Electrons and phonons in single layers of hexagonal indium chalcogenides from ab initio calculations

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=6.86×1013 cm-2, nSe=6.20×1013 cm-2, and nTe=2.86×1013 cm-2 for X=S, Se, and Te, respectively, for α-In2X2 and nS=8.32×1013 cm-2, nSe=6.00×1013 cm-2, and nTe=8.14×1013 cm-2 for β-In2X2.