Strain-modulation and service behavior of Au-MgO-ZnO UV photodetector by piezo-phototronic effect

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1 Nano Research DOI /s x Nano Res 1 Strain-modulation and service behavior of Au-MgO-ZnO UV photodetector by piezo-phototronic effect Qingliang Liao 1, Mengyuan Liang 1, Zheng Zhang 1, Guangjie Zhang 1, and Yue Zhang 1, 2 () Nano Res., Just Accepted Manuscript DOI /s x on August 7, 2015 Tsinghua University Press 2015 Just Accepted This is a Just Accepted manuscript, which has been examined by the peer-review process and has been accepted for publication. A Just Accepted manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides Just Accepted as an optional and free service which allows authors to make their results available to the research community as soon as possible after acceptance. After a manuscript has been technically edited and formatted, it will be removed from the Just Accepted Web site and published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these Just Accepted manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI ), which is identical for all formats of publication.

2 Template for Preparation of Manuscripts for Nano Research This template is to be used for preparing manuscripts for submission to Nano Research. Use of this template will save time in the review and production processes and will expedite publication. However, use of the template is not a requirement of submission. Do not modify the template in any way (delete spaces, modify font size/line height, etc.). If you need more detailed information about the preparation and submission of a manuscript to Nano Research, please see the latest version of the Instructions for Authors at TABLE OF CONTENTS (TOC) Authors are required to submit a graphic entry for the Table of Contents (TOC) in conjunction with the manuscript title. This graphic should capture the readers attention and give readers a visual impression of the essence of the paper. Labels, formulae, or numbers within the graphic must be legible at publication size. Tables or spectra are not acceptable. Color graphics are highly encouraged. The resolution of the figure should be at least 600 dpi. The size should be at least 50 mm 80 mm with a rectangular shape (ideally, the ratio of height to width should be less than 1 and larger than 5/8). One to two sentences should be written below the figure to summarize the paper. To create the TOC, please insert your image in the template box below. Fonts, size, and spaces should not be changed. Strain-modulation and service behavior of Au-MgO-ZnO UV photodetectors by piezo-phototronic effect Qingliang Liao 1,, Mengyuan Liang 1,, Zheng Zhang 1, Guangjie Zhang 1, and Yue Zhang1, 2,* 1 Department of Materials Physics and Chemistry, State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing , China 2 Key Laboratory of New Energy Materials and Technologies, University of Science and Technology Beijing, Beijing , China These two authors contributed equally to this work. The sensitivity of the UV photodetectors was enhanced by inserting an ultrathin insulating layer and the service behavior the UV photodetectors modulated by external strain were investigated. Provide the authors webside if possible. Author 1, webside 1 Author 2, webside Nano Research

3 Nano Research DOI (automatically inserted by the publisher) Research Article Nano Res. 1 Strain-modulation and service behavior of Au-MgO-ZnO UV photodetector by piezo-phototronic effect Qingliang Liao 1,, Mengyuan Liang 1,, Zheng Zhang 1, Guangjie Zhang 1, and Yue Zhang 1, 2 () Received: day month year Revised: day month year Accepted: day month year (automatically inserted by the publisher) Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2014 KEYWORDS photodetectors, ZnO, the ultrathin insulating layer, sensitivity, piezo-phototronic effect ABSTRACT Au-MgO-ZnO (AMZ) ultraviolet (UV) photodetectors were fabricated to enhance its sensitivities by an inserting ultrathin insulating MgO layer. With the insulating layer, the sensitivities of the UV photodetectors were improved by the reducing of the dark current. Furthermore, the strain-modulation was used to enhance the sensitivities of AMZ UV photodetectors. The sensitivities of the photodetectors were enhanced by the piezo-phototronic effect. But there was a limiting value of the applied strains to enhance the sensitivity of the photodetector. When the external strains exceed the limiting value, the sensitivity will decrease due to the tunneling dark current. The external stains loaded on the photodetectors will result in the degradation of photodetectors and the applied bias may accelerate the process. This work demonstrates a prospective approach to engineer the performance of a UV photodetector. In addition, the study on service behavior of the photodetectors may offer a strain range and theoretical support for safely using and studying metal-insulator-semiconductor UV photodetectors. 1 Introduction The development of ultraviolet (UV) photodetectors for imaging and switching devices applying in the invisible wavelength region is an important research issue for medical and military applications [1-3]. The key requirements in designing these UV photodetectors are selectivity, high sensitivity, responsivity, speed, and low-noise. Recently, this Address correspondence to yuezhang@ustb.edu.cn.

