Tunable UV- and Visible-Light Photoresponse Based on p-ZnO

Apr 17, 2017 - E-mail: [email protected]., *Phone: +886-6-275-7575 ext. ... (9, 10) Among such UV photodetectors, the p–n homojunction has the ...
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Tunable UV- and Visible-Light Photoresponse Based on p‑ZnO Nanostructures/n-ZnO/Glass Peppered with Au Nanoparticles Cheng-Liang Hsu,*,† Yu-Hong Lin,† Liang-Kai Wang,† Ting-Jen Hsueh,‡ Sheng-Po Chang,§ and Shoou-Jinn Chang*,§ †

Department of Electrical Engineering, National University of Tainan, Tainan 700, Taiwan National Nano Device Laboratories, Tainan 741, Taiwan § Institute of Microelectronics & Department of Electrical Engineering Center for Micro/Nano Science and Technology Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan ‡

S Supporting Information *

ABSTRACT: UV- and visible-light photoresponse was achieved via p-type K-doped ZnO nanowires and nanosheets that were hydrothermally synthesized on an n-ZnO/glass substrate and peppered with Au nanoparticles. The K content of the p-ZnO nanostructures was 0.36 atom %. The UV- and visible-light photoresponse of the p-ZnO nanostructures/n-ZnO sample was roughly 2 times higher than that of the ZnO nanowires. The Au nanoparticles of various densities and diameter sizes were deposited on the p-ZnO nanostructures/n-ZnO samples by a simple UV photochemical reaction method yielding a tunable and enhanced UV- and visible-light photoresponse. The maximum UV and visible photoresponse of the Au nanoparticle sample was obtained when the diameter size of the Au nanoparticle was approximately 5−35 nm. On the basis of the localized surface plasmon resonance effect, the UV, blue, and green photocurrent/dark current ratios of Au nanoparticle/p-ZnO nanostructures/ n-ZnO are ∼1165, ∼94.6, and ∼9.7, respectively. KEYWORDS: tunable, UV and visible photoresponse, p-type ZnO nanostructures, Au nanoparticles, p−n homojunction

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INTRODUCTION Over the past decade, ZnO nanostructures (NSs) have attracted great attention because of their wide band gap of ∼3.34 eV at room temperature, large exciton binding energy of 60 meV, great surface area to volume ratio, and quantum confinement effect. Various morphologies for ZnO NSs have been developed, such as zero-dimensional (0-D) nanoparticles (NPs), 1-D nanowires (NWs), and 2-D nanosheets for applications in nanoscale optoelectronics, optics, and electronic devices.1−7 ZnO NSs are considered one of the most promising candidate materials for ultraviolet (UV) detection. A variety of ZnO UV photodetectors have been extensively studied and broadly applied in industrial, commercial, and military fields.1−7 Until now, p−n homojunctions,7 p−n heterojunctions,8 and Schottky junctions have been widely used to realize UV photodetectors.9,10 Among such UV photodetectors, the p−n homojunction has the strongest structure, highest stability, and © XXXX American Chemical Society

best reproducibility; however, there are few articles regarding p−n ZnO homojunction photodetectors because of the difficulty in synthesizing p-type ZnO.7 Until now, the p-type ZnO dopant usually used group VA elements (N, P, As, Sb) to replace oxygen;11−14 however, in this study, potassium (K) acts as a dopant element because of its +1 valence property to substitute for the Zn +2 valence sites.15 Noble-metal NPs have been widely applied in visible-light photodetectors because of their ability to absorb visible light and produce localized surface plasmon resonance (LSPR) to enhance the photodetector.16,17 In recent years, a variety of wide-band-gap semiconducting NWs have been peppered with noble-metal NPs to enhance their UV-photodetecting performReceived: March 6, 2017 Accepted: April 17, 2017 Published: April 17, 2017 A

