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GaOOH shell-core heterojunction nanorod arrays
Accepted Manuscript Low-voltage-worked photodetector based on Cu2O/GaOOH shell-core heterojunction nanorod arrays Kai Chen, Chenran He, Daoyou Guo, Shunli Wang, Zhengwei Chen, Jingqin Shen, Peigang Li, Weihua Tang PII:
S0925-8388(18)31542-1
DOI:
10.1016/j.jallcom.2018.04.219
Reference:
JALCOM 45857
To appear in:
Journal of Alloys and Compounds
Received Date: 1 February 2018 Revised Date:
18 April 2018
Accepted Date: 19 April 2018
Please cite this article as: K. Chen, C. He, D. Guo, S. Wang, Z. Chen, J. Shen, P. Li, W. Tang, Lowvoltage-worked photodetector based on Cu2O/GaOOH shell-core heterojunction nanorod arrays, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.04.219. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Low-voltage-worked photodetector based on Cu2O/GaOOH shell-core heterojunction nanorod arrays
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Kai Chena, Chenran Hea, Daoyou Guoa,b*, Shunli Wanga, Zhengwei Chenb, Jingqin Shena, Peigang Lia,b*, Weihua Tangb a
State Key Laboratory of Information Photonics and Optical Communications & Laboratory
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b
Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, China.
of Optoelectronics Materials and Devices, School of Science, Beijing University of Posts and
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Telecommunications, Beijing 100876, China.
*Corresponding author, email: [email protected], [email protected]
ABSTRACT: Cu2O/GaOOH shell-core heterojunction nanorod arrays (NRAs)
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were constructed by coating Cu2O on GaOOH NRAs through a simple and economical chemical bath deposition route. The obtained GaOOH NRAs crystalized in orthorhombic structure, with diameter range of 80-200 nm and an
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average height of 1 µm. A p-n junction constructed with a p-type narrow
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bandgap Cu2O and a n-type wide bandgap GaOOH NRAs shows a broad photoresponse region ranging from 239 nm to 570 nm. The photodetector (PD) based on Cu2O/GaOOH heterojunction exhibited a photoresponsivity (Rλ) of 6.95 A/W and an external quantum efficiency (EQE) of 2361 % under the illumination of 365 nm ultraviolet (UV) light with a light intensity of 1.4 mW/cm2 at a bias voltage of 0.5 V. What more interesting is that the PD still shown an obvious photoelectric response to 532 nm light with a very low bias voltage of 0.5
ACCEPTED MANUSCRIPT mV. Such low-voltage-worked feature of Cu2O/GaOOH PD can be attributed to the built-in electric field formed at the interface between Cu2O and GaOOH, indicating a potential application in low power devices.
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Keywords: Shell-core NRAs; Cu2O/GaOOH; Photodetector; Low-voltage-worked.
ACCEPTED MANUSCRIPT 1. Introduction As photodetectors (PDs) have been used in various fields, such as environmental and biological analysis, sensing, detection, space research and so on [1-4], improving
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the performance of PDs attracted considerable research attention in the past decades. Up to now, most PDs require high power as the driving force to separate the photogenerated electron-hole pairs, leading to the difficulty to reduce the weight of
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integrated devices, which is not helpful for environment protecting and energy saving.
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To overcome these problems, it is highly desired to develop approaches that can efficiently promote charge separation in PDs with low power or even without power [5]. The band level difference and inner electrostatic field in the heterojunction could provide the driving force for the separation of photogenerated electrons and holes,
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which are a good tactic for designing low-power-worked PDs. In addition, the heterojunction based PDs could respond to different light ranges and broaden the light
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absorption range [6].
Gallium oxide hydroxide (GaOOH), with a higher transparency and a lower
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refractive index compared to 1.8-1.9 of Ga2O3 [7, 8], is an attractive material that could be suitable for the fabrication of PDs. So far, most research works of GaOOH focused on the nanostructures including thin films [9], nanowires [10, 11], nanorods [12, 13], nanoparticles [14, 15] and nanorod arrays (NRAs) [3, 8, 16]. Among of them, NRAs have attracted much more attentions, especially the potential applications in high performance PDs due to their high specific surface area, excellent electronic transmission performance and enhancement of light dispersion [17-20]. However, the
ACCEPTED MANUSCRIPT GaOOH is severely restricted for practical application in PDs because its wide bandgap (Eg = 4.4~5.27 eV) [9] and fast electron-hole recombination nature [21]. One possible approach to improve the efficiency of PDs is to obtain appropriate band by sensitizing the GaOOH with a narrow-bandgap
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energy configuration
semiconductor, which could increase the absorption of visible light and enhance the separation of photogenerated carriers as well [22-24].
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Cuprous oxide (Cu2O), with a direct band gap (Eg) of = 1.8~2.71 eV [25-27] and
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the characteristics of low cost, abundant reserves, good stability, and environmental friendliness, is an ideal material for fabricating p-n junction with GaOOH [26, 28]. The narrow band gap of Cu2O can also enhance the capability of light collection of the fabricated junction [26]. The oriented nanostructures have been approved highly
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helpful to facilitate the separation and transfer of photogenerated carriers due to large interface area in heterojunctions.
Herein, the Cu2O/GaOOH heterojunctions were synthesized by a facile
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chemical bath deposition route to form Cu2O shells onto GaOOH NRAs cores. The
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deposition of cuprous oxide effectively extends the absorption of GaOOH NRAs to the visible light range. The heterojunction-based PDs show a significantly photoresponse to UV and visible light illumination even with an ultra-low bias. Such high sensitive and low power consuming device has promising applications in optoelectronic devices.
2. Experimental
ACCEPTED MANUSCRIPT 2.1. Materials Ethanolamine (C2H7NO, 99 %), gallium isopropoxide (C9H21GaO3, 99 %), ethylene glycol monomethyl ether (C3H8O2, 99 %), copper nitrate [Cu(NO3)2·xH2O,
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99 %], gallium nitrate aqueous solution [Ga(NO3)3·9H2O, 10 %] were purchased from Shanghai Saen Chemical Technology Co., Ltd. Triethanolamine (C6H15NO3, 78 %) was obtained from Hangzhou Gaojing Fine Chemical Industry Co., Ltd. Hydrazine
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hydrate (N2H4·H2O, 80 %) was got from Tianjing Yongda Chemical Regent Co., Ltd.
