Brightening dark trions in WS2 monolayers via introducing atomic sulfur vacancies

Understanding the effects of atomic defects on the optical functionality of two-dimensional (2D) layered materials is critical to develop novel optical and optoelectronic applications of these ultimate materials. Herein, we correlate sulfur vacancies (VS) and luminescence properties of dark trions in monolayer WS2 through introducing VS defects and conducting a systematic optical spectroscopic characterization at cryogenic and room temperatures. It is unraveled that the VS defects can brighten the dark trions via introducing a stronger spin-orbit coupling due to the space inversion symmetry broken by the defects. Furthermore, the wavefunction localization of the dark trions bound at VS defects results in significant enhancement of the phonon scattering from the K2 valley phonons and hence makes the K2 phonon replica dominant in the emission spectrum. Theoretical calculations of the temperature-dependent photoluminescence spectra with quantum mechanics-based multimode Brownian oscillator model show strong support for the above arguments. Brightening the dark excitons not only sheds light on the understanding of the intriguing excitonic properties of 2D semiconductors, but also may open a way for regulating the optoelectronic performance of two-dimensional semiconductors.