Dattatray J. Late

MoS2 and WS2 analogues of graphene.

Following the discovery of fullerenes in 1985, it was soon recognized that inorganic layered materials such as MoS2 and WS2 can also form fullerene-like structures. [2] After the discovery of carbon nanotubes, inorganic nanotubes analogous to carbon nanotubes were prepared and characterized, nanotubes of MoS2 and WS2 being archetypal examples. [4] With the discovery and characterization of graphene, that is, two-dimensional nanocarbon, which has created great interest in last few years, it would seem natural to explore the synthesis of graphene analogues of layered inorganic materials such as dichalcogenides of molybdenum and tungsten. We aim to prepare graphene-like MoS2 and WS2, which are quasi-two-dimensional compounds in which the atoms within the layer are held together by strong covalent forces while van der Waals interaction enables stacking of the layers. Synthesis of crystals of MoS2 containing several molecular layers by micromechanical cleavage has been reported, and optical absorption and photoconductivity of these films have been studied. There is also a report on the intercalation of alkali metals with layered metal dichalcogenide crystals with controlled stoichiometry, but the products of exfoliation were not examined in this study. There is an early report on graphene-like MoS2 prepared by lithium intercalation and exfoliation, but the material was characterized only by X-ray diffraction, which is not sufficient to determine the exact nature and number of layers. Attempts were made to prepare single layers of WS2 by lithium intercalation and exfoliation as well, 12] but here again the product was only characterized on the basis of the (002) reflection in the X-ray diffraction pattern. Schumacher et al. and Gordon et al. prepared MoS2 samples by lithium intercalation followed by exfoliation and characterized the products by means of scanning force microscopy and X-ray absorption fine structure spectroscopy. Yang et al. report that the exfoliated MoS2 forms aqueous suspensions of single layers wherein sulfur atoms are bonded with molybdenum in an octahedral arrangement with 2a0 superlattice. Suspensions of layered chalcogenides have also been used to prepare inclusion compounds of various organic molecules and to fabricate light-emitting diodes. Since even MoS2 and WS2 containing five layers do not exhibit the (002) reflection prominently, layered MoS2 and WS2 produced by lithium intercalation and exfoliation must be investigated by transmission electron microscopy and other techniques. Furthermore, it seems desirable to explore alternative syntheses of these graphene-like materials. To this end, we employed three different methods to synthesize graphenelike MoS2 and WS2. In Method 1, bulk MoS2 and WS2 were intercalated with lithium and exfoliated in water. The reaction between lithium-intercalated MoS2 and WS2 and water forms lithium hydroxide and hydrogen gas and leads to separation of the sulfide layers and loss of periodicity along the c axis. In Method 2, molybdic acid and tungstic acid were treated with an excess of thiourea in an N2 atmosphere at 773 K. Method 3 involved the reaction between MoO3 and KSCN under hydrothermal conditions. The products of these reactions were characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), field-emission scanning electron microscopy (FESEM), Raman spectroscopy, and X-ray diffraction (XRD). The XRD patterns of the molybdenum sulfide samples obtained by the three methods do not exhibit the (002) reflection (Figure 1a). Energy-dispersive analysis of X-rays (EDAX) shows the products to be stoichiometric MoS2. The TEM and AFM images of the products show the presence of one or a few layers of MoS2 (Figures 2 and 3). Figure 2 a and b show graphene-like MoS2 layers obtained by methods 2 and 3 with a layer separation in the range of 0.65–0.7 nm. The highresolution image in Figure 2c shows the hexagonal structure formed by Mo and S atoms with an Mo S distance of 2.30 . The AFM images and height profiles of the products also confirm the formation of few-layer MoS2 (Figure 3a). Figure 4a compares the Raman spectra of graphene-like MoS2 samples with that of bulk MoS2. The bulk sample shows bands at 406.5 and 381.2 cm 1 due to the A1g and E2g modes with fullwidths at half maximum (FWHM) of 2.7 and 3.1 cm , respectively. Interestingly, few-layered MoS2 prepared by lithium intercalation exhibits corresponding bands at 404.7 and 379.7 cm . The sample obtained by Method 2 show these bands at 404.7 and 377.4 cm . The A1g and E2g modes in the graphene analogues of MoS2 are clearly softened. Furthermore, the FWHM values are larger in the graphene-like samples (10–16 cm 1 vs. ca. 3 cm 1 in the bulk sample). Broadening of the Raman bands is considered to be due to phonon confinement, and also suggests that the lateral dimensions of these layers are in the nanoregime. We also prepared graphene-like MoS2 by micromechanical cleavage of a MoS2 single crystal using the Scotch-tape technique. Raman spectra of these samples show progressive softening of the A1g and E2g bands with decreasing number of layers. [*] H. S. S. Ramakrishna Matte, A. Gomathi, A. K. Manna, Dr. D. J. Late, Dr. R. Datta, Prof. Dr. S. K. Pati, Prof. Dr. C. N. R. Rao Chemistry and Physics of Materials Unit, Theoretical Science Unit and International Centre for Materials Science Jawaharlal Nehru Centre for Advanced Scientific Research Jakkur P.O., Bangalore 560 064 (India) Fax: (+ 91)80-2208-2760 E-mail: cnrrao@jncasr.ac.in Angewandte Chemie

