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Supramolecules-assisted ZnO nanostructures growth and their UV photodetector application
Solid State Sciences 41 (2015) 14e18
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Supramolecules-assisted ZnO nanostructures growth and their UV photodetector application Kimleang Khun a, *, Sami Elhag a, Zafar Hussain Ibupoto b, Volodymyr Khranovskyy c, Omer Nur a, Magnus Willander a a b c
Department of Science and Technology, Campus Norrkoping, Linkoping University, SE-60174 Norrkoping, Sweden Dr. M. A. Kazi Institute of Chemistry, University of Sindh, Jamshoro, Sindh, Pakistan €ping University, SE-58183 Linko €ping, Sweden Department of Physics, Chemistry and Biology (IFM), Linko
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 October 2014 Received in revised form 23 January 2015 Accepted 26 January 2015 Available online 27 January 2015
In this work, the supramolecules (riboflavin and melamine) were used as a template for the selfassemble of ZnO nanostructures by adding into the growth solutions. After that, the composite nanostructures of nanoflowers-like p-type NiO/n-type ZnO nanosheets were grown following a two-step on the fluorine doped tin oxide substrate by the hydrothermal method. The riboflavin and melamine have a role in UV emission. Taking these advantages into account, the currentevoltage (IeV) characterization of the ZnO/NiO heterojunction was performed at room temperature and showed an obvious nonlinear and rectifying response. Strong UV absorption with fast switching was observed from the ZnO/NiO composite heterojunction. The proposed UV photodetector based on this nanocomposite is more stable, possesses fast rising and decaying time response of approximately 100 ms. Further, a low leakage current was observed for the presented device. The findings in the present study indicates that the controlled nanostructures morphology is important to develop the efficient nanodevices for various applications. © 2015 Elsevier Masson SAS. All rights reserved.
Keywords: ZnO nanosheets Supramolecules-assisted ZnO growth ZnO/NiO nano-composite UV absorption ZnO Nano-heterojunction UV photodetector
1. Introduction In the recent decade, transparent oxide semiconductors (TOSs) nanostructures have been very attractive due to their unique physical and chemical properties with potential for developing efficient nanodevices for various applications [1e5]. Among those nanodevices, UV detectors have been remained attractive because of their wide applicability in many fields such as chemical analysis, water purification, remote control, safe space to space communications and skin cancer care [6,7]. Due to chemical stability, besides being environmental-friendly with relatively low production cost the fabrication of UV detectors based on TOSs is promising [8]. ZnO belongs to the TOSs having a wide band gap of 3.34 eV, and has high charge capacity and good charge carriers transport properties [8,9]. A variety of ZnO nanostructures can be synthesized by many physical and chemical methods. Synthesis of ZnO
* Corresponding author. E-mail addresses: [email protected], (K. Khun).
[email protected]
http://dx.doi.org/10.1016/j.solidstatesciences.2015.01.011 1293-2558/© 2015 Elsevier Masson SAS. All rights reserved.
nanostructures has been followed by chemical solution methods [10]. Moreover, such synthesis has been achieved using different substrates, and organic molecules as a polymer/surfactant-assisted synthesis were incorporated to modulate the morphology of some metal oxide nanostructures [11,12]. However, p-ZnO is unstable and unreliable to be utilized for the fabrication of the UV detector i.e. when fabricating UV photodetectors based on ZnO p-n homojunctions [13]. Different UV photodetectors have been investigated using heterojunction of n-ZnO and other p-TOSs thin films [8,14e16]. The small foot-print of nanostructures enables the possibility of achieving devices of quality heterojunction with many other materials and on any substrate. Among the different p-TOSs nanostructures, NiO nanostructures have potential to fabricate p-n heterojunction with ZnO [14,17] because it's naturally p-type semiconductor, and has a wide band gap of 3.6 eV. Additionally, it has a weak absorption in the visible region, high hole mobility and can be achieved through low production cost methods [18,19]. Recently few UV photodetector devices based on heterojunction composed of ZnO and NiO nanostructures were reported [20]. However, some of these devices still have some disadvantages such as large leakage current, unstable and low
K. Khun et al. / Solid State Sciences 41 (2015) 14e18
photo-generated current and they demonstrate a relatively long recovery time. In this work, the ZnO nanosheets with high absorption ability were grown by a supramolecules-assisted hydrothermal method. Supramolecules; also called riboflavin and melamine have been used as the substance that would yield a desired morphology of ZnO nanostructures. The synthesized nanocomposite is utilized in the development of highly sensitivity UV photodetector. The prepared ZnO/NiO nanocomposite and the developed UV sensor were characterized by appropriate techniques. The fabricated ZnO/NiO UV photodetector has shown the low leakage current, high sensitivity, repeatability, stability, fast response and short delay time. 2. Experimental section Firstly, ZnO nanosheets with strong UV absorption were grown on fluorine doped tin oxide (FTO) substrate by the hydrothermal method. ZnO nanorods growth solution was 0.05 M of each zinc nitrate hexahydrate, and hexamethylenetetramine (HMT) dissolving in a 100 ml of de-ionized water. This solution was added to well-known components of supramolecules (() riboflavin and melamine) with equal concentration of 8 mM to tune the grown ZnO morphology to nanosheets instead of nanorods. A seed layer of ZnO nanoparticles [21] was coated two times on the FTO glass substrate using sin coating at 2000 r.p.m for 20 s. The seed coated substrate was annealed at 120 C for 15e20 min for binding the ZnO nanoparticles to the substrate. The annealed substrates were fixed into a Teflon sample holder and dipped into growth solution which was placed in an oven fixed at 93 C for duration of 5e6 h. After the completion of growth duration, the grown ZnO nanosheets were cleaned by de-ionized water, and then they were blown by a flow of N2 gas and were dried in air at room temperature. Secondly, NiO nanoflowers were prepared on top of the grown ZnO nanosheets substrate by the hydrothermal method. The grown ZnO nanosheets on FTO substrate were placed into the Teflon sample holder and dipped into the growth solution with 0.1 M of nickel nitrate hexahydrate and HMT in de-ionized water for a period of 4e6 h at 93 C. When the growth duration was completed, the samples were washed by de-ionized water and annealed at 450 C for 3e4 h to convert the nickel hydroxide phase into the NiO phase. Finally a top contact of 50 nm layer of silver was deposited onto the NiO to be used as an ohmic contact for electrical measurement. The schematic diagram of the presented UV photodetector device is shown in Fig. 1. The morphology of the ZnO/NiO nanocomposite was investigated by scanning electron microscopy (SEM), performed by the LEO 1550 Gemini field emission gun at 10 kV. All the XRD spectra
Fig. 1. The schematic diagram of the ZnO/NiO heterojunction UV photodetector.
