Electrons and phonons in single layers of hexagonal indium chalcogenides fromab initiocalculations

Abstract

We use density functional theory to calculate the electronic band structures, cohesive energies, phonon dispersions, and optical absorption spectra of two-dimensional In2X2 crystals, where X is S, Se, or Te. We identify two crystalline phases (α and β) of monolayers of hexagonal In2X2, and show that they are characterized by different sets of Raman-active phonon modes. We find that these materials are indirect-band-gap semiconductors with a sombrero-shaped dispersion of holes near the valence-band edge. The latter feature results in a Lifshitz transition (a change in the Fermi-surface topology of hole-doped In2X2) at hole concentrations nS = 6.86×10 13 cm −2 , nSe = 6.20×10 13 cm −2 , and nTe = 2.86 × 10 13 cm −2 for X=S, Se, and Te, respectively, for α-In2X2 and nS = 8.32 × 10 13 cm −2 , nSe = 6.00 × 10 13 cm −2 , and nTe = 8.14 × 10 13 cm −2 for β-In2X2.

Electronic and optical properties of two-dimensional InSe from a DFT-parametrized tight-binding model

“…While in its bulk form InSe [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36] is a direct gap semiconductor 37 , its electronic structure undergoes significant changes upon exfoliation to few-layer or monolayer thickness, with particularly interesting optical properties observed in recent experiments 1,38 . Density functional theory (DFT) calculations for single layer crystals of InSe 39,40 predict a large increase in the band gap as compared to bulk crystals, with the valence band maximum slightly shifted from the Γ point. Despite being a van der Waals layered material, bulk InSe has a light effective mass for electrons in the conduction and valence band across the layers.…”

“…Despite being a van der Waals layered material, bulk InSe has a light effective mass for electrons in the conduction and valence band across the layers. Therefore, it is expected that the band gap 18,39,41 and related physical properties of few-layer InSe will exhibit a strong dependence on the number of layers.…”

Electronic and thermoelectric properties of van der Waals materials with ring-shaped valence bands

“…In real III-VI materials, there is anisotropy in the Fermi surfaces, and a 6th order, angular dependent polynomial expression is provided by Zólyomi et al that captures the low-energy anisotropy 3,4 . To obtain physical insight with closed form expressions, we consider a 4th order analytical form for an isotropic Mexican hat dispersion…”

“…Another qualitative change that occurs in a number of 2D materials is the inversion of the parabolic dispersion at a band extremum into a 'Mexican hat' dispersion. [2][3][4] Mexican hat dispersions are also referred to as a Lifshiftz transition 3,5,6 , an electronic topological transition 7 or a camel-back dispersion 8,9 .…”

Ternary CuIn7Se11: Towards Ultra‐Thin Layered Photodetectors and Photovoltaic Devices

“…Previous studies have shown, however, that elemental composition plays an important role in defining the physical properties of 2D materials. For example, binary systems and their alloys exhibit a variety of behaviors and modification spaces that can be optimized to meet the requirements of applications: the 1T-2H phase transition in transition metal dichalcogenides, [24][25][26][27] similar phase transitions in InSe, [28] and the structure of junctions between MoS 2 and WS 2 , [29] are but a few examples. In sharp contrast, modification methods for the properties of single elemental systems are far more limited.…”