar results are found for other 2D TMDs. The heat formation of MoS2 is -2.86 eV or -1.43 eV per S, so the formation energy of the S vacancy drops to 0.92 eV in the S-poor limit. This energy becomes negative for Fermi energies near the CBM. This means that S vacancies would spontaneously form if MoS2 is annealed in contact with reactive metals such as Ti or Sc. It is also consistent with the predominantly n-type transport seen mostly in MoS2 . On the hand, the CNL lies lower in the gap in WSe2, which may allow ambipolar transport [6].

1. B. Radisavljevic, Nature Nano. 6 147(2011)
2. R Fivak, E Mooser, PRB 163 743 (1967)
3. S Kim, et al , Nature Comms 3 1 (2012)
4. J Robertson S Clark, PRB 82 085208 (2010)
5. W Zhou et al, Nanolett 13 2615 (2013)
6. Y Zhang, et al, Nano Lett 12 1136(2012)

Achieving ohmic contacts for layered transition metal dichalcogenides (MoS2, WS2, WSe2 and MoSe2) has been a challenge to researchers owing to the schottky barrier between metal and semiconductor. This has resulted in low on-currents, mobilities and sub-threshold slopes in the devices made with these materials. We have developed a chemical approach to reversibly transform semiconducting phase (2H) and metallic phase (1T) of these semiconductors. Taking advantage of the metallic phase, we fabricated hybrid transistors which have 1T phase of the material at the contacts and 2H phase of the material as the channel. The 1T phase significantly reduces the schottky barrier between the metal and the semiconductor thereby mitigating the high contact resistance issues. This strategy should be applicable to several other applications such as catalysis, supercapacitors and batteries. Material synthesis, compositional, optical and electrical characterization results will be discussed.

We study the metal contact properties on single-layer molybdenum disulfide (MoS2) crystals, and hence reveal the nature of switching mechanism in MoS2 transistors. We fabricate field-effect transistors on single layer MoS2 synthesized by chemical vapor deposition (CVD) method. On investigating transistor behavior with various contact length, ranging from 200 nm to 2 mu;m, we find out that the contact resistance for metal/MoS2 junctions is defined by contact area instead of contact width. We also study the gate-dependent sheet resistance, contact resistance, contact resistivity and transfer length in Ti-contacted MoS2 transistors. The minimum transfer length is ~0.63 mu;m at on-state for metal (Ti) contacted single-layer MoS2. This reveals the nature for MoS2 transistors is a Schottky barrier transistor, where the on/off states are switched by the tuning the Schottky barriers at contacts. The effective barrier height for drain and source barrier are separately controlled by gate and drain biases. We discuss the drain induced barrier narrowing effect at short channel regions, showing great potential of MoS2 Schottky barrier transistors at the channel length scaling limit.

Two-dimensional (2-D) semiconductors exhibit excellent device characteristics, as well as novel optical, electrical, and optoelectronic characteristics. In this talk, I will present our recent advancements in contact engineering, surface charge transfer doping, and heterostructure devices of layered chalcogenides. Forming Ohmic contacts for both electrons and holes is necessary in order to exploit the performance limits of enabled devices while shedding light on the intrinsic properties of a material system. In this regard, we have developed different strategies, including the use of surface charge transfer doping at the contacts to thin down the Schottky barriers, thereby, enabling efficient injection of electrons or holes into MoS2 and WSe2 mono- and multi-layers. As a result, we have been able to show high performance n- and p-FETs with both MoS2 and WSe2. Additionally, I will discuss the use of layered chalcogenides for various heterostructure device applications, exploiting charge transfer at the van der Waals heterointerfaces.

1/f noise in semiconductor device and circuit provides important information regarding quality of the interface as well as the transport mechanism. In 1D and 2D channel materials, 1/f noise also provides information on stability under ambient conditions, including effects of contaminants adsorbed on the surface. In addition, noise levels are important in evaluating suitability of the device for analog and digital applications. In this work, we have fabricated back gated field-effect transistors (FETs) using various thicknesses of mechanically exfoliated MoS₂ flakes (bilayer and 15 layer) and studied the 1/f noise under ambient conditions. The on-current of the devices scales with the number of layers. The Hooge parameters inferred from the measured noise amplitudes and calculated carrier densities are comparable to prior reports on devices such as CNTs and graphene FETs, even when measured under ambient conditions. The effect of channel and contacts on both the conductance and noise can be inferred from bias-dependent current and noise measurements.

Transition metal dichalcogenides like MoS₂ are considered an attractive alternative to graphene for fabricating low-dimensional field effect transistors (FETs). In particular, MoS₂ FETs have been demonstrated with reasonable mobility [1,2] and high on/off ratio [3]. However, contact resistance (R_C) remains a key limiting factor to existing devices. In order to scale MoS₂ FETs to sub-10 nm dimensions, R_C should be reduced by more than an order of magnitude compared to the best results available today so as not to dominate the MoS₂ channel resistance.
Here we present the first systematic study of contact resistance to MoS₂ using transfer length method (TLM) structures on devices with varying thickness (1-10 layers) over a very wide temperature range (80-500 K). We also study the MoS₂ contact with various metals under different deposition conditions. To understand the influence of work function (Phi;_M) on the Schottky barrier we choose the metals Sc, Ti, Al, Ni, and Au (Phi;_M = 3.5-5.1 eV). To understand the influence of metal deposition conditions, we compare metals deposited under different background vacuum levels (10^-9-10^-6 Torr).
Surprisingly, we uncover that R_C does not decrease with rising temperature as expected and can even increase for some samples. This temperature dependence can result from lateral current flow under the contact which is limited by phonon scattering, not thermally assisted tunneling through the Schottky barrier. Thus it is necessary to define an effective R_C consisting of both transport through the metal-MoS₂ interface and within the MoS₂ layers underneath the contact. As a result of phonon scattering and Fermi-level pinning, the choice of metal work function has little impact on the effective value of R_C. Rather, improving the quality of the metal-MoS₂ interface by evaporating under ultra-high vacuum (~10^-9 Torr) leads to a factor of 3 lower R_C. We extract both the resistivity (ρ_C asymp; 2 × 10^-6 #8486;cm²) and current transfer length (L_T asymp; 100 nm) of the various metal-MoS₂ contacts for the first time. Using resistor network and finite element models, we show that ρ_C and L_T are strongly influenced by the interlayer resistance and in-plane mobility of the MoS₂ underneath the contact. This work represents the lowest contact resistance achieved to date on MoS₂, and the results give important suggestions on how to further improve the performance of such 2-dimensional devices.
[1] S. Das et al., Nano Lett. 13, 100 (2013).
[2] S. Kim et al., Nature Communications 3, 1011 (2012).
[3] B. Radisavljevic et al., Nature Nanotechnology 6, 147 (2011).

Single to few atomic layer transition metal dichalcogenide (MX2: M = Mo or W and X=S, Se, Te) have attractive electronic and optical properties. Among them MoSe2 has an indirect band gap of 1.1eV in bulk material which increases up to 1.5 eV in monolayers. We studied the Field Effect Transistor (FET) properties of ultra-thin MoSe2 layers mechanically exfoliated onto Si/SiO2 substrates. We measured the field-effect behavior in two- and four- terminal configurations using a Si substrate as the back gate to control the carrier density. The MoSe2 FETs show ambipolar behavior with an ION/IOFF ratio exceeding 10^6. For two-terminal measurements, which are influenced by the resistance of the metallic contacts onto the MoSe2 flakes, we obtain field-effect mobilities of ~30 cm^2/Vs. Whereas when a four-terminal configuration is used, the extracted mobility from the same device reaches 80 cm^2/Vs. The temperature dependence reveals that the field-effect mobility increases significantly with decreasing temperature suggesting a dominant role for the phonon scattering. In this presentation we will also compare field effect properties with hall-effect measurements on thin MoSe2 based FETs.

The successful exfoliation of graphene and discoveries of its unique electrical and thermal properties have motivated searches for other 2D materials with unique properties. The layered van der Waals materials can be cleaved mechanically by breaking the relatively weak bonding between the layers. An interesting subgroup of inorganic van der Waals materials is the transition metal dichalcogenides. Some of these materials manifest charge density wave effects in the temperature range from ~100 K to room temperature (RT). It was recently discovered that decreasing the thickness of titanium diselenide thin films allows one to increase the transition temperature to the charge density wave phase [1]. Field-effect devices implemented with such materials often reveal unusual current - voltage (I-V) characteristics [2]. In this talk we report on promising electrical switching phenomena observed in field-effect type devices with 2H-TaSe2 channels. Single-crystal 2H-TaSe2 bulk samples were synthesized by the chemical vapor transport method. The thin films were obtained by the “graphene-like” mechanical exfoliation. The bottom-gated devices with the 2H-TaSe2 channels were fabricated with the electron beam lithography to define the top metal contacts for electrical measurements. The top contacts were fabricated with 10 nm of Ti and 100 nm of Au. The electrical measurements revealed that the RT I-V characteristics were asymmetric and strongly non-linear with the clear threshold voltage. The threshold depended on the applied gate bias. It was possible to achieve many orders of magnitude change in the current with the applied gate voltage if the device was biased near the threshold. The observed non-linear I-V characteristics in devices with such channels can possibly be used for alternative information processing [3].
Work in the Balandin group was supported, in part, by the National Science Foundation (NSF) and Semiconductor Research Corporation (SRC) Nanoelectronic Research Initiative (NRI) project “Charge-Density-Wave Computational Fabric: New State Variables and Alternative Material Implementation” NSF-1124733 and by STARnet Center for Function Accelerated nanoMaterial Engineering (FAME) - Semiconductor Research Corporation (SRC) program sponsored by MARCO and DARPA.
[1] P. Goli, J. Khan, D. Wickramaratne, R.K. Lake and A.A. Balandin, "Charge density waves in exfoliated films of van der Waals materials: Evolution of Raman spectrum in TiSe2," Nano Lett., 12, 5941 (2012); [2] J. Khan, C.M. Nolen, D. Teweldebrhan, D. Wickramaratne, R.K. Lake and A.A. Balandin "Anomalous electron transport in back-gated field-effect transistors with TiTe2 semimetal thin-film channels," Appl. Phys. Lett., 100, 043109 (2012); [3] G. Liu, S. Ahsan, A.G. Khitun, R.K. Lake and A.A. Balandin "Graphene-based non-Boolean logic circuits," J. Appl. Phys., 114, 154310 (2013).

