Decoration of Ag Nanoparticle on ZnO Nanowire by Intense Pulsed Light and Enhanced UV Photodetector

Zinc oxide (ZnO) nanowires (NWs) are wide-bandgap semiconductors that absorb ultraviolet (UV) radiation. Various post-treatment processes have been studied to improve the optical properties of the as-grown ZnO NWs. Among them, Ag nanoparticles (NPs) effectively improved the optical properties on the surface of the ZnO NWs. In this study, ZnO NWs were synthesized via the hydrothermal synthesis method. ZnO NWs were decorated with Ag NPs on the surface of the ZnO NWs in a silver nitrate (AgNO3) aqueous solution by intense pulsed light (IPL) irradiation. Ag NPs were successfully decorated under the following conditions: aqueous AgNO3 solution of 100 nM, an energy of 1 J/cm2, and an exposure time of 8 ms. The responsivity and sensitivity of the ZnO NW UV photodetectors increased by 7.43 and 3.37 times, respectively. The IPL process makes it possible to decorate Ag NPs in a simple manner within an extremely short time.

Attempts have been made to improve the properties of ZnO with annealing [24], impurity doping [25], plasma treatment [26], and UV treatment [27]. In addition, Ag NPs were incorporated to improve the properties using thermal techniques [28], UV techniques [29], evaporation [30], sputtering [31], and electrodeposition [32]. Among them, the thermal and UV techniques have employed the solution process. The advantage of a solution process is its inexpensiveness compared with the vacuum process. However, it is a time-consuming process. The reported process times for thermal and UV techniques were 3 h and 30 min, respectively [28,33]. Furthermore, the thermal technique requires Figure 1 shows the fabrication process of the Ag-decorated ZnO NW UV PDs, including hydrothermal growth of ZnO NWs, Ag-decoration with IPL, and formation of electrodes using ink-jet printing. First, the ZnO NWs were synthesized using the hydrothermal method [8,37]. The Zn seed layer was prepared on a SiO 2 /Si substrate by sol-gel spin-coating of a mixture of zinc acetate dihydrate (0.5 M) and diethanolamine (0.5 M) in ethanol at 4000 rpm for 30 s. It was followed by heat-treatment at 350 • C for 30 min on a hotplate. Subsequently, ZnO NWs were grown hydrothermally at 90 • C for 4 h in a mixture of zinc nitrate hexahydrate (25 mM) and hexamethylenetetramine (25 mM) in deionized (DI) water.
Chemosensors 2021, 9, x FOR PEER REVIEW 2 of 14 were 3 h and 30 min, respectively [28,33]. Furthermore, the thermal technique requires exposure to high temperatures amounting to several hundred degrees [28]. Conversely, the intense pulsed light (IPL) process is facile, rapid, and efficient at room temperature. Therefore, replacing the energy source from heat or UV to IPL can be advantageous.
In this study, we decorated Ag NPs using a facile and rapid IPL process to improve the properties of the hydrothermally as-grown ZnO NWs and fabricated the Ag-decorated ZnO NW UV PDs. The ZnO NWs were synthesized via a typical hydrothermal synthesis method. Hydrothermal synthesis is the most well-known method for synthesizing vertically aligned ZnO NWs directly on the substrate in a solution process [34]. Compared with the other methods, the hydrothermal method is a potential and effective way to synthesize ZnO nanowires with the advantages of mild reaction conditions, environment-friendly, and easy to control [35]. Furthermore, it is easy to manufacture single-crystalline nanostructures [36]. The Ag decoration was carried out in an AgNO3 aqueous solution by varying the concentration and irradiation energy of IPL. The IPL irradiation was performed for an extremely short time on the order of milliseconds. The Ag-decorated ZnO NWs were characterized by several analytical techniques. Finally, the Ag-decorated ZnO NWs were exploited in UV PDs to demonstrate enhanced properties, thereby improving the performance of UV PDs. Figure 1 shows the fabrication process of the Ag-decorated ZnO NW UV PDs, including hydrothermal growth of ZnO NWs, Ag-decoration with IPL, and formation of electrodes using ink-jet printing. First, the ZnO NWs were synthesized using the hydrothermal method [8,37]. The Zn seed layer was prepared on a SiO2/Si substrate by sol-gel spincoating of a mixture of zinc acetate dihydrate (0.5 M) and diethanolamine (0.5 M) in ethanol at 4000 rpm for 30 s. It was followed by heat-treatment at 350 °C for 30 min on a hotplate. Subsequently, ZnO NWs were grown hydrothermally at 90 °C for 4 h in a mixture of zinc nitrate hexahydrate (25 mM) and hexamethylenetetramine (25 mM) in deionized (DI) water.

