Tunable dielectric properties of transition metal dichalcogenides.

Since discovery of graphene, layered materials have drawn considerable attention because of their possible exfoliation into single and multilayer 2D sheets. Because of strong surface effects, the properties of these materials vary drastically with the number of layers in a sheet. We have performed first-principles density functional based calculations to evaluate the electron energy loss spectrum (EELS) of bulk, monolayer, and bilayer configurations of several transition metal dichalcogenides, which include semiconducting as well as metallic compounds. Our investigation shows that the peaks in the EELS spectra move toward larger wavelengths (red shift) with the decrease in number of layers. The π plasmon peak shifts slightly by 0.5-1.0 eV, while a significant shift of around 5.5-13.0 eV is obtained for π + σ plasmon, when exfoliated from bulk to single-layer. This underscores the importance of the interlayer coupling on the loss spectra and the dielectric properties. Our results are found to be in very good agreement with the recent measurements performed by Coleman et al. (Science2011, 331, 568).