4 2 Nano Res. research field is drawing more and more attentions to improve these properties through using different device structures or materials systems [4-6]. When these photodetectors work, the environmental factors would affect their properties. The environmental factors such as temperature, strain, ph value may have effect on the performance of photodetectors. As an effective modulation method for UV photodetector, the external strains have been investigated extensively and the performance of the UV photodetectors can be enhanced by the piezo-phototronic effect [7-10]. Most of the environmental factors were used to enhance the properties of UV photodetectors. However, some factors may have a valid values range to achieve the purpose. And there were few reports on the service behavior about the values in a particular type of UV photodetectors like mental-insulator-semiconductor (MIS) UV photodetectors. The significance of the investigation on the service behavior is that it can provide the designers of a particular type of photodetectors with operating parameters and theoretical support. This can be used to evaluate the mechanical reliability of photodetectors and predict the safe working conditions of photodetectors. ZnO is recognized as promising building blocks for future electronic, optoelectronic and electromechanical devices because of its coupled piezoelectric and semiconducting properties [11-14]. Since ZnO has a noncentral symmetric wurtzite structure, a strain along c-axis on the basic unit can cause a polarization. It results in the piezopotential inside the crystal [15-17], which would affect its properties. ZnO has diverse nanostructural morphologies [18, 19], among which one dimensional semiconducting nanostructure becomes a vital building block in UV photodetectors. This is because it has high surface to volume ratio and quantum size effect. In this study, an Au-MgO-ZnO (AMZ) UV photodetector by inserting an ultrathin insulating MgO layer between the Au electrode and ZnO nanowire (NW) arrays is reported. The sensitivity of the photodetector is improved by the insulating layer [20, 21]. A detailed explanation for the improvement will be given. Furthermore, the value range of applied strains to improve the sensitivity of the photodetector is investigated. The minimum limiting value of strain to reduce the sensitivity will be given. The reason for it is that the dark current of the photodetectors can be modulated by the strains. These studies will be helpful for promoting the development of the MIS UV photodetectors made of ZnO NW arrays and predicting their reliability. 2 Experimental Fig. 1 schematically illustrates the key steps for fabricating the AMZ UV photodetectors. Firstly, 100nm thick Au films were deposited on the FTO glass substrates (15mm 10mm) using DC sputtering. The Au films served as electrodes. Secondly, the bottom electrodes were protected with thermal tape and about 10 nm thick MgO layers were deposited on Au films by atom layer deposition (ALD) (PEALD-200A) with Mg(Cp)2 and H2O under 150. Thirdly, the top electrodes were protected by thermal tape. Then ZnO NW arrays were synthesized by using the hydrothermal method on the AZO glass substrates (15mm 7mm) [22]. At last, the AZO glass substrates with ZnO NW arrays were inverted on the FTO glass substrates with Au electrodes and MgO layers. Simultaneously, the top electrodes and the bottom electrodes were staggering placed. To illustrate the effect of the insulating MgO layer, an Au-ZnO (AZ) structured junction was fabricated to compare with the MIS one. To maintain a stable and reliable contact between the FTO glass part and ZnO NW arrays, all the fabricated devices were wrapped by tapes and fully packaged with Polydimethylsiloxane (PDMS). The morphology and structure of the ZnO NWs were characterized by scanning electron microscope (FESEM, LEO1530) and X-ray diffraction (XRD, Rigaku DMAX-RB, Japan). The fabricated photodetectors were fixed on a designed cantilever system. The system could apply axial strains to the ZnO NRAs with accurately controlled amplitudes. The electrical transport properties of photodetectors were measured by a semiconductor characterization system (Keithley4200-SCS). To characterize their photosensitive properties, a portable UV lamp (365nm, 1mW/cm 2 ) was used as the light source.

