Monolayer Molybdenum disulfide (MoS₂), a two-dimensional crystal with a direct bandgap, is a promising candidate for 2D nanoelectronic devices complementing graphene. Unlike conductive graphene and insulating h-BN, atomic layered MoS₂ is a semiconductor material with a direct bandgap, offering possibilities of fabricating high performance devices with low power consumption in a more straight-forward manner.
In this talk, we will discuss our recent efforts on the large area growth of MoS₂ atomic layers by a scalable chemical vapor deposition (CVD) method. The as-prepared samples can either be readily utilized for further device fabrication or be easily released from the growth substrate and transferred to arbitrary substrates. High resolution transmission electron microscopy and Raman spectroscopy on the as grown films of MoS₂ indicate that the number of layers range from single layer to a few layers. Our results on the direct growth of MoS₂ layers on dielectric leading to facile device fabrication possibilities show the expanding set of useful 2D atomic layers, on the heels of graphene, which can be controllably synthesized and manipulated for many applications.

Graphene&’s success has shown that it is not only possible to create stable, single-atom thick sheets of a crystalline material, but that these materials can have electronic structures that are fundamentally different than the parent 3-D material, and that are significantly influenced by the environment. Recent work at The Ohio State University has shown that unique single-layer 2D materials based on silicon and germanium can be synthesized, stabilized by appropriate ligands. In this talk, we will show theoretical calculations of the electronic properties of two-dimensional silicon and germanium sheets, which are strongly dependent on the stabilizing molecules. For our calculations, we will use density-functional theory with hybrid functionals that predict the band gaps in excellent agreement with experiment. We will examine the dependence between orbital structure and calculated bands and discuss the origin of the observed changes in the band structure with changing ligands. We will also discuss the relationship between the observed large-gap band structures for silicon and germanium and the Dirac-cone type band structure case of graphene, and the dependence of the observed properties on the geometric parameters of the atomic structure.

We have developed a novel chemical method to intercalate extremely high densities of metal atoms into 2D layered nanoribbon chalcogenides and oxides. Whereas most traditional methods of intercalation move ions into the layered host, this method moves zero-valent atoms and is specific to ultrathin 2D nanocrystalline samples. The zero-valent nature of the intercalant species allows super-stoichiometric intercalation of Ag, Au, Co, Cu, Fe, In, Ni, and Sn. We show that this method is general and can be used for a variety of 2D layered nanomaterials. This work opens new ground for accessing novel states of quantum matter such as superconducting topological insulators, unique opto-electronic & structural properties, and new energy storage applications.

Layered structures including MoS2 and WS2 are promising materials with shape, size, layer number, and interlayer distance all dictating the optoelectronic behavior. In this work, we demonstrate the synthesis of WS2 nanosheet structures through sonochemical unzipping of nanotubes. Nanosheets of different shapes and layer numbers are achieved by a chemical separation technique, and characterized using transmission electron microscopy, x-ray and electron diffraction, and Raman spectroscopy. Our separation technique isolates WS2 particles of various layer numbers, exhibiting unique and tunable optical transitions. These novel layered materials are further explored towards optoelectronic and catalytic applications.

We report CVD growth and characterization of large area crystalline Molybdenum disulfide (MoS2) layered semiconductors using sulfurization of thin molybdenum layers on different substrates. Characterization of the films indicates that the films are highly crystalline with orientation defined by the substrate. This is the first report of single crystal large area MoS2 using CVD. MoS2 has received widespread interest recently due to its promise for electronics [1-4], phototransistors [5] and sensing [6], but uniform MoS2 sheet/thin films over a large area remain a major challenge. Several groups have reported large area MoS2 by chemical synthesis [7-9], but the films were non-crystalline. MoS2 epitaxy [10] and mechanical exfoliation lead to flake-shape morphology that limits large-scale device fabrication applications. Different substrates (sapphire, GaN, Si/SiO2) were coated with Mo using e-beam evaporation, and were heated together with sulfur in a sealed quartz tube. Sulfur vapor reacted with the Mo, forming MoS2. The crystallinity and surface morphology of the films were significantly influenced by process conditions and substrates. Raman spectroscopy peaks E21g and A1g were found to be identical in intensity and ratio to mechanically exfoliated MoS2, selective area diffraction TEM and STEM image, XRD measurements indicats the high quality of the films and (002) oriented. Atomic force microscopy indicates smooth monolayer step morphology over the entire surface of the sample. In addition, MoS2 growth on several other substrates sapphire (GaN, SiO2/Si, AlGaN/GaN) at varying temperatures ranging from 500oC to 1100oC was carried out, and details of film properties will be reported. Transmission-line-measurement structures on the MoS2/sapphire showed clear linear and quadratic regions in the I-V curve, indicating ohmic and space charge transport and low background carrier density. The electron mobility, extracted using a modified Mott-Gurney equation for thin films [11], was 10.72 cm2/V.s. In conclusion, we have reported single crystal large area MoS2 using a simple CVD method that could have applications for high-quality growth of several crystalline layered 2D semiconductors for electronic and sensor applications.

Layered materials represent a diverse and largely untapped source of 2-dimensional systems with exotic electronic properties and high specific surface areas that are crucially important for applications including sensing, catalysis and energy storage. While graphene is the most well-known layered material, transition metal dichalcogenides (TMDs), transition metal oxides (TMOs) and other 2-dimensional (2D) compounds such as BN, FeTe, Bi2Te3 and Bi2Se3 are also important. If they could be easily exfoliated in large quantities, such layered materials would become an important source of 2-dimensional crystals. Here we show that layered compounds such as MoS2, WS2, MoSe2, MoTe2, TaSe2, NbSe2, NiTe2, BN, MnO2, MoO3 and Bi2Te3, can be efficiently dispersed and exfoliated in both in common solvents and in aqueous surfactant solutions. Electron microscopy shows these materials may be exfoliated down to individual layers. These dispersions can be deposited as individual flakes or formed into films. By blending with suspensions of other nano-materials or polymer solutions, we can prepare hybrid dispersions or composites which can be cast into films. Such materials demonstrate huge potential for a range of applications. For example, we show that WS2 and MoS2 effectively reinforce polymers, WS2/carbon nanotube hybrid films display promising thermoelectric properties, MoS2 films are viable candidates for battery electrodes while layered oxides are exciting for electrochemical applications. Exfoliated layered compounds are also useful as fillers in polymer based composites for applications such as mechanical reinforcement or gas barrier. We will demonstrate the utility of these materials for such applications. We note that such applications will, eventually, require large quantities of exfoliated layered materials. We will demonstrate a scalable exfoliation method for producing nanosheets in very large quantities.

Recently, Transition metal dichalcogenides (TMD), MX2 (M=Mo, W; X=S, Se, Te), have attracted considerable attention for beyond graphene. Multi-layer WS2 is an indirect gap semiconductor which is larger than that of graphene. Moreover, WS2 becomes a direct gap semiconductor same as MoS2 single layer.
Here we demonstrate the large area growth of WS2 atomic nano sheet on SiO2 substrates by a scalable chemical vapor deposition (CVD) method. WS2 nano sheet was formed in various thickness and substrate temperature. The films were confirmed with Raman spectroscopy. We fabricated bottom-gate WS2 transistors, and characterized the device performances by varying the gate insulator and the various thickness of WS2 nano sheet. Electrical properties of MoS2 films were measured by using probe station.

The fundamental and practical interest in intercalated compounds (IC) of crystalline layered solids has been stimulated by their remarkable mechanical, physical, chemical and catalytic properties as well as a possibility to use them as precursors for the exfoliation to single atomic layers. The latter was demonstrated by preparing the monolayers of graphene oxide, some metal chalcogenides and oxides, and graphene cones.
The intercalation chemistry of hexagonal boron nitride (h-BN), however, is much more challenging than that of graphite, despite h-BN and graphite are isoelectronic and have the same crystal structure with very close cell parameters. This difficulty results from the stronger interlayer interaction (due to the different interlayer registry in h-BN with eclipsing B and N atoms in the neighboring layers and polar character of the B-N bonds) and very different electronic structure. h-BN is a wide gap insulator, while graphite is a semimetal. The BN redox potential is ~ 2V more positive relative to graphite and its oxidative intercalation is much harder to achieve. Only a few ICs of h-BN have been reported.
This work demonstrates a possibility of the non-oxidative h-BN intercalation with mineral acids, such as H₂SO₄ and H₃PO₄. The h-BN- H₂SO₄ and h-BN-H₃PO₄ compounds have the interlayer distances of 6.9-7.4#8491; that implies the formation of the 1st-stage ICs of h-BN. The structure and guest-host interactions have been studied by XRD, TEM, SEM, FTIR, Raman, XPS and thermal analysis. We assume that the acid molecules interact with the h-BN layers by formation of hydrogen bonds of the type B^δ+N^δ-hellip;H-OP(S)< (or even protonation of N atoms), and dative bonds of the type
>P(S)=O:→B^δ+N^δ-.

Recent developments on 2D layered materials engineering have demonstrated that chemical or physical local modifications of the graphene atomic structure can turn its electromechanical properties into piezoelectric [1], [2]. Nanoelectromechanical (NEMS) energy harvesters based on suspended graphene nanoribons can benefit from those new transducing properties, to convert energy from ambient vibrations into useful energy on the electrical domain [3]. In this work, we present the design of non-linear NEMS energy harvesters based on bistable suspended graphene or boron nitride bridge-like nanoribons. The bistability conditions induced by a controlled compressive stress of the nanomechanical structures are analyzed by ab-initio calculations of the potential energy. From the numerical solution of the NEMS devices movement equation, mechanical and electrical power harvested from the ambient vibrations are calculated, when noise force excitations are considered.
[1] S. Chandratre and P. Sharma, "Coaxing graphene to be piezoelectric." Appl.Phys. Lett. 100, 023114 (2012).
[2] M. T. Ong, and E. J. Reed, "Engineered Piezoelectricity in Graphene." ACS Nano 6, 1387 (2012)
[3] M. Lopez-Suarez, R. Rurali, L. Gammaitoni, and G. Abadal, "Nanostructured graphene for energy harvesting." Phys. Rev. B 84, 161401 (2011).

We recently produced a new 2-D material, viz. Ti3C2, by selectively etching Al from a MAX phase Ti3AlC2 and labeled it MXene, to emphasize its similarity to graphene. MXenes represents a large family of transition metal carbides and carbonitrides, not just one phase. Unlike graphene, whose chemistry is restricted to carbon, MXenes allow a variety of chemical compositions and are establishing themselves as a new class of two-dimensional materials. MXenes possess good in-plane conductivity, which in combination with the rich surface chemistry makes them attractive for electrical energy storage. However while potential of MXenes as anode materials for Li-ion batteries has already been shown, their use in electrochemical capacitors has not been explored.
Here, we report on our latest findings regarding the capacitive properties of Ti-containing MXenes. Electrodes were fabricated by vacuum filtration of MXene sheets or by conventional film rolling with binder and conductive additives. In some cases prior to electrode fabrication MXene was delaminated in order to achieve better separation of the 2D sheets. Several different electrochemical techniques were employed to understand the mechanism of charge storage in both aqueous and organic electrolytes. Electrochemical impedance spectroscopy confirmed the low resistivity of the tested materials showing characteristics for capacitors close to 90° angle on the Nyquist plot. Cyclic voltammetry measurements showed a high rate handling ability. Moreover, a 3-5 times increase in capacitance was observed for delaminated MXenes as compared to as-produced material.

Unlike graphene, the existence of bandgaps (1-2 eV) in the layered semiconductor molybdenum disulfide (MoS₂), combined with mobility enhancement by dielectric engineering, offers an attractive possibility of using single layer MoS₂ field-effect transistors (FETs) in low-power switching devices. However, the complicated process of fabricating single layer MoS₂ with an additional high-k dielectric layer may significantly limit its compatibility with commercial fabrication. Here we show the first comprehensive investigation of process-friendly multilayer MoS₂ FETs to demonstrate a compelling case for their applications in thin-film transistors. Our multilayer MoS₂ FETs exhibited high mobilities (> 100 cm²/V s), near-ideal subthreshold swings (~70 mV/decade), and robust current saturation over a large voltage window. With simulations based on Shockley&’s long-channel transistor model and calculations of scattering mechanisms, these results provide potentially important implications in the fabrication of high-resolution large-area displays and further scientific investigation of various physical properties expected in other layered semiconductors.

Metal dichalcogenides (e.g. MoS2, WS2, NbS2, etc.) have recently attracted the attention of numerous scientists because they are layered materials that could exhibit either semiconducting or metallic properties. However, these properties could be significantly modified when these layered materials become monolayers. To the best of our knowledge, the isolation of few-layered WS2 has only been performed by mechanical and chemical exfoliation with very low yields. Recently, a CVD method has been reported to yield micron size WS2 triangular islands with unprecedented optical properties. In this work, we report for the first time the synthesis of large area (< 1 cm2) few-layer WS2 by a two step method. WOx thin films were first grown on a Si/SiO2 substrate and these films were sulfurized in a second step at high temperatures (750-950 °C). Furthermore, we have developed an efficient route to transfer these WS2 films onto different substrates, such as quartz and TEM grids. WS2 films of different thicknesses have been analyzed by Raman spectroscopy, TEM and AFM techniques. Transport properties of the films have also been measured at different temperatures. Characterization techniques demonstrate the presence of mono-, bi- and few-layered WS2 in the as-grown samples. The novel photoluminescence properties of the films will also be discussed along with high resolution TEM images exhibiting various types of stacking.

The nanosheet of layered transition metal chalcogenide--molybdenum disulfide (MoS2) has attracted great interest due to its potential applications in catalysis, optoelectronics, energy harvesting. But, studies of the nanosheet have been delayed by the lack of capabilities to prepare the nanosheet with control of physical features in atomic-scale precision. Here, we report synthesis of atomic-thin MoS2 by vapor deposition techniques on different substrates. The atomic thickness and high quality of resulting nanosheets are revealed by spectroscope and microscope (Raman, Photoluminance, XRD, XPS, TEM). This synthetic approach is facile, scalable and may generally apply to the growth of nanosheets of other layered transition chalcogenides.

As an alternative to graphene, two-dimensional layer-structured transition metal dichalcogenides (TMDs) have been expected to have a great potential in nanoscale and flexible electronic application areas. Among many available TMD materials, molybdenum disulfide (MoS2) has attracted much research interest for thin film transistor (TFT) application due to its high electron mobility and large bandgap [1]. In realizing high-performance MoS2-based TFT devices, there have been several attempts to adopt an atomic layer deposition (ALD) technique in forming high-k gate dielectrics on top of the MoS2 channel layer [1, 2]. Because the ALD process is strongly dependent on the surface identity and MoS2 has a chemically inert surface condition, it is necessary to significantly decrease the deposition temperature to obtain a uniform high-k dielectric film deposition [2, 3], which may deteriorate the dielectric properties.
In this presentation, first, we will discuss the different growth behavior of the ALD-Al2O3 and HfO2 films on mechanically-exfoliated MoS2 multilayers using trimethylaluminum and tetrakis-(ethylmethylamino)hafnium metal precursors, respectively, along with H2O oxidant. In addition, as an alternative way to obtain a uniform deposition of ALD-high-k films at a high temperature, we propose a surface treatment of the MoS2 surface using an oxygen plasma process, which resulted in a uniform growth of the Al2O3 and HfO2 films even at a deposition temperature of 250 °C. According to the XPS analysis, the MoS2 surface was partially oxidized to form an ultra-thin Mo-oxide layer, which improved the adsorption the ALD metal precursors.
[1] B. Radisavljevic et. al., Nature Nanotech. 6, 147, (2011).
[2] H. Liu et. al., Appl. Phys. Lett. 100, 152115, (2012).
[3] H. Liu et. al., IEEE Electron Dev. Lett. 33, 4, (2012).

