*2.3. Thermoluminescence Measurments*

In this study, the process of preparing the samples was carefully considered; the weighing by difference technique was applied to measure the mass of the thin films. After

that, the thin film was cut into 5 × 5 mm2. The samples were then labeled and placed in opaque containers. Before irradiating the samples, the annealing dosimeter procedure was first conducted to remove any traps. Annealing treatment is a method used to eliminate the residual signal, which may cause unwanted background readings in this work. The samples were kept under specific circumstances, to shield them against external physical and environmental effects such as dust and cleaning. Figure 1 shows the setting of the samples for irradiation using an X-ray machine (Toshiba KXO-50S) (Toshiba Medical Equipment, Tokyo, Japan) with its control panel setting, as mentioned in our previous study but with a slight modification, where the SSD and the field size were set at 80 cm and 10 × 10 cm2, respectively [35]. In the present work, the temperature–time profile was preheated at 50 ◦C, and the maximum temperature was 400 ◦C. The heating rate was carried out within the intervals of (1, 3, 5, 7, and 10 ◦C/s) for all three compositions, where the samples were irradiated by X-ray radiation, and each data point is an average of three samples to estimate the average TL intensity and standard deviation. Win REMS application software (USA) for TLD readers involved two phases of preheating.

**Figure 1.** The schematic diagram of the experimental set-up for X-ray irradiation dose using Toshiba KXO-50S X-ray.

There are two steps in the readout (acquisition phase): collecting light emitted during the heating process and converting the light into an integrated value (display glow curve). In order to measure the percentage depth doses (PDD), 10 Perspex phantom slides were used, where the thickness of each slide was 1 cm. HARSHAW TLD Model 3500 (Thermo Fisher, Waltham, MA, USA) was utilized. Initially, the reader was warmed for 30 min, and nitrogen gas was turned on at a flow rate of 3.0 to 4.0 psi. The planchet's nitrogen flow improves the accuracy of low exposure reading and extends the planchet life by eliminating the oxygen in the planchet area. The time temperature profile (TTP) included the heating cycle parameters and was set in the Win REMS software. The TL charge was collected from each reading cycle in 200 data points.

## **3. Results and Discussion**

#### *3.1. Structural, Morphological, and Chemical Composition Investigations*

Figure 2 displays the structure and crystallinity of undoped ZnO and Ag-doped ZnO thin films using XRD (20◦ ≤ 2 *θ* ≤ 80◦). The results showed polycrystalline with a hexagonal wurtzite structure for the Ag-doped sample [36]. The diffraction peaks corresponding to

ZnO and Ag appeared to agree with the standard JCPDS data card 01-089-0511 and 03-065- 2871, respectively. The peaks for all samples indicate that Ag-doped ZnO thin films have grown successfully. The peaks of 100, 002, and 101 planes correspond to 2 *θ* = 31.77◦, 34.37◦, and 36.35◦ of ZnO, respectively. The peak at (002) orientation is the highest, indicating the growth direction and c-axis orientation. In the meantime, the diffraction peak of 111 at 2 θ = 38.22◦ is consistent with Ag for Ag-doped ZnO thin films. No evidence of impurity peaks was observed from the XRD data [37].

**Figure 2.** The X-ray diffraction pattern of undoped ZnO and Ag-doped ZnO thin films.

The results showed that the diffraction peaks connected to ZnO decreased along with the Ag dopant increment, due to the disorder formed by the Ag ions in the ZnO lattice structure; moreover, the dopant's chemical reactivity plays a role in crystal growth dynamics. As a result, the added Ag atoms cause a deformation in the crystalline structure of ZnO [38]. However, this deformity may be ascribed to Ag+ having a considerably larger ionic radius (1.22 Å) than Zn+ 0.72 Å, leading to the segregation of Ag atoms to the grain boundaries of ZnO crystal, indicating Ag cluster formation that appeared as separate Ag peaks. Consequently, a metallic phase of silver was produced on the surface of ZnO [28,39]. ThesizeestimatedScherrer'sformulaandtabulatedin

