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Self-assembled growth of vertically aligned ZnO nanorods for light sensing applications
Materials Letters ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Self-assembled growth of vertically aligned ZnO nanorods for light sensing applications Noha Samir, Dina S. Eissa, Nageh K. Allam 1 Energy Materials Laboratory (EML), School of Sciences and Engineering, The American University in Cairo, New Cairo 11835, Egypt
art ic l e i nf o
a b s t r a c t
Article history: Received 2 August 2014 Accepted 20 August 2014
A facile low temperature anodization approach is presented for the first time to synthesize aligned ZnO nanorods (NRs). The NRs have diameters in the range of 150–250 nm and lengths of 50–100 mm with hexagonal structures. Electropolishing and pre-annealing of the Zn foil before the anodization step were essential to produce ordered NRs. X-ray diffraction, transmission electron diffraction, Raman and X-ray photoelectron spectroscopy analyses confirmed the formation of ZnO. The photodetector made using the prepared ZnO NRs showed an excellent responsivity of 1 A/W. & 2014 Published by Elsevier B.V.
Keywords: Anodization ZnO nanorods Raman X-ray photoelectron spectroscopy Sensor
1. Introduction Zinc oxide (ZnO) is a promising material that has been receiving a great attention in the last few years. It has a wide band gap of 3.37 eV with 60 meV exciton binding energy [1], which is higher than most of the other semiconductors. Single crystalline ZnO showed an electron mobility of 205–1000 cm2/V s, which is higher than that of TiO2 (0.1–4 cm2/V s), the most widely used semiconductor. Therefore, ZnO exhibits reduced recombination losses [2], making it an ideal material for use in optical waveguides, piezoelectric transducers, acousto-optic media, and conductive gas sensors [3]. The favorable band edge positions of ZnO with respect to the redox potentials of water suggest the use of ZnO as a photocatalyst for hydrogen evolution [4–6]. Also, the distinctive chemical, electrical and optical properties of ZnO encouraged its use in chemical and biological sensing [7]. Therefore, many reports have been devoted toward the fabrication of ZnO nanostructured materials with different shapes and sizes [8]. However, the fabrication processes are usually very complicated, expensive and irreproducible. Moreover, the exact growth mechanisms of such various structures are not fully comprehended [8]. Herein, we report a simple, yet optimized, method to fabricate well-aligned ZnO NRs via anodization of Zn foil at room temperature. Unlike other methods, the obtained nanomorphologies can be easily controlled. We focused on the NRs as they are the most effective nanostructured form of ZnO in many
E-mail address: [email protected] (N.K. Allam). Tel.: þ20 2 2615 2568; fax: þ 20 2 2795 7565.
1
applications [9], such as gas sensors, displays, MEMS devices and solar energy applications.
2. Material and methods Surface inhomogeneity of Zn foil always leads to the formation of different morphological structures within the same sample after normal anodization [10,11]. To overcome such a problem, Zn foil (0.2 mm, 99.99%) was first annealed in air at 300 1C for 1 h, followed by cleaning in ultrasonic bath using acetone and ethanol for 15 min. Subsequently, Zn foil was rinsed with ultra-pure water (UPW) and then electro-polished for 20 min in stirred solution containing both ethanol and phosphoric acid at 5 1C at 20 V and 220 mA. After that, anodization was performed in a two-electrode cell using the electropolished Zn foil as the anode and platinum as the cathode. The distance between the 2 electrodes was 3 cm, the electrolyte contained different concentrations of KHCO3 in aqueous and organic solutions. The anodization was carried at room temperature with an applied potential of 10 V for 30 min. After anodization, the sample was thoroughly rinsed with UPW and dried at 65 1C for 45 min and then postannealed at 250 1C for 45 min. Finally, the sample was stored in a vacuum desecrator cabinet for further characterizations. The morphology of the as-anodized samples was studied using a LEO Supra 55 Field Emission Scanning Electron Microscope (FESEM). Specimens were annealed in a LINDBURG Programmable Furnace model N41/M 29667. The crystal structure of as-anodized and annealed samples was studied using a PANalytical X'Pert PRO XRD diffractometer. The UV vis absorption spectral measurements
http://dx.doi.org/10.1016/j.matlet.2014.08.114 0167-577X/& 2014 Published by Elsevier B.V.
