  Recently emerging large-area single-layer MoS2 grown by chemical vapor
deposition has triggered great interest due to its exciting potential for
applications in advanced electronic and optoelectronic devices. Unlike gapless
graphene, MoS2 has an intrinsic band gap in the visible which crosses over from
an indirect to a direct gap when reduced to a single atomic layer. In this
article, we report a comprehensive study of fundamental optical properties of
MoS2 revealed by optical spectroscopy of Raman, photoluminescence, and vacuum
ultraviolet spectroscopic ellipsometry. A band gap of 1.42 eV is determined by
the absorption threshold of bulk MoS2 that shifts to 1.83 eV in monolayer MoS2.
We extracted the high precision dielectric function up to 9.0 eV which leads to
the identification of many unique interband transitions at high symmetry points
in the MoS2 momentum space. The positions of the A and B excitons in single
layers are found to shift upwards in energy compared with those of the bulk
form and have smaller separation. A very strong optical critical point
predicted to correspond to a quasi-particle gap is observed at 2.86 eV, which
is attributed to optical transitions along the parallel bands between the M and
gama points in the reduced Brillouin zone. The absence of the bulk MoS2
spin-orbit interaction peak at ~ 3.0 eV in monolayer MoS2 is, as predicted, the
consequence of the coalescence of nearby excitons. A higher energy optical
transition at 3.98 eV, commonly occurred in bulk semiconductors, is associated
with a combination of several critical points.These optical transitions herein
reported enhance our understanding of monolayer MoS2 as well as of
two-dimensional systems in general, and thus provide informative guidelines for
MoS2 optical device designs and theoretical considerations.