 crystallite was using was Table 1:

$$\mathbf{D} = 0.89\lambda / \beta \cos \theta \tag{1}$$

As seen in Table 1, the TL intensity increases as the crystallite size decreases. The change of TL is consistent with that of the surface fluorescence, which increases as the surface area to volume ratio increases, as we mentioned in the introduction. When the nanocomposite is exposed to radiation, the electron and hole will be created and trapped in these metastable states. Hence, it is obvious that the TL of the nanoparticles is proportional to the surface defects. The surface-to-volume ratio increases when the size decreases, and the particles go to more easily obtainable carriers, which increase the holes and electrons for TL emission. There is still much to learn about the aspects of the upconversion of luminescence in these nanomaterials [40–43]. However, by increasing the dopant, the intensity decreased. Researchers ascribed the decrease in luminescence intensities to the quenching effect as an increasing dopant, which is consistent with this study [44,45].


**Table 1.** Calculation of the crystallite size of ZnO and Ag-doped ZnO thin films.

Figure 3's SEM images show the surface morphology of pure ZnO and Ag-doped ZnO thin films. One-dimension nanostructure hexagonal shapes were successfully synthesized through the hydrothermal method. The effect of the amount of Ag on the morphology of ZnO nanorods is observed. Based on the obtained results, the nanorods' length is 5 μm, as confirmed from the cross section (image inset of Figure 3b). The surface morphology of Ag-doped ZnO became deformed and flattened by increasing the Ag dopant. The morphologies of thin films are affected by adding Ag+, which may be replaced with Zn2+. Further increase in impurities caused the agglomeration of Ag atoms at the grain boundary of ZnO crystal, suggesting the creation of Ag clusters that appeared as separate Ag atoms, as can be seen in Figure 3c,d [46].

**Figure 3.** FE-SEM images of the (**a**) pure ZnO and silver dopant at (**b**) 0.5%; (**c**) 1%; and (**d**) 3%, respectively.

Energy-dispersive X-ray (EDX) spectroscopy was used to evaluate the chemical compositions of pristine ZnO and Ag-doped nanorods. The results showed that the sample

comprises Zn and O elements; no additional elemental peaks were seen in the pristine ZnO analysis. Ag impurities have also been fully incorporated into the ZnO lattice. Figure 4 showed the elemental synthesis of ZnO and Ag-doped ZnO thin films via EDX and revealed a good agreemen<sup>t</sup> with the experimental composite [47]. The mapping images exhibited a uniform and homogeneous distribution of Ag+ ions in ZnO.

**Figure 4.** The EDX spectrum for undoped and Ag-doped ZnO thin films with typical mapping images.

## *3.2. Optical Properties Studies*

The PL spectra of undoped ZnO and Ag-doped ZnO thin films at room temperature were recorded. Figure 5a shows the PL spectra of pure ZnO and Ag-doped ZnO assynthesized by hydrothermal deposition at room temperature, after annealing at 400 ◦C for 2 h. The PL study was conducted at room temperature with an excitation wavelength of 325 nm xenon-lamp Laser. The obtained results revealed two peaks that included the near bandgap (NBE) (3.26 eV), due to the collision process of excitons. The other highest intensity peak in the visible region is defined as deep level emission (DLE) with a wide range (2.25 to 1.46 eV); this was mainly attributed to the electron-hole recombination, which gave rise to the surface and intrinsic defects in the crystalline structure lattice during the growth. In addition, the DLE is the result of various developmental defects, such as zinc interstitials (Zni), interstitial oxygen (Oi), zinc vacancies (VZn), and oxygen vacancies. These defects cause the DLE (VO), and the red band at 1.6 *Ev* was observed due to the interface between Oi and VZn emissions [48–51].