5 Nano Res. 3 Figure 2 Morphologies and structure characterization of the highly dense ZnO NW arrays. (a) The top-view SEM image. (b) The side-view SEM image. (c) The XRD pattern. Figure 1 Schematic diagram of the steps for the fabrication of a MIS UV photodetector. 3 Results and discussion The top-view and side-view FESEM images of the as-synthesized ZnO NW arrays are presented in Fig 2a and 2b, respectively. It can be seen that the ZnO NW arrays are highly dense and grow vertically-aligned. The length of the ZnO NW arrays is about 2µ m in average. Fig 2c shows the XRD pattern of ZnO NW arrays. The peaks center at 31.6 and 34.3 correspond to the diffraction plane (100) and the diffraction plane (002) in ZnO with the hexagonal wurtzite, respectively. However, (100) diffraction peak can be almost negligible compared with (002) diffraction peak. The high intensity (002) diffraction peak indicates that the ZnO NW arrays were highly oriented and had good crystallinity. The fabricated ZnO NW arrays are good for fabricating the MIS UV photodetectors. The response properties of the photodetectors are tested. In Fig 3a, the blue and red lines are the I-V curves of the AZ and AMZ photodetectors under dark, respectively. The I-V curves show that Schottky contacts are formed between the Au electrodes and ZnO NW arrays. At an applied bias of -1 V, the dark current of the AMZ photodetector is 0.41µA and the dark current of the AZ photodetector is 85µA. The insulating MgO layer decreases the dark current by 2 orders of magnitude compared to the AZ photodetector. The insert of Fig 3a is the I-V curve of the AMZ UV photodetector under dark. The device exhibits an excellent rectifying behavior. Fig 3b and 3c show I-V curves of AZ and AMZ photodetectors. The black and red lines are I-V curves under dark and illumination, respectively. At bias of -1V, the sensitivity (photo-to-dark current ratio) of AZ and AMZ photodetectors are 4.25 and 197, respectively. Fig 3d represents the I-t curves of the AZ and AMZ photodetectors at bias of -3V. The red and dark lines are the curves of the AZ photodetector and the AMZ photodetector, respectively. Compared with the AZ photodetector, a relatively low dark current of 0.41 µa is obtained and the photocurrent is about 92 µa on the AMZ photodetector.the sensitivity of the AMZ photodetector is found to be about 224 while the AZ photodetector with a sensitivity of 3. So the insulating MgO layer reduces the dark current and enhances the sensitivity of the device. Nano Research

6 4 Nano Res. Figure 3 (a) I-V characteristics contrast under dark of the MS UV photodetector and the MIS photodetector. The inset in (a) is I-V curve of the MIS photodetector. (b) I-V curves of the MS photodetector under dark and illumination. (c)i-v curves of the MIS photodetector under dark and illumination. (d) I-t characteristics contrast of the MS UV photodetector and the MIS photodetector. The schematic illustration of the UV photodetectors with sensitivities is shown in Fig 4. Here the mechanism for the reduction of the dark current by inserting the MgO layer is explained. Fig 4a represents the energy band diagram of the junction with a forward bias (a positive bias is applied on the Au electrode) under dark. In Fig 4a, when electrons flow into the Au electrode through ZnO, they store enough energy due to the applied electric field. Then the electrons can easily tunnel through the ultrathin insulating MgO layer and results in a large dark current. But because of the blocking of the insulator, the dark current decreased compared with AZ UV photodetector. Fig 4b shows the energy band diagram of the junction with a reverse bias (a positive bias is applied on the AZO electrode) under dark. In Fig 4b, electrons flow into the AZO electrode from the Au electrode and encounter the MgO layer immediately. Without an accelerating process, most of the electrons have insufficient energy to tunnel