Thermal annealing and laser-induced mechanisms in controlling the size and size-distribution of silver nanoparticles in Ag⁺-Na⁺ ion-exchanged silicate glasses

Highlights

• Tuning the Ag NPs size and size-distribution in silicate glasses by laser irradiation.

• 266 nm laser irradiation is very effective for forming of very uniform and relatively larger Ag NPs in silicate glasses.

• 532 nm laser irradiation is more efficient to manipulate the Ag NPs size and size-distribution in silicate glasses.

Abstract

Controlled precipitation of metal nanostructures into glassy matrices have demonstrated to be a valid way to synthesize materials suitable for photonics, optoelectronics and telecom purposes. However, the development of strategies for driving nanostructure formation, and then for tailoring the optical response of the metal doped systems, is still a challenge. Aiming at this, the present study focuses on silver state modification including nucleation/growth of Ag nanoparticles (NPs), manipulation of Ag NPs size and size-distribution in Ag⁺-Na⁺ ion exchanged silicate glasses undergone post-exchange treatments, such as thermal annealing and laser irradiation with different irradiation parameters. Raman and optical absorption spectroscopy analyses allow to follow the evolution of the Ag NPs precipitation process starting from the early stages of the overall mechanism, where the glassy matrix is permeated by a dense distribution of Ag⁰, Ag⁺ and small aggregates, which are the seeds for the formation of the larger particles upon energetic treatments. In these regards, it is observed that a proper choice of laser irradiation conditions, in terms of wavelength and pulse energy, has a direct impact on the occurrence of the overall clustering process, by specifically promoting Ag ion reduction as well as cluster nucleation, growth and fragmentation. This definitely paves the way to efficient strategies for the realization of optical materials based on controlled distribution of metal clusters with finely tuned structural and optical properties.

Introduction

In the last decades, a great deal of work has been devoted to the study of the metal-alkali thermal ion exchange process in silicate glasses. The process is usually realized by immersing silicate glass slides in a molten salt bath containing the dopant ions, which are driven into the glass by the chemical potential gradient, and replace alkali ions of the glass matrix that are released into the melt. In this way, metal concentration values far exceeding the solubility limit can be achieved in the glass without aggregation. Ion exchange found its first application for surface strengthening [1], then being studied as a suitable method in the fabrication of passive and active optical waveguides [2], [3], [4], [5], [6], [7], of stress modulated systems [8], and for basic questions concerned with the understanding of dopant-glass interactions [9]. The ion-exchange process was effectively used for doping silicate glasses with different metal species and then promoting the controlled cluster formation by means of proper subsequent treatments, e.g., particle or laser irradiation as well as heat-treatments [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. The possibility of controlling the clusterization in glasses has opened quite attractive perspectives, since nanocomposite materials made by metal clusters embedded in silicate glass exhibit striking optical properties interesting for photonic application, for example third order nonlinearity [20], finding application as well in catalysis and sensor technology and superparamagnetic materials [21], [22], [23]. Moreover, by introducing metal nanoclusters into the glass matrix, the fluorescence properties of rare-earth ions containing glasses can be greatly improved [24], [25], [26], because metal clusters can be used as sensitizers in rare earth glasses, which is very important in telecommunications technology. In addition to being interested in their technical applications, these glasses have also been studied from a more basic perspective, such as the kinetics of cluster nucleation and growth, their stability, and their structure in terms of composition, crystalline phase, size, and size distribution.

The nucleation and growth of silver clusters upon diffusion and thermal annealing of exchanged ions is a complex process, due to several factors: ion exchange is intrinsically a non-equilibrium process, since the diffusion and the site occupation occur below the glass softening point, so preventing the structure to accommodate the host dopant following structural and thermodynamical stability constraints. Moreover, three classes of phenomena take place in giving the resulting system, namely, diffusion, nucleation and aggregate growth. These processes exhibit different regimes, and are somewhat in conflict, giving rise to dynamic feedbacks that may actually stop or enhance one of them upon small changes in any of the involved variables, in particular, the local silver concentration, the temperature, the glass composition, or even the local stoichiometry. On the basis of Raman spectroscopy, the influence of alkali ions on the structure of silicate glass was studied in Ref. [27]. In our previous article, the effect of silver concentration on the significant structural changes of the ion sites of the glass network after the ion-exchange in silicate glass has been reported [19]. In another work, we studied the role of post-exchange thermal treatment in the clustering process of the incorporated silver in silicate glasses [28]. Moreover, in the aim to understand the mechanism of metal nanoparticles formation during thermal annealing of silver containing glasses we reported the use of spectroscopic techniques in a configuration that allows a cross-sectional analysis [29]. In general, precipitation of silver and subsequent cluster nucleation and growth occur (even during the silver diffusion) depending on the energy provided to the system, either by thermal treatments or by energetic beams of particles or electromagnetic radiation. The chemistry of the atmosphere in which a treatment is realized can also play a significant role, as shown by different experiments [30] in which annealing treatments in H-rich atmosphere were quite effective in promoting silver clusterization in glasses. Within this frame, a phenomenological model describing the transport phenomena including the hydrogen contribution was actually proposed for metal cluster formation [31,32], although limited in its effectiveness to specific range of the involved variables.

The control of metal nanoparticle size, as well as of size distribution is important for optimizing the operation of nonlinear optical [33] and sensor [34] devices fabricated using nanocomposite materials. In our recent article, the modification of the silica network structure and the Ag clustering process in silver-containing silicate glass caused by thermal annealing and 355 nm laser irradiation were studied by Raman spectroscopy [35]. However, the effect of laser radiation on the metal doped glasses depends on laser parameters such as wavelength, power and laser fluence, and the following mechanisms are not yet fully understood. In this framework, with the aim to control Ag nanoparticles size and size distribution in silicate glass and ultimately to tune the optical properties of this nanocomposite system, the impact of irradiation with different laser parameters, such as different wavelength and power density, on the silver containing glass is expected. In this contribution, micro-Raman spectroscopy have been used to investigate the direct impact of laser irradiation of different wavelength and laser fluence values on Ag⁺-Na⁺ ion-exchanged glasses to deepen our understanding in controlling the Ag nanoparticles size and size distribution in silicate glasses, and ultimately to provide more information on the formation of metal nanoparticles, including diffusion, aggregation, and possible cluster fragmentation.