First-principle study of the evolution of electronic and optical properties upon Tantalum doping in 2D Vanadium Telluride

Transition metal dichalcogenides containing Tellurium (Te) have attracted a great deal of attention, primarily owing to their distinctive electronic and optical characteristics that originate from strong spin-orbit coupling and Te-Te interactions. In this study, using highly accurate quantum Monte Carlo (QMC) method, we investigated VTe₂ and TaTe₂ monolayers, along with their doped derivatives. We first optimized the geometric structures of monolayer VTe₂ and TaTe₂ using QMC method, which revealed that DFT-PBE geometry relaxation tends to overestimate their lattice parameters, compared to corresponding QMC result. Our first-principles calculations, including QMC and DFT+U, reveal an indirect-direct gap transition in Ta-doped 2H-VTe₂ monolayer, where V atoms in semi-conducting 2H-VTe₂ are substituted by Ta atoms originating from metallic TaTe₂. In addition, a flat band near the Fermi level is observed in Ta-doped 1T-VTe₂ accompanied by lattice distortion. Finally, we estimated QMC formation energies and band gaps of those doped structures to accurately describe the changes in their optical and electronic properties as function of doping concentration.