Ag Decoration on as-Grown ZnO NWs
The as-grown ZnO NWs were decorated with Ag NPs using a facile and rapid IPL process. The SiO2/Si substrate on which ZnO NWs have been grown was dipped into silver nitrate (AgNO3) in DI water followed by IPL exposure. AgNO3 played the role of precursor for Ag NPs. The IPL equipment used in this study was composed of a Xenon flash

Ag Decoration on as-Grown ZnO NWs
The as-grown ZnO NWs were decorated with Ag NPs using a facile and rapid IPL process. The SiO 2 /Si substrate on which ZnO NWs have been grown was dipped into silver nitrate (AgNO 3 ) in DI water followed by IPL exposure. AgNO 3 played the role of precursor for Ag NPs. The IPL equipment used in this study was composed of a Xenon flash lamp (P4128, Heraeus Co., Hanau, Germany), pulse controller (PSTEK Co., Gunpo, Korea), power supply, capacitors, and chilling unit. The concentrations of the Ag precursor and IPL irradiation energy were adopted as process variables in the Ag-decoration process. The IPL exposure was conducted with a single exposure, which lasted 8 ms. After the IPL process, the substrate was thoroughly rinsed with DI water.

Fabrication of UV PDs
The electrodes were fabricated on the Ag-decorated ZnO NWs by printing silver ink (DGP 40LT-13C, Advanced Nano Products Co., Sejong, Korea) with an ink-jet printer (DMP-2831, FUJIFILM Dimatix Co., Santa Clara, CA, USA) for UV PDs. The electrodes had dimensions of 2 mm × 2 mm and a channel length of 120 µm.

Ag-Decorated ZnO NWs
The SEM images of the hydrothermally as-grown ZnO NWs are shown in Figure 2. The ZnO NWs were grown vertically on the SiO 2 /Si substrate and are densely packed. They have an average diameter and length of 42 nm and 840 nm, respectively. The grown ZnO NWs exhibit the hexagonal wurtzite structure, as shown in Figure 2a. This observation is quite consistent with previous studies [8,9]. However, it was difficult to identify the decorated Ag NPs with SEM, which have been done with TEM. Figure 2c,d show ZnO NWs processed for 5 and 15 s in AgNO 3 concentration of 1 mM at an IPL energy of 1 J/cm 2 per pulse and frequency of 6 Hz, respectively. ZnO NWs are covered by Ag under excessive conditions. As the IPL irradiation time increases, the surface is covered with more Ag, which prevents a UV reaction on the surface.   Figure 3b, compared to the as-grown ZnO NW (Figure 3a), that several circular particles formed on the surface of ZnO NWs processed in a AgNO3   Figure 3b, compared to the as-grown ZnO NW (Figure 3a), that several circular particles formed on the surface of ZnO NWs processed in a AgNO 3 concentration of 100 nM at IPL irradiation energy of 1 J/cm 2 . However, no discernible particle shape was observed on the surface of the ZnO NWs processed in a AgNO 3 concentration of 1 mM at 1 J/cm 2 (Figure 3c). Elemental maps were acquired to confirm the distribution of Ag elements and demonstrated in Figure 3d−f. The Zn and O were homogeneously distributed with strong and high-density signals in all three conditions. However, this is not the case for the Ag, which was distributed differently under three conditions. The background signal from the as-grown ZnO nanowires is shown in Figure 3d, playing a role of control. The Ag was concentrated at the same position with the circular particles in the ZnO NWs processed in 100 nM at 1 J/cm 2 (Figure 3e). The other part showed a slightly higher signal than the background in as-grown ZnO NWs. In the ZnO NWs, processed in 1 mM at 1 J/cm 2 , the Ag was evenly distributed on the surface of the ZnO NWs, as shown in Figure 3f. It is certain that the Ag signal is stronger than the background in the as-grown ZnO NWs. However, it does not build up large individual nanoparticles enough to be observed in TEM. It can be explained by the following reasons. The strong energy released from IPL induced nucleation from Ag + on the surface of the ZnO NWs. When the concentration of AgNO 3 was high, simultaneous nucleation occurred on all surfaces of the ZnO NWs. As shown in Figure 3f, circular particles were not yet formed, but Ag was