The effective dispersion of nanomaterials is universally acknowledged to be critical when one wishes to harness their full potential in devices or applications. Over the past number of years much work has been carried out on overcoming the inherent difficulties associated with producing high-quality dispersions of single-walled carbon nanotubes (SWNTs), graphene and other 2D nanomaterials (such as transition metal dichalcogenides) in the liquid phase.
Crucial to solvent choice for each of these low dimensional nanomaterials has been matching the surface energy of the nanomaterial with the surface energy of the solvent. Such information for organic solvents is normally contained within their solubility parameters. Hildebrand and Hansen solubility parameters have been primarily used to-date to predict which solvents will exfoliate low dimensional nanomaterials. Whilst these have been quite successful, they are unable fully describe the requirements of a solvent for such nanomaterials. In this work, we show that Kamlet Taft parameters dramatically improve our ability to predict solvents for intractable nanomaterials. We will demonstrate that specific solvent-solute interactions can be accounted for using this scale (interactions that are not possible using only Hildebrand or Hansen parameters) - interactions that may explain the ability of certain solvents to produce stable high-quality dispersions of nanomaterials.

There is little doubt that two dimensional materials are an intriguing class of materials. Some boast multifunctional properties, such as transparency, high conductivity, high thermal conductivity and high strengths simultaneously, for example. However, their integration into products and technologies will depend upon scalable and reliable ways to prepare the nanosheets with high quality. Liquid phase exfoliation of graphite and other layered inorganic crystals in solvents offers a way to produce billions of 2D nanosheets in a small volume of solvent. It also offers an avenue to prepare dispersions of 2D nanosheets with specific properties. This research aims to optimise the exfoliation of layered crystals in solvents, so that a highly concentrated dispersion of thin and large flakes can be achieved. Taking MoS2 as the model material, we recently showed that highly concentrated dispersions (~40 mg/ml) of flake size separated MoS2 nanosheets could be prepared. However, although the dispersions were highly concentrated and contained laterally large nanosheets ( ~ 2 mu;m), the flakes tended to be thick, as indicated by transmission electron microscopy. Further processing optimisation to reduce the thickness of these nanosheets, by various dilution and agitation methods were performed, with promising results. The knowledge gained throughout this research, was also successfully extended to other layered materials. Thus demonstrating the universality of this optimised preparation method.

Lack of band gap in pristine graphene as well as difficulty in its engineering has let atomic layers of MoS₂ an attractive candidate of two-dimensional materials especially in nanoelectronics because of their intrinsic direct band gap of 1.8 eV and capability of valley polarization. Nonetheless, there have been limited synthetic methods allowing formation of large-scale and patternable MoS₂ layers that is essential to avoid post-synthesis etching steps required during device fabrication due to chemical inertness of MoS₂. Herein, we report a novel way to prepare atomic layers of MoS₂ in desired geometry and dimension by CVD under mild condition: atmospheric pressure and the lowest temperature reported so far. We used Raman spectroscopy, Raman spectrum mapping, and atomic force microscopy (AFM) to check both the chemical composition and the number of layers of the product. High-resolution transmission electron microscopy (HRTEM) was also employed to check its crystal structure, grain size and the number of layers. X-ray photoelectron spectroscopy (XPS) was carried to check the oxidation state to conclude that the product is chemically MoS₂. XPS study on non-sulfidated sample provided mechanistic evidence for surface alloying of Mo on Au film as well. Lastly, we have fabricated an electronic device to measure its electronic properties as a form of field-effect transistor.

Hollow-closed fullerene-like nanoparticles of MoS₂ thereof were shown to exhibit very favourable tribological properties and are already in commercial use. However, being semiconductors, the effect of the doping of these semiconducting nanoparticles has been overlooked so far. Here the doping of these nanoparticles with rhenium atoms is studied and is shown to have a remarkable influence on their physio-chemical properties.
The Re atoms go to substitutional Mo sites in the nanoparticles lattice as shown by extended fine structure X-ray absorption (EXAFS) and high angle annular dark field (HAADF) studies. Theoretical quantum-chemical calculations show that the Re level is some 180 meV below the conduction band.
The doping leads to marked increase of the doped nanoparticles conductivity and reduced agglomeration in various suspensions, which is attributed to their negative surface charge. Furthermore, due to a robustness, flexibility and high conductivity of these nanoparticles, a precipitous reduction in friction and wear is observed.

Originally inspired by the spectacular success of graphene, interest in the 2D nanomaterials is extended well beyond graphene. Unlike graphene whose constituents are limited to be carbon atoms, layered chalcogenide materials (LCMs) comprise cations, which can be semiconductors (like Ge and Sn) or transition metals (such as Mo and W), and anions that can be -S, -Se, or -Te. This richer variety in compositions can provide the 2D LCM nanostructure with new functionality that cannot be obtained from graphene. Developing sophisticated synthetic capabilities is important for exploring the full potential of the 2D nanomaterials. Vapor deposition, as a versatile synthetic strategy, has a proven track record in enabling control of the physical features of nanomaterials with atomic precision. However, the development of controlled vapor phase synthesis of 2D LCM nanostructures necessitates a better understanding of the fundamentals involved. Knowledge on the vapor growth of nanomaterials such as nanowires, nanotubes, and graphene may not simply apply to the 2D LCM nanostructure. In this talk, we report controlled vapor growth of two kinds of LCM nanosheets. In the growth of GeS-type quasi-LCM nanosheets with high growth rate, we find that the nanosheet growth is subject to strong influences of the diffusion of source materials through the boundary layer of gas flows. This boundary layer diffusion is found to be the rate-determining step of the growth under typical experimental conditions, evidenced by a substantial dependence of the nanosheet's size on diffusion fluxes.¹ In contrast, in the growth of SnSe₂-type LCM nanosheets (nanoplates) whose growth rate is relatively slow, we find that their growth is reaction-limited. The dynamics and orientation of the nanoplate growth show substantial dependences on the strength of the interaction between the SnSe₂ materials and the substrate. A weak interaction can enable the facile migration of SnSe₂ adatoms on the substrate and facilitate the in-plane growth of the nanoplate along the substrate surface. This is evidenced by the morphological differences in the nanoplates grown on mica and on silicon. The nanoplate grown on mica, whose interaction with SnSe₂ materials is weaker, exhibits a larger size and an irregular shape compared to the nanoplates on silicon, which are smaller and hexagonal. Strong interactions, however, may induce strain energies in the nanoplate, which can subsequently suppress the in-plane growth.² These fundamental understanding of the key factors that influence the chalcogenide nanosheet growth can provide useful guidance for the development of general paradigms to control their synthesis.
Reference:
1. C. Li, et al. , Role of Boundary Layer Diffusion in Vapor Deposition Growth of Chalcogenide Nanosheets: The Case of GeS, ACS Nano 6 (2012) 8868.
2. L. Cao, et al., Substrate Mediation in the Growth of Layered Chalcogenide Nanoplates: A Case Study of SnSe₂, Submitted to ACS Nano.

Resounding from the recent discovery of graphene, there has been a significant increase involving research on layered materials. In particular, Molybdenum disulfide (MoS2) has gathered much attention for its desirable properties and varied applications. Its semiconducting behavior has led to much work on transistor and solar cell applications for example, yet there are other areas where this material&’s properties can be useful. One area of basic research that is of interest is in energy storage. Increasing the capabilities of current energy storage technologies is crucial to support the rapid growth in the needs of various devices, ranging from cell phones to electric and hybrid cars to maintaining power grids. One particular type of storage device that has seen much attention in recent years is the electrochemical double layer capacitor (EDLC). Material for the electrodes in EDLCs has traditionally been carbon or metal-oxide based, but recently it has been indicated that MoS2 can give EDLC performance comparable to that of carbon nanotube based electrodes. In order to gauge the potential of these layered materials as suitable candidates for EDLC electrodes it is necessary to understand their fundamental electrochemical properties. Here, we will present detail electrochemical characterization of thin films consisting of few-layer MoS2 flakes (synthesized via a liquid phase exfoliation). The capacitive charge storage behavior (in presence of various electrolytes) analyzed using cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy techniques will be discussed.

MoS2 is a semiconducting material consisting of sulfur-molybenum-sulfur tripledecker layers loosely bound by van der Waals interactions. Single layer MoS2 can be exfoliated mechanically similar to graphene. This poster shows an alternative avenue for the fabrication of MoS2 monolayers at comparatively low temperature and mild conditions through sulfur loading of a copper substrate using thiophenol followed by the evaporation of Mo atoms and annealing. Here we also demonstrate that another MoSx structure can be formed in this fashion, which has a far higher affinity to adsorbate interaction. Using anthraquinone and formic acid as test molecules, we titrate the various MoSx and copper-based structures presented on our substrate in order to determine the relative strength of adsorbate interaction.

Here we report a two-step synthesis of two-dimensional single- and few-layered transition metal dichalcogenides (WS₂ and MoS₂) islands with different edge geometries. The process involves the high-vacuum deposition of metal tri-oxides thin films (5-20 Å of WO₃ or MoO₃) followed by high-temperature sulfurization inside a quartz tube reactor. Different island morphologies (i.e. dendritic, triangular, hexagonal and star-shaped) have been obtained by controlling the synthesis parameters such as sulfurization temperature, carrier gas composition, S evaporation rate (or sulfur temperature), metal tri-oxide thickness and cooling-down rate. These morphologies are single-crystal islands originated from a nucleation center. Coalescence of neighboring islands lead to more complex and irregular shapes, which include grain boundaries. Atomic force microscopy and Raman spectroscopy mapping were used to determine the thickness (number of S-W-S layers) distribution at the island center and at edges. Transmission Electron Microscopy revealed the formation of zigzag edges as the most stable edges in islands with regular shapes (non-dendritic). The optical properties of the different morphologies and edges were studied through Photoluminescence and Raman mapping. For triangular islands, PL enhancement was observed near the edges; while hexagonal island presented an alternated pattern of PL enhancement, suggesting that controlling the shape of the islands as well as the structural termination and chemistry of the edge can be used to tailor the optical properties of these systems.

Recently, we discovered a new family of two dimensional, 2-D, early transition metal carbides produced by the exfoliation of the MAX phases. The latter is a large family (+60 members) of machinable layered ternary carbides and nitrides, with a M_n+1AX_n composition, where “M” is an early transition metal, “A” is a group 13 to 16 element, “X” is C and/or N, and n = 1, 2, or 3. The exfoliation process was carried out by selective etching of the A-group element from M_n+1AX_n using hydrofluoric acid (HF) at room temperature, resulting in 2-D M_n+1X_n layers, we labeled MXene to emphasis their similarity to graphene. Herein we focus on the first synthesized and most studied member of the MXenes family: Ti₃C₂ produced by HF etching of Ti₃AlC₂. The effect of synthesis parameters (temperature, time, particle size) on etching will be discussed. Not only are individual layers formed after exfoliation, but also multi-layer particles and conical scrolls of radii < 20 nm have been demonstrated. DFT simulations showed that band gap of Ti₃C₂ can be tuned from metallic to semiconductor by changing its surface termination (O, OH and F), and its elastic properties along the basal planes (c₁₁ asymp; 523 GPa) are higher than that of the binary carbide TiC. The 2-D Ti₃C₂ can be used as a precursor for other unique materials; for example rapid thermal oxidation of Ti₃C₂ in air at 1100 °C resulted in formation of nano anatase supported on graphene-like carbon sheets. One of the potential applications for 2-D Ti₃C₂ is in energy storage systems as anodes for lithium ion batteries. Additive-free Ti₃C₂ anodes, produced by filtering an aqueous dispersion of delaminated Ti₃C₂, showed capacity of 410 mAh.g^-1 at cycling rate of 1C and 110 mAh.g^-1 at 36C after 200 cycles.

Making field effect transistors (FETs) on thin flake of single crystal isolated from layered materials was pioneered by the success of graphene. To overcome the difficulties of zero band gap in graphene electronics, we report the fabrication of a electric double layer (EDL) transistor, an variation of FET based on another layered material, MoS₂. Using strong carrier tunability found in EDL coupled by ion movement, MoS₂ transistor displayed an unambiguously ambipolar operation in addition to its commonly observed n-type transport. A high on/off ratio >10⁴, large “ON” state conductivity of ~mS, and high reachable n_2D ~ 1×10¹⁴ cm^-2 confirmed the high performance transistor operation important for application. The high-density carriers of both holes and electrons can drove the MoS₂ channel to metallic states indicating that new electronic phases could be accessed using the protocol established in making EDL gated transistors on layered materials.

The discovery of graphene in 2004 [1], has drawn the interests of both industries and the scientific community on two dimensional (2D) materials. Although the enhanced carrier mobility observed in graphene is highly desirable for electronic applications, the lack of intrinsic bandgap leads to the exploration of alternative 2D materials with a natural bandgap. Molybdenum disulphide (MoS₂) is such a material with a direct bandgap of 1.9 eV in atomically thin layers [2]. Thus far the highest reported carrier mobility in single layer MoS₂ is ~217 cm²/Vs [2], in which Kis et al. employ a high dielectric (high-κ) top gate, to reduce coulomb scattering.
This work explores the capabilities of intrinsic high-κ materials as an alternative for achieving enhanced charge carrier mobilities. Molybdenum trioxide (MoO₃) is one of the transition metal oxides that has a relative dielectric constant of ~500 (~ 5 for MoS₂) and can be exfoliated to minimum resolvable atomically thin layers. But in its intrinsic nature, MoO₃ has a wide bandgap (>3 eV) which is not viable for transistor applications. However the bandgap of MoO₃ can be easily manipulated to desirable values by several techniques such as hydrogen ion (H⁺) intercalation, UV irradiation, electron beam bom-bardment etc. [3, 4]. Such techniques produce partially reduced sub-stoichiometric MoO_(3-x) which has increased carrier concentration and a high dielectric value which highly favors the enhancement in charge carrier mobility.
In this work, charge carrier mobility in a 2D high-κ material is investigated. Field effect transistors (FET) based on MoO₃ are fabricated. The MoO₃ flakes were reduced to sub-stoichiometric MoO_(3-x) in order to achieve bandgap values viable for transistor applications. Carrier mobilities > 1100 cm²/Vs were observed in a MoO_(3-x) FET. The scattering mechanisms limiting the overall mobility with respect to temperature are explored. Theoretical and experimental mobilities are compared in low and high dielectric materials.
References:
[1] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science2004, 306, 666 - 669.
[2] B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti and A. Kis, Nat. Nanotechnol.2011, 6, 147-150.
[3] J. Z. Ou, J. L. Campbell, D. Yao, W. Wlodarski and K. Kalantar-zadeh, J. Phys. Chem. C, 2011, 115, 10757-10763.
[4] J. W. Rabalais, R. J. Colton and A. M. Guzman, Chem. Phys. Lett., 1974, 29, 131-133.

Graphene has a unique combination of electrical, mechanical, and optical properties, is being actively explored for future electronic applications. High intrinsic mobility in graphene combined with the ability to modulate the carrier in graphene based field effect transistors(FET), makes graphene a promising material for nano-electronic device. Especially, in order to make top gated-graphene FET devices, a high quality uniform high-k dielectric material should be deposited on graphene surface to realize a device of high breakdown voltage and low leakage current. Dielectric layer effectively control the charge carrier movement, therefore, it should be thin, uniform without any pinholes. In a few reports, gate dielectric was deposited on graphene by physical vapor deposition such as evaporator or sputtering. And atomic layer deposition (ALD) process also used for dielectric deposition by various treatment on graphene. Hence, in the present work we address the issue of uniform deposition of Al2O3, a high-k dielectric material for top gated-graphene FET device. We deposit high-k dielectric on CVD graphene using plasma treated hexagonal boron nitride layer (h-BN) as a buffer layer for high-k dielectric deposition. Dielectric layer was conformed with Raman Spectroscopy, AFM measurement, XPS, and TEM. And graphene based top gated FET device also fabricated. We expect that h-BN exploits graphene&’s superior electronic properties through eliminating charge traps, impurity and charge carrier inhomogeneity at the graphene/h-BN-Al2O3 interface.

