Synthesis by co-sputtering of Au–Cu alloy nanoclusters in silica

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Abstract

In this work we investigated the Au–Cu binary nanophase diagram by means of structural and compositional characterization of alloy nanoclusters synthesized in silica by means of rf-cosputtering technique. In order to obtain the formation of metallic nanoclusters, as deposited samples have been annealed in reducing atmosphere at 900 °C. A good agreement with the bulk alloy phase diagram has been found: in particular the fcc cubic alloy solid solution is present in all the investigated compositions from pure Au to pure Cu, with a lattice parameter following the modified Vegard’s law typical of the bulk alloy. Three ordered phases are also present corresponding to the Au:Cu atomic ratios of 3:1, 1:1 and 1:3. The linear and non-linear optical properties of the nanocomposites have been measured: the surface plasma resonance red-shifts with increasing the Cu content in the alloy and it spans the entire region from that of pure Au (530 nm) to that of pure Cu (570 nm).

Introduction

Silica films containing metal nanoclusters (NC) exhibit peculiar optical properties that have made them attractive in several application fields. As an example, metal quantum dot composite glasses exhibit an enhanced optical Kerr susceptibility, i.e., the most important physical process for applications in all-optical switching devices. The most promising metals for constituting the nanoparticles embedded in the dielectric matrix are the noble metals. In this work, (copper + gold)-containing silica films were synthesized by radiofrequency co-sputtering deposition technique. In comparison with other techniques, largely employed in the synthesis of alloy-based nanocomposites, like ion implantation [1], [2], radiofrequency magnetron co-sputtering deposition allows to easily obtain homogeneous thicker films doped with nanoclusters [3], [4] which can be useful for the fabrication of optical waveguides. The alloy composition can be varied from pure Au to pure Cu to investigate the alloy nanostructure as a function of the composition. Compositional and structural characterization of the samples was performed by Rutherford backscattering spectrometry, X-ray diffraction, extended X-ray absorption fine structure spectroscopy, optical absorption spectroscopy and Z-scan analysis.

Section snippets

Experimental

Composite dielectric films made of (copper + gold)-doped silica were synthesized by contemporary deposition of silica, copper and gold on fused silica slides, 1 mm thick, in a radiofrequency magnetron sputtering deposition apparatus. Depositions were performed by means of three 13.56 MHz radiofrequency sources in a neutral Ar atmosphere at a pressure of 35 × 10−4 mbar. After a four-step cleaning in ultrasonic bath (deionized H2O, trichloroethylene, acetone, 2-propanol), the silica substrates were

Results

The actual composition of the samples was measured by RBS. No differences were found in the RBS signals of gold and copper between the as-deposited and the corresponding annealed samples, indicating that the diffusion of the metal species (if any) occurred only for distances in the nanometer range. The only difference was found in the O/Si atomic ratio. It was about 2.2 for the as-deposited and became 2.0 ± 0.1 for the annealed samples. The excess of oxygen that we measured in the as-deposited

Discussion

The experimental results in this work indicate that as-deposited (Au + Cu)-doped silica nanocomposites require annealing in reducing atmosphere to obtain the following two effects: (i) recovering of the correct stoichiometry of the silica matrix (which is oxygen-rich in the as-deposited systems), (ii) reduction of the Cu atoms which are mostly in oxidized forms as indicated by the EXAFS analysis. This is in agreement with previous results obtained in Au–Cu alloy nanoclusters in silica synthesized

Conclusions

In this work we reported the synthesis of Au–Cu alloy nanoclusters in silica obtained by rf-cosputtering coupled with thermal annealing in reducing atmosphere. The Au/Cu atomic ratio covered the entire Au–Cu phase diagram, from pure Au to pure Cu. We found that in the as-deposited samples most of the Cu atoms are in oxidized form and that the annealing in reducing atmosphere is mandatory to reduce them to metallic state to form Au–Cu metal alloys. From the optical point of view the annealed

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