Monolayer Indium Selenide—An Indirect Bandgap Material Exhibits Efficient Brightening of Dark Excitons

Tightly bound excitons in two-dimensional (2D) semiconductors have attracted intensive attention from the fundamental exciton science to emerging photonic and optoelectronic applications. Bright excitons exhibit great oscillator strengths, but their ultrashort radiative lifetimes on the subpicosecond timescale hinder potential optoelectronic and catalytic applications that require a long exciton lifetime. In contrast, dark excitons, such as spatial- and momentum-indirect excitons, have a higher population density and a longer lifetime of approximate nanoseconds; nevertheless, these excitons are optically inaccessible due to their weak coupling to light. Therefore, exploring mechanisms that can effectively brighten the dark exciton is crucial for extending the functionality of indirect gap materials. We demonstrate pronounced photoluminescence (PL) observed from monolayer (ML) indium selenide (InSe) [1], a two-dimensional indirect semiconductor exhibiting a sombrero-shaped valence band [2]. Prior studies on the optical properties of ML InSe have reported either weak emissions or none from the lowest excitonic state, identified as the A exciton, [2,3], leaving the excitonic phenomena in ML InSe unexplored. By observing the distinct PL from our high-quality ML InSe samples, indicating the occurrence of the brightening of the momentum-indirect dark excitons. This brightening phenomenon is attributed to the acoustic phonon-assisted radiative recombination, facilitated by strong-exciton-acoustic coupling, and in combination with the extended wavefunction in the momentum space. Furthermore, our systematic study of the PL dependence on InSe layers, temperature, and excitation power validates the application of the carrier localization model [4] to account for the asymmetric line shape of the A exciton emission. Our work shows that monolayer indium selenide is a favourable 2D material for further investigations of dark exciton phenomenon and its role in optoelectronic devices of the future.<br/><br/>References<br/>1. Paylaga, N. T., C. T. Chou, C. C. Lin, T. Taniguchi, K. Watanabe, R. Sankar, Y. H. Chan, S. Y. Chen and W. H. Wang (2024). "Monolayer indium selenide: an indirect bandgap material exhibits efficient brightening of dark excitons." Npj 2d Materials and Applications 8(1).<br/>2. Hamer, M. J. et al. Indirect to Direct Gap Crossover in Two-Dimensional InSe Revealed by Angle-Resolved Photoemission Spectroscopy. Acs Nano 13, 2136-2142 (2019).<br/>3. Bandurin, D. A. et al. High electron mobility, quantum Hall effect and anomalous optical response in atomically thin InSe. Nat Nanotechnol 12, 223-227 (2017).<br/>4. Schubert, E. F. & Tsang, W. T. Photoluminescence Line-Shape of Excitons in Alloy Semiconductors. Phys Rev B 34, 2991-2994 (1986).