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

Textured films of molybdenum disulfide have been obtained by solid state reaction between the constituents in thin films form when a (200) oriented tungsten sheet is used as substrate. The crystallites have their c axis perpendicular to the plane of the substrate. The annealing conditions are T=1073 K and t=30 min. The films are stoichoimetric and p type. Such highly textured films are achieved without foreign atom addition (Ni, Co…). It appears, as shown by x-ray photoelectron spectroscopy, that a thin WS2 layer is present at the interface W/MoS2. The crystallization process is discussed by a van der Waals texturation (pseudoepitaxy) onto dangling bond sulfur terminated surfaces, these surfaces being ordered. After characterization of the W/MoS2 structure by x-ray diffraction and x-ray photoelectron spectroscopy, an upper electrode of tungsten was deposited by sputtering. The electrical properties of these W/MoS2/W structures have been investigated by analyzing the behavior of the current–voltage characteristics as a function of the measuring temperature. It is shown that an ohmic contact is obtained with a contact resistance smaller than the resistance of the MoS2 film.

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Textured MoS2 thin films obtained on tungsten: Electrical properties of the W/MoS2 contact

Recent studies on photoconductive thin films of binary compounds

High quality crystalline MoS2 thin films obtained on Ni coated substrates: optimization of the post-annealing treatment

It is shown that photosensitive films can be obtained by solid state reaction, induced by annealing, between the constituents Mo and S sequentially deposited in thin film form if the substrate is coated with a thin (10–20 nm) NiCr layer. The thin Mo and S layers are deposited in the atomic ratio Mo : S=1 : 3. The substrates used are mica sheets. An annealing at 1073 K for 30 min under argon flow allows one to obtain highly 2H–MoS2 crystallized films. The thickness of the crystallites is similar to that of the films; they have their c-axes perpendicular to the plane of the substrate. After crystallization, X-ray photoelectron spectroscopy (XPS) depth profiles show that Ni is diffused all over the thicknesses of the films and that 1 at% of Ni is visible at the surfaces of the films. The direct current (d.c.) conductivity of these films is nearly similar to that of single crystals. The films are photosensitive. The room temperature photoconductivity, which results from interband transitions, allows one to measure the direct band gap that is similar to that of a single crystal. When bare mica substrates (without Ni) are used MoS2 films are obtained but they are poorly crystallized and not photoconductive, which shows an NiCr interfacial layer is necessary. Probably a melting phase NiSx forms, which increases the mobility of the atom at grain boundaries.

E. Gourmelon

Papers

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

Textured MoS2 thin films obtained on tungsten: Electrical properties of the W/MoS2 contact

Recent studies on photoconductive thin films of binary compounds

High quality crystalline MoS2 thin films obtained on Ni coated substrates: optimization of the post-annealing treatment

Experimental obviousness of the necessity for a thin Ni interfacial layer to obtain highly ordered photoconductive MoS2 films