Surface Science Investigations of Intercalation Reactions with Layered Metal Dichalcogenides

Abstract

Intercalation is an important solid state reaction, which can be found for many materials providing open structural units in their crystal structure. It involves two species, one being the host, the other the guest species which can be atoms, molecules or even solvation complexes. Intercalation reactions combining different guest species with different host materials have intensively been investigated in the last decades because of their fundamental interest and because of their possible technological application. Prototype host materials are the metal chalcogenides with a layered structure. The crystal structure of the layered metal dichalcogenides (LMDC; a more often used acronym is TMDC for transition metal dichalcogenide) are characterized by two-dimensional sandwich units which are separated from each other by the so-called van der Waals gap. This bonding geometry makes these materials highly anisotropic and in extreme cases two-dimensional. There is a large number of known layered chalcogenides sharing the same or similar crystal structure but composed of different elements. Thus a large variety of properties exist making these materials interesting from the theoretical point of view, e.g. charge density waves (Wilson et al., 1974; Wilson et al., 1975; Burdett, 1996; Vescoli et al., 1998), superconductivity (Nishio et al., 1994; Cai et al., 1996; Motizuki et al., 1996), two-dimensional electronic structure (Wilson et al., 1969; Grasso, 1986; Friend et al., 1987; Hughes et al., 2000b), van der Waals epitaxy (Koma, 1992; Jaegermann et al, 2000). Also various practical applications make LMDCs attractive. Due to their low shear resistance LMDCs are used as solid-state lubricants (Zonneville et al., 1988; Tenne et al., 1993; Rapoport et al., 1997; Cohen et al., 1998; Cohen et al., 1999; Golan et al., 1999). Some of them are semiconductors with a direct band gap of 1.3-2.0 eV, perfectly matching the solar spectrum with a very high absorption coefficient, in the order of 105 cm-1 (Wilson et al., 1969; Lee, 1976; Grasso et al., 1986). and have been investigated as absorber materials for solar cells (Tributsch, 1977; Aruchamy, 1992). Due to the reactivity of the plane edges TMDCs proved to be very efficient in heterogeneous catalysis. MoS2 hydrodesulfurization catalysis is one of the most widely used catalytic systems worldwide (Somorjai, 1994). By intercalation the amount of guest species in a host can be arbitrarily varied, so that the stoichiometry can be controlled in a defined manner. With intercalation structural changes occur which are accompanied by changes of the physical properties. Thus it is often possible to tailor the physical properties which can be finely tuned by variation of host and guest properties and their stoichiometric ratios.KeywordsFermi LevelWork FunctionIntercalation CompoundValence Band SpectrumGuest SpeciesThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.