Over the past few years, two-dimensional (2D) layered materials beyond graphene have received extensive research interest. Particularly, novel optical, electronic, and thermal properties, arising from reduced material dimensions, are being intensively studied in various layered materials such as MoS₂, Bi₂Se₃, and TiSe₂. The 2D nature might also lead to interesting structural properties, which might be significant in the context of phase-change memory applications but have received less attention.
Here we report the fabrication of single-crystal In₂Se₃ 2D thin layers using mechanical exfoliation and studies of phase transformations using transmission electron microscopy, correlative in-situ Raman spectroscopy and electrical measurements [1]. Our results indicate that the 2D nature of these layers contributes to the stability of crystalline phases that are unstable in bulk crystals, and that decreased layer thickness leads to enhanced crystalline-crystalline phase transition temperature. These crystalline phases have distinctive electrical properties, making them suitable for multi-level data storage within a single phase-change material. From a more fundamental perspective, these 2D layers provide an ideal system for studying the scaling behaviors of atomic motions during structural phase transformations down to a few nanometers.
[1] X. Tao and Y. Gu, Nano Lett. 13, 3501 (2013).

Unlike graphene, the existence of bandgaps in the layered semiconductor molybdenum disulfide (MoS₂) offers an attractive possibility of using single layer MoS₂ field-effect transistors (FETs) in low-power switching devices and photodetectors. Yet, the fabrication demands and the physics of MoS₂, among other reasons, suggest that multilayer MoS₂ may be more attractive than single layer MoS₂ for FET applications in a thin-film transistor (TFT) configuration. In this presentation, we explore the electrical and optoelectronic properties of multilayer MoS₂ TFTs fabricated on glass for applications in flat-panel displays. Our local bottom-gated multilayer MoS₂ TFTs achieve reasonable room temperature mobilities (> 20 cm²V^-1s^-1), a high on/off ratio (> 10⁶), current saturation, and a negligible shift in the threshold voltages during illumination. Furthermore, our multilayer MoS₂ TFTs exhibit well-defined capacitance-voltage characteristics providing useful information on the interface between MoS₂ and gate dielectric.

Layered metal dichalcogenides (MX₂) are of interest for numerous electronic and photonic device applications due to their potential to be scaled to atomic-layer thicknesses. A wide range of MX₂ materials have been investigated recently, including MoS₂ [1], MoSe₂ [2], WS₂ [3], and others. It has previously been shown that the performance of devices fabricated on MX₂ materials is sensitive to the quality of the Ohmic contacts. For this reason, MX₂ materials with high electron affinity are of interest to allow low resistance n-type contacts, as well as to enable a broad range of heterostructures to be realized. SnSe₂ is an attractive material due to its high electron affinity of 5.2 eV [4], and Si-like band gap of ~ 1 eV. However, very few investigations of thin-film SnSe₂ materials for transistor applications have been reported [5]. In this work, we report the physical and electronic properties of exfoliated thin-film SnSe₂ devices. SnSe₂ flakes were mechanically exfoliated onto an Si/SiO₂ substrate from a bulk SnSe₂ crystal purchased from a commercial vendor. The bulk crystal had the 2H polytype. Atomic force microscopy of the exfoliated flakes was performed, showing fairly uniform thicknesses ranging from 64.4 nm to 89.9 nm for five flakes on which electrical characterization was carried out. Typical flakes with this thickness were on the order of 1-5 mu;m in length. Raman spectroscopy of the films indicated a primary Raman peak at 185 cm^-1, which corresponds to the A_1g vibrational mode. Contacts consisting of Ti/Al metallization were aligned and patterned on the flakes using electron-beam lithography. The spacing between the contacts was 0.5 mu;m. The field-effect mobility was extracted on several devices and effective mobilities in the range of 3.5 to 11 cm²/Vs were determined at room temperature. Decreasing the temperature to 77 K increased the device mobilities to 4.5-29 cm²/Vs, with a corresponding increase in the variability between devices. Finally, we investigated the temperature dependence of the low-field conductivity and found that the activation energy for the metal contacts was 5.5 meV, indicating the near absence of a Schottky barrier at the Ti/SnSe₂ interface. These results are encouraging for future investigations of SnSe₂ for high-performance transistor applications.
[1] B. Radisavljevic, et al., Nature Nanotech. 6, 147 (2011); [2] Q. H. Wang, et al., Nature Nanotech. 7, 699 (2012); [3] W. S. Hwang, et al., Appl. Phys. Lett. 101 013107 (2012); [4] R. Schlaf, et al., J. Appl. Phys. 85, 2732 (1999); [5] T. S. Pan, et al., Appl. Phys. Lett. 103, 093108 (2013).

Since graphene was first discovered by A. Geim and K. Novoselov, graphene has been considered as a fascinating material due to its superior material properties. Graphene has high thermal and electrical conductivity with excellent mechanical strength. In spite of unique properties, however, graphene is faced with difficult practical application in industry owing to low processability and unease of mass production with high purity. Thus, there has been many approaches to overcome these drawbacks, graphene oxide (GO) is regarded as one of alternative material to obtain graphene. GO is synthesized from graphite with concentrated acid and strong oxidizing agents. GO is well dispersed in water and common organic solvents such as DMF, NMP and THF, and easy to scale-up. Based on these properties, GO is regarded as widely applicable graphene derivatives and actually studied for various applications. Numerous characterizations of fundamental properties of GO were conducted ahead of GO applications, however, there still remains some unclear properties of GO. One of them is the chemical structure of GO. GO consists of diverse oxygen containing functional groups on graphene sheet surfaces such as hydroxyl, epoxy and ketone groups on basal plane and carboxylic groups at the edges. However, exact stoichiometric proportion of each functional groups and oxidation mechanism is still uncertain. In this study, we synthesized a model compound analogue to GO by modified Hummers method using polycyclic aromatic hydrocarbon (PAH). PAH consists of 2-7 aromatic hydrocarbon rings with regular structure. We focused on the regular structure of PAH and regarded it as a mono-layered small graphene sheet. The model compound was successfully synthesized. The structures of GO model and original GO were characterized by using FT-IR, TGA, UV/Vis and SEM, etc. Chemical structure analysis was also conducted by NMR, liquid chromatography (LC) and mass spectrometry. As a result, we suggest stoichiometric proportion of oxygen containing functional groups and oxidation mechanism of GO. We anticipate that this study helps to clarify the exact chemical structure and oxidation mechanism of GO.

Graphene is one of strongest known material, with a theoretically estimated Young&’s Modulus value of 1.05 TPa [1]. Graphene exhibits remarkable electronic and mechanical properties. However, in its pristine form graphene is a gapless materials which precludes its use in some transistor applications. In part due to this there is a renewed interest in other graphene-like 2D materials, as evidenced by the large number of recent papers on this subject. A recent work [2] investigated a 2D material called BPC (Biphenylene carbon) [3] which presents nonzero band gap and well delocalized frontier orbitals. It has been also proposed [3] a possible route to obtain this theoretical material from the selective dehydrogenation of porous graphene, a material recently synthesized.
In this work we investigated the mechanical properties of BPC using fully atomistic reactive (ReaxFF force field) molecular dynamics simulations, as implemented in the simulation package LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator).
We analyzed the resulting stress response when a strain is applied along the two possible different topological directions of a single layer. In the simulations we considered structures finite in one direction and infinite (cycled boundary conditions) on the other. The stress is created by gradually increasing the unit cell vector along one direction. Young&’s modulus is calculated analyzing the linear region of the stress-strain curve. We estimated a value of 0.69(3) TPa for Young&’s modulus in our preliminaries calculations, a value higher than the obtained in a recent work [4]. One other interesting aspect of these structures is that exists many possible ways of stacking the layers [5], which produces quite different behaviors with relation to electronic and mechanical properties.
[1] F. Liu et al. Phys. Rev. B v76, 064120 (2007).
[2] G. Brunetto et al., J. Phys. Chem. v116, 12810 (2012).
[3] R. H. Baughman, R.; et al. J. Chem. Phys. 87, 6687 (1987).
[4] L. Yinfeng et al., arXiv:1308.4696 (2013).
[5] Y. Yu, J. Mat. Chem. A, v1,13559 (2013).

Two-dimensional layered materials have generated much interest as the mechanical, optical, and electronic properties of the bulk materials differ from the properties of the exfoliated 2D materials. Van der Waals solids share the structural property of strong covalent bonding within the layers, which are held together by weak Van der Waals forces. They can be chemically or mechanically exfoliated into 2D materials with atomic-layer thicknesses. Examples of Van der Waals solids that have been investigated include graphite, hexagonal boron nitride, and transition metal chalcogenides, but there are still a large number of unexplored materials. Here we report the results of the exfoliation of the binary pnictide GeAs. GeAs crystalizes in the monoclinic space group C2/m in a layered crystal structure with the surface of each layer terminating in As atoms. Within the layer, Ge-Ge pairs are surrounded by 6 As atoms forming a distorted trigonal antiprism. Each layer is formed through edge-sharing antiprisms rotated 90° and 180° with respect to its neighbor. The weak lone pair interactions of the terminal As atoms allow the material to be readily exfoliated through sonication. The Raman spectroscopy, photoluminescence, and atomic force microscopy techniques used to determine the size-dependent properties of the 2D GeAs will be discussed.

The effects of the induction heating on the device performance of MoS2 filed effect transistors (FETs) were investigated. In order to achieve good electrical contacts between the channel and the metal electrodes, the annealing process is required in the semiconductor fabrication processes. When the chamber or the substrate heating is used, the entire device is heated, giving the possible damage on polymer substrates since the polymers are weak at heating. The induction heating, however, can selectively heat the metal electrodes so that the annealing process can be done locally with a good controllability. For this research, the MoS2 FETs was fabricated. E-beam lithography was used to pattern drain and source electrodes, and PEDOT-PSS was used for the bottom gate electrode. The electrical characteristics were measured by using Keithley 4200. The on-off ratio and field effect mobility was enhanced after induction heating. Therefore, the device performance could be improved when using induction heating annealing. It can be applied in the industry considering the advantage of the selectivity heating.

Molybdenum disulphide (MoS2) is a novel two-dimensional semiconductor with potential applications in electronic and optoelectronic devices. However, the nature of charge transport in back-gated devices still remains elusive as they show much lower mobility than theoretical calculations and native n-type doping. In this talk I will present our recent results on charge transport In few-layer MoS2. First, we observe that the environment plays an important role in the backgated MoS2 field-effect transistors. After annealing the devices in vacuum, the on-state current of MoS2 devices increases by up to ~100 times compared to the ambient conditions[1]. Further more, we carry out electrical measurements in few-layer MoS2, together with transmission electron microscopy and density functional theory. We provide direct evidence that sulphur vacancies exist in MoS2, introducing localized donor states inside the bandgap. Under low carrier densities, the transport exhibits nearest-neighbour hopping at high temperatures and variable-range hopping at low temperatures, which can be well explained under Mott formalism [2]. We suggest that the low-carrier-density transport is dominated by hopping via these localized gap states. Our study reveals the important role of short-range surface defects in tailoring the properties and device applications of MoS2.
References:
[1] Hao Qiu, Lijia Pan, Zongni Yao, Junjie Li, Yi Shi* and Xinran Wang*, “Electrical characterization of back-gated bi-layer MoS2not; field-effect transistors and the effect of ambient on their performances”, Appl. Phys. Lett., 100, 123104 (2012).
[2] Hao Qiu, Tao Xu, Zilu Wang, Wei Ren, Haiyan Nan, Zhenhua Ni, Qian Chen, Shijun Yuan, Feng Miao, Fengqi Song, Gen Long, Yi Shi, Litao Sun, Jinlan Wang* & Xinran Wang*, “Hopping transport through defect-induced localized states in molybdenum disulfide”, Nature Comm. 4, 2642 (2013).

