## 3 Results and Discussion
### Morphology of the Printed NiO Thin Films  
On the basis of the results of electrical properties analysis, laser output was the most influential factor. Therefore, Figures 9 and 10 shows the morphologies of sintered NiO thin films, which were classified by laser output with identical scanning speed (100 mm/s) and frequency (105 k Hz). The confocal microscopy images of the thin films subjected to the 1.5 W output is shown in Figure 9a. The surface exhibited several light color spots, which were residual solution and nonuniform particle accumulation. According to the resistance values with 1.5 W laser output, the laser energy settings were too low to completely sinter the NiO thin films, which caused defects to remain on the surface and high resistance performance. On the other hand, the surface, the 2 W sintered thin film, was even and without visible defects in Figure 9b. According to the aforementioned resistance results, this smooth surface responded to high electrical properties. On the contrary, the significant cracks observed in the NiO thin films subjected to the 2.5 W output (Figure 9c) led to the increase of resistance. In Figure 10, the SEM images with higher resolution provided us with more comprehensive analyses of microscopic morphologies. The SEM morphology of NiO thin film with the 1.5 W output shown in Figure 10 had a porous and uneven surface. Since inadequate sintering energy irradiated on the NiO thin films, the incomplete sintering of original NiO nanoparticles kept the surface rough, causing high resistance. The morphologies were clearly superior in the cases of the 2 W and 2.5 W laser outputs. The surface of the thin film to which the 2 W laser output was applied showed that the particles were joined and merged, while some negligible cracks were observed. Compared with the electrical performance of 1 W and 2.5 W, the electrical properties of 2 W sintered NiO thin films were the greatest. Although the surface of the thin film, to which the 2.5 W laser output was applied, displayed a more even surface, the cracks were obvious, which were sufficiently large to reduce the electrical conduction [30]. Since the adjacent  
Figure 8: Sensitivity of all laser parameters. particles were joining together and holes and defects were left at the original positions of the merged particles, the cracks were formed by uneven stress and thus caused the resistance to increase. To observe the effect of scanning speed variation from each laser output, Figures 11-13 show the morphologies that were sintered by 1.5 W, 2 W, and 2.5 W with different scanning speeds, respectively. The SEM photographs of NiO thin films with 1.5 W output are shown in Figure 11. Since the particles were barely sintered, the surfaces of the thin films retained the shape of nanoparticle agglomerations. Moreover, according to the resistance experimental results, the resistances decreased drastically as the speed increased. The significant morphological change illustrated that the sintering process was in an initial stage, where the particles rapidly joined together [31]. Figure 12 shows the micro-surface of the NiO thin films sintered by varied scanning speeds with 2 W. The change of morphologies was not obvious under different scanning speeds. The minor cracks observed on the surface did not extend when the scanning speeds were reduced. According to the resistance experimental results, the effect of tiny cracks on electrical property was negligible. For the 2.5 W laser output, the surfaces were fairly even, and the particles were closely joined together. The level of flatness was even higher than that of the 2 W sintered surface (Figure 13, SEM images). However, the resistances' performance subject to the 2.5 W output increased, while the scanning speed slowed down. Cracks extended due to excessively high-intensity energy, which eventually affected the electrical conductivity. Figure 10: The SEM photographs (magnification 5k\(\times\) images) of thin films were sintered by same scanning speed and frequency but with different power output.