Nowdays, one-dimensional (1D) nanostructures have attracted increasing attention for its potential applications in nanodevices [1, 2]. Among numerous metal oxides, ZnO stands out owing to the wide band gap of 3.37 eV and the high exciton binding energy of 60 meV at room temperature. Because of the promising properties of ZnO, one-dimensional ZnO nanostructures such as nanowires (NWs), nanorods, nanotubes, nanobelts, etc. are extensively explored in various optoelectronic devices, e.g. light-emitting diodes (LEDs), solar cells, phototransistors, UV photodetectors, lasers etc [3–8]. Especially, the piezoelectric property of ZnO make it a promising material in piezoelectric sensors and actuators [9–11].

On one hand, considering the difficulties and instability of p-type ZnO, more focus has been laid on the fabrication of n-ZnO based heterojunctions using various p-type substrates, such as p-GaN [12], p-AlGaN [13], p-SiC [14], and p-Si [15]. On the other hand, NiO is a p-type [16] semiconductor with a bandgap of 3.7 eV and widely used in sensors, fuel cells, and antiferromagnetic devices due to their high stability and low material cost compared with p-GaN. Further more, it has been shown that NiO thin films can possess low resistance, high p-type carrier concentration, high hole mobility, low lattice mismatch (ZnO Space Group: P63mc, lattice constant: a = 0.32498 nm, c = 0.52066 nm; NiO Space Group: Fm3m, lattice constant: a = 0.41771 nm) with ZnO [17, 18] under a special orientation, which are beneficial for the formation of p-n junction with ZnO.

Recently, many works have been devoted to fibrous dye-sensitized solar cells [19–22] due to (1) less limitation of incident light angle, (2) wider range of the substrate materials, i.e. metal wires, optical fiber, carbon fiber etc., (3) low cost and (4) weavability [23]. The fibrous structure with excellent features can also apply to photodetectors on the basis that the incident angles of light are arbitrary in most cases, while conventional photodetector based on rigid substrate can only absorb light in limited directions which greatly affect the performance of photodetection. Consequently, under some circumstances where light directions are arbitrary, fibrous photodetector have more advantages over conventional ones. In this research, we fabricate a wire-shaped UV photodetector based on one-dimensional ZnO nanostructures on NiO/Ni wires using CVD method, which is both convenient and low-cost compared with using other rigid substrate. The optical and electrical properties under different rotation angles of the heterojunctions show good rectifying diode behaviors and stability which make 3D detection possible. The sensitivity of the device is observed to be enhanced by strains due to piezotronic effect of ZnO nanowires. These performances of the device demonstrates a promising approach to 3D light detecting and strain sensing and also provide a prospective application to the development of weaving single wire into fabrics technology.

The Ni wires with a diameter of 0.5 mm (99.99% purity, Laibaoli Coating Technology Co., Ltd, Beijing) were polished using an abrasive paper and cut into short lengths of 8.5 cm. Consequently, the cut wires were washed with distilled water and then ultrasonicated in acetone, ethanol, and deionized water for 10 min, respectively. After dried in air, the Ni wires were placed in a muffle furnace and were heated to 550 °C in 1 hour. The furnace was kept at 550 °C for 2 hours and then naturally cooled down to room temperature. The color of the Ni wire was changed from silver to grey indicating that the surface of the metal wires was oxidized. Energy-dispersive X-ray spectroscopy system (EDS) analysis confirmed that the Ni wire was oxidized. Moreover, the resistance of the Ni wire was ~0.2Ω. While after annealed, it became ~3kΩ. Consequently, a gold catalyst layer with nominal thickness of 3–5 nm was deposited on the oxidized Ni wires. Synthesis of the ZnO NWs was carried out by chemical vapor deposition method using a simple horizontal quartz tube furnace. The oxidized metal wires were placed at the open end of the quartz tube, while the feedstock source materials consisting of a mixture (1:1 by weight) of ZnO and graphite powders were loaded at the closed end of the same quartz tube. Flows of 0.5 sccm oxygen (purity 99.999%) and 100 sccm argon (purity 99.999%) were used as the precursor and carrier gases. Then the mixture and metal wire was heated to obtain ZnO NWs. Actually, we grew ZnO NWs under temperature range from 680 °C to 930 °C, and pressure range from 10 Pa to 1000 Pa. Finally we chose the suitable condition to get the best morphology and performance. The whole process was held for 5 min. Afterwards, the furnace was turned off automatically and cooled to room temperature while maintaining the pressure unchanged. After getting the as-deposited sample out of the furnace, a length of platinum wire with a diameter of 0.1 mm was twisted around it carefully to serve as electrode. Then a blade was used to scrape the ZnO NWs layer from one end of the oxidized metal wire in order to obtain another electrode.