Epitaxial Graphene (EG) synthesized on nominal on-axis polar planes, (0001) and (000-1), of 4H- or 6H-SiC substrates have shown great promise in high frequency and scalable graphene field effect transistors despite the challenges of step-bunch induced conductance anisotropy and interface layer (IFL) on (0001) or wrinkled multilayered graphene on (000-1). In this work, we will discuss novel synthesis steps that results in controllable large area monolayer graphene on non-polar m- and a-plane of 6H-SiC substrate.
In this work, we systematically grew graphene on substrates with square side dimensions ranging from 8 -16 mm, to elucidate and evaluate the resulting structural properties. Specifically, we will focus on the impact of the interface, strain and surface roughness.
Briefly, the growth process was carried out in a commercial hot-wall Aixtron VP508 CVD reactor. Prior to graphene formation, the substrates underwent an in situ H₂ etch at 1520-1560°C for 10 - 50 min. This etching step produces a smooth nucleation surface with RMS of ~0.11nm. The H₂ was then evacuated, and subsequent EG growth conducted at temperatures 1520 - 1620°C in a flowing Ar ambient of 10 standard liters per minute at 100 mbar. XPS, µ-Raman spectroscopy, AFM and SEM was used to extract composition, presence, thickness, strain and surface morphology variations of graphene across the samples. HRTEM was used to confirm layer thickness.
The RMS roughness of a 10 x 10 mu;m² AFM scans of non-polar EG layers had values ranging from ~0.54 to 1.1 nm. The layers did not show the step bunch and terrace morphology typically associated with the (0001) surface. The graphene layers grown on non-polar orientations were markedly smoother than growth on nominally on-axis plane. The Raman 2D mode position remains relatively constant over the mapped area (100 µm²) with an average of ~2708 ± 4.6 cm^-1, which indicates uniform strain with values approaching strain-free exfoliated graphene 2684-2696cm^-1. This contrasts with EG on polar substrates which show higher non-uniform strain values, i.e., 2D-mode positions ranging from 2735 ± 3.7 on the terrace to 2747 ± 5.0 cm^-1 at step edge. XPS C 1s core-level data shows the splitting of the SiC (283.4 eV) and EG (285.0 eV) peaks similar to (000-1) grown layers4; this and other XPS features suggest that no interface layer is present on these non-polar surfaces. The lack of an IFL for EG grown on non-polar orientations offers an attractive explanation for the recent favorable results for graphene nanoribbons formed on (1-10n) surfaces.

Recently, transition-metal dichalcogenides (TMDs) have gained renewed interest after the successful production of two-dimensional (2D) using exfoliation techniques and chemical vapor deposition methods. The TMD monolayers (MLs) prepared in such different processes may contain numerous defects which can significantly influence electronic properties of these materials. In order to fully understand and exploit defects in MoS₂ -ML, we study here the electronic properties and the quantum transport of several structural defects including point vacancies, grain boundaries and topological defects. Our simulations are performed by using the density-functional based tight-binding (DFTB) method in conjunction with the Green&’s function (GF) approach. Our results show that even at low concentration they considerably alter the quantum conductance. While the electron transport is practically isotropic in pristine MoS₂ , strong anisotropy is observed in the presence of defects. The results show that point defects cause localized mid-gap states in semiconducting MoS₂ that do not contribute to the conductivity but direction-dependent scatter the current, and that the conductivity is strongly reduced across line defects and selected grain boundary models. Our results could help to better understand the effect of the structural defects on the electronic performance of these novel 2D materials.

The controlled introduction of mechanical deformation can provide an efficient route to tune electronic properties of semiconductors, which is often called straintronics. It is found that the transition-metal dichalcogenides (TMD), as a new class of low-dimensional semiconductors, have remarkable electromechanical properties. In the case of MoS₂ monolayers, strain modifies the direct band gap into an indirect one, and substantial strain even induces an semiconductor-metal transition for isotropic elongations as large as 11% without bond breaking. This can be used to modulate electronic and transport properties of these materials for different applications in nanosensor device engineering. On the other hand, applying strain through mechanical contacts is difficult for TMD monolayers, but state-of-the-art for TMD nanotubes. We show using density-functional theory that similar electromechanical properties as in monolayer and bulk TMDs are found for large diameter TMD single- (SWNT) and multi-walled nanotubes (MWNTs). The semiconductor-metal transition occurs at elongations of 16%. Also, TMD MWNTs show twice the electric conductance compared to SWNTs, and each wall of the MWNTs contributes to the conductance proportional to its diameter. We show that Raman signals of the in-plane and out-of-plane lattice vibrations depend significantly and linearly on the strain. As a result, Raman spectroscopy supposed to be an excellent tool to determine the strain of the individual nanotubes and hence monitor the progress of nanoelectromechanical experiments in situ.

In spite of superior properties of graphene, applications of graphene in practice such as display, transistor, sensors, and electrodes are still limited due to its low processability. To overcome these drawbacks of graphene, mass producible graphene oxide (GO), functionalized graphene sheet with oxygen containing functional groups (e.g., epoxy, ketone, hydroxyl and carboxylic acid groups) has been emerged as a precursor for graphene. Especially, GO is considered as not only a precursor of graphene (i.e., reduced GO) but also a promising material for nanocomposites, films, absorbents and separation membranes due to its functionalized chemical structures, physical properties, and processibility.
Recently, several studies have been performed to use graphene or GO for gas and liquid separation membranes due to its unique characteristics such as 2D structure, atomic thickness. Although the some theoretical and experimental results clearly show the feasibility of graphene or GO applications as membrane materials (e.g., gas separation, water purification), atom-size pore formation on graphene sheet is not technically easy. In addition, typical GO membrane preparation methods (e.g., vacuum filtration) are not practical for scale-up, large area membranes with high-flux because of thick membrane nature. In this study, homogeneous and stable GO thin film composite (TFC) membranes for desalination and water purification were successfully prepared by coating of aqueous GO dispersion on the commercially available micro-porous polymeric membranes. GO layer deposited on the substrate has ultrathin thickness below 5 nm and randomly stacked layered structures made of GO nanoplatelets. Due to hydrophilic natureof GO, initial GO layers possess an amount of intercalated water in the layered structure and allow the free water or ion permeation. However, dry GO TFC membranes are formed by dewatering of stacked GO layers, resulting in more selective water and solute permeation properties. GO TFC membrane with hydrophilic, negatively charged surface and sub-nanometer scale molecular pathways exhibits low water permeability and high salt rejection properties to not only multivalent ions but also organic molecules by size exclusion and Donnan effect similarly to commercial nanofiltration (NF) membranes. The preparation method of GO TFC membranes by homogeneous coating on micro-porous substates clearly shows the possibilities to the membrane application of GO.

We report two different approaches to fabricate hybrid h-BN/graphene vertical structure: (1) direct CVD growth of graphene onto CVD-grown thin h-BN film and (2) direct CVD growth of graphene under exfoliated thick h-BN at an h-BN/Cu interface.
Firstly, a direct CVD growth of graphene on thin h-BN film on Cu is studied with sequential CVD graphene/h-BN on Cu foil was studied with scanning tunneling microscopy and scanning tunneling spectroscopy, which show the almost ideal (i.e., unperturbed) electronic properties of graphene by the high crystallinity, epitaxy, and the clean interface achieved in this sequentially CVD-grown graphene/h-BN film. After transfer, transmission electron microscopy and Raman spectroscopy confirmed that this structure is robust enough for a reliable transfer process. In addition, electrical transport measurements revealed that transistors based on this sequential CVD-grown graphene/h-BN film exhibit better electrical performance than those based on graphene mechanically transferred onto h-BN films or SiO2 substrates. Transferring graphene onto dielectric substrates can thus be avoided by this direct growth method. The interface between graphene and h-BN, therefore, is also protected from external impurities during transfer of this hybrid structure onto any substrate. Such sequentially CVD-grown graphene/h-BN structure can give a huge impact on the study of intrinsic electronic properties of graphene, a scalable template for electronic device or a fabrication of super lattice of graphene/h-BN with defect-free interfaces. Furthermore, we unveiled the catalytic growth mechanism of graphene on h-BN film supported by Cu. With experimental and theoretical analyses, we found that catalytically-activated thin h-BN film supported by Cu substrate can serve to grow graphene with the available electrons on the surface by tunneling from the underlying a Cu substrate. It is, therefore, possible to grow even patches of graphene with up to 5 nm thick h-BN and to grow high quality and large area graphene on thin h-BN film of less than 3 nm thick.
Secondly, we found a different result of the direct CVD process of graphene with thick exfoliated h-BN. Our systematic Raman measurements combined with plasma etching process indicate that a continuous graphene film is grown under exfoliated h-BN rather than on its top surface, and that an h-BN/graphene vertical hybrid structure has been fabricated. Electrical transport measurements of this h-BN/graphene, transferred on SiO2, show the carrier mobility up to approximately 3000 cm2V-1s-1. The developed method can easily be extended for direct growth of other graphene-based hybrid structures, such as MoS2/graphene.

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. While a few works report gate dielectric deposition on graphene using physical vapor deposition such as evaporator or sputtering, atomic layer deposition (ALD)-based dielectric material growth on graphene is rare. Hence, in this present work we demonstrated the deposition of high-k dielectric material on graphene using hexagonal boron nitride (h-BN) as a buffer layer. Transmission electron and atomic farce microscopy studies show that presence h-BN layer on the top of graphene facilitates the growth of high-quality aluminum oxide (Al2O3) layer by atomic layer deposition (ALD). Simulation results also support the experimental observations and provide explanation for suitability of h-BN as buffer layer. The analysis of Raman and X-ray photoelectron spectroscopy (XPS) data also confirms the importance of h-BN for growth of good-quality oxide dielectric materials. Also, h-BN works as protective shield to prevent graphene from oxidation during ALD of Al2O3 for the fabrication of top-gated graphene-based devices.

We use synchrotron-based, complementary structural and chemical in-situ characterization to investigate the growth mechanisms of hexagonal Boron-Nitride (h-BN) chemical vapor deposition under industrially relevant conditions. Using in-situ X-ray diffractometry (at ESRF) and in-situ X-ray photoelectron spectroscopy (at BESSY) under realistic reaction conditions, we elucidate the complex interplay of surface- and bulk-mediated growth mechanisms of h-BN films on transition metal catalysts (such as iron). Based on our insights from in-situ characterization, we obtain improved control over the growth of high-quality h-BN layers.

