Electronic excitations inSnS2: An electron-energy-loss-spectroscopy study

A reflection-electron-energy-loss spectroscopy study of 2H-${\mathrm{SnS}}_2$ single crystals is presented. Low-energy losses, traditionally attributed to single-electron transitions, are shown here to be of a collective character. Their interpretation as collective eigenmodes of a multiplasma electronic system is demonstrated with the aid of a simplified two-plasma model, which yields a remarkably good agreement with the experimental data. Within this framework, some of the distinct differences between the various modes are emphasized. Their hybrid (bulk and surface) behavior, as well as the physical origin of the shifts of electron losses, with respect to optically detected peaks, are coherently explained. A comparison with complementary ${\mathrm{SnS}}_2$ studies is also used for the identification of single-electron transitions from core levels. The existence of a tightly bound exciton, localized on the Sn atoms, is confirmed. Its binding energy is determined here as 1.5 eV, a value extracted from comparing various techniques having different sensitivity to local effects.