Boosting Fiber-Shaped Photodetectors via “Soft” Interfaces - ACS

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Boosting Fiber-Shaped Photodetectors via “Soft” Interfaces Zhengfeng Zhu,†,⊥ Dan Ju,†,⊥ Yousheng Zou,*,† Yuhui Dong,† Linbao Luo,‡ Tengfei Zhang,‡ Dan Shan,§ and Haibo Zeng*,† †

Key Laboratory of Advanced Display Materials and Devices, Ministry of Industry and Information Technology, Institute of Optoelectronics & Nanomaterials, College of Material Science and Engineering, Nanjing University of Science and Technology, Nanjing, 210094 China ‡ School of Electronic Science and Applied Physics and Anhui Provincial Key Laboratory of Advanced Functional Materials and Devices, Hefei University of Technology Hefei, Anhui 230009, China § School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing 210094, China S Supporting Information *

ABSTRACT: Lightweight, flexible fiber-shaped devices that can be woven into wearable electronic products have received great attention in recent years. However, the bending and poor interfaces of fiber-shaped devices typically lead to ineluctable performance degradation, which is still a great challenge yet to be dealt with. Here, taking a fiber-shaped photodetector as an example, we proposed an effective strategy, constructing inorganic−organic−graphene hybrid interfaces on a single fiber, to greatly improve the performances of fiber-shaped device. In the proposed structure, the ZnO nanorod array is grown vertically on the surface of a Zn wire (center core) and then wrapped by PVK and graphene (outmost layer) as the two outer layers. These “soft” interfaces successfully built compact contacts between various functional layers even on curved interfaces, which markedly reduced the contact resistance. Meanwhile, the whole structure also exhibited excellent durability toward the bending operations. Evidently, the Ilight/Idark ratio and photoresponsivity under bias of 0.5 V are as high as 7.2 and 0.9 A/W. In particular, the photoresponse speed has been greatly improved with the rise time of 280 ms, which was 1 order of magnitude faster than that of other fiber-shaped photodetectors without the above “soft” interfaces. KEYWORDS: fiber-shaped device, wearable device, photodetector, interface, response speed

1. INTRODUCTION With the rapid development in modern electronics, wearable electronic devices including smart skins, interfacing computers, and stretchable circuitries are playing more significant roles in our lives.1−3 With the help of a traditional weaving technique, one can weave the lightweight, flexible fiber-shaped devices into desired wearable electronic devices. As a result, various types of fiber-shaped energy storage units emerged in recent years including solar cells, supercapacitors, and lithium-ion batteries.4−6 These energy storage components would ultimately be used to power different types of devices like the display, illuminator, monitor, and sensors, and so forth.7−10 Making those devices wearable and achieving a performance level competitive with their planar counterparts remains extremely challenging. Taking sensors as an example, planar photodetectors (PDs) have been widely applied for health, safety, or some other applications, and so forth.11−15 However, only few related works have been reported about fiber-shaped photodetectors. To make things worse, as compared to the planar devices, their device performance starts to deteriorate,16−18 resulting in low responsivity, low Ilight/Idark ratio, and slow © XXXX American Chemical Society

response speed, which are mostly due to the poor contacts in the fiber-shaped structures. Interfaces are important in optoelectronic devices and directly influence the device performance, which have been extensively studied in light-emitting diodes (LEDs), PDs, and solar cells with planar structures for many years.7,8,10,19,20 Improving the film quality, optimizing the energy level alignment of layers and constructing p−n or Schottky heterojunction21−26 and so forth have all been proven to be effective interfacial engineering methods to improve the performance of planar devices. Meanwhile, the interface problems are more serious in fiber-shaped devices due to their curved and rough surface with more defects and bad contacts. Therefore, to smoothen the rough surface and achieve better contact between all layers becomes more important for fiber-shaped devices to have high performance. Received: January 17, 2017 Accepted: March 20, 2017 Published: March 20, 2017 A

DOI: 10.1021/acsami.7b00811 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Schematic diagram of the device fabrication and corresponding energy levels. (a) Schematic diagram illustrating the fabrication procedure of ZnO NRs array/PVK/graphene hybrid fiber-shaped photodetector. (b) Schematic cross-sectional morphology of the device that exhibits the p−n heterojunction and all compact soft interface in the photodetector. (c) Schematic representation of the corresponding energy levels of the materials involved in the device and the initiative transportation of electrons and holes due to the interfacial energy level alignment.