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were recorded using the same accusation parameters to enable relative comparison. The X-ray diffraction (XRD) scans were performed at the scan rate 0.1deg./sec by the Phillips PW 1729 Powder Differactometer, using CuKa radiation (l ¼ 1.5418 Å) for studying the crystal quality of the ZnO/NiO nanocomposite. The light emission features of the samples were studied by a microphotoluminescence (PL) setup. The excitation was performed by a frequency doubled Nd:YVO laser as a continuous wave excitation source, giving a wavelength l ¼ 266 nm. The PL spectra were taken for the range 320e700 nm. Via using of microlens the signal was collected from an excited area of around 1.5 mm in diameters. The emitted luminescence was collected and mirrored into a single grating 0.45 m monochromator equipped with a liquid nitrogen cooled Si-CCD camera with a spectral resolution of ~0.1 meV. Via control of the laser transmittance the power excitation density was kept ~50 mW, being then concentrated on the surface as maximal excitation power 25 W/cm2. A PerkinElmer Lambda 900 UVevisible spectrophotometer was used for the investigation of the optical absorption. The UV photo-detector response was performed using an illumination of 365 nm wavelength UV lamp with a power of 2 mW cm2 coupled to an Auto-lab. The currentevoltage (IeV) curves were measured by semiconductor characterization system (Keithley 4200-SCS). 3. Results and discussion Fig. 2 (a, c) show a typical SEM images of the ZnO nanosheets and nanorods grown on the FTO by the hydrothermal method. From both figures it can clearly be seen that well-aligned, ZnO nanorods have been changed to nanosheets due to the effect of the supramolecules. The ZnO nanosheets have highly faceted grains of an average size of approximately 200 nm, and are densely packed in a polycrystalline layer. In Fig. 2(b), they can be seen that the NiO nanoflowers are in fact a network of relatively thin sheets that cover the ZnO nanosheets surface. The cross section image of ZnO nanosheets and NiO nanoflowers exhibited that the interface of ZnO/NiO nanocomposite are in good connection as shown in Fig. 2(d). The crystal quality and phase purity of the ZnO nanorods, nanosheets, NiO nanoflowers and ZnO/NiO nanocomposite were characterized by XRD pattern as is shown in Fig. 3. The XRD pattern of ZnO/NiO nanocomposite has shown only peaks peculiar for ZnO and the NiO. However, a peak related to SnO2 was also observed too, which is due to the FTO substrate [22]. The XRD pattern of the ZnO nanostructures (nanorods and nanosheets), NiO nanoflowers and ZnO/NiO heterostructure matched the standard pattern of ZnO hexagonal crystal (JCPDS01-1136) and cubic NiO (JCPDS44-1159). Both XRD patterns of the ZnO nanorods and nanosheets show a (002) peak, although for the ZnO nanosheets the peak intensity is relatively much lower than for the nanorods. This is expected since the dominant growth direction of the ZnO nanosheets is not along caxis. This result clearly shows that the supramolecules affect the growth direction and resulted in a new morphology. Also a closer look of the NiO nanoflowers/FTO and NiO nanoflower/ZnO nanosheets indicates a relatively higher intensity when growing the NiO on top of the ZnO as shown in Fig. 3. Beside the modified morphology and growth direction of the ZnO nanostructures when using the supramolecules, the optical properties of the ZnO nanosheets and nanorods were investigated using photoluminescence (PL) for comparison. Fig. 4 shows the PL spectra of the obtained ZnO nanosheets and nanorods that were grown by the hydrothermal method. The PL spectra of both ZnO nanosheets and nanorods show the intense main peaks at approximately 380 nm, which is related to near band-edge transition of ZnO as due to the recombination of free and donor bound
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K. Khun et al. / Solid State Sciences 41 (2015) 14e18
Fig. 2. The SEM image of (a) the ZnO nanorods, (b) the NiO nanoflowers, (c) the ZnO nanosheets and (d) the cross section image of the ZnO/NiO composite nanostructures.