evenly distributed on the surface. However, at the low concentration of AgNO 3 , nucleation was not uniform on the surface. Ag adsorbed on the surface of the ZnO NW was aggregated for nucleation and formed into circular particles, as shown in Figure 3e. Figure 4 illustrates the binding energy among Ag, Zn, and O for the bare and Agdecorated ZnO NWs. Figure 4a shows the full spectrum of the bare and ZnO NWs processed in 100 nM at 1 J/cm 2 . The spectrum was calibrated based on C 1s at E b = 284.8 eV. The peaks from Zn and O were the same as the ones reported previously [26]. The peaks at 1021 and 1044 eV in Figure 4b,c are attributed to the Zn 2p 3/2 and Zn 2p 1/2 , thus confirming that Zn exists in a divalent state. There was no change in the peak of Zn with respect to the concentration of AgNO 3 solution and the IPL irradiation energy. It was confirmed that the Ag decoration process did not affect the binding energy of the Zn. The peak at 529.8 eV in Figure 4d,e was attributed to the O 1s. The spectrum of O 1s can be fitted to three Gaussian distributions representing different oxygen states. In general, the spectrum of O 1s can be represented by O lattice , O non-lattice , and O adsorption . These are O 2− ions in the ZnO crystal lattice, O 2− ions in the oxygen vacancy, and chemisorbed oxygen species on the surface, respectively [38]. Figure 5a,b demonstrate the areal ratio of O 1s with respect to the concentration of AgNO 3 solution and IPL irradiation energy, respectively. The areal ratio for O lattice was almost constant with no change. The areal ratio of O adsorption was maximized at 19.4% in ZnO NWs processed in 100 nM at 1 J/cm 2 , and 6.9% and 7.3% in ZnO NWs processed in 1 nM and 1 mM, respectively ( Figure 5a). The ratio of O non-lattice was relatively opposite to O adsorption . The real ratio of O adsorption decreased with increasing energy and was measured to be 5.9% in 100 nM at 5 J/cm 2 ( Figure 5b). Furthermore, in the result of energy change, the ratio of O non-lattice showed relatively opposite trends. This means that the chemical activity of chemisorbed oxygen species increased, owing to the high conductivity and catalytic properties of Ag NPs [39][40][41][42]. In contrast, when the concentration further increased, the surface area exposed to the atmosphere was reduced by Ag, thereby reducing the amount of adsorbed oxygen species. The decrease in O adsorption with increasing IPL irradiation energy could also be explained by the same reason. Figure 4f,g demonstrates the peaks at 367.3 and 373.3 eV from the Ag 3d 3/2 and Ag 3d 5/2 with respect to solution concentration and IPL energy, respectively. The scattered dots denote the raw data, while the line represents the fitted data. The difference (6.0 eV) in the binding energy indicates that Ag has a metallic nature [43]. The binding energy shifted significantly to a lower binding energy compared with the pure metallic Ag  [44], which is attributed to the interaction between the Ag NPs and the ZnO NW. When the Ag NPs and the ZnO NW were attached together, they adjusted the location of the corresponding Fermi energy level to the same value. Thus, there are more free electrons than Ag NPs at the Fermi level. Given that the conduction band (CB) of the ZnO NW was empty, free electrons can tunnel into the CB, and the valence of the Ag increases. The binding energy of Ag + was much lower than that of Ag 0 . Therefore, the binding energy of Ag 3d shifted to a lower binding energy [45,46]. The peak of Ag 3d decreased as the concentration and energy decreased, and the peak disappeared at 10 nM or less. The atomic percentage of Ag was calculated to be 0.174 at% in ZnO NWs processed in 1 mM at 1 J/cm 2 , 0.048 at% those processed in 100 nM at 1 J/cm 2 , and 0.055 at% in 100 nM at 5 J/cm 2 , respectively. Although Ag NPs were not observed in TEM, the Ag element exists on the surface of the ZnO NWs, which is confirmed by the elemental map result.  