Efficient brightening of momentum-indirect dark excitons in ML InSe

Manipulating bright and dark excitons in two-dimensional (2D) materials is a key to understanding many-body correlations of exciton and further developing exciton-mediated applications. On the one hand, bright excitons can directly couple to the light and exhibit great oscillator strengths. On the other hand, dark excitons have a much longer population lifetime and diffusion length, greatly enhancing the exciton-matter interactions. In this work, we investigated the photoluminescence of hexagonal boron nitride-encapsulated monolayer indium selenide (ML InSe). Remarkably, for the first time, we found ML InSe exhibits pronounced luminescence from the momentum-indirect dark excitons, comparable to the few-layer and bulk counterparts. The brightening of dark exciton in ML InSe is attributed to the efficient acoustic phonon-assisted recombination facilitated by strong exciton-phonon coupling and the extended wavefunction in momentum space. Moreover, the asymmetric line shape in PL spectra for atomically thin InSe can be well accounted for by a carrier localization model, reflecting the unique properties of long lifetime and diffusion length of dark excitons. These unique excitonic properties of atomically thin InSe provide potential avenues for manipulating the tightly-bound dark excitons of 2D material-based photovoltaic and photocatalytic applications.