In recent years layered materials such as graphene, boron nitride, transition metal dichalcogenides (TMDC) has gained a significant research interest due to their exceptional physical properties. Among this TMDC is of specific interest because members of this family have a band gap at room temperature which can be tuned by decreasing the number of layers. WS2 is one of the members TMDC family and monolayer WS2 shows extraordinary high photoluminescence at room temperature which could find diverse applications in fabrication of photovoltaic, energy efficient optoelectronic devices. However, high quality, large area growth is prerequisite for device applications. Methods like liquid exfoliation, sulphurization of transition metal coated film have been explored for batch production but the crystal quality obtained by such methods is a matter of concern. Here we report on the controlled growth of high quality, large scale monolayer WS2 using CVD process. SEM studies reveals the triangular morphology of WS2 monolayer with a dimension about 15 micron and together these triangle form large grain of different morphologies. Presence of monolayer is confirmed by Raman and room temperature photoluminescence studies. We further performed temperature dependent photoluminescence to understand the exciton dynamics.

In contrast to what is known about the ultrafast carrier dynamics and relaxation processes in two-dimensional materials [1], very little is known about the structural dynamics. Here we use nonlinear optical second harmonic generation techniques sensitive to changes in crystallographic symmetry to directly probe the dynamical response on femtosecond timescales in single layer MoS₂. Due to the lack of inversion symmetry in single layer MoS₂, optical second harmonic generation is an efficient process [2], and femtosecond optical pump-probe techniques can be applied to obtain snapshots of these dynamic processes. Here we present first measurements of the time- and polarization-dependence of the second harmonic intensity on CVD grown, monolayer MoS₂ on sapphire following above band gap excitation at 266 nm. Measurements are carried out in a microscope with focused 800 nm probe pulses so that single MoS₂ domains are probed. A large-amplitude (12%) decrease in the second harmonic intensity is observed on 300 femtosecond timescales followed by a 10 picosecond timescale recovery, with a 2% long lived decrease. The excitation intensity dependence of these effects has also been investigated. [1] Shi et al., ACS Nano 7, 2 (2013). [2] Malard et al., Phys. Rev. B 87, 20 (2013)

The transition metal dicalcogenides (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. Among them, molybdenum disulfide (MoS2) is particularly interesting. Single layer MoS2 is a direct band gap semiconductor, with a band gap of 1.8eV and mobility has been greatly enhanced by dielectric engineering. The presence of a direct band gap in single layer MoS2 makes it very interesting for application such as field effect transistor, amplifiers device, chemical sensors and integrated circuits. Significant effort have been devoted to prepare MoS2 thin layers, including scotch tape assisted micromechanical, magnetron sputtering, chemical vapor deposition, and sulfurization of MoO3 film. However, these methods have several limitations, such as difficulties in controlling the thickness and limited wafer-scale uniformity. Therefore, an improved synthesis methodology for atomically thin MoS2 nanosheets with systematic thickness controllability and wafer scale uniformity is required. In this report, we describe a novel method for the growth of nanoscale MoS2 thin films on SiO2 substrates by atomic layer deposition (ALD), which guarantees a reliable wafer-scale growth with the excellent conformality.
MoS2 films were grown by ALD using an alternating supply of molybdenum hexacarbonyl [Mo(CO)6] and H2S plasma as a precursor and reactant, respectively. The deposition temperature was varied from 150 to 225 °C. Self-limiting film growth was observed with both the precursor and reactant pulsing time and growth rate was around 0.05nm/cycle on thermally-grown SiO2 substrate and a short incubation cycle of around 13 at the deposition temperature of 175°C. The ALD- MoS2 thin films exhibit good stoichiometry with no impurity incorporation. XPS and Raman analysis showed the formation of MoS2 bonding. The step coverage of ALD-MoS2 was excellent around 70% at 250 nm sized trench and wafer-scale growth (4 inch in diameter) was demonstrated. The electrical and optical properties of ALD-MoS2 were characterized and will be reported in the conference.

The remarkable properties of graphene have made it the most widely studied two-dimensional material. However, its lack of a bandgap limits its transistor applications. Recently, two-dimensional material of molybdenum disulfide (MoS2) has attracted much interest due to its direct-gap property and potential applications in next-generation nano-electronic devices. However, the synthetic approach to obtain high quality and large-area MoS2 atomic thin layers is still rare.
In this study, Molybdenum disulfide layer was synthesized by a two-step, electro-plating synthetic method in which MoSx “precursor” layer was first electrodeposited self-limited deposition on a metal surface. These precursor molecules were then converted to MoS2 by high temperature annealing process. This synthetic approach is very simple, scalable and applicable to other two dimensional transition-metal dichalcogenides. Meanwhile, the obtained MoS2 films are transferable to arbitrary substrates, providing great opportunities to make layered composites by stacking various atomically thin layers.

Single layer transition metal dichalcogenides are 2D semiconducting systems with unique electronic band structure. Two-valley energy bands along with strong spin-orbital coupling lead to valley dependent career spin polarization, which is the basis for recently proposed valleytronic applications. These systems also exhibit unusually strong many body affects, such as strong exciton and trion binding, due to reduced dielectric screening of Coulomb interactions. Recently observed large photoluminescence helicity suggests beyond ns hole spin and valley lifetimes. But there is not much known about the impact of strong many particle correlations on spin and valley polarization dynamics. Here we report direct measurements of ultrafast valley specific relaxation dynamics in single layer MoS2 and WS2. We found that excitonic many body interactions significantly contribute to the relaxation process. Our results suggest that initial fast intervalley electron scattering and electron spin relaxation leads to loss of valley polarization for holes through an electron-hole exchange mechanism.

Our findings show that the layer number and defect density for hexagonal boron nitride (h-BN) films grown by chemical vapor deposition (CVD) changes dramatically with the growth pressure and the substrate crystallography. The effort to develop wafer-scale graphene and transition metal dichalcogenide electronic devices on solid and flexible handles requires substrates that will not degrade the fundamental properties of those materials. Recent work [1-3] has demonstrated that h-BN is an ideal substrate for these materials and can serve as spacer to enable vertical heterostructures of two-dimensional materials [4]. The need for large-area, uniform h-BN has motivated recent work synthesizing h-BN by CVD on transition metal substrates [5-8]. While these growths have synthesized submonolayer islands to very thick films, the growth mechanisms on Cu are not well understood. We study the effects of the growth pressure, hydrogen partial pressure, and the crystalline ordering of the substrate on the synthesis of h-BN on Cu foil with an ammonia borane precursor. Analysis of the growths by optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and scanning tunneling microscopy and spectroscopy (STM/S) shows that growing h-BN by low-pressure CVD (LPCVD) yields higher-quality, planar h-BN films than those grown by atmospheric-pressure CVD (APCVD). Growths by APCVD yield rougher films with polymeric amino/iminoborane chemical structure. The resulting APCVD-grown h-BN films are also thicker and more nanocrystalline than their LPCVD-grown counterparts. Growths between the two extremes of LPCVD and APCVD show that the film thickness and nanocrystalline morphology increase with the growth pressure. We also grow graphene on h-BN (G/h-BN) heterostructures. Raman spectroscopy of the G/h-BN heterostructures correlated withthe Cu crystalline structure from electron backscatter diffraction (EBSD) shows that the high-index Cu crystal surfaces lead to poorer quality h-BN and G/h-BN films, as compared with those on the low-index Cu(100) surface.
[1] J. Xue, et al., Nat. Mater. 10, 282 (2011); [2] C.R. Dean, et al., Nat. Nano. 5, 722 (2010); [3] G.-H. Lee, et al., ACS Nano 7, 7931 (2013); [4] L. Britnell, et al., Nano Lett. 12, 1707 (2012); [5] Y. Shi, et al. Nano Lett. 10, 4134 (2010); [6] L. Song, et al., Nano Lett. 10, 3209 (2010); [7] K. Kim, et al., Nano Lett. 12, 161 (2012); [8] A. Ismach, et al., ACS Nano 6, 6378 (2012).

Thin layered MoS₂ has emerged as a remarkable 2D semiconductor material with great potential in electronic applications due to the presence of a direct band-gap, high rectification, and high current density. Here, the carrier doping and electrical properties of few layer sheets of MoS₂ were controllably modulated by interfacing it with polyanionic and polycationic polymers: polystyrene sulfonate (PSS) and polyallylamine hydrochloride (PAH). PSS interfacing results in blue shift of the Raman E¹_2g peak in MoS₂ by 1.3 cm^-1 (383.5 cm^-1 to 382.2 cm^-1) and the Raman A_1g peak by 1.3 cm^-1 (406.9 cm^-1 to 405.6 cm^-1). This corresponds to hole doping, as expected for negative potential gating. PAH adsorption on PSS coated MoS₂ led to a recovery of the A_1g peak to 406.9 cm^-1, while the E¹_2g peak red shifted slightly to 382.6 cm^-1 attributed to over deposition of PAH and electron doping. Similar results were attained with PAH interfacing on MoS₂ few layer sheets resulting in electron doping. Electrical characterization confirmed electron doping for PAH and hole doping for PSS. Chemical modification or interfacing provides a path for tuning the electronic properties of few layer sheets of MoS₂ for use in future devices.

Transition metal dichalcogenides (TMDC) materials that have layered structures like graphene are of considerable interest as promise electronic materials for future electronics. [1] Among TMDC materials, MoS2 that has an indirect bandgap of 1.29 eV is attractive materials as a complementary of graphene that are lack of band gap. Multilayer MoS2 FET with a bottom gate structure was reported and this showed excellent field effect mobility of >100 cm2 V-1 s-1 and subthreshold swing of ~70 mV/decade. [2] MoS2 FET based on the excellent electrical characteristics have been studied in a number of areas such as logic circuit, memory devices, gas sensor, and phototransistor for future electronics and optoelectronics. The electrical instability of practical transistor is important as a device parameter. Specially, threshold voltage shift by gate bias stress limits applications such as memory device and display.
In this work we focus on threshold voltage shift under the positive and negative gate bias stress of multilayer MoS2 FET. First, multilayer MoS2 flakes were mechanically exfoliated from bulk MoS2 crystals and transferred on highly doped p-type Si substrate grown thermal 300 nm SiO2. Au/Ti (50/10 nm) electrodes deposited by evaporator were patterned using lift-off technique. The device was annealed at 200 oC for 2 h (100 sccm Ar and 10 sccm H2) to decrease contact resistance. We measured electrical characteristics of MoS2 FET using Keithley 4200-SCS and extracted the field effect mobility ~4.52 cm2 V-1 s-1, on/off ratio ~106, subthreshold swing ~0.8 V/dec from transfer curve at VD = 10 V. To investigate the effect of the gate bias stress time, IDS-VG curves at different stress time measured after +20 V and -20V gate bias stress was applied in the vacuum state. The transfer curve shifts by the positive gate bias of 20V in the positive direction, whereas it shifts by the negative gate bias of -20 V in the negative direction. Threshold voltage shift as a function of bias stress time fitted a stretched exponential equation. [3] From these experiments, we will discuss about relations among threshold voltage instability under the gate bias and charge trapping at the semiconductor/dielectric interface.
Acknowledgement
"This research was supported by the MSIP(Ministry of Science, ICT and Future Planning), Korea, under the “IT Consilience Creative Program” (NIPA-2014-H0201-14-1001) supervised by the NIPA(National IT Industry Promotion Agency)
Reference
[1] Q.H. Wang et al., Nat. Nanotech., 7, 699 (2012)
[2] S. Kim et al. Nat. Comm., 3, 1011 (2012)
[3] F.R. Libsch et al. Appl. Phys. Lett. 62, 1286 (1992)

