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Piezo-Phototronic Effect Enhanced UV Photodetector Based on Cui/Zno

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Piezo-Phototronic Effect Enhanced UV Photodetector Based on Cui/Zno Liu et al. Nanoscale Research Letters (2016) 11:281 DOI 10.1186/s11671-016-1499-1 NANO EXPRESS Open Access Piezo-phototronic effect enhanced UV photodetector based on CuI/ZnO double- shell grown on flexible copper microwire Jingyu Liu1†, Yang Zhang1†, Caihong Liu1, Mingzeng Peng1, Aifang Yu1, Jinzong Kou1, Wei Liu1, Junyi Zhai1* and Juan Liu2* Abstract In this work, we present a facile, low-cost, and effective approach to fabricate the UV photodetector with a CuI/ZnO double-shell nanostructure which was grown on common copper microwire. The enhanced performances of Cu/CuI/ZnO core/double-shell microwire photodetector resulted from the formation of heterojunction. Benefiting from the piezo-phototronic effect, the presentation of piezocharges can lower the barrier height and facilitate the charge transport across heterojunction. The photosensing abilities of the Cu/CuI/ZnO core/double-shell microwire detector are investigated under different UV light densities and strain conditions. We demonstrate the I-V characteristic of the as-prepared core/double-shell device; it is quite sensitive to applied strain, which indicates that the piezo-phototronic effect plays an essential role in facilitating charge carrier transport across the CuI/ZnO heterojunction, then the performance of the device is further boosted under external strain. Keywords: Photodetector, Flexible nanodevice, Heterojunction, Piezo-phototronic effect, Double-shell nanostructure Background nanostructured ZnO devices with flexible capability, Wurtzite-structured zinc oxide and gallium nitride are force/strain-modulated photoresponsing behaviors resulted gaining much attention due to their exceptional optical, from the change of Schottky barrier height (SBH) at the electrical, and piezoelectric properties which present metal-semiconductor heterojunction or the modification of potential applications in the diverse areas including the band diagram of semiconductor composites [15, 16]. field effect transistors, energy harvesters, photodetectors, Piezo-phototronic effect utilizes external stress-induced solar cells, pressure sensors, and chemical sensors [1–14]. piezopotential to modulate photo-generated carrier trans- In particular, one-dimensional (1D) ZnO micro/nanowires port, which has promising applications for pressure- have attracted great attention due to its advantages in fa- triggered sensors and human-machine interfacing [17]. cile synthetic procedure, long-term chemical stability, and The presence of piezocharges at one side of the hetero- environmental friendliness. Coupling the semiconducting junction can considerably regulate the behaviors of the and piezoelectric properties of ZnO, applying force/strain charge carriers and tune performances of flexible elec- along the ZnO polar c-axis can generate piezoelectric tronic and optoelectronic devices by controlling external polarization-induced piezopotential at two ends of 1D strain conditions. micro/nanowire, which facilitates the modulation of Recently, considerable attention has been paid to charge carrier transportation process [2]. By endowing piezotronic and piezo-phototronic nanodevices based on ZnO micro/nanowires array and their strain-modulated * Correspondence: [email protected]; [email protected] † photoresponsing behaviors [18, 19]. However, it should Equal contributors be noted that the presentation of free electrons in ZnO 1Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences; National Center for Nanoscience and Technology (NCNST), Beijing can partially screen piezopocharges under applied strain 100083, People’s Republic of China which unavoidably reduce the performances of piezotronic 2 College of Environmental Sciences and Engineering, Peking University, and piezo-phototronic nanodevice [20, 21]. In order to Beijing, China © 2016 The Author(s). Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Liu et al. Nanoscale Research Letters (2016) 11:281 Page 2 of 7 suppress electron screening effect, substantial efforts had ZnO can be considered as the idea building block for the already been devoted to the synthesis of p-type ZnO or formation of heterojunction and UV detector. the formation of heterojunction. Previous work reported The Cu microwire-based devices have been reported the improved performance of ZnO/Au Schottky junction- such as piezoelectric nanogenerator, dye-sensitized solar based UV detector [10] because the Fermi energy of Au is cells, and directional gas collection/transportation [24–26]. lower than that of ZnO, which facilitated electron transfer The surface modification of Cu microwire with unique from the ZnO to Au via the Schottky junction. Under morphologies or nanostructured functional materials external strain, the enhanced photocurrent is related to extensively broadened practical application in flexible charge redistribution which induced by piezoelectric devices. In this work, we present a facile and environ- polarization at the ZnO/Au heterojunction. Another mentally friendly approach for fabricating a flexible UV strategy for efficient separation of photo-generated elec- photodetector with a Cu/CuI/ZnO core/double-shell trons to prevent charge carrier recombination is the forma- structure. The performances of Cu/CuI/ZnO microwire tion of semiconductor heterojunction. Involving multi-step PD can be tuned by changing strain and illumination synthetic processes for constructing complex nanostruc- conditions, which indicated this type of PD has a potential tures, several reports exhibited enhanced photoresponse of application in flexible nanodevices. The CuI/ZnO hetero- ZnO nanowires achieved with CdS or Cu2Ounderstrain junction plays an essential role in modulating the photo- conditions [19, 22]. It can avoid the problems in the generated charge carrier transport with regard to the band synthesis of p-type ZnO, which are known as the low alignment of wide band gap CuI and ZnO. Compared to solubility of dopants or poor stability [23]. Neverthe- the strain-free conditions, applying compressive strains less, the as-fabricated photodetectors (PDs) exhibited leads to the photoresponsivity, relative changes of photo- the photoresponse in the visible region as well, which responsivity, and the detectivity of Cu/CuI/ZnO photo- considerably influenced the direct detection of UV light. detector was improved at different light intensities. Our Therefore, p-type semiconductor with band gap close to work might provide an alternative strategy for utilizing Fig. 1 SEM images and EDS characterizations of a Cu microwire, b Cu/CuI core/shell microwire, and c Cu/CuI/ZnO core/double-shell microwire; all scale bars are 100 μm Liu et al. Nanoscale Research Letters (2016) 11:281 Page 3 of 7 piezo-phototronic effect for boosting photoresponsing double-shell microwire are presented in Fig. 1. After performance of PD in a low-cost and convenient manner. carrying out one-step iodization procedure, a uniform gray-white CuI shell was observed on surface of Cu Methods microwire, see Fig. 1b. As shown in Fig. 1c, outer shell The preparation of inner shell CuI was conducted by ex- ZnO was deposited on the top of CuI layer by a stand- posing the Cu microwire surface to iodine vapor. The outer ard RF magnetron sputter deposition subsequently. For shell ZnO was grown by a standard RF magnetron sputter a bare Cu microwire, only Cu signals were detected in deposition using a high purity ZnO target (99.99 %) and the energy dispersive spectroscopy (EDS), see Fig. 1a. Ar/O2 (flow rate was fixed at 8:1) as the sputtering gases at As shown in Fig. 1b, iodine signals were unambiguously room temperature. The morphology of the Cu microwire, identified for Cu/CuI core/shell microwire. Further- Cu/CuI core/shell microwire, and Cu/CuI/ZnO core/ more, the presentation of ZnO coating on the top sur- double-shell microwire was examined by scanning face of Cu/CuI microwire was further confirmed by the electron microscope (SEM, Hitachi 8020). The X-ray EDS characterization, as seen in Fig. 1c. The crystal diffraction (XRD) spectra of Cu/CuI and Cu/CuI/ZnO structure of Cu/CuI core/shell microwire and Cu/CuI/ microwire were recorded on a Bede D1 ZM-SJ-001 ZnO core/double-shell microwire was studied by XRD diffractometer. The X-ray photoelectron spectroscopy measurement; see Fig. 2. It can be seen from Fig. 2a (XPS) characterization was performed at Thermo Scientific that three peaks can be assigned to (111), (200), and ESCALAB 250Xi. Ag paste was employed to fix the end of (220) planes of face-centered cubic (fcc) Cu, which Cu/CuI/ZnO core/double-shell microwire on the polyethyl- were labeled by filled circle. After ionization process, ene terephthalate (PET) flexible substrate as electrodes. A the surface of Cu microwire was fully covered by CuI thin layer of polydimethylsiloxane (PDMS) was used to shell. The XRD spectrum of Cu/CuI core/shell micro- protect the device during the test. The measurements wires reveals characteristic (111), (220), (311), (400), of photoresponse properties were conducted by a semi- (331), (420), and (422) peaks, which indicating a cubic conductor characterization system (Keithley 4200 SCS). phase γ-CuI structure, which
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