1. Introduction

Over the last decade, wide bandgap semiconductors materials have attracted extensive attentions in optical and electrical devices due to their unique properties [1–4]. Among the wide bandgap semiconductor, zinc oxide (ZnO) has attracted more attention due to its large exciton binding energy (60 meV), wide bandgap (3.37 eV at room temperature), bandgap engineering tunable, piezoelectric effect and easy processing properties [5–7]. ZnO has been applied in an extensive field of nanogenerators [8], nanolasers [9], filed-effect transistors [10], light-emitting diodes [11, 12], field emitters [13, 14], and ultraviolet (UV) photosensors [15, 16].

For photodetectors, fast response and recovery time, together with high responsibility, are commonly desired characteristics [17, 18]. One-dimensional semiconductor nanowires (NWs) are considered as one of the most sensitive and fast responsive photodetector candidates for the large surface-to-volume ratio and Debye length comparable to their small size [19]. Furthermore, the reported ZnO single-NW device has very high internal photoconductivity gain due to the surface-enhanced electron-hole separation efficiency [20–22]. Therefore, ZnO NWs become a promising material for UV photodetector application. However, the single-NW UV photodetector is hard to be manipulated and measured, due to its complex fabrication and measurement procedures [23, 24]. Comparatively, vertically aligned NW arrays devices show more advantages in the light scattering and absorption, which is of much importance for optimizing photodetector performance. To meet the requirements of real-time application, vertically aligned ZnO NW arrays based UV photodetectors have been intensively investigated [25]. However, due to the presence of a carrier-depletion layer on NWs surface caused by surface trap states, the recovery time of photodetectors is very long based on the variation of ZnO NW photoconductivity [26]. Many efforts, such as heterostructure improvement [26–28], schottky type contact [29] and surface modification [30, 31] have been endeavored to improve the sensitivity, response and recovery time of this kinds of devices. Recently, noble metal coated semiconductor nano-materials, as a composition functional devices, have been intensively studied because they have shown Localized Surface Plasmon Resonance (LSPR) effect, leading to unique light absorption, efficient photo excited electron-holes separation, and charge carriers transfer properties at metal/semiconductor interface. NPs of noble metal Ag have especially attracted intense interest because of their efficient LSPR in blue-violet and visible region, which would strong couple with the large bandgap of ZnO applied in optoelectronic devices.

Here, we reported a new type of UV photodetector fabricated by loading Ag nanoparticles (NPs) onto ZnO NW arrays surface. This Ag@ZnO NW composite arrays showed obvious enhancements in light absorption, photocurrent, fast response and recovery time at a high frequency of 100 Hz on/off shift of UV light. The results confirmed that Ag@ZnO NW photodetector exhibited good reproducibility and high sensitivity to the UV light.

2. Experimental

Synthesis of the vertically aligned ZnO NW arrays was carried out by chemical vapor deposition (CVD) route using a simple horizontal tube furnace. Gallium nitride (GaN) thin films (p-type and with c-plane expose) on sapphire was chosen as the substrate to grow the vertically aligned ZnO NW arrays for the same crystal structure (wurtzite) and the small lattice mismatch in plane (1.8%) of ZnO and GaN. And a layer of gold particles, 3-5 nm in thickness on GaN surface, was deposited as catalyst by sputtering. Highly pure ZnO powder and graphite powder (99.9%) with a weight ratio of 1:1, was mixed thoroughly and used as the precursors. The Au coated p-GaN substrate (size in 5 mm × 5 mm) was placed about 10 cm away from precursors, with flows rates of 1 sccm Oxygen (purity 99.999%) and 49 sccm Argon (purity 99.999%) as the reaction and carrier gases, respectively. The source materials and substrate were heated to 1000 °C and 880 °C at a rate of 50 °C/min and held for 20 min, followed by cooling down naturally. The as-prepared samples were in grey color with uniform and vertically aligned ZnO NW arrays covered on the p-GaN substrate. Afterwards, Ag NPs with size of 10 nm, were coated onto ZnO NWs surface by a simple thermal evaporation route.

Morphology of as-obtained products was characterized by high-resolution field emission SEM (FEI Nova Nano-SEM 450) and high-resolution TEM (Tecnai G² 20 UTwin) with energy-dispersive X-ray spectroscopy system (EDS). Photo absorption properties of samples was carried out by a high sensitivity fiber optic spectrometer equipped with a 1024 × 122 pixel TE cooled CCD detector (AvaSpec-HS1024x122TEC-USB2) and an integrating reflection sphere. To characterize the ultraviolet photoresponse properties, two ITO slides were fixed on the fabricated Ag@ZnO NW arrays samples to build photodetector devices. I-V characteristics of the devices were measured using an Agilent source meter (B2901A). And photoconductive transients measurement was carried out at a fixed bias voltage of 5 V under switched illumination by a UV LED light source (365 nm, 100 mW) controlled by a function generator. Distance between photodetector and UV LED was fixed at 5cm and all of the measurement was carried out in ambient condition.