Sensing behavior of atomically thin-layered MoS2 transistors.

Most of recent research on layered chalcogenides is understandably focused on single atomic layers. However, it is unclear if single-layer units are the most ideal structures for enhanced gas-solid interactions. To probe this issue further, we have prepared large-area MoS2 sheets ranging from single to multiple layers on 300 nm SiO2/Si substrates using the micromechanical exfoliation method. The thickness and layering of the sheets were identified by optical microscope, invoking recently reported specific optical color contrast, and further confirmed by AFM and Raman spectroscopy. The MoS2 transistors with different thicknesses were assessed for gas-sensing performances with exposure to NO2, NH3, and humidity in different conditions such as gate bias and light irradiation. The results show that, compared to the single-layer counterpart, transistors of few MoS2 layers exhibit excellent sensitivity, recovery, and ability to be manipulated by gate bias and green light. Further, our ab initio DFT calculations on single-layer and bilayer MoS2 show that the charge transfer is the reason for the decrease in resistance in the presence of applied field.

Hysteresis in Single-Layer MoS2 Field Effect Transistors

Field effect transistors using ultrathin molybdenum disulfide (MoS(2)) have recently been experimentally demonstrated, which show promising potential for advanced electronics. However, large variations like hysteresis, presumably due to extrinsic/environmental effects, are often observed in MoS(2) devices measured under ambient environment. Here, we report the origin of their hysteretic and transient behaviors and suggest that hysteresis of MoS(2) field effect transistors is largely due to absorption of moisture on the surface and intensified by high photosensitivity of MoS(2). Uniform encapsulation of MoS(2) transistor structures with silicon nitride grown by plasma-enhanced chemical vapor deposition is effective in minimizing the hysteresis, while the device mobility is improved by over 1 order of magnitude.

GaS and GaSe Ultrathin Layer Transistors

Room-temperature, bottom-gate, field-effect transistor characteristics of 2D ultrathin layer GaS and GaSe prepared from the bulk crystals using a micromechanical cleavage technique are reported. The transistors based on active GaS and GaSe ultrathin layers demonstrate typical n-and p-type conductance transistor operation along with a good ON/OFF ratio and electron differential mobility.

Band-like transport in high mobility unencapsulated single-layer MoS 2 transistors

Ultra-thin MoS2 has recently emerged as a promising two-dimensional semiconductor for electronic and optoelectronic applications. Here, we report high mobility (>60 cm2/Vs at room temperature) field-effect transistors that employ unencapsulated single-layer MoS2 on oxidized Si wafers with a low level of extrinsic contamination. While charge transport in the sub-threshold regime is consistent with a variable range hopping model, monotonically decreasing field-effect mobility with increasing temperature suggests band-like transport in the linear regime. At temperatures below 100 K, temperature-independent mobility is limited by Coulomb scattering, whereas, at temperatures above 100 K, phonon-limited mobility decreases as a power law with increasing temperature.