Figure 4 illustrates the binding energy among Ag, Zn, and O for the bare and Agdecorated ZnO NWs. Figure 4a shows the full spectrum of the bare and ZnO NWs processed in 100 nM at 1 J/cm 2 . The spectrum was calibrated based on C 1s at Eb = 284.8 eV. The peaks from Zn and O were the same as the ones reported previously [26]. The peaks at 1021 and 1044 eV in Figure 4b,c are attributed to the Zn 2p3/2 and Zn 2p1/2, thus confirming that Zn exists in a divalent state. There was no change in the peak of Zn with respect   The PL spectra from ZnO NWs are demonstrated in Figure 6. Figure 6a presents the PL spectra of the bare ZnO NWs processed in DI water, and the Ag-decorated ZnO NWs processed in 100 nM at 1 J/cm 2 . The PL spectra of the ZnO NWs yielded a sharp peak intensity at 381 nm, mainly owing to the band-band exciton recombination associated with near-band edge emission [47][48][49]. Figure 6b,c displays the PL spectra with respect to AgNO3 concentration and IPL irradiation energy, respectively. As the concentration of     The PL spectra from ZnO NWs are demonstrated in Figure 6. Figure 6a presents the PL spectra of the bare ZnO NWs processed in DI water, and the Ag-decorated ZnO NWs processed in 100 nM at 1 J/cm 2 . The PL spectra of the ZnO NWs yielded a sharp peak intensity at 381 nm, mainly owing to the band-band exciton recombination associated with near-band edge emission [47][48][49]. Figure 6b,c displays the PL spectra with respect to AgNO3 concentration and IPL irradiation energy, respectively. As the concentration of  The PL spectra from ZnO NWs are demonstrated in Figure 6. Figure 6a presents the PL spectra of the bare ZnO NWs processed in DI water, and the Ag-decorated ZnO NWs processed in 100 nM at 1 J/cm 2 . The PL spectra of the ZnO NWs yielded a sharp peak intensity at 381 nm, mainly owing to the band-band exciton recombination associated with near-band edge emission [47][48][49]. Figure 6b,c displays the PL spectra with respect to AgNO 3 concentration and IPL irradiation energy, respectively. As the concentration of AgNO 3 solution increases, the intensity of PL increases up to 100 nM and then decreases again. Since the ZnO NW receives electrons from Ag NPs, the intensity of the PL increases. However, an excessive decoration process lowered the intensity of PL [50]. The PL peak decreased with an increase in the IPL irradiation energy, which is the same reason as in excessive decoration. AgNO3 solution increases, the intensity of PL increases up to 100 nM and then decreases again. Since the ZnO NW receives electrons from Ag NPs, the intensity of the PL increases. However, an excessive decoration process lowered the intensity of PL [50]. The PL peak decreased with an increase in the IPL irradiation energy, which is the same reason as in excessive decoration.  Figure 7 shows the current measurement results at a bias voltage of 1 V. The UV PDs used here employed the bare and Ag-decorated ZnO NWs processed in DI water and various concentrations of AgNO3 solution. UV PDs were irradiated with a UV LED at a wavelength of 365 nm and a power density of 850 μW/cm 2 . The temporal response curves were obtained for a time of 50 s. The basic UV detection mechanism of the ZnO NWs is as follows. Electrons in the absence of UV moved to the center of the ZnO NW owing to the O2 − molecules adsorbed on the surface of the ZnO NW. It forms a thin conduction channel in the center and a thick depletion region on the surface. In the presence of UV, the adsorbed O2 − molecules were desorbed, and the ZnO NW conduction channel expanded, while the depletion region narrowed. The widened conduction channel increased the current flowing between electrodes. If the O2 − molecules were not desorbed, the current should be saturated. When the UV was turned off again, O2 − molecules were adsorbed on the surface, and the current decreased [7]. According to the experimental results, the highest light current was shown in the ZnO NWs processed in 100 nM at 1 J/cm 2, as shown in Figure 7d. Figure 8 shows the responsivity and sensitivity with respect to the concentration of AgNO3 solution and IPL irradiation energy. The responsivity and sensitivity were calculated by the following equations [51,52]:

