## 3 Results and Discussion
### XRD Analysis  
Besides morphology, XRD analyses assisted us to obtain a better understanding of the reaction of UV laser to the NiO particles and the mechanism of cracks. The XRD patterns of NiO thin films are shown in Figure 15. These detected 2\(\theta\) at 37.2\({}^{\circ}\), 43.3\({}^{\circ}\), 62.9\({}^{\circ}\), 75.5\({}^{\circ}\), and 79.5\({}^{\circ}\) respond to the diffraction signals of crystal planes (111), (200), (220), (311), and (222), respectively. Since XRD penetrated the thin films and reflected the signal of ITO electrodes, there were redundant peaks that represented the diffraction of the ITO electrodes and glass substrate at 2\(\theta\) at 30.32 and 35.22, although the XRD operation was a grazing incidence small angle. Yet, these unwanted signals did not take effect on the NiO peaks and could be ignored. The 2\(\theta\) results illustrated that there were no redox reactions in NiO, and unpredictable peaks did not appear. Moreover, the diffraction intensities enhanced as the UV laser energy absorption increased. This phenomenon indicated the signals of crystal planes were enhanced because of the increasing combination of particles; however, Figure 15 shows the intensity slightly declined in the experimental group of 2.5 W laser output since the uneven thickness, defects, and strains affected the diffraction intensity [32], causing peak shifting and broadening [33]. Therefore, according to SEM photographs, the micro-strains of 2.5 W sintered thin films were more likely to reduce the diffraction intensity (Figure 16). Figure 14: SEM morphologies (magnification 5k\(\times\)) of 2W sintered thin films with scanning speed 150 mm/s, and frequencies 90 k Hz (**a**), 105 k Hz (**b**), and 120k Hz (**c**). Figure 15: XRD patterns of sintered NiO thin films, which were sintered by different laser power outputs and scanning speeds. The results illustrated that there were no redox reactions in NiO, and unpredictable peaks. In Figure 17, the crystal plane distances exhibited that there was no significant increase while laser energy increased. The maximum difference of crystal plane distances was only approximately 3 nm. There was no obvious recrystallization or growth of the grain in the films since the excessive laser energy damaged the fragile printed thin films easily. However, there was a trend that crystal plane distances slightly declined at laser power 2.5 W while scanning speed accelerated. According to [PERSON]'s equation (Equation (2)), the longer crystal plane distances can be stretched; thus, the larger \(\beta\) (FWHM) of peaks are broadened [33]. In the status of no recrystallization occurrence, the strain was the reason that broadens the \(\beta\) (FWHM). Therefore, the strains were built in the NiO thin films during the sintering process in the range of laser energy from 1.5 W to 2 W, causing the crystal plane distances to raise slightly. However, as significant cracks appeared, the stress between boundaries was released. In conclusion, the cause of the varied crystal plane distance was not recrystallization but strains. It eventually formed the cracks, which decreased the electrical conduction of the thin films. For the mechanism of the strain analyses, the Williamson-Hall plots were drawn to analyze the strain in the thin films. By applying this method, the strain can be acquired  
Figure 16: Intensity of main picks of all laser output parameters. Figure 17: Crystal plane distance of all laser parameters. from the slope of the linear regression in the plots. Figure 18 shows the examples of the Williamson-Hall plot from results of varied laser output with 100 mm/s scanning speed. The positive slope of linear regression indicated tensile stress in the Williamson-Hall plot. In previous NiO thin film studies, tensile strain frequently occurred while the furnace sintering processes were applied [34; 35; 36] because of the long calcination time and high furnace temperature, which provided adequate conditions for grain growth.