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

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 $e_T^{*}\sim 4e$; in Mo${\mathrm{S}}_2$, Mo${\mathrm{Se}}_2$, W${\mathrm{S}}_2$, and W${\mathrm{Se}}_2$, weaker reststrahlen with $e_T^{*}\sim 0.5e$; 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.