Please cite this article as: Samir N, et al. Self-assembled growth of vertically aligned ZnO nanorods for light sensing applications. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.08.114i
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were performed using a Shimadzu UV vis NIR spectrophotometer with a solid-sample holder for reflectance measurements and an integrating sphere. The surface composition of the samples was analyzed by X-ray photoelectron spectroscopy using a Thermo Scientific K-alpha XPS with an Al anode. Spectra were charge referenced to O 1s at 532 eV. Raman measurements were performed on a Raman microscope (ProRaman-L Analyzer) with an excitation laser beam wavelength of 532 nm. The photoluminescence (PL) spectra of the samples were measured with a He–Cd laser (325 nm) at room temperature.
3. Results and discussion Fig. 1a–c shows field emission scanning electron microscope (FESEM) images of the obtained ZnO films upon the anodization of Zn foil in solutions containing different concentrations of KHCO3. In aqueous solution containing 10 mM KHCO3 (Fig. 1a), the image
shows the initiation of ZnO nanostructures. Upon increasing the KHCO3 concentration to 50 mM (Fig. 1b), well aligned ZnO NRs with diameter in the range of 150–250 nm and length of 50–100 mm have been obtained. Fig. 1c shows a bottom view image of the obtained radiating ZnO NRs with the inset showing a close up view. Fig. 1d shows the EDX spectrum of the ZnO NRs, revealing Zn and O peaks. X-ray diffraction (XRD) was also performed for the ZnO NRs using Cu Kα1 radiation source obtained in 2θ range from 251 to 801 on an X-ray diffractometer with λ ¼ 1.5406 Å (Fig. 2a). The diffraction peaks reveal the hexagonal wurtzite structure of ZnO [12], with the pronounced ZnO diffraction peaks corresponding to (100), (002) and (101) lattice planes. Other lower intensity peaks were also observed that are characteristic of (102), (110), (103), (112) and (202) lattice planes, confirming the polycrystalline nature of the ZnO NRs [3,13]. Transmission electron diffraction (TED) was also performed (Fig. 2a inset). The results are in a good agreement with the XRD results, revealing the poly-crystallinity of the obtained NRs. The lattice fringes spectra showed a spacing of
Fig. 1. FESEM images of ZnO NRS synthesized by anodization of Zn foil in (a) 10 mM KHCO3 aqueous solution, (b and c) 50 mM KHCO3 aqueous solution, and (d) EDX spectra of the sample shown in (b).
Fig. 2. (a) XRD patterns of the anodized Zn in 50 mM KHCO3 aqueous solution, with the inset showing the TEM electron diffraction pattern of the NRs, and (b) the corresponding Raman spectra of the NRs.
Please cite this article as: Samir N, et al. Self-assembled growth of vertically aligned ZnO nanorods for light sensing applications. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.08.114i
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4.1 Å, characteristic of ZnO [3,13]. Fig. 2b shows the obtained Raman spectra for the fabricated ZnO NRs, showing a significant peak at 437 cm 1 that can be related to the E2 (high) phonon mode [14], a characteristic of the hexagonal wurtzite phase [15]. The E2 (high) forbidden mode was also observed at 332 cm 1 and the suppressed E1 mode was observed at 530 cm 1. Fig. 3a shows the absorption spectra of the fabricated ZnO NRs. ZnO absorbs in the UV region 250–400 nm, after which it declines. The optical absorption coefficient (α) was determined using the Kubelka–Munk transfer model [14], where f(R)¼ (1 R)2/2R¼ α/s, with f(R) representing the Kubelka–Munk function, and assuming that Reflectance (R) and scattering coefficient (s) are λ-independent. The optical band gap (Eg) was obtained by extrapolation of the linear region of the plot (αhν)2 versus photon energy (hν) as shown in the inset of Fig. 3a. The obtained band gap of the prepared ZnO NRs is very close to the ideal bandgap of ZnO (3.37 eV). Fig. 3b depicts the room-temperature photoluminescence (PL) emission spectra of the fabricated ZnO NRs. The PL emission was observed in the wavelength range of 350o λ o600, with two peaks observed between 350 and 400 nm, i.e. in the near band edge (NBE) region. The NBE peak suggests the presence of exciton recombination through exciton-to-exciton collision [15,16]. Broad green emission was also observed, which is usually attributed to sub-band transition that is intrinsic in nature [15,16]. Fig. 4 shows the X-ray photoelectron spectra (XPS) of the ZnO NRs. Well- resolved Zn 2p and O 1s peaks were observed that are characteristic of hexagonal ZnO NRs, in agreement with the Raman findings. The O 1s spectra can be deconvoluted into two peaks. While the peak observed at 531 eV is characteristic of Zn–O bonding, the peak at 532.5 eV can be attributed to the O–H bonding. Two peaks for Zn were observed at 1022 and 1045 eV,