Figure 2. Morphology characterization of synthetic ZnO NRs and graphene. (a) Full-view SEM image of synthetic ZnO NRs on Zn wire. (b) Sideview SEM image of the ZnO NR array. (c) Low-resolution TEM image of single ZnO NR. (d) Corresponding High-resolution TEM image of ZnO NR. (e) Optical microscope image of large-area graphene film transferred onto silicon substrate. (f) Corresponding Raman spectrum of the graphene on silicon substrate. Inset of (a) is the amplifying SEM image of ZnO NR array. Inset of (d) is the selected area electron diffraction (SAED) of the single ZnO NR.

Hence, we construct an inorganic−organic−graphene hybrid fiber-shaped PD with “soft” interfaces of all layers. Here, the unconstrained organic semiconductor fully covers the inorganic functional layer and form a “soft” contact to smooth its rough surface. Subsequently, ultrasoft graphene would wrap the surface tightly and form another “soft” interface. Then, on the basis of a Zn wire, a fiber-shaped PD composed of ZnO NRs array, organic PVK, and ultraflexible graphene was fabricated, with a built-in electric field attributed to the p−n heterojunction of PVK/ZnO. In this PD, the ZnO NRs array was uniformly grown on Zn wire with good crystallinity, and the PVK layer was in tight contact with ZnO NRs array. The seamless contacts and precise interfacial energy level alignment were in favor of the separation and transportation of

photoinduced carriers. Meanwhile, by using soft graphene as surface electrode with conformal contacts instead of hard metal wire with line contacts, the device performance is expected to be further improved. Indeed, the responsivity (Rλ) of this PD could reach 0.9 A/W at 340 nm, and the Ilight/Idark ratio was about 7.2 at −0.5 V bias. More excitingly, the rise and decay time were only 280 ms and 2.2 s, respectively, indicating an order of magnitude improvement of response speed when compared to that of the previous reports. Hence, by effective interface optimization, the properties of fiber-shaped PDs could be improved remarkably, and the similar strategy of utilizing soft interfaces could be applied in other fiber-shaped devices to get better performance. B

DOI: 10.1021/acsami.7b00811 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. Structure characterization of the PD composed of ZnO/PVK/graphene. (a) SEM image of the fabricated fiber-shaped photodetector. (b) Locally amplified top-view SEM image of the junction. (c) Amplified SEM image of PVK film on ZnO NR array. (d) Cross-sectional SEM image of ZnO and PVK layer. (e) Side-view SEM image of the fabricated device. (f) Amplifying side-view SEM image of the contact surface.

