Effect of Isotopes on Photoluminescence in Single Layer MoS₂

We have performed calculations based on DFT and TDDFT to elucidate the effect of Mo isotope mass on photoluminescence (PL) of single-layer MoS_2. In particular, we have computed the electron and the phonon spectra of the system, as well as the strength of the electron-phonon coupling of the system with ⁹²Mo, ⁹⁵Mo and ¹⁰⁰Mo. Contrary to most bulk semiconductors in which isotope shift of the PL peak results from electron-phonon and hole-phonon interactions induced change of the bandgap, we show that in single-layer MoS₂ the observed PL frequency decreases with mass increasing is due to a renormalization of the exciton energy because of the exciton-phonon coupling. Such a strong effect of exciton-phonon coupling can be explained by the large excitonic binding energy in the system that corresponds to a strong interaction of a very stable “e-h dipole” with ionic vibrations, and to reduced effects of screening. The results for the isotope shift of the PL peak are in a good agreement with experimental data[1].

[1] Kai Xiao et.al. private communication