## 3 Results and Discussion
### Optical Properties of the AZ Films  
Figure 8 shows the UV-Vis results of the ZnO films obtained using different deposition cycles. The adsorption spectrum was recorded in the wavelength range 200-800 nm. All the ZnO films exhibited UV absorbance at wavelengths of less than 400 nm. The AZ-I film exhibited an absorption edge at 325 nm. As for the AZ-II and AZ-III, the UV absorbance edge was at 432 nm and 339 nm, respectively. The result indicates that the optical properties of the ZnO films change with the number of deposition cycles. During AD process, fragmentation of the raw particles could cause a reduction in the crystallites (1/10 th of the original raw material particles) [29]. Therefore, as the size of the ZnO used in this study was approximately 100 nm, it is thought that particle fragments generated could have resulted in different optical properties due to the abovementioned phenomenon.  
Figure 6: The thickness of ZnO films obtained with different deposition cycles.  
Figure 7: Schematic representing the formation of ZnO films using the AD method under different deposition cycles (**a**) AZ-I, (**b**) AZ-II, and (**c**) AZ-III.  
The shift in absorbance edge of UV for the ZnO films may be caused by the defects generated within the ZnO crystals during the high impact collision during the RTIC. The presence of defects in ZnO films, which act as recombination centres, can be identified using room temperature photoluminescence (RTPL). Figure 9a shows the RTPL spectra of the aerosol-deposited ZnO films, which exhibited a typical near band edge emission (NBE) at approximately \(\sim\)380 nm and a broad visible light emission (deep level emission, DLE). The NBE is associated with the release of energy due to transition of electrons from the conduction band (CB) to valence band (VB) of ZnO [41]. The maximum peak of NBE for the ZnO films produced at one, two and three deposition cycles are 387, 384 and 383 nm, respectively. In addition, the intensity of NBE was not dominant for all the ZnO films, suggesting a rather low crystallinity of ZnO films.  
Figure 8: Absorbance spectra of the aerosol deposited ZnO films obtained using different cycles.  
Figure 9: (**a**) RTPL spectra of the aerosol-deposited ZnO films, and (**b**) \(\rm I_{UV}/I_{Vis}\) ratio.  
On the other hand, the dominant DLEs observed are defect related emissions. The excited charge carriers recombination at the crystal defects would then produce luminescence signals with less energy than the band gap energy [42]. The DLE of ZnO could originate from oxygen interstitials (O\({}_{\text{i}}\)), oxygen vacancies (V\({}_{\text{O}}\)), oxygen substitution in Zn site (O\({}_{\text{Zn}}\)), Zn interstitials (Zn\({}_{\text{i}}\)), Zn vacancies (V\({}_{\text{Zn}}\)), or Zn substitution in the oxygen site (ZnO) [43]. In this study, the maximum peaks of DLE for AD-I, AD-II and AD-III are 558, 560 and 541 nm, respectively. The green emission, i.e., 558-560 nm obtained is associated with oxygen vacancies (V\({}_{\text{O}}\)) in the ZnO films [44], whereas the yellow emission (541 nm) was related to the interstitial oxygen (O\({}_{\text{i}}\)) [45]. The strong emission intensity of the AZ-III sample in the visible region indicates an existence of high defects concentration in the ZnO film. This could be caused by repetitive collisions during AD that generated the crystal defects, loose agglomerated particles and rough film's surface as shown in Figure 5e,f. Figure 9b shows the ratio of UV peak intensity to visible light emission intensity (I\({}_{\text{UV}}\)/I\({}_{\text{Vis}}\)). A low I\({}_{\text{UV}}\)/I\({}_{\text{Vis}}\) ratio indicates that the ZnO films possess poor crystal quality. In comparison, the AZ-III sample exhibited the lowest crystal quality as the smallest I\({}_{\text{UV}}\)/I\({}_{\text{Vis}}\) ratio (0.53) was obtained while the I\({}_{\text{UV}}\)/I\({}_{\text{Vis}}\) for AD-I and AD-II are comparable (0.82 and 0.84).