## 2 Method  
### NiO Ink Preparation and Fabrication Process of Thin Film  
For NiO nanoparticle ink preparation, the ink formula consisted of 0.5 g of NiO nanoparticles (Sigma-Aldrich, Burlington, MA, USA), 8 g of deionized water, and 2 g of propylene glycol methyl ether. The ink formula was referred to that of [PERSON] et al. [19]. However, the particle size distribution was altered for the aerosol jet printer (AJP) with an average particle size of \(<\) 50 nm. NiO nanoparticles are insoluble in water, and the pH value of the recipe was adjusted to avoid particle aggregation and precipitation, achieving a stable suspension of nanoparticles. In a previous study [20], the suggested pH value for AJP ink was 3 to 5. An AJP with a nozzle diameter of 150 \(\upmu\)m (Optomec, Albuquerque, NM, USA) was used to print NiO nanoparticle thin films in this study. An AJP features an atomizer that turns ink into small droplets; then, N\({}_{2}\) pressure transfers the droplets to the nozzle, and the nitrogen flow is used to focus the aerosol stream by controlling the width of the print track [26].The AJP used in this study has two atomizer modules, namely, an ultrasonic atomizer (Figure 1 left) and a pneumatic atomizer (Figure 1 right). Viscosity is the main determinant of which module should be employed. An ultrasonic atomizer is only capable of operating with low viscosity (1-10 cP), whereas a pneumatic atomizer is made for high-viscosity ink (1-2500 cP); the ultrasonic atomizer module was applied in this study. An AJP can operate on curved surfaces, the printed ink tracks can be formed without aligning the nozzle on the substrate; aerosol jet printing differs starkly from conventional printing techniques. For the detailed information of printing process, the width of printing the NiO track was 30 \(\upmu\)m, the printing speed was 2 mm/s, and the spacing of tracks was 20 \(\upmu\)m; therefore, the overlap of the tracks was 10 \(\upmu\)m, which produced even NiO thin films. In Table 1, the detailed information of the raw NiO thin films was exhibited. The substrates used for printing NiO thin films were commercial flat glasses with indium tin oxide (ITO) interdigital electrodes that were designed for resistance signal detection, as shown in Figure 2b. In Figure 2a, the printed NiO thin films are shown by 4 mm\({}^{2}\) gray dashed square. These ITO interdigital electrodes were open circuit, and thus the resistance signal can only be detected after sintered NiO thin films were fabricated on the electrodes. In our previous study [21], NiO thin films were fabricated over the \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Raw NiO Thin Films** & \
\begin{tabular}{c} **Average Thickness of** \\ **NiO Thin Films** \\ \end{tabular} & **Resistance at 100 \({}^{\circ}\)C** &
\begin{tabular}{c} **Height of** \\ **Satellite Particles** \\ \end{tabular} & **Average of Roughness** \\ \hline
15 \(\upmu\)m & 106.8 M\(\Omega\) & 1–5 \(\upmu\)m & 1.63 \(\upmu\)m \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Detailed information of non-sintered NiO nanoparticle thin films as fabricated by aerosol jet. Figure 1: The aerosol jet printer modules. Ultrasonic atomizer (**left**) and pneumatic atomizer (**right**). silver electrodes and the substrates were thermal resistant tape, Kapton(r) (DuPontTM, Inc., Wilmington, DE, USA). The study found that the thermal resistant tape could be deformed under 80% UV LED light irradiation. Furthermore, in Figure 3b, height differences were clearly visible on the surface. In this color map, the red color indicated the highest spot, while blue represented lowest spot. According to the result, the thickness of NiO thin films was only 15 microns thick, yet the sliver electrodes were approximately 15\(\sim\)17 microns. The overlap of NiO and sliver caused a significant height difference, which might have affected the results of UV laser sintering. Therefore, the commercial ITO glass substrates were applied to eliminate the factor of height difference and substrate deformation in this study (see Figure 3a).