Concurrent Improvement of Photocarrier Separation and Extraction in

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C: Physical Processes in Nanomaterials and Nanostructures

Concurrent Improvement of Photocarrier Separation and Extraction in ZnO Nanocrystals UV Photodetector Jun Yuan, Liang Hu, Zhenyu Xu, Yiyue Zhang, Hui Li, Xingzhong Cao, Huawei Liang, Shuangchen Ruan, and Yu-Jia Zeng J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 23 May 2019 Downloaded from http://pubs.acs.org on May 23, 2019

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The Journal of Physical Chemistry

Concurrent Improvement of Photocarrier Separation and Extraction in ZnO Nanocrystals UV Photodetector Jun Yuan,†∥ Liang Hu,†∥ Zhenyu Xu,† Yiyue Zhang,† Hui Li,† Xingzhong Cao,‡ Huawei Liang,†* Shuangchen Ruan† and Yu-Jia Zeng†*

†Shenzhen

Key Laboratory of Laser Engineering, College of Physics & Optoelectronic

Engineering, Shenzhen University, Shenzhen, 518060, P. R. China. ‡Beijing

Engineering Research Center of Radiographic Techniques and Equipment,

Institute of High Energy Physics, CAS, Beijing 100049, China KEYWORDS: ZnO NCs, UV photodetector, two-step annealing, zinc vacancy, photocarrier separation and extraction.

ABSTRACT: Polycrystalline film photodetectors often suffer from several drawbacks, such as uncontrollable defect species, grain boundary scattering and surface oxygen trapping/detrapping, hindering their practical applications in high performance UV photodetection. In this work, we induce acceptor-type zinc vacancy (VZn) defect in zero

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dimensional ZnO nanocrystals by a dual thermal annealing process, which have been closely examined by defect sensitive electron paramagnetic resonance (EPR) technique. The optimization of annealing parameters can well tune the VZn concentration and induce a considerable self-powered behavior, which is believed to be result from ionized acceptor enhanced charge separation. On the other aspect, the capping of metallic Zn can lead to the formation of abundant interface conducting channels for the highly-efficient charge transport and extraction. The optimized responsivity is enhanced from 5.3×10-3 to 15.3 A W-1 and the average rise and decay time remains at 36.6 ms and 101.8 ms. Conductive atomic force microscopy (CAFM) confirms a uniform photocurrent distribution in the annealed polycrystalline films, suggesting the significance of synergies of lattice defects and grain boundary conductive channels. This study unambiguously demonstrates a new route to engineer the photodetection in nanocrystalline oxides, providing a promising prospect for future low-cost, low-powerconsumption micro/nano optoelectronic devices.

INTRODUCTION Detection of ultraviolet (UV)-A band (320-400 nm) has a wide range of commercial applications, such as flame sensing, missile detection, communications, medical diagnostics.1-6 However, several challenges remain for the current commercial UV photodetectors, including low responsivity, poor recovery speed, high cost, and/or bad


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selectivity.7-8 In specific, silicon-based photodetectors that are constructed by complex ion implantation process, often require the integration of expensive long-pass filters or phosphors to narrow the incident light to a certain frequency band. Even so, long-term exposure to UV light also boosts their degradation. In order to reduce the dark current, cooling equipment is commonly utilized, undoubtedly complexing the device structure.9-11 UV-resistant wide-bandgap semiconductors such as GaN, ZnO and SiC, have been proposed as alternative for UV photoresponse materials.12-13 Among them, ZnO (Eg~3.37 eV) has been paid more attention owing to its flexibility of preparation and tuning of properties.12,

14-18

Despite these, the performance of ZnO-based UV

photodetectors is still lower than expected.19-20 In general, the presence of defect and/or surface species will adsorb a large amount of oxygen molecules and affect the separation process of photocarriers, which result in a huge photo gain, but at the expense of the response time.21 Many groups have tried to assemble heterojunction to obtain faster optical response by taking the advantage of Schottky contacts. However, high operating biases of up to 20 V are typically required to achieve adequate responsiveness.22-23 The synthesis of novel ZnO-based nanocomposites by modifying ZnO

surface

is

another

promising

way

to

realize

high

performance

UV

photodetectors.24-25 So far, their response speed is still relatively slow. Therefore, how to


