  Atomically thin two-dimensional molybdenum disulfide (MoS2) sheets have
attracted much attention due to their potential for future electronic
applications. They not only present the best planar electrostatic control in a
device, but also lend themselves readily for dielectric engineering. In this
work, we experimentally investigated the dielectric effect on the Raman and
photoluminescence (PL) spectra of monolayer MoS2 by comparing samples with and
without HfO2 on top by atomic layer deposition (ALD). Based on considerations
of the thermal, doping, strain and dielectric screening influences, it is found
that the red shift in the Raman spectrum largely stems from modulation doping
of MoS2 by the ALD HfO2, and the red shift in the PL spectrum is most likely
due to strain imparted on MoS2 by HfO2. Our work also suggests that due to the
intricate dependence of band structure of monolayer MoS2 on strain, one must be
cautious to interpret its Raman and PL spectroscopy.
