Heat treatment effects on the surface morphology and optical properties of ZnO nanostructures

solidi status physica pss www.interscience.wiley.com reprints www.pss-rrl.com www.pss-c.com www.pss-b.com www.pss-a.com T N I R P E R Heat treatment effects on the surface morphology and optical properties of ZnO nanostructures solidi status pss physica Phys. Status Solidi C 7, No. 9, 2286–2289 (2010) / DOI 10.1002/pssc.200983722 c www.pss-c.com current topics in solid state physics M. Zainizan Sahdan*,1,3, M. Hafiz Mamat1, M. Salina1, Zuraida Khusaimi2, Uzer M. Noor1, and Mohamad Rusop1 1 Faculty of Electrical Engineering, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia 3 Faculty of Electrical and Electronics Engineering, Universiti Tun Hussein onn Malaysia, 86400 Batu Pahat, Johor, Malaysia 2 Received 4 June 2009, accepted 29 April 2010 Published online 14 June 2010 Keywords ZnO, CVD, nanocrystals, surface morphology, structure, annealing, photoluminescence * Corresponding author: e-mail zainizno@gmail.com, Phone: +6019 727 6903, Fax: +607 453 6060 Zinc oxide (ZnO) nanostructures have received broad attention due to its wide applications especially for thin-film solar cells and transistors. In this paper, we report the effects of heat treatment on the structural and optical properties of ZnO nanostructures. Zinc oxide nanostructures were synthesized using thermal chemical vapour deposition (CVD) method on glass substrate. The surface morphologies which were observed by scanning electron microscope (SEM) show that ZnO nanostructures change its shape and size when the annealing temperature increases from 400oC to 600oC. Structural measurement using X-ray diffraction (XRD) has shown that ZnO nanostructures have the highest crystallinity and smallest crystallite size (20 nm) when annealed at 550 oC. Furthermore, the samples were optically characterized using Photoluminescence (PL) spectrometer. The PL spectra indicate that ZnO nanostructures have the highest peak at UV wavelength when annealed at 550 oC. The mechanism of the PL properties of ZnO nanostructures is also discussed. We conclude that ZnO nanostructures deposited using thermal CVD have the optimum structural and PL properties when annealed at 550 oC. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction The reducing of size in electronic devices has produced self-assembled micro and nanostructured materials for commercial applications. There is also significant academic research interest in nano-systems as their properties are remarkably different from their bulk materials due to quantum confinement effect [1,2]. In quantum confinement theory, 1-dimensional confinement such as rods and wires increase carrier transport in light harvesting cells [3,4]. Therefore, much attention has been paid recently to the nanostructured materials such as zinc oxide (ZnO) and gallium nitrate (GaN) due to their ability to exhibit near ultra-violet emission. ZnO is an n-type semiconductor which has wide bandgap energy (~3.37 eV) and large exciton binding energy (~60 meV) [5,6]. Nanostructures and heterostructures made of ZnO have already been used as a transparent conductor in solar cells, varistors and sensors [7,8]. Recently, ZnO nanostructures have draw attentions for possible applications in optoelectronic devices such as nanostructured solar cells, ultra sensitive optical fiber sensors and UV laser diodes [9-11]. ZnO nanostructures are required to have high crystalline quality for most applications. Thermal annealing is a widely used technique to improve the crystal quality, which affect the structural, optical and electrical properties by reducing defects in material [1,12]. Therefore, understanding the effects of heat treatment on the structural and optical properties of ZnO nanostructures is of interest for various technologies employing this material. ZnO nanostructures can be prepared using various deposition techniques including spray pyrolysis, r.f. magnetron sputtering, pulse laser deposition, sol-gel and chemical vapour deposition (CVD) [13-15]. Among these techniques, CVD is a promising technique for its simple process flow, produce high conductivity films with fewer defects and at low cost. A gas blocker was introduced by the author in previous research to synthesis ZnO nanostructures [16]. Samples produced using this technique was annealed at different temperature ranging from 450 oC to 600 oC. The effects of heat treatment on the surface morphology of ZnO nanostructures was studied using field © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Contributed Article Phys. Status Solidi C 7, No. 9 (2010) 2287 emission scanning electron microscope (Zeiss Supra-Ultra) and the structural property was measured using X-ray diffraction (D8 Advance Bruker). The photoluminescence (PL) and transmittance properties were measured using PL (Horiba Jobin Vyon) and UltraViolet-Visible-Near Infra Red (Lambda 750), respectively. In this paper, we focus our discussion on the optical properties of ZnO nanostructures. a 2 Experimental setup The experimental setup to prepare ZnO nanostructures using thermal CVD by introducing gas blocker is shown in Fig. 1. The equipment consists of two thermal furnaces, temperature controller, horizontal alumina tube, and gas inlet and outlet. b Figure 1 The schematic diagram of thermal CVD system. Glass substrate was cleaned using acetone and sonicated for 10 minutes. 