Abstract
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.
Highlights
The electronic band structure of van der Waals layered crystals has properties that depend on the composition, thickness and stacking of the component layers
Still more work is needed to deepen our understanding of the unique electronic band structure of this 2D crystal, which underpins future research, and applications of InSe and its competitiveness with other van der Waals (vdW) crystals, such as transition metal dichalcogenides (TMDCs) and black phosphorus (bP)
We demonstrate PL emission from InSe layers exfoliated in air over an extended range of layer thicknesses, down to L = 2 layers, a tuneable band gap energy ranging from ~1.3 eV (L > 20 layers) to 2 eV (L = 2 layers), and quantum dot-like optical emissions distributed over a wide range of energy
Summary
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. Near the VB edge, the constant energy contours have the form of a ring in k-space and the 2D density of states develops a one-dimensional Van Hove singularity[7,8,9], a property that could lead to tuneable magnetism, superconductivity, and enhanced thermoelectricity[10,11] These findings have the potential to extend further the prospects of InSe as a material for several technologies and devices, which range from field effect transistors (FETs)[12,13] with record high room temperature electron mobility (μ = 0.2 m2V−1s−1)[13] to bendable[14] and high-gain[15] photodetectors, image sensors[16], and photon sources[17]. These properties combined with the chemical stability in air of InSe expands significantly the range of potential applications of 2D vdW crystals
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