2. RESULTS AND DISCUSSION Due to their excellent optoelectronic properties, ZnO NRs have shown intriguing applications in various electronic devices. Similarly, graphene has shown excellent electrical and mechanical properties including high conductivity, high transmittance, and super flexibility,27−29 and is thereby a suitable transparent electrode material. To exploit the superiority of both materials, we proposed an inorganic−organic−graphene hybrid fiber-shaped PD with soft interfaces between all layers. On the basis of a Zn wire, a fiber-shaped PD composed of ZnO NRs array, organic PVK, and ultraflexible graphene was fabricated, with a built-in electric field attributed to the p−n heterojunction of PVK/ZnO. The seamless contact of all layers and precise interfacial energy level alignment were both expected to be in favor of the separation and transportation of photoinduced carriers. Figure 1(a) is a schematic diagram illustrating the synthesis procedure. In brief, after the ZnO thin film was first deposited on the Zn wire (diameter: 400 μm) as a seed layer via atomic layer deposition (ALD), the ZnO NRs array was subsequently synthesized by hydrothermal method. Then, PVK was dippedcoated, and the poly(methyl methacrylate) (PMMA) supported graphene film was finally wrapped around the modified Zn wire to finish the fabrication of the PD. Figure 1(b) schematically illustrates that soft interfaces among ZnO, PVK, and graphene ensure close contact between layers and a coaxial p−n heterojunction of PVK/ZnO will also be built. Meanwhile, the energy band diagram shown in Figure 1(c) presents the corresponding energy levels of the materials involved in the device and is crucial for efficient carrier separation and transport. In this band alignment, built-in electric field (E) in the p−n heterojunction of PVK/ZnO is beneficial to the rapid separation and transportation of photoinduced carriers. When electron−hole pairs were generated in ZnO NRs with irradiation of light source, the holes migrated from the valence band to the highest occupied molecular orbital (HOMO) of PVK30,31 and subsequently transported to graphene smoothly. Furthermore, PVK acted as an electron blocking layer in the fabricated PD to impede the electrons transferring from ZnO to PVK, which greatly reduced the recombination of electrons and holes. Thus, with the high

separation and transportation efficiency of photoinduced electrons and holes, excellent photoresponse performance of the PD would be achieved. Figure 2(a) displayed the full morphology of ZnO NR array grown on Zn wire in our work, in which the ZnO NR array covered the whole Zn wire completely without any flaws. Moreover, as observed from the top-view SEM image in inset of Figure 2(a) and side-view SEM image presented in Figure 2(b), the high-quality ZnO NRs array orientated uniformly with the homogeneous length of ∼3.5 μm and diameter of ∼150 nm. These were attributed to the highly crystalline ZnO thin film (in Figure S1 of the Supporting Information) that was beforehand deposited on Zn wire by atomic layer deposition (ALD). As observed from the cross-sectional SEM image of the ZnO NRs (Figure S2), the ∼150 nm thick film (the seed layer) also helped smoothen the surface of Zn wire. The lowresolution transmission electron microscope (TEM) image of single ZnO NR is given in Figure 2(c), and the growth orientation along the crystal orientation [0001] was demonstrated by the corresponding select area electron diffraction (SAED) pattern. The interplanar spacing of 2.6 Å observed from high-resolution TEM image (Figure 2(d)) is found to be consistent with (002) lattice plane. Moreover, its excellent crystallinity can also be verified by the X-ray Diffraction (XRD) pattern of ZnO NRs displayed in the Figure S2(b), with all peaks corresponding to wurtzite phase ZnO (JCPDS No. 361451). The high quality ZnO NRs achieved here are expected to have excellent optoelectronic properties and will build the foundation for high performance PD. Alternatively, to achieve a flexible and transparent electrode, a large-area and continuous graphene film was needed. The optical microscope image in Figure 2(e), shows that a large-area and continuous graphene film was obtained and could be transferred perfectly. Moreover, the Raman spectrum in Figure 2(f) exhibits two major scattering peaks: 2D-band peak at ∼2735 cm−1 and G-band peak at ∼1580 cm−1. The intensity ratio I2D/IG of ∼2.66 and the weak D-band scattering peak at ∼1360 cm −1, confirms the high quality of the monolayer graphene. With all the important materials ready, we are in position to fabricate the PD. The interface problems of the electron devices C

DOI: 10.1021/acsami.7b00811 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. Schematic representation of the graphene film self-assembled onto Zn wire based ZnO NRs array via the surface tension of water. (a) The graphene film was taken out from the water. (b) Force condition of the graphene film. (c) The graphene film covered the modified Zn wire closely.