DOI: 10.1021/acsami.7b03216 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces ance.18,19 The interface of the noble-metal NPs and NWs creates a Schottky barrier, which produces a depletion region on the NW surface.10 Accordingly, the conductivity and dark current of the photodetector reduce because of surface depletion, while the photocurrent of the photodetector increases under UV illumination and is enhanced by LSPR. This means that the UV photocurrent to dark current ratio of the photodetector is enhanced by the peppered noble-metal NPs.10 Au, Pt, Ag, and Pd NPs are often used as noble-metal materials for application in photodetectors.18−21 In this investigation, the p-type ZnO:K NWs and nanosheets were mixed and hydrothermally grown on an n-type ZnO/glass substrate. Under UV irradiation, the HAuCl4 was photochemically reacted to generate Au NPs, after which the Au NPs were peppered on the p-type K-doped ZnO (p-ZnO:K) NSs. The pZnO NSs have a very large surface area to accommodate a greater amount of Au NPs compared with those described in previous articles. These novel UV- and visible-light photodetectors based on Au NPs/p-ZnO:K NSs/n-ZnO/glass were fabricated and their photoresponses under UV, blue, and green LED illumination examined. The fabrication processing steps and physical properties of the novel UV- and visible-light photodetector are discussed in detail.

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after which the p-ZnO:K NWs and nanosheets were hydrothermally synthesized on the prepared substrate. The hydrothermal method involved immersing the n-ZnO/glass substrate in a solution composed of 6 mM potassium nitrate (KNO3), 80 mM zinc nitrate hexahydrate [Zn(NO3)2·6H2O], and 80 mM hexamethylenetetramine (HMTA, C6H12N4). It should be noted that the source of the synthesized ZnO is the zinc nitrate hexahydrate and HMTA, while the source of the dopant K is potassium nitrate (KNO3 ↔ K+ + NO3−). The chemical reactions and synthesis steps of p-ZnO:K NSs are similar to those of the p-ZnO:La NWs, which were previously hydrothermally synthesized.5 The immersed n-ZnO/glass substrate and solution were then placed in a sealed autoclave and heated in a constant-temperature oven at 100 °C for 6 h. Afterward, the samples were removed from the oven, cleaned with deionized (DI) water, and then dried in air. The Au NPs synthesis method involved hydrogen tetrachloroaurate(III) (HAuCl4· 4H2O, 99.99%) diluted 1000, 5000, and 10 000 times with ethanol and subsequently irradiated under UV exposure (Kinsten KVB-30D UV box, 380 nm, 100 W) for 5 min. The UV photochemically reacted Au NPs were then deposited on p-ZnO:K NSs/n-ZnO/glass. Next, the Au NPs/p-ZnO:K NSs/n-ZnO/glass specimens were annealed in a furnace under Ar flow at 350 °C for 15 min to remove chlorine. Finally, the electrodes of the samples were coated with Ag paste. The photodetector consisted of the common metal−semiconductor−metal (MSM) structure and had Ohmic contacts. Field emission scanning electron microscopy (FE-SEM, JEOL7000F) was used to measure the surface morphology and element composition ratio. Photoluminescence (PL, Jobin Yvon-Spex Fluorolog-3 spectroscopy system) measurements employed a Xe lamp (wavelength 254 nm) as the excitation source. The electroluminescence (EL) spectra confirmed that the green, blue, and UV LED illumination had peak wavelengths of ∼520, ∼465, and ∼365 nm, respectively. The fabricated photodetectors were put in a dark box and then current−voltage (I−V) curves measured via source-measure instruments (Keithley model-4200) under LED illumination and in the dark. These measurements were performed at room temperature and under ambient atmospheric conditions.

EXPERIMENTAL SECTION

The schematic in Figure 1 illustrates the synthesized processing and fabrication steps of the proposed Au NPs/p-ZnO:K NSs/n-ZnO/glass. The 50 nm thick n-ZnO thin film was deposited on a glass substrate (Corning EAGLE XG) by direct current (dc) magnetron sputtering,

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RESULTS AND DISCUSSION Figure 2a,b presents the I−V measurements of the n-ZnO NWs/n-ZnO/glass and p-ZnO:K NSs/n-ZnO/glass samples in the dark and under green, blue, and UV LED illumination. The UV, blue, and green photocurrent to dark current ratios of the n-ZnO NW and p-ZnO:K NS samples are IUV/Idark = 476, Iblue/ Idark = 4.3, and Igreen/Idark = 1.9, and IUV/Idark = 743, Iblue/Idark = 7.4, and Igreen/Idark = 3.2, respectively. This means that the pZnO:K NS sample featured a photoresponse approximately 2 times better than that of the n-ZnO NW sample. Although the UV- and visible-light photocurrents of the n-ZnO NW and pZnO:K NS samples were roughly within the same range, the 1.39 μA dark current of the n-ZnO NWs was around 2 times higher than that of the p-ZnO:K NS sample at 0.642 μA at an applied bias of 10 V. The p-type property of the p-ZnO:K NS sample was verified by Hall-effect measurement. The major carrier of the p-ZnO:K NS sample was holes, and its measured results featured the lower hole carrier concentration of 7.19 × 1013 cm−3 and higher resistivity of 862.2 Ω cm and hole mobility of 100.1 cm2/(V s). In general, ZnO is, by nature, an n-type material, the major carrier of which is the electron. However, the K acts as a dopant element because of its +1 valence property to substitute for the Zn +2 valence sites. These K dopants increase the hole concentration and reduce the electron concentration because most of the K-dopantgenerated holes have been recombined with the original electrons of ZnO, thereby changing the n-type property into ptype. The conductivity of the p-ZnO:K NS sample was lower because a majority of the carrier holes were recombined with

Figure 1. Schematic illustration of the fabricated Au NPs/p-ZnO:K NSs/n-ZnO/glass photodetector processes and steps. B

DOI: 10.1021/acsami.7b03216 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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sample, after repeating the dripping step 6 times with the 1:10 000 dilution ratio, had the maximum UV (IUV/Idark = 1168), blue (Iblue/Idark = 94.6), and green (Igreen/Idark = 9.7) responses. This demonstrates that the Schottky barrier and LSPR effect of the Au NPs enhanced the UV- and visible-light photoresponse. Figure 3a shows the I−V curves of the p-ZnO:K NSs/nZnO/glass sample peppered with Au NPs after repeating the

Figure 2. I−V curves of (a) ZnO NWs/ZnO/glass and (b) p-ZnO:K NSs/n-ZnO/glass in the dark and under green, blue, and UV LED illumination.

electrons. In addition, the p-ZnO:K NSs/n-ZnO/glass has a high p−n homojunction barrier, which decreases the dark current and increases the photocurrent/dark current ratio. The diluted HAuCl4 solutions (ratios of 1:1000, 1:5000, and 1:10 000) were dripped on the p-ZnO:K NSs/n-ZnO/glass substrate under UV illumination for Au NP synthesis. The diameter and density of Au NPs were proportional to the different concentrations of HAuCl4 solution under UV exposure. Table 1 presents comparisons of the photocurrent/ dark current ratios of n-ZnO NWs and p-ZnO:K NSs/n-ZnO, and the various densities of the Au NPs/p-ZnO:K NSs/n-ZnO samples. The dripped diluted HAuCl4 step was repeated several times to increase the Au NP coverage and density. The Au NP

Figure 3. (a) I−V curves of p-ZnO:K NSs/n-ZnO peppered with Au NPs via 6 repetitions of the 1:10 000 dilution ratio in the dark and under green, blue, and UV LED illumination. (b) PL spectra of n-ZnO NWs, p-ZnO:K NSs, and Au NPs/p-ZnO:K NSs.

Table 1. Comparison of Photocurrent/Dark Current Ratio of ZnO NWs, p-ZnO:K NSs/n-ZnO, and Various Densities of the Au NPs/p-ZnO:K NSs/n-ZnO Samples materials

UV LED (365 nm, 6.0 mW/cm2)

blue LED (465 nm, 29.0 mW/cm2)

green LED (520 nm, 10.8 mW/cm2)

n-ZnO NWs/n-ZnO/glass p-ZnO:K NSs/n-ZnO/glass Au NPs (1:1000) 1 time/p-ZnO:K NSs/n-ZnO/glass Au NPs (1:5000) 1 time/p-ZnO:K NSs/n-ZnO/glass Au NPs (1:5000) 2 times/p-ZnO:K NSs/n-ZnO/glass Au NPs (1:5000) 3 times/p-ZnO:K NSs/n-ZnO/glass Au NPs (1:10 000) 1 time/p-ZnO:K NSs/n-ZnO/glass Au NPs (1:10 000) 2 times/p-ZnO:K NSs/n-ZnO/glass Au NPs (1:10 000) 4 times/p-ZnO:K NSs/n-ZnO/glass Au NPs (1:10 000) 6 times/p-ZnO:K NSs/n-ZnO/glass Au NPs (1:10 000) 10 times/p-ZnO:K NSs/n-ZnO/glass

476 743 653 358 293 339 171 470 703 1168 310

4.3 7.4 26.0 36.2 23.2 16.0 21.7 16.3 43.9 95.2 29.0

1.9 3.2 7.3 7.9 5.0 3.0 4.5 3.1 7.0 9.8 7.9

C

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Figure 4. (a) Schematics of current paths (A and B) of the photodetector. Band diagrams of (b) current path A in the dark, (c) path B in the dark, and (d) sample under light illumination.