through MgO layer expect a small part (the green arrow). The small part mainly composes of electrons by thermionic emission. As a result, most of the electrons are blocked on the Au/MgO interface. It greatly reduces the dark current. Thus, the AMZ photodetector exhibits a good rectifying behavior in the dark current and the dark current is smaller. On UV illumination, the photogenerated electrons are produced in ZnO NW arrays in large amounts. So the original existing electrons can be neglected. The photogenerated electrons will go through an accelerating process under the reverse bias or the forward bias. Therefore, all of the photogenerated electrons have sufficient energy to flow to the corresponding electrode. As a result, the sensitivity of the AMZ UV photodetector is enhanced under a reverse bias compared to the AZ photodetector. Figure 4 The schematic illustration of the MIS UV photodetectors with sensitivities. (a) The energy band diagram of the junction with a forward bias under dark. The blue arrow is the direction of current and the green one is the tunneling current. The black dash dot line is the Fermi level at thermal equilibrium. (b) The energy band diagram of the junction with a reverse bias under dark. The green arrow is electrons by thermionic emission. To further improve UV sensing property and the

7 Nano Res. 5 investigate the strains effects on it, I V characteristic of the photodetect is measured under different compressive strains. The result is shown in Fig 5a. Fig 5b represents how the dark current changes with the increase of the applied compressive strains at bias of -3V. These figures in Fig 5a and Fig 5b illustrate that the dark current decreases first and then increases along with the applied compressive strains increasing. At bias of -3V, the sensitivity of the device is 19.9 under 1.6% strains in contrast to under 1.0% strains. The data prove the existence of a range value from 0% to 1.0% of the applied strains. Otherwise, there is a limiting value of strains corresponding to 1.0% [23]. When the applied strains are over the limiting value, the sensitivity of the photodetector gradually decreases to the failure of device. Figure 5 (a) I-V curves of the MIS UV photodetectors with a series of compressive strains under dark. (b)the curve about how the dark current changes with the increase of the applied compressive strains at bias of -3V. Firstly, we explain the mechanism for the first reduction of the dark current by increasing the applied compressive strains. Fig 6a shows the energy band diagram in the junction without a bias or compressive strains. Fig 6b shows the energy band diagram in the junction with a reverse bias. Compared Fig6a and 6b, the reverse bias makes the width of depletion region (marked as W) wider. The conduction band bends downward and becomes sharper near the MgO/ZnO interface. Fig 6c shows the energy band diagram in the junction with both a reverse bias and smaller compressive strains, respectively. The dark current of AMZ UV photodetectors mainly composes of electrons by thermionic emission. Under compressive strains, the photodetector can be regarded as that a negative potential (Em) is produced on the Au/ZnO interface by the piezoelectric effect of ZnO. The negative potential makes the Schottky barrier height (SHB) increase, as shown in Fig 6c. With strains increasing, the SHB becomes higher and higher. Because of it, the thermionic emission electrons gradually reduce and the dark current becomes smaller and smaller [24]. Secondly, the mechanism for the later increase of the dark current with sequentially increasing the applied compressive strains is explained. The schematic diagram of the mechanism is illustrated in Fig 6. Fig 6d represents the energy band diagrams in the junction with both a reverse bias and bigger compressive strains. Compared Fig 6b and Fig 6a, the built-in barrier height (marked as Ψi) increases by Φbias after applying a negative bias on the Au electrode. Meanwhile, W becomes wider. The conduction band near the MgO/ZnO interface becomes more downward bending and sharper. Ψi increases by Φm again with sequentially applied compressive strains between Fig 6d and 6b. In consequence, W became narrower. The conduction Nano Research