Figure 5. Device performance of the PD with structure of ZnO/PVK/graphene. (a) Responsivity of the PD at the reverse bias of −0.5 V and the absorption spectrum of ZnO NRs, respectively. (b) Current−voltage (I−V) curves of fabricated PD in the dark condition and under the different irradiation at the wavelength of 325 nm. (c) The energy band diagram of the PVK/ZnO coaxial p−n heterojunction (d) Corresponding photoresponse characteristic of the PD with and without bending at the reverse bias of −0.5 V under the same illumination.

by the amplified side-view SEM image in Figure 3(f). Then, the protrusive part of graphene film could be easily connected with Ag paste or slender Ag wire, and there was no influence on the device performance due to the ohmic contact between Ag and graphene, which was verified by the linear current−voltage (I− V) curve in Figure S3(a). Hereto, we confirmed that the “soft” and smooth interfaces among the ZnO NR array, PVK, and graphene led to the seamless contacts of all layers in the fabricated PD, which was vital to the device performance. More importantly, different from the traditional line electrode twined around the PD manually, graphene film can be self-assembled onto the modified Zn wire assisted by the surface tension of water without causing any factitious damage to the contact interface. The schematic diagram was illustrated in Figure 4. First, the modified Zn wire was cautiously taken out of water with the PMMA supported graphene film stuck on its surface (Figure 4(a)). Then, there was some remaining water sandwiched between the modified Zn wire and graphene (Figure 4(b)). Due to the water infiltration, a concave meniscus would be

mainly arise from the defects due to the bad contacts of inner layers and the outer electrode. Given the curved structure of the fiber-shaped PDs, intrinsic roughness of all-inorganic layers would lead to more serious contact defects. Thus, we proposed inorganic−organic−graphene hybrid structure with soft interfaces should solve the contact problems perfectly. The fabricated fiber-shaped device is shown in Figure 3. As illustrated in Figure 3(a), the PMMA supported graphene film was wrapped around the modified Zn wire tightly. And the locally amplified SEM image of the junction of PVK and graphene presented in Figure 3(b) further indicated that graphene film covered the surface closely and smoothly. This was attributed to PVK layer (∼280 nm) that covered ZnO NRs array as a thin reticular film uniformly (shown in Figure 3(c)) to smooth the surface of ZnO NR array as shown in Figure 3(d). Additionally, the soft graphene film played a significant role in this device. As demonstrated by Figure 3(e), the ultraflexible graphene film wrapped the modified Zn wire completely and formed close conformal contacts with the curve surface. The compact contacts could be further demonstrated D

DOI: 10.1021/acsami.7b00811 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. Photoresponse characteristics of the PD with structure of ZnO/PVK/graphene. (a) Schematic representation of the transport property of the photoinduced carriers on the graphene. (b) Corresponding current−time (I−t) curves that reflect the response time. Inset of (a) is photographic image of the fabricated fiber-shaped photodetector.

Table 1. Summary of Device Performance and Comparison with Some Fiber-Shaped and Planar Photodetectors in Recent Yearsa

a

device structure

type

bias (V)

Ilight/Idark

response time (s)

ref.

ZnO NR array/PVK/graphene ZnO NR array/PVK/PEDOT:PSS/Ag wire NiO/ZnO NR array/Au wire Pt/ZnO NR array/Pt wire ZnO NR array/CdS/ITO/Ag Co3O4/graphene Ag/NiO/ZnO NR array/Ag Au/ZnO NR array/Au Al/ZnO film/NiO/Al ITO/NiO/ZnO film/Al

fiber-shaped fiber-shaped fiber-shaped fiber-shaped fiber-shaped fiber-shaped planar planar planar planar

−0.5 −0.5 −3.5 1 2 1.5 −1 5 1 −5

7.2 1.5 4.9 ∼4 ∼1.3 ∼2.5 2.5 9 11.56 ∼102

0.28/2.2 6/7 >10/18.1 7.5/8.6 ∼10/ ∼18/∼17 /30

this work this work 16 17 18 35 36 37 38 39

Here, the response time was uniformly estimated at 90% of maximum of Ilight.