3. Results and discussion

Morphology of as-prepared ZnO NW arrays and the Ag NPs coated ones were examined by FE-SEM and high-reslution TEM. Figure 1(a) and 1(b) are the low and high magnification SEM images of ZnO NW arrays, which reveals that the as-prepared ZnO NW arrays have the high-yield, uniform, and vertically well aligned properties. The diameter of NWs is in the range of 60-100 nm and the length is about 2 µm. Figure 1(c)-1(d) shows the low and high-resolution morphology of Ag NPs coating on ZnO NW arrays, respectively. After Ag NPs coated on, the NW arrays remained the same uniform and well aligned morphology, but showed a slightly increased diameter, a uniform Ag coated stem with a ragged surface, and a huge Ag caps appeared at the tip of NWs which is helpful for the followed top ITO electrodes contact.

 

Fig. 1 Morphology characterization of as-prepared Ag@ZnO NW arrays. (a) Low-magnification and (b) high-magnification SEM images of ZnO nanowire arrays grown on GaN thin film before Ag NPs coating. (c) Low-magnification and (d) high-magnification SEM image of after 10 nm Ag NPs coated on ZnO NW arrays. (e) low-magnification TEM image of Ag@ZnO NW with the composition shown by EDS spectrum in the inset. (f) High-resolution TEM image illustrates clearly a 10 nm Ag nanoparticle attaching to the surface of ZnO NW.

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A representative TEM image of an individual Ag NPs/ZnO nanowire was shown in Fig. 1(e). Ag NPs with a size of 5~16.5 nm were coated uniformly along the stem of individual NWs, and the recorded EDS spectrum confirmed the well deposition of Ag element. Figure 1(f) shows the high resolution TEM (HRTEM) image of the interface between an Ag NP and ZnO NW, where the Ag NP with a face-centered cubic (fcc) crystal structure, directly attached to the hexagonal ZnO NW.

Photodetectors involves several successive or simultaneous mechanisms, namely absorption of the incident light, carrier photogeneration and carrier transport. Well understanding of optical properties of the assembled Ag NPs modified ZnO NW arrays is of large importance for optimizing NW-based photodetectors performance [32, 33]. Figure 2(a) shows the reflectivity of GaN thin films sample with 10 nm Ag deposited layer (sample A), random grown ZnO NW thin films(sample B), well vertically aligned ZnO NW arrays(sample C), and 10 nm Ag NPs deposited ZnO NW arrays (sample D). Inset of Fig. 2(a) is the digital photograph of these four sample named as A, B, C and D, respectively. In the wavelength ranged from 200 to 1000 nm, the Ag modified GaN thin films shows a high reflectivity, indicated that the Ag deposition on flat films will decrease the light absorption and reduce the photoresponse of devices. But semiconductor NW with small diameter can reduce the light reflectivity, i.e. light absorption enhancement, evidenced by the experimental data that random grown ZnO NWs thin films (sample B) has much less light reflectivity than that of sample A. Periodic vertically aligned NWs will further trap the incident light, as the reflectivity of ZnO NW arrays demonstrating a 15% lower than that of sample B in the region of UV to near infrared light. After Ag NPs coating, the color of ZnO NW arrays change from grey to black (sample D). The corresponding absorption spectra were shown in Fig. 2(b). For GaN thin film, after 10 nm Ag coated, the absorption showing the lowest level, which is resulted from the metal reflector of the deposited 10 nm Ag. The peak located at 364 nm was from the intrinsic absorption of GaN thin film. For both random oriented ZnO NW and vertically aligned ZnO NW arrays, the spectra have a wide absorption region from 350 nm to 700 nm, and include two peaks located at 383 nm and 573 nm, corresponding to the intrinsic absorption and oxygen defect bandgap level absorption, respectively. The well vertically aligned ZnO NW arrays show higher absorption than the random oriented ZnO NW, attributing to the light trapped effect in uniform nanowires forest. Furthermore, after Ag NPs loaded, the Ag@ZnO NW arrays show the highest absorption and an obvious broad peak centered at 490 nm. This broad peak is probably due to the surface plasmon resonance absorption band of Ag NPs with irregular sizes or aggregated Ag NPs, and the resonance of ZnO NW and Ag NPs. In additional, in the UV and IR region, the Ag@ZnO NW also shows absorption enhancement, which is attributed to light scattering by NW top-end Ag NPs and trapping absorbed by ZnO NWs, as shown inset of Fig. 3(a). This result clearly indicated that the Ag NPs modification can enhance the light absorption of NW arrays effectively as expected. The ~5% reflectivity of Ag@ZnO NW arrays sample confirms the light absorption enhancement of the metal NPs/semiconductor NW arrays heterostructures.

 

Fig. 2 Optical properties measurement. (a) Photo reflectivity spectra of Ag@ZnO NW arrays (sample D). For compare, GaN thin films/10 nm Ag (sample A), random oriented ZnO NW thin films (sample B) and pure ZnO NW arrays (sample C) were introduced. Inset shows the digital images of four samples, respectively. (b) Absorption spectra of sample A, B, C and D. Peak located at 364 nm was from the intrinsic absorption of GaN thin film. Peaks at 383 nm and 573 nm correspond to the intrinsic absorption and crystal-defect absorption of ZnO, respectively. The peak of Ag@ZnO at 490 nm was attributed to the surface plasmons absorption. (c) Surface-Enhanced Raman Scattering of Ag NPs loaded ZnO NWs arrays. (d) Room temperature PL spectra form ZnO NW arrays and after Ag NPs coated, excited by 325 nm UV laser.

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Fig. 3 (a) the schematic diagrams of the Ag NPs/ZnO NW arrays photodetector. (b) I-V characteristics of the photodetectors with and without UV LED (365 nm, 0.24 mW/cm²) shed from the top side. (c) Time-dependent photocurrent response under UV light at 5 V bias. (d) High speed photocurrent transient performance of devices under UV LED illumination with on/off shift at 100 Hz high frequency.

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In order to give a further impression on optical properties, the Raman spectrum and photoluminescence (PL) of the ZnO NW arrays before and after Ag NPs coated were shown in Fig. 2(b) and 2(c), respectively. Figure 2(c) shows ZnO NWs Raman scattering enhancement by Ag NPs coating. Two typical Raman active mode of E₂^low and E₂^high, and the weak mode of A₁(E₂^high -E₂^low), A₁(LO) and 2 A₁(LO) were observed. Each mode were increased in the intensities and shows the Surface-Enhanced Raman Scattering, which was the evidence of interaction between Ag NPs and ZnO NWs. Furthermore, the PL spectra were showing in Fig. 2(d). With excited under 325 nm UV laser, the pure ZnO NWs have a typical near-bandgap emission peak located at 382.9 nm and a crystal-defect related visible peak around 530 nm. After Ag NPs coating, the intensity of near bandgap emission peak shows an obvious increasing and the peak position shifted to 378.8 nm. The visible peak was suppressed (inset of Fig. 2(d)). This optimization of PL spectra was due to the Localized Surface Plasmon Resonance (LSPR) effect originated from the Ag NPs decoration, which will discuss in detail in the following mechanism description.

The LSPR UV photodetector was prepared by a simple and effective method of direct contacting ITO electrodes with Ag@ZnO NW arrays surface. Figure 3(a) shows the schematic diagrams of the assembled device, and the photocurrent measurements were conducted in air under illumination of a 365 nm LED light source at room temperature. Figure 3(b) shows I-V characteristics of the fabricated Ag NPs/ZnO NW arrays photodetector measured in dark and under UV illumination environment. The nearly linear and symmetric behavior of the recorded I-V plots demonstrated the Ohm contact between ZnO NW arrays and ITO electrode. Compare with the pure ZnO NW arrays sample, the Ag NPs coating layer would tremendously enlarge the photocurrent intensity of the prepared detector. At an applied bias of 5 V, the fabricated photodetector could be turned on/off reversibly, by switching the UV illumination with an interval of 20 s (Fig. 3(c)). For the samples with or without Ag NPs coating layer, the on-off ratio were 89.7 and 13.2, respectively. Response time (${\tau }_R$) and recovery time (${\tau }_D$) are two key characteristic parameters to estimate the performance of photodetector, where ${\tau }_R$ represents the need time to approach 63% (≈1-e⁻¹) value of the maximum photocurrent from dark current, and ${\tau }_D$ is defined as the need time for recovery to 37% (≈e⁻¹) of maximum photocurrent [34]. The calculated ${\tau }_R$ and ${\tau }_D$ are 320 ms and 3.02 s for the pure ZnO NW-based device, and 80 ms / 3.27 s for the Ag NPs decorated ZnO NW arrays device, respectively. We have also calculated the response and recover times in terms of 90-10% scale, for pure ZnO NW device, the response and recover time were 8.7 s and 13.25 s, as after Ag NPs loaded ZnO NW device, the response and recover time were 1.02 s and 15.5 s, respectively. The faster response time illustrated the Ag NPs modification effect on the performance of photodetector and could be attributed to the change of the surface trapped states mediated by Ag NPs. Generally, at the photo response mode, the Ag@ZnO hetreojunction could trap and separate electron-hole pairs excited by the incident UV light at the interface, and resulted a fast response time. But when the UV light tune off, the trapped electrons cannot immediately captured by oxygen molecules in the air, which resulted a slow recovery time. Furthermore, to estimate the ultrafast response to the UV illumination, photocurrent transient properties measured under UV LED on/off shift at high frequency of 100 Hz was shown in Fig. 3(d). The photocurrent reproducibility following the high-speed chopped UV light, means that the fabricated photodetector could have response with high speed cut-off signal, which is attributed to high migration rate of carries at interface between Ag NPs and ZnO NW.