8 6 Nano Res. band becomes greater downward bending and sharper. When the strains are large enough to ensure a very sharp conduction band, a considerable part of the previous blocked electrons in Au electrode can tunnel through MgO and barrier. These tunneling electrons make the dark current increase. Figure 6 The schematic diagram of the MIS photodetector with compressive strain under dark. They are the energy band diagrams in the junction (a) at 0V without applied strains, (b) at a reverse bias without applied strains and (c) at a reverse bias with insufficient compressive strains, (d) at a reverse bias with enough compressive strains, respectively. The green bent arrow is electrons by thermionic emission and the straight one is the tunneling current. Ebias is the applied bias. The black dash dot line is the Fermi level. On the other hand, a calculation is exhibited to interpret the increase of the dark current as mentioned above. As a MIS device with an ultrathin insulating layer, it has a current at a certain bias. The current is non-equilibrium, that is to say, the Fermi energy levels of electron and hole are separate. There are three main different points between the MIS devices and the MS devices. Firstly, the dark current of the MIS device is smaller. The MIS device has a lower barrier height than the MS device. This is because of the existence of a certain voltage drop though the insulator. The MIS device has a higher ideality factor. The tunneling current of the MIS device can be described as following [25]: J * A B qv exp exp 1 kt (1) where J is the dark current density at a fixed bias (marked as V), A* is the effective Richardson constant, B is a constant related to temperature, ζ is the effective barrier, δ (it is measured in Å) is the thickness of the insulating layer, q is the unit electronic charge, η is the ideality factor, k is the Boltzmann constant, and T is the temperature. And η here can be described as following [25]: 1 s W qdits i 1 i qditm (2) where εi is dielectric permittivity, εs is semiconductor permittivity, WD is the width of depletion region, Dits is the trap density of the semiconductor, and Ditm is the trap density of the metal. From the above two formulas, we can see that J becomes bigger along with the broadening of W. The total of the built-in potential can be described as following [26]: 2 W NDq kt i V 2 s q (3) where Ψi is the total of the built-in potential, and

9 Nano Res. 7 ND is donor impurity concentration. As shown in 5c, Ψi is defined as: i mbi bias (4) where Ψbi is the built-in potential in the thermal equilibrium state without compressive strains, Φm is the increase of built-in barrier induced by compressive strains, and Φbias is the increase of built-in barrier induced by the applied reverse bias. When the device is under compressive strains, it can be regarded as a negative potential producing on the Au/ZnO interface. The negative potential is due to the piezoelectric effect of ZnO and makes the built-in barrier increase by Φm. The build-in barrier increases as compressive strains increasing. Similarly, Φbias increased as the applied reverse bias being enhanced. Combining the two formulas, a new formula can be obtained as following: 2 W NDq 2 s kt m bias bi V q (5) where Ψbi can be regarded as a constant without applied reverse bias and compressive strains. When Φm or Φbias increases, W will be wider. This leads the tunneling current to increase. Thus, both applied reverse bias and compressive strains can enhance the dark current when strains are large enough. To prove this point, Fig 7a shows I S (compressive strains) characteristic of the device measured at different reverse biases. The compressive strains corresponding to the turning point of the dark current are 0.8%, 0.6%, 0.4% at bias of -1V, -2V and -3V, respectively. According to them, the conclusion is that the corresponding compressive strains became smaller as the applied reverse bias increasing. The S S (sensitivity compressive strains) characteristic of the device at bias of -3V is represented in Fig 7b. The sensitivity of the device becomes very low with large enough compressive strains. Finally, the AMZ UV photodetector degrades and fails. Figure 7 (a)the curves of the AMZ UV photodetectors about how the dark current changed with the increase of the applied compressive strains (I-S curve) at different biases. (b) The curves of the AMZ UV photodetectors about how the sensitivity changed with the increase of the applied compressive strains (S-S curve) at bias of -3V. Consequently, the dark current decreases first along with the increase of applied compressive strains and the sensitivity of the AMZ UV photodetector is enhanced. But there is a limiting value of strains. When the compressive strain is over the limiting value, it will lead to the performance degradation and even the failure of the photodetector. 4 Conclusions In summary, an Au-MgO-ZnO UV photodetector is successfully fabricated. Inserting the ultrathin insulating MgO layer is to improve the photo sensing property of the photodetector. To demonstrate this point, Au- ZnO UV photodetectors are fabricated. The performances of the fabricated photodetectors are characterized and compared. Compared to the AZ UV photodetectors, the AMZ photodetectors with inserted ultrathin insulating MgO layers exhibit good rectifying behaviors under dark and have higher sensitivities to UV light. The ultrathin insulator reduces the dark current due to the selective tunneling current transport. In order to improve its property, compressive strains are applied on the AMZ UV photodetector. At first, the increase of applied compressive strains enhances the SBH. The increase of SBH leads the dark current to decrease and sensitivity to increase. The property is really improved by the piezo-phototronic effect. But the current increases as strains increasing continuously. This is because that the tunneling probability of electrons increases. The increase of the dark current will lead the sensitivities of photodetectors decrease. The study demonstrates the existence of the limiting value of applied Nano Research

10 8 Nano Res. compressive strains. Above the limiting value, the strains finally make their performance degrade and fail. These results can offer a particular type of photodetectors for promoting the photonics or optoelectronic technology. Simultaneously, data and theoretical supporting for the study on the safe working conditions of photodetectors are provided. It will promote the study on the service and failure of different photodetectors. Acknowledgements This work was supported by the National Major Research Program of China (No. 2013CB932601), the Major Project of International Cooperation and Exchanges (No. 2012DFA50990), the Program of Introducing Talents of Discipline to Universities (B14003), National Natural Science Foundation of China (Nos , , and ), the Fundamental Research Funds for Central Universities, State Key Lab of Advanced Metals and Materials (2014Z-11), and Program for New Century Excellent Talents in Universities (NCET ). References [1] Zhai, T.; Fang, X.; Liao, M.; Xu, X.; Zeng, H.; Yoshio, B.; Golberg, D. A comprehensive review ofon e-dimensional metal-oxide nanostructure photodetectors. Sensors 2009, 9, [2] Zhai, T..; Li, L.; Xang, X.; Fang X.; Yoshio, B.; Golberg, D. Recent developments in one-dimensional inorganic nanostructures for photodetectors. Adv. Funct. Mater. 