Strong quantization effects and tuneable near‐infrared photoluminescence emission are reported in mechanically exfoliated crystals of γ‐rhombohedral semiconducting InSe. The optical properties of InSe nanosheets differ qualitatively from those reported recently for exfoliated transition metal dichalcogenides and indicate a crossover from a direct to an indirect band gap semiconductor when the InSe flake thickness is reduced to a few nanometers.

High broad‐band photoresponsivity of mechanically formed InSe–graphene van der Waals heterostructures is achieved by exploiting the broad‐band transparency of graphene, the direct bandgap of InSe, and the favorable band line up of InSe with graphene. The photoresponsivity exceeds that for other van der Waals heterostructures and the spectral response extends from the near‐infrared to the visible spectrum.

The electronic band structure of van der Waals (vdW) layered crystals has properties that depend on the composition, thickness and stacking of the component layers. Here we use density functional theory and high field magneto-optics to investigate the metal chalcogenide InSe, a recent addition to the family of vdW layered crystals, which transforms from a direct to an indirect band gap semiconductor as the number of layers is reduced. We investigate this direct-to-indirect bandgap crossover, demonstrate a highly tuneable optical response from the near infrared to the visible spectrum with decreasing layer thickness down to 2 layers, and report quantum dot-like optical emissions distributed over a wide range of energy. Our analysis also indicates that electron and exciton effective masses are weakly dependent on the layer thickness and are significantly smaller than in other vdW crystals. These properties are unprecedented within the large family of vdW crystals and demonstrate the potential of InSe for electronic and photonic technologies.

We demonstrate that β-In 2 Se 3 layers with thickness ranging from 2.8 to 100 nm can be grown on SiO 2 /Si, mica and graphite using a physical vapour transport method. The β-In 2 Se 3 layers are chemically stable at room temperature and exhibit a blue-shift of the photoluminescence emission when the layer thickness is reduced, due to strong quantum confinement of carriers by the physical boundaries of the material. The layers are characterised using Raman spectroscopy and x-ray diffraction from which we confirm lattice constants c = 28.31 ± 0.05 Å and a = 3.99 ± 0.02 Å. In addition, these layers show high photoresponsivity of up to ∼2 × 10 3 A W −1 at λ = 633 nm, with rise and decay times of τ r = 0.6 ms and τ d = 2.5 ms, respectively, confirming the potential of the as-grown layers for high sensitivity photodetectors.

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layers, [ 4,5 ] while metal chalcogenides such MoS 2 and WSe 2 are attracting increasing interest as optoelectronic materials. [ 3,[7][8][9][10] It is now established that these dichalcogenides have a thickness-dependent electronic band structure, including a transition from indirect-to direct-band gap for single monolayers, a property that has stimulated recent studies of photoconductivity and photovoltaic effects. [7][8][9][10] By exploiting the ease with which atomically thin layers of these materials can be produced by mechanical exfoliation, it is now possible to assemble van der Waals heterostructures layer by layer, with properties that are quite distinct from those of the bulk starting materials. Among the vdW crystals, the III-VI layered semiconductors, such as GaSe and InSe, provide an important class of direct-band gap semiconductors. [11][12][13][14][15][16] Recent work has included the exfoliation [ 12 ] and growth by a vapor phase technique [ 13 ] of thin fi lms of GaSe, the demonstration of strong quantum confi nement effects in InSe, [ 14,15 ] whose direct-band gap can be tuned in the near infrared spectral range when the crystals are exfoliated into nanometer-thick fl akes, [ 14 ] and the direct-indirect band gap crossover occurring in the single monolayer limit. [ 16 ] These properties differ from those of other vdW crystals (e.g., MoS 2 , WS 2 …), which become direct only as single monolayers. [ 17,18 ] Despite their promise as optoelectronic materials, electroluminescence (EL) has to date not been reported in the III-VI layered crystal junctions, and, to our knowledge, has not yet been reported for any vdW heterostructure fabricated using exfoliation and mechanical adhesion between layered crystals.In this work we demonstrate room temperature electroluminescence from van der Waals semiconductor junctions with atomically fl at interfaces, which are fabricated by exfoliation and direct mechanical adhesion of III-VI layered crystals. Homojunction diodes formed from layers of p -and n -type InSe exhibit EL at energies ( hv ∼ 1.23 eV) close to the band gap energy of InSe ( E g = 1.26 eV). In contrast, heterojunction diodes formed by combining layers of p -type GaSe and n -type InSe emit photons at lower energies ( hv = 1.1-1.2 eV), which we attribute to the generation of spatially indirect excitons and a staggered valence band lineup for the holes at the GaSe/InSe interface. Our results demonstrate the technological potential

Van der Waals (vdW) layered crystals and heterostructures have attracted substantial interest for potential applications in a wide range of emerging technologies. An important, but often overlooked, consideration in the development of implementable devices is phonon transport through the structure interfaces. Here we report on the interface properties of exfoliated InSe on a sapphire substrate. We use a picosecond acoustic technique to probe the phonon resonances in the InSe vdW layered crystal. Analysis of the nanomechanics indicates that the InSe is mechanically decoupled from the substrate and thus presents an elastically imperfect interface. A high degree of phonon isolation at the interface points toward applications in thermoelectric devices, or the inclusion of an acoustic transition layer in device design. These findings demonstrate basic properties of layered structures and so illustrate the usefulness of nanomechanical probing in nanolayer/nanolayer or nanolayer/substrate interface tuning in vdW heterostructures.

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