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A fluorine doped tin oxide (FTO) with 350 nm conductive layer is used as a substrate. FTO conductive glass (14 Ω·cm-2, size: 10 × 20 × 2.2 mm3) was bought from Japan Nippon Sheet Glass Co., Ltd. All chemicals are analytical grade. 2.2. Preparation of GaOOH NRAs
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FTO substrates were ultrasonically cleaned in acetone, alcohol, and deionized water sequentially. The ordered GaOOH NRAs were prepared by the hydrothermal growth method on the seed layer. 0.1 M (18 µL) ethanolamine and 0.1 M (0.0741 g)
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gallium isopropoxide were first dissolved in 2.98 mL ethylene glycol monomethyl
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ether solution, and the above mixture solution was stirred continuously in 60 ℃ water bath for 1 h. The seed layer was acquired by spin coating ethylene glycol monomethyl ether solution of ethanolamine and gallium isopropoxide onto the FTO substrate at 3000 rpm for 15 s, and subsequently annealed at 450 ℃ in air for 30 min. In the hydrothermal process, 0.30 g Ga(NO3)3·9H2O was dissolved in 30 mL DI water to form a solution with a concentration of 0.0239 M. The substrate coated with
ACCEPTED MANUSCRIPT Ga2O3 seed layer was placed in the growth solution of Ga(NO3)3·9H2O and heated at 150 ℃ for 12 h in an oven. After the growth, the product was washed by DI water, dried in air at 80 ℃.
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2.3. Preparation of Cu2O/GaOOH shell-core NRAs 0.1 M (0.9664 g) Cu(NO3)2 and 1 M (6.796 mL) triethanolamine were dissolved by ultrasound for 5 min in 40 mL DI water. The FTO substrates covered with GaOOH
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NRAs were leaned on the wall of the beaker with the above mixture solution inside,
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and followed by adding 0.05 M (121.27 µL) hydrazine hydrate to form a flocculent suspension drop by drop. After ageing for 3 h, Cu2O/GaOOH shell-core NRAs were obtained.
The formation process of Cu2O/GaOOH shell-core NRAs is shown in Scheme 1.
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First, Ga2O3 seed layer was acquired by a spin coating method. GaOOH NRAs were directly grown on a Ga2O3 seed layer via hydrothermal synthesis in a solution of Ga(NO3)3·9H2O. Then, the Cu2O shell thin film layer was grown on GaOOH NRAs
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to form Cu2O/GaOOH shell-core NRAs via a chemical precipitation method in mixed
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solution of Cu(NO3)2, hydrazine hydrate and triethanolamine. 2.4. Characterization
The morphologies of GaOOH NRAs and Cu2O/GaOOH NRAs were observed by
a Hitachi S-4800 field-emission scanning electron microscope (SEM) and a JEOL JEM-2100 transmission electron microscopy (TEM). The crystal structure of samples was analyzed by a Bruker D8 Advance X-ray diffractometer (XRD). The ultraviolet-visible (UV-vis) absorption spectrum was taken using a Hitachi U-3900
ACCEPTED MANUSCRIPT UV-vis spectrophotometer. The surface composition of samples was characterized by a Thermo Scientific K-Alpha X-ray photoelectron spectroscopy (XPS). For transport measurement, a small point electrode (∼0.02 mm diameter) was fabricated
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by silver pulp on the top of Cu2O/GaOOH NRAs as the positive electrode, and FTO conductive glass worked as the negative electrode, as shown in Scheme 1. The optoelectronic property of the Cu2O/GaOOH NRAs based PDs were investigated by
3. Results and discussion
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lamp as the 254 nm and 365 nm light sources.
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an I–V semiconductor characterization system (Keithley-4200) equipped with a 7 W
3.1. Morphology and crystal structure of Cu2O/GaOOH shell-core NRAs
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The SEM result shows that a large-area, highly dense, and vertically aligned GaOOH NRAs have been successfully grown on the FTO glass substrate [Fig. 1(a-f)]. Fig. 1(a) displays a representative top-view SEM micrograph of as-synthesized
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GaOOH nanorods. Fig. 1(c) is a magnified image of Fig. 1(a). Fig. 1(b) is a top view
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SEM image in the boundaries of NRAs, and the inset of Fig. 1(b) is the bright-field TEM image of a single GaOOH nanorod. The nanorods have diameters ranging from 80 to 200 nm estimated from the Fig. 1(b) and (c). The tips of the nanorods reveal rhombus-shaped cross sections in Fig. 1(c), originating from its orthorhombic crystal symmetry, which is consistent with the XRD observation in Fig. 2(a). Figure 1(d) shows the surface mophorlogy of the GaOOH NRAs coated by Cu2O. Figure 1(e) shows the cross-section image of GaOOH NRAs on FTO substrate, we can estimate
ACCEPTED MANUSCRIPT that the average length of nanorods is ~1 µm. Figure 1 (f) is the cross-section image of Cu2O/GaOOH shell-core NRAs. Fig. 2(a) shows the XRD patterns of the FTO, FTO/GaOOH and
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FTO/GaOOH/Cu2O, respectively. In addition to the diffraction peak of the FTO substrate, four peaks located at 35.3°, 37.4°, 62.3° and 66.7° were observed in FTO/GaOOH, which can be indexed to (021), (111), (002) and (070) crystal planes of
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orthorhombic GaOOH (JCPDS file No.06-0180). The NRAs show a preferred growth
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orientation of [111] direction [15, 29, 30]. After the depositing of Cu2O shell thin film layers, three additional peaks located at 29.5°, 36.4°, 42.2° were observed, which can be ascribed to (110), (111) and (200) planes of cubic Cu2O (JCPDS file No.05-0667). The characteristic peaks of Cu and CuO are not observed for all the samples,
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suggesting that no metallic copper or CuO formed in the chemical bath deposition process.
3.2. Optical properties of Cu2O/GaOOH shell-core NRAs UV-vis
absorbance
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The
spectra
of
the
FTO/GaOOH
NRAs
and
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FTO/GaOOH/Cu2O NRAs are shown in Fig. 2(b). It shows that the absorption onset of FTO/GaOOH NRAs is at ~300 nm. After the deposition of Cu2O thin film shell layer, the absorption edge of the FTO/GaOOH/Cu2O NRAs shows an obvious redshift, meaning that the introduction of Cu2O thin film in GaOOH NRAs extends the absorption range to the visible light range. The optical bandgaps of FTO/GaOOH NRAs and FTO/GaOOH/Cu2O can be determined based on the equation: (αhν)2 = A(hν- Eg). The energy bandgap (Eg) is
ACCEPTED MANUSCRIPT measured by linear extrapolation to the hv-axis. The (αhν)2 versus hv curve of the FTO GaOOH/Cu2O NRAs is shown in the inset of Fig. 2(b), the band-gap of FTO/GaOOH/Cu2O NRAs was estimated to be ~2.50 eV, which is almost identical
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with the pure Cu2O [27]. The UV-vis absorbance spectra of GaOOH nanorod powder was shown in Fig. 2(c), and the estimated bandgap of GaOOH is ~5.18 eV.
