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