4 samples were prepared and sputtered with gold target (6 nm) using d.c. sputter coater. The sample was placed in the thermal furnace 2. The precursor was prepared using ZnO nanopowder mixed with graphite using same ratio (1:1). It was then placed in the thermal furnace 1 attached with a gas blocker. The parameters for the deposition process are adjusted and shows in Table 1. The deposition process is carried out for 1 hour. After that, the sample was cooled down at room temperature. The samples were annealed at different temperature ranging from 450 oC to 600 oC. After the annealing process was done, samples were characterized to study the effects of annealing temperature on the surface morphology, structural and optical properties of ZnO nanostructures. c Table 1 The parameters for ZnO nanostructures deposition. Parameter Value Temperature (T1) Temperature (T2) Ar gas flow rate 1000 oC 400 oC 1 litre/min 3 Results and discussion Figure 2 shows the surface morphology of ZnO nanostructures when annealed at different temperature. In Fig. 2(a), a worm-like ZnO nanostructure was synthesized when 450 oC was used as the annealing temperature. The diameter of the structure is approximately 72 nm (inset). Figure 2(b) shows the ZnO forms many grains on the substrate when annealed at 500 o C. The grain size is approximately 200 nm. In Fig. 2(c), ZnO needles were formed with diameter size www.pss-c.com d Figure 2 The SEM images of ZnO nanostructures annealed at different temperature, inset is the diameter of the structure; (a) 450 oC; (b) 500 oC; (c) 550 oC; (d) 600 oC. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim solidi physica c 2288 status pss M. Z. Sahdan et al.: On the surface morphology and optical properties of ZnO nanostructures of approximately 14 nm when annealed at 550 oC. In Fig. 2(d), a nearly similar structure as 2(c) was grown on the sample when it was annealed at 600 oC. However, the needles formed are shorter than in Fig. 2(c). By observing the surface morphology, we found that the annealing temperature has significant effects on the growth of the ZnO nanostructures. However, the growth pattern is not uniform. Figure 3 shows the structural property of ZnO nanostructures after annealing at different temperature using Xray diffraction (XRD). We focused on the (002) plane since this direction has effective ionic charges between the alternating Zn and O layers [17]. Stronger preferable orientation on (002) plane will increase the Hall mobility and therefore is suitable for optoelectronic applications [18]. The crystallite size (D) is determined using Scherrer formula using FWHM (full width half modulation) [21]. D = 0.9.λ/ (FWHM).Cos θ (1) Using formula Eq. (1) where λ is the X-ray wavelength (0.154 nm) and θ is the Brag diffraction angle, the calculated crystallite size is summarized in Table 2. Table 2 The crystallite of ZnO nanostructures at (002) plane. Temperature (oC) 450 500 550 600 FWHM 0.273 0.302 0.424 0.396 Crystallite size (nm) 31 28 20 21 We found that, as the annealing temperature increased from 450 oC to 550 oC, the crystallite size also increased. However, when the annealing temperature was increased to 600 oC, the crystallite size was decreased. We suppose that, for the structural growth of ZnO nanostructures, the optimum annealing temperature is at 550 oC. Figure 3 The (002) peaks of ZnO nanostructures annealed at different temperature; (a) 450 oC; (b) 500 oC; (c) 550 oC; (d) 600 oC. In Fig. 3, the (002) peak for the sample annealed at 450oC is diffracted at 34.5o. The diffraction angle for the sample annealed at 500 oC is 34.56o (shifted to right). However, the intensity of the peak is slightly decreased. When the sample was annealed at 550 oC, the intensity of (002) peaks has increased sharply and the diffraction angle is at 34.6o (shifted to right). Further increased on the annealing temperature to 600 oC has results on the decreasing of (002) peak intensity and also the diffraction angle has shifted to left at 34.57o. Referring to JCPDS card No. 361451, we found that as we increased the annealing temperature, the residual stress of the ZnO nanostructures also increased [19]. This stress is due to the thermal expansion coefficient (α) between ZnO (α = 7 x 10-6.C-1) and gold catalyst (α = 14 x 10-6.C-1) [20]. We supposed that due to large different in the thermal expansion coefficient, as the annealing temperature increased, the residual stress also increased and shifting the diffraction angle to right. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Figure 4 shows the photoluminescence (PL) spectra of ZnO nanostructures when annealed at different temperatures. As can be seen from the figure, each spectrum has sharp peak at UV region and broad peak at indigo and green-yellow regions. The PL spectrum centred at about 3.31 eV and 2.14 eV. The PL emission at indigo region is difficult to determine the centre point since it has many small peaks. Observation from PL spectra, we found that at the visible wavelength, as the crystallite size of ZnO decreased, the PL intensity will increase. However, at UV wavelength, although the crystallite size for annealing temperature at 500 oC is bigger (28 nm) than the 600 oC (21 nm), the PL intensity at UV emission for 500 oC is higher than 600 oC. We suggest that shape of the ZnO nanostructures also affect its optical properties as being reported by Buhro et al. [22]. As for the mechanism, the emission of ZnO nanostructures has been studied using full-potential liner muffin-tin orbital method. Peng has calculated the energy defect level and the result is shown in Fig. 5 [21]. The ultraviolet emission of ZnO (~3.31 eV) which is nearly the value of bandgap energy (3.37 eV) is due to the radiative recombination between electron and holes [22]. We supposed that, the blue-green emission (2.9, 2.8, 2.7 eV) is due to the electron transition from conduction band to the oxygen interstitial (Oi) which is the acceptor defect in intrinsic ZnO. The yellow-green emission is due to the electron transition from oxygen vacancy (Vo) which is the donor defect in intrinsic ZnO to the top level of the valence band. From the PL emission, the optimum annealing temperature is 550 oC. www.pss-c.com Contributed Article Phys. Status Solidi C 7, No. 9 (2010) 2289 Acknowledgements The authors would like to thank Universiti Tun Hussein Onn Malaysia and Ministry of Higher Education Malaysia for the financial support. References [1] F. E. Bacaksiz, S. Yılmaz, M. Parlak, A. Varilci, M. Altunbas, J. Alloys Compd. 478, 367-370 (2009). [2] K.M. Whitaker, S.T. Ochsenbein, V.Z. Polinger, D.R. Gamelin, J. Phys. Chem. C 112, 14331-14335 (2008). [3] W.E. Buhro, V.L. Cohvin, Nature Mater. 2, 138-139 (2003). [4] Huynh, W.U. Dittmer, J.J. Alivisatos, Appl. Phys. Sci. 295, 2425-2427 (2002). [5] M. R. Islam, J. Podder, Cryst. Res. Technol. 44, 286-292 (2009). [6] R. Ding, H. Zhu, Y. Wang, Mater. Lett. 62, 498-500 (2008). 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Rafaie, Z. 4 Conclusion ZnO nanostructures were synthesized Khusaimi, U. M. Noor, and M. Rusop, Am. Inst. Phys. Conf. using thermal CVD by employing a gas blocker. The efProc. 1136, 307-311 (2009); doi:10.1063/1.3160153. fects of annealing temperature on the structural and optical properties were studied by varying the temperature from [17] Y. Gu, X. Li,W. Yu, X. Gao, J. Zhao, C. Yang, J. Cryst. Growth 305, 36-39 (2007). 450 oC to 600 oC. Different surface morphologies were ob[18] Hua-Chi Cheng, Chia-Fu Chen, Chien-Yie Tsay, Jih-Perng tained when different annealing temperature was used in Leu, J. Alloys Compd. 475, 46-49 (2009). the heat treatment of ZnO nanostructures. Using Scherrer [19] Y. Zhu, Y. Chen, X. Zhang, Q. Chen, Y. Shao, Mater. Lett. formula, the crystallite size of each sample can be obtained. 63, 1242–1244 (2009). The crystallite size decreases as the temperature increased from 450 oC to 550 oC, but decreases when annealed at 600 [20] Y.C. Lee, S.Y. Hu, W. Water, K.K. Tiong, Z.C. Feng, Y.T. o Chen, J.C. Huang, J.W. Lee, C.C. Huang, J.L. Shen, M.H. C. Study on the photoluminescence has found that the Cheng, J. Lumin. 129, 148-152 (2009). ZnO nanostructures have optimum UV emission when ano [21] M. Kumar, R.M. Mehra, S.Y. Choi, Curr. Appl. Phys. 9, nealed at 550 C. It is found that, the photoluminescence 737-741 (2009). has strong correlation with the crystallite size. The UV [22] X. Peng, H. Zang, Z. Wang, J. Xu, Y. Wang, J. Lumin. 128, emission of ZnO nanostructures is inversely proportional 328-332 (2008). with the crystallite size. The absorbance of ZnO nanostructures also is very sensitive at UV region. From the absorb- [23] Q. Li, Z. Kang, B. Mao, E. Wang, C. Wang, C. Tian, S. Li, Mater. Lett. 62, 2531-2534 (2008). ance spectra, the transmittance of ZnO nanostructures also can be found. The optimum value of ZnO transmittance is obtained when the annealing temperature is 550 oC. Therefore, by observing the structural and optical properties of ZnO nanostructures, we suggest that using 550 oC as the annealing temperature will result the optimum structural and optical properties. www.pss-c.com © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim