## 3 Results  
Because of the poor wettability of Ag nanofilms on silicon substrates, a large number of voids and gaps exist in the ultrathin Ag nanofilms created using physical sputtering fabrication methods, and thus the photothermal effects can be relatively large [17]. As shown in Figure 1a,b, Ag nanofilms with a large number of voids and gaps segregated and conglobated together within a short time period (about 15 min) under light illumination. Reference test samples covered with Al foil (thickness 1 mm) are provided in the insets framed by the yellow dotted line. The Al foil-covered samples here can reflect the light and maintain a similar temperature as the samples under light illumination. Interestingly, the conglobations of Ag nanofilms with thicknesses below 10 nm display mutual bonding under light that is different from the characteristics of the sample under the cover. As a result, the average particle size of the Ag nanoparticles counted using ImageJ were much larger than without the light illumination when the thickness of the Ag nanofilms was below 10 nm, as shown in Figure 2a,b. As the thickness of the Ag nanofilm was increased to 20 nm, the light-induced conglobation of the Ag nanofilms exhibited an increase in the size of the voids and gaps in the sample. As shown in Figure 1c, the conglobation of the Ag nanofilms occurs at the voids and gaps--the light-induced conglobation is much larger than that induced only with the heat treatment in the reference tests. The results show clearly that the energy provided by the light illumination is not sufficient for the thorough conglobation of the Ag nanofilms into Ag nanoparticles on the silicon substrate when the thickness of the Ag nanofilms was increased to 20 nm. When the thickness was further increased to 30 nm, the Ag nanofilms became continuous films, with few voids or gaps observable. As shown in Figure 1d, few distinct conglobations occur on the 30-nm-thick Ag nanofilms via either light illumination or heat treatment below 140 \({}^{\circ}\)C.  
Figure 1: Conglobation of Ag nanofilms with various thicknesses with and without light illumination; (**a–d**) Ag nanofilms with thickness 5 nm, 10 nm, 20 nm and 30 nm, respectively, under light and under Al foil covers (insets). The scale bar in the inset is 250 nm. Figure 2: Statistical results of the diameters of Ag nanoparticles generated by Ag nanofilms with and without light illumination; (**a**) Ag nanofilms with thickness 5 nm; (**b**) Ag nanofilms with thickness 10 nm. From a thermodynamics perspective, the conglobation of metallic materials far below their melting points \(T_{m}\) can be primarily attributed to atomic diffusion [18]. The atomic diffusion temperature called Tamman temperature \(T_{tm}\) is only about 0.3 \(T_{m}\)[19]. Meanwhile, the melting points of the nanoparticles and nanofilms will decrease along with their size [20]. The surface energy of Ag nanofilms increases as their thickness is reduced, which reduces the energy barrier and leads to surface atomic diffusion [21]. As shown in Figure 3, atomic diffusion in Ag nanofilms with 5 nm thickness generally requires a temperature of 230 \({}^{\circ}\)C, which is much higher than the surface temperature of the specimens in our case. To further understand such thickness-dependent light-induced conglobations, a model based on FDTD method was used to study the light enhancement of Ag nanofilms on silicon substrates. Herein, surface gap and void structures in Ag nanofilms at different depths were used to simulate actual grain boundaries (gaps) and holes (voids) in ultrathin Ag nanofilms under light illumination. The incident light adopted in this case was a plane wave with a wavelength of 800 nm that vertically illuminated the Ag nanofilms. According to the experimental results, a large number of voids exist when the thickness of the Ag nanofilms was reduced to 10 nm. Voids in the Ag nanofilms increased in size as the thickness of the Ag nanofilms was decreased, as shown in Figure 4a.