Solution-Processed UV and Visible Photodetectors Based on Y

royal blue, blue, green, yellow, and red LED illumination, as shown in Figures 6(a) and 6(b), respectively. It should be noted that these I-V curves a...
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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Solution-Processed UV and Visible Photodetectors Based on Y‑Doped ZnO Nanowires with TiO2 Nanosheets and Au Nanoparticles Cheng-Liang Hsu,*,† Hsin-Yu Wu,† Chung-Cheng Fang,† and Sheng-Po Chang*,‡ †

Department of Electrical Engineering, National University of Tainan, Tainan 700, 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

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ABSTRACT: Various chemical solutions were employed to fabricate the Au nanoparticles (NPs), TiO2 nanosheets, and Ydoped ZnO nanowires on glass substrates via the photochemical, sol−gel, and hydrothermal methods, respectively. The core−shell of the TiO2/ZnO:Y nanowires was dusted with Au nanoparticles. The ultrathin (2.48−7.02 nm) TiO2 nanosheet has a noncrystalline structure, and the various sizes (3.0−33.91 nm) of Au NPs enhanced the light absorption ability. The Au nanoparticles/TiO2 nanosheet/ZnO:Y nanowires featured a high UV on/off ratio of 1786.0 and various visible photoresponses of 154.1−27.3 with a variety of LEDs. Apparently, the coated noncrystalline TiO2 nanosheet improved the response speed. The external quantum efficiency of the sample was ∼5.1% and offered a higher responsivity in the visible light region due to the localized surface plasmon resonance effect. KEYWORDS: Y-doped, ZnO nanowires, Au nanoparticles, TiO2 nanosheets, photodetector

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INTRODUCTION In recent years, the optical properties of semiconductors have attracted great attention due to their wide application in commercial, industrial, and military products,1−4 such as solar cells,1 light-emitting diodes (LEDs),2 charge-coupled devices (CCDs), and photodetectors.3,4 Ultraviolet (UV) photodetectors are fabricated with wide-band-gap semiconductors, such as SiC, Ga2O3, GaN, SnO2, ZnO, and TiO2.5−13 Among these, ZnO is the most common UV-photodetector material due to its wide band gap of 3.37 eV, large exciton binding energy of 60 meV, simple synthesis, stable physical and chemical properties, and high UV response and sensitivity.10 In addition, the optical properties of ZnO nanostructures (NSs) exhibit good performance, which is several times higher than that of ZnO thin film.14 Currently, several methods are used to improve the performance of ZnO NS-based UV photodetectors, the most common of which include doping various elements into the ZnO NSs and decorating with noble metal nanoparticles.15,16 More specifically, it has been demonstrated that doping with rare-earth elements improves the optical properties.17 Further, noble metal nanoparticles (NPs) feature the localized surface plasmon resonance (LSPR) effect, which increases the visible light performance of ZnO NS-based photodetectors.18 Although ZnO NSs exhibit various morphologies via simple synthesis processes,10 the UV response speed of ZnO NSs is quite slow compared with other wide-band-gap semiconductors.5−13 The surface defects of ZnO absorb atmospheric oxygen to form O2−, which by desorption reacts with UV light.4 © XXXX American Chemical Society

The oxygen absorption and adsorption of ZnO NSs is delayed because of the UV photoresponse speed. In a previous report, a SiO2 isolation layer was coated on the ZnO nanowires (NWs) to increase the photoresponse speed.19 In the current study, TiO2 was applied as an oxygen barrier layer due to its good photoresponse and antireflective properties.20 TiO2 NSs have been used in many commercial products, such as UV photodetectors,11 UV photoelectrochemical photodetectors,21 photocatalysis,22 solar cells, and photochromic devices.11,23 Yttrium-doped ZnO (ZnO:Y) NWs were synthesized on a glass substrate by the hydrothermal process; then, a TiO2 layer was deposited via the sol−gel method on the ZnO:Y NWs, after which it was decorated with Au NPs to increase the visible light response. To date, most nanocomposite structures have been fabricated using high-temperature processes, expensive equipment, and complex technologies. However, this study employed a photochemical, sol−gel, and hydrothermal method to synthesize a low-cost nanocomposite at low temperature. The UV photoresponse speed and visible light detection ability of this nanocomposite photodetector have greatly increased compared with previous ZnO NW reports.10 The crystal structure as well as optical and electrical properties of the Au NPs/TiO2/ZnO:Y NWs/glass are discussed in detail. Received: February 6, 2018 Accepted: April 19, 2018 Published: April 19, 2018 A

