Negative Poisson's ratio and thickness-dependent optoelectronic response in two-dimensional thermoelectric TlCuSe

In one of the latest accomplishments in the field of materials for energy conversion, layered TlCuSe with a relatively high thermoelectric figure of merit has been designed and successfully fabricated. Inspired by this exciting advance, we herein conduct first-principles calculations to explore the dynamical and thermal stability, mechanical properties, and thickness dependent electronic and optical properties of TlCuSe nanosheets. Analysis of mechanical deformation reveals that TlCuSe monolayer shows a negative in-plane Poisson's ratio of −0.29 and is thus an auxetic material. This novel monolayer also exhibits an intrinsically p-type character with an appreciable hole mobility of 1528 cm²V⁻¹s⁻¹, an HSE06 indirect gap of 1.41 eV, and a multi-valley conduction band. It is found that electronic band gap in TlCuSe considerably decreases with increasing the number of layers and reaches to 0.47 eV for the bulk lattice, indicating strong quantum confinement effects. The mutli-valley character of the conduction and valence bands is also boosted in multilayer TlCuSe systems. Analysis of optical absorption of monolayer to tri-layer TlCuSe indicates that they possess remarkably large absorption coefficients within the visible and UV range of light spectrum. The acquired results provide useful information on physicochemical and electronic properties of TlCuSe nanomaterials for advanced applications.