Dimensionality-driven phonon softening and incipient charge density wave instability in TiS ₂

Abstract

Density functional theory and density functional perturbation theory are used to investigate the electronic and vibrational properties of TiS2. Within the local density approximation the material is a semimetal both in the bulk and in the monolayer form. Most interestingly we observe a Kohn anomaly in the bulk phonon dispersion, which turns into a charge density wave instability when TiS2 is thinned to less than four monolayers. Such instability, however, disappears when one calculates the electronic structure with a functional, such as the LDA+U, which returns an insulating ground state. In this situation charge-doping or strain does not bring back the charge density wave instability, whereas the formation of the TiSSe alloy does.

Two-Dimensional Metal Dichalcogenides and Oxides for Hydrogen Evolution: A Computational Screening Approach

“…in TiS 2 . 31,32 In the case of TiS 2 we found that for a 12 atoms unit cell (2X2 unit cell), the distorted and the perfectly symmetric structure differ by only ∼ 0.04 eV (∼ 0.004 eV per atom). It turns out that due to similar energy differences, the CDW phases of other compounds e.g.…”

Excitonic effects in two-dimensional TiSe2 from hybrid density functional theory

“…charge doping suppresses the instability [48,49]), it is unclear to what extent this prediction is robust or resulting partially from a cancellation of errors. We note that for TiS 2 , PBE calculations predict a metallic electronic structure for both bulk and monolayer forms and phonon softening, with imaginary frequencies in the monolayer case [51]. Experimentally, the bulk material exhibits a gap of ∼ 0.7 eV [52] and no sign of CDW or phonon softening were reported.…”

Vibrational and dielectric properties of monolayer transition metal dichalcogenides

“…In some of these compounds, our phonon band structures show Kohn anomalies due to their small or zero band gaps [136][137][138][139]. When compared to their bulk counterparts [60], the Kohn anomalies appear at M for both bulk and monolayer TiS 2 and TiSe 2 .…”