condition and under the irradiation with different energy density excited by a 325 nm laser, respectively. In addition to the good photoresponse, the obvious rectifying behavior of the PD could be observed. We attributed this rectifying characteristic to coaxial p−n heterojunction of PVK/ZnO, and the corresponding band diagram was given in Figure 5(c). When ptype PVK is in contact with n-type ZnO, the energy band of them bent spontaneously near their interface to generate a built-in electric field E pointing from ZnO to PVK. Referred to the reported work function data of ZnO NRs (∼4.45 eV)32 and PVK (∼5.5 eV),33 the theoretical energy barrier was ∼1.1 eV. With the reverse bias increasing across the two electrodes, the depletion region became wider and induced a high potential barrier at the p−n heterojunction, which would suppress the transportation of carriers along the high impedance path and lead to a low dark current at a reverse bias. Therefore, larger Ilight/Idark ratio would emerge at reverse bias when compared to positive bias, and then a low reverse bias of −0.5 V was used to characterize the photoresponse properties of the fiber-shaped PD. As shown in Figure 5(d), the current−time (I−t) curves were measured by a 325 nm laser with irradiation energy of 0.5 mW/cm2, and the dark and illumination state was controlled by a light shutter. When the laser was turned on, the current increased from 0.19 to 1.37 μA with the Ilight/Idark ratio of 7.2, where the Ilight and Idark were light and dark current, respectively. Meanwhile, there was no obvious change when the PD was bent with the deformation of 5%, well indicating flexibility of this PD. ZnO was a typical piezoelectric material and exhibited piezoelectric effects external deformation and stress is applied. However, in this PD, the external graphene film was ultraflexible and ultralight, and there was not any other

formed at the interface of water and graphene. Following the schematics in Figure 4(b), we defined the surface tension of water, friction force of flowed water, reactive force to graphene as F, f, Ff, respectively. F was tangential to the concave meniscus and formed an angle θ with graphene (Figure 4(b)). Then, water outflowed continuously along the surface due to the component of F parallel to graphene which is marked as F//G (F//G = F × cos θ). Accordingly, water would experience an equal friction force (f). Therefore, graphene film would be under a reactive force (Ff) pointing to the end of Zn wire. Thus, under the tension of Ff at the two ends, the initial wrinkled graphene film became smooth as the water outflowed gradually. Finally, the graphene film naturally clung to the surface of the modified Zn wire smoothly and seamlessly after all the whole water flowed away (Figure 4(c)). In this PD, a slender silver wire was also covered by the graphene film directly and then it could act as an electrode convenient for measurements, and the whole photograph of the PD was presented in the inset of Figure 5(a). As for device performance, we first studied the photoresponse properties related to the excitation wavelength of light source. Responsivity (Rλ) is a critical parameter of the PD, representing a ratio of photocurrent to light intensity at a specific wavelength. In this fiber-shaped PD, the spectral responsivity was measured at a reverse bias of −0.5 V, as shown in Figure 5(a), an obvious response arose in the ultraviolet (UV) range of 300−370 nm, and the maximum peak of 0.9 A/W was obtained at 340 nm. This response wavelength was consistent with the absorbance of ZnO NRs (Figure 5(a)). Figure 5(b) shows the current−voltage (I−V) curves of the fabricated PD measured from −2.5 to 2.5 V both in the dark E

DOI: 10.1021/acsami.7b00811 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 7. Photoresponse characteristics of the PD with structure of ZnO/PVK/PEDOT:PSS/Ag wire. (a) SEM image of the fiber-shaped device with twined Ag wire as electrode. (b) Amplifying SEM image of the contact point between Ag wire and the surface. (c) Schematic representation of the transport property of the photoinduced carriers along Ag wire. (d) Corresponding current−time (I−t) curves that reflect the response time. Inset of (a) is cross-sectional SEM image of the PD.

