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Geometrical confinement effect in exfoliated sheets of layered materialsleads to significant evolution of energy dispersion with decreasing layerthickness. Molybdenum disulphide (MoS2) was recently found to exhibit indirectto direct gap transition when the thickness is reduced to a single monolayer.This leads to remarkable enhancement in the photoluminescence efficiency, whichopens up new opportunities for the optoelectronic applications of the material.Here we report differential reflectance and photoluminescence (PL) spectra ofmono- to few-layer WS2 and WSe2 that indicate that the band structure of thesematerials undergoes similar indirect to direct transition when thinned to asingle monolayer. Strong enhancement in PL quantum yield is observed formonoayer WS2 and WSe2 due to exciton recombination at the direct band edge. Incontrast to natural MoS2 crystals extensively used in recent studies, few-layerWS2 and WSe2 show comparatively strong indirect gap emission along withdistinct direct gap hot electron emission, suggesting high quality of syntheticcrystals prepared by chemical vapor transport method. Fine absorption andemission features and their thickness dependence suggest strong effect of Sep-orbitals on the d electron band structure as well as interlayer coupling inWSe2.

Evolution of Electronic Structure in Atomically Thin Sheets of WS2 and WSe2