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 τ-point, forming an indirect gap, with the conduction band edge known to be at the τ-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 τ, 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.

Indirect to direct gap crossover in two-dimensional InSe revealed by angle-resolved photoemission spectroscopy / Hamer, M. J.; Zultak, J.; Tyurnina, A. V.; Zolyomi, V.; Terry, D.; Barinov, A.; Garner, A.; Donoghue, J.; Rooney, A. P.; Kandyba, V.; Giampietri, A.; Graham, A.; Teutsch, N.; Xia, X.; Koperski, M.; Haigh, S. J.; Fal'Ko, V. I.; Gorbachev, R. V.; Wilson, N. R.. - In: ACS NANO. - ISSN 1936-0851. - 13:2(2019), pp. 2136-2142. [10.1021/acsnano.8b08726]

Indirect to direct gap crossover in two-dimensional InSe revealed by angle-resolved photoemission spectroscopy

Giampietri A.;
2019

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 τ-point, forming an indirect gap, with the conduction band edge known to be at the τ-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 τ, 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.
2019
2D materials; ARPES; Density functional theory; Indium selenide; Photoluminescence; Spin-orbit coupling
01 Pubblicazione su rivista::01a Articolo in rivista
Indirect to direct gap crossover in two-dimensional InSe revealed by angle-resolved photoemission spectroscopy / Hamer, M. J.; Zultak, J.; Tyurnina, A. V.; Zolyomi, V.; Terry, D.; Barinov, A.; Garner, A.; Donoghue, J.; Rooney, A. P.; Kandyba, V.; Giampietri, A.; Graham, A.; Teutsch, N.; Xia, X.; Koperski, M.; Haigh, S. J.; Fal'Ko, V. I.; Gorbachev, R. V.; Wilson, N. R.. - In: ACS NANO. - ISSN 1936-0851. - 13:2(2019), pp. 2136-2142. [10.1021/acsnano.8b08726]
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11573/1550090
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