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.

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.

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We report resonant Raman scattering of MoS 2 layers comprising of single, bi, four and seven layers, showing a strong dependence on the layer thickness. Indirect band gap MoS 2 in bulk becomes a direct band gap semiconductor in the monolayer form. New Raman modes are seen in the spectra of single-and few-layer MoS 2 samples which are absent in the bulk. The Raman mode at 230 cm À1 appears for two, four and seven layers. This mode has been attributed to the longitudinal acoustic phonon branch at the M point (LA(M)) of the Brillouin zone. The mode at~179 cm À1 shows asymmetric character for a few-layer sample. The asymmetry is explained by the dispersion of the LA(M) branch along the Γ-M direction. The most intense spectral region near 455 cm À1 shows a layer-dependent variation of peak positions and relative intensities. The high energy region between 510 and 645 cm À1 is marked by the appearance of prominent new Raman bands, varying in intensity with layer numbers. Resonant Raman spectroscopy thus serves as a promising non invasive technique to accurately estimate the thickness of MoS 2 layers down to a few atoms thick.

There has been emerging interest in exploring single‐sheet 2D layered structures other than graphene to explore potentially interesting properties and phenomena. The preparation, isolation and rapid unambiguous characterization of large size ultrathin layers of MoS2, GaS, and GaSe deposited onto SiO2/Si substrates is reported. Optical color contrast is identified using reflection optical microscopy for layers with various thicknesses. The optical contrast of these thin layers is correlated with atomic force microscopy (AFM) and Raman spectroscopy to determine the exact thickness and to calculate number of the atomic layers present in the thin flakes and sheets. Collectively, optical microscopy, AFM, and Raman spectroscopy combined with Raman imaging data are analyzed to determine the thickness (and thus, the number of unit layers) of the MoS2, GaS, and GaSe ultrathin flakes in a fast, non‐destructive, and unambiguous manner. These findings may enable experimental access to and unambiguous determination of layered chalcogenides for scientific exploration and potential technological applications.