Optical study of local strain related disordering in CVD-grown MoSe₂ monolayers

We present temperature dependent micro-photoluminescence and room temperature photoreflectance spectroscopy studies on aged MoSe₂ monolayers with high surface roughness. A⁰ and B⁰ exciton bands were detected at 1.512 eV and 1.72 eV, respectively, which are 50-70 meV lower than those commonly reported for high-quality samples. It is shown that the difference can be accounted for using a model of localized excitons for disordered MoSe₂ monolayers where the optical band gap energy fluctuations could be caused by random distribution of local tensile strain due to surface roughness. The density of localized exciton states is found to follow the Lorentzian shape, where the peak of this distribution is about 70 meV from the energy of delocalized states.