C. A. Papageorgopoulos

Li intercalation across and along the van der Waals surfaces of MoS2(0001)

Abstract During Li deposition on single crystals of MoS2 at RT, Li is intercalated into MoS2 while two reactions take place: (a) an intercalation reaction according to the rigid band model, (b) an intercalation reaction accompanied by a phase transition 2H → 1T. The latter reaction dominates during Li intercalation across the van der Waals planes, while it is negligible and the first reaction prevails when (i) Li is intercalated along the van der Waals planes, (ii) the substrate temperature is elevated, and (iii) the Li deposition flux is relatively low. A relation of the minimum flux of Li deposition, at a certain temperature, across the van der Waals planes of MoS2 necessary to prevent the phase transition and to dominate the rigid band intercalation reaction, is given as a function of the distance between successive molecular layers and the diffusion coefficient of Li.

Adsorption of Li on Ni(110) surfaces at low and room temperature

Abstract We present the investigation of Li adsorption on Ni(110) surfaces at low temperature (LT) and room temperature (RT) by means of Auger electron spectroscopy, low-energy electron diffraction, thermal desorption spectroscopy and work function measurements, in ultrahigh vacuum. At RT, from the early stages of deposition (Θ≤0.1xa0ML) Li induces a (1×2) reconstruction of the Ni(110) surface. The (1×2) reconstruction is lifted at 0.5xa0ML. A further increase of Li coverage leads to a disordered surface, with an intermixing of Li adatoms and the Ni substrate with a tendency toward Li–Ni alloy formation. A gradual heating of the Li-covered Ni(110) surface gives an Li(3×1), an Li(5×1) structure and the (1×2) reconstruction before the complete desorption of Li. Near saturation (Θ≈2xa0ML), Li forms a nearly metallic overlayer. At LT (150xa0K), deposition of Li up to 1xa0ML gives a sequence of c(2×2), (7×2), (3×2), (4×2), (5×4) and finally (1×1) structures, which have been determined using structure factor calculations. These structures are attributed to a continuous stretching of deposited Li atoms along the [110] direction of the Ni(110) surface. The Li–Ni interaction is a thermally activated process and decreases the stability of the Ni surface, leading to its reconstruction at RT. However, at LT the Li–Li interaction dominates and the effect of Li adatoms on the substrate structure is limited. The change of the (1×2) reconstruction to the ideal (1×1) bulk-like surface occurs at the beginning of the transition of ionic to metallic bonding of the Li overlayer.

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.

Adsorption and decomposition of C60 on Ni(110) surfaces

Abstract In the present paper we study the adsorption of C 60 on Ni(1xa01xa00) surfaces at RT and 650 K, as well as the desorption/decomposition process. The investigation took place in UHV by means of Auger electron spectroscopy, low energy electron diffraction, thermal desorption spectroscopy and work function (WF) measurements. The observed overlayer structures during C 60 deposition at 650 K are in complete agreement with reported results. The sticking coefficient is nearly the same for both RT and 650 K. Heating of the 1 ML C 60 -covered Ni(1xa01xa00) surface, however, at 750 K, causes a partial fragmentation of C 60 to a 2D graphite-like layer, while the remaining C 60 molecules are rearranged in the (5×3) structure, in contrast to reported results. Further heating to 800 K results in the complete fragmentation of C 60 accompanied by carbon desorption, possibly in molecular C n ( n >10) form. Above 800 K, the C of the graphite-like layer is desorbed in both atomic and molecular form. The WF at saturation coverage was found to be 5.61 eV for both deposition at RT and 650 K. This value is substantially greater than the average value of 5 eV, which has been suggested to hold for C 60 overlayer systems, regardless of the metal substrate.