other reason for the strong UV absorption of the grown ZnO nanosheets may be due to the presence of many thin leaves leading to relatively larger surface-to-volume ratios than the ZnO nanorods. The UVevisible absorption spectra of the ZnO nanosheets and nanorods are shown in Fig. 5(a). The absorption spectra of ZnO nanostructures indicate that the ZnO nanosheets have strong absorption in UV region compare to the ZnO nanorods. In Fig. 5(b), the relationship between the (ahn)2 and hn photon energy is plotted. The optical band-gap value of the ZnO nanosheets was smaller “2.80 eV” than ZnO nanorods “2.98 eV”. Furthermore, for the ZnO nanosheets, two new bands with a non-pronounced intensity have
Fig. 3. The XRD spectra of ZnO nanostructures, the NiO nanoflowers and the ZnO/NiO nano-composites growth on FTO substrate.
excitons [23]. The other two peaks observed, with relatively lower intensity as compared to the main UV peaks are so called “visible” or “green-yellow” emissions and are due to recombination on the deep level defects in ZnO, oxygen vacancies and/or other interstitial defects. The PL spectra of ZnO nanorods and nanosheets are represented by red dotted line and blue solid line, respectively in Fig. 4. From that, it can be clearly seen that the ZnO nanosheets have more intense UV emission compared to ZnO nanorods. This also is in agreement with [24] where they attributed the enhancement in PL spectra to the effect of the two components. In our study, the two components are represented by HMT and the supramolecular. The
Fig. 4. The PL spectra of ZnO nanosheets (solid black line) and ZnO nanorods (dotted red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
K. Khun et al. / Solid State Sciences 41 (2015) 14e18
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Fig. 6. The IeV data obtained in the dark current (solid red line) and under UV light (dotted black line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. (a) The absorption spectra for the ZnO nanorods and nanosheets and (b) the plot of (ahn)2 versus photon energy for the ZnO nanorods and nanosheets.
been observed at around “2.56 and 2.79 eV”, respectively. Simply, all these modulation in the UV/visible spectra could be due to impurities as a direct influence of the supramolecular species added. They can result in electronic energy states within the band gap of the ZnO nanosheets [25]. Fig. 6 shows the rectifying IeV characteristics of the p-NiO/nZnO heterojunction devices measured in the dark (UV-OFF) and under UV with constant illumination of the wavelength 360 nm power 2 mWcm2 at room temperature. The IeV response of the pn heterojunction device is highly rectifying and nonlinear. From Fig. 6, it can be seen that the fabricated ZnO/NiO UV photodetector device is highly sensitive to UV illumination by increasing the photocurrent response in the forward region because of the more generation of electronehole pairs in the fabricated photodetector device when UV illumination is applied. The IeV curves for the AgeNiO nanoflowers and the FTO-ZnO nanosheets are both linear, showing the contacts are good ohmic as an inset in Fig. 6. Therefore, the characteristic rectifying and nonlinear behavior of the fabricated UV photodetector are due to the ZnO/NiO heterojunction [14]. The time response of the ZnO/NiO UV photodetector device was measured by using Auto-lab in the condition dark (UV off) and light (UV on) as shown in Fig. 7. The space between each illumination was 1e2 min to obverse the time response and the resetting of the developed photodetector device to reach its equilibrium position. We have repeated a number of cycles during the on/off of the UV
light and the photocurrent was observed to be consistent, repeatable, and stable. The enlarged rising and decaying edge time response of the proposed UV photodetector device was shown in Fig. 8. It can be seen clearly that the rising response time is about 100 ms and the decaying time response is also found to be 100 ms. The fast response time, repeatability and stability of the presented UV photodetector device are attributed to the high surface area to volume ratio of the nanocomposite of ZnO nanosheets/NiO nanoflowers. This is the main difference between these devices and UV photodetectors having a similar architecture have been reported in [14,17]. The mechanism of photoconduction in the NiO/ZnO hetrojunction includes the photo-generation of free carriers and the electrical transfer through the interface between the ZnO and the NiO. The free carrier generation of the ZnO and the NiO heterojunction related to the adsorption/desorption of oxygen and hydroxide molecules. The oxygen molecules are adsorbed onto the nanostructure surface and capture free electrons from ZnO. This generates a low conductivity depletion layer near the nanosheets interface (O2 þ e / O2). This depletion layer and the junction barrier of the ZnO/NiO heterojunction are the reason for the low
Fig. 7. The time response of the ZnO/NiO UV photodetector in condition on/off of UV light.