The understanding of metal and transition metal dichalcogenides (TMDs) interface is critical for future electronic device technologies based on this new class of two dimensional semiconductors. Here, we investigate the initial growth of nanometer-thick Pd, Au, and Ag films on monolayer MoS2. Distinct growth morphologies are identified by atomic force microscopy (AFM): Pd forms a uniform contact, Au clusters into nanostructures, and Ag forms randomly distributed islands on MoS2. The formation of these different interfaces is elucidated by large scale spin polarized density functional theory calculations. Using Raman spectroscopy, we find that the interface homogeneity shows characteristic Raman shifts in E_2g^1 and A1g modes. Interestingly, we show that insertion of graphene between metal and MoS2 can effectively decouple MoS2 from the perturbations imparted by metal contacts (e.g., strain), while maintaining an effective electronic coupling between metal contact and MoS2, suggesting that graphene can act as a conductive buffer layer in TMD electronics.
The work at UT Dallas was supported partly by NSF (CHE-1300180), and the Center for Low Energy Systems Technology (LEAST), one of six centers supported by the STARnet phase of the Focus Center Research Program (FCRP), a Semiconductor Research Corporation program sponsored by MARCO and DARPA. The MoS2 synthesis performed at the University of Washington (C.H., D.C., and X.X.) was supported by US, DoE, BES, Materials Science and Engineering Division.

Graphene has launched a revolution in revolution in scientific and engineering community due to the unique properties which stem from its 2D structure. Now as other 2D materials have become to emerge, the integration of these materials must be studied in order to move 2D electronics to realizable applications. Here we have synthesized single layer films of graphene, h-Boron Nitride and MoS2, by chemical vapor deposition and have been able to stack them together in different arrangements. Multiple stacking geometries of graphene, MoS2 and Boron Nitride is presented here. Raman Spectroscopy is presented for varying heterostructure stacking, as well as the tunneling properties of these different configurations. Finally Field Effect transistors are configured to probe possible improvements and its charactistic IV curves and mobilities are presented.

Recently molybdenum disulfide (MoS₂) has become an intense research focus due to their exceptional electronic and optical properties. Unlike conductive graphene, MoS₂ is a semiconductor material with a wide band gap (1.2 ~ 1.8 eV). These properties indicate that MoS₂ can be a potential candidate in the fields of electronics, optoelectronics, sensing, and photovoltaics. Various synthesis methods of MoS₂ layer were researched for device applications for example micromechanical exfoliation, intercalation assisted exfoliation, hydrothermal method, and chemical vapor deposition (CVD). However, Uniform MoS₂ sheet/thin films over a large area remain a major challenge yet. Several groups have reported large area MoS₂ by CVD, but CVD method required high temperature process. Here, we report the large scale synthesis of MoS₂ thin films by atomic layer deposition (ALD) process. MoS₂ thin films were grown on several other substrates (SiO₂/Si, graphene, sapphire, mica) at varying temperatures ranging from 200^oC to 300^oC. Molybdenum(V) chloride, MoCl₅, and Bis(trimethylsilyl)surfide, S(SiEt₃)₂, were used as precursors for deposition of Mo and S, respectively, and details of process will be reported. The crystallinity and surface morphology of the films were significantly influenced by process conditions and substrates. We used Raman spectroscopy 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) and X-ray diffraction (XRD) were also employed for its crystalline structure and grain size measurement. X-ray photoelectron spectroscopy (XPS) was carried out for the measurement of the oxidation state in as-deposited MoS₂.

We report on the first demonstration of p-type doping in large area few-layer films of (0001)-oriented chemical vapor deposited (CVD) MoS2. Nb, which has one less electron than Mo, was used as a substitutional defect in a MoS2 thin film, and was found to behave as an acceptor with low ionization. Using this method, we achieved high hole sheet density (up to 10^20 cm-3 with associated Hall mobility of up to 10 cm^2/Vs) in large area p-type MoS2 films, in addition p-type WS2 films. Theoretical scattering rate calculations for mobility were found to match well with the measured hole mobility over a range of acceptor concentrations. This is the first report of in-situ grown substitutional p-type acceptor for thin film MoS2, thus fulfilling a critical requirement for useful electronic and optoelectronic devices.
The reported work utilizes a previously reported method to achieve large area (0001) oriented MoS2 with good crystalline and structural qualities by chemical vapor transport on single crystal (0001) sapphire . Such CVD-grown MoS2 overcomes limitations associated with the commonly used mechanical exfoliation approach (control of thickness and area) and is therefore viable for large-scale device fabrication.
In the present work, a metal stack of 2.5 nm Mo/0.2 nm Nb/2.5 nm Mo was deposited on (0001) sapphire templates by electron beam evaporation. The substrate was vacuum sealed in a quartz tube with 20 mg of sulfur, placed in a furnace and heated at 900C for ten minutes. Niobium was found to act as an efficient acceptor up to relatively high density in MoS2 films, as predicted by density functional theorem calculations . For a hole density of 4 x 10^20 cm^-3 Hall mobility of 8.5 cm^2/Vs was determined, which matches well with the theoretically expected values. AFM and optical images show that smooth continuous films of few-layer MoS2 were formed. XRD spectra showa the (002) peak associated with 2H-MoS2, and Raman spectra display the characteristic in-plane (E12g) and out-of-plane (A1g) vibrational modes of MoS2 at 381 and 407 cm^-1, respectively. Absorption measurements showed that the doped sample had similar characteristics to high-quality undoped samples and exfoliated samples, with a clear absorption edge at 1.8 eV. These measurements suggest that the inclusion of Nb acceptors does not degrade the energy band structure or structural quality of the MoS2 . This demonstration of p-type doping in large area epitaxial MoS2 enable a wide variety of electrical and opto-electronic devices based on layered metal dichalcogenides.
1 M.R. Laskar, L. Ma, S. Kannappan, P.S. Park, S. Krishnamoorthy, D.N. Nath, W. Lu, Y. Wu, S. Rajan, Appl. Phys. Lett. 102(25), 252108, (2013).
2 K. Dolui, I. Rungger, C. D. Pemmaraju and S. Sanvito, (http://arxiv.org/pdf/1304.8056.pdf).

Single layer MoS2 can be exfoliated mechanically similar to graphene. This presentation 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. In addition, this method allows the growth of other ordered structures, such as Mo2S3 films and Mo6S6 nanowires. 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 their affinity for adsorbate interactions. We will also report on ongoing catalytic/TPD investigations of these films.

Two decades ago, it was theoretically predicted that the thermoelectric figure of merit of semiconductor materials will be enhanced by quantum confinement effects when characteristic sizes are comparable to or smaller than the electron wavelength, on the order of 5-10 nm for bismuth telluride, however, realizing materials which exhibit quantum confinement effects has been limited due to experimental difficulty. One promising method for achieving three dimensional bulk nanostructured materials with characteristic lengths on this order is through the use of block copolymer assisted templating. In this poster, we present the use of the amphilic copolymer, Polystyrene-block-polyethylene oxide (PS-b-PEO), as a sacrificial scaffold to template self-assembled 3D networks of nanostructured Bi2Te3. The synthesis-structure relationship will be discussed.

Field effect transistors (FETs) are fabricated to characterize exfoliated MoS2 films. Prior to fabrication, exfoliated films are characterized using high resolution atomic force microscopy and Raman spectroscopy. Exfoliated MoS2 films are characterized by back gate and top gate FETs including the investigation of the FET design parameters such as channel length, gate dielectric material type and thickness. Design parameters are selected to vary between aluminum oxide (Al2O3) to hafnium oxide (HfO2) and 30 to 60 nm and 3 to 6 µm for dielectric material type, dielectric material thickness and channel length, respectively, to observe the effects on device performance. Higher mobility values are obtained for shorter channel length of 3 µm and thinner HfO2 (30 nm) gate dielectric. IDS-VDS curves are recorded for different values of VGS and IDS-VGS curves are plotted with a bias voltage ranging from 50mV to 500 mV where channel length, gate dielectric material and thickness are changed accordingly. A suspended top gate FET has also been fabricated and significant improvement has been observed in current on/off ratio and mobility values.

Fuelled by the astonishing success of graphene, layered materials have been enjoying phenomenal attention in recent years because of their dimension-dependent properties. Many layered materials have recently been explored with the aim of unraveling their thickness related properties. Transition metal chalcogenides are the most heavily studied class of materials next to graphene because of their potential applicability in microelectronics, spin and valley-tronics, thermoelectrics, etc. Copper antimony sulfide (CuSbS2) is a ternary layered semiconductor with a band gap between 1.38-1.52 eV. CuSbS2 has been widely investigated as an absorber material in thin film solar cells due to its optimal band gap with high absorption coefficient of over >104 cm-1. We have for the first time developed solution phase approaches for the synthesis of mono-, few and multiple layers of CuSbS2. This includes the colloidal bottom-up approach by which we have synthesized nanoplates of CuSbS2 with thickness from six layers to several layers, and hybrid bottom-up-top-down approach for mesobelts of CuSbS2. The latter can be exfoliated by Li-ion intercalation and sonication process to obtain down to monolayers. In order to understand the thickness-dependent optical and electrical properties, we have calculated the electronic structures of mono-, and multiple layers of CuSbS2 using the hybrid functional method (HSE 06). We find that the monolayer exhibits noticeably distinct properties from the multilayered or bulk system, with a markedly increased band gap that is compromised by the presence of localized surface states. The latter is predominantly composed of the energetically-favorable Sb pz states, which break off from the rest of the Sb p states that would otherwise be the top of the gap. The computed absorption spectra for the monolayer and bulk indicate gaps of around 2.5 eV and 1.6 eV, respectively, with additional peaks at lower energies in the former due to the split-off Sb states. The details of the syntheses methods, structural and optical characterizations and band structure calculation will be presented.

