Impact of substrate induced band tail states on the electronic and optical properties of MoS₂

Substrate, environment, and lattice imperfections have a strong impact on the local electronic structure and the optical properties of atomically thin transition metal dichalcogenides. We find by a comparative study of MoS₂ on SiO₂ and hexagonal boron nitride (hBN) using scanning tunneling spectroscopy (STS) measurements that the apparent bandgap of MoS₂ on SiO₂ is significantly reduced compared to MoS₂ on hBN. The bandgap energies as well as the exciton binding energies determined from all-optical measurements are very similar for MoS₂ on SiO₂ and hBN. This discrepancy is found to be caused by a substantial amount of band tail states near the conduction band edge of MoS₂ supported by SiO₂. The presence of those states impacts the local density of states in STS measurements and can be linked to a broad red-shifted photoluminescence peak and a higher charge carrier density that are all strongly diminished or even absent using high quality hBN substrates. By taking into account the substrate effects, we obtain a quasiparticle gap that is in excellent agreement with optical absorbance spectra and we deduce an exciton binding energy of about $0.53 eV$ on $SiO2$ and $0.44 eV$ on hBN.