Electronegativity, phase transition, and ferroelectricity of TeSe₂ few-layers

We use first-principles calculations to demonstrate that γ TeSe₂ few-layers (FLs) are significantly more stable than α and β FLs due to the difference in the electronegativity of two kinds of atoms, while γ Te FLs are not due to the unfavorable multivalency of Te atoms. The quasiparticle single-shot G₀W₀ band gaps are 1.13 and 2.30 eV for γ and β monolayers (MLs), respectively. Therefore, they will be useful for optoelectronics operating at room temperature, which is further supported by their dynamic and thermal stability. The γ ML and bilayer (BL) are expected to undergo phase transitions to β ML and α BL under hole doping. Furthermore, the ionicity brings about spontaneous electric polarization in the α BL that is approximately 60% larger than that in the α Te BL. Its ferroelectricity (FE) is comparable to that of SnTe ML, the only 2D FE material experimentally identified up to now. The polarization can be further enhanced by more than 75% under uniaxial tensile strain.