## 2 Materials and Methods  
### Fabrication of the AD ZnO (AZ) Films  
ZnO powder (powder size - 100 nm, Sigma Aldrich, Hamburg, Germany) was used for the AD of ZnO thin films. The ZnO powder was dried for 4 days at 100 \({}^{\circ}\)C for moisture removal to avoid the agglomeration of ZnO powder. Fluorine-doped tin-oxide (FTO) conductive glasses with size of 20 mm \(\times\) 30 mm and thickness of 1.6 mm (Nippon Sheet Glass-FTN 1.6, Osaka, Japan) were used as substrates for ZnO thin film deposition.  
The details of the AD equipment used are reported in our previous study [36]. In the AD process, high purity nitrogen gas (99%) was used to carry and accelerate the ZnO powder onto the FTO substrate. The flow rate of carrier gas was fixed at 30 L/min. The pressure of the AD chamber was maintained at 20 Pa during the AD process. The deposition time per cycle was 4 min and 15 s. The area of the deposited film was 10 \(\times\) 20 mm\({}^{2}\). In order to investigate the effect of the number of deposition cycles, ZnO films were aerosol-deposition for one, two, and three cycles. The ZnO films deposited with different cycles were labelled as AZ-I, AZ-II and AZ-III, respectively. The geometry of the nozzle used in AD is displayed in Figure 1. All other deposition parameters were kept constant as listed in Table 2.  
After aerosol deposition, the ZnO films obtained were dried at room temperature and characterized. X-ray diffraction (XRD) (RINT 2500, Rigaku, Tokyo, Japan) with a Cu K\(\alpha\) radiation (\(\alpha\) = 1.54059 A) was carried out to investigate the crystallinity of the films. The size of the ZnO crystallite was calculated using the Debye-Scherrer formula as shown in Equation (1) [37].  
\[D=\frac{0.9\lambda}{\beta\cos\theta} \tag{1}\]  
\begin{table}
\begin{tabular}{l l} \hline \multicolumn{2}{c}{**Deposition Parameters**} \\ \hline Raw material & ZnO (\(\sim\)100 nm) commercial powder \\ Carrier gas & Nitrogen \\ Substrate & FTO [20 \(\times\) 30 mm\({}^{2}\), 1.6 mm (t)] \\ Working distance (mm) & 10 \\ Carrier gas flow rate (L/min) & 30 \\ Deposition cycles & 1, 2, 3 \\ Deposition time (min)/cycle & 4.25 \\ Pressure (Pa) & 20 \\ Deposition Temperature & Room temperature \\ \hline \end{tabular}
\end{table}
Table 2: Deposition parameters used for the ZnO films during AD process.  
Figure 1: Schematic and photograph showing the geometry of the nozzle used for the aerosol deposition.  
where \(D\) is the crystallite size (nm), \(\beta\) is the broadening of the diffraction line measured at half of its maximum (FWHM) intensity (rad.) and \(\lambda\) is the X-ray wavelength (1.5406 A). The morphology and thickness of ZnO films were characterized using field emission scanning electron microscope (FESEM) (S-4800, HITACHI, Tokyo, Japan) at an accelerating voltage of 40 kV. The optical properties of ZnO films were evaluated using a UV-Visible spectrometer (V-670, JASCO Corporation, scanning range: 350-700 nm) and Raman spectroscopic (NRS-3100, JASCO Corporation, Tokyo, Japan). The photoluminescence (PL) characteristics were investigated using a He-Cd laser (KIMMON KOHA, Tokyo, Japan, excitation wavelength, \(\lambda\) = 325 nm) as an excitation source, and the spectra were recorded using a monochromator (Nikon G250, Tokyo, Japan).