Accordingly, the diameter and density of the Au NPs enable the adjustment and improvement of the photodetector’s visiblelight photoresponse. Figure 3b reveals the PL spectra measured at room temperature for the n-ZnO NWs/n-ZnO, p-ZnO:K NSs/nZnO, and p-ZnO:K NSs/n-ZnO/glass samples peppered with the Au NPs via 6 repetitions of dripping the 1:10 000 dilution ratio. The n-ZnO NW sample shows a strong, narrow UV emission (381 nm) and a broad green emission (556 nm). The band gap of the n-ZnO NWs can be used to calculate the peak position of the UV emission to obtain ∼3.292 eV, which corresponds to the exciton−exciton collisions in ZnO.23 The UV emission of the p-ZnO:K NS sample is weaker than that of the n-ZnO NW sample because the K dopants have built the recombination sites in the ZnO, which decay the UV-emission efficiency. The green/UV emission peak ratio of the p-ZnO:K NS sample reached a maximum of 1.67, which indicates that the K dopant provided recombination sites and increased oxygen vacancies, thereby increasing the green emission.9 The UV emission of the Au NP sample was larger than those of the nZnO NW and p-ZnO:K NS samples by around 4.1- and 2.1fold. The green/UV emission ratio of the Au NP sample had a minimum of 0.13. According to previous reports,24 the Au-NPenhanced UV emission is attributed to several mechanisms, such as the LSPR enhancement issue, the spontaneous recombination caused by the Au NPs/p-ZnO:K NSs’ interactions, and the increased light absorption because of the Au NPs’ light scattering. The green emission of the Au NPs/pZnO:K NS sample was the weakest, suggesting that the Au NPs absorbed the most light, with the remainder irradiating the pZnO:K NSs.

dripping step 6 times with the 1:10 000 dilution ratio in the dark and under UV, blue, and green LED illumination. The I− V curves of the 6 repeated drippings for the Au NPs/p-ZnO:K NSs/n-ZnO photodetectors are linear curves in the dark and under light illumination, as shown in Figure S1 (Supporting Information). The linear I−V characteristics indicate Ohmic contact. According to a previous report,22 the resistance of Ohmic contact is around 10−100 kΩ, which, in comparison to the photodetector resistance of ∼15.6 MΩ, is sufficiently small to be ignored. The dark current of this Au NP sample was only 0.375 μA, which is smaller than the 0.642 μA value for the pZnO:K NSs/n-ZnO/glass sample. Low dark current implies that the Au NPs directly contact the p-ZnO:K NSs to form a Schottky barrier,19 which causes band-bending of the p-ZnO:K NSs and creates a depletion region in the Au/p-ZnO:K interface. The UV photocurrent of 4.38 × 10−4 A for the Au NP sample is smaller than that for the p-ZnO:K NSs/n-ZnO/glass, which was 4.77 × 10−4 A. It may be that the Au NPs partially block and scatter the UV light irradiating the p-ZnO:K NSs/nZnO thereby reducing the UV photocurrent; nevertheless, the blue (3.57 × 10−5 A) and green (3.67 × 10−6 A) photocurrents of the Au NP sample are higher than those of the p-ZnO:K NSs/n-ZnO sample by 7.35- and 1.59-fold, respectively. Moreover, the UV (∼1168), blue (∼95.2), and green (∼9.8) photoresponses of the Au NP sample are higher than those of the p-ZnO:K NSs/n-ZnO sample by ∼1.57-, ∼12.78-, and ∼3.03-fold, respectively. This means that the method with peppered Au NPs is successful at enhancing the UV and visible response of the photodetector. The increasing visible-light photocurrent is attributed to the LSPR effect by the Au NPs’ absorption and transference of the visible-light energy. D

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Figure 5. (a) Top-view and (b) cross-sectional FE-SEM images of p-ZnO:K NSs/n-ZnO peppered with Au NPs via 6 repetitions with the 1:10 000 dilution ratio. Inset images of part b are a zoomed-in image and EDX spectra.