Due to its unique and remarkable properties graphene is considered a very promising material for a new electronics. However, in its pristine form graphene is a zero bandgap semiconductor which poses serious limitations in done transistors applications. Because of that there is a renewed interest in other possible non-zero bandgap similar structures.
One of this structures is the biphenylene carbon (BPC) [1] (also known as graphenylene [2]), a bidimensional pure carbon structure with a bandgap about 0.8 eV. Recently, it was proposed [3] that the BPC can be obtained from the selective dehydrogenation of porous graphene [4].
In this work we propose a new two-dimensional boron nitride structure which we called inorganic BPC. Our ab initio molecular dynamics showed that similarly to the BPC case if can be spontaneously formed from the dehydrogenation of porous boron nitride sheets. It also important to mention that the inorganic BPC is thermally stable even at high temperatures. Its unique electronic and mechanical properties are also addressed.
[1] R. H. Baughman, H. Eckhardt, and M. Kertesz, J. Chem. Phys. 87, 6687 (1987).
[2] Q. Song, B. Wang, K. Deng, X. Feng, M. Wagner, J. D. Gale, K. Müllen, and L. Zhi, J. Mater. Chem. C (2013).
[3] G. Brunetto, P. A. D. S. Autreto, L. D. Machado, B. I. Santos, R. santos, and D. S. Galvao, J. Phys. Chem. C 120508112932001 (2012).
[4] M. Bieri, M. Treier, J. Cai, K. Aït-Mansour, P. Ruffieux, O. Gröning, P. Gröning, M. Kastler, R. Rieger, X. Feng, K. Müllen, and R. Fasel, Chem. Commun. 6919 (2009).

In addition to Graphene 2D transition metal chalcogenide, as for example MoS2 and WS2, nanostructures are promising materials for applications in electronics and mechanical engineering. Though the structure of these materials results in a highly inert surface with a low defect concentration, defects and edge effects can strongly influence the properties of these nanostructures. Therefore, a basic understanding of the interplay between electronic and mechanical properties and the influence of defects, edge states and doping is needed.
We demonstrate on the basis of atomistic quantum mechanical simulations of several types of MoS2 nanostructures how the topology, the edge structure, local defects, dislocations, doping and mechanical deformation can vary the mechanical behavior, the electronic properties, and also determine the electronic device characteristics of such systems.

In this talk, we will discuss the approaches to control electronic and optical properties of two-dimensional materials, graphene and monolayer molybdenum disulfide (MoS2).
First, we address the possibility of controlling the effective dielectric constant k of 2D materials by fabricating graphene or MoS2 devices that are suspended inside liquids ranging from hexane (k~1.9) to water (k~80). With increasing k, we observe robust signatures of screening of electrostatic interactions between charge carriers in both graphene and MoS2. For graphene, we observe a rapid increase of carrier mobility with k, with room temperature mobility reaching 60,000 cm2/Vs for high-k liquids. For MoS2, increasing k causes an upshift in photoluminescence energy, consistent with decreasing binding energy of excitons in this material.
Second, we investigate the influence of uniform mechanical strain fields on both monolayer MoS2 and graphene. In suspended graphene, the presence of strain quenches flexural phonons, a strong scatterer of charge carriers. We observe an experimental signature of this quenching -- room temperature resistance of graphene decreasing with strain. In monolayer MoS2, we observe large changes in intensity and positions of photoluminescence peaks. These changes are consistent with predicted transition from direct to indirect band gap character in this material at strain levels of ~2%.

It is well known that carbon-based nanomaterials such as graphene and carbon nanotubes exhibit remarkable mechanical, electrical, thermal and optical properties which has stirred a great deal of excitement for considering them for a wide variety of applications ranging from nanoscale transistors, interconnects, field-emission displays, photo-voltaics and nano-electro-mechanical-systems (NEMS). The investigation of graphene as a model 2D system has impacted a diverse array of fields spanning physics, chemistry, materials science, and engineering. While great strides have been made recently for applications that have stemmed from graphene&’s unique properties, in electronics applications, specifically digital electronics, the absence of a band-gap in graphene poses concerns for its attractiveness to enable high ON/OFF ratios. Although a band-gap in graphene is induced through quantum confinement by creating graphene nanoribbons, the band gaps nonetheless are small (few hundred meV) and it is challenging to maintain pristine edge chirality due to defects that are induced during nanofabrication of the ribbons. Recently, layered 2D crystals of other materials similar to graphene have been realized which include insulating hexagonal-BN (band gap ~5.5 eV) and transition metal di-chalcogenides which display properties ranging from superconducting NbS2 to semiconducting MoS2. In addition, it has been shown that bulk MoS2 films transform from an indirect band-gap semiconductor with a band gap of ~1.2 eV to a direct band gap semiconductor with a gap of ~1.8 eV for single atomic layers. The device applications of such systems show promising characteristics; for example transistors derived from 2D monolayers of MoS2 show ON/OFF ratios many orders of magnitude larger than the best graphene transistors at room temperature, with comparable mobilities. In this talk, I will provide an overview of the Electronics, Photonics and Magnetic Devices (EPMD) program in the Electronics, Communications and Cyber Systems (ECCS) division where graphene, as well as other layered 2D nanomaterials, are playing an important role for enabling innovative device applications in electronics, photonics and sensing.

Atomically thin layers can be formed by carbon, BN, metal chalcogenides, oxides and hydroxides, but no carbides have been reported to exist in 2-D form until recently. Transition metal carbides are known for many outstanding properties, but their strong bonds render their exfoliation into 2-D layers quite difficult. Nevertheless, we have synthesized a new family of early transition metals carbides in 2-D form, starting with the MAX phases. The latter is a large family (+60 members) of layered hexagonal ternary metal carbides and/or nitrides, where “M” stands for an early transition metal, “A” stands for a group 13 to 16 element, and “X” stands for carbon and/or nitrogen. Etching the “A” layer from MAX phase using hydrofluoric acid or other etchants at room temperature results in weekly bonded M_n+1X_n layers that can be separated by sonication yielding 2-D layers. To emphasize their similarity to graphene, we labeled them MXenes. The following compounds have already been synthesized: Ti₂C, Ti₃C₂, Nb₂C, Ta₄C₃, TiNbC, (V_0.5Cr_0.5)₃C₂, and Ti₃CN. A review of the published information about synthesis and properties of MXenes will be presented. DFT simulations showed that their band gaps can be tuned by changing their surface termination, and that their elastic properties along the basal planes are superior to their 3-D binary counterparts. MXene powders are ductile enough to be cold pressed into free-standing discs with conductivities comparable to graphite. Water contact angle measurements showed hydrophilic behavior due to oxygen or OH termination of MXene surfaces. Among a multitude of possible applications, herein we focus on electrical energy storage systems, such as lithium ion batteries, lithium ion capacitors and electric double layer capacitors. The simplicity of the synthesis process enables large-scale production of MXenes in quantities from grams to kilograms.

Besides graphene and hexagonal boron nitride, transition metal chalcogenides (TMC) such as MoS2, WS2, NbS2 and WSe2 also exhibit a layered structure in which the layers weakly interact via Van der Waals forces, and for this reason these materials exhibit excellent lubrication properties. For TMC, the layers are formed by the transition metal atom sandwiched by the sulfur atoms. MoS2 and WS2 in bulk are indirect band gap semiconducting materials. However, an isolated sheet of MoS2 or WS2 becomes a direct gap semiconductor [1]. This particular behavior makes them very attractive in terms of optical properties such as spin polarization, in which the lack of center of inversion of one layer plays a crucial role [2,3]. Therefore, it is important to study the properties of different configurations of bi-layer TMC systems, generated by displacing or rotating one layer with respect to the other, thus breaking the inversion symmetry of the bulk stacking. In order to first assess this issue, Density functional theory (DFT) calculations were carried out for different bilayer WS2 geometries considering different rotation angles and different displacements. It was found that for particular geometries of the bilayer systems, the indirect and direct band gaps “compete”, thus exhibiting different electronic and optical properties. As in graphene, Raman spectroscopy provides relevant information about the different layer stackings and twistings in these materials [4]. From the experimental standpoint, we will demonstrate that chemical vapor deposition (CVD) routes could result in the growth of TMC twisted layers, thus making this technique adequate to study the optical properties of new configurations experimentally. We will discuss these issues and also possible applications of these twisted bi-layers.
References
1.- Fai Mak, K., Lee, C., Hone, J., Shan, J., Heinz, T.F., Physical Review Letters, Vol. 105, 136805 (2010).
2.- Zeng, H., Dai, J., Yao, W., Xiao, D., Cui, X., Nature Nanotechnology, Vol. 7, 490-493 (2012).
3.- Fai Mak, K., He, K., Shan, J., Heinz, T.F., Nature Nanotechnology, Vol. 7, 494-498 (2012).
4.- Sato, K., Saito, R.,Cong,C., Yu, T., Dresselhaus, M.S., Physical Review B, Vol. 86, 125414 (2012).

Graphene's success has shown that it is not only possible to create stable, single-atom thick sheets from a crystalline solid, but that these materials have fundamentally different properties than the parent material. In this talk, we will discuss our recent results on the creation of two-dimensional single atom thick hydrogen-terminated and organic-terminated germanium (GeR) analogues of graphane (CH). Contrary to the indirect gap of Ge at 0.67 eV, these materials have direct band gaps centered at 1.5 eV for GeH, and are tunable depending on the nature of the surface ligand. These materials represent a new class of covalently-terminated single atom thick derivatives of Group 14 semiconductors, and have great potential for a wide range of optoelectronic and sensing applications.

SnS2 and SnS2/SnS superlattice nanotubes were synthesized in copious amounts using both sealed ampoules and a horizontal/vertical flow systems with Bi and Sb2S3 as catalysts [1]. The outer diameter of the tubes ranged from 13 to 165 nm, and their length varied from 90 nm to 3.2 micrometers. Careful examination in HRTEM revealed that most of the tubular structures consisted of layers showing periodic or almost periodic patterns which can be interpreted as a superstructure of SnS2 and SnS layers, whereas others exhibited evenly spaced fringes of SnS2. This observation was further confirmed by Raman measurements of individual nanotubes after the examination in HRTEM.
Chemical profiling showed a constant profile of the catalysts along the tube axis. It is believed that the high sulfur affinity of Bi causes partial decomposition of the pseudo hexagonal SnS2 precursor producing the sulfur deficient SnS and the misfit layered tubular structures of a type (SnS)n(SnS2)m. The scrolling process was promoted by adding miniscule amounts of Sb2S3 powder to the ampoules.
The tubular morphology of the SnS2/SnS ordered superstructure nanotubes is a result of the lattice mismatch between the two sublattices comprising the host-guest structure. This driving force comes in addition to the already established closure mechanism, i.e., annihilation of dangling bonds at the periphery of the layers of the inorganic nanotubes (INT) nanostructures.
The diversity of the structures manifests itself through different stacking orders of SnS2 and SnS layers along their common c-axis and their relative in-plane orientation. Folding vectors and chiral angles of both subsystems can be determined [2].
The scaling-up of the nanotubes synthesis was accomplished in a horizontal/vertical flow reactor. These reactors have produced 10-20 mg of either SnS2 or SnS/SnS2 ordered superstructure nanotubes phase in 30% yields.
1. G. Radovsky, R. Popovitz-Biro, M. Staiger, K. Gartsman, C. Thomsen, T. Lorenz, G. Seifert and R.Tenne, Angew. Chem, Intl. Ed. 50 (51), 12316-12320, (2011)
2. G. Radovsky, R. Popovitz-Biro, and R. Tenne, Chem. Mater. 24, 3004-3015, (2012).

This talk will describe our ongoing work on understanding the properties of CVD-grown films of 2D electronic materials, in particular graphene and molybdenum disulfide, and demonstrating their potential in functional devices. We have reported that CVD-grown graphene can achieve electronic performance equivalent to that of exfoliated material1. In more recent work, we demonstrate that CVD graphene can also achieve mechanical strength equal or close to that of the pristine lattice. Moreover, we directly study the mechanical properties of grain boundaries and point defects to understand their role in determining the mechanical behavior of defective graphene. The superior electrical and mechanical properties of CVD graphene combine to enable graphene FETs with performance in the GHz regime at strains of close to 2 percent. We also have recently achieved CVD growth of highly crystalline molybdenum disulfide. We characterize its structural, optical, and electronic properties by a number of techniques. We are able to correlate the shape of crystallites to their crystal structure, and identify common grain boundary morphologies. These large single crystals allow for careful study of electronic transport, in particular separate treatment of contact effects and bulk mobility. Finally, this talk will briefly discuss new techniques for creating layered heterostructures of these materials, and some of their properties and applications.
1. Nicholas Petrone et al., Nano Letters 12, 2751 (2012).

The recent interest in layered compounds and especially in thin films of layered compounds is the focus of extensive research. The properties of single layers are calculated and measured when these layers are embedded within devices, while other applications require the use of thin films of a few layers. The layered structure, by definition, consists of anisotropy when comparing the chemical and physical properties within the layers and across them. In addition, in some devices edge effects produce even more complex systems.
It is therefore extremely important to characterize the basic features of the layers within the structure in order to predict and explain the measured properties and in order to design new applications. Important parameters include: determining the stacking of the layers one relative to the other and the induced disorder; Estimating the rotation of layers relative to each other; Imaging the atomic structure of the edges; Measuring physical properties by spectroscopy and associated them with local structural features.
Today, with the development of the high resolution electron microscope into a versatile experimental set-up, these issues can be addressed with unprecedented resolution. Recent results f MoS2 and WS2 will be presented and discussed.

Two-dimensional sp2-bonded hexagonal nanomaterials such as graphene and hexagonal boron nitride (h-BN) have received significant research interest due to their excellent mechanical strength, chemical inertness, stability, high thermal conductivity, and a range of electronic properties ranging from conducting to insulating. Understanding the atomic structure of defects in these materials is essential for their future use in devices and other applications. Prior Transmission Electron Microscopy (TEM) work has determined the structure of grain boundaries in CVD graphene, but the structure of many defects in CVD h-BN, which is molecularly isoelectronic with graphene, are still unknown. We have utilized atomic resolution TEM to image defects in CVD h-BN nanosheets, including vacancy defects as well as the interfaces and grain boundaries between adjacent domains.

Recently layered semiconductors based on metal dichalcogenide have drawn quite intense research focus. Among these mechanically exfoliated small flakes of monolayer and few layers of MoS2 were found to exhibit intriguing electronic properties such as high-performance field-effect transistors, electric field induced superconductivity, luminescence etc. These findings indicates that these materials can be potential candidate for future optoelectronic applications. However, for many applications availability of large area atomic layers of these materials are needed. In this regard the recent success in the growth of large area 2D MoS2 layers through chemical vapor deposition method is very encouraging and opens up a wide range of possibility for a variety of electronic applications. Here, we report on the electrical transport properties of large area few layers of MoS2 synthesized by chemical vapor deposition under the application of a D.C as well as an A.C bias. Our results indicate 2D Mott-type variable range hopping transport under D.C. bias. The frequency-dependent conductivity (<100 KHz) increases linearly with frequency. Detail analysis of the A.C. transport data show an evidence of universal A.C transport where all the measurements collapse into one single master curve with appropriate scaling. These results will be discussed within the framework of existing theoretical models and scaling theories.
This work is supported by the U.S. Army Research Office MURI grant W911NF-11-1-0362

Following the initial results on graphene, many other 2-D layered structures have been explored by researchers around the world. This has generated genuine excitement for interdisciplinary research given the intriguing geometric form(s), defect structures and potential for technologically useful properties anchored by the 2-D confinement effects in such atomically-layered systems.
The fundamental and technological opportunities for 2-D atomically-layered structures are plentiful but so are the scientific and engineering challenges. These require requisite control over synthesis (especially large-scale), lateral conformation of layers (layer size), site-specific patterning and solution-based assembly as well as unambiguous measurements of their true intrinsic properties - all mediated by unconventional defect structure in atomically-layered systems.
Our efforts in this rapidly evolving field are focused on synthesis, assembly and atomic-scale characterization of nanoscale versions of these layered systems, and exploration of their self-assembly and intrinsic transport properties. On one front, nanoscale version of graphene oxide (nGO) is synthesized that is amenable to functionalization which provides exquisite control over self- and supramolecular assembly. Such assemblies have been shown to exhibit excellent sensing behavior for biomolecular structures, such as protein identification. On another front, have developed surface functionalization strategies of atomically-layered chalcogenides (e.g., MoS2) that allow for colloidal stability of atomic sheets for further processing, patterning and measurements. We have developed and introduced classical optical and Raman characterization methods for rapid identification of atomically-layered chalcogenides that are sensitive to individual sheet number, thereby expanding the structural “palette” for exploration of intrinsic properties of single, double and multiple atomic-layered sheets (and heterostructures) with single-layer discrimination. We have fabricated transistor geometries to assess the intrinsic semiconducting characteristics of several chalcogenides and explored their sensitivity to gas and chemical interactions. The initial findings point to truly unusual behavior of not only single and isolated atomic layers but their multiple layer counterparts. The presentation will cover ongoing research focused on isolation and characterization of single and multiple atomic sheets of nGO and chalcogenides; the unusual defect structures as well as their colloidal assembly and transport properties.