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K. Khun et al. / Solid State Sciences 41 (2015) 14e18
formation of the p-n heterojunction is forming a space-charge region at the interfaces between p-type NiO and n-type ZnO because of the diffusion and electrons and holes. This region creates an electric field that allows the electrons and holes to move in the opposite direction [19]. When the device was illuminated by the UV light with energy higher or equal to the energy band gap of ZnO and NiO, immediately the photo-generated electronehole pairs was separated and create the electric field in the space charge area. The electrons and holes are moving to the conduction band of the ntype ZnO and the valence band of the p-type NiO by electric force, respectively. 4. Conclusions In the present work, the desired size, shapes and optical properties of ZnO nanostructures were controlled by the supramolecules (riboflavin and melamine) which they use as the surfactant. The composite nanostructures of the obtained nanosheets-like n-type ZnO/p-type NiO nanoflowers-like were grown on the FTO substrate by the hydrothermal method. The photocurrent of the fabricated NiO/ZnO heterojunction was measured at room temperatures. The photodetector diode showed clearly a nonlinear and rectifying IeV behavior. The proposed UV photodetector has a high sensitivity, fast rising UV response approximately 100 ms, stable performances, low leakage current, large photocurrent and short delay time of 100 ms after the illumination of UV light. Acknowledgment We are thankful to International Science Programme (ISP), Uppsala University, Sweden and the Royal University of Phnom Penh (RUPP), Cambodia (IPPS CAM:01), who financially supported this research work. Fig. 8. The enlarged response time of the proposed ZnO/NiO UV photodetector for (a) the rising edge response and (b) the decaying edge response.
dark current response. When the UV light is shined on the UV photodetector devices, electrons (e) in the VB was exited to the CB with simultaneous created the same amount of holes (hþ) in the VB (ħn / hþ þ e). The electronehole pairs were separated under the influence of the electrostatic field induced by difference work function. Thus the photo-generated holes move toward the ZnO interface to desorb the oxygen ions, and due to this, the width of depletion layer at surface is reduced resulting in an increase of the free carrier concentration (O2 þ hþ / O2) The free electrons are collected with the passage of time at the interval of desorption and reabsorption of oxygen and hydroxide ions to increase the photocurrent until a saturation limit of the UV light is reached. The schematic diagram for the band structure of the developed ZnO/ NiO heterojunction is shown in Fig. 9. The process for the generation of photocurrent is explained in terms of the formation of p-n heterojunction between the ZnO and the NiO nanostructures. The
Fig. 9. The energy band structures of the ZnO/NiO nano-heterojunction.
References [1] M. Imada, A. Fujimori, Y. Tokura, Rev. Mod. Phys. 70 (1998) 1039. [2] H.Y. Hwang, Y. Iwasa, M. Kawasaki, B. Keimer, N. Nagaosa, Y. Tokura, Nat. Mater. 11 (2012) 103. [3] K. Khun, Z.H. Ibupoto, M. Willander, Phys. Status. Solidi A 210 (2013) 2720. [4] X. Ren, D. Chen, X. Meng, F. Tang, X. Hou, D. Han, L. Zhang, J. Colloid. Inter. Sci. 334 (2013) 183. € derlind, C. Vahlberg, M. Ahren, P.O. Ka €ll, [5] L. Selegard, V. Khranovskyy, F. So R. Yakimova, K. Uvdal, ACS Appl. Mater. Interface 2 (2010) 2128. [6] E. Munoz, E. Monroy, J.L. Pau, F. Calle, F. Omnes, P. Gibart, J. Phys. Cond. Matter 13 (2001) 7115. [7] Y. Xie, L. Wei, G. Wei, Q. Li, D. Wang, Y. Chen, S. Yan, G. Liu, L. Mei, J. Jiao, Nano. Res. Lett. 8 (2013) 188. [8] H. Ohta, H. Hosono, Mater. Today 7 (2004) 42. [9] K. Liu, M. Sakurai, M. Aono, Sensors 10 (2010) 8604. [10] L. Vayssieres, Adv. Mater. 15 (2003) 464. [11] Q.X. Jia, T.M. Mccleskey, A.K. Burrell, Y. Lin, G.E. Collis, H. Wang, A.D.Q. Li, S.R. Foltyn, Nat. Mater. 3 (2004) 529. [12] A.I. Inamdar, S.H. Mujawar, V. Ganesan, P.S. Patil, Nanotech 19 (2008) 325706. [13] W.W. Liu, B. Yao, B.H. Li, Y.F. Li, J. Zheng, Z.Z. Zhang, C.X. Shan, J.Y. Zhang, D.Z. Shen, X.W. Fan, Solid State Sci. 12 (2010) 1567. [14] H. Ohta, M. Hirano, K. Nakahara, H. Maruta, T. Tanabe, M. Kamiya, T. Kamiya, H. Hosono, Appl. Phys. Lett. 83 (2003) 1029. [15] D.S. Kim, T.J. Park, D.H. Kim, S.Y. Choi, Phys. Stat. Sol. A 203 (2006) R51. [16] I.S. Jeong, J.H. Kim, H.H. Park, S. Im, Thin Solid Films 447 (2004) 111. [17] H. Ohta, M. Kamiya, T. Kamiya, M. Hirano, H. Hosono, Thin Solid Films 445 (2003) 317. [18] A. Fujimori, F. Minami, Phys. Rev. B 30 (1984) 957. [19] Z. Zhang, C. Shao, X. Li, C. Wang, M. Zhang, Y. Liu, ACS Appl. Mater. Inter 2 (2010) 2915. [20] M.A. Abbasi, Z.H. Ibupoto, A. Khan, O. Nur, M. Willander, Mater. Lett. 108 (2013) 149. [21] C. pacholski, A. Kornowski, H. Weller, Angew. Chem. Int. Ed. 41 (2002) 1188. [22] S. Yagi, Appl. Phys. Lett. 76 (2000) 345. [23] X.L. Zhang, K.S. Hui, K.N. Hui, Mater. Res. Bull. 48 (2013) 305. [24] S. Manna, A. Saha, A.K. Nandi, Chem. Commun. 45 (2006) 4285. [25] V.E. Henrich, Prog. Surf. Sci. 50 (1995) 77.