Li on S covered Ni(100) surfaces

Abstract In this paper we study the interaction of Li with S-covered Ni(1xa00xa00) surfaces at room and elevated temperatures, with low energy electron diffraction, Auger electron spectroscopy, thermal desorption spectroscopy and work function measurements. On clean Ni(1xa00xa00) Li forms an order layer up to 0.5 ML, whereas at higher coverages it causes a restructuring of the surface with possible formation of Li–Ni alloy. At high coverages the Li overlayer becomes metallic. Lithium on S-covered Ni(1xa00xa00) interacts strongly and destabilizes the S–Ni bound. The Li–S interaction increases the ionicity of Li on the surface and leads to Li–S compounds formation. The Li–S compound formation give a low work function surface system on Ni(1xa00xa00). Sulphur passivates the Ni(1xa00xa00) surface against the restructuring caused by the Li deposition.

Coadsorption of cesium and elemental sulfur on Ni(100) surfaces

Abstract Adsorption of cesium on clean Ni(100) at room temperature produces an hcp metallic structure at the completion of a single layer [0.3 monolayers (ML)]. Independently of the sequence of cesium and sulfur deposition, the latter is bound directly to the Ni(100) substrate giving a (2×2) and a c(2×2) structure similar to those of sulfur alone on clean Ni(100). Cesium deposition on preheated c(2×2)-S structure changes it successively to (2×2) and (3×3) by reducing the density of the sulfur structure which is contact with the nickel. The presence of sulfur causes a disordering and dimetallization of the well-ordered uniform cesium layer on clean Ni(100). Apparently these mutual effects of the coadsorbates are due to a strong Cs–S interaction. The presence of sulfur also increases the binding energy and the amount of cesium in multilayers. The binding energy of cesium on sulfated Ni(100) was found to be 2.7xa0eV as compared with 2.8xa0eV on MoS 2 . There is no indication for the formation of any Cs–S compound.

Adsorption of Br2 on Na-intercalated layered compounds: Bromine-induced deintercalation

This work refers to a study of the behavior of Br2 adsorbed on Na-intercalated 1T-TaSe2(0001) and TiSe2(0001) surfaces at 100 K and during subsequent warming up to 300 K. The investigation was performed in UHV with LEED and photoemission with synchrotron radiation measurements. At 100 K, the bromine forms molecular multilayers on Na/TaSe2. The Br2 layer, close to the substrate, interacts weakly with Na near the surface, forming . At 300 K, part of the Br2 overlayer desorbs while the remainder on both 1T-TaSe2 and Tise2 surfaces interacts strongly with Na, leading to deintercalation of Na to the surface in the tendency to form NaBr. The formation of NaBr overlayers causes the transition of the incommensurate surface structure of 1T-TaSe2(0001) to 1×1. ESD from the Br/Na/1T-TaSe2 surface causes the restoration of the .

Coadsorption of Na and elemental S on Ni(100)

Abstract In this work we study the coadsorption of sulfur and sodium on Ni(100) surfaces at room temperature by LEED, AES, TDS and WF measurements. The adsorption of Na on clean Ni(100) produces a metallic uniform overlayer with a c(2×2) structure at a saturation coverage of 0.5xa0ML. The presence of S increases the maximum amount and binding energy of Na on Ni(100) surfaces. Sodium interacts strongly with the adsorbed S and causes the diffusion of the submerged S to the surface of Ni(100) and a tendency to form Na x S y compounds. The formation of these compounds causes a substantial decrease in the work function to a value of 1.0xa0eV.

The Behavior of Cs on S-Covered Si(100)-(2 × 1) and Si(100)-(1 × 1) Surfaces

In this work we study the adsoption of Cs on (a) clean Si(100)-(2 × 1), (b) 0.5 ML of S-covered Si(100)-(2 × 1) and (c) 1 ML of S-covered Si(100)-(1 × 1) in ultrahigh vacuum (UHV). LEED and AES measurements suggest that the array of a Cs monolayer on clean and S-covered Si(100)-(2 × 1) surfaces was that of the double layer model, according to which, half of the Cs atoms reside on the raised sites of the dimers and the other half in the troughs. However, Cs on 1 ML of S-covered Si(100)-covered Si(100)-(1 × 1) forms initially a coplanar monolayer with the Cs atoms residing only on the same kind of sites. The presence of S on the Si surfaces increases the subsequently deposited (at RT) coverage of Cs to more than 1 ML. Structural models of Cs on clean and S-covered Si(100) surfaces are proposed.

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