Characterizing defects and crystal structure is critical for controlling quality and materials properties in the expanding array of 2D materials growths. In particular, while defects such as grain boundaries have dramatic effects on the electrical and mechanical properties of graphene (1), much less is known about the atomic structure of these defects for other 2D materials such as MoS₂. MoS₂ has received recent attention as a 2D direct band-gap semiconductor suitable for building electronics from layered materials (2). Here, we present an in-depth study of the structure of monolayer MoS₂ grown by chemical vapor deposition (CVD) using a combination of atomic-resolution scanning transmission electron microscopy (STEM) and dark-field TEM (DF-TEM).

First, we demonstrate that with CVD growth on SiO2 substrates, (3) we can achieve single crystals of monolayer 2H MoS₂ up to 100 microns across. The edges of these CVD MoS₂ crystals are strongly faceted along 2 distinct zig-zag [-1010] directions: the S-terminated and Mo-terminated faces. By correlating the 3-fold symmetry of the monolayer MoS₂ diffraction pattern (4) to the shape of MoS₂ single crystals, we further demonstrate that the orientation and edge termination of MoS₂ crystals can frequently be approximately deduced from their shapes. In polycrystalline films, we use DF-TEM (1) to show that both overlapping and atomically abrupt rotational grain boundaries form with grain sizes from 1-100 micron scales, and that the sizes and types of boundaries present depend strongly on growth conditions.

Importantly, we report the discovery of a common line defect in CVD MoS₂ systems—mirror twin boundaries. In monolayer MoS₂, mirror twins are the intersections of two MoS₂ crystals with a relative in-plane rotation of 60 (or 180) degrees, a half-unit cell lattice shift that effectively swaps the positions of Mo and S sites. We show that, while these boundaries can be easily missed in a simple diffraction analysis, mirror twins appear clearly in DF-TEM. Atomic-resolution STEM images of the twin boundary show that the line defect is formed primarily from 4 and 8-membered rings that avoid homo-elemental bonds, but require changes in atomic bonding states. The MoS₂ twin boundary thus contrasts with twin (5) and rotational boundaries (1) in graphene, which are more commonly formed by 5- and 7- membered rings without dangling bonds. In summary, our large-grain, high-quality growths and analysis of atomic structure of edges, grain boundaries, and twin boundaries in CVD MoS₂ demonstrates the immense utility in applying TEM techniques to further the growth and understanding of CVD materials beyond graphene.
1. P. Y. Huang et al., Nature 469, 389-392 (2011).
2. B. Radisavljevic, et al., Nat. Nanotechnol. 6, 147-150 (2011).
3. Y.-H. Lee et al., Adv. Mater. 24, 2320-2325 (2012).
4. J. Brivio et al., Nano Lett. 11, 5148-5153 (2011).
5. J. Lahiri, Y. et al., Nat. Nanotechnol. 5, 326-329 (2010).

Exploring new plasmonic materials with functionally more diverse and potentially superior properties compared to the popular noble metals such as gold and silver is actively ongoing. Correspondingly, two-dimensional (2D) conductors such as graphene have gained great interest as new plasmonic materials [1]. Here, we demonstrate that 2D layered metal chalcogenides such as Bi2Se3 and Bi2Te3 in the form of ultrathin nanoplates [2] can be tunable metamaterials that can be engineered at atomic-scale. The anisotropic 2D bonding nature and the van der Waals gap present new ways to tune the plasmonic and photonic modes of the chalcogenide nanoplates.
Monochromated electron energy-loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM) has proven to be a powerful technique to study the local spatial variations of plasmons of nanostructures, as demonstrated by the plasmon mappings of silver triangular nanoplates [3] and plasmon resonances of silver nanoparticles [4,5]. EELS can also probe photonic density of states [6,7]. Using STEM-EELS, we examine and directly visualize the photonic and plasmonic modes that are supported in the chalcogenide nanoplates. We show that these modes can be widely tuned by various methods such as geometric control and compositional tuning at nano- and atomic-scale. The wide tuning range and the co-existence of the photonic and plasmonic modes afforded in these metal chalcogenide nanoplates will be useful for various applications.
[1] Jablan, M., Buljan, H., and Solja#269;icacute;, M., Phys. Rev. B 80, 245435 (2009).
[2] Kong, D. et al., Nano Lett. 10, 2245 (2010).
[3] Nelayah, J. et al., Nature Phys. 3, 348 (2007).
[4] Ouyang, F., Batson, P. E., and Isaacson, M., Phys. Rev. B 46, 15421 (1992).
[5] Scholl, J. A., Koh, A. L., and Dionne, J. A., Nature 483, 421 (2012).
[6] García de Abajo, F. J., Rev. Mod. Phys. 82, 209 (2010).
[7] Cha, J. J. et al., Phys. Rev. B 81, 113102 (2010).

Hexagonal Boron-Nitride (h-BN) has been identified as an extremely attractive material for multiple technological applications due to its exceptional mechanical, thermal and electronic properties. The highly stable and mechanically strong structure of hexagonal boron nitride exhibits extremely high chemically stability and resistance to oxidation until temperatures of over 800 oC thus making h-BN highly attractive for a range of applications in the bulk-phase where chemical stability and structural integrity are demanded. It is therefore particularly challenging to initiate chemical reactions involving atoms in the bulk h-BN lattice whilst retaining the intrinsic properties of the material. It is apparent that for the application of exfoliated h-BN nanosheets, novel chemical strategies will be necessary in order to integrate and utilize BNNSs in applications such as surface coatings and nanocomposites.
Here we present the covalent functionalization of exfoliated h-BN nanosheets (BNNSs) via solution phase radical addition chemistry. We present functionalization approaches which involve the generation of reactive radical species within a solution of exfoliated BNNSs. In one case, we describe the covalent functionalization of BNNSs via the addition of Nitrene radicals to B atoms within the h-BN sp2 hybridized lattice. We demonstrate the modification of the surface chemistry of BNNSs by forming polycarbonate analogue composites with the modified BNNSs. We also extend this functionalization methodology to covalently bind polycarbonate analogue chains directly to the BNNSs.
In a second case, we present the covalent functionalization of BNNSs via oxygen radicals. In a two-step procedure we demonstrate the oxygen radical functionalization of BNNSs via the thermolysis of an organic peroxide molecule in the presence of exfoliated BNNSs. Oxygen radical functionalization results in the grafting of organic butoxy groups grafted to boron atoms in the h-BN sp2 hybridized lattice. The successful grafting of the organic functional groups to the BNNSs is then followed by the hydrolytic de-functionalization of the groups to yield hydroxyl groups covalently bound to the B atoms in the lattice. The hydroxyl functionalized BNNSs (OH-BNNSs) are then demonstrated to exhibit hydrophilic character and are further chemically derivitized via carbamate linkages to form borocarbamate functional groups. Modification of the surface chemistry of the BNNSs based on the hydroxyl functional groups is utilized in order to form BNNS-polymer composites which are mechanically analyzed.
Tuning the chemistry of h-BN in these ways demonstrates the rational design of these 2-D nanosheets in order to enhance the integration and chemical compatibility within polymer matrices. This in turn provides the basis for enhanced pathways for mechanisms of stress transfer to optimize the efficient reinforcement of next generation of polymer nanocomposites.

We present chemical vapor deposition methodologies for the controlled synthesis of mono- and few- layer h-BN and the sequential growth of stacked layers of h-BN/few-layer graphene. Growth of such thin h-BN films was achieved using solid (ammonia-borane) and gaseous (ammonia and diborane) precursors. The growth rate and its mechanism(s) were studied with the “stop-growth” method in which the reaction is stopped after running under various conditions, and then the resulting films are characterized by Raman spectroscopy including micro-Raman mapping spectroscopy, by scanning and transmission electron microscopy (SEM, TEM), atomic force microscopy (AFM), and photo and cathode-luminescence. Emphasis was given to the growth methods that yielded large h-BN domain size (up to hundreds of microns), as one of the most important parameters that typically has a strong influence on the electronic, mechanical, thermal, chemical, and optical properties of polycrystalline thin films is the grain size.
The direct synthesis of h-BN/few-layer graphene stacked layers was done by a combination of deposition of h-BN on a metal substrate enriched with carbon in its bulk, followed by the controlled segregation of C to the h-BN-metal interface. 13C-enriched methane was used as the carbon source to minimize overlap between the graphene Raman D-band (red-shifted about 50 cm-1 by the use of 13C) and the h-BN Raman band. We expect to be able to present the assembly of mono and few-layer graphene/h-BN stacked films into functional devices at the conference.
Acknowledgements: We appreciate support from the Army Research Office (grant W911NF1010428) and the NRI SWAN. Work at TTU was supported by DOE under Contract No. FG02-09ER46552. H.X.J. and J.Y.L. would like to acknowledge the support of Whitacre endowed chair positions through the ATT foundation.

Freestanding oxide layers approaching two-dimensionality allow us to probe the fundamentals of carrier transport in confined structures and the role of surface proximity in influencing conductance transitions. They also serve as useful test structures to probe emergent phases under high electric fields. The role of space charge boundary zones may also become more pronounced in such layers and in turn may be of interest in design of low power sensors and field effect structures. I will present our on-going efforts in this area with emphasis on two classes of oxide materials: one that displays metal-insulator conductance transition by electric bias such as vanadium dioxide and the other related to ionic-electronic conductance transitions induced by chemical doping. Especially for transition metal oxides that display strong electronic correlations, point defect induced conduction or local stoichiometry changes becomes crucial in decoupling the physics from electrochemically-induced processes. Particular emphasis will be on experimental methods to fabricate quasi-2D oxide membranes and building integrated devices with such layers, including field effect transistors and sensor devices. Given the typical stress relaxation that occurs during phase transitions (especially during structural transitions or lattice distortion during redox processes involving oxygen exchange), one has to fabricate suspended structures with great care to retain mechanical integrity and we will consider these in detail. We will then present experimental efforts to investigate transistor action in such ultra-thin self-supported membranes and the role of surface proximity. Custom instrumentation to study the properties of such ultra-thin membranes will also be briefly discussed.

Insulating compounds that are isostructural to graphene may have a significant degree of ionic character. This brings up the important question of whether "polar discontinuities''
might play a role in these low-dimensional systems. Here, we perform extensive density-functional theory calculations for coplanar zigzag interfaces between II-VI, III-V and IV-IV two-dimensional honeycomb insulators. Our results confirm that these interfaces are polar, with a net charge of excess charge that, according to the interface theorem, is uniquely determined from bulk properties of the participating materials, and is related to the nontrivial topology of their respective polarization lattice. It is shown that, in the limit of increasingly wide domains, the logarithmically divergent electrostatic potential produced by the lines of charge triggers a Zener-like breakdown mechanism not dissimilar to what happens at insulating oxide interfaces. Such mechanism gives rise to spin-polarized one-dimensional electron/hole gases.

Theoretical and experimental results about the nonlinear optical response of a transition metal dichacogenides Van Der Waals solid are presented. A two-wave mixing configuration with nanosecond pulses at 532 nm wavelength was employed in order to study the third order optical nonlinearities of the samples. Strong optical Kerr effects were observed by intense optical excitation with and without the contribution of nonlinear optical absorption. An electronic response together with thermal effects was identified as the dominant physical mechanism responsible for the resulting nonlinear refractive index. Using a vectorial nanosecond technique, we were able to control the modification of the optical phase and the self-focusing effect associated with one of the interacting beams in propagation through the samples. This control of the optical phase and the deflection of the signal beam can be achieved by the separate excitation of the nonlinear refracting phenomena exhibited by the samples with large sensibility to the polarization of the optical pumping in a multi-wave mixing experiment. Our results indicated that the nonlinear refractive index and the optical beam direction can be simultaneously tailored by an electronic nonlinear refractive index, and by a thermal effect, both resulting from optical irradiation excitation. The different participation of the Kerr nonlinearity related to the samples seems to promise potential application for developing laser-induced controlled ultrafast devices.
C. Torres-Torres acknowledges the financial support from the Instituto Politécnico Nacional

Without an efficient method to prepare thin layer hexagonal boron nitride (h-BN), research of h-BN devices and applications have been limited compared to its sister nanomaterial, graphene. This is unfortunate because h-BN is a promising substrate for graphene due to its flatness and analog hexagonal structure. We investigated a systematic method to select and transfer a thin h-BN flake from one substrate onto another using a polyester polymer. This flexible method not only allows researchers to pick particular flakes and thicknesses of h-BN from exfoliation, but also reduces surrounding tape residue and flakes. Optical microscopy and atomic force microscopy confirmed 2-30nm thick h-BN flakes after tape method exfoliation. The polymer transfer process results in a thin coating with 20-30nm particles over the target flake. Annealing the sample in hydrogen significantly reduces polymer residue. Overall, this new process has the potential of adding user control to the preparation of cleaner thin layered h-BN for use in devices.

Growth mechanisms of 2D nanostructures (eg: graphene, h-BN) have so far been described by rather simplistic models of elemental solubility of the constituent elements (eg: C, B etc.) in metallic catalysts eg: Ni (high solubility - precipitation from bulk) and Cu (low solubility - surface reaction). These models are speculated based on ex-situ experiments but in-situ experimental evidence remains elusive.[1]
Using a combination of environmental scanning electron microscopy (ESEM)[2], high-pressure depth and time resolved in-situ X-ray photoelectron spectroscopy (XPS)[2,3] and in-situ X-ray diffraction (XRD)[2,3] at realistic CVD conditions, i.e. pressure (~0.001 - 0.5 mbar) and extreme temperatures (800-1000oC) we study the behaviour of polycrystalline transition metal catalyst films during 2D nanostructure growth. These experiments allow us to study the elemental incorporation in the 2D nanostructure on the catalyst surface as it happens by identifying the state of the catalyst and the elemental species at any point of time during CVD. The growth of 2D nanostructures is actually found to be isothermal along with some precipitation for both Ni[3] and Cu[2], i.e. we observe catalyst behaviour during growth as a complex interplay of isothermal and precipitation based growth mechanisms with kinetic effects playing an important role.
We highlight the use of this approach by proposing a generic framework to study the growth of 2D nanostructures during CVD which allows for the development of a comprehensive understanding of the fundamental growth mechanisms based on direct in-situ evidence avoiding the need for any speculation.
1. Kidambi et al. J. Phys.Chem. C. 116, 42, 22492-22501 (2012).
2. Kidambi et al. (in preparation)
3. Weatherup et al. Nano Letters. 11, 4154 (2011).

