The study of external electric field effect on the growth of ZnO crystal

Transcription

1 ISBN The study of external electric field effect on the growth of ZnO crystal Evi Maryanti 1, B. Prijamboedi 2 *, Ismunandar 2 1 Chemistry Departement, University of Bengkulu 2 Inorganic and Physical Chemistry Group, Institut Teknologi Bandung * boedi@chem.itb.ac.id Abstract ZnO is one of group II-VI compound semiconductor material with the wide energy gap and large exciton binding energy of 60 mev at room temperature. ZnO has been developing as a promising material for new electronic and photonic devices. In this study, ZnO nanorods were grown using chemical solution deposition method under external electrical field at various precursor concentrations using zinc nitrates as precursor and temperature of 90 o C. We found that the growth of ZnO crystals on ITO substrate was affected by electrodes configuration through the ITO surfaces. The observed XRD pattern and SEM images showed that the concentrations of precursor solution as well as electrodes configuration affect the homogeneity, crystal size and orientation of ZnO nanorods. The highest homogeneity of size and growth orientation (c-axis) could be obtained when precursor concentration is 100 mm, with configuration where positive electrode faced on ITO surfaces and deposition time of 8 hours. The photoluminescence spectra show some sharp peak at 360 and 381 nm and its intensity increases if the direction of the external electrical field goes onto ITO surfaces. The increasing intensity of these photoluminescence spectra, which identified as transition of exciton state in ZnO indicated that ZnO nanorods have a good crystalline phase and optical properties. Keywords: ZnO nanorods, Chemical Solution Deposition method, electric field, photoluminescence Introduction ZnO has important electronic and photonic properties because it has a good thermal and chemical stability, large exciton binding energy. ZnO also has a lot of interesting application, such as solar cell, tranparent conductive coating, gas sensor, electric and photoluminescence materials (4, 7, 11). Physics and chemicals synthesis method to produce one dimensional ZnO nanostructure have been developed, including vapor phase method (thermal evaporation, chemical vapour deposition, metal organic chemical vapour deposition (MOCVD), electrodeposition), solution method (sol gel synthesis, solution deposition, hydrothermal synthesis, microemulsion technique and direct growth on water-alcohol solution (8, 11, 13). Among the solution methods, chemical solution deposition (CSD) was an effective method to produce one dimensional nanomaterial. This technique has a lot of advantages such as structure control ability, can be performed at low temperature, applicable to a lot of type of substrates and needs a simple and low cost equipment. There are many publications about ZnO nanostructure grown with solution method. Vayssieres et al has prepared the highly oriented ZnO microrods and microtubes (10). In that study, Vayssieres report on the ZnO nanorods synthesis by decreasing of reagent concentration to produce a lower ZnO rod diameter from 1-2 µm reduced to nm (11). Meanwhile, Sugunan et al reported a hexamine action on ZnO nanowires growth through hydrothermal route. He showed that hexamine could work as a shape controller molecule by coating a nonpolar crystalography side of ZnO crystal, selectively (9). Yi et al already synthesis of ZnO nanorods homogeneity configuration by chemical bath deposition at low temperature (14). Another report also mentioned the synthesis of ZnO nanorods that growth on ITO glass substrate by solution deposition method (13), it showed that ZnO nanorods size increased when precursor concentration increase and give different density and nonorods morphology if different of time of reaction is applied. There is no publication about a method that can control size homogeneity and orientation of ZnO crystal nanosize. This research aims to synthesis ZnO nanorods by solution deposition method on ITO glass at low temperature and studying the effect of external electric field on ZnO crystal growth in order to have highly oriented ZnO crystal with high homogeneity. In the future, nanorods ZnO with size homogeneity and c-axis orientation will have a great potential as a optoelectronic nanodevice materials. Materials and Methods In this study, 1 mm, 10 mm, 100 mm of Zinc nitrate hexahydrate aqueous solution [Zn(NO 3 ) 2.6H 2 O, 98% of purity] and methenamine (C 6 H 12 N 4, 99 % of purity) were used. Zinc nitrate hexahydrate and methenamine solution (1:1) were filled in a glass bottle (14 x 6 x 2 cm). The ITO glass was used as subtrate, which were cleaned ultrasonically with 125

