Tunable dielectric response of transition metals dichalcogenides MX2 (M=Mo,W; X=S,Se, Te): Effect of quantum confinement

The first principle calculations have been performed to study the influence of number of layers on the dielectric properties of dichalcogenides of Mo and W for in-plan (E⊥c)$(E\perp c)$ as well as out-of-plan polarization (E∥c)$(E\parallel c)$. We have taken bulk, mono, bi, four and 6-layer setup for this study. The EELS shows significant red shift in the energies of π$\pi$ plasmons, while prominent red shift has been found for the energies of (π+σ)$(\pi +\sigma )$ plasmons of all the studied materials by reducing the number of layers from bulk to monolayer limit. The ?_s${?}_s$ has been found to red shifted by 62.5% (66.3%), 48.5% (62.1%), 52.7% (66.2%), 61.7% (64.6%), 61.5% (66.7%) and 62.5% (70.5%) from bulk values of MoS₂, MoSe₂, MoTe₂, WS₂, WSe₂, WTe₂ respectively for E⊥c$E\perp c$(E∥c)$(E\parallel c)$ as one goes from bulk to monolayer of these materials. The interband transitions are found to remain independent of the number of layers, however their intensity decreases with decrease in the number of layers. The dielectric functions are highly anisotropic in low energy range and becomes isotropic in high energy range.