Recently, hexagonal boron nitride (h-BN) has attracted increased interest as a dielectric material to graphene electronics. Traditional dielectrics, such as SiO2 or various high-k materials, can introduce scattering from charged surface states, impurities, surface optical phonons, and substrate roughness; significantly degrading the transport properties of graphene. [1, 2] Hexagonal boron nitride boasts several key advantages over SiO2 and high-k dielectrics. Most notably, is an atomically smooth surface that is expected to be free of dangling bonds, leading to an interface that is relatively free of surface charge traps and adsorbed impurities. Additionally, h-BN&’s high energy surface optical phonon modes lead to reduced phonon scattering from the dielectric. Using h-BN (grown via CVD on copper foil) as a gate dielectric to epitaxial graphene devices, we report a >2.5x increase in intrinsic current gain cut-off frequency and a >3x increase in mobility over HfO2 gated devices.[3] However, small domain sizes and surface defects resulting from the h-BN CVD growth process result in a rough film morphology (RMS roughness > 1nm) and significant leakage currents (VBreakdown < 1V) for films <10nm thick.
Here, we present a detailed investigation of the CVD growth mechanisms of h-BN on various transition metals and show that the choice of substrate has a significant effect on the domain size and layer thickness of the h-BN films as determined via TEM. On Cu substrates, we find that after formation of a single monolayer, growth proceeds via nucleation of nano-domains over the original monolayer. This is due to the poor surface reactivity of borazine on h-BN compared to the Cu surface. We show that tailoring of the borazine precursor concentration (sublimed from ammonia borane) and use of a copper overpressure results in an increased domain size and a reduction of 3D islanding for h-BN on Cu substrates. Conductive AFM verifies a reduction of leakage current with increasing h-BN domain size, indicating that control of the domain size is required for scalable 2D h-BN dielectrics. Finally, we discuss the incorporation of these same h-BN films into nanomechanical structures. The vibrational response of nanomechanical resonators reveals the level of internal stress, as well as variations in mechanical properties, of the films as a function of growth conditions.
References:
[1] Hollander M. J. et al. Enhanced Transport and Transistor Performance with Oxide Seeded High-k Gate Dielectrics on Wafer-Scale Epitaxial Graphene. Nano Lett. 2011, 11, 3601-3607.
[2] Robinson, J. A. et al. Epitaxial Graphene Materials Integration: Effects of Dielectric Overlayers on Structural and Electronic Properties. ACS Nano 2010, 4, 2667-2672.
[3] Bresnehan, M. S. et al. Integration of Hexagonal Boron Nitride with Quasi-freestanding Epitaxial Graphene: Toward Wafer-Scale, High-Performance Devices. ACS Nano. 2012, 6, 5234-5241.

Hexagonal boron nitride (h-BN) is known to have a large optical band-gap and is considered an insulating material. Here, we find that altering pristine h-BN platelets via acoustic irradiation in deionized water changes the material from a white, near-UV bandgap electrical insulator, to a dark, electrically conducting material. A drastic optical band-gap reduction from 5.46eV to 3.97eV and a three order increase in conductivity is observed. We will present on the experimental study of the inversion in the material properties of h-BN, and elucidates the underlying mechanisms that result in fragmentation and partial oxidization of h-BN. This provides a facile avenue for the realization of two-dimensional advanced nanomaterials with tuned physical and chemical properties that depart from the intrinsic behavior of pristine h-BN. We employ ToF-SIMS, SEM-EDX, UV-Vis, Raman, TEM, XPS and XRD to examine the acoustically irradiated h-BN.

We have exfoliated hexagonal Boron Nitride (BN) by sonication in solutions of polyvinylalcohol (PVA) in water. The resultant nanosheets are sterically stabilised by adsorbed polymer chains. Such dispersions can be used to produce PVA-BN composite films. We find both the modulus, Y, and strength, σB, of these composites to increase linearly with volume fraction, Vf, up to Vf ~ 0.1vol% BN before falling off. The rates of increase are extremely high - dY/dVf = 670 GPa and dσB/dVf = 47 GPa. The former value matches shear lag theory modelling while the latter value is consistent with remarkably high polymer-filler interfacial strength. However, because the mechanical properties increase over such a narrow volume fraction range, the maximum values of both modulus and strength are only ~40% higher than the PVA itself. This phenomenon has also been observed for graphene-filled composites and represents a serious challenge in the production of high performance polymer-nanosheet composites.

Providing a simple route towards achieving high quality optical and nano-mechanical material systems would be beneficial for many scientific and industrial research areas. One way to accomplish this is through the manipulation of 2-D material systems (Ref 1). Here we present the first process-property relationship study on the heterogeneous integration of hexagonal boron nitride (h-BN) and graphene oxide (GO), referred to as BoroCarbon OxyNitride (BCON).
Our group has developed a fundamental understanding of the chemical exfoliation process required for the heterogeneous integration of graphene oxide and hBN. We have found that forming a heterogeneous suspension between GO and h-BN is primarily dependent upon the pH of the GO material. Fourier transform infrared spectroscopic (FTIR) data indicated the degradation in the B-N-B bending of the daughter BCON solution as the pH of the parent GO solution decreased. This represents a reduction in particle isolation which results in agglomeration of loosely bonded h-BN sheets in the GO suspension. Optimizing this suspension near a pH of 5 provides a means for forming very stable and uniform GO suspensions. Subsequently this suspension was mixed, sonicated and dried to form a paper material. X-ray photoelectron spectroscopy (XPS) shows an organic amide functional group that is formed during this synthesis process which could be the result of localized heating from the sonication process. These changes in chemistry, along with variations in GO:hBN concentrations, are expected to impact the electronic bandgap (Ref 1). We will present ellipsometry and UV-Vis measurements correlating optical and material properties of BCON.
Imaging techniques, such as scanning electron microscopy (SEM), have shown that the h-BN particles are intercalated between the agglomerated sheets of GO. The intercalation of these particles warp the GO structure and induces high stress points at these sites. When the BCON material is hBN rich (> 50%), it easily tears into micro-ribbon formations (~30um). Electrical measurements have shown that the impedance decreases from 100Omega; to 14Omega; as the material is compressed in-plane, indicating cross-linking between GO nano-platelets in the BCON structure, which is recoverable upon a return to the non-compressed state. We will discuss a quantitative correlation between the compressive force and electrical properties. Additionally, we will show impedance values of BCON as a function of hBN percentage and temperature.
(Ref 1) Gao, Guanhui, Wei Gao, E. Cannuccia, Jaime Taha-Tijerina, Luis Balicas, Akshay Mathkar, T. N. Narayanan, et al. “Artificially Stacked Atomic Layers: Toward New Van Der Waals Solids.” Nano Letters 12, no. 7 (2012): 3518-3525.

Hexagonal boron nitride (h-BN) has received a great deal of attention as a substrate material for high-performance graphene electronics because it has an atomically smooth surface, lattice constant similar to that of graphene, large optical phonon modes, and a large electrical band gap. Herein, we report the largescale synthesis of high-quality h-BN nanosheets in a chemical vapor deposition (CVD) process by controlling the surface morphologies of the copper (Cu) catalysts. It was found that morphology control of the Cu foil is much critical for the formation of the pure h-BN nanosheets as well as the improvement of their crystallinity. For the first time, we demonstrate the performance enhancement of CVDbased graphene devices with large-scale h-BN nanosheets. The mobility of the graphene device on the h-BN nanosheets was increased 3 times compared to that without the h-BN nanosheets. The onminus;off ratio of the drain current is 2 times higher than that of the graphene device without h-BN. This work suggests that high-quality h-BN nanosheets based on CVD are very promising for high-performance large-area graphene electronics.

The elastic properties of edges are among the most fundamental properties of finite two-dimensional (2D) crystals. Using a combination of the first-principles density functional theory calculations and a continuum elasticity model, we present an efficient technique to determine the edge stresses of non-stoichiometric orientations in multicomponent 2D crystals. Using BN and MoS2 as prototypical examples of 2D binary monolayers with threefold in-plane symmetry, we unambiguously compute unique edge stresses of commonly observed non-stoichiometric edges. Our results show that the edge stresses for these structurally distinct orientations can differ significantly from the average values that have been typically reported to date.
[1] J. Deng, I. Fampiou, J. Z. Liu, A. Ramasubramaniam and N. V. Medhekar, Applied Physics Letters 100, 251906 (2012).

We present an ab initio approach to the electron dynamics through the calculation of the
total polarizability matrix, including the off-diagonal elements. The charge density
induced in a system by an external perturbation is computed in real space and time,
following the idea of Abbamonte et al. [1]. The difference between our approach and the
one from Ref. [1] is that we can calculate not only the diagonal response chi;(q,q,omega;), but
also the off-diagonal elements of the matrix chi;(q,q',omega;). Hence, we have access to the
microscopic charge oscillations which are induced by the local-field effects.
We have studied these charge oscillations at various frequencies comprising interband-
transition and plasmon-excitation energies. The real-space approach allows us to see
which electrons (or orbitals) contribute to which kind of excitation. The final goal is to
offer theoretical support and benchmark to future inelastic x-ray scattering experiments
that may measure also the off-diagonal elements of the polarizability.
The method is applied to hexagonal boron nitride (h-BN) which is the most stable of the
three existing structures (hexagonal, cubic, and wurtzite) at room temperature and
ambient pressure. Because of its thermal stability it is a widely used material in vacuum
technology. The interest in h-BN has been renewed by the possibility of preparing boron-
nitride nanotubes that are far more resistant to oxidation than carbon nanotubes and,
therefore, suitable for high-temperature applications [2].
Adela Nicolaev acknowledges support from the ESF through the project POSDRU 107/1.5/S/80765.
[1] P. Abbamonte et al., Phys. Rev. Lett. 92, 237401 (2004)
[2] B. Arnaud et al., Phys. Rev. Lett. 96, 026402 (2006)

Graphene's success has shown that it is not only possible to create stable, single-atom thick sheets from a crystalline solid, but that these materials have fundamentally different properties than the parent material. We have created two-dimensional single atom thick hydrogen-terminated germanium (GeH) analogues of graphane (CH). A combination of absorbance measurements and high-level theory simulations show that the 0.67 eV indirect band gap of bulk germanium is converted into a 1.55 eV direct gap in GeH. These GeH sheets are resilient towards oxidation in air for at least 60 days based on fourier transform infrared spectroscopy (FTIR) measurements. These sheets can be mechanically exfoliated as single layers onto SiO2/Si surfaces. This material represents a new class of covalently-terminated single atom thick graphane analogues and has great potential for a wide range of optoelectronic applications.

Ab initio calculations are performed in the framework of density functional theory on Mn-doped boron nitride sheets, which are candidates for two-dimensional diluted magnetic semiconductors (DMSs). Each type of substitution reveals a qualitatively different magnetic behavior encompassing ferromagnetic, anti-ferromagnetic and spin glass ordering. The ability of formation of these defects is also discussed. We analyze the dependence of the exchange couplings on the distance between impurities and the typical range and distribution are extracted. Multiple-impurity configurations are considered and the results are mapped on an Ising-type Hamiltonian with higher order exchange interactions, revealing deviations from the standard two-spin models. The percolation of interacting magnetic moments is discussed and the critical concentration is determined for the underlying transition from a ferromagnetic to a super-paramagnetic state. We conclude our study by providing the optimal conditions for doping in order to obtain a ferromagnetic DMS. In addition we investigate the changes in the magnetic behavior in the context of finite width BN nanoribbons.
These types of structures can constitute the building blocks of future spintronic devices.
References: T.L. Mitran, Adela Nicolaev, G.A. Nemnes, L. Ion and S. Antohe, J. Phys.: Condens. Matter 24, 326003 (2012)

The development of small electronic components is fundamental in our highly technology dependent society. Currently, the electronic industry is rapidly approaching the limit of silicon-based complementary metal-oxide-semiconductor (CMOS) technology. As a consequence, the development of new technology to replace silicon has rapidly become a hot topic not only in the academic community but also in industry. This new Holy Grail of electronic materials has to perform better than silicon at smaller scales (>10 nm) and if possible add new functionalities for electronic devices such as flexible electronics. In this direction, Prof. Kis has recently showed that single layers of MoS2 exhibit electrical properties and notably field-effect transistor characteristics that are comparable to silicon thin films with the development of the first MoS2-based transistor.
We have studied by means of periodic density functional calculations a wide variety of transition metal-based single-layer materials (including transition metal dichalcogenides). The band structures of the studied materials allowed us to identify the ones that could be used in the development of new technologically oriented materials. Additionally, we investigated doping effects on the electronic structure of these materials.

Transition metal chalcogenides are layered materials, similar to graphite. Their layers weakly interact via van der Waals forces and they have been used in hydrogenation catalysis and as high temperature solid lubricants. Inspired in recent experiments on the synthesis and photoluminescence enhancement of single-layer WS2 sheets and triangular islands [1], in the present work, first-principles density functional theory calculations are carried out on different WS2 nanostructures. In addition, we have studied WS2 clusters with different 2-D morphologies (triangles, hexagons, stars, etc), nanoribbons with zigzag and armchair edges, as well as single- and few-layered WS2. The electronic density of states, scanning tunneling microscopy simulations, structural and magnetic ordering stability, and edge chirality are studied. Bethe-Salpeter equation for the electron-hole two particle Green function has been solved in order to calculate the in-plane polarized optical spectrum and exciton wave functions. In addition, the role of spin-orbit coupling on the electronic properties of single layer WS2 is discussed.
[1] H. R. Gutierrez, N. Perea-Lopez, A. L. Elias, A. Berkdemir, B. Wang, R. Lv, F. Lopez-Urias, V. H. Crespi, H. Terrones, M. Terrones. Extraordinary room-temperature photoluminescence in WS2 monolayers, Submitted to Nano Lett. (2012).

Individual monolayers of metal dichalcogenides are atomically thin two-dimensional crystals with attractive physical properties different from their bulk layered counterpart. Here we describe the direct synthesis of WS2 monolayers with triangular morphologies and strong room-temperature photoluminescence (PL). The Raman response as well as the luminescence as a function of the number of S-W-S layers is also reported. The PL becomes weaker with the increase of S-W-S layers number due to a transition from direct (in a monolayer) to indirect band gap (in multilayers). The edges of WS2 monolayers exhibit PL signals with extraordinary intensity, around 25 times stronger than the platelets center. The structure and composition of the platelet edges appear to be critical for the PL enhancement effect. Electron diffraction revealed that platelets present zigzag edges, while first-principles calculations indicate that sulfur-rich zigzag WS2 edges possess metallic edge states, which might tailor the optical response reported here. These novel 2D nanoscale light sources could find diverse applications including the fabrication of flexible/transparent/low-energy optoelectronic devices.

The synthesis of monolayer WS2 islands has been recently reported. These highly crystalline 2-dimensional islands sometimes exhibit cracks when quenched during synthesis. In this work, we have performed theoretical modeling in order to study and explain those cracked patterns. Our theoretical analysis shows that the unexpected anisotropic crack behavior in 2D WS2 with isotropic elastic properties is attributed to the monotonic dependence of edge energy on edge orientation with respect to the lattice. Monolayer WS2 islands show various crack patterns, depending on different S compositions along the edges of two flakes, with crack energy for armchair (AC) direction about 40% higher than that for the zigzag (ZZ) direction. The islands and the cracks were carefully characterized by SEM, HRTEM, ED and AFM techniques. These experimental results are in agreement with our theoretical calculations.

New applications using two-dimensional (2D) sheets obtained from bulk layered materials are very promising as evidenced from graphene. Boron nitride nanosheets (BNNSs) are one example. These nanosheets have an identical crystal structure and similar lattice parameter to those of graphene sheets. However, growing quality BNNSs consisting of only several atomic layers remains a challenge.
In this presentation, we report on the catalyst/solvent free, low temperature (300 oC) synthesis of large scale, high-purity few atomic layers BNNSs onto different metal substrates using CO2-Pulsed laser deposition technique. The obtained BNNSs appear highly flat and transparent. The thickness of a signal sheet is recorded down to 5nm and diameter is even more than 200nm, from which no defects and impurities are detected based on high-resolution TEM measurements. Though we operated the TEM at 200keV, we mitigated the appearance of defects due to knock-on damage by examining the materials as quick as possible with reduced illumination. After functionalizing/doping with carbon, BNNSs-based metal/semiconductor/metal junction diode and sensing device were fabricated. Cycling test of sensing behavior of BNNSs-based gas sensor toward methane (CH4) diluted with dry air at 150 oC indicates that the response time is less than 1s. The signal out of sensor became large following an increase of methane concentration. Repetition is also excellent. In contrast, poor response to oxygen was observed.
In the case of BNNSs-based Schottky diode, current-voltage (IV) characteristics at different temperatures 25 oC, 50 oC and 75 oC indicates that slight doping carbon into BNNSs would bring a significant change in its characteristics. A rapid increase in the forward current (35µA) and breakdown voltage lower than -70V is achieved, indicating the characteristics of BNNSs as PN-junction diode.