XPS is a useful tool to investigate the surface chemical compositions and the
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valence states of materials. Fig. 2(d) is the high-resolution XPS spectra of
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GaOOH/Cu2O NRAs. The peaks located at 932.2 and 952.0 eV can be ascribed to those of Cu 2p3/2 and Cu 2p1/2 from Cu2O, respectively. The characteristic peaks for Cu2+ at 934.5 eV (2p3/2) and 953.4(2p1/2) were not observed [20, 22, 31, 32], indicating that no CuO was mixed in NRAs. Fig. 2(e) shows the enlarge view of the
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XPS valence-band spectra of GaOOH at the range of 1~4.5 eV. By the linear extrapolation of the slope of the peak, the VBM of pure GaOOH NRAs was determined to be at 2.88 eV. According to the literature, the VBM of Cu2O is 0.1 eV
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[33]. Combined with the optical bandgap shown in Fig. 2(b) and (c), the band
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structure diagram of Cu2O/GaOOH heterojunction can be schematically drawn [Fig. 2(f)]. When Cu2O contacts GaOOH, the electrons in n-GaOOH would move to the p-Cu2O, and then the energy level near the n-type GaOOH will bend upward, causing the formation of built-in electric field at the interface of Cu2O/GaOOH. Under the light irradiation, the electron-hole pairs will be excited and separated by the built-in electric field [34]. After applied a fixed bias, the photogenerated electrons and holes can be separated rapidly and then transfer to the corresponding electrodes.
ACCEPTED MANUSCRIPT 3.3. Photoelectronic characteristic of Cu2O/GaOOH shell-core NRAs The I-V curves of the Ag-Cu2O/GaOOH-FTO heterojunction are plotted in Fig. 3(a), which shows a quasi-Ohmic behavior due to the low energy barrier between
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Cu2O and GaOOH. The I-V curves under 254 nm light, 365 nm light and 532 nm light illuminations show that the resistances under light were obvious lower than that under dark, indicating that the PD is sensitive to the spectrum range from 254 nm to 532 nm.
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In order to explore the photoresponse behaviours of the PD, the I-t curves of the
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device measured under light illumination with a various wavelength are shown in Fig. 3(b). The dark current is approximately 23.5 µA under the bias of 0.5 V. Under 254 nm light (light intensity of 1400 µW/cm2), 365 nm light (light intensity of 1400 µW/cm2) and 532 nm light illumination, the currents rapidly increase to the values of
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approximately 23.65 µA, 24.52 µA, and 49.19 µA respectively. When the UV light turns off, the current will decrease down to the original value. The Ion/Ioff ratios are calculated to be about 1.01, 1.04 and 2.09 for 254 nm, 365 nm and 532 nm light
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respectively. After several circles, the I-t curves exhibit a nearly identical
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photoresponse behavior, indicating the stability and reproducibility of the device under the light illumination. Photoresponsivity (Rλ) and external quantum efficiency (EQE) are two important
parameters to evaluate the sensitivity of PDs. Rλ is defined as the photocurrent generated by per unit power of incident light on the effective area of a PD and EQE is bound up with the number of electron-hole pairs excited by a PD per adsorbed photon and per unit time. Rλ and EQE can be expressed in the following equations:
ACCEPTED MANUSCRIPT Rλ=∆Iλ/(PλS)
(1)
EQE=hcRλ/(eλ)
(2)
where ∆Iλ = Iλ- Idark is the difference between photocurrent and dark current, Pλ is the
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incident light intensity, S is the effective illuminated area, h is the Planck’s constant, c is the velocity of light, e is the electron charge, and λ is the incident light wavelength. For the 254 nm light illumination, Pλ = 1400 µW/cm2, S = 1.13×10-4 cm2, ∆Iλ =
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0.17×10-6A, then Rλ and EQE can be estimated to be about 1.07 A/W and 522 %,
6.95 A/W and 2361 %, respectively.
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respectively. While for the 365 nm light, ∆Iλ = 1.1×10-6A, the Rλ and EQE are about
To study the effect of light intensity on the photoresponse properties of the PDs [35], the measurement under 365 nm UV light illumination with the intensity ranging
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from 200 to 2400 µW/cm2 was performed [Fig. 3(c)]. The photocurrent as a function of the light intensity is presented in Fig. 3(d). Notably, the photocurrent increases linearly with the increase of the light intensity, because a higher light intensity would
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photocurrent.
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excite more pairs of the photogenerated electron-hole, resulting in a higher
Fig. 4(a) plots the I-t curves of the Cu2O/GaOOH heterojunction under 532 nm
light illumination at different bias voltages in the range from 0.00005 to 0.5 V. It is clear that the dark current and the photocurrent both increase with the increase of the bias voltage. When a bias higher than the built-in electric field applied, the space charge region would become narrow and the multiplicative diffusion motion would increase, leading to a larger diffusion current. In a certain range, the stronger the
ACCEPTED MANUSCRIPT external electric field applied, the greater the forward dark current obtained. This evolution of the photocurrent is attributed to that a higher bias can promote more photogenerated electron-hole pairs separation and suppress them recombination.
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Interestingly, the Cu2O/GaOOH PD also has an obvious photoelectric response to 532 nm light at a bias voltage of 0.5 mV, exhibiting a low-voltage-worked characteristic.
For a more detailed comparison of the response time of the Cu2O/GaOOH
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heterojunction, the quantitative analysis of the process of the current rise and decay
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process involves the fitting of the photoresponse curve with a bi-exponential relaxation equation of the following type.
I=I0+Ae-t/τ1+Be-t/τ2
(3)
Where I0 is the steady state photocurrent, t is the time, A and B are constant, τ1 and
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τ2 are two relaxation time constants. As is shown in Fig. 4(b), the photoresponse processes are well fitted. τr and τd are the time constants for the rising edge and fall edge, respectively. We note that the rise time constants τr1 and τr2 are estimated to be
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0.38 s and 3.05 s under 532 nm light illumination with a bias of 0.5 V, and the decay
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time constants τd1 and τd2 are 0.24 s and 1.79 s. The photoresponse of semiconductors is a complicate process of electron-hole pair separation, trapping/releasing, and recombination. Generally, the fast-response components (τr1, τd1) can be attributed to the rapid change of carrier concentration as soon as the light turns on or off, while the slow-response components (τr2, τd2) are caused by the carrier trapping/releasing, owing to the existence of defects in GaOOH NRAs, such as oxygen vacancies. For comparison, we list the photoresponse parameters of Cu2O/GaOOH
ACCEPTED MANUSCRIPT heterojunction PD and other types of devices reported in the literature in Table 1. It can be seen that the properties of our device including Rλ and EQE are better than that of other devices based on Ga2O3 nanowire [36], IZO/β-Ga2O3/IZO M-S-M structure
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[37], Ga2O3/SnO2:Ga core-shell nanowire [38]. Compared with the ZnO/Ga2O3 core-shell microwire based device, the nanorods shell-core structure of Cu2O/GaOOH based device shows a characteristic of the lower working voltage [39]. The
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Cu2O/GaOOH heterojunction PD with a simple structure renders relatively high
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performance, wide absorption region and low fabrication cost, promising building high performance optoelectronic devices with a low-voltage-worked characteristic in
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the future.
ACCEPTED MANUSCRIPT 4. Conclusions In conclusion, a heterojunction based PD was fabricated by simply deposited a layer of Cu2O thin film on GaOOH NRAs. The fabricated Cu2O/GaOOH
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heterojunction based PD was sensitive to 254 nm, 365 nm and 532 nm light illumination with a high stability and reproducibility, and shown a good photoresponse properties, such as response time of 0.38 s, recovery time of 0.24 s
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(under 532 nm light illumination), Rλ of 6.95 A/W and EQE of 2361 % at 0.5 V
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applied bias (under the 365 nm illumination with intensity of 1400 µW/cm2). Moreover, with a low bias voltage of 0.5 mV, the PD still shows an obvious photoresponse to 532 nm light, which is important for designing low power device. Based on the above results, the low-voltage-worked PD based on Cu2O/GaOOH
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shell-core NRAs has numerous potential applications in UV and visible detection.
ACCEPTED MANUSCRIPT Acknowledgements The project was supported by Open Fund of IPOC (BUPT), the National Natural Science Foundation of China (No. 61704153, 51572241, 61774019 and 51572033),
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the Scientific Research Project for the Education Department of Zhejiang Province (No. Y201738294), Science and Technology Department of Zhejiang Province
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Foundation (No. 2017C37017).
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solar-blind
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β-Ga2O3 ultraviolet
transparent graphene electrodes, ACS Photonics 5 (2018) 1123-1128. [25] A.H. Jayatissa, P. Samarasekara, G. Kun, Methane gas sensor application of
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cuprous oxide synthesized by thermal oxidation, Phys. Status Solidi A 206 (2009)
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[26] H. Xu, J. Dong, C. Chen, One-step chemical bath deposition and photocatalytic
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activity of Cu2O thin films with orientation and size controlled by a chelating agent, Mater. Chem. Phys. 143 (2014) 713-719.
[27] P.B. Ahirrao, B.R. Sankapal, R.S. Patil, Nanocrystalline p-type-cuprous oxide thin films by room temperature chemical bath deposition method, J. Alloys Compd. 509 (2011) 5551-5554.
ACCEPTED MANUSCRIPT [28] X. Zou, H. Fan, Y. Tian, M. Zhang, X. Yan, Chemical bath deposition of Cu2O quantum dots onto ZnO nanorod arrays for application in photovoltaic devices, RSC Adv. 5 (2015) 23401-23409.
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[29] G. Sinha, K. Adhikary, S. Chaudhuri, Optical properties of nanocrystalline α-GaO(OH) thin films, J. Phys.: Condens. Matter 18 (2006) 2409-2415.
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[30] L.S. Reddy, Y.H. Ko, J.S. Yu, Hydrothermal synthesis and photocatalytic property of beta-Ga2O3 nanorods, Nanoscale Res. Lett. 10 (2015) 1-7.
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[31] Y. Yang, J. Han, X. Ning, W. Cao, W. Xu, L. Guo, Controllable morphology and conductivity of electrodeposited Cu2O thin film: effect of surfactants, ACS Appl. Mater. Interfaces 6 (2014) 22534-22543.
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[32] B.G. Ganga, S.M. Seetharaman, P.C.R. Varma, M.A.G. Namboothiry, P.N. Santhosh, Photovoltaic properties of low temperature solution processed earth abundant CuO nanocrystal-based hybrid solar cells, Phys. Status Solidi A 214
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[33] K. Eom, S. Kim, D. Lee, H. Seo, Physicochemical interface effect in Cu2O-ZnO heterojunction on photocurrent spectrum, RSC Adv. 5 (2015) 103803-103810.
[34] D.Y. Guo, Z.P. Wu, Y.H. An, X.C. Guo, X.L. Chu, C.L. Sun, L.H. Li, P.G. Li, W.H. Tang, Oxygen vacancy tuned ohmic-schottky conversion for enhanced performance in β-Ga2O3 solar-blind ultraviolet photodetectors, Appl. Phys. Lett. 105 (2014) 1-5.
ACCEPTED MANUSCRIPT [35] D.Y. Guo, H.Z. Shi, Y.P. Qian, M. Lv, P.G. Li, Y.L. Su, Q. Liu, K. Chen, S.L. Wang, C. Cui, C.R. Li, W.H. Tang, Fabrication of β-Ga2O3/ZnO heterojunction for solar-blind deep ultraviolet photodetection, Semicond. Sci. Technol. 32 (2017)
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1-6. [36] Y.L. Wu, S.J. Chang, W.Y. Weng, C.H. Liu, T.Y. Tsai, C.L. Hsu, K.C. Chen, Ga2O3 nanowire photodetector prepared on SiO2/Si template, IEEE Sens. J. 13
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(2013) 2368-2373.
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[37] T.C. Wei, D.S. Tsai, P. Ravadgar, J.J. Ke, See-through Ga2O3 solar-blind photodetectors for use in harsh environments, IEEE J. Sel. Top. Quantum Electron. 20 (2014) 1-6.
[38] C.L. Hsu, Y.C. Lu, Fabrication of a transparent ultraviolet detector by using
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n-type Ga2O3 and p-type Ga-doped SnO2 core-shell nanowires, Nanoscale 4 (2012) 5710-5717.
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[39] B. Zhao, F. Wang, H. Chen, Y. Wang, M. Jiang, X. Fang, D. Zhao, Solar-blind
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avalanche photodetector based on single ZnO-Ga2O3 core-shell microwire, Nano Lett. 15 (2015) 3988-3993.