The Performance of Ag-Decorated ZnO NW UV PDs
And: where R is the responsivity, S is the sensitivity, Ilight is the light current, Idark is the dark current, A is the area between two electrodes, and P is the UV power. While the bare ZnO NWs have a responsivity of 14 A/W, the most responsivity was 104 A/W found in ZnO NWs processed in 100 nM at 1 J/cm 2 , which is 7.43 times increased, as shown in Figure 8a. Meanwhile, the ZnO NWs processed in 1 mM at 5 J/cm 2 exhibited the lowest responsivity of 5.03 A/W, which was reduced by 64.1% compared to the bare ZnO NWs. The sensitivity was calculated to be the highest of 28,951% for ZnO NWs processed in 100 nM at 1 J/cm 2 , which is 3.39 times increased, as shown in Figure 8b. In addition, the external quantum efficiency (EQE), which is the ability of electrons generated by the excitation of photons, is defined as follows [52].  − molecules were desorbed, and the ZnO NW conduction channel expanded, while the depletion region narrowed. The widened conduction channel increased the current flowing between electrodes. If the O 2 − molecules were not desorbed, the current should be saturated. When the UV was turned off again, O 2 − molecules were adsorbed on the surface, and the current decreased [7]. According to the experimental results, the highest light current was shown in the ZnO NWs processed in 100 nM at 1 J/cm 2, as shown in Figure 7d. Figure 8 shows the responsivity and sensitivity with respect to the concentration of AgNO 3 solution and IPL irradiation energy. The responsivity and sensitivity were calculated by the following equations [51,52]:

The Performance of Ag-Decorated ZnO NW UV PDs
And: where R is the responsivity, S is the sensitivity, I light is the light current, I dark is the dark current, A is the area between two electrodes, and P is the UV power. While the bare ZnO NWs have a responsivity of 14 A/W, the most responsivity was 104 A/W found in ZnO NWs processed in 100 nM at 1 J/cm 2 , which is 7.43 times increased, as shown in Figure 8a. Meanwhile, the ZnO NWs processed in 1 mM at 5 J/cm 2 exhibited the lowest responsivity of 5.03 A/W, which was reduced by 64.1% compared to the bare ZnO NWs. The sensitivity was calculated to be the highest of 28,951% for ZnO NWs processed in 100 nM at 1 J/cm 2 , which is 3.39 times increased, as shown in Figure 8b. In addition, the external quantum efficiency (EQE), which is the ability of electrons generated by the excitation of photons, is defined as follows [52].
where λ is the wavelength of UV light. It depends on the responsivity (R) at the same wavelength (λ). At the largest responsivity, the EQE is calculated to be 35,312%. The increase in responsivity and sensitivity by the Ag decoration process results from the formed Ag NPs. The mechanism of UV detection by Ag NPs is described in Section 3.3. The decrease in responsivity decreased the photocurrent, which is the difference between the light current and dark current, owing to the decrease in oxygen species adsorbed on the surface of the ZnO NWs, like the decrease in the peak of O adsorption of XPS.
where λ is the wavelength of UV light. It depends on the responsivity (R) at the same wavelength (λ). At the largest responsivity, the EQE is calculated to be 35,312%. The increase in responsivity and sensitivity by the Ag decoration process results from the formed Ag NPs. The mechanism of UV detection by Ag NPs is described in Section 3.3. The decrease in responsivity decreased the photocurrent, which is the difference between the light current and dark current, owing to the decrease in oxygen species adsorbed on the surface of the ZnO NWs, like the decrease in the peak of Oadsorption of XPS. The performance of Ag-decorated ZnO by IPL was compared with the previous literature and is summarized in Table 1. The performance includes the responsivity, sensitivity, and response/recovery time of PDs based on the ZnO nanostructures decorated with metal NPs. It is confirmed that our study demonstrated a significant improvement compared to other literature.