The great variety of possible carbon-based structures can be attributed to the multiple and different carbon chemical hybridizations. A similar versatility can be attributed to another tetravalent element, the silicon, which has been proven to be able to form many different structures. Nevertheless, one reason for the great importance of silicon today resides in the fact that our present electronics is silicon-based.
Amidst the emergence of graphene as one of the most promising materials for technological applications, many have hypothesized the substitution of silicon by carbon in future electronic devices. However, the fact remains that our existing processing plants are designed towards dealing with silicon. Besides that there is the zero bandgap challenge of graphene, which poses serious limitations to its use in some kind of transistors. In this way, there is a renewed interest in novel possible silicon structures that could prove to be the valuable for future electronics. Silicene could be one of these structures.
Silicene, a two-dimensional planar honeycomb silicon sheet, analogous to graphene, was first theoretically predicted [1] via ab initio calculations and experimentally realized recently by different groups [2-5]. It has been demonstrated that silicon can indeed form a honeycomb sheet, altough not totally planar as graphene, but exhibiting some level of buckling. Silicene has very interesting electronic and magnetic properties, making it a very promising material for electronic and spintronics applications.
In this work we have investigated, using reactive classical molecular dynamics (ReaxFF [6]) simulations, the mechanical properties and fracture dynamics of silicene membranes under strain. We calculated their Young modulus values along different directions. Our results show that the strained membranes exhibit two different regimes, one at the beginning, in which there is some structural buckling present, and when the strain is increased, a second regime appears in which the structures become planar, until before being fractured. The dynamics of the whole fracturing processes of silicene membranes was also analyzed, revealing unique characteristics, quite distinct from the ones observed for the case of graphenes.
[1] K. Takeda and K. Shiraishi Phys. Rev. B 50 20 14916 (1994).
[2] B. Lalmi et al. Appl. Phys. Lett. 97 223109 (2010).
[3] B. Aufray et al. Appl. Phys. Lett. 96 183102 (2010).
[4] P. D. Padova et al. Appl. Phys. Lett. 96 261905 (2010).
[5] P. Vogt et al. Phys. Rev. Lett. 108 155501 (2012).
[6] A. C. T. van Duin , S. Dasgupta , F. Lorant , and W. A. Goddard III, J. Phys. Chem. A v105, 9396 (2001).

Graphene has outstanding electrical properties, flexibility, large surface area and high chemical tolerance. It has also motivated a renewal of interest in two-dimensional (2-D) nanomaterials, including analogues made of heteroelements. Exfoliated layered boron nitride (hexagonal h-BN) and boron carbon nitrides (BCN) are the flagships of such hetero compounds, with high chemical stability, much higher oxidation resistance than graphene, high thermal conductivity, excellent mechanical properties, and above all, a finite electronic band gap which clearly distinguishes these materials from graphene.
Recent works on graphene-based nanocomposites showed their exciting potential for energy storage and catalysis, which was attributed to the large surface area, chemical stability, and high electrical conductivity of graphene. Few-layers BCN nanosheets, as analogues of graphene, might exhibit complementary properties, especially because of the polarity of B-N bonds. In particular, they could show interesting properties as environmentally friendly, metal-free electrodes in lithium-ion batteries. Up to now, little research has been conducted on the electrochemical behaviour of BCN nanostructures,14 and to our knowledge, no study has reported on the Li insertion properties of nanostructured BCN.
In this work, we have used a simple salt melt approach for the synthesis of few-layers BCN nanosheets. Most of the BCN sheets are composed of 2-6 atomic layers with a large surface area. They show promising storage performance in lithium batteries which is clearly superior to bulk BCNs. Moreover, the nanostructured material exhibits high-rate performance and stable capacity (~100 mAh gminus;1) at 2 A g-1 for 5000 cycles. This work opens the door to the design of innovative light elements anode materials, which could meet the needs of the next generation of rechargeable batteries for light-weight and high power applications.

Recently, high-quality exfoliated multilayer graphene devices on single-crystal hexagonal boron nitride (h-BN) substrates have been fabricated [1-4]. In this work, we have performed first-principles calculations based on density functional theory and planewave pseudopotential method to investigate the atomic and electronic structure of monolayer, bilayer, and trilayer graphene on a monolayer h-BN (MLBN). We found that the energetically most stable configuration for monolayer graphene on MLBN substrate is with one carbon atom to lie on top of a boron atom and a neighbouring carbon atom to lie at the centre of the h-BN. We have used this configuration for multilayer graphene on MLBN systems. The graphene sheet is weakly adsorbed on the h-BN substrate with a binding energy of 40 meV per C atom. We show that the electronic band gap is tunable for different multilayer graphene on h-BN substrate. The origin of the gap tuning this gap is a charge transfer from the h-BN to the graphene which leads to the development of a dipole moment across graphene. The resulting dipole moment generates an internal dipole potential across graphene. We calculate LDA band gaps of 57 meV, 287 meV, and 41 meV at the K point for monolayer, bilayer, and trilayer graphene on MLBN, respectively. The dispersion of the lowest unoccupied band near the K point changes from linear to a mixture of linear, quadratic, and cubic for multilayer graphene on MLBN.
[1] D. Usachov et al., Phys. Rev. B 82, 075415 (2010).
[2] J. Xue et al., Nature Mater. 10, 282 (2011).
[3] J. H. Warner et al., ACS Nano 4, 1299 (2010).
[4] C. R. Dean et al., Nature Nanotechnol. 5, 722 (2010).

Hexagonal boron nitride (hBN), also known as inorganic graphite, is the structural analogue of graphite, presenting the same morphology of pilled up honeycomb sheets. Even the interlayer and bond distances are almost identical, the only important structural difference being the stacking order. As a consequence, almost all carbon nanostructures derived from graphite have been found to have a BN analogous. Despite these similarities, the lower symmetry of hBN significantly changes its electronic properties, in a way that while graphene sheets are zero gap semiconductors, hBN sheets are wide bandgap insulators. This is also reflected on the BN nanostructures.
In an effort to produce graphene-based nano electronic devices, many approaches to create a bandgap in graphene have been tried. One of the most interesting is the conversion from graphene to graphane (hydrogenated graphene), via plasma hydrogenation of pristine graphene membranes [1, 2]. It has been observed [2] the formation of correlated hydrogenated domains leading to the structural shrinkage of the graphene membranes.
A natural question is what is dynamics and consequences of the hydrogenation of the inorganic graphene, i. e., the hydrogenation of boron nitride membranes leading to the formation of the equivalent inorganic graphane.
We simulated the complete process of the hydrogenation of BN membranes using reactive classical molecular dynamics simulations (ReaxFF) [3]). The simulations were carried out exposing both sides of the BN membranes to atomic hydrogen atmospheres. Our results show that the rate of hydrogen bonded to the membrane atoms is highly dependent on the temperature and that only at low temperatures there is a preferential hydrogen bonding to boron or nitrogen sites. Unlike graphane, hydrogenated BN membranes do not exhibit the formation of correlated domains and become increasingly defective proportional to the number of incorporated hydrogen atoms. After a critical incorporation rate the membranes collapse and completely lost their 2D-like topologies. The diffusion of hydrogen atoms on the sheet surface, as well as, their radial pair distribution and the second-nearest neighbor correlations were also analyzed.
[1] D. C. Elias et al., Science v323, 610 (2009).
[2] M.Z.S. Flores et al., Nanotechnology v20, 465704 (2009).
[3] A. C. T. van Duin , S. Dasgupta , F. Lorant , and W. A. Goddard III, J. Phys. Chem. A v105, 9396 (2001).

Since the first isolation of single-layer graphene in 2004, this novel 2D material has continued to receive intense interest. Recently, attention has shifted to an expanded set of exfoliatable materials such as boron nitride and the transition metal dichalcogenides. Boron nitride has been identified as an ideal substrate on which to build graphene devices, improving mobilities close to that of suspended structures. The transition metal dichalcogenides are wide band-gap 2D semiconductors, whose applications are just now being explored. New layered heterostructures, fabricated by alternately stacking different sets of these materials, offers unlimited possibilities for exploring new device functionalities. The fabrication of multi-layered structures, however, has been problematic owing to two issues: i) As devices are built up in a layer-by-layer approach, polymer residues accumulate at interfaces resulting in bubbles, wrinkles and poorly contacted interfaces. ii) metal contact to buried layers requires a multi-step fabrication process. In this talk I describe recent advancements we have made to address both of these issues. First I will describe a new polymer-free transfer process that allows unlimited stacking of multi-layer device structures. Using this technique we report BN-encapsulated graphene devices of unprecedented quality. We also demonstration an extension of this technique to realize high mobility MoS2. Second, I will describe a novel method to electrically contact fully encapsulated graphene devices. The contact resistance is comparable to the lowest value measured among the conventional metal-graphene contact in literature. Our techniques open up the possibility for pursuing new directions in 2D materials.

The existence of silicene, the graphene-like form of silicon, has been recently demonstrated by combining theoretical modeling and experimental evidences. [1,2] Due to the honeycomb lattice, silicene exhibits an electronic dispersion resembling that of relativistic Dirac fermions similarly to graphene. [2] Unlike graphene, silicene arranges in stable buckled configurations due to the large ionic radius of silicon atoms. The vertical distortion of the honeycomb lattice is expected to alter many physical properties of silicene which, consequently, might exhibit relevant advantages over graphene such as a band gap opening and its tunability.
The present work is intended to rationalize basic concepts of the epitaxial silicene on Ag(111) by means of a systematic in situ scanning tunneling microscopy-spectroscopy investigation. A position dependent density of states is reported [3] and interpreted as a buckling related electronic band structure which - similarly to the case of graphene [4] - can derive from the symmetry breaking in the silicene honeycomb lattice. In a more applicative view, silicene encapsulation is here successfully shown in order to prevent easy oxidation and to access ex situ characterization. New hints at the silicene growth on non-metal substrates with proper surface arrangement are also discussed aiming at device-oriented applications.
These outcomes disclose exceptionally novel issues in the physics of the emerging silicene and promote a renewed interest in nanoscaled silicon as active material for electronic devices.
[1] M. Houssa, G. Pourtois, V. V. Afanas&’ev, A. Stesmans, Appl. Phys. Lett. 97, 112106 (2010).
[2] P. Vogt, P. De Padova, C. Quaresima, J. Avila, E. Frantzeskakis, M. C. Asensio, A. Resta, B. Ealet, G. Le Lay, Phys. Rev. Lett. 108, 155501 (2012).
[3] D. Chiappe, C.Grazianetti, G. Tallarida, M. Fanciulli and A. Molle, Adv. Mater. 24, 37, 5088 (2012).
[4] S. Y. Zhou, G. H. Gweon, A. V. Fedorov, P. N. First, W. A. De Heer, D. H. Lee, F. Guinea, A. H. C. Neto, A. Lanzara, Nat. Mater. 6, 770, (2007).

One of the new focuses in nano science and technology is the search for graphene-like atomic layered materials, with recent efforts aiming at novel electronic properties and device functionalities beyond graphene. Among the various candidates, the layered transition metal dichalcogenides such as MoS2 have attracted great interest in terms of valleytronics and electric-field induced superconductivity. In this context electric-double-layer transistors (EDLTs) are powerful tools to tune and even create novel electronic states in these fascinating materials. The ability of EDLTs to access high sheet carrier densities (ns) at gated liquid/solid interfaces up to ~ 1015 cm-2, beyond the regime of chemical doping or solid dielectric field effect transistors, has enabled the realization of ionic liquid gated superconductivity in MoS2. However, the character of this superconducting state, as well as its origin - an electrostatic field effect or electrochemical intercalation - remain open questions.
Here we present comprehensive studies on the dimensionality of the superconductivity in ionic liquid gated EDLTs on single crystal MoS2 flakes. By controlling the external bias of the EDLT gate, we succeeded in observing superconductivity at low temperatures following an insulator-to-metal transition. The superconducting transition temperature Tc showed a maximum of Tc = 10 K centered around ns ~ 4 X 1014 cm-2, decreasing systematically as ns deviated from this optimal value. More importantly, the angular dependence of the superconducting upper critical field showed strong anisotropy, with a sharp peak when the magnetic field was applied parallel to the layers. These data could be well described the Ginzburg-Laudau theory for two dimensional (2D) superconductors, indicating that the field effect induced superconductivity in MoS2 has 2D character, not the anisotropic 3D character of the bulk. These findings not only help us to understand the 2D nature of the induced superconducting state in MoS2 bulk crystals, but also demonstrate a strong evidence for its electrostatic nature.

The development of 2D electronic semiconductors prepared from two-dimensional layered metal-chalcogenides has renewed the interest in the electronic structure of these materials in dependence of the number of layers, but in particular of those of single layer thiockness. We will present in this contribution a summary of our previous investigations on the electronic structure of layered chalcogenide epitaxial films. The determination of the electronic structure of single layers is often impeded by the interaction of the layers with the substrate material. In the case of metal chalcogenides, the interaction between layers is governed by weak van der Waals-like interactions. When using a substrate material with a dissimilar electronic structure and strong lattice mismatch, the electronic coupling between electronic states of film and substrate can be even weaker.
We have analyzed the electronic structure of metal-chalcogenides using energy dependent and angle-resolved ultraviolet photoelectron spectroscopy. We present results of three different layer/substrate combinations: (i) InSe on highly oriented pyrolytic graphite; (ii) WS2 on single crystalline graphite; (iii) GaSe half-sheets on Si(111). The films have been prepared by using the concept of van der Waals epitaxy in growing well defined layers of different thickness as analysed by STM. In all three cases, for single layers the electronic structure of the films parallel to the surface can well be described in terms of wave function symmetry and energy band dispersion of bulk band structures. The electronic structure of a single layer is hence not influenced by the substrate, indicating an electronic decoupling of the layers from the substrate. A clear transition is observed from single to double layer film thickness and to thicker bulk like films for the layered chalcogenides grown on graphite substrates. Of specific interest is the GaSe half-sheet which forms a chemically stable nearly perfect electronic passivation layer on the Si(111) surface, which provides a straight forward connection between single layer metal-chalcogenide electronics and Si technology.

In the example of graphene and other atomically-thin materials, two-dimensional (2D) electronic states form when the material becomes thin enough to confine the electron movement in plane. In contrast, certain materials known as topological insulators (TI) can accommodate 2D electronic states on the surface of bulk due to their unusual bulk property of strong spin-orbit effect. The exemplar TI material bismuth selenide (Bi₂Se₃), layered metal chalcogenide, possesses the 2D electronic states on the surface. The gapless surface bands, residing in the bulk insulating gap, have Dirac cone band structure similar to graphene, which has attracted great attention in science and applications. In this presentation, we report detailed transport studies on the topological surface states of Bi₂Se₃. Vapor-liquid-solid grown Bi₂Se₃ nanoribbons are protected with Se layer against environmental degradations and fabricated into nanodevices for electron transport at cryogenic temperature. With large magnetic field up to 9T, we observe clear magneto-oscillations known as Shubnikov-de Haas (SdH) oscillations from high mobility carriers in the nanoribbon device. Angle dependent measurements confirm that the high mobility electronic states are of 2D nature and careful analysis of Landau levels manifests the Berry&’s phase from Dirac band structure. Multi-frequency oscillatory features in SdH oscillations and gate-tunability of each surface band will be discussed as well.

Two dimensional crystalline layered materials such as MoS2, WS2, have recently become an intense focus of research activities due to their exceptional electronic and optical properties as well as their availability. A single- or a few atomic layers of these materials show quite promising charge conduction characteristics, such as large mobility or fast on/off switch ratios, which lead to a few recent examples of integrated circuits based on these materials. Here, we will present a comparison among the electronic transport properties of, either mechanically exfoliated or CVD grown MoS2 under different substrates, i.e. on SiO2, on exfoliated or on CVD grown h-BN, and suspended. We will also discuss results obtained from back and top gated configurations with different dielectrics.

