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Indirect to direct gap crossover in two-dimensional InSe revealed by angle-resolved photoemission spectroscopy
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Hamer, Matthew J., Zultak, Johanna, Tyurnina, Anastasia V., Zólyomi, Viktor, Terry, Daniel, Barinov, Alexei, Garner, Alistair, Donoghue, Jack, Rooney, Aidan P., Kandyba, Viktor, Giampietri, Alessio, Graham, Abigail J., Teutsch, Natalie C., Xia, Xue, Koperski, Maciej, Haigh, Sarah J., Fal’ko, Vladimir I., Gorbachev, Roman V. and Wilson, Neil R. (2019) Indirect to direct gap crossover in two-dimensional InSe revealed by angle-resolved photoemission spectroscopy. ACS Nano, 13 (2). pp. 2136-2142. doi:10.1021/acsnano.8b08726 ISSN 1936-0851.
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Official URL: http://dx.doi.org/10.1021/acsnano.8b08726
Abstract
Atomically thin films of III–VI post-transition metal chalcogenides (InSe and GaSe) form an interesting class of two-dimensional semiconductors that feature a strong variation of their band gap as a function of the number of layers in the crystal and, specifically for InSe, an expected crossover from a direct gap in the bulk to a weakly indirect band gap in monolayers and bilayers. Here, we apply angle-resolved photoemission spectroscopy with submicrometer spatial resolution (μARPES) to visualize the layer-dependent valence band structure of mechanically exfoliated crystals of InSe. We show that for one-layer and two-layer InSe the valence band maxima are away from the Γ-point, forming an indirect gap, with the conduction band edge known to be at the Γ-point. In contrast, for six or more layers the band gap becomes direct, in good agreement with theoretical predictions. The high-quality monolayer and bilayer samples enable us to resolve, in the photoluminescence spectra, the band-edge exciton (A) from the exciton (B) involving holes in a pair of deeper valence bands, degenerate at Γ, with a splitting that agrees with both μARPES data and the results of DFT modeling. Due to the difference in symmetry between these two valence bands, light emitted by the A-exciton should be predominantly polarized perpendicular to the plane of the two-dimensional crystal, which we have verified for few-layer InSe crystals.
Item Type: | Journal Article | |||||||||||||||||||||||||||||||||
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Subjects: | Q Science > QC Physics | |||||||||||||||||||||||||||||||||
Divisions: | Faculty of Science, Engineering and Medicine > Science > Physics | |||||||||||||||||||||||||||||||||
Library of Congress Subject Headings (LCSH): | Photoelectron spectroscopy -- Research, Photoemission, Oxide minerals -- Synthesis | |||||||||||||||||||||||||||||||||
Journal or Publication Title: | ACS Nano | |||||||||||||||||||||||||||||||||
Publisher: | American Chemical Society | |||||||||||||||||||||||||||||||||
ISSN: | 1936-0851 | |||||||||||||||||||||||||||||||||
Official Date: | 26 February 2019 | |||||||||||||||||||||||||||||||||
Dates: |
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Volume: | 13 | |||||||||||||||||||||||||||||||||
Number: | 2 | |||||||||||||||||||||||||||||||||
Page Range: | pp. 2136-2142 | |||||||||||||||||||||||||||||||||
DOI: | 10.1021/acsnano.8b08726 | |||||||||||||||||||||||||||||||||
Status: | Peer Reviewed | |||||||||||||||||||||||||||||||||
Publication Status: | Published | |||||||||||||||||||||||||||||||||
Access rights to Published version: | Restricted or Subscription Access | |||||||||||||||||||||||||||||||||
Date of first compliant deposit: | 10 October 2019 | |||||||||||||||||||||||||||||||||
Date of first compliant Open Access: | 24 January 2020 | |||||||||||||||||||||||||||||||||
RIOXX Funder/Project Grant: |
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