Transition metal dichalcogenides (TMDCs) have attracted great attention recently because of their fascinating physical properties and potential applications in optoelectronics and spintronics. We have successfully demonstrated a new method for large area growth of single- and few-layer tungsten disulfide (WS2) sheets. This method is based on the sulfurization of hydrothermally synthesized WO3 nanoplates (2-10 microns in length, 2-5 microns in width, with a thickness in the range 10-30 nanometers) dispersed on SiO2/Si substrates. Raman and photoluminescence spectroscopy are used to study the electronic structure of WS2 layers and to examine the performance of the synthesis technique. Surface morphology and quality of the sheets are studied by SEM, AFM and OM. Furthermore, we have studied the electrical transport properties of the synthesized WS2 layers. Electron mobility based on back-gate field-effect transistors is measured to be approximately 0.16 cm2/Vs. The electronic band structures of bulk and monolayer WS2 and hybrid WS2-MoS2 have been investigated with ab-initio density-functional-theory computations.

In this paper, the strain-gate technology that has been implemented in Si MOSFETs manufacturing is adapted to the most widely studied 2D material, Molybdenum Disulfide (MoS2) for carrier mobility enhancement. Because of the lowered effective mass for both electrons and holes, a >50% increment of carrier mobility has been achieved in Si MOSFETs through a nitride stress capping layer deposited on the top of the transistor channel. Compared with conventional bulk materials such as silicon, 2D semiconductor like MoS2 is more suitable for strain engineering, due to its flexibility for highly anisotropic strain and chemically inactive surface. Earlier published theoretical studies on the electronic properties of monolayer MoS2 have shown two important results 1) tensile strain reduces the band gap energy and effective masses of both electrons and holes 2) two characteristic transitions are predicted, namely, direct-to-indirect transition at a tensile strain of 1% and semiconductor-to-metal transition at a tensile strain above 10%. Here, we fabricate strain-gate MoS2 field-effect transistors (FET) to exploit the potential of the material for logic devices. Following the mature strain technology used in the industry, the FETs are made into a top-gate configuration and a nitride stress capping layer wrapping the top gate is deposited to induce strain to the channel. Mobility and current on/off ratio are measured before and after the deposition of the nitride capping layer. Channel length and width are varied to test which channel geometry gives the best response to stress. The semiconductor-to-metal transition is assessed through the channel conductance measurement.

2D MoS2 has emerged as the cutting edge of materials science. It carries an tantalizing prospect to scale all the semiconductor technologies down to the atomic level. However, little has been known about the surface properties and dielectric constants of 2D MoS2 materials. This knowledge is important for a wide range of fundamental and applied research on these materials, such as the design of electronic and photonic devices. Here we report our studies on the surface energy and dielectric constant of 2D MoS2. Very interestingly, our results show strong layer dependence in both surface energy and dielectric constant. We will also discuss the fundamental physics underlying the experimental observations.

The electron beam in a scanning transmission electron microscope (STEM) can be used to modify the structure of a material and simultaneously provide atomic-scale images of the dynamical processes that occur. In recent work, we used the electron beam to generate nanoscale holes in transition-metal dichalcogenide (TMDC) monolayers that are then used to sculpt metallic nanowires connecting designated points within the semiconducting monolayer sheet [1,2].
In the present work, we use the electron beam to generate, excite, and, through sequential atomic-scale Z-contrast imaging, monitor defects and defect dynamics in semiconducting MoSe2 monolayers. We find that single Se vacancies are randomly created by radiation damage and then preferentially agglomerate into line defects under the energy transferred from the electron beam. Density functional theory (DFT) calculations show that such agglomeration is due to the reduced migration barrier of Se vacancies in the vicinity of line defects and results in lowering of the system energy. Moreover, we show that successive evolution of the line defects can create distinct triangular inversion domains within the MoSe2 layer and generate conducting mirror-symmetry grain boundaries within the semiconducting matrix. Migration of such grain boundaries can be further activated by deformation of the nearby regions. Our results provide an atomic-scale understanding of the dynamical behavior of defects in MoSe2 monolayers, which can also be applied to other TMDC materials for defect engineering.
[1] J. Lin, O. Cretu, W. Zhou, K. Suenaga, D. Prasai, K.I. Bolotin, N.T. Cuong, M. Otani, S. Okada, A.R. Lupini, J.C. Idrobo, D. Caudel, A. Burger, N.J. Ghimire, J. Yan, D.G. Mandrus, S.J. Pennycook, S.T. Pantelides, submitted for publication (2013).
[2] J. Lin et al., MRS Fall Meeting, RR14.02 (2013)
This research was supported in part by U.S. DOE grant DE-FG02-09ER46554 (JL, STP), a Wigner Fellowship through the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. DOE (WZ), the Office of Basic Energy Sciences, Materials Sciences and Engineering Division, U.S. DOE (SJP, STP), and through a user project supported by ORNL&’s Center for Nanophase Materials Sciences (CNMS), which is sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. DOE. This research used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the US Department of Energy under Contract No.DE-AC02-05CH11231.

Wafer scale single layer Molybdenum Disulphide (MoS2) film was prepared by an extended growth ambient pressure chemical vapor deposition (APCVD) process. These films were grown using e-beam evaporated Mo material on a SiO2 substrate. These films are easily transferable and great for heterojunction application on other 2D materials such as graphene. Chemical doping is known to be very effective to change carrier density of materials and has been previously been used in doping of graphene. P-doping of the MoS2 films was accomplished through a chemical drop-cast method giving a significant blue shift in the PL. This chemical method can easily be repeatable and further p-dope the material. Mobility of the as grown film followed by the chemical doping are significantly improved compared to un-doped material.

Atomically thin Molybdenum Disulphide (MoS2) triangles and hexagrams were prepared by a two-step regrowth ambient pressure chemical vapor deposition (APCVD) process. MoO3 nanobelts, a few microns in length and width, were prepared using a hydrothermal technique and utilized as the starting material. High temperature treatment of the MoO3 nanobelts followed by a rigorous sulfurization via APCVD processing provided different morphologies of MoS2 monolayers and bi-layer morphologies. Triangle and hexagram morphologies were characterized using scanning electron microscopy, Raman spectroscopy, photoluminescence, and atomic force microscopy. MoS2 layers have a hexagonally packed structure and are held together by Van Der Waals forces. Zipping effect of triangular and hexagram domains is a process where large area domains form when nucleation sites are increased. PL, Raman Spectroscopy, and Raman mapping peak shifts confirm the presence of monolayer and bilayer regions in our regrowth process.

Recently, we reported on a new family of two-dimensional early transition metal carbides and carbonitrides, so called MXenes. They were produced by selective etching of atomically thin layers of aluminum from MAX phases. The latter are a large family (>60 phases) of ternary layered transition metal carbides and/or nitrides with composition of M_n+1AX_n where M is an early transition metal (e.g. Ti, V, Cr, Nb, Mo), A is group A elements (most of group 13 and 14 elements in the periodic table; e.g. Al, Si, Sn), X is carbon and/or nitrogen, and n =1, 2, or 3. The etching of the Al layer from MAX phases was carried out by using aqueous hydrofluoric acid at room temperature. So far, the following MXenes were produced: Ti₃C₂, Ti₂C, Ta₄C₃, TiNbC, (V_0.5Cr_0.5)₃C₂, and Ti₃CN. Unlike graphene, MXenes are hydrophilic; and unlike clays, MXene are electrically conductors, thus MXenes can be considered as a new class of conductive clays. Ab initio calculations predicted that MXenes band gap can be tuned by changing the surface termination, and they have good elastic properties. One of the main applications of MXenes that has been explored is using MXenes as a hosting material for ions in energy storage systems, as they showed an excellent capability to handle high cycling rates. Herein we report on the recent progress in the synthesis of new MXenes (Nb₂C and V₂C) and their performance as electrode materials in lithium ion batteries in comparison with Ti₂C.

Two-dimensional materials such as graphene, molybdenum disulfide (MoS2), or hexagonal boron nitride (h-BN) have numerous extreme properties. They are the thinnest materials know, but at the same time the strongest, the lightest and, in many cases, they have excellent electronic and optical properties. In addition, their all-surface structure offers new opportunities to tune their properties chemically or mechanically, at the same time that maximizes the surface-to-volume ratio. In this talk, I will describe how my group at MIT is taking advantage of these properties to develop some of the first micro- and nano-systems based on 2D materials. Specifically, we will analyze three main application areas: large area flexible electronics, tunable infrared detectors, and chemical and biological sensors.
The success of 2D semiconductor devices and applications first requires the development of scalable synthesis techniques that allow the production of high quality 2-D materials. Most of our work is based on chemical vapor deposition (CVD) on metallic and insulating substrates. Then, the 2D material can be transferred to arbitrary substrates to enable complex heterostructures.
After the synthesis and stacking of the 2D materials, the different devices need to be fabricated. For this, it is very important to develop processing technologies that keep the surface of the materials clean. This is much more difficult than in conventional semiconductor technologies, as a single-atom-thick layer of impurities can completely alter the properties of 2D materials. In addition, many of the conventional techniques to clean surfaces are not compatible with materials that are only one atom thick.
The large variety of 2D materials enables many different opportunities in future applications. This talk will describe, for example, the use of MoS2 in the development of flexible and transparent digital electronics. We will also describe the potential of tuning the properties of graphene to demonstrate a new generation of infrared imaging systems. Finally, we will summary the potential of 2D materials to increase the signal-to-noise ratio in chemical and biological sensors.
Acknowledgements.- This work has been partially supported by the ONR GATE MURI project, the Army Research Laboratory, the MIT-Army Institute for Soldier Nanotechnologies, and the ONR PECASE project.

The rise of 2D crystals has opened various possibilities for future electrical and optoelectronic applications. MoS2 n-type transistors have shown great potential in ultra-scaled and low-power electronics, but this material cannot satisfy the demand for complementary logic devices. Here, we introduce black phosphorus, a new 2D material with layered structure, which can be used as the channel material for p-type transistors. Our band structure calculations based on density functional theory indicate that few-layer black phosphorus is a semiconductor with a direct band gap. We observe anisotropic behavior of the conductance and mobility in the 2D plane. High on-current of 194 mA/mm, hole mobility up to 286 cm2/Vs and on/off ratio up to 1E4 was achieved with few-layer black phosphorus transistors at room temperature. We determined the Schottky barrier height at the Ti/black phosphorus junction to amount to ~0.21 eV based on a temperature dependent study. We demonstrate a CMOS inverter based on black phosphorus PMOS and MoS2 NMOS transistors, which shows great potential for semiconducting 2D crystals in future electronic, optoelectronic and flexible electronic devices.