3
with a spin orbit splitting of 23 eV correspond to Zn2 þ species [17,18]. A metal–semiconductor–metal (MSM)-based UV photodetector (PD) was fabricated, which consists of two interdigitated contacts with four fingers each. Each finger has a width of 200 μm, a length of 3 mm, and a spacing of 300 μm between the length and the width. Silver contacts were deposited by vacuum thermal evaporation. After deposition, the device was annealed in N2 atmosphere at 400 1C for 5 min. Responsivity (R), the ratio of the device photocurrent (Ip) to the incident optical power (Popt), is a performance metric used to characterize photodetectors [18] R¼
Ip P opt
ð1Þ
Fig. 5a shows the variation of R as a function of wavelength for the ZnO NRs PD with Ag contact. The photodetector R increased slightly until it reached the maximum at 386 nm, followed by a gradual decrease. At shorter wavelength, absorption coefficient increased and the penetration depth of the UV light became shallower, thereby increasing only the carrier concentration near the film surface and consequently producing carriers with shorter lifetime due to surface recombination and hence, led to the observed drop in R. A sharp cut-off at 388 nm was observed that matches the absorption edge of ZnO and agrees with the PL results shown in Fig. 3b. R dropped considerably above the cut-off wavelength, indicating that a photoconductive UV detector with high sensitivity was achieved. The obtained R in this study was 0.96 A/W, which is 10 times higher than that reported for ZnO nanorods-based photodetectors [19]. To test the reversibility of the detector, the transient response of the two photodiodes was measured by turning on and off the light source, Fig. 5b. The fabricated device demonstrates
Fig. 3. (a) Absorption spectra (inset is Tauc plot), and (b) the room temperature PL spectra of the fabricated ZnO NRs.
Fig. 4. XPS spectra of the ZnO NRs.
Please cite this article as: Samir N, et al. Self-assembled growth of vertically aligned ZnO nanorods for light sensing applications. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.08.114i
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Fig. 5. (a) The room temperature responsivity spectra of the ZnO NRs MSM photodetector under an applied bias of 3 V, and (b) the corresponding photocurrent–time response.
shorter rise and decay times compared to other materials used to fabricate UV photodetectors [20].
4. Conclusions We demonstrate a facile and inexpensive method for the growth of ZnO NRs via the anodization of Zn foil. Surface heterogeneity and structural irreproducibility were eliminated via electropolishing and pre-annealing of the Zn foil before anodization. We demonstrate the ability to fabricate well aligned ZnO NRs with diameters in the range of 150–250 nm and lengths of 50–100 mm, depending on the anodization conditions. XRD, XPS and Raman analysis confirmed the fabrication of hexagonal ZnO nanostructures. The photodetector made using the synthesized ZnO NRs showed a responsivity of 1 A/W, which is 10 times higher than that reported for ZnO nanorods-based photodetectors.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Acknowledgment
[18]
The financial support from The American University in Cairo is highly appreciated.
[19] [20]
Zhao J, Wang X, Liu J, Meng Y, Xu X, Tang C. Mater Chem Phys 2011;126:555–9. Seol M, Kim H, Tak Y, Yong K. Chem Commun 2010;46:5521–3. Xu F, Lu Y, Sun L, Zhi L. Chem Commun 2010;46:3191–3. Brewster MM, Zhou X, Lu MY, Gradečak S. Nanoscale 2012;4:1455–62. Cheng C, Fan HJ. Nano Today 2012;7:327–43. Tian C, Zhang Q, Wu A, Jiang M, Liang Z, Jiang B, et al. Chem Commun 2012;48:2858–60. Hahn YB, Ahmad R, Tripathy N. Chem Commun 2012;48:10369–85. Ramirez-Canon A, Miles DO, Cameron PJ, Mattia D. RSC Adv 2013;3:25323. Pan ZW, Dai ZR, Wang ZL. Science 2001;291:1947–9. Zhang X, Qin J, Xue Y, Yu P, Zhang B, Wang L, et al. Sci Rep 2014;4:4596. Hu Z, Chen Q, Li Z, Yu Y, Peng LM. J Phys Chem C 2010;114:881–9. Hassan NK, Hashim MR, Allam NK. Sens Actuators A Phys 2013;192:124–9. Diaz Cano AI, El Filali B, Torchynska TV, Casas Espinola JL. Physica E: Lowdimens Syst Nanostruct 2013;51:24–8. Dinh HN. Photoelectrochemical water splitting: standards experimental methods and protocols. . Hassan NK, Hashim MR, Allam NK. Chem Phys Lett 2012;549:62–6. Chen Z-G, Ni A, Li F, Cong H, Cheng HM, Lu GQ. Chem Phys Lett 2007;434: 301–5. Huang SH, Chen Z, Shen XC, Zhu ZQ, Yu K. Solid State Commun 2008;145: 418–22. Ngo-Duc T, Singh K, Meyyappan M, Oye MM. Nanotechnology 2012;23: 194015. Lu X, Wang G, Xie S, Shi J, Li W, Tong Y, et al. Chem Commun 2012;48:7717–9. Chen M, Wang X, Yu Y, Pei Z, Bai X, Sun C, et al. Appl Surf Sci 2000;158: 134–40.