Subsequently, the morphology and structure features of the device were characterized by means of a scanning electron microscopy (FESEM, FEI NOVA NanoSEM 450) and a Transmission electron microscope (Tecnai G2 20 U-TWIN). A photoluminescence (PL) measurement was implemented on a LabRam HR800 UV spectrometer using a 30 mW He–Cd laser (λ = 325 nm). UV photoresponse measurement was conducted between two electrodes at a fixed bias voltage of 1 V by switching the light from a high pressure mercury lamp (λ = 365 nm, 7.5 mW/cm²) ‘on’ and ‘off’. Autolab electrochemic workstation (modelAUT84315, The Netherlands) was used to measure the I-T (current-time) and I-V (current-voltage) characteristics of the sample. Piezotronic investigations were conducted by using a homemade probe table. The distance between the sample and the lamp was fixed. All of the measurements were carried out at room temperature in ambient condition.

Figure 1(a) shows the optical images of the as-prepared product, where ZnO nanowires were grown on the oxidized Ni wire with 60 mm length. The dark part of the product are ZnO NWs grown cylindric on the metal wire. Due to its unique morphology, the device can absorb light in all directions and be bent reversibly under small strains. Figure 1(b) shows the schematic diagram of the device, Pt wire was twisted around the as-deposited product to act as the electrode and ZnO NWs were scraped off from one end of product to obtain another electrode. In order to make sure that the pn junction characteristics of device does not originate from the contact between Pt wire and ZnO NWs, the contact property between Pt wire and ZnO must be identified. The I-V curve is shown in Fig. 1(c) indicating that the contact is almost ohmic. The inset depicts the schematic diagram of the device used to measure the I-V properties of the contact between Pt and ZnO, two isolated Pt wires were twisted around the as-prepared product and then switched in the electrochemic workstation. Due to the high work function of Ni, the contact between Ni and NiO is generally ohmic contact [24, 25]. Consequently, the influence of Pt/ZnO and Ni/NiO contact on the electric properties of the device can be neglected. In order to get the absorption data of the as-grown NWs, ZnO NWs were scraped off from the wire and then dispersed in alcohol. The result as shown in Fig. 1(d) reveals that the NWs have a strong absorption around 375nm. The FESEM images in Fig. 2 exhibit the general morphologies of the as-prepared product in different magnifications. Figure 2(a) depicts that the diameter of the wire-shaped device is about 0.5 mm. Figure 2(b) shows the ZnO NWs grown on the surface of the oxidized Ni wire. The ZnO NWs are randomly distributed with a length of about 5 μm and a diameter of 30 nm as shown insert. Figure 2(b) and (c) show the TEM image and selected area electron diffraction (SAED) of the as-grown ZnO NWs, which illustrates good crystallization of the NWs. Room-temperature PL properties of the ZnO NWs grown on oxidized Ni wires were measured and the results are shown in Fig. 2(d). It has been observed that ZnO generally exhibits two emission peaks. One peak centered at 380 nm is the near-band-edge emission (NBE) transition or UV emission, the other yellow–green band peak located at 520 nm is the deep-level emission (DLE). The strong UV emission at about 380 nm and the negligible deep-level emission at about 520 nm also indicate that the as-prepared ZnO NWs are well crystallized and low concentration of defects [26], which is suitable for fabrication of UV laser devices and UV detectors.

 

Fig. 1 (a) Optical images of the as-prepared ZnO nanowires grown on oxidized Ni wire. (b) Schematic diagram of the device. (c) I-V characteristics of the Pt/ZnO/Pt structure used to measure the contact properties, inset shows the schematic diagram of the Pt/ZnO/Pt structure.(d) UV-VIS absorption pattern of the NWs.

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Fig. 2 (a) Top-view FESEM micrograph of the wire-shaped device. (b) FESEM micrograph of the ZnO NWs grown on oxidized Ni wire in different magnifications. (c) TEM image of the as-grown ZnO NWs, insert is the SAED pattern. (d) Room-temperature photoluminescence spectra of the ZnO NWs.

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The schematic diagram of the ZnO NWs photodetector is shown in Fig. 1(b), the ZnO NWs directly contact with the Pt wire without depositing and coating any material on the NWs tips to form the electrode. In addition, the ZnO layer is not too thick so that we can scrape the ZnO NWs from the as-prepared product to obtain the electrode. Besides, metal wires have better electrical and thermal conductivity compared with metal oxides. The UV response of the devices was conducted on a 3D homemade probe table as shown in Fig. 3(a) by a high pressure mercury lamp in ambient condition. Taking the unique fibrous structure of the photodetector into consideration, the device should be capable of responding to UV light in all directions. Thus, rotating the device would not change the I-V and I-T characteristics so much. The current–voltage characteristics of the photodetector under dark and UV illumination (365 nm, 7.5 mW/cm²) with different rotation angles are shown in Fig. 3(b), while the inset is the enlarged I-V curve without UV illumination. The highest on-off ratio reached ~40. The I–V curves were measured with bias from −5 to 5 V at room temperature in ambient condition, clear rectifying behavior can be observed with and without UV illumination which manifested the pn junction characteristics of the device. The photocurrent and dark background current at 5V was 17 μA and 0.92 μA, respectively indicating that the photocurrent was enhanced evidently. Changing the rotation angles of the sample have little effect on the I-V characteristics of the device implying that ZnO NWs are radically homogenously grown around the oxidized Ni wire.