DOI: 10.1021/acsaem.8b00180 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Energy Materials

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The morphologies, element composition ratio, and crystal structure were analyzed by field emission scanning electron microscopy (FESEM, JEOL-7000F), high-resolution transmission electron microscopy (HR-TEM, JEOL JEM-2100F), and X-ray diffraction (XRD, MACMXP18). The elemental composition of the Au NPs/TiO2/ZnO:Y NWs was analyzed with energy-dispersive X-ray spectrometry (EDS) and X-ray photoelectron spectroscopy (XPS, VG Scientific Microlab 350), for which a Xe lamp with a wavelength of 254 nm was the photoluminescence excitation source (PL, Jobin Yvon-Spex fluorolog3 spectroscopy). The current−voltage (I−V) curves were measured in the dark and under varied LED illumination by source-measure instruments (Keithley Model-4200).

EXPERIMENTAL SECTION

Figure 1a schematically illustrates the synthesized flow of the Au NPs/ TiO2/ZnO:Y NWs/glass photodetector. Photographs of the TiO2/

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RESULTS AND DISCUSSION The FE-SEM images of the TiO2 and Au NPs synthesized on the ZnO NWs/glass and ZnO:Y NWs/glass are shown in Figure 2a,b, respectively. The TiO2 nanosheet layer and Au

Figure 1. (a) Schematic illustration of the fabrication processes and steps of the Au NPs/TiO2/ZnO:Y NWs/glass photodetector. (b) Photograph of the Au NPs/TiO2/ZnO:Y NWs/glass and TiO2/ ZnO:Y NWs/glass photodetectors. ZnO:Y NWs/glass photodetectors with and without the Au NPs are shown in Figure 1b. As can be seen, the colors of TiO2/ZnO:Y NWs/ glass and the sample containing the Au NPs sample are white and light pink, respectively. The ZnO:Y NWs were hydrothermally synthesized on a 50 nm thick ZnO seed layer, which was sputtered on a glass substrate (Corning EAGLE XG). The hydrothermal solution of the ZnO:Y NWs was composed by an Y source [8 mM Y(NO3)3] and Zn source [80 mM Zn(NO3)2·6H2O] with hexamethylenetetramine [80 mM C6H12N4]. The ZnO seed layer sample was immersed in hydrothermal solution and placed in a sealed autoclave at 100 °C for 6 h. The synthesis mechanism and chemical reactions to grow the ZnO:Y NWs are similar to previous reports.15,17 A TiO2 nanosheet layer was coated on the ZnO:Y NWs by the sol− gel method,24 the solution precursors of which were titanium(IV) butoxide (TIB) diluted in a 1:100 ratio with isopropanol and stirred for 15 min until fully mixed. Deionized (DI) water was then gradually added to the diluted TIB solution in a molar ratio of 4:1 for the subsequent 30 min hydrolysis reaction. The reactions in the TiO2 synthesis are as follows.24

Ti(OC4H 9)4 + 4H 2O → Ti(OH)4 + 4C4H 9OH

Figure 2. FE-SEM images of (a) ZnO NWs/glass, and (b) ZnO:Y NWs/glass. Inset images are the corresponding magnified SEM images.