Please cite this article as: Samir N, et al. Self-assembled growth of vertically aligned ZnO nanorods for light sensing applications. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.08.114i
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Self-assembled growth of vertically aligned ZnO nanorods for light sensing applications Noha Samir, Dina S. Eissa, Nageh K. Allam 1 Energy Materials Laboratory (EML), School of Sciences and Engineering, The American University in Cairo, New Cairo 11835, Egypt
art ic l e i nf o
a b s t r a c t
Article history: Received 2 August 2014 Accepted 20 August 2014
A facile low temperature anodization approach is presented for the first time to synthesize aligned ZnO nanorods (NRs). The NRs have diameters in the range of 150–250 nm and lengths of 50–100 mm with hexagonal structures. Electropolishing and pre-annealing of the Zn foil before the anodization step were essential to produce ordered NRs. X-ray diffraction, transmission electron diffraction, Raman and X-ray photoelectron spectroscopy analyses confirmed the formation of ZnO. The photodetector made using the prepared ZnO NRs showed an excellent responsivity of 1 A/W. & 2014 Published by Elsevier B.V.
Keywords: Anodization ZnO nanorods Raman X-ray photoelectron spectroscopy Sensor
1. Introduction Zinc oxide (ZnO) is a promising material that has been receiving a great attention in the last few years. It has a wide band gap of 3.37 eV with 60 meV exciton binding energy [1], which is higher than most of the other semiconductors. Single crystalline ZnO showed an electron mobility of 205–1000 cm2/V s, which is higher than that of TiO2 (0.1–4 cm2/V s), the most widely used semiconductor. Therefore, ZnO exhibits reduced recombination losses [2], making it an ideal material for use in optical waveguides, piezoelectric transducers, acousto-optic media, and conductive gas sensors [3]. The favorable band edge positions of ZnO with respect to the redox potentials of water suggest the use of ZnO as a photocatalyst for hydrogen evolution [4–6]. Also, the distinctive chemical, electrical and optical properties of ZnO encouraged its use in chemical and biological sensing [7]. Therefore, many reports have been devoted toward the fabrication of ZnO nanostructured materials with different shapes and sizes [8]. However, the fabrication processes are usually very complicated, expensive and irreproducible. Moreover, the exact growth mechanisms of such various structures are not fully comprehended [8]. Herein, we report a simple, yet optimized, method to fabricate well-aligned ZnO NRs via anodization of Zn foil at room temperature. Unlike other methods, the obtained nanomorphologies can be easily controlled. We focused on the NRs as they are the most effective nanostructured form of ZnO in many
E-mail address: [email protected] (N.K. Allam). Tel.: þ20 2 2615 2568; fax: þ 20 2 2795 7565.
1
applications [9], such as gas sensors, displays, MEMS devices and solar energy applications.
