The properties of heterogeneous interfaces are modified essentially by the presence of nanoparticles. We provide a model and give spectroscopic evidence that nanoscale clusters exist which have a metallic core and a shell of an almost perfect oxide. Such clusters produce a large dipole moment which manifests itself as shifts in core levels as seen by photoelectron spectroscopy, as well as non-Ohmic rectifying behavior in the device electrical properties. PACS numbers: 73.20.Hb, 79.60.Dp, 82.80.Pv The existence of nanoparticles at heterogeneous interfaces, the associate dipole moment as well as its effect on the electronic structure, and thus significance to electronic applications are addressed. The nanoparticle dipole moment acts as the driving force for a variety of electrical and physical mechanisms, which have been otherwise ascribed to the properties of bulk oxidic semiconductors and to particular reactions of metallic interface contacts such as Schottky barriers [1,2]. The dipole contribution at inhomogeneous interfaces is of importance in the fields of catalysis to discussions of strong-metal support interactions, in material science and device technology to explain nonlinear I͑V ͒ characteristics, and, in photoelectron spectroscopy (PES) it provides a novel explanation of the observed core level shifts.Nanoscale particles have amazingly different properties from their bulk counterparts and consequently are of considerable technological interest [3][4][5][6][7][8]. We give spectroscopic evidence that nanoscale particles show up at the interfaces of semiconducting oxide materials and are formed when metallic clusters interact with a spurious amount of oxygen. A high interface dipole moment is produced by the outer rim of an oxidic shell covering a nanoscale metal cluster. The size of the clusters influences the value of the dipole moment formed.Our model is based on the existence of metal clusters ͑,10 nm͒ wrapped in an oxidic coating. Such nanoclusters have a dipole moment caused by charge differences between the metallic core of the nanoparticle and its substrate or environment (see Fig. 1). In PES spectra a dipole moment arising from the interface of the nanoscale particles will shift its spectroscopic contributions against those being in equilibrium with the substrate.Two mechanisms are responsible for the nanoscale dipole moment to become active. The first mechanism represents a structural contribution to the interface dipole and depends on the oxide thickness and quality. It arises, as within the oxidic shell the oxide close to the metallic core is almost stoichiometric, whereas at the rim of that oxide layer there is a considerable increase in the number of defects. In a similar manner a dipole moment results from the softening of the outer oxygen bonds. The ability of oxidic layers to be considerably relaxed has been addressed recently. In epitaxial oxide layers formed on Fe single crystals a relaxation of the first layer up to 50% from the ideal bulk values has been established by lowenergy elec...

No abstract

No abstract

By means of synchrotron radiation photoemission spectroscopy, we have investigated the adsorption and desorption processes of the SO 2 molecule on a rutile TiO 2 (110)-(1ϫ1) surface. We have recorded the S 2p core-level photoemission peaks for different SO 2 exposures at a substrate temperature of 120 K in order to get information about the divers species formed on the surface. We have also recorded real-time photoemission spectra to study the adsorption from the early stages to large exposures and to follow the chemical transformations occurring with the adsorbed species as the temperature increases. We have seen that the first arriving molecules react with the oxygen atoms of the surface forming SO x species, both at low and room temperature. Doses higher than the saturation dose ͑6 L͒ lead to the dissociation of the molecule generating adsorbed S. SO 2 multilayer has been found for exposures higher than around 250 L. We have found a progressive reduction of the SO x species with the temperature and the formation of sulphide as the most stable phase. We have not found any signature of molecular ordered species at the interface.