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Indium selenide (InSe) is a two-dimensional III-VI post-transition metal chalcogenide semiconductor with promising electronic and optoelectronic properties. The large layer-dependence of its band gap and its tightly bounded excitons make atomically thin InSe a unique material for achieving an appreciable bandwidth ranging from the whole visible region to the near-infrared region for various applications in optoelectronics. It is known that the direct-to-indirect band gap transition of atomically thin InSe leads to a weaker emission from the lowest-energy bright exciton (A exciton) as the layer number decreases. However, the A exciton emis-sion of monolayer (ML) InSe was observed to be either absent or very weak in recent experi-mental studies, rendering the nature of the lowest-energy excitonic states of ML InSe and its optical properties largely unknown. In this work, we systematically measure and analyze the photoluminescence of hexagonal boron nitride-encapsulated InSe from ML InSe to bulk InSe. Remarkably, ML InSe exhibits pronounced luminescence near the A exciton excitation energy despite its indirect band structure. We uncover the mechanism for brightening the momentum-indirect dark excitons of ML InSe, which can be attributed to the efficient acoustic phonon-assisted recombination facilitated by strong exciton-phonon coupling and the extended wave-function in momentum space. Moreover, the asymmetric line shape of the lowest-energy exci-tonic emission for atomically thin InSe can be well accounted for by a carrier localization model. Our work demonstrates the unique excitonic properties of atomically thin InSe, which can pro-vide potential avenues for manipulating the tightly bound dark excitons of two-dimensional ma-terial-based optoelectronic devices.

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• Creative Commons BY Attribution 4.0 International
  

Keyword: Atomically thin, InSe, dark excitons

Date published: 2024-02-20

Publisher: Springer Science and Business Media LLC

Journal:

• npj 2D Materials and Applications (ISSN: 23977132) vol. 8 issue. 1 12

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Manuscript type: Publisher's version (Version of record)

MDR DOI:

First published URL: https://doi.org/10.1038/s41699-024-00450-3

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Updated at: 2025-02-28 08:30:58 +0900

Published on MDR: 2025-02-28 08:30:58 +0900