ZnO:K NS sample was calculated to be ∼15.58 MΩ, which indicates that the influence of path A can be ignored because of the high impedance. Path B shows that the current passes through the intertwined p-ZnO:K NWs and nanosheet link. The interface of these pZnO:K NSs has a grain-boundary barrier layer that creates a surface depletion region.25 This means that path B contains thousands of depletion regions, as shown in Figure 4c. Figure 4d reveals the band diagram of the p-ZnO:K NSs under UVand visible-light illumination. UV photons generated many electron−hole pairs on the p-ZnO:K NSs, which reduced or eliminated the depletion region. Under visible-light illumination, the surface of the Au NPs produced the LSPR effect, which generated a strong electromagnetic-field enhancement that reduced the depletion region and increased the photocurrent. Figure 5a,b displays the top-view and cross-sectional SEM images of p-ZnO:K NSs/n-ZnO peppered with Au NPs via the 6 dripping repetitions of the 1:10 000 dilution ratio. The topview SEM image shows numerous p-ZnO:K nanosheets grown

Figure 4a schematically depicts the current paths (A and B) of the photodetector at the applied bias. Path A shows that the current passes through the p-ZnO NSs, n-ZnO film, and p-ZnO NS interface, as shown in Figure 4b. Path A is the p−n−p homojunction structure, and its equivalent circuit is two diodes connected back-to-back, which is a forward p−n bias in series connected to a reverse n−p bias. In general, path A should feature the diode I−V characteristic; however, the I−V characteristic of the Au NPs/p-ZnO:K NSs/n-ZnO is a linear property in the dark and under illumination. It is speculated that the reverse bias caused a high impedance that reduced the influence of path A. It was noted that the resistance of the sputtered n-ZnO thin film (thickness 50 nm) had high impendence (over 1 GΩ), compared with those in previous reports (∼100 kΩ to ∼10 MΩ), because the n-ZnO thin film possessed good crystal quality and low oxygen-vacancy defects. Path A is serial connection forward bias, high resistance n-ZnO film, and reverse bias. The impedance of path A should be several times higher than the resistance of the n-ZnO film. On the basis of the I−V curves in Figure 2b, the resistance of the pE

DOI: 10.1021/acsami.7b03216 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces on the ZnO:K NWs. The width and thickness of the p-ZnO:K nanosheets are around 3−10 μm and 3−60 nm, respectively. The edge of p-ZnO:K nanosheets is observed to be very thin, with the NW profile being visible through them. The length and diameter of ZnO:K NWs are about 2.1 μm and 100−500 nm, respectively, by cross-sectional SEM images. The inset in the top-left corner of Figure 5b is the zoomed-in SEM image. As can be seen, the p-ZnO:K NSs have been covered and surrounded by a large amount of Au NPs. The inset in the topright corner of Figure 5b is the energy dispersive X-ray spectroscopy (EDX) spectra, and demonstrates that the pZnO:K NSs contain 0.36 atom % K. The Au signal (1.88 atom %) in the EDX spectra was observed, and confirms the existence of Au NPs. Figure 6a,b shows the TEM and HR-TEM images, respectively, of the 6 repeated-dripping (6-TRD) Au NPs/pZnO:K NSs/n-ZnO sample. The TEM samples were prepared by finely dispersing the Au NPs/ZnO:K NSs by ultrasonication for 30 min. Although the SEM image revealed a large amount of Au NPs adhered to the ZnO:K NSs, the TEM image displays only a small amount of Au NPs. This difference is attributed to the ultrasonic process causing damage to the Au NPs/p-ZnO:K NSs, leading to a large amount Au NPs dislodging from the pZnO:K NSs. The diameter distribution of the 6-TRD Au NPs is around 5−35 nm, which demonstrates the challenges inherent in photochemically synthesizing uniform Au NPs. Nevertheless, the various sizes of the Au NPs can provide photodetectors with a wider light-wavelength absorption ability because the specific diameter sizes of the Au NPs correspond to specific light-wavelength absorptions.26 The top-left and top-right inset images of Figure 6b are the selected area electron diffraction (SAED) patterns of the Au NPs and ZnO:K NWs, respectively, and indicate that the former has an amorphous structure while the latter is single crystalline with wurtzite structure. Figure 6c presents the TEM image and EDX mapping images, which correspond to O, Zn, K, and Au atom distributions. The highdensity Zn spots are distributed throughout the p-ZnO:K NW, while the distribution of the Au spots is focused in one small area, which corresponds to a Au NP’s place. However, the distribution of the K spots is very sparse, because of a lower relative K atom %. Nevertheless, the K spots of the p-ZnO:K NW area are still slightly more numerous than elsewhere. Figure S3 reveals the X-ray photoelectron spectroscopy (XPS) spectra of a Au NPs/p-ZnO:K NSs/n-ZnO sample. The K 2P3/2 and K 2P1/2 signal peaks are of much lower intensity compared with those of Zn 2P3/2 and Zn 2P1/2, which demonstrates that K dopants of ZnO exist at low concentration. Although the EDX mapping and XPS are not rigorous enough to prove the existence of K dopants, the Hall-effect measurement verified that K dopants cause the p-type property of the pZnO:K NS sample. The PL of the p-ZnO:K NS sample also proved that the K dopant provided recombination sites and increased oxygen vacancies, thereby increasing the green emission. Figure 7a shows the I−V curves of the 6-TRD Au NPs/pZnO:K NSs/n-ZnO/glass substrate measured under various relative humidity (RH) values. The I−V curves of the photodetector indicate Ohmic contact. The measured current of the device clearly decreases with increasing RH. According to a previous report,9 many electrons are produced by large amounts of water molecules directly in contact with the semiconductor. The conductivity of the p-type semiconductor is reduced by the decrease in hole concentration because the

