Indirect to Direct Gap Crossover in Two-Dimensional InSe Revealed by Angle-Resolved Photoemission Spectroscopy.
ACS Nano
; 13(2): 2136-2142, 2019 Feb 26.
Article
en En
| MEDLINE
| ID: mdl-30676744
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.
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1
Colección:
01-internacional
Base de datos:
MEDLINE
Tipo de estudio:
Prognostic_studies
Idioma:
En
Revista:
ACS Nano
Año:
2019
Tipo del documento:
Article
País de afiliación:
Reino Unido