Tunable Optical Properties of Cu/VSe₂ from the Visible to Terahertz Spectral Range: A First-Principles Study

In this study, Density Functional Theory (DFT) and Density Functional Tight-Binding (DFTB) calculations were used to study two different interfaces of Cu/VSe₂ as well as four nanodiodes of VSe₂ bulk including/excluding the Cu layer. We calculated the electronic and optical properties of two systems of two Cu/VSe₂ in which Cu atoms are positioned on the top and at the corner of the VSe₂ monolayer lattice. The electronic band structure calculations revealed that the metallic properties of the VSe₂ monolayer did not change with the interface of Cu atoms; however, the peak around the Fermi level (E_F) in Cu/VSe₂(Top) shifted downward to lower energies. The optical properties showed that in the visible range and the wavelengths related to the interband transition/intraband excitation of Cu atoms, the enhancement of Re(ω) values could be observed for both Cu/VSe₂(Top) and Cu/VSe₂(Corner) nanostructures, while in infrared/terahertz ranges, less/more negative values of Re(ω) were predicted. Through the effect of Cu atoms on the VSe₂ monolayer, the intensity of the peaks in the Im(ω) part of the dielectric constant was increased from 0.2 THz for Cu@VSe₂(Top) and 2.9 THz for Cu@VSe₂(Corner) instead of the zero constant line in the pure system of VSe₂. Refractive index (n) calculations indicated the higher indices at 5.4 and 4.6 for Cu/VSe₂(Top) and Cu@VSe₂(Corner), respectively, in comparison to the value of 2.9 for VSe₂. Finally, DFTB calculations predicted higher current values from I(V) characteristic curves of Au/Cu/VSe₂/Au and Ag/Cu/VSe₂/Ag nanodiodes concerning two other devices without the presence of the Cu layer.