Figure 6. Cross-sectional (a) TEM and (b) HR-TEM images of Au NPs/p-ZnO:K NWs. (c) TEM image and EDX mapping images of Au NPs/p-ZnO:K NWs.

high humidity provides electrons to recombine with holes, which demonstrates that the p-ZnO:K NSs have the p-type property. Figure S2 shows the optical absorption spectra measured at room temperature for the n-ZnO NWs/n-ZnO, pZnO:K/n-ZnO NSs, and 6-TRD Au NPs/p-ZnO:K NSs/nZnO samples by a UV/vis/IR spectrophotometer (JASCO, V600). In the UV range, the absorption values of p-ZnO:K F

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ZnO sample. One reason for this result could be that the K dopant increased the oxygen vacancies and nanosheets covering the NWs, thereby increasing the visible-light absorption.9 The 6-TRD Au NPs/p-ZnO:K NSs/n-ZnO sample featured the highest visible-light absorption. Apparently, the Au NPs increased the visible-light absorption compared with that of the p-ZnO:K NSs/n-ZnO sample. Figure 7b,c displays the transient response of the p-ZnO:K NSs/n-ZnO and 6-TRD Au NPs/p-ZnO:K NSs/n-ZnO with the green, blue, and UV LED excitations switched on and off with a 2.0 V applied bias. The reproducibility and stability property of these photodetectors were examined by green, blue, and UV LED illumination. The UV photocurrent values of the 6-TRD Au NPs/p-ZnO:K NSs/n-ZnO and p-ZnO:K NSs/nZnO samples are in the same current region of ∼10 mA; however, the blue and green photocurrents at ∼20 and ∼8 μA, respectively, for the Au NP sample are higher than those of the non-Au NP sample by around 10-fold. This confirms that the Au NPs under visible light can enhance the photocurrent of the photodetector via the LSPR effect.26 The photoresponse time of the Au NP sample is higher than that of the non-Au NP sample by ∼2 times. It is speculated that the interface barrier of the Au NPs and p-ZnO:K caused the depletion region, which had limited electron and hole recombination and delayed the response time. In general, the Au NPs are used in the photodetector for enhancing the visible-light response by LSPR and decreasing the dark current by forming a Schottky barrier. Under visiblelight illumination, free electrons of Au NPs were of collective coherent oscillation to produce an oscillating electromagnetic field, which causes the charge separation and dipole oscillation on the surrounding Au NP’s surface. According to a previous report,26 various Au NPs have the high absorbance band in the wavelength range 450−550 nm of incident light, which is the visible-light region. The wavelength and bandwidth of LSPR depend on the size of the Au NPs. The TEM images have demonstrated that the diameter distribution of Au NPs is approximately 5−35 nm, which can provide a wider lightwavelength absorption ability for generated LSPR and enhanced photodetectors. Figure 8a−c displays the spectra responsivities of the n-ZnO NWs/n-ZnO, p-ZnO:K NSs/n-ZnO, and 6-TRD Au NPs/pZnO:K NSs/n-ZnO samples measured with a 1−10 V applied bias at room temperature. The responsivities of these samples increased with increasing voltage, the values of which were 2.62 × 10−2, 3.71 × 10−2, and 2.54 × 10−2 A/W for the n-ZnO NW, p-ZnO:K NS, and 6-TRD Au NP samples, respectively, at 1 V and a 360 nm incident light. The quantum efficiency (η) can also be calculated with the equation19 Figure 7. (a) I−V curves of a 6-TRD Au NPs/p-ZnO:K NSs/n-ZnO/ glass substrate measured under various RH values. Transient response of (b) p-ZnO:K NSs/n-ZnO and (c) 6-TRD Au NPs/p-ZnO:K NSs/ n-ZnO with the green, blue, and UV LED excitation switched on and off.

