Electronegativity, phase transition, and ferroelectricity of TeSe 2 few-layers

We use first-principles calculations to demonstrate that gamma TeSe2 few-layers (FLs) are significantly more stable than alpha and beta FLs due to the difference in the electronegativity of two kinds of atoms, while gamma 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 gamma and beta 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 gamma ML and bilayer (BL) are expected to undergo phase transitions to beta ML and alpha BL under hole doping. Furthermore, the ionicity brings about spontaneous electric polarization in the alpha BL that is approximately 60% larger than that in the alpha 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.