Free clusters studied by core-level spectroscopies

In this review we describe the development and current status of free clusters studied using core-level spectroscopies. This topic ranges from simple model systems, such as rare gas clusters, to molecular clusters, and clusters of solids, held together by ionic, covalent and metallic bonding. (C) 2009 Elsevier B.V. All rights reserved

Core level photoelectron spectroscopy probed heterogeneous xenon/neon clusters

Binary rare gas clusters; xenon and neon which have a significant contrariety between sizes, produced by a co-expansion set up and have been studied using synchrotron radiation based x-ray photoelectron spectroscopy. Concentration ratios of the heterogeneous clusters; 1%, 3%, 5% and 10% were controlled. The core level spectra were used to determine structure of the mixed cluster and analyzed by considering screening mechanisms. Furthermore, electron binding energy shift calculations demonstrated cluster aggregation models which may occur in such process. The results showed that in the case of low mixing ratios of 3% and 5% of xenon in neon, the geometric structures exhibit xenon in the center and xenon/neon interfaced in the outer shells. However, neon cluster vanished when the concentration of xenon was increased to 10%

Core-shell structure disclosed in self-assembled Cu-Ag nanoalloy particles.

Core-shell segregation of copper and silver in self-assembled, free nanoparticles is established by means of photoelectron spectroscopy in a wide range of relative Cu-Ag concentrations. These conclusions are based on the analysis of the photon-energy-dependent changes of the Cu 3d and Ag 4d photoelectron spectra. The nanoparticles are formed from mixed Cu-Ag atomic vapor created by magnetron sputtering of a bimetallic sample in a gas-aggregation cluster source. Even at similar Cu and Ag fractions in the primary vapor the surface of the nanoparticles is dominated by silver. Only at low Ag concentration copper appears on the surface of nanoparticles. For the latter case, a threefold decrease in the Ag 4d spin-orbit splitting has been detected. The specific component distribution and electronic structure changes are discussed in connection with the earlier results on Cu-Ag macroscopic and surface alloys

Tuning the oxidation degree in sub-10 nm silver-oxide nanoparticles: From Ag2O monoxide to AgOx(x > 1) superoxide

A preparation method for silver-oxide containing nanoparticles is described. The suggested approach allows varying the oxidation degree from monoxide to an oxygen-rich state resembling superoxide The method is based on magnetron reactive sputtering and vapour aggregation, and utilizes silver-oxygen interaction in the vapour phase - prior to the nanoparticle aggregation from the vapour. Synchrotron-based photoelectron spectroscopy - a local probing method with high chemical sensitivity - allows 'on-the-fly' analysis of the composition and oxidation degree in nanoparticles, before any deposition of the particles. Preparation and properties of oxygen-rich nanoparticles are discussed in connection to the catalytic processing of economically important hydrocarbons. (C) 2014 Elsevier B.V. All rights reserved

Ag-Cu oxide nanoparticles with high oxidation states : towards new high Tc materials

In Ag-Cu oxides possible to fabricate so far, superconductivity has not been detected, but high conductivity was. In the quest for superconductivity the demand is to create a high and peculiar copper-oxygen coordination. Such coordination makes it non-trivial to determine Cu oxidation states, which may be several and co-existing. Another reason for uncertainty is in oxygen deficiency typical for superconducting crystals. Finally, Cu oxidation is influenced by the other metals in the substance. For chemical fabrication the difficulty is to tune the relative abundances of elements in a fine way. Ag-Cu oxides have been also produced by reactive co-sputtering of Cu and Ag, but the composition with high Cu oxidation states necessary for high conductivity has not been realized. In the present work we have fabricated Ag-Cu-oxide nanoparticles containing Cu and Ag in high oxidation states actual for superconductivity. The fabrication includes reactive sputtering of Ag and Cu metals, their vapour oxidation and aggregation into nanoparticles. The ability to create different and high oxidation states, also co-existing, is demonstrated. The fabrication approach also allows overcoming the poor miscibility of Cu and Ag. The nanoparticle composition and the oxidation states could be determined due to an experimental arrangement in which photoelectron spectroscopy is applied to free nanoparticles in a beam in vacuum, what allows avoiding any contact of the particles to a substrate or atmosphere. The combination of the fabrication and characterization methods has proven to be a powerful approach when fine composition tuning and control are desirable

Size-varied photoelectron spectroscopy of metal clusters using the Exchange Metal Cluster Source

The recently developed Exchange Metal Cluster Source (EXMEC) is described. The source is designed for electron-spectroscopic studies of neutral metal clusters. Preliminary results on the rubidium and tin clusters using synchrotron radiation excited photoelectron spectroscopy show the capability for cluster production in the size regime of few tens to few hundred of atoms per cluster. A wide range of metal evaporation temperatures is covered applying resistive and induction heating methods. (C) 2010 Elsevier B.V. All rights reserved

Free silver nanoparticles doped by potassium : Work-function change in experiment and theory

The composition-dependent change in the work-function (WF) of binary silver-potassium nanoparticles has been studied experimentally by synchrotron-based x-ray photoelectron spectroscopy (PES) and theoretically using a microscopic jellium model of metals. The Ag-K particles with different K fractions were produced by letting a beam of preformed Ag particles pass through a volume with K vapor. The PES on a beam of individual non-supported Ag-K nanoparticles created in this way allowed a direct absolute measurement of their WF, avoiding several usual shortcomings of the method. Experimentally, the WF has been found to be very sensitive to K concentration: Already at low exposure, it decreased down to ≈2 eV - below the value of pure K. In the jellium modeling, considered for Ag-K nanoparticles, two principally different adsorption patterns were tested: without and with K diffusion. The experimental and calculation results together suggest that only efficient surface alloying of two metals, whose immiscibility was long-term textbook knowledge, could lead to the observed WF values

Core-shell structure in self-assembled lead/lead-oxide nanoclusters revealed by photoelectron spectroscopy

Nanoclusters containing metallic lead and lead oxide have been produced by self-assembly out of a primary mixture of lead atoms and oxygen in a reactive sputtering-based cluster source. Comparison of the valence and core-level responses in the photoelectron spectra shows that clusters have a core-shell structure with a lead-oxide core coated by an outer shell of metallic lead. This core-shell order is opposite to that typical for most microscopic and macroscopic materials. We explain this by the peculiarities of the cluster production kinetics and by the system's energy minimization striving due to what lead oxide is placed in the core of the mixed cluster. DOI: 10.1103/PhysRevB.87.03540