Contents lists available at ScienceDirect
Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie
Supramolecules-assisted ZnO nanostructures growth and their UV photodetector application Kimleang Khun a, *, Sami Elhag a, Zafar Hussain Ibupoto b, Volodymyr Khranovskyy c, Omer Nur a, Magnus Willander a a b c
Department of Science and Technology, Campus Norrkoping, Linkoping University, SE-60174 Norrkoping, Sweden Dr. M. A. Kazi Institute of Chemistry, University of Sindh, Jamshoro, Sindh, Pakistan €ping University, SE-58183 Linko €ping, Sweden Department of Physics, Chemistry and Biology (IFM), Linko
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 October 2014 Received in revised form 23 January 2015 Accepted 26 January 2015 Available online 27 January 2015
In this work, the supramolecules (riboflavin and melamine) were used as a template for the selfassemble of ZnO nanostructures by adding into the growth solutions. After that, the composite nanostructures of nanoflowers-like p-type NiO/n-type ZnO nanosheets were grown following a two-step on the fluorine doped tin oxide substrate by the hydrothermal method. The riboflavin and melamine have a role in UV emission. Taking these advantages into account, the currentevoltage (IeV) characterization of the ZnO/NiO heterojunction was performed at room temperature and showed an obvious nonlinear and rectifying response. Strong UV absorption with fast switching was observed from the ZnO/NiO composite heterojunction. The proposed UV photodetector based on this nanocomposite is more stable, possesses fast rising and decaying time response of approximately 100 ms. Further, a low leakage current was observed for the presented device. The findings in the present study indicates that the controlled nanostructures morphology is important to develop the efficient nanodevices for various applications. © 2015 Elsevier Masson SAS. All rights reserved.
Keywords: ZnO nanosheets Supramolecules-assisted ZnO growth ZnO/NiO nano-composite UV absorption ZnO Nano-heterojunction UV photodetector
1. Introduction In the recent decade, transparent oxide semiconductors (TOSs) nanostructures have been very attractive due to their unique physical and chemical properties with potential for developing efficient nanodevices for various applications [1e5]. Among those nanodevices, UV detectors have been remained attractive because of their wide applicability in many fields such as chemical analysis, water purification, remote control, safe space to space communications and skin cancer care [6,7]. Due to chemical stability, besides being environmental-friendly with relatively low production cost the fabrication of UV detectors based on TOSs is promising [8]. ZnO belongs to the TOSs having a wide band gap of 3.34 eV, and has high charge capacity and good charge carriers transport properties [8,9]. A variety of ZnO nanostructures can be synthesized by many physical and chemical methods. Synthesis of ZnO
* Corresponding author. E-mail addresses: [email protected], (K. Khun).
[email protected]
http://dx.doi.org/10.1016/j.solidstatesciences.2015.01.011 1293-2558/© 2015 Elsevier Masson SAS. All rights reserved.
nanostructures has been followed by chemical solution methods [10]. Moreover, such synthesis has been achieved using different substrates, and organic molecules as a polymer/surfactant-assisted synthesis were incorporated to modulate the morphology of some metal oxide nanostructures [11,12]. However, p-ZnO is unstable and unreliable to be utilized for the fabrication of the UV detector i.e. when fabricating UV photodetectors based on ZnO p-n homojunctions [13]. Different UV photodetectors have been investigated using heterojunction of n-ZnO and other p-TOSs thin films [8,14e16]. The small foot-print of nanostructures enables the possibility of achieving devices of quality heterojunction with many other materials and on any substrate. Among the different p-TOSs nanostructures, NiO nanostructures have potential to fabricate p-n heterojunction with ZnO [14,17] because it's naturally p-type semiconductor, and has a wide band gap of 3.6 eV. Additionally, it has a weak absorption in the visible region, high hole mobility and can be achieved through low production cost methods [18,19]. Recently few UV photodetector devices based on heterojunction composed of ZnO and NiO nanostructures were reported [20]. However, some of these devices still have some disadvantages such as large leakage current, unstable and low
K. Khun et al. / Solid State Sciences 41 (2015) 14e18
photo-generated current and they demonstrate a relatively long recovery time. In this work, the ZnO nanosheets with high absorption ability were grown by a supramolecules-assisted hydrothermal method. Supramolecules; also called riboflavin and melamine have been used as the substance that would yield a desired morphology of ZnO nanostructures. The synthesized nanocomposite is utilized in the development of highly sensitivity UV photodetector. The prepared ZnO/NiO nanocomposite and the developed UV sensor were characterized by appropriate techniques. The fabricated ZnO/NiO UV photodetector has shown the low leakage current, high sensitivity, repeatability, stability, fast response and short delay time. 2. Experimental section Firstly, ZnO nanosheets with strong UV absorption were grown on fluorine doped tin oxide (FTO) substrate by the hydrothermal method. ZnO nanorods growth solution was 0.05 M of each zinc nitrate hexahydrate, and hexamethylenetetramine (HMT) dissolving in a 100 ml of de-ionized water. This solution was added to well-known components of supramolecules (() riboflavin and melamine) with equal concentration of 8 mM to tune the grown ZnO morphology to nanosheets instead of nanorods. A seed layer of ZnO nanoparticles [21] was coated two times on the FTO glass substrate using sin coating at 2000 r.p.m for 20 s. The seed coated substrate was annealed at 120 C for 15e20 min for binding the ZnO nanoparticles to the substrate. The annealed substrates were fixed into a Teflon sample holder and dipped into growth solution which was placed in an oven fixed at 93 C for duration of 5e6 h. After the completion of growth duration, the grown ZnO nanosheets were cleaned by de-ionized water, and then they were blown by a flow of N2 gas and were dried in air at room temperature. Secondly, NiO nanoflowers were prepared on top of the grown ZnO nanosheets substrate by the hydrothermal method. The grown ZnO nanosheets on FTO substrate were placed into the Teflon sample holder and dipped into the growth solution with 0.1 M of nickel nitrate hexahydrate and HMT in de-ionized water for a period of 4e6 h at 93 C. When the growth duration was completed, the samples were washed by de-ionized water and annealed at 450 C for 3e4 h to convert the nickel hydroxide phase into the NiO phase. Finally a top contact of 50 nm layer of silver was deposited onto the NiO to be used as an ohmic contact for electrical measurement. The schematic diagram of the presented UV photodetector device is shown in Fig. 1. The morphology of the ZnO/NiO nanocomposite was investigated by scanning electron microscopy (SEM), performed by the LEO 1550 Gemini field emission gun at 10 kV. All the XRD spectra
Fig. 1. The schematic diagram of the ZnO/NiO heterojunction UV photodetector.