ACCEPTED MANUSCRIPT Figure and Table Captions Scheme 1. Schematic illustration of preparation of the Cu2O/GaOOH shell-core NRAs
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Fig. 1. The top views of SEM images at low (a) and high (c) magnification of GaOOH NRAs grown on the FTO substrate. (b) The edge view of GaOOH NRAs, inset shows the bright-field TEM image of a single GaOOH nanorod. (d)
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The top views of Cu2O/GaOOH shell-core NRAs at high magnification, the
Cu2O/GaOOH NRAs (f).
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inset is an enlarged view. The cross-section of GaOOH NRAs (e) and
Fig. 2. (a) XRD patterns of the FTO, FTO/GaOOH and FTO/GaOOH/Cu2O. (b) UV-vis absorption spectra of the FTO/GaOOH NRAs and FTO/GaOOH/Cu2O
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NRAs, inset is the plot of (αhν)2 versus the energy of light (hν). (c) UV-vis absorption spectra of the GaOOH powder. (d) High-resolution XPS spectra of Cu 2p for Cu2O. (e) The enlarge view of the XPS valence-band spectra of
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GaOOH at 1~4.5 eV. (f) The band structure alignment of Cu2O/GaOOH
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heterojunction.
Fig. 3. (a) I-V curves and (b) I-t curves of the Cu2O/GaOOH shell-core NRAs in dark and under different light illumination. (c) Photoresponse of the device under 365 nm light with various intensities. (d) The relationship between the light intensity and photocurrent. Fig. 4. (a) Current of the Cu2O/GaOOH shell-core NRAs under 532 nm light illumination at bias voltages of 0.00005, 0.0005, 0.005, 0.05, and 0.5 V. (b) The
ACCEPTED MANUSCRIPT enlarged view of the rise/decay edges and the corresponding exponential fitting of the Cu2O/GaOOH heterojunction PD measured with a bias of 0.5 V under 532 nm light illumination.
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Table 1. Comparison of the device parameters of the present Cu2O/GaOOH
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heterojunction PD and other Ga2O3 nanostructures based devices.
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Scheme 1
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Table 1
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Highlights 1. The Cu2O/GaOOH shell-core heterojunction nanorod arrays were
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constructed. 2. A built-in electric field was formed at the interface of Cu2O and GaOOH.
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low-voltage-worked feature.
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3. The photodetector based on Cu2O/GaOOH heterojunction shows a
S0925-8388(18)31542-1
DOI:
10.1016/j.jallcom.2018.04.219
Reference:
JALCOM 45857
To appear in:
Journal of Alloys and Compounds
Received Date: 1 February 2018 Revised Date:
18 April 2018
Accepted Date: 19 April 2018
Please cite this article as: K. Chen, C. He, D. Guo, S. Wang, Z. Chen, J. Shen, P. Li, W. Tang, Lowvoltage-worked photodetector based on Cu2O/GaOOH shell-core heterojunction nanorod arrays, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.04.219. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Low-voltage-worked photodetector based on Cu2O/GaOOH shell-core heterojunction nanorod arrays
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Kai Chena, Chenran Hea, Daoyou Guoa,b*, Shunli Wanga, Zhengwei Chenb, Jingqin Shena, Peigang Lia,b*, Weihua Tangb a
State Key Laboratory of Information Photonics and Optical Communications & Laboratory
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b
Department of Physics, Zhejiang Sci-Tech University, Hangzhou 310018, China.
of Optoelectronics Materials and Devices, School of Science, Beijing University of Posts and
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Telecommunications, Beijing 100876, China.
*Corresponding author, email: [email protected], [email protected]
ABSTRACT: Cu2O/GaOOH shell-core heterojunction nanorod arrays (NRAs)
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were constructed by coating Cu2O on GaOOH NRAs through a simple and economical chemical bath deposition route. The obtained GaOOH NRAs crystalized in orthorhombic structure, with diameter range of 80-200 nm and an
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average height of 1 µm. A p-n junction constructed with a p-type narrow
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bandgap Cu2O and a n-type wide bandgap GaOOH NRAs shows a broad photoresponse region ranging from 239 nm to 570 nm. The photodetector (PD) based on Cu2O/GaOOH heterojunction exhibited a photoresponsivity (Rλ) of 6.95 A/W and an external quantum efficiency (EQE) of 2361 % under the illumination of 365 nm ultraviolet (UV) light with a light intensity of 1.4 mW/cm2 at a bias voltage of 0.5 V. What more interesting is that the PD still shown an obvious photoelectric response to 532 nm light with a very low bias voltage of 0.5
ACCEPTED MANUSCRIPT mV. Such low-voltage-worked feature of Cu2O/GaOOH PD can be attributed to the built-in electric field formed at the interface between Cu2O and GaOOH, indicating a potential application in low power devices.
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Keywords: Shell-core NRAs; Cu2O/GaOOH; Photodetector; Low-voltage-worked.
ACCEPTED MANUSCRIPT 1. Introduction As photodetectors (PDs) have been used in various fields, such as environmental and biological analysis, sensing, detection, space research and so on [1-4], improving
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the performance of PDs attracted considerable research attention in the past decades. Up to now, most PDs require high power as the driving force to separate the photogenerated electron-hole pairs, leading to the difficulty to reduce the weight of
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integrated devices, which is not helpful for environment protecting and energy saving.
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To overcome these problems, it is highly desired to develop approaches that can efficiently promote charge separation in PDs with low power or even without power [5]. The band level difference and inner electrostatic field in the heterojunction could provide the driving force for the separation of photogenerated electrons and holes,
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which are a good tactic for designing low-power-worked PDs. In addition, the heterojunction based PDs could respond to different light ranges and broaden the light
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absorption range [6].
Gallium oxide hydroxide (GaOOH), with a higher transparency and a lower
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refractive index compared to 1.8-1.9 of Ga2O3 [7, 8], is an attractive material that could be suitable for the fabrication of PDs. So far, most research works of GaOOH focused on the nanostructures including thin films [9], nanowires [10, 11], nanorods [12, 13], nanoparticles [14, 15] and nanorod arrays (NRAs) [3, 8, 16]. Among of them, NRAs have attracted much more attentions, especially the potential applications in high performance PDs due to their high specific surface area, excellent electronic transmission performance and enhancement of light dispersion [17-20]. However, the
ACCEPTED MANUSCRIPT GaOOH is severely restricted for practical application in PDs because its wide bandgap (Eg = 4.4~5.27 eV) [9] and fast electron-hole recombination nature [21]. One possible approach to improve the efficiency of PDs is to obtain appropriate band by sensitizing the GaOOH with a narrow-bandgap
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energy configuration
semiconductor, which could increase the absorption of visible light and enhance the separation of photogenerated carriers as well [22-24].
