Ab initio study of alkali diffusion and intercalation on transition metal dichalcogenides
The aim of this work was to understand how alkali metal atoms intercalate into transition metal dichalcogenides (TMDCs). To this end, a detailed investigation into the geometrical and electronic effects on TMDCs caused by adsorption and intercalation of alkali atoms was carried out by means of ab initio density-functional theory. The geometrical and electronic properties of clean TiSe_2 (0001) surface were studied. Also, the adsorption sites and diffusion of alkali atoms on TiSe_2 (0001) surface were computed and compared with those obtained at the van der Waals gap. The electron charge distribution caused by alkali atoms on TiSe_2 (0001) surface was analysed. The alkali coverage dependence of the physical properties adsorption energy, work function, and valence electron distribution was also studied. The question of whether the alkali atoms can reach the van der Waals gap from the plain surface through the atomic planes was addressed. Because of the huge energy barrier involved in this process (3.81 eV), this type of intercalation can be excluded. Thus, the experimentally found intercalation processes must be associated with defects, such as steps or dislocations at the surface. A model of alkali intercalation into TMDCs was obtained, namely via a (1 0 -1 0) step.