Enhanced Field‐Emission Behavior of Layered MoS2 Sheets

Field emission studies are reported for the first time on layered MoS₂ sheets at the base pressure of ∼1 × 10⁻⁸ mbar. The turn-on field required to draw a field emission current density of 10 μA/cm² is found to be 3.5 V/μm for MoS₂ sheets. The turn-on values are found to be significantly lower than the reported MoS₂ nanoflowers, graphene, and carbon nanotube-based field emitters due to the high field enhancement factor (∼1138) associated with nanometric sharp edges of MoS₂ sheet emitter surface. The emission current-time plots show good stability over a period of 3 h. Owing to the low turn-on field and planar (sheetlike) structure, the MoS₂ could be utilized for future vacuum microelectronics/nanoelectronic and flat panel display applications.

NO2 AND HUMIDITY SENSING CHARACTERISTICS OF FEW-LAYER GRAPHENES

Sensing characteristics of few-layer graphenes for NO2 and humidity have been investigated with graphene samples prepared by the thermal exfoliation of graphitic oxide, conversion of nanodiamond (DG) and arc-discharge of graphite in hydrogen (HG). The sensitivity for NO2 is found to be highest with DG. Nitrogen-doped HG (n-type) shows increased sensitivity for NO2 compared with pure HG. The highest sensitivity for humidity is observed with HG. Sensing characteristics of graphene have been examined for different aliphatic alcohols and the sensitivity is found to vary with the chain length and branching.

Thermal Expansion, Anharmonicity and Temperature‐Dependent Raman Spectra of Single‐ and Few‐Layer MoSe2 and WSe2

We report the temperature-dependent Raman spectra of single- and few-layer MoSe2 and WSe2 in the range 77-700 K. We observed linear variation in the peak positions and widths of the bands arising from contributions of anharmonicity and thermal expansion. After characterization using atomic force microscopy and high-resolution transmission electron microscopy, the temperature coefficients of the Raman modes were determined. Interestingly, the temperature coefficient of the A(2)(2u) mode is larger than that of the A(1g) mode, the latter being much smaller than the corresponding temperature coefficients of the same mode in single-layer MoS2 and of the G band of graphene. The temperature coefficients of the two modes in single-layer MoSe2 are larger than those of the same modes in single-layer WSe2. We have estimated thermal expansion coefficients and temperature dependence of the vibrational frequencies of MoS2 and MoSe2 within a quasi-harmonic approximation, with inputs from first-principles calculations based on density functional theory. We show that the contrasting temperature dependence of the Raman-active mode A(1g) in MoS2 and MoSe2 arises essentially from the difference in their strain-phonon coupling.

Superior Field Emission Properties of Layered WS2-RGO Nanocomposites

We report here the field emission studies of a layered WS2-RGO composite at the base pressure of ~1 × 10−8 mbar. The turn on field required to draw a field emission current density of 1 μA/cm2 is found to be 3.5, 2.3 and 2 V/μm for WS2, RGO and the WS2-RGO composite respectively. The enhanced field emission behavior observed for the WS2-RGO nanocomposite is attributed to a high field enhancement factor of 2978, which is associated with the surface protrusions of the single-to-few layer thick sheets of the nanocomposite. The highest current density of ~800 μA/cm2 is drawn at an applied field of 4.1 V/μm from a few layers of the WS2-RGO nanocomposite. Furthermore, first-principles density functional calculations suggest that the enhanced field emission may also be due to an overalp of the electronic structures of WS2 and RGO, where graphene-like states are dumped in the region of the WS2 fundamental gap.

A study of the synthetic methods and properties of graphenes

Abstract Graphenes with varying number of layers can be synthesized by using different strategies. Thus, single-layer graphene is prepared by micromechanical cleavage, reduction of single-layer graphene oxide, chemical vapor deposition and other methods. Few-layer graphenes are synthesized by conversion of nanodiamond, arc discharge of graphite and other methods. In this article, we briefly overview the various synthetic methods and the surface, magnetic and electrical properties of the produced graphenes. Few-layer graphenes exhibit ferromagnetic features along with antiferromagnetic properties, independent of the method of preparation. Aside from the data on electrical conductivity of graphenes and graphene-polymer composites, we also present the field-effect transistor characteristics of graphenes. Only single-layer reduced graphene oxide exhibits ambipolar properties. The interaction of electron donor and acceptor molecules with few-layer graphene samples is examined in detail.