After quantum dots, nanotubes and nanowires, two-dimensional materials in the shape of sheets with atomic-scale thickness represent the newest addition to the diverse family of nanoscale materials. Single-layer molybdenum disulphide (MoS₂), a direct-gap semiconductor is a typical example of new graphene-like materials that can be produced using the adhesive-tape based cleavage technique originally developed for graphene. The presence of a band gap in MoS₂ allowed us to fabricate transistors that can be turned off and operate with negligible leakage currents. Furthermore, our transistors can be used to build simple integrated circuits capable of performing logic operations and amplifying small signals.
I will report here on high-performance 2D MoS₂ transistors with increased currents and transconductance due to enhanced electrostatic control. Our devices also show current saturation for the first time in a 2D semiconductor. Electrical breakdown measurements of our devices show that MoS₂ can support very high current densities, exceeding the current carrying capacity of copper by a factor of fifty. Next, I will show optoelectronic devices based on MoS₂ that have a sensitivity surpassing that of similar graphene devices by several orders of magnitude. Finally, I will present temperature-dependent electrical transport and mobility measurements that show clear mobility enhancement due to the suppression of the influence of charge impurities with the deposition of an HfO₂ capping layer.

Transition-metal dichalcogenides (TMDs) are ones of the most famous layered materials. In particular, semiconducting TMDs are nowadays attracting great interests after the invention of so-called “Scotch-tape method” established in graphene research. Semiconducting TMDs are front-runners of “post graphene” since they endow finite band gap which is crucial for device applications. Recently high-performance monolayer TMD-FET with high-k gate dielectric is reported using channel material of MoS2, molybdenum disulfides [1]. MoS2 is the most common TMD because of the practical usage as solid lubricant. It has also been a target for material scientist because its alkali or alkaline-earth doped compounds shows superconductivity with highest TC around 7 K.
We fabricated MoS2 transistor adopting electric double layer (EDL) as gate dielectric. EDL is a nano-sized capacitor formed at the interface between solid and ions in electrolyte, providing more than one order in the magnitude higher capability for charge accumulation, thus enables field-effect phase transitions such as superconductivity, magnetism, and Mott insulator. So far, EDL has realized p-type conducting MoS2 in addition to well-known n-type conduction showing ambipolar operation [2]. In our study, field-effect superconducting transition of MoS2 was realized with maximum TC around 10 K. This TC is the highest not only within MoS2 compounds but also among whole TMDs. The highest TC discovered in this study lies in the carrier density region much smaller than chemically investigated region. Such compounds with small doping level have never been successfully synthesized by chemical method.
Furthermore, by combining HfO2 (typical high-k material for FETs) gating with EDL gating, continuous control of carrier density, and thus quantum phase, was demonstrated. As a result, we successfully obtained the phase diagram of MoS2 [3]. Interestingly, the TC exhibits strong carrier density dependence, showing dome-shaped superconducting phase which is a well-known feature of cuprates. Superconducting dome in other materials than cuprates has been reported only a few times in doped 2D semiconductors. Since FET charge accumulation is basically two dimensional, our result implies the existence of common mechanism for superconducting dome in 2D band insulators.
[1] B. Radisavljevic et al. Nat. Nanotechnol. 6, 147 (2011)
[2] Y. J. Zhang et al. Nano Lett. 12, 1136 (2012)
[3] J. T. Ye, Y. J. Zhang et al. Science in press

We report a label-free sensitive biosensor platform using few-layer molybdenum disulfide (MoS2) nanosheet field effect devices for cancer marker detection. To the best of our knowledge, this is the first report on the biofunctionalization of MoS2 nanosheet field effect device and the application of the sensor in liquid-phase. The detection of cancer markers at the earliest stage is of paramount importance to cancer diagnostics and treatments. Various nanowire, nanotube, and graphene -based biosensors in the configuration of field-effect transistors (FETs) have been extensively demonstrated. In contrast to graphene, other 2D nanomaterials, such as MoS2, have a direct energy bandgap which requires no further gap opening. It was reported by B. Radisavljevic et al. in 2011 that when single-layer MoS2 was employed as the active materials with HfO2 as the gate dielectric in FETs, a high current On/Off ratio exceeding 1x10^8 at room temperature can be achieved.
In our work, first, we fabricated few-layer MoS2 nanosheet FET with HfO2 gate oxide, and characterized its transport properties both in air via backgate and in liquid solution via Ag/AgCl wire in contact with the solution. We note that a high in-solution transconductance-to-noise ratio is important for achieving high sensitive biosensors. Second, to transform the as-fabricated field effect device into biologically active sensor, we proposed and implemented a silane-based covalent coupling method to functionalize HfO2-coated MoS2 surface with monoclonal antibodies targeting specific analytes. The covalent biofunctionalization scheme enables the reversible operation of sensors for multiple tests in flowing fluids and for longer sensor life-time. Lastly, we used the MoS2 biosensor for real-time, label-free, electrical detection of prostate-specific antigen (PSA), the biomarker identified for screening and diagnosis of prostate cancer, as the specific bindings of PSA molecules to the sensor surface gave rise to detectable drain current changes according to the field-effect mechanism. The test was performed in a microfluidic channel system where there was continuous flow of different sample solutions. The device showed a concentration-dependent change in its drain current in response to PSA solutions, and in addition to the sub-picomolar sensitivity, the sensor appeared to have good specificity by showing no significant signals to non-target serum protein. The demonstration of the fast and sensitive detection of PSA using MoS2 nanosheet field-effect devices with versatile HfO2-silane-based bio-functionalization scheme promises their wide application in detection of other disease markers and the potential for point-of-care diagnostics.

Two dimensional materials such as graphene and Molybdenum disulfide (MoS2) have drawn intense attention due to their ultimate scaling limit in the thickness. Compared to zero band gap of graphene, the wide band gap (~1.2-1.8eV) of MoS2 enables switching and logic device. Combined with the reduced short channel effect afforded by the efficient gate control of ultrathin channel material, it is suitable for low power consumption application. Recent experimental results have demonstrated that MoS2 transistor can achieve silicon-level mobility and higher on/off ratio (more than 10^7). In addition to these attractive characteristics, the atomic-scale thickness and high breaking strength of MoS2 make it promising for flexible electronics.
In this work, we have demonstrated the bendability of MoS2 thin film transistor on flexible substrate. We used polyimide as the flexible substrate, Ti/Pd as the bottom gate, and Al2O3 or HfO2 deposited by atomic layer deposition method as the gate dielectric. Commercial MoS2 crystal was exfoliated onto target substrate by the scotch-tape method. After flakes with thickness less than 15nm were carefully selected, source/drain patterns were defined by e-beam lithography, Ti/Au deposited by e-beam evaporation, and then followed by the lift off process. Raman and AFM were used to verify the dimension and the thickness of the MoS2 flakes.
Bending measurement were applied to examine the stability of each device parameters under strain condition, such as mobility, on current, and on/off ratio, and the curvature radius of this bending test was down to 0.7mm. Multi-cycles of the bending procedure were applied repeatedly to study the reliability issue. In-situ Raman measurement was used to study the strain effect of MoS2 flakes. The results indicate that MoS2 is suitable for high-performance mechanically flexible nanoelectronics.

Recent advances in monolayer and few layered MoS₂ have shown the potential use of layered semiconductors for high performance n-type field-effect transistors (n-FETs), which hold promises for future sub-5 nm gate length FETs since layered semiconductors exhibit advantageous surfaces with minimal roughness, dangling bonds, defect states and native oxides. Here, we report both high electron and hole mobility monolayer WSe₂ FETs with excellent I_ON/I_OFF, and more importantly routes for controllable doping of chalcogenide layered semiconductors at the source/drain (S/D) contacts for low parasitic resistances. Specifically, by developing a surface dopant-profiling technique, we demonstrate the first high mobility WSe₂ ML-FETs (body thickness of ~0.7 nm) with degenerately doped contacts. The use of heavily p- and n-doped contacts is essential in lowering the metal contact resistances to WSe₂ by orders of magnitude, and enabling the demonstration of p-FETs with peak effective mobility of ~250 cm²/Vs, near ideal subthreshold swing (SS) of ~60 mV/decade, high I_ON/I_OFF of >10⁶, and n-FETs with peak effective mobility of ~110 cm²/Vs. Along with the previously demonstrated MoS₂ single layer transistor, the results here encourage further investigation of layered semiconductors, especially the transition metal dichalcogenide family, for future high performance electronics. As emphasized in this work, surface doping is a necessity for obtaining high performance ML-FETs, and in this regard exploration of other dopant species for both n- and p-doping is needed in the future.
References:
1. Hui Fang, Steven Chuang, Ting-Chia Chang, Kuniharu Takei, Toshitake Takahashi and Ali Javey, Nano Letters, 12, 3788-3792, 2012.
2. Hui Fang et al., submitted.

Silicene recently attracts considerable attention, due to its exotic electronic structure and promising applications in Si-nanoelectronics. It has two honeycomb sublattices are displaced vertically with respect to each other, having large spin orbit coupling energy. An external electric field couple with spin orbit coupling induces a transition between a topological and a band insulator. In this talk, I will discuss a combined effect of external electric and magnetic filed on silicene. Transition metals adsorbed silicene exhibit a valley-electronic.

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 in the past seven years on chemistry and physics of 2D chalcogenide nanoribbons and nanoplates. First, 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. Second, 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.

The industrial development of the 21st century requires new smart and miniaturized devices, strong and light materials, materials with wide variety of new properties. Nanotechnology based on nanocompounds comes to address this need. Inorganic fullerene-like (IF) nanoparticles and inorganic nanotubes (INT) from MoS2 and WS2 were discovered in 1992 in the laboratory of Prof. R.Tenne, Weizmann Institute of Science. This study reports the synthesis of a pure phase of multiwall WS2 nanotubes in substantial amounts. Careful parameterization of the conditions within the reactor leads to the comprehensive understanding of the growth mechanism and facilitates the synthesis of large amounts (0.5 kg/week) of pure nanotubes with high crystalline order. The majority of the nanotubes range from 10 to 20 micron in length and 50-120 nm in diameter. Special efforts were committed to produce loosely agglomerated nanotubes that can be easily dispersed in organic solvents and different polymer matrices. The excellent mechanical properties of INT-WS2, nanosize, good dispersion and adhesion to the composite&’s matrix have favorable influence on its properties, reinforcing variety of polymers, and improving their thermal and mechanical properties. In view of semiconducting characteristics of INT-WS2 large number of applications could be anticipated for such nanotubes, especially in sensors, actuators, photodetectors, rechargeable batteries, etc. It was shown that WS2 nanotubes are prospective candidates for applications in high-sensitivity and high-speed nanoscale photodetectors for visible and near-infrared lights, and as humidity sensors. Recently, efficient resonant energy transfer from quantum dots (QD) to the INT-WS2 upon photoexitation was observed, providing evidence that this system holds potential for photovoltaic, luminescence tagging, and optoelectronics applications. Interestingly, the size-depending Resonance Raman profiles of WS2 nanotubes were also recently reported. It was shown that the A excitonic transition energy lies below the bulk value and is increasingly redshifted with decreasing diameter of the nanotubes. This observation indicates that the A excitonic transition energy could be used to probe the INT size- and shape-dependence being compared to the bulk material.

Polymorphism in single-layer transition metal disulfides (SL-TMD) MS ₂
(M = Mo or W), semiconducting 2H and metallic 1T phases, creates rich interfaces in this new flatland: either grain boundaries between the same phase or unique electronic heterostructures between different phases. Based on first-principles calculations, we explore the structures and properties of dislocations, grain boundaries (GB) and electronic heterostructrual interfaces. In sharp contrast to other two-dimensional (2D) materials (truly planar graphene and h-BN), here the edge dislocations extend in third dimension. They include homoelemental bonds and, by reacting with point defects, yield families of the derivative cores for each Burgers vector. The overall structures of GB are controlled by both local-chemical and far-field mechanical energies and display different combinations of dislocation cores. Two distinct electronic behaviors of GB are observed. The structures of electronic heterostructures depends not only on the interfacing angles but also the orientaions (either M- or S-oriented), brings in different migration patterns for different interfaces, which explains both the low activation barrier during transition and appreciable amounts of remnant 1T phase after thermal annealing.

Wide band gap 2-dimentional (2D) crystalline semiconductor materials emit or absorb light in the deep ultraviolet (UV) regions are promising for various applications particularly in medical treatment and environment protection. Hexagonal boron nitride nanosheets (BNNSs) are one example of such materials. However, growing quality BNNSs consisting of only few atomic layers remains a challenge.
In this presentation, we report on the synthesis of few atomic-layers BNNSs and study of its optical and electrical properties. The synthesis process is carried out by irradiating hexagonal boron nitride (h-BN) 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 were measured (5nm) using HRTEM by imaging the edges of nanosheets. Additionally, doping in BNNSs with C atoms can adjust the bandgap and make it feasible to prepare UV photodetectors with different cut-off wavelengths. BNNSs-based photoconductors and metal-semiconductor-metal photodiodes have been developed. UV lamps of three different wavelengths 254nm, 302nm and 365nm were used to put light on active region of photodiode. A rapid increased in the forward current (.25mA) has been recorded when light of 254nm wavelength was shined on the device indicating excellent UV detection property of BNNSs. The low temperature cathodoluminescence spectroscopy revealed several sharp excitonic peaks between 215nm and 260nm confirmed the strong excitonic spectral feature of BNNSs in the deep UV spectral range.

Graphene and other two-dimensional layers have emerged as suitable materials for the
development of a new generation of transistors, solar cells, and other optoelectronic devices.
An interesting example of those materials is MoS2, a direct semiconductor of 1.8 eV in the single-layer version that turns into an indirect semiconductor in the few-layer and bulk versions [1].
We present a theoretical study of the electronic structure of single-layer, double-layer
and bulk MoS2, in the framework of the GW method and the Bethe-Salpeter equation. We pay
special attention to the excitonic effects on the optical properties.
We relate changes in the excitonic properties (binding energy, absolute position)
to differences in the screening of the Coulomb interaction and to the interaction between layers [2]. Additionally, the implemented ab-initio methodology allows
us to determine the weights of the electron-electron and electron-hole interaction
in the photoluminescence and absorption spectra.
[1] Phys. Rev. Lett. 105, 136805 (2010) ; Nat. Nanotech. 6, 147 (2011).
[2] A. Molina-Sanchez and L. Wirtz, in preparation.

Highly deformable yet chemically stable atomically thin materials animate a wide range of novel structural, optical, and electronic phenomena. For example, the division of surrounding space into two disconnected zones by an impenetrable suspended sheet enables adsorption of otherwise highly co-reactive species in opposite subspaces, with an intense cross-sheet charge transfer that can generate a nanoscale Stark effect, strong non-adiabatic effects and broad band gap tuning. New physics also results when the same species is adsorbed to both sides, with unusual "which-side" symmetry breaking. Growth kinetics and edge structure provides other avenues for property tuning in both transition metal dichalcogenide and boron nitride systems. This talk will survey recent theoretical advances in these areas.

We report the temperature-dependent photoluminescence (PL) and Raman spectra of single-layer MoS2 as a function of various substrate environments. Mechanical exfoliation from bulk MoS2 is used to prepare single-layer flakes which are then transferred to either sapphire (with and without ALD HfO2 overcoating) or suspended over holes on a Si/Si3N4 substrate We measure the temperature dependence of PL and Raman spectra from 100K to 400K using HeNe 632.8 nm (PL) and Ar+-ion 514.5 nm (Raman) laser excitations coupled to a microscope and grating spectrometer. Photoluminescence shows a single, narrow peak corresponding to a direct-band transition approximately centered at 1.9 eV with a width of 50 meV. The PL peak redshifts and broadens with increasing temperature, which is the result of the thermal expansion induced bandgap narrowing for the MoS2 film. Based on the differences in the PL peak intensity and position between samples with and without HfO2 on top, it is proposed that the HfO2 adheres strongly and exerts tensile stress on the MoS2 flake. Raman spectra reveal two strong phonon vibrational modes, the planar E12g and out-of-plane A1g, both of which soften with increasing temperature. The HfO2 covered sample shows a red shift of A1g mode and a negligible change in E12g mode compared to the one without HfO2 on top in the temperature range explored. The HfO2 deposited using ALD method softens A1g phonons by inducing external charges on the thin MoS2 film. We extract a linear temperature coefficient not;for both Raman modes, in the range of (-0.011 to -0.015) cmminus;1/K, comparable to the G-mode of graphene. This approximately linear change of phonon frequency verses temperature is a result of anharmonic behavior of the lattice vibrations, which is determined by the anharmonic potential constants, the phonon occupation number and the thermal expansion of the crystal. A comparison of not; the dependence of Raman peak position on incident optical power for the suspended flake shows moderate heat flux efficiency for the single-layer MoS2. The impact of dielectric and substrate on the extraction of thermal conductivity and temperature dependent behavior of MoS2 will be discussed.