The concrete process of the LSPR effect in our NWs can be expressed as follows: when the wavelength of irradiated UV laser is larger than the size of Ag NPs, the high-density electrons in Ag NPs formed an oscillating electron cloud. Along with the electrons accumulation at the interface between Ag NPs and ZnO NWs, band edge level of the composite bended downward at the ZnO side, and thus facilitating the electron transfer from Ag NPs to ZnO NWs. As a result, the Fermi level of the ZnO goes up, thus the near bandgap emission is blue-shifted and the visible peak is suppressed. In order to understand the mechanism of Ag NPs loading treatment enhancing ZnO NW arrays UV photocurrent, we performed the energy band diagram of Ag/ZnO heterojunction before and after UV light irradiation. It is known that the work function of ZnO (W_ZnO = 5.2 eV) is larger than that of Ag (W_Ag = 4.26 eV), implying that the electrons will migrate from the conduction band of ZnO to Ag NPs to achieve the Fermi level (E_F) equilibration, while the holes can remain on the surface which formed a low Schottky barrier when they get into contact, as shown in Fig. 4(a). In the Ag-ZnO heterojunction system, this deflexed energy band formed the space charge region, which could suppress the recombination rate of photogenerated electron-hole pairs. Under UV light irradiation, the oscillating electrons, also called hot electrons, on the Ag NPs can be photoexicited to a high energy level. The high energy hot electrons can subsequently transfer from the Ag NPs to the conduction band of ZnO NW arrays to achieve a new thermal equilibrium, as shown in Fig. 4(b). The transfer of Ag NPs surface electrons to ZnO causes an increase of electron intensity in the conduction band of ZnO and then a higher PL intensity comparing to pure ZnO NW arrays. Figure 4(c)-4(d) are the schematic of photocurrent response process of ZnO NW before and after Ag NPs coating, respectively. Owing to large surface to volume ratio of nanowire, the surface trapped oxygen play an important role in photocurrent response process [25]. In ambient conditions, oxygen molecules would be adsorbed on the NW surface by capturing free electrons to form ionized oxygen ($O_2(g)+e^{-}\to O_2^{-}$) and a surface depletion layer. After UV light irradiation, the photogenerated holes can migrate to surface, react with ionized oxygen and released free oxygen molecules. Due to the limited amount of ionized oxygen, the surface depletion layer is not sufficient to separate all of photogenerated electron-hole pairs. The photogenerated electron-hole pairs could recombine at the ZnO NW crystal defect location, which results lower photocurrent. However, after Ag NPs loading, the heterojunction barriers formed large space charge region, which can cooperate with the surface depletion layer to separate the electron-hole pairs more efficiently, and result the remarkable photocurrent increment, as shown in Fig. 4(d). In the other hand, the electrons transfer from Ag NPs to ZnO NWs by LSPR effect can increase the photocurrent directly. Furthermore, the electron transfer speed is very fast because the hot electron was oscillated with illuminate light at a very high frequency. Therefore, the ZnO nanowire arrays UV photodetector could be greatly enhanced by coating the Ag nanoparticles on the surface of the ZnO nanowires.

 

Fig. 4 Energy band structure and photocurrent response process schematic diagram of Ag@ZnO NW arrays UV photodetector. (a) Energy band structure of Ag@ZnO heterojunction. (b) LPSR excited electrons transfer from Ag NPs to ZnO NWs. (c-d) UV photocurrent response process in ZnO NW photodetector and Ag NPs@ZnO NW LSPR photodetector.

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4. Conclusions

In summary, Ag nanoparticles@ZnO NW arrays composite were well assembled on GaN thin film by a convenient strategy. Owning to the light trapping on the metal/semiconductor nanowire interface, photo absorption was enhanced observably from UV to near infrared region. The Ag@ZnO composite UV photodetector shows doubled photocurrent enhancement and fourfold accelerated photoresponse time than that of pure ZnO NW arrays device. This work demonstrated a simple route to obtain high performance UV photodetectors.