where λ is the wavelength of UV light. It depends on the responsivity (R) at the same wavelength (λ). At the largest responsivity, the EQE is calculated to be 35,312%. The increase in responsivity and sensitivity by the Ag decoration process results from the formed Ag NPs. The mechanism of UV detection by Ag NPs is described in Section 3.3. The decrease in responsivity decreased the photocurrent, which is the difference between the light current and dark current, owing to the decrease in oxygen species adsorbed on the surface of the ZnO NWs, like the decrease in the peak of Oadsorption of XPS. The performance of Ag-decorated ZnO by IPL was compared with the previous literature and is summarized in Table 1. The performance includes the responsivity, sensitivity, and response/recovery time of PDs based on the ZnO nanostructures decorated with metal NPs. It is confirmed that our study demonstrated a significant improvement compared to other literature.      Figure 9 shows the various performances of the UV PDs with ZnO NWs processed at optimal conditions. Figure 9a shows I-V curves on a voltage range of −5 to 5 V in the absence and presence of UV. It is noted that there was an ohmic contact between the Ag electrode and the Ag-decorated ZnO NWs. The conductivity of the channel between two electrodes was greatly increased in the presence of UVs. This was ascribed to an expanded conductive channel at the center of ZnO NWs. The results of the repeated measurements with a 50-s interval are shown in Figure 9b. The response/recovery time and repeatability were excellent. The time to reach 90% and 10% of the saturation state is defined as the response and recovery time, respectively. The response/recovery times of the Ag-decorated ZnO NWs PDs were and 5.6/2.2 s, respectively. The current was measured at increasing powers of UV with 200, 410, 625, and 850 µW/cm 2 , and is shown in Figure 9c. As the UV light power increases, the light current increases, and the response time decreases. The higher power of UV, the more O 2 − molecules desorbed from the surface of NWs. The decreased response time is attributed to the increased desorption rate of the O 2 − molecules on the surface.  Figure 9 shows the various performances of the UV PDs with ZnO NWs processed at optimal conditions. Figure 9a shows I-V curves on a voltage range of −5 to 5 V in the absence and presence of UV. It is noted that there was an ohmic contact between the Ag electrode and the Ag-decorated ZnO NWs. The conductivity of the channel between two electrodes was greatly increased in the presence of UVs. This was ascribed to an expanded conductive channel at the center of ZnO NWs. The results of the repeated measurements with a 50-s interval are shown in Figure 9b. The response/recovery time and repeatability were excellent. The time to reach 90% and 10% of the saturation state is defined as the response and recovery time, respectively. The response/recovery times of the Ag-decorated ZnO NWs PDs were and 5.6/2.2 s, respectively. The current was measured at increasing powers of UV with 200, 410, 625, and 850 μW/cm 2 , and is shown in Figure 9c. As the UV light power increases, the light current increases, and the response time decreases. The higher power of UV, the more O2 − molecules desorbed from the surface of NWs. The decreased response time is attributed to the increased desorption rate of the O2 − molecules on the surface.

UV-Detection Mechanism
Herein, a detection mechanism is proposed for Ag-decorated ZnO NW UV PDs, as shown in Figure 10. Ag NPs play a dual role in UV PDs in enhancing the performance of UP PDs. At first, the catalytic properties of Ag NPs increase the chemical activity of the chemisorbed oxygen species, as shown in Figure 4d,e [39][40][41][42]. The increase in chemically activated oxygen species widens the depletion region in ZnO NWs, as shown in Figure   (

UV-Detection Mechanism
Herein, a detection mechanism is proposed for Ag-decorated ZnO NW UV PDs, as shown in Figure 10. Ag NPs play a dual role in UV PDs in enhancing the performance of UP PDs. At first, the catalytic properties of Ag NPs increase the chemical activity of the chemisorbed oxygen species, as shown in Figure 4d,e [39][40][41][42]. The increase in chemically activated oxygen species widens the depletion region in ZnO NWs, as shown in Figure 10b. More oxygen species are desorbed in the presence of Ag NPs upon exposure to UV light than in the absence of Ag NPs, resulting in more photocurrent in the presence of Ag NPs, as shown in Figure 10c,d. In general, ZnO NWs synthesized by the hydrothermal method are known to have a work function of 5.28 eV, an electron affinity of 4.35 eV, and a bandgap of 3.37 eV [64,65]. The work