Infrared-to-violet tunable optical activity in atomic films of GaSe, InSe, and their heterostructures

Abstract

Two-dimensional semiconductors - atomic layers of materials with covalent intra-layer bonding and weak (van der Waals or quadrupole) coupling between the layers - are a new class of materials with great potential for optoelectronic applications. Among those, a special position is now being taken by post-transition metal chalcogenides (PTMC), InSe and GaSe. It has recently been found that the band gap in 2D crystals of InSe more than doubles in the monolayer compared to thick multilayer crystals, while the high mobility of conduction band electrons is promoted by their light in-plane mass. Here, we use Raman and PL measurements of encapsulated few layer samples, coupled with accurate atomic force and transmission electron microscope structural characterisation to reveal new optical properties of atomically thin GaSe preserved by hBN encapsulation. The band gaps we observe complement the spectral range provided by InSe films, so that optical activity of these two almost lattice-matched PTMC films and their heterostructures densely cover the spectrum of photons from violet to infrared. We demonstrate the realisation of the latter by the first observation of interlayer excitonic photoluminescence in few-layer InSe-GaSe heterostructures. The spatially indirect transition is direct in k-space and therefore is bright, while its energy can be tuned in a broad range by the number of layers.

arXiv comment8 pages 4 figures

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• Raman spectroscopy of GaSe and InSe post-transition metal chalcogenides layers
• Polytypism in Few-Layer Gallium Selenide
• Evidence of Direct Electronic Band Gap in two-dimensional van der Waals Indium Selenide crystals
• Review of Nanolayered Post-transition Metal Monochalcogenides: Synthesis, Properties, and Applications
• Probing Hyperbolic and Surface Phonon-Polaritons in 2D materials using Raman Spectroscopy
• Van Der Waals Heteroepitaxy of GaSe and InSe, Quantum Wells and Superlattices
• Resonance and antiresonance in Raman scattering in GaSe and InSe crystals
• Band Gap Opening in Bilayer Graphene-CrCl${}_3$/CrBr${}_3$/CrI${}_3$ van der Waals Interfaces
• Tunable spin-orbit coupling in two-dimensional InSe
• Thickness and twist angle dependent interlayer excitons in metal monochalcogenide heterostructures
• Structures of bulk hexagonal post-transition-metal chalcogenides from dispersion-corrected density-functional theory
• Crossover from weakly indirect to direct excitons in atomically thin films of InSe
• Light sources with bias tunable spectrum based on van der Waals interface transistors
• Quasiparticle and Excitonic Structures of Few-layer and Bulk GaSe: Interlayer Coupling, Self-energy, and Electron-hole Interaction