η=R

(hc) (qλ)

(1)

where R denotes the responsivity, h is the Planck constant, c represents the speed of light, q is the electron charge, and λ signifies the incident light wavelength. The quantum efficiency values of the n-ZnO NW, p-ZnO:K NS, and 6-TRD Au NP samples were around 9.0%, 12.8%, and 8.7%, respectively, at 1 V and 360 nm incident light. The p-ZnO:K sample presented the maximum UV responsivity and quantum efficiency, which corresponded to the UV (365 nm) photocurrent/dark current ratio in Table 1. The incident blue (460 nm) and green (520 nm) light of the 6-TRD Au NP sample had responsivities of

NSs/n-ZnO and 6-TRD Au NPs/p-ZnO:K NSs/n-ZnO samples are slightly higher than that of n-ZnO NWs/n-ZnO. Because of these nanosheets covering the NWs and their coverage being around 40% of the sample area, it is speculated that the p-ZnO:K nanosheets increase the UV-absorption ability. In the visible-light range, the absorption of the p-ZnO:K NSs/n-ZnO sample was higher than that of the n-ZnO NWs/nG

DOI: 10.1021/acsami.7b03216 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

ZnO:K NSs constitute a metal−semiconductor interface that contains a Schottky barrier,10 which creates a depletion region to reduce the dark current. Under UV LED illumination (365 nm, 6.0 mW/cm2), a large amount of electron−hole pairs are photogenerated on the surface of the p-ZnO:K NSs. Accordingly, the depletion region of the Schottky barrier disappears and is replaced by the photogenerated electron− hole pairs. The O2− ion absorbs a hole and breaks away from the ZnO surface to become oxygen. The photogenerated electron concentration of the p-ZnO:K NSs is higher than that of the Au NPs, which causes a portion of their electrons to diffuse to the Au NPs. When the UV illumination is turned off, the depletion region of the Schottky barrier increases and isolates the Au NPs and p-ZnO:K NSs.10 Initially, the electrons from the p-ZnO:K NSs recombine with the holes, but there are still many electrons distributed in the Au NPs. These diffusion electrons from the Au NPs do not easily return to the p-ZnO:K NSs. The remaining holes of p-ZnO:K NSs have a longer lifetime and slower recombination with electrons. As such, the photocurrent rise and fall is slower than previously reported, and the I−V curves of the photodetector are similar to those of a capacitor, which contains Au NPs and a Schottky barrier. Because the photon energy of visible light (3.18−1.65 eV) is lower than that of the ZnO band gap (3.34 eV) at room temperature, visible light alone can not generate a large amount of electron−hole pairs in ZnO. On the basis of the PL measurement, the K dopants increased the oxygen vacancies and built a deep-level trap (ET) for electron−hole recombination. The photon energy values of the blue (465 nm, 2.67 eV) and green (520 nm, 2.38 eV) LEDs were enough to excite electrons from the valence band (EV) to pass through the ET to the conduction band (EC). The blue and green photocurrents of the p-ZnO:K NSs/n-ZnO photodetector were 7.4- and 3.2fold greater than the dark current. To improve the visible-light detection ability of the photodetector, the Au NPs were photochemically synthesized on the p-ZnO:K NSs/n-ZnO/ glass substrate. The blue (Iblue/Idark = 94.6) and green (Igreen/ Idark = 9.7) responses of the Au NP sample were increased by 1278.4% and 303.1% compared with those of the non-Au NP sample. Moreover, the Au NPs absorbed the blue and green irradiation energy to generate the LSPR effect, which is a strong electromagnetic field that changed the Au NPs and ZnO status to increase the photocurrent.27 The electron−electron relaxation roots of the Au NPs were built from the collision of photoexcited high-energy electrons with other electrons. Such electron collisions of the Au NPs caused an electronenergy redistribution by the Fermi−Dirac distribution, which then formed a higher Fermi level EF′ in Au NPs.28 The blue light has larger photoenergy compared with that of green light. Accordingly, the LSPR E′F level of the Au NPs under blue illumination is higher, and so the EV′ electrons are easier to excite and pass through ET to EC, which enables the blue photocurrent to be higher than the green photocurrent by about 10-fold.