15
were recorded using the same accusation parameters to enable relative comparison. The X-ray diffraction (XRD) scans were performed at the scan rate 0.1deg./sec by the Phillips PW 1729 Powder Differactometer, using CuKa radiation (l ¼ 1.5418 Å) for studying the crystal quality of the ZnO/NiO nanocomposite. The light emission features of the samples were studied by a microphotoluminescence (PL) setup. The excitation was performed by a frequency doubled Nd:YVO laser as a continuous wave excitation source, giving a wavelength l ¼ 266 nm. The PL spectra were taken for the range 320e700 nm. Via using of microlens the signal was collected from an excited area of around 1.5 mm in diameters. The emitted luminescence was collected and mirrored into a single grating 0.45 m monochromator equipped with a liquid nitrogen cooled Si-CCD camera with a spectral resolution of ~0.1 meV. Via control of the laser transmittance the power excitation density was kept ~50 mW, being then concentrated on the surface as maximal excitation power 25 W/cm2. A PerkinElmer Lambda 900 UVevisible spectrophotometer was used for the investigation of the optical absorption. The UV photo-detector response was performed using an illumination of 365 nm wavelength UV lamp with a power of 2 mW cm2 coupled to an Auto-lab. The currentevoltage (IeV) curves were measured by semiconductor characterization system (Keithley 4200-SCS). 3. Results and discussion Fig. 2 (a, c) show a typical SEM images of the ZnO nanosheets and nanorods grown on the FTO by the hydrothermal method. From both figures it can clearly be seen that well-aligned, ZnO nanorods have been changed to nanosheets due to the effect of the supramolecules. The ZnO nanosheets have highly faceted grains of an average size of approximately 200 nm, and are densely packed in a polycrystalline layer. In Fig. 2(b), they can be seen that the NiO nanoflowers are in fact a network of relatively thin sheets that cover the ZnO nanosheets surface. The cross section image of ZnO nanosheets and NiO nanoflowers exhibited that the interface of ZnO/NiO nanocomposite are in good connection as shown in Fig. 2(d). The crystal quality and phase purity of the ZnO nanorods, nanosheets, NiO nanoflowers and ZnO/NiO nanocomposite were characterized by XRD pattern as is shown in Fig. 3. The XRD pattern of ZnO/NiO nanocomposite has shown only peaks peculiar for ZnO and the NiO. However, a peak related to SnO2 was also observed too, which is due to the FTO substrate [22]. The XRD pattern of the ZnO nanostructures (nanorods and nanosheets), NiO nanoflowers and ZnO/NiO heterostructure matched the standard pattern of ZnO hexagonal crystal (JCPDS01-1136) and cubic NiO (JCPDS44-1159). Both XRD patterns of the ZnO nanorods and nanosheets show a (002) peak, although for the ZnO nanosheets the peak intensity is relatively much lower than for the nanorods. This is expected since the dominant growth direction of the ZnO nanosheets is not along caxis. This result clearly shows that the supramolecules affect the growth direction and resulted in a new morphology. Also a closer look of the NiO nanoflowers/FTO and NiO nanoflower/ZnO nanosheets indicates a relatively higher intensity when growing the NiO on top of the ZnO as shown in Fig. 3. Beside the modified morphology and growth direction of the ZnO nanostructures when using the supramolecules, the optical properties of the ZnO nanosheets and nanorods were investigated using photoluminescence (PL) for comparison. Fig. 4 shows the PL spectra of the obtained ZnO nanosheets and nanorods that were grown by the hydrothermal method. The PL spectra of both ZnO nanosheets and nanorods show the intense main peaks at approximately 380 nm, which is related to near band-edge transition of ZnO as due to the recombination of free and donor bound
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Fig. 2. The SEM image of (a) the ZnO nanorods, (b) the NiO nanoflowers, (c) the ZnO nanosheets and (d) the cross section image of the ZnO/NiO composite nanostructures.
