282 research outputs found
Near-infrared room temperature luminescence of few-atom Au aggregates in silica: a path for the energy-transfer to Er3+ ions
No full textUltra-small molecule-like AuN nanoclusters made by a number of atoms N less than 30 were produced by ion implantation in silica substrates. Their room temperature photoluminescence properties in the visible and near-infrared range have been investigated and correlated with the Er sensitization effects observed in Er–Au co-implanted samples. The intense photoluminescence emission under 488 nm laser excitation occurs in three different spectral regions around 750 nm (band A), 980 nm (band B) and 1150 nm (band C) as a consequence of the formation of discrete energy levels in the electronic structure of the molecule-like AuN nanoclusters. Indeed, energy maxima of bands A and C scale with N−1/3 as expected for quantum confined systems. Conversely, the energy maximum of band B appears to be almost independent of size, suggesting a contribution of electronic surface states. A clear correlation between the formation of band B in the samples and Er-related photoemission is demonstrated: the band at 980 nm related to AuN nanoclusters resonant with the corresponding Er3+ absorption level, is suggested as an effective de-excitation channel through which the Au-related photon energy may be transferred from Au nanoclusters to Er ions (either directly or mediated by photon absorption), eventually producing the Er-related infrared emission at 1540 nm
Enhanced optical functionalities in silica by doping with Au-based nanostructures
No full textThe sensitization mechanism of Er3+ ions by sub-nanometric AuN aggregates formed in silica by sequential ion implantation has been investigated by room temperature photoluminescence characterizations. Er–Au co-implanted samples with different Au and Er relative concentrations have been studied. The results allowed to point out a correlation between the luminescence properties of the molecule-like AuN nanoclusters and the Er sensitization effect and to definitely prove the short-range nature of the energy-transfer process
Structure and thermal stability of Au-Fe alloy nanoclusters formed by sequential ion implantation in silica
No full textSequential ion implantation of Au and Fe ions has been performed in silica. Despite the fact that the two species are not miscible in the bulk, structural characterizations show that the nanoparticles produced are a Au–Fe alloy. The crystalline structure of the nanoparticles is fcc with a lattice parameter of 0.395 nm. The nanocomposite is ferromagnetic at 3 K, with a magnetic moment per Fe atom equal to 1.4μB and it shows a hysteresis loop with a coercive field of 24 mTorr. Due to alloying, the optical absorption spectrum does not exhibit the typical surface plasmon resonance of gold nanoparticles. The sample exhibits a change of its optical and structural properties when is annealed at 600 °C
Effect of ultrasmall Au–Ag aggregates formed by ion implantation in Er-implanted silica on the 1.54 m Er3+ luminescence
No full textMultiple implantations of Au and Ag ions in pure silica and Er-doped silica matrices have been performed to promote the formation of small Au–Ag alloy clusters. Upon annealing in N2 atmosphere at 600 C the structural investigation based on X-ray absorption spectroscopy at Au L3-edge detected for both cases the formation of Au–Ag alloy clusters, whose size is likely below 1 nm. The alloy composition is rich in Au; a minor part of Au atoms remains dispersed into the matrix and oxidized. In the Er-doped silica, the presence of these small alloy aggregates promotes a strong enhancement of the Er3+ luminescence at 1.54 microm, that is more marked with respect to similarly produced layers where only pure Au sub-nanometer clusters were present
Thermal evolution of cobalt nanocrystals embedded in silica
No full textThe structural evolution of cobalt nanoclusters synthesized in silica glass by ion implantation has been investigated upon thermal annealing. The samples were characterized by in-situ grazing incidence X-ray diffraction, exploiting a synchrotron radiation beam and following their evolution during thermal treatments in vacuo up to T = 800 °C. Before heating, the system is composed of hcp Co nanocrystals; we have not detected the transition from hcp to fcc structure that in the bulk phase occurs around 420 °C; nevertheless, the differences in the diffraction pattern recorded at T = 800 °C with respect to the corresponding one at room temperature suggest the presence of a second crystalline phase
Amorphous intermixing of noble and magnetic metals in thin film-based nanostructures
No full textIn nanostructures made of a mixture of bulk-immiscible metallic species, the alloy formation down to the atomic scale is a crucial and debated point. We report on the first experimental evidence of an amorphous metallic phase in Au-Co thin films and 2D array of nanostructures, that is constituted by a fine mixing of single-metal (sub)-nm domains, as shown by experiments coupling short- and long range- order characterization techniques, as X-ray Absorption Spectroscopy-XAS, X-ray Diffraction-XRD, Diffraction Anomalous Fine Structure-DAFS. Despite the mixing does not reach the atomic scale, the extended Au-Co interface can entail about half of atoms, and is responsible for the previously measured magnetic moment of Au in these systems. This amorphous nanomixed phase coexists with a minor fraction of fcc AuxCo1-x nanocrystals, preferentially oriented with the 111 crystallographic planes parallel to the film surface. 2D patterned Au-Co films with very similar structure can be easily obtained, but with smaller and randomly oriented nanocrystals. The thermal stability of the system (amorphous and crystalline) is limited to below 250 °C. At higher temperatures an extended decomposition occurs and Au and fcc Co nanocrystals coexist
Sub-nanometric metallic Au clusters as efficient Er3+ sensitizers in silica
No full textSilica films co-implanted with Er and Au ions show an enhancement of rare earth
photoluminescence after gold introduction in the matrix. Er excitation originates in a broad spectral
region, from the red to the near ultraviolet. We have investigated the influence of gold aggregation
on the optical properties of co-doped samples by varying the temperature of post-Au implantation
annealing in the 400–900 °C range. Optical measurements and extended x-ray absorption analysis
support the hypothesis of an energy transfer process mediated by sub-nanometric Au aggregates
with metallic character that are optically activated mostly through electron interband transitions
between d and sp-conduction levels
Structural modification of Au-Co thin films induced by annealing in oxidizing atmosphere
No full text
Extrinsic chirality tailors Stokes parameters in simple asymmetric metasurfaces
No full textMetasurfaces tailor electromagnetic confinement at the nanoscale and can be appropriately designed for polarization-dependent light-matter interactions. Adding the asymmetry degree to the desing allows for circular polarizations of opposite handedness to be differently absorbed or emitted, which is of interest in fields spanning from chiral sensing to flat optics. Here, we show that simple, low-cost asymmetric metasurfaces can control Stokes parameters in the transmitted far-field. With only 50 nm of asymmetric plasmonic shells on self-assembled polystyrene nanospheres, our metasurfaces allow for great spectral and incident angle tunability. We first investigated broadband extrinsic chirality in metasurfaces with asymmetric plasmonic semishells; we found high extinction circular dichroism (CD) in the near-infrared range. We then excited it with linear polarization and performed hyperspectral Stokes polarimetry on the transmitted field. We showed that the S3 parameter follows the behavior of CD in extinction, and that the output field position on the Poincaré sphere can be widely controlled by using the incidence angle and wavelength. Furthermore, simulations agreed well with the experiments and showed how the near-field chiro-optical response influences the extrinsic chiral behavior in absorption and the polarization state of the transmitted field
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