## 3 Results and Discussion  
### Film Morphology and Material Characterization  
An optical microscope image of a typical ZnO thin film coating formed using PBM nanoink and doctor blading is shown in the inset of Figure 2a. The coatings displayed good uniformity, stability, and adhesion under ambient conditions and up to 200 \({}^{\circ}\)C. The SEM image in Figure 2a shows that the milled ZnO films consist of fine nanostructured particles with distributed pores. The ZnO film surface topography was further characterized using AFM (Figure 2b). As expected, particles were ground into finer sizes as grinding speed was increased, and we observed a reduction in root mean square (RMS) film roughness, as measured with AFM (Figure 2c), which drops below 80 nm for grinding at 1000 rpm for 10 min. Higher grinding speed also concurrently reduced particle size below 100 nm (Figure 2d). A similar decreasing trend in particle size/film roughness was observed as grinding time was increased (at constant rpm) (see Figure 2e) [60]. The photoluminescence spectrum of a typical film is shown in Figure 2f. The milled ZnO thin films showed five different peaks of various intensity levels at different wavelength ranges, which can be correlated with the electronic and structural properties of the milled particles. Consistent with previous studies, the 465 nm peak (blue emission band) is attributed to deep level emission originating from oxygen vacancies or interstitial zinc ions of ZnO [76]. The green emission (525 nm) is due to singly ionized oxygen vacancy or oxygen antisite defect O\({}_{\text{Zn}}\)[55]. Similarly, the peak at 577 nm is most likely due to disorder at the surface of nanoparticles after milling [55]. Lastly, the other peaks near 590 nm and 655 nm emission are usually attributed to oxygen vacancies [77; 78]. Raman spectra for both PBM nanoink thin films and bulk starting powder are shown in Figure 2g and h. ZnO most often has a wurtzite structure, and there are two A\({}_{1}\), two E\({}_{1}\), two E\({}_{2}\), and two B\({}_{1}\) modes in the Raman spectra of its crystal structure [79]. The most common Raman intensive E\({}_{2}\) (low) mode at \(\sim\)99 cm\({}^{-1}\) is just beyond the range of our detection; however, the other Raman mode, E\({}_{2}\) (high), at \(\sim\)437 cm\({}^{-1}\) is visible, which is assigned to oxygen vibrational modes [80]. E\({}_{2}\) (high) mode is most prominent in the starting material; after milling, the intensity of the peak decreases and becomes broadened. Lowered intensity and peak broadening of the 437 cm\({}^{-1}\) peak indicate a change in band structure and crystallinity of nanostructures after milling. The Raman spectra of both ground and bulk powder display three different peaks at about 206, 329, 379, and 412 cm\({}^{-1}\), where these peaks are attributed to 2 TA:2E\({}_{2}\), E\({}_{2}\) (high)-E\({}_{2}\) (low), A\({}_{1}\)(TO), and E\({}_{1}\) (TO) symmetry, respectively [80]. However, broader Raman peaks and lower intensity are attributed to size effects, lattice strain, and lower crystallinity [55; 81]. The decrease in crystallinity is attributed to defects produced during ball milling. For the milled nanoparticles, we observed a broad peak around 580 cm\({}^{-1}\) that became more prominent for longer grinding time or higher speeds. It is predicated that peaks with high intensity from 560 to 580 cm\({}^{-1}\) indicate the presence of defects related to oxygen vacancy (V\({}_{\text{o}}\)) and/or zinc interstitial (Zn\({}_{\text{i}}\)) [80; 82], which could be advantageous for gas sensing [19]. The milling media (grinding beads, solvents) can also affect the particles, but this islikely only significant for longer grinding times and/or higher speeds (see red curve in Figure 2h--the peak at 189 cm\({}^{-1}\) is assumed to be related to the zirconia grinding beads [83]). The EDX results for our ZnO gas sensor films (Figure 2i) confirmed the presence Zn and O elements, with average atomic percentages of 48.05 and 51.95, respectively.