Structural and optical properties of Cu:silica nanocomposite films prepared by co-sputtering deposition

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Abstract

Copper-containing silica films were synthesized by radiofrequency (rf) co-sputtering deposition technique, and then heat-treated in different annealing atmospheres, i.e. either oxidizing or reducing, with the aim to develop suitable preparation methodologies for controlling the composite structure. Characterization of the samples along the various preparation steps was performed by Rutherford backscattering spectrometry (RBS), transmission electron microscopy and optical absorption spectroscopy. The nonlinear optical coefficient n2 of the nanocomposite films was estimated by the Z-scan technique. Experimental observations showed that copper migration and aggregation depend critically on the annealing conditions, giving rise to quite different stable structures. In particular, for samples heat-treated first in air and then in a H2–Ar gas mixture, the oxidizing atmosphere drives copper towards the surface while the reducing one promotes the subsequent clusterization in a well defined region.

Introduction

Metal nanoclusters embedded in fused silica exhibit peculiar optical properties that have made them attractive in several application fields. In particular, metal nanocluster composite glasses (MNCGs) are expected to exhibit features that can be exploited in integrated-optical devices [1], owing to their transparency in the optical fibers transmission window. Prescribed Cu:silica composite features require the control of the cluster formation and growth, and therefore the definition of effective preparation protocols. Sputtering deposition technique is particularly suitable for co-depositing the matrix together with more than one dopant element, also possibly creating core-shell or alloy aggregates [2], [3], [4], [5], [6]. Moreover, by sputtering deposition one can prepare homogeneous films several microns thick, so operating in the field of light waveguiding structures. In this work, copper-containing silica films were synthesised by radiofrequency (rf) co-sputtering deposition technique, and then heat-treated (in some cases sequentially) in different annealing atmospheres, i.e. either oxidizing or reducing.

Section snippets

Experimental

Samples were synthesized at room temperature by radiofrequency magnetron multitarget co-sputtering deposition of silica and copper, in pure Ar atmosphere at a working pressure of 35×10−4 mbar, using two different radiofrequency sources for silica and copper, respectively. Substrate were fused silica slides. The rf-power to the 2′′ diameter targets was 250 W for silica and 12 W for copper. Co-deposition time was 40 min, preceeded and followed by a 5 min deposition of sole silica. The final composite

Results and discussion

Optical absorption spectroscopy is the first indication of the possible formation of copper nanoclusters. In Fig. 1, the optical spectra are shown for the various heat-treated samples, compared with the spectrum for the as-deposited sample. The peak at about 560 nm is related to the surface plasmon resonance (SPR) of copper metallic nanoclusters, and is evident in the two cases of H2-annealing. Fig. 2 shows the RBS spectra for the same samples. From the observation of these spectra, it is

Conclusions

The radiofrequency magnetron co-sputtering deposition, combined with suitable subsequent thermal treatments, provides effective MNCG preparation protocols.

The hydrogen-annealed films formed by copper nanoparticles embedded in silica exhibit very large values of the nonlinear refractive index, n2, thus making them interesting as composite materials for photonic applications.

The optical properties of the films annealed in reducing atmosphere are promising compared to those of similar copper MNCGs

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