**Figure 5.** (**a**) Room temperature PL spectra of pure ZnO and Ag-doped ZnO 1%, 0.5% mol thin films, and (**b**) Gaussian deconvolution of Ag-doped ZnO thin film.

For the Ag-doped ZnO thin film, there is an increase in the intensity of NBE without shifting the peak's position. The enhancement in UV intensity after annealing may be attributed to excitonic recombination; we can explain it as due to the implanted Ag atoms; thermal annealing provides energy to occupy Zn atom sites in the lattice of ZnO. When no Ag atoms are stuck in non-equilibrium positions, the occupying probability could increase the temperature and gradually become steady at a specific value. The UV light in ZnO crystal can excite photocarriers [51,52]. However, the optimized intensity was 0.5 mol%, due to the electron-hole recombination as an electron transporting layer and the oxygen vacancies mechanism. With the increase in Ag molarity beyond 0.5% of the Ag amount, the PL intensity tended to reduce, which may be due to the surface plasmon resonance (SPR) of Ag, as reported in a prior study by [53].

The cause of the red region is still controversial. Due to the diverse array and complexity of defects existing in ZnO, some authors have attributed it to ZnO structure defects. In contrast, some other authors attributed it to an excess of oxygen impurities [54]. To understand the characteristics of the broad visible emission band, the deconvolution of the components band was applied via Gaussian fitting, see Figure 5b. The deconvolution peak at 2.10 (eV) may be ascribed to the VZn–VO vacancies that other scholars assigned to

positively charged oxygen vacancy VO++ [54–57]. Moreover, we observed other peaks at 1.84 (eV) and 1.7 (eV) assigned as the yellow and orange emissions in ZnO, respectively, attributed to neutral VO and interstitial zinc atoms Zni [58–61]. The emission bands in the 1.7 (eV), 1.56 (eV), and 1.46 (eV) were caused by different types of defects, such as interstitial zinc, zinc vacancy, oxygen vacancy, and interstitial oxygen [56,62–64]. The emission bands have been improved by oxygen heat treatment and Ag-doping, suggesting that this emission band is due to Ag–O clusters [18].

Figure 6 shows the transmittance spectra of ZnO and Ag-doped ZnO over 200 to 800 nm. The results revealed that the transmittance decreased with the increment of Ag, which may be ascribed to the scattering of photons by crystal defects formed by the Ag dopant, agglomeration density, and adsorption of free carriers [65].

**Figure 6.** Optical transmittance for undoped ZnO and Ag-doped ZnO thin films.

*α*

Figure 7 shows the bandgap (*Eg*) of the samples. From the above transmittance, the absorption coefficient (*α*) was determined based on the following equation:

$$
\dot{\rho} = -LnT/d\tag{2}
$$

where *T* is the transmittance, and *d* is the thickness of the thin films. Meanwhile, the bandgap can be measured by extrapolating the linear portion curve of (αhν)<sup>2</sup> versus the photon energy (*hν*), according to the following equation:

$$
abla \upsilon = (\hbar \upsilon - E\mathfrak{g})^{\mathfrak{m}} \tag{3}$$

where *Eg* is the gap energy; *hν* is the energy of the photon; *α* is the calculated absorption coefficient from the raw transmittance data; and *m* = 1/2 for the plotted direct transition bandgap (*αhv*)<sup>2</sup> vis photon energy (*hv*).

**Figure 7.** The bandgap for undoped ZnO and Ag-doped ZnO thin films.

The energy gap for ZnO recorded 3.2 eV, slightly decreasing with the addition of Ag. The decrease in *Eg* with the increase in Ag may signify Ag+ was substituting for Zn+ in the lattice [66]. The production of oxygen vacancies plays a vital role in reducing the bandgap, as they serve as trap centers that minimize the recombination of charge carriers by capturing the electrons. Furthermore, the addition of Ag may cause defects in the bandgap, which can broaden the spectrum and promote emission in the visible range, leading to improved luminescence characteristics of Ag-doped ZnO nanocomposites [67].
