## 3 Results and Discussion
### Surface Morphology
#### 3.2.2 Effect of Pulse Width  
Thermal effects in the laser process are also considered to be an important factor in removing excess PVP on the surface and implanting silver nanoparticles in the pore. To investigate the effect of the pulse width which significantly affects the thermal effect during the process, line irradiation was performed on the YSZ surface using three different pulse widths: femtosecond (fs), picosecond (ps), and nanosecond (ns). The scanning speed was adjusted to become the same pulse overlap rate for each laser. The SEM images of the irradiated surface at different parameters are shown in Figure 8. When irradiated with low laser fluence (Figure 8(a1-c1)), there were many embedded nanopores on the YSZ surface at 256 fs. Materials in the pore were still observed at 10 ps. On the other hand, they were completely removed after irradiating nanosecond pulses of 26 ns. White-colored spheres with diameters on the order of hundreds and tens of nanometers were densely adhered to the surface of the original grains around the pores. When irradiated with high laser fluence (Figure 8(a2-c2)), LIPSS were formed on the surface, and almost no embedded materials were in the pores at 256 fs. By increasing pulse width to 10 ps, LIPSS formation was suppressed and only the aperture of the pore split, keeping materials in the pore. At 26 ns, the YSZ surface was melted and each grain was difficult to identify. Some microcracks were also generated along the original grain boundaries. No residue remained in the pores, but very small particles were attached to the surface. Figure 7: EDX analysis of YSZ surface after embedding silver nanoparticles by laser irradiation: (a) SEM image, (b) elemental mapping images, (c) distribution map of nanopore (orange color) and silver (blue color), (d) magnified image of silver-embedded nanopore surface. To compare differences in PVP response due to thermal effects, a sample was heated in an electric furnace at 450 \({}^{\circ}\)C to remove surface deposits instead of using a laser. The heating temperature was set to slightly exceed the decomposition temperature of the PVP. Figure 9 shows the surface morphology of the sample. Figure 9a shows many spheres attached to the YSZ surface, and their size seemed quite larger than those observed on the surface irradiated by a nanosecond pulsed laser (Figure 8c). By the backscattered electron (BSE) image of Figure 9b, the spheres were detected in a brighter color than the YSZ substrate, which indicates that it is composed of heavier elements than zirconium, i.e., silver without PVP. It was found that all of the larger silver spheres were attached to the pore-forming region but did not penetrate the interior of the pore. The results in Figures 8 and 9 suggest that laser processing, with a shorter pulse width, is suitable for implantation into the pore, rather than simply heating the surface. Figure 8: SEM images of the irradiated surface with varying pulse width: (**a**) 256 fs (10 mm/s), (**b**) 10 ps (10 mm/s), (**c**) 26 ns (53 mm/s). (**a1–c1**) are irradiated at laser fluence of 0.3 J/cm\({}^{2}\), and (**a2–c2**) are irradiated at laser fluence of 0.7 J/cm\({}^{2}\). Figure 9: YSZ surface after heating in an electric furnace: (**a**) secondary electron (SE) image, (**b**) backscattered electron (BSE) image. According to the abovementioned results, the surface morphology of dried nano-silver dispersion film and YSZ substrate after laser irradiation under different parameters were summarized. Figure 10 shows parameter maps. The range of the scanning speed in each graph was unified so that the overlap rate was the same. At 256 fs, the dried film was able to be removed in a limited range of laser fluence and scanning speeds. There was a tendency for the laser beam not to affect the surface at a higher scanning speed for each laser fluence, and the YSZ surface structure changed to LIPSS at a lower speed. The range of conditions in which film was removed was further reduced by increasing pulse width.