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Cuprous oxide (Cu2O), with a direct band gap (Eg) of = 1.8~2.71 eV [25-27] and
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the characteristics of low cost, abundant reserves, good stability, and environmental friendliness, is an ideal material for fabricating p-n junction with GaOOH [26, 28]. The narrow band gap of Cu2O can also enhance the capability of light collection of the fabricated junction [26]. The oriented nanostructures have been approved highly
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helpful to facilitate the separation and transfer of photogenerated carriers due to large interface area in heterojunctions.
Herein, the Cu2O/GaOOH heterojunctions were synthesized by a facile
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chemical bath deposition route to form Cu2O shells onto GaOOH NRAs cores. The
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deposition of cuprous oxide effectively extends the absorption of GaOOH NRAs to the visible light range. The heterojunction-based PDs show a significantly photoresponse to UV and visible light illumination even with an ultra-low bias. Such high sensitive and low power consuming device has promising applications in optoelectronic devices.
2. Experimental
ACCEPTED MANUSCRIPT 2.1. Materials Ethanolamine (C2H7NO, 99 %), gallium isopropoxide (C9H21GaO3, 99 %), ethylene glycol monomethyl ether (C3H8O2, 99 %), copper nitrate [Cu(NO3)2·xH2O,
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99 %], gallium nitrate aqueous solution [Ga(NO3)3·9H2O, 10 %] were purchased from Shanghai Saen Chemical Technology Co., Ltd. Triethanolamine (C6H15NO3, 78 %) was obtained from Hangzhou Gaojing Fine Chemical Industry Co., Ltd. Hydrazine
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hydrate (N2H4·H2O, 80 %) was got from Tianjing Yongda Chemical Regent Co., Ltd.
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A fluorine doped tin oxide (FTO) with 350 nm conductive layer is used as a substrate. FTO conductive glass (14 Ω·cm-2, size: 10 × 20 × 2.2 mm3) was bought from Japan Nippon Sheet Glass Co., Ltd. All chemicals are analytical grade. 2.2. Preparation of GaOOH NRAs
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FTO substrates were ultrasonically cleaned in acetone, alcohol, and deionized water sequentially. The ordered GaOOH NRAs were prepared by the hydrothermal growth method on the seed layer. 0.1 M (18 µL) ethanolamine and 0.1 M (0.0741 g)
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gallium isopropoxide were first dissolved in 2.98 mL ethylene glycol monomethyl
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ether solution, and the above mixture solution was stirred continuously in 60 ℃ water bath for 1 h. The seed layer was acquired by spin coating ethylene glycol monomethyl ether solution of ethanolamine and gallium isopropoxide onto the FTO substrate at 3000 rpm for 15 s, and subsequently annealed at 450 ℃ in air for 30 min. In the hydrothermal process, 0.30 g Ga(NO3)3·9H2O was dissolved in 30 mL DI water to form a solution with a concentration of 0.0239 M. The substrate coated with
ACCEPTED MANUSCRIPT Ga2O3 seed layer was placed in the growth solution of Ga(NO3)3·9H2O and heated at 150 ℃ for 12 h in an oven. After the growth, the product was washed by DI water, dried in air at 80 ℃.
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2.3. Preparation of Cu2O/GaOOH shell-core NRAs 0.1 M (0.9664 g) Cu(NO3)2 and 1 M (6.796 mL) triethanolamine were dissolved by ultrasound for 5 min in 40 mL DI water. The FTO substrates covered with GaOOH
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NRAs were leaned on the wall of the beaker with the above mixture solution inside,
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and followed by adding 0.05 M (121.27 µL) hydrazine hydrate to form a flocculent suspension drop by drop. After ageing for 3 h, Cu2O/GaOOH shell-core NRAs were obtained.
The formation process of Cu2O/GaOOH shell-core NRAs is shown in Scheme 1.
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First, Ga2O3 seed layer was acquired by a spin coating method. GaOOH NRAs were directly grown on a Ga2O3 seed layer via hydrothermal synthesis in a solution of Ga(NO3)3·9H2O. Then, the Cu2O shell thin film layer was grown on GaOOH NRAs
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to form Cu2O/GaOOH shell-core NRAs via a chemical precipitation method in mixed
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solution of Cu(NO3)2, hydrazine hydrate and triethanolamine. 2.4. Characterization
The morphologies of GaOOH NRAs and Cu2O/GaOOH NRAs were observed by
a Hitachi S-4800 field-emission scanning electron microscope (SEM) and a JEOL JEM-2100 transmission electron microscopy (TEM). The crystal structure of samples was analyzed by a Bruker D8 Advance X-ray diffractometer (XRD). The ultraviolet-visible (UV-vis) absorption spectrum was taken using a Hitachi U-3900
ACCEPTED MANUSCRIPT UV-vis spectrophotometer. The surface composition of samples was characterized by a Thermo Scientific K-Alpha X-ray photoelectron spectroscopy (XPS). For transport measurement, a small point electrode (∼0.02 mm diameter) was fabricated
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by silver pulp on the top of Cu2O/GaOOH NRAs as the positive electrode, and FTO conductive glass worked as the negative electrode, as shown in Scheme 1. The optoelectronic property of the Cu2O/GaOOH NRAs based PDs were investigated by
3. Results and discussion
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lamp as the 254 nm and 365 nm light sources.
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an I–V semiconductor characterization system (Keithley-4200) equipped with a 7 W
3.1. Morphology and crystal structure of Cu2O/GaOOH shell-core NRAs
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The SEM result shows that a large-area, highly dense, and vertically aligned GaOOH NRAs have been successfully grown on the FTO glass substrate [Fig. 1(a-f)]. Fig. 1(a) displays a representative top-view SEM micrograph of as-synthesized
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GaOOH nanorods. Fig. 1(c) is a magnified image of Fig. 1(a). Fig. 1(b) is a top view
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SEM image in the boundaries of NRAs, and the inset of Fig. 1(b) is the bright-field TEM image of a single GaOOH nanorod. The nanorods have diameters ranging from 80 to 200 nm estimated from the Fig. 1(b) and (c). The tips of the nanorods reveal rhombus-shaped cross sections in Fig. 1(c), originating from its orthorhombic crystal symmetry, which is consistent with the XRD observation in Fig. 2(a). Figure 1(d) shows the surface mophorlogy of the GaOOH NRAs coated by Cu2O. Figure 1(e) shows the cross-section image of GaOOH NRAs on FTO substrate, we can estimate
ACCEPTED MANUSCRIPT that the average length of nanorods is ~1 µm. Figure 1 (f) is the cross-section image of Cu2O/GaOOH shell-core NRAs. Fig. 2(a) shows the XRD patterns of the FTO, FTO/GaOOH and
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FTO/GaOOH/Cu2O, respectively. In addition to the diffraction peak of the FTO substrate, four peaks located at 35.3°, 37.4°, 62.3° and 66.7° were observed in FTO/GaOOH, which can be indexed to (021), (111), (002) and (070) crystal planes of
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orthorhombic GaOOH (JCPDS file No.06-0180). The NRAs show a preferred growth
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orientation of [111] direction [15, 29, 30]. After the depositing of Cu2O shell thin film layers, three additional peaks located at 29.5°, 36.4°, 42.2° were observed, which can be ascribed to (110), (111) and (200) planes of cubic Cu2O (JCPDS file No.05-0667). The characteristic peaks of Cu and CuO are not observed for all the samples,
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suggesting that no metallic copper or CuO formed in the chemical bath deposition process.