2. Material and methods Surface inhomogeneity of Zn foil always leads to the formation of different morphological structures within the same sample after normal anodization [10,11]. To overcome such a problem, Zn foil (0.2 mm, 99.99%) was first annealed in air at 300 1C for 1 h, followed by cleaning in ultrasonic bath using acetone and ethanol for 15 min. Subsequently, Zn foil was rinsed with ultra-pure water (UPW) and then electro-polished for 20 min in stirred solution containing both ethanol and phosphoric acid at 5 1C at 20 V and 220 mA. After that, anodization was performed in a two-electrode cell using the electropolished Zn foil as the anode and platinum as the cathode. The distance between the 2 electrodes was 3 cm, the electrolyte contained different concentrations of KHCO3 in aqueous and organic solutions. The anodization was carried at room temperature with an applied potential of 10 V for 30 min. After anodization, the sample was thoroughly rinsed with UPW and dried at 65 1C for 45 min and then postannealed at 250 1C for 45 min. Finally, the sample was stored in a vacuum desecrator cabinet for further characterizations. The morphology of the as-anodized samples was studied using a LEO Supra 55 Field Emission Scanning Electron Microscope (FESEM). Specimens were annealed in a LINDBURG Programmable Furnace model N41/M 29667. The crystal structure of as-anodized and annealed samples was studied using a PANalytical X'Pert PRO XRD diffractometer. The UV vis absorption spectral measurements
http://dx.doi.org/10.1016/j.matlet.2014.08.114 0167-577X/& 2014 Published by Elsevier B.V.
Please cite this article as: Samir N, et al. Self-assembled growth of vertically aligned ZnO nanorods for light sensing applications. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.08.114i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102
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were performed using a Shimadzu UV vis NIR spectrophotometer with a solid-sample holder for reflectance measurements and an integrating sphere. The surface composition of the samples was analyzed by X-ray photoelectron spectroscopy using a Thermo Scientific K-alpha XPS with an Al anode. Spectra were charge referenced to O 1s at 532 eV. Raman measurements were performed on a Raman microscope (ProRaman-L Analyzer) with an excitation laser beam wavelength of 532 nm. The photoluminescence (PL) spectra of the samples were measured with a He–Cd laser (325 nm) at room temperature.
3. Results and discussion Fig. 1a–c shows field emission scanning electron microscope (FESEM) images of the obtained ZnO films upon the anodization of Zn foil in solutions containing different concentrations of KHCO3. In aqueous solution containing 10 mM KHCO3 (Fig. 1a), the image
shows the initiation of ZnO nanostructures. Upon increasing the KHCO3 concentration to 50 mM (Fig. 1b), well aligned ZnO NRs with diameter in the range of 150–250 nm and length of 50–100 mm have been obtained. Fig. 1c shows a bottom view image of the obtained radiating ZnO NRs with the inset showing a close up view. Fig. 1d shows the EDX spectrum of the ZnO NRs, revealing Zn and O peaks. X-ray diffraction (XRD) was also performed for the ZnO NRs using Cu Kα1 radiation source obtained in 2θ range from 251 to 801 on an X-ray diffractometer with λ ¼ 1.5406 Å (Fig. 2a). The diffraction peaks reveal the hexagonal wurtzite structure of ZnO [12], with the pronounced ZnO diffraction peaks corresponding to (100), (002) and (101) lattice planes. Other lower intensity peaks were also observed that are characteristic of (102), (110), (103), (112) and (202) lattice planes, confirming the polycrystalline nature of the ZnO NRs [3,13]. Transmission electron diffraction (TED) was also performed (Fig. 2a inset). The results are in a good agreement with the XRD results, revealing the poly-crystallinity of the obtained NRs. The lattice fringes spectra showed a spacing of
Fig. 1. FESEM images of ZnO NRS synthesized by anodization of Zn foil in (a) 10 mM KHCO3 aqueous solution, (b and c) 50 mM KHCO3 aqueous solution, and (d) EDX spectra of the sample shown in (b).
Fig. 2. (a) XRD patterns of the anodized Zn in 50 mM KHCO3 aqueous solution, with the inset showing the TEM electron diffraction pattern of the NRs, and (b) the corresponding Raman spectra of the NRs.