The naturally layered structure of 2D materials allows intercalation of foreign atoms, ions or molecules, which may dramatically change their physical and chemical properties. Here we present our discovery of extraordinary light transmission through nanoplates of bismuth chalcogenides achieved by intercalation of copper atoms, which is on the contrary to most transparent materials where doping reduces the light transmission. The intercalation of copper introduces enormous amount of free electron into the material, which is evidenced by the blue-shift of free carrier plasma frequency. In addition to great enhancement of electric conductivity, large electron density actually caused a substantial widening of the optical band gap. Therefore optical absorption of the material is dramatically reduced over a wide frequency range. The transmission is further improved by “zero-wave anti-reflection effect” due to the ultra-small thickness of the nanoplates. Such an extraordinary phenomenon is the synergy of nanophotonic effect and drastic material property tuning via intercalation, and can lead to real applications such as transparent electrodes.

By combining first-principles simulations with high-resolution transmission electron microscopy (HR-TEM) experiments, we study the evolution of atomically thin layers of transition metal dichalcogenides (TMDs) under electron irradiation. We show that vacancies produced by the electron beam agglomerate and form line structures, which can be used for engineering materials properties. We also study the radiation hardness of 2D TMD materials [1]. We further show that TMDs can be doped by filling the vacancies with impurity atoms. We also study the stability and electronic properties of single layers of mixed TMDs, such as MoS2x Se2(1minus;x), which can be referred to as 2D random alloys [2]. We demonstrate that 2D mixed ternary MoS2/MoSe2/MoTe2 compounds are thermodynamically stable at room temperature, so that such materials can be manufactured by CVD or exfoliation techniques. By applying the effective band theory approach we further study the electronic structure of the mixed ternary 2D TMD compounds and show that the direct gap in these material can continuously be tuned.
[1] H-P. Komsa et al. PRL 109 (2012) 035503;
[2] H-P. Komsa et al., submitted.

The Raman scattering of single- and few-layered WS2 is systematically studied as a function of the number of S-W-S layers and the excitation laser wavelength in the visible range (488, 514 and 647 nm). For the three excitation wavelengths used in this study, the frequency of the A1g(Γ) phonon mode monotonically decreases with the number of layers, while the E12g(Γ) frequency increases. For single-layer WS2, 514.5 nm excitation generates a second-order Raman resonance for the longitudinal acoustic mode at the M point. This 2LA(M) resonance results from a double-resonant Raman coupling between the electronic band structure and lattice vibrations, an effect not previously seen in any single-layered metal dichalcogenide. We performed ab initio calculations to determine the electronic and phonon band structures of single-layer and bulk WS2, these results were used to compute the reduced intensity of the 2LA mode from the fourth-order Fermi golden rule. The reduced intensity of the 2LA double resonant mode at different q-points was obtained as a function of the laser wavelength for both the bulk and the monolayer. The monolayer exhibits a pronounced resonant peak close to 514.7 nm. This mechanism may be more broadly applicable in characterizing the structural, electronic and vibrational properties of other layered systems. Our observations also establish an unambiguous and nondestructive Raman fingerprint for identifying single- and few-layered WS2 films.

We study the possibility of making ultra-thin (1-10 nm) solar cells using two-dimensional (2D) monolayer materials such as graphene, graphene-BN (CBN), MoS2, and other transition-metal oxides and dichalcogenides. Using a combination of first principles calculations (density functional theory and the GW-Bethe Salpeter approach) we demonstrate an extremely large sunlight absorbance for 2D monolayers and bilayers of the aforementioned materials. In some cases (e.g. MoS2 and MoSe2) the calculated absorbance of a single monolayer is as high as 10% of the infinite-thickness limit.
In addition, we demonstrate the possibility of forming type-II heterojunctions with highly tunable band offsets and power conversion efficiency (PCE) at interfaces between 2D monolayers, or between a monolayer and a common acceptor such as PCBM fullerene [1,2]. Such interfaces have the potential to achieve a significant PCE of up to 10-20% within ultra-thin active layers.
In closing, we show how monolayers of reduced graphene oxide (rGO) can be used as electrodes with a work function tunable in a window of 2.5 eV by changing the functional group composition. Our work guides the realization of ultra-thin solar cells made exclusively of 2D monolayer materials.
[1] M. Bernardi, M. Palummo, and J. C. Grossman, Phys. Rev. Lett. 108, 226805 (2012).
[2] M. Bernardi, M. Palummo, and J. C. Grossman, ACS Nano (2012). DOI: 10.1021/nn303815z. Published on-line.

Molybdenum disulfide has attracted intense interest as a long-awaited two-dimensional semiconducting analog of graphene. Bulk molybdenum disulfide is an indirect bandgap semiconductor that becomes a direct bandgap semiconductor when thinned down to one molecular layer, making it an extremely promising candidate for atomically thin electronic, optical and photovoltaic devices. To date, the majority of experiments have been on exfoliated material. These exfoliated samples are only a few microns in size, which precludes many scientific studies and all serious technological applications. Just as for graphene, growth by chemical vapor deposition has emerged as the practical route to large-area synthesis of MoS2. While several promising reports of monolayer films grown by solid-source CVD have recently been published[1], there has been no systematic characterization of the fundamental materials properties of these CVD-grown films.
Here, we combine transmission electron microscopy (TEM) characterization of atomic structure with optical spectroscopy and electrical transport to provide a comprehensive study of the grain structure of CVD-grown MoS2, and to determine the optoical and electronic properties of its grain boundaries. We achieve CVD growth of single crystals up to 120 µm across whose optical properties are superior to those of exfoliated samples. We use diffraction filtered TEM imaging to correlate crystal shape with grain boundary structure.
With the knowledge gained about the crystal grain structure, we identify and perform photoluminescence and electrical transport measurements on individual grain boundaries. We find that mirror grain boundaries result in strong quenching of the photoluminescence while tilt boundaries result in strong enhancement. Meanwhile, we built field effect transistors out of the MoS2 on single crystals, and with the grain boundary parallel and perpendicular to the flow of electrons. The single-crystal device mobility ranges from 3-8 cm2V-1s-1, depending on the growth. Meanwhile, grain boundaries perpendicular to the flow of electrons do not noticeably affect the resistance, while grain boundaries parallel to the flow will slightly increase (~25-60%) the conductivity.
[1] “Grains and grain boundaries in single layer graphene atomic patchwork quilts” P. Huang, C.S. Vargas, A.M van der Zande et al. Nature 469, 389 (2011)
[2] “Synthesis of large-area MoS2 atomic layers with chemical vapor deposition”, Y.-H. Lee, Z.-Q. Yang et al. Adv. Mat. 24 17, (2012)

In recent times, Bi2Se3 has received a lot of attention as a model Topological Insulator material. The strong spin-orbit coupling in Bi2Se3 is known to result in topologically protected co-existence of gapless metallic surface states and semiconducting bulk states. An important question of considerable fundamental interest is how the electronic properties of this material modifies under nanoscale confinements. In this work, we present optical absorption studies of single-crystal Bi2Se3 nano-flakes. Samples of high-quality single-crystalline Bi2Se3 were fabricated on Si and quartz substrates using a catalyst-free chemical vapor deposition technique. Uniformly flat, hexagonal or triangular nanoflakes of Bi2Se3 could be fabricated with growth-condition-dependent thicknesses and lateral sizes. Experiments were also performed on separately prepared samples mechanically exfoliated from bulk Bi2Se3. In both cases, photon energy-dependent optical absorbance (1.1 eV < E\_{photon} <6.5 eV) were measured for samples with different thickness-ranges, from bulk all the way down to a few quintuple layers (QLs). Bulk samples show strong optical absorbance at the lowest photon energies, in good agreement with our theoretical calculations. As the layer-thickness reduces, we find a dramatic change in their optical absorbance - with an optical gap of ~3 eV appearing in the thinnest samples (~few QLs). The results will be discussed in the framework of the evolution of its electronic structure and optical absorbance as the layer-thickness approaches a single QL limit.

Continuous large area films of monolayered MoS₂ synthesized by chemical vapor deposition (CVD) were used as light sensing devices. Current-Voltage (I-V) and photo response measurements were performed on the film samples electrically connected between two Au/Cr contacts. The photocurrent measurements were carried out at room temperature using discrete wavelengths corresponding to 405 nm (3.06 eV), 488 nm (2.54 eV), 514 nm (2.41 eV) and 635 nm (1.95 eV). Interestingly, the material response is proportional to the photon energy of the excitation laser, when the excitation energy is below the direct band gap, no photo response is observed. The MoS₂ samples were structurally characterized by Raman spectroscopy, atomic force microscopy, scanning electron microscopy and high-resolution transmission electron microscopy. Raman spectra confirms the presence of monolayers and UV-visible absorbance spectra revealed the resonance peaks at the energy matching the direct gap expected for single layers of MoS₂ (~1.85 eV ). Further experiments on time response and continuous spectral response are now underway and will be presented.

Molybdenum disulfide (MoS2), a member of the family of layered transition metal dichalcogenides, has long been used as lubricants. The interest in electronic application of MoS2 thin films has been kindled by recent demonstrations of high mobility transistors with MoS2 active layer [1,2]. In contrast to the most popular layered material graphene, MoS2 thin films have a large bandgap ranging 1.3-1.8 eV which makes them a promising candidate for electronic devices. One important aspect of layered materials in general is the fact that they are particularly amenable to heterostructure formation. This is because, due to their layered structure and therefore chemically inert nature, formation of interface states is expected to be suppressed [3]. Here we demonstrate an ultra sensitive metal-semiconductor-metal (MSM) photodetector made of MoS2 amorphous silicon (a-Si) heterojunction. This van-der-waals heterojunction yields a photoresponsivity of 233 mA/W, the largest reported for thin film a-Si devices. The ‘doping-free&’ heterojunction also increases the dynamic range of its counterparts, i.e. a-Si and MoS2 MSM devices, by 20 and 48 dB, respectively. The high photoresponsivity along with a low dark current of 66 fA/um render the device a promising photodetector for low-level light sensing such as bioanalytical and biomedical imaging.
[1] Radisavljevic, B., Radenovic, A., Brivio, J., Giacometti, V. & Kis, A. Single-layer MoS2
transistors. Nat. Nanotechnol. 6, 147-150 (2011).
[2] Lee, H. S. et al. MoS2 Nanosheet Phototransistors with Thickness-Modulated Optical Energy
Gap. Nano Lett. 12, 3695-3700 (2012).
[3] Yang, H. et al. Graphene Barristor, a Triode Device with a Gate-Controlled Schottky Barrier.
Science 336, 1140-1143 (2012).

Recently, 2D compounds besides graphene have attracted much attention due to their unique properties.[1-2] The advance in experimental synthesis[1] or theoretical predication[2] of 2D structures, such as Ge, Sn, BN, GaN, and transition metal dichalcogenides (TMDs) monolayers, have led us to explore the properties by chemical functionalizing them. In this talk, we briefly review our studies on the electronic and magnetic structures of these materials modulated by external effects, such as foreign element adsorption, applying strain, introducing point defects and interacting with graphene. For Ge and Sn monolayers under halogenation, the p and p* bands of remain crossed at the Fermi level - despite the crossing point shifting from K to G points;[3-4] moreover, appreciable gaps in halogenated Ge and Sn monolayers can be opened at Dirac-like points, several orders of magnitude larger than that in pure germanene due to the robust spin-orbit coupling. While for BN and GaN monolayers under half-fluorination, they exhibit intriguing magnetic transitions between ferromagnetism and antiferromagnetism by applying strain; and a new mechanism that can provide a concise physical understanding of this interesting property is proposed.[5] This mechanism also well holds for VX2 (X=S, Se) monolayers.[6] VX2 monolayers exhibit exciting ferromagnetic behavior, offering evidence of the existence of magnetic behavior in pristine 2D monolayers; furthermore, interestingly, both the magnetic moments and strength of magnetic coupling increase rapidly with increasing isotropic strain for VX2 monolayers. When introducing vacancy and nonmetal adsorption, MoSe2, MoTe2 and WS2 monolayers would exhibit intriguing magnetic properties.[7] On the other hand, when MoX2 adhered with graphene, it has little affect on the electronic properties of graphene, indicating that MoX2 monolayer emerges as a potentially suitable substrate material for graphene.[8,9] Engineering such variety properties in these 2D compounds could pave the way for a new generation of devices for electronics and spintronics.
References
[1] Jonathan N. Coleman, et al. Science 331, 568 (2011).
[2] H. ahin, et al. Physical Review B 80, 155453 (2009).
[3] Y. D. Ma, Ying Dai, et al. J. Mater. Chem. 22, 12587 (2012).
[4] Y. D. Ma, Ying Dai, et al. J. Phys. Chem. C 116, 12977 (2012).
[5] Y. D. Ma, Ying Dai, et al. Nanoscale, 3, 2301 (2011).
[6] Y. D. Ma, Ying Dai, et al. ACS Nano, 6, 1695 (2012).
[7] Y. D. Ma, Ying Dai, et al.Phys. Chem. Chem. Phys.13, 15546 (2011).
[8] Y. D. Ma, Ying Dai, et al. J. Phys. Chem. C 115, 20237 (2011).
[9] Y. D. Ma, Ying Dai, et al. Nanoscale 3, 3883 (2011).

Recently, the discovery of the allotropic phase of graphene-like silicon, namely silicene [1], opened new prospective for the integration of 2D materials in micro- and nano-electronic devices in analogy with the prototypical integration of graphene and MoS2 layers in RF transistor and logic gate, respectively.
Although huge efforts have been devoted to the morphological and electronic characterization of silicene by means of in-situ techniques such as Scanning Tunneling Microscopy and Angle Resolved Photoemission spectroscopy [1, 2], no evidence of the vibrational properties has been up to now reported for epitaxially grown silicene which can unambiguously consolidate the honeycomb lattice in analogy with the graphene counterpart.
Here we present the first Raman identification of silicene, by combining Raman spectroscopy and Scanning Tunneling Microscopy with ab-initio density functional theory calculations. We demonstrate how the Raman fingerprint of epitaxial silicene is strictly connected to the geometrical arrangement of the hexagonal silicene lattices, which in turn is determined by the silicene/substrate interaction. Remarkably, beyond the presence of the double degenerate E2g mode (analogous to the G peak for graphene), typical for honeycomb lattices, the non-uniform bond length and the non-symmetric buckling in epitaxial silicene induce broader features and intense D-like peaks respectively, even for defect-free domains. As a consequence we show how the Raman spectrum of highly distorted reconstructions is dominated by D-like peaks, which can be thus exploited to discriminate the different phases of the epitaxial silicene[1, 3].
Furthermore, in contrast to graphene, we show how the Raman spectrum of epitaxial silicene undergoes resonance effect by tuning the excitation energy, thus confirming the presence of a band gap in its electronic structure as measured by means of angle resolved photoemission spectroscopy experiments [2], hence opening new perspectives for future implementation of silicene-based nanodevices.
Furthermore, based on complementary in-situ X-Ray photoemission spectroscopy, we show that engineering an adequate unreactive encapsulation layer for silicene is mandatory to hinder easy oxidation in air and hence to perform any ex-situ characterization as well as functionalization.
[1]P. Vogt, P. De Padova, C. Quaresima, J. Avila, E. Frantzeskakis, M. C. Asensio, A. Resta, B. Ealet, G. Le Lay, Phys. Rev. Lett. 2012, 108, 155501.
[2]D. Chiappe, C.Grazianetti, G. Tallarida, M. Fanciulli and A. Molle, Adv. Mater.2012, 24, 37, 5088.
[3]B. Feng, Z. Ding , S. Meng, Y. Yao, X. He, P. Cheng, L. Chen, and K. Wu, Nano Lett., 2012, 12 (7), 3507.