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Toward an Understanding the Role of VZn Defect on the Photoconductivity of Surface Passivated ZnO NRs Dipanwita Sett, and Durga Basak J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06393 • Publication Date (Web): 16 Oct 2017 Downloaded from http://pubs.acs.org on October 23, 2017
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Toward an Understanding the Role of VZn Defect on the Photoconductivity of Surface Passivated ZnO NRs Dipanwita Sett and Durga Basak* Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India.
*
Corresponding author:
[email protected] 1 ACS Paragon Plus Environment
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Abstract The point defects in ZnO nanorods (NRs) play very crucial role in its photoconductivity (PC) properties and thus it is essential to understand the sub-band gap carrier dynamics in order to have efficient ultraviolet (UV) photodetection. In order to understand the role of a dominant point defect, Zn vacancy (VZn) which is prevalent on the surface of the NRs, we employ a high temperature annealing step in air and also an excess hydration step for one set of annealed NRs, each followed by a final surface passivation step by poly-vinyl butyral (PVB). A comprehensive study on the photocurrent spectra, photocurrent transients under different sub-band gap excitations and power dependence of photocurrent of aqueous chemically grown ZnO NRs treated under various conditions have been carried out and demonstrates the superiority (extraordinarily high and fast UV response) of the point defect rich but surface passivated NRs as compared to ones with absence or less VZn defects. Further a good support on the major role of VZn has been obtained from the PC results of a set of ZnO NRs treated in excess Zn with a concurrence from the enhanced UVto-VIS ratio. The experimental results in harmony with the reported theoretical calculations reveal that both trapping and/or recombination centers nature of an acceptor-like VZn and their defect complexes are likely to be the most important role playing in the PC. The study offers new insight in understanding the advanced mechanism of PC of ZnO NRs controlled by VZn–related sub-band gap defects.
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1. Introduction
One dimensional (1D) nanostructures are the most promising materials for various optoelectronic applications ranging from sensing devices, LEDs to solar cells.1-8 The nanorods (NRs) array films offer the most appropriate design which can effectively modulate the optical and electronic characteristics due to availability of their high surface area. 1D nanostructures of different morphologies of ZnO is one of the most studied wide band gap semiconductors (Egap=3.36 eV at 300 K) owing to its outstanding physicochemical properties controlled by the surface defects as a result of their high surface to bulk ratio whereas the role of the bulk properties is comparatively negligible.9-11 One such example is very high ultraviolet (UV) response properties of ZnO NRs due to its low photoexcited electron-hole recombinations controlled by its large surface defects.12-14 The photoresponse of ZnO can also be tuned within the visible spectral region by controlling the intrinsic surface defect levels, which makes it an excellent photodetector in the sub-UV regime. Therefore, understanding the behavior of surface defects is essential to the successful application of ZnO as UV and visible photodetector since the surface defects often capture the electron-hole pairs and makes the response slow. The intrinsic defects in ZnO are usually categorized into four types, namely, O vacancies (VO) and Zn vacancies (VZn) which are generally surface defects, and interstitials (Zn and O) and antisites which exist in the bulk of the material. The defect states due to surface adsorbed species such as O2 and H2O likely to modify the PC by influencing the charge carrier concentration and their life time.15-17 The growth procedure of ZnO nanostructures by large controls the formation of its intrinsic defects. The number of defects is known to be dependent on a post-growth sample treatment,18-22 which may substantially alter its properties. However, lack of control over defects stands as an obstacle for the utilization of ZnO in many practical devices. Thus, to achieve high and fast UV photodetection, one way to get rid of point defects located in the bulk and/or at the surface is 3 ACS Paragon Plus Environment
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to post-anneal the ZnO nanostructures. High temperature annealing in air results a good control over the surface related defects mainly VO and reduces surface radiative recombinations.23-24 Further surface of ZnO NRs is extensively modified after annealing in air as O2 dissociates at the VO sites filling it with one O atom and depositing another O as adatom at a five coordinate Zn2+ site, which interacts with H2O forming O-H---O and this fact changes it’s nature from hydrophobic to hydrophilic.25-27 Therefore, the interaction of H2O with ZnO not only leads to molecular adsorption on surfaces, but also to dissociation of H2O forming a number of OH species
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which influences the e-h recombination paths. This
certainly implies that surface passivation would manipulate the e-h recombination. In fact high UV photo response of ZnO nanostructure is obtained due to surface passivation via polymer capping.29-31 Both post-growth high temperature annealing in air and surface passivation are particularly very efficient to control the sub-band gap defects when ZnO 1D nanostructures are grown at low temperature and in aqueous medium. However, low temperature aqueous chemical growth (ACG) is known to be a facile, cheap and capable of large scale production. Therefore, understanding the sub band gap carrier dynamics appears essential to utilize the ACG ZnO NRs for UV and visible photodetection. The sub-band gap states can only be detected under the excitation of various lights with energy less than the band gap. In this work we have particularly focused on the surface point defects on the PC of surface passivated ZnO NRs. We report detailed sub-band gap photocarrier dynamics of the as-grown, annealed in different ambience and excess hydrated ACG ZnO NRs passivated with poly vinyl butyral (PVB). The surface passivation with PVB has been a common final step for as-grown, annealed and hydrated NR samples in order to eliminate the effect of surface adsorbed species. However, as-grown NRs without surface passivation have also been studied as a control sample. The NRs have been illuminated by each red, green and UV lights to vacate the trap levels having energies within the band gap and the subsequent
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photorcurrent transient properties have been noted. The dark conductivity of air and excess Zn annealed NRs increases respectively by two and six orders of magnitude in comparison to as-grown NR films. Interesting differences have been observed by illuminating the annealed NRs with successive red, green and UV lights as compared to the as-grown ones revealing clear influence of VZn band gap states which is not reported yet to the best of our knowledge. Compiled experimental results clearly point out to an acceptor like VZn defects playing a major role in both dark and photoconductivity of ZnO NRs film.