3.2. Optical properties of Cu2O/GaOOH shell-core NRAs UV-vis
absorbance
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The
spectra
of
the
FTO/GaOOH
NRAs
and
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FTO/GaOOH/Cu2O NRAs are shown in Fig. 2(b). It shows that the absorption onset of FTO/GaOOH NRAs is at ~300 nm. After the deposition of Cu2O thin film shell layer, the absorption edge of the FTO/GaOOH/Cu2O NRAs shows an obvious redshift, meaning that the introduction of Cu2O thin film in GaOOH NRAs extends the absorption range to the visible light range. The optical bandgaps of FTO/GaOOH NRAs and FTO/GaOOH/Cu2O can be determined based on the equation: (αhν)2 = A(hν- Eg). The energy bandgap (Eg) is
ACCEPTED MANUSCRIPT measured by linear extrapolation to the hv-axis. The (αhν)2 versus hv curve of the FTO GaOOH/Cu2O NRAs is shown in the inset of Fig. 2(b), the band-gap of FTO/GaOOH/Cu2O NRAs was estimated to be ~2.50 eV, which is almost identical
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with the pure Cu2O [27]. The UV-vis absorbance spectra of GaOOH nanorod powder was shown in Fig. 2(c), and the estimated bandgap of GaOOH is ~5.18 eV.
XPS is a useful tool to investigate the surface chemical compositions and the
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valence states of materials. Fig. 2(d) is the high-resolution XPS spectra of
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GaOOH/Cu2O NRAs. The peaks located at 932.2 and 952.0 eV can be ascribed to those of Cu 2p3/2 and Cu 2p1/2 from Cu2O, respectively. The characteristic peaks for Cu2+ at 934.5 eV (2p3/2) and 953.4(2p1/2) were not observed [20, 22, 31, 32], indicating that no CuO was mixed in NRAs. Fig. 2(e) shows the enlarge view of the
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XPS valence-band spectra of GaOOH at the range of 1~4.5 eV. By the linear extrapolation of the slope of the peak, the VBM of pure GaOOH NRAs was determined to be at 2.88 eV. According to the literature, the VBM of Cu2O is 0.1 eV
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[33]. Combined with the optical bandgap shown in Fig. 2(b) and (c), the band
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structure diagram of Cu2O/GaOOH heterojunction can be schematically drawn [Fig. 2(f)]. When Cu2O contacts GaOOH, the electrons in n-GaOOH would move to the p-Cu2O, and then the energy level near the n-type GaOOH will bend upward, causing the formation of built-in electric field at the interface of Cu2O/GaOOH. Under the light irradiation, the electron-hole pairs will be excited and separated by the built-in electric field [34]. After applied a fixed bias, the photogenerated electrons and holes can be separated rapidly and then transfer to the corresponding electrodes.
ACCEPTED MANUSCRIPT 3.3. Photoelectronic characteristic of Cu2O/GaOOH shell-core NRAs The I-V curves of the Ag-Cu2O/GaOOH-FTO heterojunction are plotted in Fig. 3(a), which shows a quasi-Ohmic behavior due to the low energy barrier between
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Cu2O and GaOOH. The I-V curves under 254 nm light, 365 nm light and 532 nm light illuminations show that the resistances under light were obvious lower than that under dark, indicating that the PD is sensitive to the spectrum range from 254 nm to 532 nm.
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In order to explore the photoresponse behaviours of the PD, the I-t curves of the
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device measured under light illumination with a various wavelength are shown in Fig. 3(b). The dark current is approximately 23.5 µA under the bias of 0.5 V. Under 254 nm light (light intensity of 1400 µW/cm2), 365 nm light (light intensity of 1400 µW/cm2) and 532 nm light illumination, the currents rapidly increase to the values of
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approximately 23.65 µA, 24.52 µA, and 49.19 µA respectively. When the UV light turns off, the current will decrease down to the original value. The Ion/Ioff ratios are calculated to be about 1.01, 1.04 and 2.09 for 254 nm, 365 nm and 532 nm light
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respectively. After several circles, the I-t curves exhibit a nearly identical
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photoresponse behavior, indicating the stability and reproducibility of the device under the light illumination. Photoresponsivity (Rλ) and external quantum efficiency (EQE) are two important
parameters to evaluate the sensitivity of PDs. Rλ is defined as the photocurrent generated by per unit power of incident light on the effective area of a PD and EQE is bound up with the number of electron-hole pairs excited by a PD per adsorbed photon and per unit time. Rλ and EQE can be expressed in the following equations:
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(1)
EQE=hcRλ/(eλ)
(2)
where ∆Iλ = Iλ- Idark is the difference between photocurrent and dark current, Pλ is the
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incident light intensity, S is the effective illuminated area, h is the Planck’s constant, c is the velocity of light, e is the electron charge, and λ is the incident light wavelength. For the 254 nm light illumination, Pλ = 1400 µW/cm2, S = 1.13×10-4 cm2, ∆Iλ =
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0.17×10-6A, then Rλ and EQE can be estimated to be about 1.07 A/W and 522 %,
6.95 A/W and 2361 %, respectively.
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respectively. While for the 365 nm light, ∆Iλ = 1.1×10-6A, the Rλ and EQE are about
To study the effect of light intensity on the photoresponse properties of the PDs [35], the measurement under 365 nm UV light illumination with the intensity ranging
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from 200 to 2400 µW/cm2 was performed [Fig. 3(c)]. The photocurrent as a function of the light intensity is presented in Fig. 3(d). Notably, the photocurrent increases linearly with the increase of the light intensity, because a higher light intensity would
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photocurrent.