Furthermore, to demonstrate the enormous advantages of our ZnO/PVK/graphene hybrid PD, we also fabricated another fiber-shaped PD for comparison. It is composed of ZnO/PVK/ PEDOT:PSS/Ag wire similar to the previous reports as shown in Figure 7(a). In the PD, the layers could be clearly observed form the cross-sectional SEM image (inset of Figure 7(a)). Here, PEDOT:PSS was used as hole transport layer to improve the transport of the photoinduced holes, and Ag wire was twined around the as-prepared PD acting as an outer electrode, From the I−V curve presented in Figure S3(b), the ohmic contact of Ag and PEDOT:PSS was confirmed. As schematically shown in Figure 7(c), the twined Ag wire only provided a narrow path due to its line contacts with the PD when compared to the conformal contacts in our graphene hybrid PD, leading to a much slower transport of the photoinduced carriers. In addition, the Ag wire could not get full contact with the surface due to the hardness of metals and led to more gaps on the surface at microscale (Figure 7(b)), which further impeded the transport of carriers. As the I−V curves exhibited in Figure S6(a), this PD could work under the UV irradiation with effective photoresponse, but the performance was so poor when compared with the hybrid graphene PD, and the low responsivity of 0.12 A/W was observed (Figure S6(b)). When measured under the same conditions, the rise and decay time of the PD were as long as 6 and 7 s, respectively (Figure 7(d)). Moreover, without timely separation and transportation, electron−hole pairs would recombine due to the short lifetime of photoinduced carriers, thus the photocurrent decreased obviously with a low Ilight/Idark ratio of 1.5. Hence, we demonstrated that the contact of the surface electrode was vital to the device performance and interface optimization of all layers with “soft” interfaces was an effective

packaging involved as shown in Figure S4. Therefore, when the PD was bent, the graphene film was naturally bent without causing obvious stress on the ZnO NRs array and the piezoelectric effect could be ignored when compared to the photoelectric effect in this PD. Moreover, the stability test (Figure S5), shows no obvious change of the device performance when the PD was exposed in air for a week, and the photocurrent could also keep 84% of the maximum value after the PD was bent for 2000 times. Excitingly, as the photoresponse characteristics of the PD shown in Figure 6(b), the rise and decay time (estimated at 90% of maximum of Ilight) were just 280 ms and 2.2 s, respectively. They were the shortest response time when compared to other reported fiber-shaped PDs, and the response speed was improved by 1 order of magnitude.16−18 Here, we illustrated some corresponding device performances of these fiber-shaped PDs in Table 1. These excellent properties were mainly attributed to the compact contact of ZnO, organic PVK, and flexible graphene. Certainly, the conformal contacts of the graphene electrode also played significant roles in the device. When the device was under the illumination of light source, the photoinduced electron−hole pairs from ZnO quickly separated and were transported to electrodes as a photocurrent, and this process was greatly influenced by the transport path of the carriers. To demonstrate the significance of the interface in fiber-shaped PDs clearly, we described the transport performance of the photoinduced carriers through graphene via the schematics in Figure 6(a). Due to the tight surface contact of the graphene electrode, a broad path was provided for fast transport of all of the induced carriers, and then led to large photocurrent and high response speed. F

DOI: 10.1021/acsami.7b00811 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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strategy to improve the photoresponse properties of fibershaped PDs.