function of Ag is 4.26 eV; however, the smaller the NP size, the larger the work function [66,67]. The work function is found to be 5.3 eV when the Ag NP size decreases below 30 nm. Consequently, a Schottky barrier was formed at the interface of Ag NP and ZnO NW owing to the higher work function of Ag NP, as shown in Figure 10e [33,65,66,68]. This also leads to the narrowing of the conducting channel of the ZnO NWs, as shown in Figure 10b. The electrons were generated in the exposure of UV light, and the holes moved toward the Ag NPs owing to the Coulombic force [69]. Some holes were trapped at the interface, and the rest passed through the Schottky barrier and recombined with electrons, as shown in Figure 10f. This results in the narrowing of the depletion region. The photogenerated electrons were secured by the recombination of generated holes with the electrons from Ag NPs, which enhances lifetime. Additionally, Ag NPs enhance the light absorption efficiency by particle-induced light scattering [70]. It increases the number of photogenerated electron-hole pairs. 10b. More oxygen species are desorbed in the presence of Ag NPs upon exposure to UV light than in the absence of Ag NPs, resulting in more photocurrent in the presence of Ag NPs, as shown in Figure 10c,d. In general, ZnO NWs synthesized by the hydrothermal method are known to have a work function of 5.28 eV, an electron affinity of 4.35 eV, and a bandgap of 3.37 eV [64,65]. The work function of Ag is 4.26 eV; however, the smaller the NP size, the larger the work function [66,67]. The work function is found to be 5.3 eV when the Ag NP size decreases below 30 nm. Consequently, a Schottky barrier was formed at the interface of Ag NP and ZnO NW owing to the higher work function of Ag NP, as shown in Figure 10e [33,65,66,68]. This also leads to the narrowing of the conducting channel of the ZnO NWs, as shown in Figure 10b. The electrons were generated in the exposure of UV light, and the holes moved toward the Ag NPs owing to the Coulombic force [69]. Some holes were trapped at the interface, and the rest passed through the Schottky barrier and recombined with electrons, as shown in Figure 10f. This results in the narrowing of the depletion region. The photogenerated electrons were secured by the recombination of generated holes with the electrons from Ag NPs, which enhances lifetime. Additionally, Ag NPs enhance the light absorption efficiency by particle-induced light scattering [70]. It increases the number of photogenerated electron-hole pairs.

Conclusions
In conclusion, we employed a facile and rapid IPL method to decorate ZnO NWs with Ag NPs on the surfaces of the ZnO NWs grown vertically on the substrate. It was confirmed that Ag was successfully decorated on the ZnO NW surface through TEM, elemental maps, and XPS investigation. When the concentration of AgNO3 solution was lower than 100 nM, the Ag NPs were not formulated efficiently. The ZnO NWs processed in 100 nM at 1 J/cm 2 exhibited the most superior decoration of Ag NPs along with the highest photocurrent. Further increase in the concentration and exposure energy of IPL resulted in a decrease in electrical performance of the ZnO NW PDs. The decoration process in this study boasts of extremely short (~8 ms) duration. Our results clearly indicate that the IPL process used in the current work is an efficient way to decorate Ag on ZnO NWs. Considering that it is difficult to improve the crystal quality of the ZnO NW grown at low temperatures, this can act as an exemplary guideline for the improvement of UV PD performance.

Conclusions
In conclusion, we employed a facile and rapid IPL method to decorate ZnO NWs with Ag NPs on the surfaces of the ZnO NWs grown vertically on the substrate. It was confirmed that Ag was successfully decorated on the ZnO NW surface through TEM, elemental maps, and XPS investigation. When the concentration of AgNO 3 solution was lower than 100 nM, the Ag NPs were not formulated efficiently. The ZnO NWs processed in 100 nM at 1 J/cm 2 exhibited the most superior decoration of Ag NPs along with the highest photocurrent. Further increase in the concentration and exposure energy of IPL resulted in a decrease in electrical performance of the ZnO NW PDs. The decoration process in this study boasts of extremely short (~8 ms) duration. Our results clearly indicate that the IPL process used in the current work is an efficient way to decorate Ag on ZnO NWs. Considering that it is difficult to improve the crystal quality of the ZnO NW grown at low temperatures, this can act as an exemplary guideline for the improvement of UV PD performance.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:
The authors declare no conflict of interest.