  We present a photoluminescence study of single-layer MoS2 flakes on SiO2
surfaces. We demonstrate that the luminescence peak position of flakes prepared
from natural MoS2, which varies by up to 25 meV between individual as-prepared
flakes, can be homogenized by annealing in vacuum, which removes adsorbates
from the surface. We use HfO2 and Al2O3 layers prepared by atomic layer
deposition to cover some of our flakes. We clearly observe a suppression of the
low-energy luminescence peak observed for as-prepared flakes at low
temperatures, indicating that this peak originates from excitons bound to
surface adsorbates. We also observe different temperature-induced shifts of the
luminescence peaks for the oxide-covered flakes. This effect stems from the
different thermal expansion coefficients of the oxide layers and the MoS2
flakes. It indicates that the single-layer MoS2 flakes strongly adhere to the
oxide layers and are therefore strained.