other reason for the strong UV absorption of the grown ZnO nanosheets may be due to the presence of many thin leaves leading to relatively larger surface-to-volume ratios than the ZnO nanorods. The UVevisible absorption spectra of the ZnO nanosheets and nanorods are shown in Fig. 5(a). The absorption spectra of ZnO nanostructures indicate that the ZnO nanosheets have strong absorption in UV region compare to the ZnO nanorods. In Fig. 5(b), the relationship between the (ahn)2 and hn photon energy is plotted. The optical band-gap value of the ZnO nanosheets was smaller “2.80 eV” than ZnO nanorods “2.98 eV”. Furthermore, for the ZnO nanosheets, two new bands with a non-pronounced intensity have
Fig. 3. The XRD spectra of ZnO nanostructures, the NiO nanoflowers and the ZnO/NiO nano-composites growth on FTO substrate.
excitons [23]. The other two peaks observed, with relatively lower intensity as compared to the main UV peaks are so called “visible” or “green-yellow” emissions and are due to recombination on the deep level defects in ZnO, oxygen vacancies and/or other interstitial defects. The PL spectra of ZnO nanorods and nanosheets are represented by red dotted line and blue solid line, respectively in Fig. 4. From that, it can be clearly seen that the ZnO nanosheets have more intense UV emission compared to ZnO nanorods. This also is in agreement with [24] where they attributed the enhancement in PL spectra to the effect of the two components. In our study, the two components are represented by HMT and the supramolecular. The
Fig. 4. The PL spectra of ZnO nanosheets (solid black line) and ZnO nanorods (dotted red line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
K. Khun et al. / Solid State Sciences 41 (2015) 14e18
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Fig. 6. The IeV data obtained in the dark current (solid red line) and under UV light (dotted black line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 5. (a) The absorption spectra for the ZnO nanorods and nanosheets and (b) the plot of (ahn)2 versus photon energy for the ZnO nanorods and nanosheets.
been observed at around “2.56 and 2.79 eV”, respectively. Simply, all these modulation in the UV/visible spectra could be due to impurities as a direct influence of the supramolecular species added. They can result in electronic energy states within the band gap of the ZnO nanosheets [25]. Fig. 6 shows the rectifying IeV characteristics of the p-NiO/nZnO heterojunction devices measured in the dark (UV-OFF) and under UV with constant illumination of the wavelength 360 nm power 2 mWcm2 at room temperature. The IeV response of the pn heterojunction device is highly rectifying and nonlinear. From Fig. 6, it can be seen that the fabricated ZnO/NiO UV photodetector device is highly sensitive to UV illumination by increasing the photocurrent response in the forward region because of the more generation of electronehole pairs in the fabricated photodetector device when UV illumination is applied. The IeV curves for the AgeNiO nanoflowers and the FTO-ZnO nanosheets are both linear, showing the contacts are good ohmic as an inset in Fig. 6. Therefore, the characteristic rectifying and nonlinear behavior of the fabricated UV photodetector are due to the ZnO/NiO heterojunction [14]. The time response of the ZnO/NiO UV photodetector device was measured by using Auto-lab in the condition dark (UV off) and light (UV on) as shown in Fig. 7. The space between each illumination was 1e2 min to obverse the time response and the resetting of the developed photodetector device to reach its equilibrium position. We have repeated a number of cycles during the on/off of the UV
light and the photocurrent was observed to be consistent, repeatable, and stable. The enlarged rising and decaying edge time response of the proposed UV photodetector device was shown in Fig. 8. It can be seen clearly that the rising response time is about 100 ms and the decaying time response is also found to be 100 ms. The fast response time, repeatability and stability of the presented UV photodetector device are attributed to the high surface area to volume ratio of the nanocomposite of ZnO nanosheets/NiO nanoflowers. This is the main difference between these devices and UV photodetectors having a similar architecture have been reported in [14,17]. The mechanism of photoconduction in the NiO/ZnO hetrojunction includes the photo-generation of free carriers and the electrical transfer through the interface between the ZnO and the NiO. The free carrier generation of the ZnO and the NiO heterojunction related to the adsorption/desorption of oxygen and hydroxide molecules. The oxygen molecules are adsorbed onto the nanostructure surface and capture free electrons from ZnO. This generates a low conductivity depletion layer near the nanosheets interface (O2 þ e / O2). This depletion layer and the junction barrier of the ZnO/NiO heterojunction are the reason for the low
Fig. 7. The time response of the ZnO/NiO UV photodetector in condition on/off of UV light.
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formation of the p-n heterojunction is forming a space-charge region at the interfaces between p-type NiO and n-type ZnO because of the diffusion and electrons and holes. This region creates an electric field that allows the electrons and holes to move in the opposite direction [19]. When the device was illuminated by the UV light with energy higher or equal to the energy band gap of ZnO and NiO, immediately the photo-generated electronehole pairs was separated and create the electric field in the space charge area. The electrons and holes are moving to the conduction band of the ntype ZnO and the valence band of the p-type NiO by electric force, respectively. 4. Conclusions In the present work, the desired size, shapes and optical properties of ZnO nanostructures were controlled by the supramolecules (riboflavin and melamine) which they use as the surfactant. The composite nanostructures of the obtained nanosheets-like n-type ZnO/p-type NiO nanoflowers-like were grown on the FTO substrate by the hydrothermal method. The photocurrent of the fabricated NiO/ZnO heterojunction was measured at room temperatures. The photodetector diode showed clearly a nonlinear and rectifying IeV behavior. The proposed UV photodetector has a high sensitivity, fast rising UV response approximately 100 ms, stable performances, low leakage current, large photocurrent and short delay time of 100 ms after the illumination of UV light. Acknowledgment We are thankful to International Science Programme (ISP), Uppsala University, Sweden and the Royal University of Phnom Penh (RUPP), Cambodia (IPPS CAM:01), who financially supported this research work. Fig. 8. The enlarged response time of the proposed ZnO/NiO UV photodetector for (a) the rising edge response and (b) the decaying edge response.
