D. Tonti

Exchange reaction between Li and Na intercalated into TiS2

Single crystals of TiS2(0001) were intercalated sequentially by Li and Na. The intercalation occurred across the Van der Waals planes, by deposition onto the basal plane of TiS2. The investigation took place in UHV by soft X-ray photoelectron spectroscopy. Deposition of Na onto Li-intercalated TiS2(0001) caused the Li to move further into the bulk of the substrate. However, deposition of Li onto Na-intercalated TiS2(0001) forced Na to move out towards the surface by an exchange mechanism. These interactions between intercalated Li and Na into TiS2(0001) and their subsequent behaviour have been explained by a correlation of thermodynamic (Gibbs free energy), kinetic (diffusion coefficient) and electrostatic effects.

In-situ photoelectron spectroscopy study of a TiS2 thin film cathode in an operating Na intercalation electrochemical cell

TiS2 thin films were prepared and intercalated in UHV either chemically or electrochemically and investigated by photoelectron spectroscopy. The chemical reaction was induced by Na evaporation. For the electrochemical reaction, the film was deposited on a Na solid electrolyte and the voltage between the TiS2 and a graphite layer on the back of the plate was controlled during PES investigations.With both methods the same effects on the substrate are observed. The Na Auger peak appears and increases at a kinetic energy typical for intercalated Na. Due to the electron transfer to the conduction band an increase of the electron density at the Fermi level is clearly observed. The progressive filling of higher energy states shifts the Fermi level as reference for the PES spectra, and as a result the S 2p core levels and valence bands are shifted to higher binding energies. A shoulder appears at the lower binding energy side of the Ti 2p peak, indicating a higher negative charge density on the Ti atoms.It is also shown how the in-situ electrochemical intercalation allows experimentally to extrapolate the variation of the ionic contribution to the battery voltage.

Electronic passivation of Si(111) by Ga–Se half-sheet termination

A Si(111):GaSe van der Waals surface is prepared using sequential deposition of Ga and Se at elevated temperature on a Si(111)-7×7 surface. Surface properties were investigated by soft x-ray photoelectron spectroscopy and low-energy electron diffraction. The Si(111)-1×1:GaSe surface remains with electronic surface potentials near flatband condition.

Na and Cl2 interaction on 1T and 2H-TaSe2 (0001) surfaces

In this paper we study the interaction ofCl2and Na on 1T–TaSe2and 2H–TaSe2(0001)surfaces in the temperature range of 100–300 K. The experiments are performed in UHV with the use of LEED and SXPS by synchrotron radiation measurements. Deposition of Na onCl2-covered 1T–TaSe2at 100 K forms initially a

Alkali displacements in intercalated 1T-TaSe2

{\rm Na}\mbox{-}{\rm Cl}_2^-

Surface Science Investigations of Intercalation Reactions with Layered Metal Dichalcogenides

, which with increasing temperature to 300 K leads to NaCl formation. Adsorption ofCl2on Na-intercalated 1T and 2H–TaSe2surfaces at 100 K formsCl2multilayers. The firstCl2layer, in contact with the substrate, interacts with the Na near the surface and forms

Interaction between Li and Na intercalated into 1T-TaSe2 layer compounds

{\rm Na}\mbox{-}{\rm Cl}_2^-

A SYNCHROTRON RADIATION STUDY OF THE FORMATION OF CuxSey AND NaxCuySez THIN FILMS ON Cu SUBSTRATES: Cl2-INDUCED OUT-DIFFUSION OF Na

. Warming up to 300 K leads to partial desorption ofCl2, while the remaining chlorine interacts strongly with Na, causing the deintercalation of Na to the surface in the tendency to form NaCl. The intercalation–deintercalation process takes place across the van der Waals planes and it is much faster on 2H than on 1T–TaSe2, which is attributed to the different crystal structure of 2H and 1T ofTaSe2.

Cesium deintercalation by Li or Na deposited on 1T-TaSe2 (0001) surfaces

Single crystals of 1T-TaSe2 have been intercalated with different alkali metals by deposition in ultra-high vacuum onto in situ-cleaved (0001) surfaces. In a second step a different alkali metal, or Cl2, has been deposited on top. The interactions among the deposited species and the substrate have been investigated using soft x-ray photoelectron spectroscopy (SXPS). Li and Na appear to compete in the intercalation process. Li replaces Na, by pushing it deeper into the crystal. Cs on Li:TaSe2 does not intercalate, but stays on top, repelling intercalated Li+ deeper inside. For Li on Cs:TaSe2 an exchange reaction takes place, and Cs is deintercalated. The same effect is induced by Na deposited on intercalated Cs. 1T→2H phase transition for TaSe2 has been observed only for Li deposition. Cl2 deposition on Na intercalated substrate induced deintercalation of Na. The experimental results are discussed in relation to thermodynamic, electronic and electrostatic effects.

Synchrotron radiation studies on the growth of TSe2 (T=Ta, Ti) thin films on Ta substrates: intercalation and de-intercalation of Na

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.