Available online at ScienceDirect. Procedia Chemistry 19 (2016 )

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1 Available online at ScienceDirect Procedia Chemistry 19 (2016 ) th International Conference on Recent Advances in Materials, Minerals and Environment (RAMM) & 2nd International Postgraduate Conference on Materials, Mineral and Polymer (MAMIP), 4-6 August 2015 High Responsivity IR Photodetector Based on CuO Nanorod Arrays/AAO Assembly Khaled M. Chahrour a, *, Naser M. Ahmed a, M.R. Hashim a,b, Ahmad M. Al-Diabat a a Nano-Optoelectronics Research and Technology Laboratory, School of Physics, Universi Sains Malaysia, Penang, Malaysia b Institute of Nano-optoelectronics Research & Technology Laboratory (INOR), School of Physics, Universiti Sains Malaysia, USM, Penang, Malaysia Abstract Fast with high-performance CuO nanorod arrays IR photodetector was fabricated using CuO nanorod arrays/aao assembly. The IR photodetector based on MSM with Al contact electrodes and its optoelectronic properties were examined. The CuO nanorod arrays used in the experiment were synthesized by DC electrodeposition method into Si-based/AAO template. The electrical performance and photoelectric response performance were studied, and the results showed that IR photodetector exhibited a high sensitivity to 808 nm infrared diode laser source. Both the response and recovery time were found to be fast; 0.19 and 0.15 s, respectively, which are shorter time compared to other IR photodetectors reported in the literature The The Authors. Authors. Published Published by Elsevier by Elsevier B.V. B.V. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia. Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia Keywords: CuO nanorods; self-assembly AAO; Electrodeposition; IR photodetector *Corresponding author. Tel.: ; fax: address: skhaled_66@yahoo.com The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia doi: /j.proche

2 312 Khaled M. Chahrour et al. / Procedia Chemistry 19 ( 2016 ) Nomenclature AAO anodic aluminum oxide NMOSs nanostructured metal oxides semiconductors DC direct current CuO cupric oxide IR infrared FESEM field emission scanning electron G current gain EDX electron dispersive x-ray spectrometer Iph photo current XRD x-ray diffraction Id dark current R light responsivity 1. Introduction Among the available nanostructured metal oxides semiconductors (NMOSs), such as Fe 2O 3, NiO, CuO and ZnO. Cupric oxide (CuO) nanostructures were of particular interest of research, because of their interesting properties as a p-type semiconductor with a narrow band gap (1.4eV) and promising applications in lithium-ion batteries 1, solar cell 2, bio sensor 3, removal of inorganic pollutants 4 and photodetectores 5. Several approaches have been used in the preparation of CuO nanostructures, such as chemical vapor deposition 6, hydrothermal 7, sol-gel method 8, laser vaporization 9 and AAO template synthesis 10. Among these methods the AAO template synthesis is considered as a flexible and promising method, because of its simplicity and versatility, since it does not utilize expensive and sophisticated lithographic process for defining nanostructures 11. In particular, AAO template is considered as an ideal template for synthesis of CuO nanorod arrays as it has good mechanical strength, thermal stability and self-assembled honeycomb array of uniformly sized parallel channels with good control of aligned nanorod dimensions at high density of pores ( cm 2 ) 12. In this paper, we reported a low cost and efficient electrochemical deposition method was developed to prepare uniform CuO nanorod arrays embedded in Si-based AAO template. Moreover, CuO with relatively narrow band gap would be a perfect metal oxide that can be used to detect IR radiation. However, no report on the fabrication by using AAO template of aligned CuO nanorod arrays/aao assembly IR photodetector based on MSM can be found in the literature. Electrical and optoelectronic properties of the fabricated device will be also presented and discussed. 2. Experimental Method The CuO nanorod arrays / AAO assembly were prepared by the following experimental procedure. At first, a layer of thin Al film with a purity of 99.99% was deposited onto the Si substrate (p-type, 5 Ω cm, and <100> oriented) using e-beam evaporation in a high vacuum chamber (based pressure 7.5 x10-6 mbar, average deposition rate 10 Å s -1 ). For one continuous run, the deposited Al film has a thickness up to 1μm. All samples were annealed at 400 o C for 2h in a conventional furnace under nitrogen ambient. The experimental results show that the use of 20 nm of a Ti interfacial layer prevented peeling-off the Al film from the Si substrate during anodizing process. The anodizing process was carried out using a two-step method in 0.3M oxalic acid solution at 20 o C and anodizing voltage 50V. In this process, a specially designed electrochemical cell using a platinum rod as a cathode and the Si substrate as an anode. Afterward, the AAO template was immersed into a 5 wt% phosphoric acid solution at room temperature for 45 min to widen the nanopores and to remove the bottom barrier layer of AAO template. The Cu nanorods were synthesized from aqueous solution prepared from 0.5M of Cu sulfate dissolved in 3M lactic acid. The solution s ph was adjusted around 6 using sodium hydroxide. The electrodeposition was performed potentiostatically, with Si-based AAO template serving as working electrode, platinum rod and Ag/AgCl (sat) as the counter and reference electrodes, respectively using EDAQ Model potentiostat at an applied potential of -1.2V for 5 min, the temperature was maintained at 65 o C. Upon completion, the Cu nanorod arrays/aao assembly was rinsed