The nature of the metal-MoS2 contact (Ohmic or Schottky, n- or p-type) is still in much debate with contrasting experimental outcomes, and the underlying mechanisms for contact formation are unclear. Density functional theory calculations are performed to unravel the nature of the contact between metal electrodes and monolayer MoS2. Schottky barriers are shown to be present for a variety of metals with the work functions spanning over 4.2 - 6.1 eV. Except for the p-type Schottky contact with platinum, the Fermi levels in all of the studied metal-MoS2 complexes are situated above the midgap of MoS2.
The mechanism of the Fermi level pinning at metal-MoS2 contact is shown to be unique for metal-2D-semiconductor interfaces, remarkably different from the well-known Bardeen pinning effect, metal induced gap states (MIGS) and defect/disorder induced gap states (DIGS) which are applicable to traditional metal-semiconductor junctions. At metal-MoS2 interfaces, the Fermi level is partially pinned as a result of two interface behaviors: first by a metal work function reduction by interface dipole formation due to the surface charge repulsion, and second by the production of gap states mainly of Mo d-orbitals character by the weakened intralayer S-Mo bonding due to the interface metal-S interaction. This finding would provide guidance to develop approaches to form Ohmic contact to MoS2.
This work was supported by the Center for Low Energy Systems Technology (LEAST), one of six centers supported by the STARnet phase of the Focus Center Research Program (FCRP), a Semiconductor Research Corporation program sponsored by MARCO and DARPA.

Inspired by the knowledge of isolation of graphene, other ultra-thin single crystal semiconductor including two-dimensional (2D) transition-metal dichalcogenide materials in the form of MX2 (where M=transition metal such as Mo, W, Ti, Nb, etc. and X=S, Se, or Te) have drawn considerable attention. The MX2 family material consists of one or more sets of triple layers with one M and two X in a sandwich structure (X-M-X). Atoms within each layer are strongly held together by covalent-ion mixed bonds, while interlayer van der Waals forces are weak. The introduction of these 2D materials in the channel significantly improves the gate electrostatics comparing with conventional Si, resulting in higher gate capacitance and higher performance. In addition, the lack of out of plane bonds leads to low chance of interface trap which causes lower subthreshold swing. Nano-scale semiconductors including 2D materials have been extensively investigated as the channel materials of transistor for low power logic devices. However, nano-scale materials are not introduced in high-voltage power devices area which is on the opposite end of low power transistor. Here we report the fabrication and demonstration of 2D MoS2, WS2, and MoTe2 FETs for low power application, while Ga2O3 of ultra-thin single crystal semiconductor is used as channel materials for power device application.
Ultra-thin single crystal semiconductor materials are transferred on top of back-gated SiO2 dielectrics. The source and drain contacts are defined by electron beam lithography (EBL) using Ti/Au (5/100 nm) contacts. The electron diffraction patterns reveal that ultra-thin materials retain the crystal symmetry and lattice constant. The transport properties and materials properties of each material will be discussed. The larger bandgap of materials leads to higher on/off current ratio and higher breakdown voltages. Mechanical exfoliation has been used in this work in order to generate ultra-thin materials.
In summary, ultra-thin single crystal semiconductor transistors were fabricated and characterized for both low power and high power application. Electron diffraction patterns confirm the crystal nature of the material. These nano-scale materials offer opportunities for efficient heat removal and such approach enable the integration of materials on conventional platform for low and high power devices.
This work was supported by the Semiconductor Research Corporation (SRC), Nanoelectronics Research Initiative (NRI) and the National Institute of Standards and Technology (NIST) through the Midwest Institute for Nanoelectronics Discovery (MIND), STARnet, an SRC program sponsored by MARCO and DARPA, and by the Office of Naval Research (ONR) and the National Science Foundation (NSF).

Molybdenum disulfide (MoS₂) is a promising n-type semiconductor material for transistor applications. It combines high thermal and mechanical stability with excellent electrical properties. The layered transition metal dichalcogenide consists of covalently bonded S-Mo-S atoms that are stacked in planes which in turn are held together by van-der-Waals interaction. This weak interlayer bonding allows micromechanical exfoliation from bulk crystalline MoS₂ yielding flake sizes in the order of 20x30µm and thicknesses between 1 and 100 nm. While transistors based on single-layer MoS₂ have shown superior performance to their multilayer counterparts production of multilayer flakes is significantly easier. The injection and transport properties of thin flake MoS₂ have not been fully investigated up to now.
In this study we conduct temperature dependent field-effect transistor measurements in a temperature range from 11K to 300K on lithography-free patterned multilayer MoS₂ transistors (69-95nm thickness). Annealing of the samples leads to a 4-fold increase in room temperature mobility of up to 29 cm²/Vs. At room temperature the drain current does not saturate. We attribute this to a high bulk doping concentration. Lowering the temperature freezes out the dopants as denoted by a decrease in the threshold voltage and a clear saturation of the drain current thus marking the transition from bulk to monolayer transport.
Furthermore, we observe different trends of mobility µ and drain current I_D with temperature depending on the magnitude of the drain voltage. For low voltages on one hand, both µ and I_D significantly decrease with decreasing temperature indicating a strong current limitation by the contacts. On the other hand, for high drain voltages we observe a slight increase of both µ and I_D, as is expected for band transport.
Our combined measurements clearly allow us to distinguish between the effect of contact resistance, 3D and 2D transport on the electrical transport properties.

Two-dimensional materials hold promise for both the continuation of conventional Moore scaling and new emerging applications such as flexible and transparent electronics [1]. MoS2 transistors have already been demonstrated to have a reasonable mobility and excellent on-off ratios [2]. However, more work is needed to better understand the transport physics of MoS2 transistors. Existing theoretical studies differ in terms of the band structure and importance of various scattering mechanisms, leading to a broad range of estimates for low-field mobility and other transport characteristics [2-4].
Here we investigate for the first time both low- and high-field transport in MoS2 transistors using a two-valley band structure, from a theoretical perspective calibrated against experimental data. We use the first two moments of the Boltzmann transport equation to balance momentum and energy gained from the electric field with that relaxed through various scattering mechanisms. We implement acoustic and optical phonon scattering using deformation potential formalism, polar phonon scattering using Frölich formalism, and use Lindhard screening theory for ionized impurity scattering. We also model device self-heating at high fields, finding that it plays a significant role especially at lower temperatures.
We calibrate our model to current-voltage data for several MoS2 devices of different thickness ranging from single to multiple layers, in the temperature range from 80 to 500 K. In particular, we quantitatively describe the effect of negative differential conductance (NDC), which can be experimentally observed in most of our devices at high electric fields and temperatures below 200 K [5]. In our simulations, we uncover that the 6-fold degenerate Q-type valley must be above the 2-fold degenerate K-type valley to represent experimental data with NDC. This valley alignment is typical for single-layer MoS2 and found in some calculations for multilayer MoS2 [6,7], but in disagreement with other computed band structures [8]. We also show how changes in band structure (such as the relative position of the two valleys, and the change of deformation potentials for phonon scattering) affect the temperature dependence of mobility and velocity saturation in MoS2, which is responsible for the experimentally observed NDC. Our study highlights the importance of combining transport theory and measurements to elucidate high-energy band structure effects, and provides crucial insights for practical device operating conditions.
[1] G-H Lee et al., ACS Nano 7(9), 7931 (2013)
[2] S. Kim et al., Nature Comm. 3, 1011 (2012)
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Silicene, an analogue of graphene as 2-D material, is composed of silicon atoms arranged in a buckled honey comb lattice. Due to its structural similarity to graphene, Dirac zone was predicted in the band diagram of silicene based on theoretical calculation.^[1] The buckled nature of the silicene film leads to greater interface interaction (than graphene) that can result in a sizeable band gap and afford desirable electronic and optoelectronic properties.^[2] However, despite these expectations, no experimental transistor devices have been reported to support the wealth of theoretical predictions. One of the key challenges is how to preserve the material integrity of silicene post-synthesis and during its transfer process and device fabrication.
Here, we present first experimental data indicating Dirac-like behavior of silicene-like films and investigation on its stability. The silicene film used in this study was deposited on Ag (111) film on mica or SiO₂/Si supporting substrates at elevated temperature in a molecular beam epitaxy system. The grown sample was characterized using in-situ scanning tunneling microscopy as well as post-deposition Raman spectroscopy, indicating silicene domain formation altogether.
The transfer and handling of grown silicene film on device substrate are the key technical steps to successful fulfillment of a device. Although silicene exhibits stability on original Ag film even at ambient condition, it tends to oxide rapidly after separating from Ag layer. This makes wet transfer process, similar to graphene, not a practical way for device making. To circumvent this issue, silicene and underline Ag film were detached from supporting substrates. Raman spectroscopy was taken to confirm the presence of silicene after transfer. E-beam lithography was used to define a complementary pattern serving as a mask to create source and drain pads directly on the original Ag layer. Electrical characterization on these back gated devices were measured. A linear output characteristics and ambipolar transfer characteristics were observed, revealing clear evidence of charge transport arising from the Dirac-like band structure. The extracted field effect mobility ~100 cm²/V-s is in agreement with theoretical calculations that suggest acoustic phonon limited transport.^[3] To our knowledge, above results represent the first experimental measurement of silicene based field-effect transistors.
*We acknowledge Army Research Office (ARO) and Army Research Laboratories (ARL) for funding support.
References:
1. Cahangirov, S. et al., Two- and One-Dimensional Honeycomb Structures of Silicon and Germanium, Physical Review Letters 2009, 102, 236804.
2. Cinquanta, E. et al., Getting through the Nature of Silicene: An Sp2-Sp3 Two-Dimensional Silicon Nanosheet, The Journal of Physical Chemistry C 2013, 117, 16719.
3. Xu, M. et al., Graphene-Like Two-Dimensional Materials, Chemical Reviews 2013,113, 3766.

Two-dimensional (2D) layer-structured chalcogenides host many interesting physical and chemical phenomena such as topological insulator and intercalation. Their nanostructures represent novel candidates to host those phenomena. Here we present our study on chemistry and physics of 2D chalcogenide nanomaterials. First, we have synthesized a rich morphologies of 2D layered nanomaterials. Second, we have developed a new method of zero-valent intercalation which allows unprecedented high levels of various metal intercalants inserted into the van der Waals gaps. The resulted optical properties and electrical conductance change drastically. Third, we have fabricated single nanostructure electrical transport devices. Using topological insulator nanostructures, we observed interesting physical phenomena including Aharonov-Bohm oscillations from topological surface electrons, ambipolar transport with effective control of the Fermi level into the bulk bandgap and across the Dirac Point, and localization effects emerging from the surface electrons in confined dimensions. Lastly, we also demonstrate the interesting tunable catalytic property.

Recently, much work has described liquid exfoliation of a range of layered crystals to give exfoliated nanosheets of materials such as graphene, BN and MoS2. Of particular interest are the family of transition metal dichalcogenides (TMD) which includes members such as MoS2, WS2, MoSe2, MoTe2, ReS2 etc. When exfoliated, TMD nanosheets can be used for a range of applications in optoelectronics, energy and catalysis. For many applications, control of lateral size and thickness of nanosheets is crucial. However, both control and measurement of dimensions are challenging. Here we present a simple method for separating small, thin nanosheets from large, thick ones. More importantly, we will demonstrate an in-situ spectroscopic technique for simultaneously measuring both lateral size and thickness of liquid dispersed MoS2 nanosheets. The lateral size measurement relies on the effect of nanosheet edges on the electronic properties of nanosheets and so the shape of their absorption spectra. The thickness measurement is based on quantifying the confinement effect of thickness on the excitonic positions. The combination of size selection and measurement techniques allows us to prepare dispersions rich in MoS2 monolayers. This allows us to measure, for the first time, the photoluminescence of solvent exfoliated MoS2 mono-, bi- and tri-layers. We demonstrate that these techniques are general and can also be applied to WS2, MoSe2, ReS2 etc.