Ti(OH)4 → TiO2 · x H 2O + (2 − x)H 2O The TiO2·xH2O was deposited on the ZnO NWs and dried in air. TiO2·xH2O then transformed into anatase TiO2 by calcination at 450 °C for 60 min. The TiO2 sol−gel procedure was repeated twice to increase the TiO2 nanosheet thickness and avoid the uniformity issue. Next, HAuCl4·4H2O was diluted 10 000 times with ethanol, after which 50 μL was dropped on the TiO2/ZnO NWs/glass sample. The Au NPs were then synthesized with diluted HAuCl4·4H2O solution under UV illumination (Kinsten KVB-30D UV box, 380 nm, 100 W) for 500 s. For removal of chlorine and an increase in the AuNPs/TiO2 interface adhesion, the samples were placed in a furnace with Ar gas flow at 400 °C for 10 min. The synthesis procedure of the Au NPs was also performed twice to increase the density of the Au NPs. Finally, Ag paste was coated to form electrodes on the sample and to construct a metal−semiconductor−metal (MSM) structure.

NPs cannot be clearly observed in the FE-SEM images. Although the lengths of the ZnO NWs and ZnO:Y NWs are uniform, their diameters are uneven. The average length and diameter of the ZnO NWs were around 1.62 μm and 120 nm, respectively, yielding an aspect ratio smaller than the ZnO:Y NWs, which correspondingly measured at ∼2.65 μm and ∼90 nm. In the ZnO:Y NW sample, yttrium comprised 0.24 atom % by EDS. The large aspect ratio provided more light scattering and lower reflectivity, which enhanced the photodetector performance. B

DOI: 10.1021/acsaem.8b00180 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

TiO2-coated samples are derived from TiO2, and so the ZnO signal is slight. The NBE/DLE intensity (INBE/IDLE) ratios of the TiO2-coated samples are higher than those of the ZnO:Y NWs. It was found that the Au NPs/TiO2/ZnO:Y NWs had the maximum INBE/IDLE = 7.04, which indicates that their defect density is the lowest. Figure 4 shows the XPS spectra of the Au NPs/TiO2/ZnO:Y NW sample. The Zn 2p3/2, Zn 2p3/2 and O 1s signals constitute the strong peaks, which demonstrates that the NW material composition is ZnO. The Y 3d signal is weak, which indicates that the Y element has a low concentration, as found by SEMEDS. The Au NPs and TiO2 nanosheet layer were detected, as indicated by the strong Au 4f and Ti 2p3/2 signals. Figure 5a reveals the TEM and EDS mapping images of the Au NPs/TiO2/ZnO:Y NWs. The Zn, O, and Ti elements in the EDS mapping are distributed throughout the entire NW; however, the bright spot of Y mapping is sparse, and the Y signal of the EDS spectrum is weak. The Au element is in a localized area, which is mapped to the Au NPs. The HR-TEM images of the Au NPs/TiO2/ZnO:Y NWs are presented in Figure 5b−d. The HR-TEM images and the selected area electron diffraction (SAED) patterns confirm that the ZnO:Y NWs are single-crystal and have the wurtzite structure. However, the TiO2 nanosheet layer is not uniformly covered on the ZnO:Y NWs, and its thickness is ultrathin, ranging from 2.48 to 7.02 nm. In addition, the HR-TEM images indicate that the TiO2 did not have an obvious crystalline structure, and that the Au NP size is widely distributed with a range from 3.0 to 33.91 nm, which allows the absorption of a variety of visible light wavelengths. According the previous report,25 the absorption of 9−99 nm Au NPs is strong in visible light and weaker in the UV region because of the LSPR effect. The smaller Au NPs reveal the higher absorbance in the UV region compared with larger Au NPs. It is speculated that 3.0−33.91 nm Au NPs have enlarged the UV absorbance. The I−V curves of the ZnO NWs and ZnO:Y NWs were measured in the dark and under UV, royal blue, blue, green, yellow, and red LED illumination, as shown in Figure 6a,b, respectively. It should be noted that these I−V curves are the ohmic contact. The emission wavelengths of the UV, royal blue, blue, green, yellow, and red LEDs are 365, 445−450, 465−470, 520−525, 520−525, and 620−630 nm, respectively. The input voltage and current of these LEDs are around 3.8 V and 7 mA, respectively, which yields an LED input power of ∼26.6 mW. The input power of these LEDs is not equal to output light power, because of the different energy conversion efficiency (η) of these LEDs, but the photoresponse performance of differentstructure photodetectors can be roughly examined and compared by LED-excited photocurrent. The lighting power densities of UV, royal blue, blue, green, yellow, and red LEDs are measured as 1.8, 19.3, 10.3, 57.9, 36.3, and 52.7 mW/cm2 by photometers. The photocurrent increased with shorter wavelengths of LED illumination. The ∼3.4 eV photoenergy of the 365 nm UV-LED is higher than the ZnO band gap of 3.3 eV, which absorbed UV to generate most of the electron and hole pairs, and increased conductivity. The UV photocurrent to dark current (IUV/Idark) ratio of the ZnO:Y NWs was 53.7, which is higher than that of the ZnO NWs (30.3). This means that the Y dopant increased the ZnO NW photoresponse performance by 177.2%. Figure 7a,b reveals the I−V curves of the TiO2/ZnO:Y and Au NPs/TiO2/ZnO:Y NW samples, respectively, in the dark and under UV, royal blue, blue, green, yellow, and red LED

