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

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 (and) 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 nₒ=6. 8610^13 cm^-2, nₒ₄=6. 2010^13 cm^-2, and nₓ₄=2. 8610^13 cm^-2 for X=S, Se, and Te, respectively, for -In₂X₂ and nₒ=8. 3210^13 cm^-2, nₒ₄=6. 0010^13 cm^-2, and nₓ₄=8. 1410^13 cm^-2 for -In₂X₂.