No abstract

Time-resolved x-ray excited optical luminescence (XEOL) and x-ray absorption near edge structures have been employed to study the origin of the multicolor luminescence from SnO2 nanoribbons. The authors find that the yellow-green luminescence has a long lifetime while the blue luminescence a short one. The luminescence is attributed to the radiative decay of trapped electrons in oxygen vacancies just below the conduction band and electrons in the conduction band to intrinsic surface states in the band gap.

Using Pd and Ag L 3,2 -edge x-ray-absorption near edge structures (XANES) and x-ray photoemission spectroscopy, we have investigated the charge distribution in a series of Pd-Ag alloys (Pd 3 Ag, PdAg, and PdAg 3 ) from a local perspective. It is found that, relative to the pure element, both Pd and Ag gain d and lose non-d (s and p) charge upon alloying. The results are discussed in terms of band filling, rehybridization, electronegativity, and electroneutrality considerations. The possibility of using the L 3,2 -edge XANES white line to circumvent the need of an appropriate volume for charge transfer discussions and for other applications are noted. [S0031-9007(96)01801-7] PACS numbers: 78.70.Dm, 71.55.AkThe electronic properties of 4d noble metals, Pd and Ag, are determined largely by their band structure, particularly the energy distribution of the d electrons and the interplay between the d and the non-d (s and p) conduction electrons [1]. Thus d-charge redistribution upon alloy formation has a profound effect on the electronic behavior of the alloy. It is generally recognized that the Pd d band extends above the Fermi level, e F , and exhibits high unoccupied densities of d states at the Fermi level while the Ag d band is entirely below e F [2]. Experimentally, the occupied portion of the d band can be studied with photoemission, and, to a lesser extent, x-ray emission techniques, while the unoccupied densities of states of d character can be investigated with the L 3,2 -edge x-ray-absorption near edge structure (XANES). This is because L 3,2 XANES probes the d holes via dipole transitions. It has been shown that, owing to the presence of localized densities of states (DOS) of the unoccupied d band at the Fermi level, the Pd L 3,2 -edge XANES exhibits an intense spike (commonly known as a white line) at the onset of the L 3,2 -edge XANES, while the XANES of Ag, of which the d band is nominally full, does not [3]. In the latter, however, close examination reveals that there exist some unoccupied states of 5d character above the Fermi level in Ag as the result of sp-d rehybridization [4].The Pd-Ag alloy system, which has complete miscibility, fcc structure, and negligible volume effects, is of particular interest in that the center of gravity of the d band of the pure elements, which can be determined by an atomiclike one-electron energy ͑e d ͒, is well separated and that Pd-Ag alloy is known to have two separate dband components which closely correspond to that of the pure constituents [5,6]. The energy position of e d changes only slightly in the alloys, although the individual d component narrows as the constituent becomes more dilute. In this paper, we examine the nature and the direction of charge transfer in Pd-Ag alloys in connection with the behavior of the alloy d band from a local perspective. Pd and Ag L 3,2 -edge XANES [3,7], which is ideally suited as a local probe for the study of unoccupied d states above the Fermi level in Pd-Ag alloys, was used together with photoemission in this study. A chan...

No abstract

No abstract

Utilizing porous silicon as a reducing agent and a substrate, gold complex ions [AuCl4]- were reduced from aqueous solution to produce nanoparticles of gold upon the surface of porous silicon. Scanning electron microscopy (SEM) was utilized to study the morphology of the porous silicon layers and the deposits of gold nanoparticles. It is found that preparation conditions have a profound effect on the morphology of the deposits, especially on porous silicon prepared from a p-type wafer. The gold nanoparticles, varying from micrometric aggregates of clusters of the order of 10 nm, to a distribution of nearly spherical clusters of the order of 10 nm, to strings of ~10 nm were observed and compared to bulk gold metal using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS). These techniques confirm and complement the SEM findings. The potential for this reductive deposition technique is noted.Key words: gold nanostructures, reductive deposition, porous silicon, morphology, X-ray spectroscopy.

No abstract

We report M3,2-edge x-ray absorption near-edge structure (XANES) measurements for a series of 5d metals, Ta, W, Pt, and Au using a total electron yield technique. It is found that the M3,2 XANES absorption features and their systematics are comparable to their L3,2 counterparts. These results clearly indicate that M3,2-edge XANES can be used as an alternative probe for the unoccupied densities of d states of the 5d metals and their compounds. The implication of these results is discussed.