## 3 Results
Accordingly, light in the voids and gaps at different depths in the Ag nanofilms are presented in Figure 4c,d. Significant light enhancement can be observed in the voids and gaps in the Ag nanofilms. Additionally, the optical density enhancement in the voids is much higher than that in the gaps. Owing to the significant absorbance of Ag nanofilms, localized heat can be generated in these voids and gaps, causing the atoms to shift in these voids and gaps. A schematic diagram is presented in Figure 4b to illustrate the evolution of the transformation of the voids and gaps into Ag nanoparticles. The gaps grow into voids and the Ag nanofilms separate into small pieces; then, the small pieces conglobate into Ag nanoparticles because of their high surface energies. For the thicker Ag nanofilms with thicknesses larger than 30 nm, few voids are observed; these films are continuous. Therefore, light is rarely enhanced in these nanofilms, as shown in Figure 4e,f. Meanwhile, the surface energy of the Ag nanofilms is greatly reduced as the thickness is increased. As a result, surface conglobation can barely be observed for Ag nanofilms with thicknesses of 30 nm. It should be noted that the effects of surface fluctuations on the Ag nanofilms were also been studied using FDTD. However, the surface fluctuations cannot couple light into the highly localized surface plasmons on the ultrathin Ag nanofilms because of the films' irregular morphologies, as shown in Figure 4g,h. Therefore, plasmonic enhancements in the void and gap structures are likely the main reason for the conglobation of Ag nanofilms. Figure 3: Tamman temperature of Ag nanoparticles and Ag nanofilms. When the Ag nanoparticles conglobate from the Ag nanofilms, plasmonic enhancement occurs at the interfaces, as shown in Figure 5a. Analogous to Ostwald ripening, small Ag nanoparticles can merge together with the assistance of plasmonic heating at the interfaces because of the nanoparticles' low \(T_{m}\) values, as shown in Figure 2. As the merging process proceeds, the plasmonic enhancement at the interfaces gradually reduces and finally causes the Ag nanoparticles to cease merging, as shown in Figure 5b. The plasmonic enhancement-assisted merging process of the Ag nanoparticles is also presented in a schematic diagram in Figure 5c. Compared with only the heat treatment, plasmonic heating yields Ag nanoparticles with larger particle sizes for Ag nanofilms with similar thicknesses. Figure 4: Voids induced plasmonic enhancement on Ag nanofilms with various thicknesses; (**a**) schematic diagram of voids in Ag nanofilms with various thicknesses; (**b**) schematic diagram of the evolution of gaps/voids in Ag nanofilms with light illumination; (**c**–**f**) plasmonic enhancement of gaps/voids on Ag nanofilms with 6 nm, 10 nm, 20 nm and 25 thicknesses; (**g**), (**h**) plasmonic enhancement of surface fluctuations on Ag nanofilms (thickness 10 nm) with average \(\pm\)2 nm and \(\pm\)4 nm. To demonstrate the practicality of the conglobation process for use with the complex topological structures presented by carbon spheres, carbon sphere samples with smooth and porous surfaces were first deposited before sputter coating 20-nm-thick Ag nanofilms on top. Carbon spheres and their core-shell structures with noble-metal nanoparticles have a broad range of applications in fields such as drug delivery [22], hydrogen storage [23], biodagnostics [24] and photothermal conversion [25]. After 15 min of light illumination, the Ag nanofilms fully and uniformly segregated, forming nanoparticles with average diameters of about 30 nm on the top of both the carbon sphere samples with smooth and with porous surfaces, as shown in Figure 6a-f. The ambient temperature of the specimen was lower than 140 \({}^{\circ}\)C, which is far below the melting points of the Ag nanofilms.