3 Khaled M. Chahrour et al. / Procedia Chemistry 19 ( 2016 ) by deionized water and dried. The clean and dry Cu nanorod arrays/aao assembly was oxidized at optimized conditions (500 o C and 45 min). CuO nanorod arrays/aao assembly was obtained to fabricate IR photodetector based on MSM as shown in the Fig.1. Al contacts were deposited by e- beam evaporation with a base pressure of 7.5 x10-6 mbar. The metallic Al was evaporated on a metal mask based on the pattern structure of the two contacts (electrodes) with four fingers in each side. The Al contacts have a thickness of 250 nm with a finger width and finger spacing of 0.35 mm. The length of each electrode is approximately 5 mm. The fabricated IR photodetector was characterized using current-voltage (I-V) measurements. The measurements were conducted in the dark and under IR laser radiation. The photocurrent response for the fabricated device was measured at various bias voltages of 2, 3, 4 and 5V by using Keithley source meter (model No. 4200) at ambient conditions. During photocurrent measurement, we used a laser diode source at 808 nm (1.5 ev) as the excitation source due to the fact that energy of the emitted photons is slightly larger than the bandgap energy of CuO. Fig. 1. Schematic diagram of the IR photodetector device. The surface morphology of the fabricated AAO template and the deposited CuO nanorod arrays were analyzed using Carl Zeiss field emission scanning electron microscope (FESEM) Leo-Supra 50VP equipped with electron dispersive X-ray spectrometer (EDX) for elemental chemical analysis, as well as Bruker atomic force microscope (AFM) Dimension Edge in tapping operation mode. The structure was characterized by X-ray diffraction (XRD) using PANalytical X Pert PRO MRD PW3040 diffractometer equipped with Cu-Kα radiation (λ= Å). 3. Results and discussion Fig.2 (a, b) shows a typical top view of AAO template and its three-dimensional AFM image, respectively. An ordered honeycomb microstructure with uniform distribution of pore-diameter and high pore density can be observed. The mean value of pore diameter was 80 nm and the mean value of inter-pores distance was 120 nm. The cross-section view of AAO template (Fig.2 (c)) indicates the formation of vertically aligned nanotubes with a bottom barrier-free AAO template grown onto Si substrate. Fig.2 (d) illustrates the analyzed EDX spectrum confirming the chemical composition of the cross-section of AAO template onto Si substrate.