Please cite this article as: Samir N, et al. Self-assembled growth of vertically aligned ZnO nanorods for light sensing applications. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.08.114i
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4.1 Å, characteristic of ZnO [3,13]. Fig. 2b shows the obtained Raman spectra for the fabricated ZnO NRs, showing a significant peak at 437 cm 1 that can be related to the E2 (high) phonon mode [14], a characteristic of the hexagonal wurtzite phase [15]. The E2 (high) forbidden mode was also observed at 332 cm 1 and the suppressed E1 mode was observed at 530 cm 1. Fig. 3a shows the absorption spectra of the fabricated ZnO NRs. ZnO absorbs in the UV region 250–400 nm, after which it declines. The optical absorption coefficient (α) was determined using the Kubelka–Munk transfer model [14], where f(R)¼ (1 R)2/2R¼ α/s, with f(R) representing the Kubelka–Munk function, and assuming that Reflectance (R) and scattering coefficient (s) are λ-independent. The optical band gap (Eg) was obtained by extrapolation of the linear region of the plot (αhν)2 versus photon energy (hν) as shown in the inset of Fig. 3a. The obtained band gap of the prepared ZnO NRs is very close to the ideal bandgap of ZnO (3.37 eV). Fig. 3b depicts the room-temperature photoluminescence (PL) emission spectra of the fabricated ZnO NRs. The PL emission was observed in the wavelength range of 350o λ o600, with two peaks observed between 350 and 400 nm, i.e. in the near band edge (NBE) region. The NBE peak suggests the presence of exciton recombination through exciton-to-exciton collision [15,16]. Broad green emission was also observed, which is usually attributed to sub-band transition that is intrinsic in nature [15,16]. Fig. 4 shows the X-ray photoelectron spectra (XPS) of the ZnO NRs. Well- resolved Zn 2p and O 1s peaks were observed that are characteristic of hexagonal ZnO NRs, in agreement with the Raman findings. The O 1s spectra can be deconvoluted into two peaks. While the peak observed at 531 eV is characteristic of Zn–O bonding, the peak at 532.5 eV can be attributed to the O–H bonding. Two peaks for Zn were observed at 1022 and 1045 eV,
3
with a spin orbit splitting of 23 eV correspond to Zn2 þ species [17,18]. A metal–semiconductor–metal (MSM)-based UV photodetector (PD) was fabricated, which consists of two interdigitated contacts with four fingers each. Each finger has a width of 200 μm, a length of 3 mm, and a spacing of 300 μm between the length and the width. Silver contacts were deposited by vacuum thermal evaporation. After deposition, the device was annealed in N2 atmosphere at 400 1C for 5 min. Responsivity (R), the ratio of the device photocurrent (Ip) to the incident optical power (Popt), is a performance metric used to characterize photodetectors [18] R¼
Ip P opt
ð1Þ
Fig. 5a shows the variation of R as a function of wavelength for the ZnO NRs PD with Ag contact. The photodetector R increased slightly until it reached the maximum at 386 nm, followed by a gradual decrease. At shorter wavelength, absorption coefficient increased and the penetration depth of the UV light became shallower, thereby increasing only the carrier concentration near the film surface and consequently producing carriers with shorter lifetime due to surface recombination and hence, led to the observed drop in R. A sharp cut-off at 388 nm was observed that matches the absorption edge of ZnO and agrees with the PL results shown in Fig. 3b. R dropped considerably above the cut-off wavelength, indicating that a photoconductive UV detector with high sensitivity was achieved. The obtained R in this study was 0.96 A/W, which is 10 times higher than that reported for ZnO nanorods-based photodetectors [19]. To test the reversibility of the detector, the transient response of the two photodiodes was measured by turning on and off the light source, Fig. 5b. The fabricated device demonstrates
Fig. 3. (a) Absorption spectra (inset is Tauc plot), and (b) the room temperature PL spectra of the fabricated ZnO NRs.
Fig. 4. XPS spectra of the ZnO NRs.
Please cite this article as: Samir N, et al. Self-assembled growth of vertically aligned ZnO nanorods for light sensing applications. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.08.114i
67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132
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Fig. 5. (a) The room temperature responsivity spectra of the ZnO NRs MSM photodetector under an applied bias of 3 V, and (b) the corresponding photocurrent–time response.
shorter rise and decay times compared to other materials used to fabricate UV photodetectors [20].
4. Conclusions We demonstrate a facile and inexpensive method for the growth of ZnO NRs via the anodization of Zn foil. Surface heterogeneity and structural irreproducibility were eliminated via electropolishing and pre-annealing of the Zn foil before anodization. We demonstrate the ability to fabricate well aligned ZnO NRs with diameters in the range of 150–250 nm and lengths of 50–100 mm, depending on the anodization conditions. XRD, XPS and Raman analysis confirmed the fabrication of hexagonal ZnO nanostructures. The photodetector made using the synthesized ZnO NRs showed a responsivity of 1 A/W, which is 10 times higher than that reported for ZnO nanorods-based photodetectors.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
Acknowledgment
[18]
The financial support from The American University in Cairo is highly appreciated.
[19] [20]
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Please cite this article as: Samir N, et al. Self-assembled growth of vertically aligned ZnO nanorods for light sensing applications. Mater Lett (2014), http://dx.doi.org/10.1016/j.matlet.2014.08.114i
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