Modifications in silver-doped silicate glasses induced by ns laser beams
Research highlights
▶ Ion exchange from molten salt bath can be used to dope silicate glasses (soda-lime and borosilicate) with Ag. ▶ With a suitable choice of the post-exchange treatments (thermal annealing and/or UV laser irradiation) it is possible to induce the silver aggregation, namely to obtain metal nanocluster composite glasses. ▶ The role of irradiation parameters and of the host matrix structure in the aggregation phenomena are investigated to better understand the formation and the dissolution of the nanoparticles. ▶ Thermal treatments are able to induce formation of aggregates of few silver ions (for low T annealing) and silver metal particles 2–3 nm in size (for high T annealing). Laser irradiation was able to induce Ag nanoparticles formation in the as-exchanged soda-lime samples. ▶ In the previously annealed Ag-doped soda-lime samples, the laser irradiation with a very large number of consecutive pulses induces a fragmentation of the silver nanoparticles. ▶ No effects were observed by laser irradiation of the Ag-doped BK7 glass samples.
Introduction
Nanoparticles embedded in suitable host matrices give origin to a class of composite materials having possible application in photonic devices as light waveguides and optical switches [1], [2]. These systems can be realized by different methods based, for instance, on a matrix doping technique, like metal ion implantation in dielectrics [1], [3]; or chemical synthesis such as the sol–gel procedure [4]; usually followed by material processing to induce a controlled metal precipitation. Actually, in metal-doped glasses, energy treatments such as thermal annealing and/or irradiation by laser beams may be used both to promote the aggregation of metal atoms, with consequent nanoparticle precipitation, and to select the nanoparticle size, thus giving suitable methodologies for the controlled preparation of nanocomposite glasses [5], [6], [7], [8], [9], [and refs therein] Depending on mechanisms that have not been yet completely addressed and described, the effect of the interaction between the electromagnetic wave and the metal-doped glass changes with wavelength, power density and repetition rate of the laser beam, as well as with the structure of the glass.
An alternative approach for the synthesis of metal nanocluster composite glasses (MNCGs) regards the ion exchange process as the first step to dope a matrix with metal atoms. Subsequently, the embedded metal atoms can be forced to aggregate by means of suitable treatments such as thermal annealings and/or low-mass ion beam or laser irradiations [1]. Metal precipitation to form particles in the nanometer range of size is promoted with many degrees of freedom in the multistep preparation [5] for effectively defining the cluster structure and, eventually, the composite optical performances [10]. The effect of the laser irradiation on the doping metal depends on the wavelength as well as on the power and energy density, following mechanisms not yet completely understood [9], [11].
The aim of this paper is to investigate some of these effects by analyzing the impact of high power laser irradiation on Ag+–Na+ ion exchanged glasses (as-exchanged and annealed in air) related to the formation of metal nanoparticles, including diffusion and aggregation phenomena and possible cluster fragmentation. The laser processing was done by using the third harmonic of a Nd:YAG laser (wavelength of 355 nm) as the light source. A thermal annealing in air for 1 h was performed in some cases before the laser irradiation, at temperature T in the 300–550 °C range. After laser irradiation the different silicate samples were characterized by optical absorption spectroscopy (OA), photoluminescence spectroscopy (PL), and Rutherford backscattering spectrometry (RBS), in order to determine the role of irradiation parameters and of the host matrix structure in the aggregation phenomena.
Section snippets
Experimental
Ag+–Na+ ion exchanged samples were realized by immersing commercial soda-lime optical glass (SLG) or Schott BK7 borosilicate optical glass slides in a molten salt bath of AgNO3:NaNO3, with silver nitrate molar concentration of 1%. The bath temperature was 320 °C, for 20 min of immersion duration. The atomic % composition for SLG is 59.6 O, 23.9 Si, 10.1 Na, 2.6 Mg, 2.4 Ca, 0.7 Al, 0.5 K, 0.2 S + Ti + other elements in traces; for borosilicate glass, BK7, it is 60.2 O, 22.4 Si, 11.0 B, 3.8 Na, 1.8 K,
Results and discussion
RBS measurements (Fig. 1) showed the diffusion of Ag inside the sample during the exchange process: silver is present in the whole analyzed region for the SLG samples (RBS sampling depth ∼2–3 μm), and up to about 1.3 μm for the BK7. The estimated Ag concentration at the SLG sample surface is almost 6 at%, and it decreases to 3 at% at a depth of ∼2 μm. Comparing the Ag and Na detected concentrations, at the investigated depths the sum of the Ag and Na concentrations is always around 10%, namely the
Conclusions
The binary ion exchange technique allows the doping of glasses in a controlled way in order to fabricate optical dielectrics and glass waveguides (planar, channel, buried). We synthesized Ag-doped glass films by Ag–Na ion exchange of different silicate glasses in molten salt baths, and we induced in the exchanged glasses silver nanoparticle formation by thermal treatments in air and/or laser irradiation in the near UV region. Depending on the glass used and on the annealing temperature, the
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