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In this work, we report the growth of MoO3 nanowires (NWs) by thermal evaporation with the aim of evaluating their use as catalysts for oxygen evolution reaction (OER) in water splitting application. Growth was performed on two different kinds of substrates, Si(100) and Si(111), and at different growth temperatures between 200 and 400 °C. The effect of such experimental parameters on the morphological, structural, and compositional properties was investigated. The deposited materials were characterized by in situ reflection high-energy electron diffraction, scanning electron microscopy, transmission electron microscopy, Rutherford backscattering spectroscopy, and X-ray diffraction. For a growth temperature of 200 °C, a flat and amorphous oxide layer is obtained, while as the temperature is increased, one-dimensional (1D) growth prevails on 2D layer formation. In particular, at 400 °C, the deposited material consists of a high-density array of MoO3 crystalline NWs a few nm in diameter and up to about 0.5 μm in length. Only in the case of Si(111) substrates have small three-dimensional structures formed and coexisted with NWs since the beginning of the growth process. Finally, the OER activity of MoO3 NWs was evaluated. The nanostructures were deposited on graphene paper, and their electrochemical activity was investigated and compared to other nonprecious metal-based catalysts for alkaline OER. Our MoO3-based electrodes showed overpotentials of 330 and 340 mV at 10 mA/cm2 (η10) depending on the average length of NWs and a quite high mass activity associated with a low catalyst loading, thus fitting the best performing electrocatalysts. Our results demonstrate that MoO3 NWs are suitable candidates for the development of anode electrodes for water splitting application. [ABSTRACT FROM AUTHOR]

Abstract The usage of semiconductor nanostructures is highly promising for boosting the energy conversion efficiency in photovoltaics technology, but still some of the underlying mechanisms are not well understood at the nanoscale length. Ge quantum dots (QDs) should have a larger absorption and a more efficient quantum confinement effect than Si ones, thus they are good candidate for third-generation solar cells. In this work, Ge QDs embedded in silica matrix have been synthesized through magnetron sputtering deposition and annealing up to 800°C. The thermal evolution of the QD size (2 to 10 nm) has been followed by transmission electron microscopy and X-ray diffraction techniques, evidencing an Ostwald ripening mechanism with a concomitant amorphous-crystalline transition. The optical absorption of Ge nanoclusters has been measured by spectrophotometry analyses, evidencing an optical bandgap of 1.6 eV, unexpectedly independent of the QDs size or of the solid phase (amorphous or crystalline). A simple modeling, based on the Tauc law, shows that the photon absorption has a much larger extent in smaller Ge QDs, being related to the surface extent rather than to the volume. These data are presented and discussed also considering the outcomes for application of Ge nanostructures in photovoltaics. PACS: 81.07.Ta; 78.67.Hc; 68.65.-k

The influence of bismuth on erbium optical properties at 1.54 µm has been investigated in yttrium-erbium disilicate thin films synthesized by magnetron co-sputtering and implanted with two Bi different doses. The Bi depth distribution and the evolution of its oxidation states after annealing treatments at 1000 °C in two atmospheres, O2 and N2, have been investigated. It was found that only in O2 the Bi3+ valence state is prevalent, thanks to the enhanced Bi mobility in the oxidizing ambient, as demonstrated by Rutherford backscattering spectrometry. At lower Bi content, although the formation of Bi0 metallic nanoparticles that are deleterious non radiative channels for Er luminescence, efficient energy transfer from Bi to Er has been obtained only in O2. It is due to the excitation of ultraviolet broad Bi3+ absorption band and the energy transfer to Er ions. We have evaluated that in this case, Er effective excitation cross section increased by a factor of 5 in respect with the one for direct Er absorption at 488 nm. At higher Bi dose, this mechanism is absent, but an increased Er optical efficiency at 1.54 µm has been observed under resonant excitation. It is due to the contribution of a fraction of Er ions having an increased lifetime. This phenomenon is associated with the formation of Bi agglomerates, induced at higher Bi doses, which well isolate Er from non-radiative quenching centers. The increased decay time assures higher optical efficiency at 1.54 µm. [ABSTRACT FROM AUTHOR]

We investigated the properties of light emitting devices whose active layer consists of Er-doped Si nanoclusters (nc) generated by thermal annealing of Er-doped SiOx layers prepared by magnetron cosputtering. Differently from a widely used technique such as plasma enhanced chemical vapor deposition, sputtering allows to synthesize Er-doped Si nc embedded in an almost stoichiometric oxide matrix, so as to deeply influence the electroluminescence properties of the devices. Relevant results include the need for an unexpected low Si excess for optimizing the device efficiency and, above all, the strong reduction of the influence of Auger de-excitation, which represents the main nonradiative path which limits the performances of such devices and their application in silicon nanophotonics. [ABSTRACT FROM AUTHOR]