The XRD patterns and PL spectra of the TiO2/ZnO, ZnO:Y, TiO2/ZnO:Y, and Au NPs/TiO2/ZnO:Y NW samples are presented in Figure 3a,b, respectively. The XRD patterns of

Figure 3. (a) XRD patterns and (b) PL spectra of the Au NPs/TiO2/ ZnO:Y, TiO2/ZnO:Y, ZnO:Y, and TiO2/ZnO NWs.

these samples showed a strong ZnO (002) peak, which indicates the hexagonal wurtzite structure. Although, the XRD signals did not display TiO2 and Au peaks, the EDS spectrum of the SEM equipment did detect the Ti and Au elemental signals. It is speculated that the TiO2 nanosheet layer and Au NPs were noncrystalline and ultrathin. The inset enlarged image of the XRD pattern in Figure 3a shows the notable ZnO (002) peak. By observing the ZnO (002) peak, it was revealed that the ZnO:Y NW samples had shifted to a larger angle compared with the TiO2/ZnO NW sample. This means that the ZnO lattice constants were decreased by the Y dopant. Because of the ionic radius of Y3+ = 0.88 Å being larger than that of Zn2+ = 0.74 Å, the ZnO lattice constants should be increased by the larger ionic radius of Y3+ replacing the Zn2+. It is believed that an Y3+ ion is interstitially doped, thereby forming an Y−O−Zn binding on the ZnO surface.17 The UV and green emissions of these PL curves are present at the near band edge (NBE) and deep level emissions (DLE) peaks. The NBE peak position of the ZnO:Y NWs had a wavelength of 381.04 nm; however, the NBE peaks of the TiO2-coated samples were distributed in the 385.18−388.29 nm region. At room temperature, the band gaps of ZnO and TiO2 are 3.3 and 3.2 eV, respectively, which correspond to 375.76 and 387.5 nm. This means that the PL spectra of the C

DOI: 10.1021/acsaem.8b00180 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

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ACS Applied Energy Materials

Figure 4. (a) Zn, (b) O, (c) Y, (d) Au, and (e) Ti signals from the XPS spectrum of the Au NPs/TiO2/ZnO:Y NWs.