 

Fig. 3 (a) 3D home made probe table used to measure the piezotronic properties of the device. (b) I-V characteristics of the device under dark and UV illumination conditions with different rotation angles, the inset shows the magnified I-V curves in dark conditions. (c) I-T characteristics of the device under different angles with a 1 V forward bias. (d) Enlarged I-T curve of the device in one cycle.

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The photodetector can reversibly be turned ‘on’ and ‘off’ with a forward bias of 1 V by removing and mounting the metal lid block the UV illumination from the lamp. The time interval and on–off ratio are 150 s and 4, respectively as shown in Fig. 3(c). In Fig. 3(d), the photocurrent grew very fast initially and then slowly increased and saturated at last. The response and recovery time for the curve in Fig. 3(d) are calculated as 7.5 s and 8.6 s, respectively. PL data show negligible defect emission, and the photoresponse is relatively slow. This is probably on account of that the photoresponse speed was a statistical result of number of NWs. Although, there are some slight changes in I-T curves in different rotation angles, especially in each peak of the curves, the I-T characteristics remained stable with different rotation angles. The slight changes in I-T characteristics could be explained by the fact that the ZnO NWs are homogenously grown and distributed in general, but in some small areas there may be variations due to its long size. Water molecules and ionized oxygen adsorption and desorption at the surface defect sites [27, 28] are known to interfere the UV response and recovery time of ZnO NWs. As the ZnO NWs were grown using CVD method, the influence of water molecules on the photoresponse was eliminated [29]. Consequently, oxygen is believed to play a critical role in the observed photoresponse.

Piezotronic properties of the device were measured using a 3D homemade probe table which can adjust movement precisely as a micrometer in Fig. 3(a). Here, we just showed 4 states of the device under strains. The variation of current due to different strains applied can be attributed to the piezoelectric nature of the ZnO nanowire [30]. Figure 4 shows the photosensing measurements of the flexible device under strains with a forward bias of 1 V. The I-V curve in Fig. 4(b) shows that the current increase with the increase of compressive strains. P0 represents the original state of the device, p1, p2, p3, and p4 represents one end of the wire-shaped device moving 0.5mm, 1.0mm, 1.5mm and 2.0mm while keeping the other unchanged, the schematic measurement diagram is displayed in Fig. 4(a). The I-T curve in Fig. 4(c) also shows the same trend that the photocurrent increases with the increase of strains. In our series experiments, the photocurrents were enhanced by about 2 times under strains. This is comparable with others work [31]. Figure 4(c) shows the current-time relationship of the device under different strains, which indicates that the response time is not significantly changed by the bending of the device. These strain-dependent photoresponse characteristics are based on a series of factors including band structure change and piezophototronic effects coupled with piezoelectric effects, photon excitation, and semiconductor properties [32]. Figure 4(d) could help explain the phenomenon that the photocurrent increases with the increase of tensile strains, which shows the schematic energy diagram of both the unstrained and strained device. The piezopotential is related to the pointing direction of + c-axis [33], when a compression along the NiO/ZnO heterjunction device leads to a tensile strain along the ZnO NWs, the c-axis [0001] of the ZnO NW positioned toward the outside which results in a positive and negative piezopotential generated at the outer and inner ends of ZnO NWs, respectively. The effect of the local positive piezopotential at the NiO/ZnO interface will lower the conduction and valence bands of ZnO and result in a decrease of the barrier height at the heterojunction interface which is shown in the red curve in Fig. 4(d). Consequently, the transport of photogenerated electrons from excited ZnO will be facilitated, thus enhancing the photoresponse of the device. Here we should mention that the length of the wire device is about 8.5cm and more strain could be produced with a shorter length or a bigger displacement.

 

Fig. 4 (a) Schematic measurement diagram of piezotronic properties. (b) I-V characteristics of the device under different strains. (c) I-T characteristics of the device under different strains with a forward bias of 1 V. (d) Energy band diagram of a the NiO/ZnO NWs heterjunction based device with and without strain.

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In conclusion, a novel wire-shaped n-ZnO nanowires (NWs) /p-NiO heterojunction UV photodetector has been fabricated with ZnO NWs grown by chemical vapor deposition method. The device, exhibits good crystallinity and low concentration of defects. The current–voltage (I-V) and current-time (I-T) characteristics under different rotation angles of the heterojunctions show good rectifying diode behaviors and stability under different angles. The response and recovery time are 7.5s and 8.6s respectively. Furthermore, the photocurrent is enhanced by the application of a tensile strain on the device showing good piezoelectric properties and strain-dependent photoresponse characteristics. This phenomenon could be explained by the modification of energy band diagram at the p−n heterojunction by strain-induced piezoelectric polarization. These performances of the device demonstrates a promising approach to 3D light detection and strain sensing and also provide a prospective application to the development of weaving single wire into fabrics technology.