Atomically thin indium selenide (InSe) exhibits a sombrero-like valence band, leading to distinctive excitonic behaviors. It is known that the indirect band gap of atomically thin InSe leads to a weak emission from the lowest-energy excitonic state (A peak). However, the A peak emission of monolayer (ML) InSe was observed to be either absent or very weak, rendering the nature of its excitonic states largely unknown. Intriguingly, we demonstrate that ML InSe exhibits pronounced PL emission because of the efficient brightening of the momentum-indirect dark excitons. The mechanism is attributed to acoustic phonon-assisted radiative recombination facilitated by strong exciton-acoustic phonon coupling and extended wavefunction in momentum space. Systematic analysis of layer-, power-, and temperature-dependent PL demonstrates that a carrier localization model can account for the asymmetric line shape of the lowest-energy excitonic emission for atomically thin InSe. Our work reveals that atomically thin InSe is a promising platform for manipulating the tightly bound dark excitons in two-dimensional semiconductor-based optoelectronic devices.

原子薄的硒化铟 (InSe) 表现出类似宽边帽的价带，从而导致独特的激子行为。众所周知，原子级薄的 InSe 的间接带隙导致最低能量激子态（A 峰）的弱发射。然而，观察到单层 (ML) InSe 的 A 峰发射要么不存在，要么非常弱，导致其激子态的性质很大程度上未知。有趣的是，我们证明了 ML InSe 由于动量间接暗激子的有效增亮而表现出明显的 PL 发射。该机制归因于强激子-声子耦合和动量空间中的扩展波函数促进的声子辅助辐射复合。对层、功率和温度相关 PL 的系统分析表明，载流子局域化模型可以解释原子薄 InSe 的最低能量激子发射的不对称线形状。我们的工作表明，原子级薄的 InSe 是操纵二维半导体光电器件中紧密束缚的暗激子的有前途的平台。