Effect of quantum confinement on electronic and dielectric properties of niobium dichalcogenides NbX2 (X=S, Se, Te)

We have performed first principle pseudopotential and numerical pseudoatomic orbitals basis set based calculations to study the influence of slab thickness on the electronic and dielectric properties of niobium dichalcogenides (S, Se, Te). We have taken bulk, mono, bi, quadri and six-layer setup for this study. The importance of semicore states in Nb pseudopotential has been investigated. Electronic band structures of studied materials show splitting of the bands of the order of 0.7 eV at Γ point due to interlayer interactions for all the materials under consideration. Electron energy loss spectra (EELS) for in-plane polarization shows 0.65 eV (6.5 eV), 0.75 eV (7.5 eV) and 0.55 eV (6.9 eV) red shift in energy of π(π + σ) plasmons peaks respectively for NbS2, NbSe2 and NbTe2 by reducing the slab thickness from bulk to monolayer, while for out-of-plane polarization, approximately ≈6.3 eV red shift in π + σ plasmons peak has been found for all the studied materials on reducing the number of layers from bulk to monolayer. Interband transitions energies are found to remain same irrespective of the quantum confinement effect for all the dichalcogenides of Nb with an additional interband transition at 3.2 eV for NbTe2.