Thickness and defect dependent electronic, optical and thermoelectric features of WTe ₂

Transition metal dichalcogenides (TMDs) receive significant attention due to their outstanding electronic and optical properties. In this study, we investigate the electronic, optical, and thermoelectric properties of single and few layer WTe ₂ in detail utilizing first-principles methods based on the density functional theory (DFT). Within the scope of both PBE and HSE06 including spin orbit coupling (SOC), the simulations predict the electronic band gap values to decrease as the number of layers increases. Moreover, spin-polarized DFT calculations combined with the semi-classical Boltzmann transport theory are applied to estimate the anisotropic thermoelectric power factor (Seebeck coefficient, S) for WTe ₂ in both the monolayer and multilayer limit, and S is obtained below the optimal value for practical applications. The optical absorbance of WTe ₂ monolayer is obtained to be slightly less than the values reported in literature for 2H TMD monolayers of MoS ₂, MoSe ₂, and WS ₂. Furthermore, we simulate the impact of defects, such as vacancy, antisite and substitution defects, on the electronic, optical and thermoelectric properties of monolayer WTe ₂. Particularly, the Te-O ₂ substitution defect in parallel orientation yields negative formation energy, indicating that the relevant defect may form spontaneously under relevant experimental conditions. We reveal that the electronic band structure of WTe ₂ monolayer is significantly influenced by the presence of the considered defects. According to the calculated band gap values, a lowering of the conduction band minimum gives rise to metallic characteristics to the structure for the single Te(1) vacancy, a diagonal Te line defect, and the Te(1)-O ₂ substitution, while the other investigated defects cause an opening of a small positive band gap at the Fermi level. Consequently, the real (ε₁(ω)) and imaginary (ε₂(ω)) parts of the dielectric constant at low frequencies are very sensitive to the applied defects, whereas we find that the absorbance (A) at optical frequencies is less significantly affected. We also predict that certain point defects can enhance the otherwise moderate value of S in pristine WTe ₂ to values relevant for thermoelectric applications. The described WTe ₂ monolayers, as functionalized with the considered defects, offer the possibility to be applied in optical, electronic, and thermoelectric devices.