The nonlinear absorption of Ag atomic clusters and nanoparticles dispersed in a transparent oxyfluoride glass host has been studied. The as-prepared glass, containing 0.15 at.% Ag, shows an absorption band in the UV/violet attributed to the presence of amorphous Ag atomic nanoclusters with an average size of 1.2 nm. Upon heat-treatment the Ag nanoclusters coalesce into larger nanoparticles that show a surface plasmon absorption band in the visible.Open aperture z-scan experiments using 480 nm nanosecond laser pulses demonstrated nonsaturated and saturated nonlinear absorption with large nonlinear absorption indices for the Ag nanoclusters and nanoparticles, respectively. These properties are promising, e.g., for applications in optical limiting and object's contrast enhancement.

A major drawback of state-of-the-art proton exchange membrane fuel cells is the CO poisoning of platinum catalysts. It is known that CO poisoning is reduced if platinum alloys are used, but the underlying mechanism therefore is still under debate. We study the influence of dopant atoms on the CO adsorption on small platinum clusters using mass spectrometry experiments and density functional theory calculations. A significant reduction in the reactivity for Nb and Mo doped clusters is attributed to electron transfer from those highly coordinated dopants to the Pt atoms and the concomitant lower CO binding energies. On the other hand Sn and Ag dopants have a lower Pt coordination and have a limited effect on the CO adsorption. Analysis of the density of states demonstrates a correlation of dopant induced changes in the electronic structure with the enhanced tolerance to CO poisoning.The development of efficient fuel cells is a promising strategy to diminish the dependence on fossil fuel by making use of environmentally friendly energy sources. [1] Proton exchange membrane fuel cells (PEMFCs) are highly susceptible to CO poisoning of the platinum catalyst. [2] CO molecules, present as trace components in the fuel, preferentially adsorb on Pt nanoparticles, thereby blocking the active sites and degrading the cell's performance. Several Pt alloys, such as Pt-X (X = Sn, Ru, Mo, Nb, W, Ag, and Ni), are known for an enhanced tolerance to the CO poisoning and thus improve the performance of the fuel cell. [3][4][5][6][7] The physical mechanism responsible for the tolerance has been extensively studied and is ascribed to an alteration of the local electronic structure at the reaction site upon alloying and/or to a bi-functional mechanism, in which OH groups adsorbed on the alloying agent interact with CO and form CO2 and H2, which are released from the catalyst, regenerating the active sites. [3] The interaction of transition metal surfaces with CO is a complex problem which has been described by three qualitative models: the d-band center model, [8] the Blyholder model, [9] and the π-σ model. [10] In the Blyholder model, the Pt-CO interaction is described by donation of electron density from the CO 5σ orbital to empty Pt 5d states and back-donation from occupied Pt 5d states to the CO 2π* antibonding orbital. The bottom-line of local electronic structure modifications as explanation for the CO tolerance in Pt-X alloy nanoparticles is that electron transfer from the alloying agent to the empty Pt 5d states reduces the Pt-CO bonding strength. [11] Although few-atom platinum clusters in the gas phase differ significantly in size and in environmental conditions from the nanoparticles used in PEMFCs, they can provide a better understanding for the enhanced tolerance to CO poisoning. The CO binding is a local event, which poisons a Pt active site. Clusters in molecular beams are ideal model system for complex processes that depend on local chemistry. Conditions (cluster size, composition, and charge state) are well c...

different from those of bulk materials. [6]

The thermal radiation from small, laser heated, positively charged niobium clusters has been measured. The emitted power was determined by the quenching effect on the metastable decay, employing two different experimental protocols. The radiative power decreases slightly with cluster size and shows no strong size-to-size variations. The magnitude is 40-50 keV/s at the timescale of several microseconds, which is the measured crossover time from evaporative to radiative cooling.

A new approach to enhance the sensitivity of existing SAW UV-visible photo-detectors based on the monitoring of the multiple reflections of the acoustic waves in the Fourier transform of the frequency response (S21) is demonstrated. By using this concept, it is possible to monitor the UV-visible light in a wide intensity range from very high to ultralow. We present a strategy to obtain an ultrafast SAW UV photo-detector with millisecond response by tuning the deposition conditions of the ZnO film and using the multiple-reflections concept.

The combination of x-ray spectroscopy methods complemented with theoretical analysis unravels the coexistence of paramagnetic and antiferromagnetic phases in the Zn 0.9 Mn 0.1 O shell deposited onto array of wurtzite ZnO nanowires. The shell is crystalline with orientation toward the ZnO growth axis, as demonstrated by X-ray linear dichroism. EXAFS analysis confirmed that more than 90% of Mn atoms substituted Zn in the shell while fraction of secondary phases was below 10%. The value of manganese spin magnetic moment was estimated from the Mn Kβ X-ray emission spectroscopy to be 4.3μ B which is close to the theoretical value for substitutional Mn Zn . However the analysis of L 2,3 x-ray magnetic circular dichroism data showed paramagnetic behaviour with saturated spin magnetic moment value of 1.95μ B as determined directly from the spin sum rule. After quantitative analysis employing atomic multiplet simulations such difference was explained by a coexistence of paramagnetic phase and local antiferromagnetic coupling of Mn magnetic moments. Finally, spin-polarized electron density of states was probed by the spin-resolved Mn K-edge XANES spectroscopy and consequently analyzed by band structure calculations.

Core-shell ZnO/ZnMnO nanowires on a-Al 2 O 3 and GaN ͑buffer layer͒/Si ͑111͒ substrates were fabricated by pulsed laser deposition using a Au catalyst. Two ZnO targets with a Mn content of 10% were sintered at 1150 and 550°C in order to achieve the domination in them of paramagnetic MnO 2 and ferromagnetic Mn 2 O 3 phases, respectively. Cluster mechanism of laser ablation as a source of possible incorporation of secondary phases to the wire shell is discussed. Raman spectroscopy under excitation by an Ar + laser revealed a broad peak related to the Mn-induced disorder and a redshift in the A 1 -LO phonon. Resonant Raman measurements revealed an increase in the multiphonon scattering caused by disorder in ZnO upon doping by Mn. Besides the UV emission, a vibronic green emission band assisted by a ϳ71 meV LO phonon is also observed in the photoluminescence spectra. Core-shell structures with smooth shells show a high exciton to green band intensity ratio ͑ϳ10͒ even at room temperature.

Photoabsorption spectra of gas phase Au(n)(+) and Au(n-1)Pd(+) (13 ≤ n ≤ 20) clusters were measured using mass spectrometric recording of wavelength dependent Xe messenger atom photodetachment in the 1.9-3.4 eV photon energy range. Pure cationic gold clusters consisting of 15, 17, and 20 atoms have a higher integrated optical absorption cross section than the neighboring sizes. It is shown that the total optical absorption cross section increases with size and that palladium doping strongly reduces this cross section for all investigated sizes and in particular for n = 14-17 and 20. The largest reduction of optical absorption upon Pd doping is observed for n = 15.