dark current response. When the UV light is shined on the UV photodetector devices, electrons (e) in the VB was exited to the CB with simultaneous created the same amount of holes (hþ) in the VB (ħn / hþ þ e). The electronehole pairs were separated under the influence of the electrostatic field induced by difference work function. Thus the photo-generated holes move toward the ZnO interface to desorb the oxygen ions, and due to this, the width of depletion layer at surface is reduced resulting in an increase of the free carrier concentration (O2 þ hþ / O2) The free electrons are collected with the passage of time at the interval of desorption and reabsorption of oxygen and hydroxide ions to increase the photocurrent until a saturation limit of the UV light is reached. The schematic diagram for the band structure of the developed ZnO/ NiO heterojunction is shown in Fig. 9. The process for the generation of photocurrent is explained in terms of the formation of p-n heterojunction between the ZnO and the NiO nanostructures. The
Fig. 9. The energy band structures of the ZnO/NiO nano-heterojunction.
References [1] M. Imada, A. Fujimori, Y. Tokura, Rev. Mod. Phys. 70 (1998) 1039. [2] H.Y. Hwang, Y. Iwasa, M. Kawasaki, B. Keimer, N. Nagaosa, Y. Tokura, Nat. Mater. 11 (2012) 103. [3] K. Khun, Z.H. Ibupoto, M. Willander, Phys. Status. Solidi A 210 (2013) 2720. [4] X. Ren, D. Chen, X. Meng, F. Tang, X. Hou, D. Han, L. Zhang, J. Colloid. Inter. Sci. 334 (2013) 183. € derlind, C. Vahlberg, M. Ahren, P.O. Ka €ll, [5] L. Selegard, V. Khranovskyy, F. So R. Yakimova, K. Uvdal, ACS Appl. Mater. Interface 2 (2010) 2128. [6] E. Munoz, E. Monroy, J.L. Pau, F. Calle, F. Omnes, P. Gibart, J. Phys. Cond. Matter 13 (2001) 7115. [7] Y. Xie, L. Wei, G. Wei, Q. Li, D. Wang, Y. Chen, S. Yan, G. Liu, L. Mei, J. Jiao, Nano. Res. Lett. 8 (2013) 188. [8] H. Ohta, H. Hosono, Mater. Today 7 (2004) 42. [9] K. Liu, M. Sakurai, M. Aono, Sensors 10 (2010) 8604. [10] L. Vayssieres, Adv. Mater. 15 (2003) 464. [11] Q.X. Jia, T.M. Mccleskey, A.K. Burrell, Y. Lin, G.E. Collis, H. Wang, A.D.Q. Li, S.R. Foltyn, Nat. Mater. 3 (2004) 529. [12] A.I. Inamdar, S.H. Mujawar, V. Ganesan, P.S. Patil, Nanotech 19 (2008) 325706. [13] W.W. Liu, B. Yao, B.H. Li, Y.F. Li, J. Zheng, Z.Z. Zhang, C.X. Shan, J.Y. Zhang, D.Z. Shen, X.W. Fan, Solid State Sci. 12 (2010) 1567. [14] H. Ohta, M. Hirano, K. Nakahara, H. Maruta, T. Tanabe, M. Kamiya, T. Kamiya, H. Hosono, Appl. Phys. Lett. 83 (2003) 1029. [15] D.S. Kim, T.J. Park, D.H. Kim, S.Y. Choi, Phys. Stat. Sol. A 203 (2006) R51. [16] I.S. Jeong, J.H. Kim, H.H. Park, S. Im, Thin Solid Films 447 (2004) 111. [17] H. Ohta, M. Kamiya, T. Kamiya, M. Hirano, H. Hosono, Thin Solid Films 445 (2003) 317. [18] A. Fujimori, F. Minami, Phys. Rev. B 30 (1984) 957. [19] Z. Zhang, C. Shao, X. Li, C. Wang, M. Zhang, Y. Liu, ACS Appl. Mater. Inter 2 (2010) 2915. [20] M.A. Abbasi, Z.H. Ibupoto, A. Khan, O. Nur, M. Willander, Mater. Lett. 108 (2013) 149. [21] C. pacholski, A. Kornowski, H. Weller, Angew. Chem. Int. Ed. 41 (2002) 1188. [22] S. Yagi, Appl. Phys. Lett. 76 (2000) 345. [23] X.L. Zhang, K.S. Hui, K.N. Hui, Mater. Res. Bull. 48 (2013) 305. [24] S. Manna, A. Saha, A.K. Nandi, Chem. Commun. 45 (2006) 4285. [25] V.E. Henrich, Prog. Surf. Sci. 50 (1995) 77.