2. Experimental procedure 2.1 Preparation of ZnO NRs and annealing ZnO NRs were synthesized by aqueous chemical method using zinc acetate as a precursor following a method described elsewhere.32 In brief, NRs were grown on a self seeded glass substrate by dipping the substrate for an hour in a mixture of zinc acetate and
Figure 1. Schematic representation of synthesis procedure of various ZnO NRs samples. 5 ACS Paragon Plus Environment
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hexamethylenetetramine (CH2)6N4 (10 mM), the temperature of which was 80 0C and then drying in an oven at 120 0C (known as as-grown NRs). A batch of as-grown NRs was annealed at 500 0C in air ambient (known as annealed NRs). Another batch of as-grown NRs were annealed at 500 0C in air ambient and then dipped in distilled water inside a beaker at 90 0
C for 30 min for excess hydration (known as annealed and water vapor treated NRs).
Finally, the NRs of all these batches were capped with PVB solution by spin coating twice at a speed of 2500 rpm for 25 s (as per the schematic shown in the Figure 1). Therefore, asgrown (AZN), PVB coated (ABZN), annealed and PVB coated (ABCZN), annealed, water vapor treated and PVB capped NRs (ABCDZN) are the four kinds of NRs samples (Table.1) which were undertaken for investigation in this study. Table 1. Nomenclature with detailed specifications of various ZnO NRs samples
Sample name
Specifications
AZN ABZN
As-grown ZnO NRs As-grown ZnO NRs are coated with PVB
ABCZN
As-grown ZnO NRs are annealed and coated with PVB As-grown ZnO NRs are annealed, then water vapor treated and coated with PVB
ABCDZN
2.2 Characterization techniques 2.2.1 Structural and Morphological characterizations The crystalline structure of the NRs were investigated by X-ray diffractometry (XRD, Bruker, model D8) using Cu Kα radiation at 40 kV and 40 mA phase. The morphology and microstructure of the NRs were characterized using field emission scanning electron microscopy (FESEM, JEOL, model JEM2010) and high-resolution transmission electron microscopy (HRTEM, JEOL, model JEM 2010).
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2.2.2
Photoconductivity,
photoluminescence
and
time-resolved
luminescence
measurements For photoconductivity measurements, two Al metal electrodes (40 nm thickness) were deposited in circular form with a diameter of 1 mm through a shadow mask at a separation of 3 mm on the top surface of the NRs using a thermal evaporator. The photocurrents were measured by illuminating the NRs with a monochromatic light from a 300 W Xenon (model 66902) lamp fitted with a monochromator (Newport model: 74125) under 5 V bias conditions using a Keithley source meter (model 2400). The photocurrent spectrum was measured by illuminating the sample for 1 minute with a light of varying wavelength from 800 nm to 300 nm and recording the corresponding current. Prior to photocurrent measurement, the samples were brought to equilibrium by keeping them in dark for several hours. A He-Cd laser (Kimmon Koha Co., Ltd.; model KR1801C) as a 325 nm excitation source was used for the optical excitation of the sample. A high-resolution spectrometer (Horiba Jobin Yvon, Model: iHR 320) together with a photomultiplier tube was used to detect Photoluminescence (PL) emissions from the samples. The decay transients were measured following the time correlated single photon counting (TCSPC) technique using a Horiba Jobin Yvon timeresolved spectrofluorimeter with picoseconds time resolution fitted with a FluoroHub single photon counting controller and a FC-MCP-50SC MCP-PMT detection unit. The samples were excited using a 280 nm light emitting diode (pulse duration