Two-dimensional (2D) semiconductors are of interest due to their exotic physical properties that are strongly thickness-dependent. In particular, transition from indirect band gap in the bulk to direct band gap in the 2D monolayer drastically enhances the photoluminescence (PL) intensity of semiconducting transition metal dichalcogenides (TMDs). It is widely known that point defects in semiconductors can efficiently trap excitons and potentially introduce new radiative recombination mechanisms, thus strongly influencing optical properties of the host materials. Such effects by point defects become even stronger in the 2D limit owing to tighter localization of the electron wavefunction. In this work we seek to develop a new approach to determine the configuration of point defects and quantify their optical effects present in single layer TMDs.
In our work, point defects (mostly anion vacancies) were introduced into monolayer TMDs using high-energy particle irradiation (3 MeV alpha particles) or thermal annealing at sub-decomposition temperatures. By systematically investigating the PL at 77 K, these intentionally introduced defects were found to strongly modify the optical properties of 2D TMDs, such as MoS2, WSe2, and WS2. The overall integrated PL intensity is enhanced and new PL peaks were observed below the band gap. In addition, these effects require interaction with environmental gas molecules, such as N2 and O2, and the intensity of the defect-related PL peaks strongly depends on the irradiation dose that can be linearly scaled to point defect density.

Our observations provide a good foundation for identification of dominant defects in 2D TMDs when combined with calculated formation energies and associated localized energy states of various point defects, e.g., mono-sulfur and di-sulfur vacancies in monolayer MoS2. We will also present how defect density can be estimated through optical probing of TMD monolayers, assisted by high-resolution transmission electron microscopy study. Our study provides new approaches to diagnose crystal quality of 2D semiconductors and tailor their physical properties via defect engineering.

The transition metal dichalcogenides (TMDs) have recently emerged as a fascinating class of materials that can be prepared as crystals of single- or few-layer thickness by exfoliation and chemical vapor growth [1]. Here we review recent progress in understanding the distinctive electronic and optical properties of these systems. One of the key results is the crossover in typical TMDs like MoS2 from dark, indirect-gap semiconductors in the bulk to bright, direct-gap semiconductors at monolayer thickness [2]. A significant optical second-harmonic response is also observed in the monolayer MoS2, but not in bilayers, reflecting the thickness-dependent crystal symmetry [3]. The strong many-body effects in monolayers of these materials are manifest in the robust excitonic emission/absorption and the spectroscopic signatures of charged excitons (trions) [4]. The predicted production of valley-selective excitations using circularly polarized light has also been verified experimentally in monolayers of these materials [5].
[1] A. M. van der Zande, et al., Nature Mater. 12, 554-561 (2013).
[2] K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, Phys. Rev. Lett. 105, 136805 (2010).
[3] Y. Li, Y. Rao, K. F. Mak, Y. M. You, S. Wang, and T. F. Heinz, Nano Lett. 13, 3329 (2013).
[4] K. F. Mak, K. He, C. Lee, G. H. Lee, J. Hone, T. F. Heinz, and J. Shan, Nature Mater. 12, 207 (2013).
[5] K. F. Mak, K. L. He, J. Shan, and T. F. Heinz, Nature Nano. 7, 494 (2012).

Graphene research has inspired the study of other two-dimensional (2D) electronic materials, particularly those based on transition-metal dichalcogenides. Group III-VI semiconductors represent a lesser investigated class of 2D materials. Among these, gallium telluride (GaTe) is a layered monochalcogenide with a room-temperature direct band gap of 1.7 eV. It is typically a p-type semiconductor and possesses a unique monoclinic crystal structure. Previous studies have indicated that GaTe undergoes a surface transformation from the monoclinic structure to a metastable hexagonal structure similar to that observed for GaS and GaSe. However, other reports suggest that the surface is stable or that the surface transformation is just due to surface oxidation and the formation of tellurium and gallium oxides. Here, we report on the surface properties of GaTe exposed to ambient conditions using Raman, photoluminescence (PL) and photomodulated reflectance (PR) spectroscopy as well as X-ray photoelectron spectroscopy (XPS). While partial surface oxidation is confirmed, we find that extended exposure to air results in the formation of a disordered phase which is characterized by the quenching of the room-temperature PL and PR signals as well as the complete transformation of the Raman spectrum. Interestingly, Hall Effect measurements indicate that the carrier concentration and carrier mobility remain relatively constant even after the surface transformation, while atomic force microscopy shows that the layered structure and layer flatness are preserved even after extend air exposure of several weeks. Partial reversibility of the surface transformation is achieved after annealing at low temperature (< 250° C) in an argon or nitrogen atmosphere. Though the mechanism is not fully understood, we attribute intercalation of oxygen or water molecules between the layers to be responsible for the formation of the disordered phase. Mechanically exfoliated flakes display the properties of monoclinc GaTe but are transformed over time into the disordered phase, which is seemingly dominant in all exfoliated flakes below a thickness threshold around 20 nm. This work was principally supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1106400. XPS work done at the Molecular Foundry was supported by the DOE&’s Office of Basic Energy Sciences under Contract No. DE-AC02-05CH11231.

Topological insulators are characterized by a narrow bandgap in the bulk and a Dirac cone-like conducting surface state. However, identification of gapless surface electronic states measurements (i.e.,Angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy(STM) measurements) used particularly specialized instruments, and their procedures are excessively complex.
We demonstrate that a terahertz (THz) wave can be generation from the (001) surface of single crystalline Bi2Se2Te1 thin film under the excitation of a using the 790nm femtosecond optical pulses. The multilayer films composed of alternately layers of Bi and Se (Te) ( [Bi3Te9]n/[Bi3Se9]n,) were fabricated by controlling the layered thickness within atomic scale using thermal evaporation techniques. The as-grown thin film is amorphous phase, p-type THz polarity. However, annealed thin film is single crystalline Bi2Se2Te1, n-type THz polarity. It is because of Se and Te vacancies behave as electron donor site, resulting in n-type characteristics. THz polarity is coincident with major carrier type by Hall measurement. In topological insulator, THz emission is caused by optical rectification of the optical pulse at the topological insulator surface and the transient current within the surface depletion region. The surface effects arising from just a few sample surface layer on single crystalline phase due to resonance enhancement of the quasi-real optical transitions between the surface metallic states and insulating bulk state will be discussed.
In summary, we show that fabrication of Bi2Se2Te1 single crystal thin film through the thermal evaporator on Si substrate. We observed efficient THz emission from the metallic surface state by using the femtosecond laser pumping of a topological insulator Bi2Se2Te1. This work gives useful information for valuable spectroscopic method for future devices and studies of the dynamics of the surface electrons in topological insulator.

Often dubbed white graphite, hexagonal boron nitride (h-BN) is a III-V compound and an insulating isomorph to graphite consisting of sp2 hybridized bonding in layered hexagonal basal planes. It is a unique material considering its very high thermal conductivity and electrically insulating behavior. The advent of 2D materials has rekindled the interest on h-BN. While studies on exfoliated materials are impactful for fundamental insights, integration and application driven research has focused more on large area growth. This comes with its own cost, which is relative loss in crystallinity through the introduction of defects and grain boundaries due to the substrates and the deposition process. Since nanoscale materials are more sensitive to defects, there is a strong need for accurate characterization of large area 2D materials.
In this study, we focus on the thermal conductivity of chemical vapor deposited h-BN films utilizing a single ammonia borane (NH3BH3) precursor. This is motivated by the tremendous potential of thermal management in next generation nanoelectronics, arising from its very high thermal conductivity and low electrical conductivity. We employed both steady state and transient techniques that use freestanding 10 nm and 20 nm thick specimens transferred on a micro-electro-mechanical heater and non-contact infrared thermometry. The measured value for both thicknesses, about 100 ± 10 W m-1K-1, is lower than the bulk basal plane value (390 W m-1K-1) due to the imperfections in the specimen microstructure. Impressively, this value is still 100 times higher than conventional dielectrics. Since CVD growth is more pertinent to real life implementation of h-BN in 2D electronics, this study highlights the role of microstructural defects on thermal transport that can be useful in future applications

Silicene, the graphene analogue for silicon, has recently attracted intense attention mainly due its compatibility with current Si-based nanoelectronic devices. Free-standing silicene does not exist in Nature, but it is possible to engineer it on suitable substrates such as silver (Ag). Although there is evidence that several configurations of buckled silicene with honeycomb lattices and mixed sp2/sp3 hybridization are formed on Ag(111), the question about the existence of a Dirac cone is wide open. Evidence for Dirac fermions in 4x4 silicene on Ag(111) has been provided by Angle-Resolved UV Photoelectron Spectroscopy (ARPES) [1] and Scanning Tunneling Spectroscopy (STS) [2] measurements. However the experimental evidence is debated by several other (mostly) theoretical works claiming that the observed [1] steep linearly dispersing band is related either to Ag bulk [3] or to a new state that emerges due to the strong hybridization between silicon and Ag orbitals [4]. In this work, the electronic band structure of monolayer (4x4) silicene on Ag(111) is in-situ imaged by ARPES, in an attempt to resolve the issue of the existence of Dirac cone. We observe [5] a new dominant surface metallic band located near the bulk Ag(111) sp-band which is also faintly visible. The two-dimensional character of the surface band has been distinguished against the bulk character of the Ag(111) sp-band by means of photon energy variation experiments. We have attributed the surface band (which appears only after Si deposition) to a hybridization of Si and Ag orbitals in line with predictions in ref 4. Using ARPES we have examined a large portion of the k-space and found that the hybrid surface band exhibits a steep linear dispersion around the K point of Ag(111) and a saddle point at the M point of Ag(111), resembling the π-band dispersion in graphene. Although a Dirac cone in silicene has not been confirmed here, our work shows that it is possible to modify the surface properties of metals, and possibly other materials, to obtain new graphene-like electronic band structure. As we have already showed [6] Ge on Ag(111) similarly produces rich and highly dispersing bands, albeit with a stronger hybridization consistent with a Ag2Ge ordered surface alloy. Here we present our most recent unpublished results showing that mixing Si and Ge, produces a new 2D material Si0.5Ge0.5 on Ag(111) which differs with either one of Si/Ag or Ge/Ag systems.
Acknowlegement: The authors acknowledge financial support from the EU through the FET project 2D NANOLATTICES-270749.
References:
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[5] D. Tsoutsou et al., Appl. Phys. Lett. , to be published (2013)
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