4 314 Khaled M. Chahrour et al. / Procedia Chemistry 19 ( 2016 ) Fig. 2. (a, b) FESEM image and AFM image show top view of the AAO template, (c, d) FESEM image shows a cross- section view of the AAO template and EDX spectra, respectively. With the help of AAO template and electrochemical deposition followed by thermal oxidation of the deposited Cu, highly ordered and vertical arrays of CuO nanorods can be synthesis. Fig.3 (a, b) shows a cross-section view for AAO template onto Si substrate with embedded CuO nanorods and confirmed with EDX spectrum. It can be observed that almost all the nanopores were filled by CuO nanorod arrays. The lengths of CuO nanorods grown into the AAO template were longer than the depth of AAO template whereby all nanorods showed sign of overgrowth. Fig. 3. (a, b) FESEM image shows cross-sectional of AAO template filled with CuO and EDX spectra, respectively. Fig.4 Shows the X-ray diffraction pattern of CuO nanorods embedded into AAO template. The nanorods were polycrystalline with a monoclinic structure, as indicated by the presence of two prominent peaks close to 2θ angles of 35.6 o, and o, corresponding to CuO (-111), and CuO (111) reflections, respectively (ICSD card No ). Additionally, it was found that no AAO feature peak can be detected in Fig.4, this is because AAO template exists in an amorphous state, which would crystallize upon heating above 800 o C 13.

5 Khaled M. Chahrour et al. / Procedia Chemistry 19 ( 2016 ) Fig. 4. XRD spectrum of CuO nanorod arrays embedded in AAO template. The photodetection properties of the fabricated Al/CuO nanorod arrays/al IR photodetector were studied by measuring the current-voltage (I-V) relationships in the dark and under IR laser light of 808 nm (Fig.5). The (I-V) characteristics show a nonlinear relationship and rectifying behavior is observed. This behavior, which indicates that Schottky barrier has formed at the metal-semiconductor contact. The current gain (G) was calculated from (I-V) characteristics using the following ratio [G = I ph /I d] 14 at various bias voltages, as shown in Table 1. Fig. 5. Logarithm of current-voltage characteristics of Al-CuO nanorod arrays/aao assembly-al photodetector Similar to other metal oxides, oxygen absorption on the surface should also play an important role in carrier transport of the CuO nanorod arrays IR photodetector, we thus observed the photoresponse for the IR photodetector by turning the (808 nm) IR laser source on and off. This phenomenon involved the oxygen-adsorption process in the dark and oxygen-desorption process upon IR illumination that are generally thought to be associated with the generation of free carriers 5. In ambient conditions, oxygen molecules are absorbed on the nanorods surface and capture free electrons from the nanorods making a low-conductivity depletion layer near the surface of CuO nanorods [O 2 (g) + e - O 2- (ads)]. When illuminated with IR light, electron-hole pairs are generated. The holes migrate to the surface to desorb the oxygen adsorbents, resulting in a reduction in the depletion barrier thickness and an increase in the freecarrier concentration. Hence, photocurrent increases upon the IR light illumination [O 2- (ads) + h + O 2 (g)]. After turning off the illumination, oxygen molecules re-adsorb on the nanorods surface, returning the nanorods to their initial low-conductivity state. Light responsivity (R) is a critical parameter used to determine the capability of a photodetector. It is defined as the ratio of the device photocurrent generator to the incident optical power on the effective area of the device (A) and can be expressed using Eq. (1) 14. R = = (1)

