# Towards a Glass New World: The Role of Ion-Exchange in Modern Technology
## 2 Glass Strengthening: The Ion-Exchange Contribution
### Ion-Exchanged Noble Metal Nanoparticles in Glasses for Sers Applications
#### 3.2.4 Laser Irradiation Techniques  
Besides the aforementioned thermal post-processes, other annealing techniques involve the use of continuous [102; 165; 166; 167] and/or pulsed lasers [98; 101; 168; 169; 170; 167; 168; 169; 171] working at different wavelengths of the spectrum (from X-ray to ultraviolet (UV) and visible (VIS) regions up to infrared (IR) ones) and able to effectively reduce the noble metal ions in metal NPs. In this case, the mechanism of nanoparticle formation depends on the power of the laser source at the surface of the ion-exchanged specimen. In fact, in many cases the interaction between the laser beam and the sample is of thermal nature, as occurs for silver-ion-doped glasses when the working wavelength of the light source falls in the sample absorption region (i.e., from the UV to visible band, depending on the electronic status of silver). If the power level of the laser is relatively low, so as to ensure that the temperature of the irradiated area remains close to the glass transition temperature \(\mathrm{T_{g}}\) during the whole process, then the reduction in silver atoms follows a trend almost similar to that of the thermal annealing mentioned above (in O\({}_{2}\) environment): laser irradiation produces free electrons from the non-bridging oxygens (NBOs) which are captured by the Ag\({}^{+}\) ions to form neutral Ag\({}^{0}\) atoms (metal silver atoms), starting their following growth process in metal NPs. As in the previous case, the nanoparticles thus formed are completely embedded within the glass matrix and a further etching step is required to make them partially exposed on the surface. This makes them available for SERS applications. Conversely, at higher laser power levels, the temperature of the glass surface becomes greater than the glass \(\mathrm{T_{g}}\) and the sample, in the irradiated region, behaves as a molten glass. In this liquid state, some of the silver nanoparticles are formed in a deeper region of the irradiated area while others may migrate to the glass surface where they are constrained to remain under the effect of the surface tension. Here, following the temperature distribution, they move towards the edges of the laser beam where they continue to aggregate forming larger structures. Once the laser is turned off, these nanoparticles remain frozen and partially exposed to the air [102; 165; 167]. In this way, these metal nanostructures are ready for SERS sensing and no further etching step is required. If compared with the standard thermal annealing process in air, the one induced by laser irradiation is faster in terms of nanoparticle formation and offers the non-negligible advantage of being able to draw ordered structures of nanoparticles--equally spaced from each other--so that the SERS response of the substrate is enhanced [102; 166]. Figure 6 sketches the mechanism of metal nanostructure formation by high power laser beam in the case of silver ion-exchanged glass (Figure 6a). Figure 6b reports a SEM image of the edge region between irradiated and not-irradiated area. The lack of visible nanostructures on the surface of the exposed region suggests the possibility to obtain silver nanoparticles buried beneath the glass surface. Conversely, densely packed spherical silver NPs, whose size distribution ranges from a few to several tens of nanometres, are clearly visible on the ion-exchanged glass surface at the border of the irradiated area. Figure 6c shows a SEM image of a quasi-periodic arrangement of silver NPs obtained with this technique. Finally, Figure 6d represents a zoom image of an array of silver nanoparticles. The alignment of the nanostructures appears quite regular. Obviously, once exposed, these silver NPs can undergo oxidation effects that affect their performance in terms of SERS response.