2010, 20, [3] Razeghi, M.; Rogalski, A. Semiconductor ultraviolet detectors; Spie- Int Soc Optical Engineering: Bellingham, [4] Chen, M.; Lu, M.; Wu, Y.; Song, J.; Lee, C.; Lu, M.; Chang, Y.; Chou, L.; Wang, Z.; Chen, L. Near UV LEDs made with in situ doped p-n homojunction ZnO nanowire arrays. Nano Lett. 2010, 10, [5] Hu, P.; Han, N.; Zhang, X.; Yao, M.; Cao, Y.; Zuo, A.; Yang, G.; Yuan, F. Fabrication of ZnO nanorod-assembled multishelled hollow spheres and enhanced performance in gas sensor. J. Mater. Chem. 2011, 21, [6] Ahmad, M.; Pan, C. F.; Zhu, J. Electrochemical determination of L-Cysteine by an elbow shaped, Sb-doped ZnO nanowire-modified electrode. J. Mater. Chem. 2010, 20, [7] Wang, Z.; Yu R.; Wen X.; Liu Y.; Pan C.; Wu W.; Wang, Z. Optimizing performance of silicon-based pn junction photodetector by piezo-phototronic effect. ACS Nano 2014, 8, [8] Zhang, F.; Niu, S.; Guo, W.; Zhu, G.; Liu, Y.; Zhang, X.; Wang, Z. L. Piezo-phototronic effect enhanced visible/uv photodetector of a carbon-fiber/zno-cds double-shell microwire. ACS Nano 2013, 7, [9] Zhang, F.; Ding, Y.; Zhang, Y.; Zhang, X.; Wang, Z. Piezo-phototronic effect enhanced visible and ultraviolet photodetection using a ZnO-CdS core-shell micro/nanowire. ACS Nano 2012, 6, [10] Yang, Q.; Liu, Y.; Pan, C.; Chen, J.; Wen, X.; Wang, Z. L. Largely enhanced efficiency in ZnO nanowire/p-polymer hybridized inorganic/organic ultraviolet light-emitting diode by piezo- phototronic effect. Nano Lett. 2013, 13, [11] Zhang, Y.; Yan, X. Q.; Yang, Y.; Huang, Y. H.; Liao, Q. L.; Qi, J. J. Scanning probe study on the piezotronic effect in ZnO nanomaterials and nanodevices. Adv. Mater. 2012, 24, [12] Lin, P.; Yan, X.; Zhang, Z.; Shen, Y.; Zhao, Y.; Bai, Z.; Zhang, Y. Self-powered UV photosensor based on PEDOT:PSS/ZnO micro/nanowire with strain-modulated photoresponse. ACS Appl. Mater. Inter. 2013, 5, [13] Zhang, Z.; Liao, Q.; Yan, X.; Wang, Z. L.; Wang, W.; Sun, X.; Lin, P.; Huang, Y.; Zhang, Y. Functional nanogenerators as vibration sensors enhanced by piezotronic effects. Nano Res. 2014, 7, [14] Liao, Q.; Zhang, Z.; Zhang, X.; Mohr, M.; Zhang, Y.; Fecht, H.-J. Flexible piezoelectric nanogenerators based on a fiber/zno nanowires/paper hybrid structure for energy harvesting. Nano Res. 2014, 7, [15] Wang, Z. L.; Song, J. Piezoelectric nanogenerators based on zinc oxide nanowire arrays. Science 2006, 312, [16] Wang, Z. L. Piezotronic and piezophototronic effects. J. Phys. Chem. Lett. 2010, 1, [17] Wang, Z. L. Progress in piezotronics and piezo-phototronics. Adv. Mater. 2012, 24, [18] Wang, Z. L. Zinc oxide nanostructures: growth, properties and applications. J. Phys.-Condens. Mat. 2004, 16, R829-R858. [19] Wang, Z. L. ZnO nanowire and nanobelt platform for nanotechnology. Mater. Sci. Eng. R 2009, 64, [20] Kim, D.; Jung, B.; Lee, J.; Cho, H.; Lee, J. Dramatically enhanced ultraviolet photosensing mechanism in a n-zno nanowires/i-mgo/n-si structure with highly dense nanowires and ultrathin MgO layers. Nanotechnology 2011, 22, [21] Jung, B.; Kim, D.; Kong, B.; Kim, D.; Cho, H. Fully transparent vertically aligned ZnO nanostructure-based ultraviolet photodetectors with high responsivity. Sensor. Actuat. B-Chem. 2011, 160, [22] Lin, W.; Yan, X.; Zhang, X.; Qin, Z.; Zhang, Z.; Bai, Z.; Lei, Y.; Zhang, Y. The comparison of ZnO nanowire detectors working under two wavelengths of ultraviolet. Solid State Commun. 2011, 151, [23] Zhang, Z.; Liao, Q.; Yu, Y.; Wang, X.; Zhang, Y. Enhanced photoresponse of ZnO nanorods-based self-powered photodetector by piezotronic interface engineering. Nano Energy 2014, 9, [24] Zhou, J.; Fei, P.; Gu, Y.; Mai, W.; Gao, Y.; Yang, R.; Bao, G.; Wang, Z. L. Piezoelectric-potential-control led polarity-reversible schottky diodes and switches of ZnO wires. Nano Lett. 2008, 8,

11 Nano Res. 9 [25] Sze, S. M.; Ng, K. K. Physics of Semiconductor Device; A John Wiley & Sons: California, [26] Neaman D.; Pevzner B. Semiconductor Physics and Device: Basic Principles; McGraw-Hill: New York, Nano Research