6 316 Khaled M. Chahrour et al. / Procedia Chemistry 19 ( 2016 ) Where, E is the irradiance of the light, which is measured by a standard power meter (Newport power meter, Model 2936-C USA). The photoresponse spectrum of the fabricated IR photodetector based on MSM was measured under front illumination within the range of 790 nm into 900 nm at bias voltage of 2V, as illustrated in Fig.6. Fig. 6. Responsivity spectra of the Al-CuO nanorod arrays/aaoassembly-al photodetector under ambient conditions. Fig.6 demonstrates the photocurrent increases with wavelength up to 850 nm, and then falls sharply near 850 nm, which is close to an energy band gap of CuO. Upon illumination, the incident light excites the charge carriers from the valance band to the conduction band, and thus, the photoconduction sensitivity of the device is enhanced. At higher wavelength, the light has insufficient energy to excite, electrons to the conduction band, which contribute to a decrease in the photocurrent. Therefore, the responsivity sharply decreased beyond 850 nm. The observed sharp cut-off indicates that IR photodetector with high sensitivity was obtained. The device exhibited a responsivity value of 2.5 A/W. This higher responsivity (R) could be attributed that CuO nanorod arrays providing high density with larger and rougher surface areas, as well as a good Schottky contact was formed in the Al-CuO nanorod arrays/aao assembly- Al photodetector 15. Repeatability of the IR photodetector was studied by investigating the optical response using dynamic response time measurement by illuminating the photodetector by a pulsed IR laser radiation (808 nm) under different bias voltages. Fig.6 shows the corresponding increase in photocurrent as a function of time at different bias voltages when the IR laser source was repeatedly turned on and off 9 times (partially shown in Fig.6). The results show acceptable difference with time. All curves reveal rectangular shaped profiles, and the magnitude of photocurrent in every on/off cycle is steady and repeatable. The response time (i.e., the time in which current increased from 10% to 90% of its saturation value) and recovery time (i.e., the time in which current decreased from 90% and back to 10% of its saturation value) for IR photodetector were measured at various bias voltages, as shown in Table 1. The response time and recovery time of CuO nanorod arrays in this study were clearly by far the shortest time achieved in various CuO IR detectors reported in the literature 5, 16. The extremely fast photoresponse of IR photodetector in the present study was related to the high quality and large photoactive surface areas of the CuO nanorod arrays. The fast response indicated that the vertical CuO nanorod arrays were useful in high-speed operation.

7 Khaled M. Chahrour et al. / Procedia Chemistry 19 ( 2016 ) Fig. 7. Repeatability performance of the Al-CuO nanorod arrays/aao assembly-al photodetector under On/Off IR laser radiation Table 1. Photodetection characteristics of the IR photodetector with increasing bias voltage. Voltage (V) Photocurrent x 10-3 (A) Response time (s) Recovery time (s) Current Gain Conclusion Hexagonal highly-ordered thin nanoporous anodic alumina (AAO) template of pre-deposited Al thin film onto Si substrate was fabricated under controllable conditions by using a two-step anodization to grow Cu nanorod arrays onto Si-based AAO template by electrodeposition method. The Cu nanorod arrays/aao assembly was oxidized inside a conventional furnace under optimized conditions to obtain CuO nanorod arrays/aao assembly. The evaluation of CuO nanorod arrays/aao template photodetector based on MSM in the IR region (808 nm) showed fast response and recovery time. These results demonstrated that the as-fabricated high quality of CuO nanorod arrays/aao assembly could be a promising candidate as a low-cost IR photodetector for industrially integrated optoelectronic application. Knowledgements The authors gratefully acknowledge the financial support provided by the University Saince Malaysia (USM) fellowship and School of Physics, under grant no. 1001/PFIZIK/ and grant no. 304/PFIZIK/ References 1. Wang L, Gong H, Wang C, Wang D, Tang K, Qian Y. Facile synthesis of novel tunable highly porous CuO nanorods for high rate lithium battery anodes with realized long cycle life and high reversible capacity. Nanoscale 2012; 4: Kislyuk VV, Dimitriev OP. Nanorods and nanotubes for solar cells. J Nanosci Nanotechnol 2008; 8: Rahman MM, Saleh Ahammad Aj, Jin J-H, Ahn SJ, Lee J-J. A comprehensive review of glucose biosensors based on nanostructured metaloxides. Sensors (Basel, Switzerland) 2010; 10: Ali I. New generation adsorbents for water treatment. Chem Rev 2012; 112: Wang SB, Hsiao CH, Chang SJ, Lam KT, Wen KH, Hung SC, et al. A CuO nanowire infrared photodetector. Sensor Actuat A: Phys 2011; 171:

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