## 3 Results and Discussion  
Spontaneous Growth of Silver Nanoparticles on the Surface of Ion-Exchanged Photo-Thermo-Refractive Glass Heat-Treated in Air Atmosphere  
Plasmonic metal nanoparticles have attracted significant interest in photonics due to their unique optical properties. Potential applications of metal nanoparticles include but are not limited to biosensing [15], surface-enhanced Raman spectroscopy (SERS) [16], solar cells [17], targeted drug delivery [18], saturable absorbers [19], and photocatalysis [20]. SERS is one of the most widely studied applications of the metal nanoparticles. The phenomenon was firstly observed from pyridine molecules adsorbed on roughened silver electrodes by [PERSON] et al. in 1974 [21]. Despite SERS mechanisms still being under discussion, it has become a powerful and ultrasensitive tool due to the possibility of enhancing Raman scattering magnitude by 10-14 orders. This allows one to detect even a single molecule [22; 23]. Roughened metal surfaces can be obtained in different ways, including thermal, electron beam, chemical, and plasma deposition methods. Recently, it was shown that a SERS-active silver nanoisland film (SNIF) can be easily formed on the surface of Na\({}^{+}\)-Ag\({}^{+}\) ion-exchanged glasses. Two different approaches were developed to grow SERS-active SNIF on a glass surface (Figure 1). The first is based on the growth of SNPs in the ion-exchanged glass layers with subsequent chemical etching to obtain SNPs directly on the glass surface [24; 25]. The concentration of NPs on glass surface can be controlled by silver content in a salt melt during the IE process and by the thickness of the etched glass layer. The second approach involves thermal treatment of Na\({}^{+}\)-Ag\({}^{+}\) ion-exchanged glass in a reducing atmosphere, for example, hydrogen atmosphere or water vapor [26; 27]. Growth of SNPs in the subsurface glass region can be suppressed by low temperature of the treatment (at least 100 \({}^{\circ}\)C below the glass transition temperature). Moreover, the poling technique can be applied to Na\({}^{+}\)-Ag\({}^{+}\) ion-exchanged glasses to obtain single NPs on the glass surface [28; 29]. It should be noted that the considered approaches show similar performance as SERS-active substrates. Our preliminary research revealed the spontaneous growth of SNIF at the surface of Na\({}^{+}\)-Ag\({}^{+}\) ion-exchanged undoped matrix-based PTR glasses. To our best knowledge, this is the first observation of growth of SNIF at the surface of ion-exchanged glass after heat treatment in air. Figure 2 shows photos of BK-7 optical glass and PTR matrix-based glass with no dopants (hereafter denoted as PTR-M; see Table 2) after the Na\({}^{+}\)-Ag\({}^{+}\) IE and heat treatment at a temperature of 50 \({}^{\circ}\)C above corresponding Tg values. BK-7 exhibited the yellowish coloration typical for most silver ion-exchanged and heat-treated glasses, which results from absorption of the SNPs formed in silver-enriched glass layers [30; 31]. PTR-M glass, on the contrary, possessed no coloration, due to the absence of electrons for reducing  
Figure 1: Schemes of two processes of formation of SNIF at the surface of Na\({}^{+}\)–Ag\({}^{+}\) ion-exchanged glass. silver ions inside the glass [32]. Moreover, a gray semitransparent film was observed on the PTR-M glass surface. For further detailed research, PTR-M glass with no dopants was studied. Silver ions were introduced by immersing the PTR glass samples into the bath containing a mixture of 5 AgNO3/95 NaNO3 (mol.%) at 320 \({}^{\circ}\)C for 15 min. Ion-exchanged PTR glass samples were then heat-treated in air at temperatures in the range 350-500 \({}^{\circ}\)C, with a treatment duration of up to 7 h. An Na\({}^{+}\)-Ag\({}^{+}\) ion-exchanged glass layer was mechanically removed from one side of the samples prior the heat treatment. Glass samples were placed in a muffle furnace with the silver-containing side facing upward.