IUV/Idark ratio of 1786.0 due to its ultralow dark current. The Au NPs deposited on the TiO2/ZnO:Y NWs created a Schottky barrier at the Au NPs/TiO2 interface,28 which decreased the conductivity of the sample. The low photocurrent is attributed to the scattering effect of the Au NPs. The current working area of photodetector device is an important factor. The darkcurrent-to-photocurrent regions of ZnO, ZnO:Y, TiO2/ZnO:Y, and Au NPs/TiO2/ZnO:Y NWs are 6.6−210, 3.1−173, and 0.39−23.2 μA and from 4.7 nA to 9.1 μA at applied bias 2 V, respectively. Table 1 shows the Iphoto/Idark ratios of the ZnO, ZnO:Y, TiO2/ZnO:Y, and Au NPs/TiO2/ZnO:Y NW samples. As can be seen, the Iphoto/Idark ratios for these samples increased with smaller wavelengths of LED illumination. The TiO2 nanosheet layer of the TiO2/ZnO:Y NWs enhanced the visible light (445−630 nm) Iphoto/Idark ratio by ∼2-fold compared with that of the ZnO:Y NWs. It is speculated that the noncrystalline TiO2 may have a wide light wavelength absorption ability. The Au NPs/TiO2/ZnO:Y NWs featured the maximum Iphoto/Idark ratios, namely, IUV/Idark = 1786.0, Iroyal‑blue/Idark = 154.1, Iblue/ Idark = 135.0, Igreen/Idark = 67.3, Iyellow/Idark = 37.3, and Ired/Idark = 27.3. These high photoresponses are contributed by the

illumination. As before, these I−V curves are the ohmic contact. The applied voltage and current of these LEDs are the same as previously mentioned. The IUV/Idark ratio of the TiO2/ZnO:Y NWs was ∼54.0, which is similar to that of the ZnO:Y NWs. At an applied bias of 10 V, the UV photocurrent of ∼0.123 mA for the TiO2/ZnO:Y NWs was lower than that of the ZnO:Y NWs (∼0.865 mA) by around 7 times; interestingly, the dark current of ∼2.28 μA for the TiO2/ZnO:Y NWs was also lower than that for the ZnO:Y NWs (∼16.1 μA) by about 7-fold. The resistances of the TiO2/ZnO:Y NWs and ZnO:Y NWs were 4.39 and 0.62 MΩ, respectively. The coated TiO2 sample had higher resistance due to the potential barriers at the TiO2/ZnO heterojunction by the grain boundaries band bending.26 Despite the TiO2 nanosheet layer being very thin, according to the HR-TEM analysis, the structure of the TiO2 nanosheet layer was noncrystalline. The absorption coefficient of the noncrystalline TiO2 was much higher than the crystalline TiO2,27 which means that a large portion of the UV light was absorbed by the ultrathin TiO2. The UV photocurrent of ∼45.9 μA and dark current of ∼25.7 nA for the Au NPs/TiO2/ZnO:Y NWs were the lowest compared with the other samples, which yielded the highest D

DOI: 10.1021/acsaem.8b00180 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Article

ACS Applied Energy Materials

Figure 6. I−V curves of the (a) ZnO NWs/glass and (b) ZnO:Y NWs/glass measured in the dark and under a variety of LED illuminations.

Figure 5. (a) TEM image and EDX mapping, and (b−d) HR-TEM images of the Au NPs/TiO2/ZnO:Y NWs. Inset image is the SAED pattern.

sample’s lowest dark current, which is attributed to the Au NPs causing the Schottky barrier and decreasing the conductivity. The transient photoresponses of the Au NPs/ZnO:Y, TiO2/ ZnO:Y, and Au NPs/TiO2/ZnO:Y NWs were measured by I− V curves under UV, royal blue, and green LED illumination switched on/off, as shown in Figure 8a−c, respectively. It is obvious that the noncrystalline TiO 2 nanosheet layer dramatically increases the UV photoresponse speed, which is consistent with the previous experimental design. The photoresponses of the Au NPs/TiO2/ZnO:Y NWs were higher than those of the TiO2/ZnO:Y NWs because of Au NPs generating the Schottky barrier in the dark and absorbing incident light to produce the LSPR effect.24 The photoresponses of these samples were stable and reproducible with the UV, royal blue, and green LED switched on/off. The response and recovery time can be calculated by a transient fit with formula.29,30 The response speed (