Research Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b00811. Detailed characterizations, including additional SEM, XRD, optical microscope image, Raman spectrum, and energy band diagram (PDF)

3. CONCLUSIONS In summary, based on a Zn wire, an inorganic−organic− graphene hybrid fiber-shaped UV photodetector was fabricated, composed of ZnO/PVK/graphene with all soft interfaces and precise band alignment. By using graphene instead of twined Ag wire as surface electrode, the photoresponse performance of PD was significantly improved. A large Ilight/Idark ratio of 7.2, high responsivity (Rλ) of 0.9 A/W, and fast response speed with the rise time of 280 ms was achieved and is far superior to the traditional fiber-shaped PDs. These excellent device performances were attributed to compact contacts of all layers, the p−n heterojunction of PVK/ZnO and smooth energy band alignment, which were in favor of the separation and transport of photoinduced electrons and holes. More importantly, as a universal strategy that utilized interface engineering, the majority of the fiber-shaped electron devices could get better performance by similar interface optimization.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.Z.). *E-mail: [email protected] (H.Z.). ORCID

Haibo Zeng: 0000-0002-0281-3617 Author Contributions ⊥

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Key Basic Research Program of China (2014CB931702), NSFC (51672132, 51572128, 21403109), NSFC-RGC (5151101197), the National Key Research and Development Program of China (2016YFB0401701), the Fundamental Research Funds for the Central Universities, and PAPD of Jiangsu.

4. EXPERIMENTAL SECTION 4.1. Synthesis of Zn Wire-Based ZnO NRs Array. In a typical procedure, the Zn wire with a diameter of 0.4 mm and length of 5 cm was ultrasonically cleaned with absolute ethanol for 30 min twice and then dried on the hot plate. Subsequently a thin and compact ZnO film with the thickness of ∼150 nm was deposited on the Zn wire by atomic layer deposition (ALD) technology. Then, ZnO NRs array was grown on the treated Zn wire with 200 mL growth solution, composed of 4 mol Zn(NO3)2·6H2O and 6 mL NH3·H2O and supplemental deionized water in an oven at 95 °C for 6 h. Finally, the prepared ZnO NRs array based on Zn wire was rinsed with deionized water and then dried for use. 4.2. Synthesis of Monolayer Graphene Film. The monolayer graphene film was synthesized via a chemical vapor deposition (CVD) method on Cu foils, with a mixed gas of CH4 (40 SCCM) and H2 (20 SCCM) reacted at 1000 °C.34 Then, the Cu substrates were removed by etchant solution (CuSO4/HCl/H2O = 10 g: 50 mL: 50 mL). After that the prepared sample was spin-coated with poly(methyl methacrylate) (PMMA) solution (5 wt % in chlorobenzene) to obtain PMMA supported graphene film. Here, the spin-coating speed and time of PMMA solution were 2500 r/min and 40 s, respectively, and the obtained film was ∼2 μm thickness. 4.3. Device Fabrication. Synthetic ZnO NRs array based on Zn wire was dipped into a chlorobenzene solution of PVK (10 g/L) for 2 h, and subsequently put under UV irradiating (365 nm) as postprocessing for 30 min. Then, the sample was wrapped with PMMA supported graphene film on its surface, and Ag wire was sandwiched between the graphene convenient for measurement. 4.4. Material Characterization. TEM, HRTEM, and SAED images of ZnO NRs were taken on a TECNAI G2 20 LaB6 TEM instrument operated at an acceleration voltage of 200 kV. SEM images of ZnO NRs array were obtained on a Quant 250 FEG SEM instrument. The optical absorption spectra of ZnO NRs suspensions were measured by a Shimadzu UV-3600 UV/vis/NIR spectrophotometer. XRD patterns were acquired using a Bruker D8 Advance Xray diffractometer operating with Cu Kα radiation (λ = 1.5406 Å). The optical photograph of graphene was taken on an OLYMPUS BX52 optical microscope. The Raman spectrum of the graphene was obtained on an Aramis Raman instrument. 4.5. Photoelectronic Measurements. The measurements of I−V (current−voltage) curve and I−t (current−time) curve of the photodetector were carried out by a Keithley 6487, in which 325 nm wavelength laser was used as the light source. The responsivity measurement was obtained by Zolix DSR101UV−B UV detector spectral responsivity measurement system.

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DOI: 10.1021/acsami.7b00811 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX