Defects activated photoluminescence in two-dimensional semiconductors: interplay between bound, charged and free excitons

TLDR

Point defects in semiconductors trap free carriers and localize excitons, and their interaction with charge carriers is stronger in reduced dimensionalities, profoundly affecting the host material’s properties. We investigated the effects of anion vacancies in monolayer transition metal dichalcogenides, controlling vacancy density by α‑particle irradiation or thermal annealing. Vacancy generation in monolayer TMDs produces a new sub‑bandgap emission peak and boosts overall photoluminescence, an effect that disappears in vacuum and shows that room‑temperature optical quality does not reliably indicate crystal quality, thereby revealing defect engineering as a viable means to tune 2D semiconductor optics.

Abstract

Abstract Point defects in semiconductors can trap free charge carriers and localize excitons. The interaction between these defects and charge carriers becomes stronger at reduced dimensionalities and is expected to greatly influence physical properties of the hosting material. We investigated effects of anion vacancies in monolayer transition metal dichalcogenides as two-dimensional (2D) semiconductors where the vacancies density is controlled by α-particle irradiation or thermal-annealing. We found a new, sub-bandgap emission peak as well as increase in overall photoluminescence intensity as a result of the vacancy generation. Interestingly, these effects are absent when measured in vacuum. We conclude that in opposite to conventional wisdom, optical quality at room temperature cannot be used as criteria to assess crystal quality of the 2D semiconductors. Our results not only shed light on defect and exciton physics of 2D semiconductors, but also offer a new route toward tailoring optical properties of 2D semiconductors by defect engineering.