Single-layer metal dichalcogenides are two-dimensional semiconductors that present strong potential for electronic and sensing applications complementary to that of graphene.

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Electronics and optoelectronics of two-dimensional transition metal dichalcogenides.

Novel physical phenomena can emerge in low-dimensional nanomaterials. Bulk MoS2, a prototypical metal dichalcogenide, is an indirect bandgap semiconductor with negligible photoluminescence. When the MoS2 crystal is thinned to monolayer, however, a strong photoluminescence emerges, indicating an indirect to direct bandgap transition in this d-electron system. This observation shows that quantum confinement in layered d-electron materials like MoS2 provides new opportunities for engineering the electronic structure of matter at the nanoscale.

Emerging Photoluminescence in Monolayer MoS2

Graphene-Like Two-Dimensional Materials

Molybdenum disulfi de (MoS 2 ) is systematically studied using Raman spectroscopy with ultraviolet and visible laser lines. It is shown that only the Raman frequencies of E 1 and A1g peaks vary monotonously with the layer number of ultrathin MoS 2 fl akes, while intensities or widths of the peaks vary arbitrarily. The coupling between electronic transitions and phonons are found to become weaker when the layer number of MoS 2 decreases, attributed to the increased electronic transition energies or elongated intralayer atomic bonds in ultrathin MoS 2 . The asymmetric Raman peak at 454 cm − 1 , which has been regarded as the overtone of longitudinal optical M phonons in bulk MoS 2 , is actually a combinational band involving a longitudinal acoustic mode (LA(M)) and an optical mode ( A2u ). Our fi ndings suggest a clear evolution of the coupling between electronic transition and phonon when MoS 2 is scaled down from three- to two-dimensional geometry.

From bulk to monolayer MoS2: evolution of Raman scattering

Controlling surface structure at the atomic scale is paramount to developing effective catalysts. For example, the edge sites of MoS(2) are highly catalytically active and are thus preferred at the catalyst surface over MoS(2) basal planes, which are inert. However, thermodynamics favours the presence of the basal plane, limiting the number of active sites at the surface. Herein, we engineer the surface structure of MoS(2) to preferentially expose edge sites to effect improved catalysis by successfully synthesizing contiguous large-area thin films of a highly ordered double-gyroid MoS(2) bicontinuous network with nanoscaled pores. The high surface curvature of this catalyst mesostructure exposes a large fraction of edge sites, which, along with its high surface area, leads to excellent activity for electrocatalytic hydrogen evolution. This work elucidates how morphological control of materials at the nanoscale can significantly impact the surface structure at the atomic scale, enabling new opportunities for enhancing surface properties for catalysis and other important technological applications.

Engineering the surface structure of MoS2 to preferentially expose active edge sites for electrocatalysis

MS2 (M = W, Mo) photosensitive thin films for solar cells

MoS2 thin films obtained by a new technique: Solid state reaction between the constituents in thin film form

MoS 2 films were obtained by solid state reaction—induced by annealing—between the Mo and S constituents of a multilayer structure (Mo/S/Mo…Mo/S). The films were investigated by X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM, TEM), optical absorption and electrical measurements. It was found that, at the end of the process, the films were well crystallized and stoichiometric. MoS 2 , in hexagonal form, was obtained by annealing for 24 h at 853 K. However, it can be seen by SEM that, if the first 100 nm of the films were perfectly oriented, the upper part of the films was not. The first 100 nm were textured, the crystallites had their c axis perpendicular to the plane of the substrate. The upper part was more randomly oriented and more porous. Its crystallites were very small and preferentially oriented with their c axis parallel to the plane of the substrate. XRD, TEM and conductivity characterization were in good accordance with SEM results.

Physicochemical characterization of MoS2 films obtained by solid state reaction between the constituents of a multilayer Mo/S…/Mo/S structure

The conductivity of WSe 2 , MoS 2 , WS 2 thin films, obtained by solid state reaction between the metal and the chalcogen sequentially deposited, is controlled by grain boundary scattering mechanisms. A mild annealing treatment at 373 K for 1/2 h to 6 h under iodine atmosphere improves the conductivity of the films. In the case of WSe 2 , which gives the better results, the room temperature conductivity can be multiplied by 25. If the conductivity is improved, its dependence with the temperature demonstrates that the conductivity process is still controlled by grain boundary scattering mechanisms. All the conductivity results are interpreted with the help of a grain boundary model. Barrier height fluctuations at the grain boundary are introduced to have experimental and theoretical data fit together. After iodine treatment the mean barrier height has decreased and the homogeneity of the films is improved. These improvements are very promising for photovoltaic application since it appears that iodine passivates defects at the grain boundaries.

H. Hadouda

Papers

MS2 (M = W, Mo) photosensitive thin films for solar cells

MoS2 thin films obtained by a new technique: Solid state reaction between the constituents in thin film form

Physicochemical characterization of MoS2 films obtained by solid state reaction between the constituents of a multilayer Mo/S…/Mo/S structure

Improvement of the Electrical Properties of Layered Semiconductor Thin Films by Iodine Treatment