Figure 8. Measured spectra responsivities of (a) n-ZnO NWs/n-ZnO, (b) p-ZnO:K NSs/n-ZnO, and (c) the 6-TRD Au NPs/p-ZnO:K NSs/n-ZnO photodetector biased at 1−10 V.

4.03 × 10−4 and 5.80 × 10−4 A/W, respectively, at 1 V bias, which were higher than those of the n-ZnO NW sample by ∼8.1 times and the p-ZnO:K NS sample by ∼2.6 times. Accordingly, it appears that the Au NPs can enhance the visible-light responsivities; in addition, the visible-light responsivities of the p-ZnO:K NS sample were higher than those of the n-ZnO NW sample. Figure 9a−d plots the band diagrams of the 6-TRD Au NPs/ p-ZnO:K NSs in the dark, and under green, blue, and UV LED illumination, respectively. The atmospheric oxygen absorbs an electron from the ZnO surface to form the O2− ion.2 The pZnO:K NS surface has fewer electrons and increases the hole concentration. Because the major carriers of p-type semiconductors are holes, the surface of p-ZnO:K NSs should increase the conductivity by O2−-ion-captured electrons, which causes the band-bending of p-ZnO:K NSs. The Au NPs and p-

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CONCLUSION High-density p-ZnO:K NWs and nanosheets were hydrothermally synthesized on an n-ZnO/glass substrate. The K element of p-ZnO:K NSs had an atom % of 0.36. The p-ZnO:K NS sample featured a better photocurrent/dark current ratio (IUV/Idark = 1165, Iblue/Idark = 7.4, Igreen/Idark = 3.2), which was higher than that of the ZnO NW sample by ∼2-fold. The maximum UV- and visible-light photoresponse of the Au NP H

DOI: 10.1021/acsami.7b03216 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 9. Band diagrams of Au NPs/p-ZnO:K NSs photodetectors: (a) in the dark, and under (b) UV, (c) blue, and (d) green LED illumination.

Notes

sample was achieved via the preparation that involved repeating the UV photochemical reaction with 1:10 000 diluted HAuCl4 solution 6 times to coat the p-ZnO:K NSs/n-ZnO/glass substrate. The diameter-size distribution of Au NPs was around 5−35 nm, which allows for a wider visible-light-wavelength absorption. The p-ZnO:K NSs demonstrated the p-type property by I−V measurement at various RH values. On the basis of the localized surface plasmon resonance effect, the measured 6-TRD Au NPs/p-ZnO:K NSs/n-ZnO/glass had a higher UV photocurrent/dark current ratio (IUV/Idark = 1165) and larger blue and green photocurrent/dark current ratios (Iblue/Idark = 94.6, Igreen/Idark = 9.7) in comparison with those in previous reports.

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The authors declare no competing financial interest.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b03216. I−V curves, UV−vis absorption spectra, and XPS spectra (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Phone: +886-6-260-6123 ext. 7785. Fax: +886-6-260-2305. Email: [email protected]. *Phone: +886-6-275-7575 ext. 62391. Fax: +886-6-276-1854. E-mail: [email protected]. ORCID

Cheng-Liang Hsu: 0000-0001-7318-9738 Funding

The authors would like to thank the Ministry of Science and Technology, Taiwan, for financially supporting this research under Contract MOST 105-2221-E-024-015. I

DOI: 10.1021/acsami.7b03216 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsami.7b03216 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX