Dipole Moment of Nanoparticles at Interfaces

Abstract

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...

Size dependence of nanostructures: Impact of bond order deficiency

“…XPS spectra in Fig. 33c shows the Ta-4f spectra taken after removing about 30% (upper) and 50% (lower) of the surface nanoparticles of a gate device by sputtering [496]. The first pair of doublets (Ta-4f 5/2 and Ta-4f 7/2 ) at (23.4 and 26.8 eV) arises from TaSi x and Ta 2 O 5 , respectively.…”

“…This suggestion means that an oxygen atom bonds more strongly to the Cu atoms in a nanosolid than does the oxygen atom to the Cu atoms inside the bulk. (iii) The size-enhanced Sn-3d, Sn-4d and Ta-4f core-level shift of the OeSn and OeTa covered metallic clusters was considered as the contribution from the interfacial dipole formation between the substrate and the particles [496]. The number of dipoles or the momentum of the dipoles was expected to increase with reducing particle size.…”

“…(iii) The energy levels of the core bands and the adsorbate-induced chemical shifts move simultaneously towards higher binding energy (in absolute value). XPS measurements revealed that the main core-level peaks and the oxide satellites of Cu-2p 3/2 [495] (ÿ932.1, ÿ940.1 eV), Sn-3d (ÿ484.4, ÿ486.7 eV) [496], Sn-4d (ÿ26, ÿ31 eV), Ta-4f 5/2 (ÿ23.4, ÿ26.8 eV) and Ta-4f 7/2 (ÿ31.6, ÿ36.5 eV) [496], move simultaneously up and the amounts of change depend on the original core-level position and particle size. These dedicated observations confirm that the particle size and oxidation have important effects on the core-level shift of nanometric compounds, which is of great value in understanding the nature of nanometric systems.…”

Langmuir monolayers of gold nanoparticles: from ohmic to rectifying charge transfer

“…4), which seems to imply a memory effect of the nanoparticle thin films. Again, these unique charge-transfer behaviors can be attributable to the interfacial dipole properties of the nanoparticle molecules where the alignment of the nanoparticle dipoles generated during electron transfer process might not be readily switched by the bias polarity [17,18]. From the above studies (Figs.…”

Oxidation electronics: bond–band–barrier correlation and its applications

“…On the other hand, XPS investigations of O-Cu [462], O-Sn, O-Ta [463], ZnS [464] and CdS [465] nanoclusters have revealed additional features at higher binding energy besides the well-known chemical shift (see Fig. 40).…”