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excite more pairs of the photogenerated electron-hole, resulting in a higher
Fig. 4(a) plots the I-t curves of the Cu2O/GaOOH heterojunction under 532 nm
light illumination at different bias voltages in the range from 0.00005 to 0.5 V. It is clear that the dark current and the photocurrent both increase with the increase of the bias voltage. When a bias higher than the built-in electric field applied, the space charge region would become narrow and the multiplicative diffusion motion would increase, leading to a larger diffusion current. In a certain range, the stronger the
ACCEPTED MANUSCRIPT external electric field applied, the greater the forward dark current obtained. This evolution of the photocurrent is attributed to that a higher bias can promote more photogenerated electron-hole pairs separation and suppress them recombination.
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Interestingly, the Cu2O/GaOOH PD also has an obvious photoelectric response to 532 nm light at a bias voltage of 0.5 mV, exhibiting a low-voltage-worked characteristic.
For a more detailed comparison of the response time of the Cu2O/GaOOH
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heterojunction, the quantitative analysis of the process of the current rise and decay
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process involves the fitting of the photoresponse curve with a bi-exponential relaxation equation of the following type.
I=I0+Ae-t/τ1+Be-t/τ2
(3)
Where I0 is the steady state photocurrent, t is the time, A and B are constant, τ1 and
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τ2 are two relaxation time constants. As is shown in Fig. 4(b), the photoresponse processes are well fitted. τr and τd are the time constants for the rising edge and fall edge, respectively. We note that the rise time constants τr1 and τr2 are estimated to be
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0.38 s and 3.05 s under 532 nm light illumination with a bias of 0.5 V, and the decay
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time constants τd1 and τd2 are 0.24 s and 1.79 s. The photoresponse of semiconductors is a complicate process of electron-hole pair separation, trapping/releasing, and recombination. Generally, the fast-response components (τr1, τd1) can be attributed to the rapid change of carrier concentration as soon as the light turns on or off, while the slow-response components (τr2, τd2) are caused by the carrier trapping/releasing, owing to the existence of defects in GaOOH NRAs, such as oxygen vacancies. For comparison, we list the photoresponse parameters of Cu2O/GaOOH
ACCEPTED MANUSCRIPT heterojunction PD and other types of devices reported in the literature in Table 1. It can be seen that the properties of our device including Rλ and EQE are better than that of other devices based on Ga2O3 nanowire [36], IZO/β-Ga2O3/IZO M-S-M structure
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[37], Ga2O3/SnO2:Ga core-shell nanowire [38]. Compared with the ZnO/Ga2O3 core-shell microwire based device, the nanorods shell-core structure of Cu2O/GaOOH based device shows a characteristic of the lower working voltage [39]. The
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Cu2O/GaOOH heterojunction PD with a simple structure renders relatively high
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performance, wide absorption region and low fabrication cost, promising building high performance optoelectronic devices with a low-voltage-worked characteristic in
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the future.
ACCEPTED MANUSCRIPT 4. Conclusions In conclusion, a heterojunction based PD was fabricated by simply deposited a layer of Cu2O thin film on GaOOH NRAs. The fabricated Cu2O/GaOOH
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heterojunction based PD was sensitive to 254 nm, 365 nm and 532 nm light illumination with a high stability and reproducibility, and shown a good photoresponse properties, such as response time of 0.38 s, recovery time of 0.24 s
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(under 532 nm light illumination), Rλ of 6.95 A/W and EQE of 2361 % at 0.5 V
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applied bias (under the 365 nm illumination with intensity of 1400 µW/cm2). Moreover, with a low bias voltage of 0.5 mV, the PD still shows an obvious photoresponse to 532 nm light, which is important for designing low power device. Based on the above results, the low-voltage-worked PD based on Cu2O/GaOOH
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shell-core NRAs has numerous potential applications in UV and visible detection.
ACCEPTED MANUSCRIPT Acknowledgements The project was supported by Open Fund of IPOC (BUPT), the National Natural Science Foundation of China (No. 61704153, 51572241, 61774019 and 51572033),
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the Scientific Research Project for the Education Department of Zhejiang Province (No. Y201738294), Science and Technology Department of Zhejiang Province
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Foundation (No. 2017C37017).
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ACCEPTED MANUSCRIPT Figure and Table Captions Scheme 1. Schematic illustration of preparation of the Cu2O/GaOOH shell-core NRAs
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Fig. 1. The top views of SEM images at low (a) and high (c) magnification of GaOOH NRAs grown on the FTO substrate. (b) The edge view of GaOOH NRAs, inset shows the bright-field TEM image of a single GaOOH nanorod. (d)
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The top views of Cu2O/GaOOH shell-core NRAs at high magnification, the
Cu2O/GaOOH NRAs (f).
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inset is an enlarged view. The cross-section of GaOOH NRAs (e) and
Fig. 2. (a) XRD patterns of the FTO, FTO/GaOOH and FTO/GaOOH/Cu2O. (b) UV-vis absorption spectra of the FTO/GaOOH NRAs and FTO/GaOOH/Cu2O
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NRAs, inset is the plot of (αhν)2 versus the energy of light (hν). (c) UV-vis absorption spectra of the GaOOH powder. (d) High-resolution XPS spectra of Cu 2p for Cu2O. (e) The enlarge view of the XPS valence-band spectra of
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GaOOH at 1~4.5 eV. (f) The band structure alignment of Cu2O/GaOOH
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heterojunction.
Fig. 3. (a) I-V curves and (b) I-t curves of the Cu2O/GaOOH shell-core NRAs in dark and under different light illumination. (c) Photoresponse of the device under 365 nm light with various intensities. (d) The relationship between the light intensity and photocurrent. Fig. 4. (a) Current of the Cu2O/GaOOH shell-core NRAs under 532 nm light illumination at bias voltages of 0.00005, 0.0005, 0.005, 0.05, and 0.5 V. (b) The
ACCEPTED MANUSCRIPT enlarged view of the rise/decay edges and the corresponding exponential fitting of the Cu2O/GaOOH heterojunction PD measured with a bias of 0.5 V under 532 nm light illumination.
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Table 1. Comparison of the device parameters of the present Cu2O/GaOOH
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heterojunction PD and other Ga2O3 nanostructures based devices.
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Scheme 1
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Table 1
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Highlights 1. The Cu2O/GaOOH shell-core heterojunction nanorod arrays were
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constructed. 2. A built-in electric field was formed at the interface of Cu2O and GaOOH.
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low-voltage-worked feature.
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3. The photodetector based on Cu2O/GaOOH heterojunction shows a