*2.1. Materials*

All of the compounds chosen for this research were conducted without additional purification. Zinc acetate dihydrate (Zn(CH3COO)2·2H2O) (BHD, Poole, UK), silver nitrate AgNO3 (Sigma, Ronkonkoma, NY, USA), hexamethylenetetramine (C6H12N) (HMTA, Merck, Darmstadt, Germany). The ammonia (NH3) and ionized water were available in the laboratory of School of Physics Universiti Sains Malaysia.

#### *2.2. Preparation of Ag-Doped ZnO Thin Films and Measurements*

The simple hydrothermal method was employed to synthesize Ag-doped ZnO thin film growth on the glass. The solution was prepared by mixing 0.1 mol of Zinc acetate dihydrate and 0.1 mol of hexamethylenetetramine in 100 mL of ionized water. After that, the mixture was vigorously stirred for half hour. AgNO3 (0, 0.5, 1.0, 3.0, and 5.0 wt%) was introduced to the prepared solution and continuously stirred for 1 h to obtain a homogeneous solution. Then, NH3 was added dropwise into the solution until the transparent solution was acquired. The solution was then poured into 150 mL Teflon autoclave, with the immersion of the glass substrate (slab glass that deposited the thin films on it), before it was closed tightly and heated at 180 ◦C in the furnace for 12 h, after which it was cooled to ambient temperature. The thin film was rinsed several times using deionized water. Finally, the sample was annealed at 400 ◦C for 1 h in the furnace for impurity removal. This procedure was performed several times for different concentrations of Ag, as previously mentioned.

Meanwhile, the structure analysis of pure ZnO and Ag-doped ZnO thin films was measured by X-ray diffraction (XRD) (Malvern PANalytical, Malvern, UK) and equipped with a Cu-K emission wavelength at (0.154 nm), for the range of 2θ at 20◦ ≤ 2 θ ≤ 80◦. Besides that, the morphology of the thin films was studied by scanning electron microscope (SEM) (JSM-6460LV SEM JEOL Ltd, Tokyo, Japan). The nanocomposite of the thin films was depicted via energy-dispersive X-ray (EDX) (Tokyo, Japan) analysis. In addition, photoluminescence (PL) (Jobin Yvon HR 800 UV Jobin Yvon, Kyoto, Japan) was investigated using a laser with a power of 0.18 mW, and the excitation source's wavelength was 325 nm. Ultraviolet visible (UV–vis) spectrophotometry (Model Cary 5000 UV-VIS-NIR Agilent Technologies, Santa Clara, CA, USA) was applied to measure the transmittance.