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balance the responsivity and the response speed is a technical challenge worthy of investigation. In addition, the preparation method also has a great influence on the photoelectric properties of materials. Sputtering or pulse laser deposition (PLD) method derived ZnO films have less surface states or defect species than chemically derived ZnO nanostructures.19,

24, 26

However, on the practical aspect, as an alternative to the

mainstream thin film technique, colloidal chemistry route is fully compatible with inkjet process and can be applied on any substrates with large areas. This method can be used to delicately control the atomic ratio and dimension of materials, which are beneficial for tuning point defects, doping and for confining carrier transport.11, 20, 27-31 However, how to improve the crystalline quality of chemically derived ZnO materials that would increase the response speed,12 remains as one of the biggest obstacles to the practical application of colloidal ZnO-based UV photodetectors.24 ZnO is a material that is very sensitive to defects,32 which mainly include interstitial (Zni, Oi), vacancy (VZn, VO) and inversion (ZnO, OZn) defects.30,

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These defects can

greatly affect the photoelectric properties of ZnO.34-36 However, very limited work has been reported to reveal the intrinsic link between defects and the photoresponse properties. Furthermore, due to the instability of the defects, how to regulate the concentration of defects is also an important issue.37 VZn is one of the typical acceptor


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defects in ZnO,38 which can capture electrons.39-41 Therefore, engineering and in-depth understanding of VZn are critical ZnO ultraviolet photodetector. Herein, we propose a dual annealing process to control the concentration of VZn acceptor and to introduce a metallic Zn layer between adjacent ZnO nanocrystals (NCs). To the best of our knowledge, this is the first demonstration of directly tuning VZn defects to accelerate the separation of photocarriers in zero-dimensional ZnO NCs, which is believed to induce a substantial self-powered behavior in such junctionless photodetector. The presence of metallic Zn films effectively reduces the scattering in the grain boundary and results in a faster transfer of photocarriers. Thanks to synergies, the optimized polycrystalline film devices give a considerate UV photocurrent level on the magnitude of 10-4 A at a low bias of 2V, a high on/off ratio of approximately 3 × 105 and a fast response recovery time less than one second. The responsivity is greatly enhanced approximately 3000 times compared with that of the pristine device, which highlights the important role of VZn as well as the dual annealing process. EXPERIMENTAL SECTION Chemical and Materials. Anhydrous zinc acetate (Zn(Ac)2, Alfa Aesar, 99.9+ %), potassium hydroxide pellets (KOH, Alfa Aesar, 85+ %), methanol (Aladdin, chromatographically pure), home-made deionized water (R = 16.2 MΩ), chloroform


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(Aladdin, chromatographically pure) and n-octylamine (OA, Aladdin, 99%) . All chemicals were used as received. Synthesis of Colloidal ZnO NCs. ZnO NC colloidal inks were prepared according to our previous work.10 Specifically, a methanol solution of Zn(Ac)2 (0.1 M, 42 mL) was loaded into a three-neck flask and refluxed at 60 °C for an hour. As mineralizer of ZnO formation, stoichiometric KOH-methanol solution (0.24 M, 22 mL) was dropwise added into the flask and the whole system was maintained at 60 °C for another two hours. During the reaction process, a small quantity of deionized water was selectively injected into the above precursors to control the nucleation rate and the size of NCs. When the reaction was terminated by quenching the flask in cold water, white precipitates were observed and further collected through multi-step centrifugation washing process. If the size of NCs was well monodispersed, they can be easily suspended in non-polar chloroform and OA addition can further improve their dispersity for a long-term storage of colloidal inks. Thermal Treatment and Device Fabrication of ZnO NCs. Prior to spin-coating, the colloidal inks were filtered through a 0.45 μm Nylon-66 filter to remove the possible aggregated particles. Afterwards, the inks were spin-coated onto pre-cleaned p++-Si (

Concurrent Improvement of Photocarrier Separation and Extraction in

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