**1. Introduction**

Thermoluminescence dosimeter, well-known as TLD, is a device that can keep radiation energy for a specific time and then read it out after being induced by heating. The process behind this device depends on the emission of light. TLD is a tool for measuring radiation doses in the clinical, radiotherapy, environmental, irradiated food, industrial, and quality assurance fields [1–8]. TLD remains the most powerful method to measure radiation doses due to its reliability, non-intricacy, and portability [1,3,9–11]. The thermoluminescence (TL) mechanism depends on impurities defects in the crystal structure, which increment the capacity of the materials to store radiation energy [6,11–13].

In recent years, many studies have been performed to establish novel high-performance TLDs that show a linear response at a wide range of doses, since most dosimeters exhibit nonlinear responses in a wide range of doses. TLD materials are microcrystalline powders

**Citation:** Thabit, H.A.; Kabir, N.A.; Ismail, A.K.; Alraddadi, S.; Bafaqeer, A.; Saleh, M.A. Development of Ag-Doped ZnO Thin Films and Thermoluminescence (TLD) Characteristics for Radiation Technology. *Nanomaterials* **2022**, *12*, 3068. https://doi.org/10.3390/ nano12173068

Academic Editors: Wei Chen and Derong Cao

Received: 1 August 2022 Accepted: 27 August 2022 Published: 3 September 2022

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or chips that can disperse light, where the near-surface light produced can approach the photon detector more than the light from within its depth. This result depends on the dosimeter's ability to keep light emitting through it. Most researchers focus on block form dosimeters such as pellets, chips, and discs [4,14–17]. In contrast, a few researchers have used limited efforts to investigate nanocomposites with a creative design of 2D thin films to serve as TL materials [18]. We see that it is of the utmost importance to consider modifying the dimensions of the dosimeter, to ensure that it is compatible with various applications and the critical regions for radiation measurements.

The development of thin films with dosimetric characteristics is of grea<sup>t</sup> importance in calculating low penetrating radiation doses, including in the study of dose distribution at interfaces. However, knowing the dose in the particular region and beyond clinics are significant. These are essential assets of knowledge to prevent unnecessary issues in the skin and treat the lymphatic system at a depth of 0.5 mm. According to the literature review, several attempts have been made to design two dimensional (2D) dosimeters [19–22]. Additionally, the 2D design of the dosimeter with a high resolution is essential in modern radiotherapy, for modalities where steep gradients of dose distributions occur. Moreover, overcoming the problems associated with measuring depth dosage distributions is desirable [14,23,24].

Hence, Zinc oxide, ZnO, is an excellent advantage in improving efficient TL phosphor tailored to be used in dosimetry applications, due to its wide bandgap of 3.37 eV, exciton binding energy of 60 MeV, and high transparency of 90% in the visible region [23]. Based on the characterization of ZnO, the surface area to volume ratio increases with the decrease in the nano-range of grain sizes, which changes the optical properties such as transmittance. Moreover, ZnO has inherent structures with different morphologies, i.e., nanoparticles, nanowires, and nanorods [24]. However, ZnO has weaknesses, such as a high electron-hole recombination rate. To address this limitation, doping with foreign atoms is crucial for modifying the characteristics and proposed uses of semiconductor nanocrystals. Adding impurities to ZnO will change the emission luminescence, as it creates defects in materials and increases the charge carriers. The results of ZnO doped by transition metals showed ZnO as a promising composite material in dosimetry, based on the TL glow curves, per the most recent literature review indicated in our previous work [25]. The introduction of Ag to ZnO caused the substitute of Ag2+ in the Zn2+ lattice, which increased the oxygen vacancies due to variation in the ionic radius between Ag (1.15 Å) and ZnO (.72 Å). These vacancies act as the sub-bandgap donor sites, producing traps; these sub-bandgap act as traps for the electrons during irradiation. Later by stimulating the nanocomposite, the trapped electrons tend to relax in the recombination center. A typical recombination center may be created by dislocating a negative ion that works as an electron trap; if this trap is shallow may be released by thermal vibrations of the lattice. On the other side, if the trap is deep (high activation energy), the electrons will recombine with the holes' trap at the recombination center, giving rise to light emission (TL).

Therefore, adding selective elements to ZnO offers a vital way to enhance and control optical and luminescence properties. According to the theory of valence control in oxide semiconductors, the Debye length (*LD*) of ZnO is reduced when it is doped with acceptor elements such as Au, Cu, and Ag; Ag can promote the separation of spatially generated charge carriers. Furthermore, the unique interface interactions between Ag metal and ZnO may be related to the presence of the Schottky barrier, which promotes charge carrier separation. For instance, Huang observed that when Ag (NPs) is added to ZnO, electrons concentrate in ZnO along the Ag–ZnO interface until the electron-rich islands link. This difference in electron transport at Ag–ZnO is caused by the fact that the work function of ZnO (4.62 eV) is greater than that of Ag (4.24 eV), increasing electric conductivity up to 1000 times.

Furthermore, when the Ag content in the ZnO matrix increases, the electron concentration rises to 2.4 1020/cm, causing the electron accumulation zones to overlap and forming a percolation channel for electron transport, without reducing electron mobility [26]. Similarly, Corro and their research group reported that adding Ag to pure ZnO increased the number of electrons in the conduction band (CB) of ZnO. This is caused by the interfacial

electronic interactions between the metals and the ZnO. These impurities may increase the probability of localized electrons being trapped in de-traps (close to conduction band CB); this might occur because the electron transfer that emerged from Ag to ZnO caused the shifted absorption to a higher wavelength [27]. Saboor's findings demonstrated that the Ag-doping concentration significantly impacted the morphology, structure, and intrinsic defects of ZnO nanorods. As the modifier shifts the conduction band and Fermi level of the ZnO nanorods, it creates vacancies and forms ionic bonds with the oxygen atom rather than covalent bonds [28]. Likewise, incorporating silver into ZnO creates surface defects, which act as effective charge carrier traps to reduce the recombination rate of the photogenerated charge carriers. Thus, the addition of Ag to ZnO caused an increase in oxygen vacancies sites, due to the electron sensitization effect of Ag; as a result, an improvement in TL intensity can be obtained [29]. However, scholars are still making intensive efforts to improve the TL properties of these phosphors, either by preparing them in various ways, doping them with different impurities, or introducing new matrices, with ZnO being one of these new host materials. Although many researchers have been dedicated to developing a ZnO-based dosimeter, to our knowledge, no study has been reported that covers all of the features [30–34]. This study comprehensively investigated the dosimeter characteristics of Ag-doped ZnO thin films grown via the hydrothermal method.
