Infrared-Reflectance Spectra of Layered Group-IV and Group-VI Transition-Metal Dichalcogenides

Abstract

Infrared-reflectivity spectra have been obtained for a large number of group-IV (Ti, Zr, Hf) and group-VI (Mo, W) transition-metal dichalcogenides. Three distinct types of spectra have been identified: In Zr${\mathrm{S}}_{2}$, Hf${\mathrm{S}}_{2}$, and Hf${\mathrm{Se}}_{2}$, very strong reststrahlen with effective charges ${e}_{T}^{*}\ensuremath{\sim}4e$; in Mo${\mathrm{S}}_{2}$, Mo${\mathrm{Se}}_{2}$, W${\mathrm{S}}_{2}$, and W${\mathrm{Se}}_{2}$, weaker reststrahlen with ${e}_{T}^{*}\ensuremath{\sim}0.5e$; and in Ti${\mathrm{S}}_{2}$, Zr${\mathrm{Te}}_{2}$, and Hf${\mathrm{Te}}_{2}$, plasmon-type reflection bands corresponding to free-carrier densities in excess of ${10}^{20}$/${\mathrm{cm}}^{3}$. These observations, coupled with considerations of ion size, have led to a new description of the chemical bonding in these layered materials that is not consistent with a simple rigid-band scheme.