2 aceton, ethanol and deionized water for 20 minutes. Then, the substrates were immersed and tilted against the wall of bottle in the precursor solution at 90 o C in an oven for 8 hours without any stirring. Synthesis were done at different concentration (1, 10 and 100 mm) at 0, 2.5, and 4 kv of voltage and two position of positive electrode at substrate. They are B configuration (positive electrode was on the top of substrate) and A configuration (positive electrodes was on the bottom of substrate). Subsequently, the thin films were cooled to room temperature, washed with deionized water and dried in air. XRD (PAnalytical PW 3373), SEM (JEOL,JSM 6360), and spektrofluorometer (Shimadzu RF-5301 PC He-Cd 325 nm of`laser), were used to characterize its crystal structure, surface morphology and optical properties of ZnO crystal. From Fig. 1 it can be seen that the diffraction peak intensity increasing when precursor concentration increased. Diffraction pattern shows that ZnO crystal deposited on the substrate have hexagonal wurtzite structure. Figure 2 show different shape and size of ZnO crystal obtained during deposition process. Nanorods with the size nm, nm, 600 nm were obtained at the concentration of 1, 10, and 100 mm precursor. In general, the sizes of the ZnO nanorods become larger with the increasing of precursor concentration. It indicates that the precursor concentration was the important factor that governed the nanorods size and morphology, due to the critical diffusion of the monomers, limited growth, precipitation condition and ionic strength in ageing process and interfacial tension (3-14). Results and Discussions The effect of precursor concentration toward size and morphology of ZnO crystal Fig 1 shows XRD patterns of ZnO crystals deposited on ITO glass substrate with different concentration of precursor without external electric field effect (0 kv of voltage). The effect of precursor concentration toward optical properties of ZnO crystal Figure 3 show PL spectrum of ZnO nanorods at room temperature in excitation wavelength of 325 nm with different precursor concentration. Intensity (a.u) (002) (101) ZnO 100 mm, 0 V/cm, 8 hours ZnO 10 mm, 0 V/cm, 8 hours ZnO 1 mm, 0 V/cm, 8 hours Figure 1 XRD patterns of ZnO with different concentration of precursor without external electric field effect 2 θ ( 0 ) Figure 2 SEM images of ZnO nanorods with different concentration of precursor when the reaction time is 8 hours without electric field effect. (a) 1 mm; (b) 10 mm; (c) 100 mm. 126

3 PL Intensity (a.u) Wave Length (nm) Figure 3 PL spectrum of ZnO nanorods on ITO substrate with different precursor concentration and 8 hour of deposition time at room temperature. The spectrum show some sharp UV peak at 360 and 381 nm, and broad blue-green peak emission at nm. UV peak emission of ZnO at 360 nm (3.45 ev) is attributed to an exciton-related activity, which is a free exciton emission (2) and UV peak emission at 381 nm (3.25 ev) was a donor-bound exciton (1). Green peak emission was due to the point defect such as oxygen vacancy or impurity (13). Blue emission at 415 nm was due to exciton annihilation or donoracceptor pair (DAP) recombination (2). It is shown in Figure 3 that the UV emission has much sharper peak if the precursor concentration increases, while the blue-green emission decrease. It shows that the higher precursor concentration will give a result in better quality of ZnO nanorods (5). The effect of external electric field on ZnO crystal growth mm 0 V/cm 10 mm 0 V/cm 100 mm 0 V/cm In Fig. 4 we could see that intensity of the difraction peak is higher when we apply an electric field with configuration A compared with the result from sample that was prepared without an electric field. In sample prepared using configuration A, the intesity of (002) reflection peak is higher than other reflection peak, while in sample that was prepared without an electric field, the intensity of reflection peak is more dominant. Significant changes occurs when we used configuration B and the (002) reflection peak dominates the diffraction pattern. This result shows that the ZnO crystals tend to grow perpendicular to the ITO glass substrate surface (c-axis orientation). It can be concluded that external electric field at 100 mm of precursor concentration can produce highly c- axis oriented crystal. Intensity (a.u) (002) (101) θ ( 0 ) ZnO 100 mm, 2500 V/cm B, 8 hours ZnO 100 mm, 2500 V/cm A, 8 hours ZnO 100 mm, 0 V/cm, 8 hours Figure 4 ZnO diffraction pattern at 100 mm precursor concentration with and without external electric field. Figure 4 shows the XRD patterns of ZnO crystal deposited on ITO glass with precursor concentration of 100 mm. Figure 5 ZnO nanorods SEM images with and without external electric field at 100 mm precursor concentration. (a) without electric field, (b) with A electric field (positive electrode at bottom of substrate, (c) B electric field (positive electrode face on substrate). 127

4 Jum lah Blue peak emission was probably due to exciton annihilation or donor-acceptor pair (DAP) recombination (2), while green peak emission was due to oxygen vacancy in ZnO (4-6). The increasing intensity and the sharpness of UV emission shown by sample prepared in external electric field especially with configuration B (positive electrode on the top of substrate) could indicate that the external electric field improves the ZnO crystal quality. Diameter (nm) Figure 6 Distribution of diameter of ZnO nanorods at 100 mm precursor concentration, (a) configuration B, (b) configuration A, (c) without electric field. PL Intensity (a.u) mm 0 V/cm 100 mm 2500 V/cm A 100 mm 2500 V/cm B Fig. 6 show that there is a different of size (diameter) distribution of ZnO nanorods at 100 mm precursor concentration with and without electric field using. The diameter of ZnO nanorods with configuration A (~1-1.2 µm) is higher than B configuration (~ nm) and without electric field using (~ nm). It also be seen that sharper distribution is found in the ZnO nanorods that recieve electric field during its grown. These results strongly indicate that the electric field could improve the crystalinity and homogeneity of ZnO crystals. This could be expected since the ZnO cyrstal structure along the c-axis consist of polar surface of Zn and O atoms, therefore the electric field could allign the growth of ZnO crystal with the internal dipole of Zn and O atom in parallel with the electric field. The effect of external electric field on optical properties of ZnO crystal Photoluminescence spectra of ZnO nanorods at 100 mm concentration with and without external electric field is showed in Fig. 7. From Fig. 7 we could see sharp emission peak at 359 and 383 nm, broad emmition peak at 390 nm and broad emission bluegreen peak at nm. UV peak emission at 359 nm was a free exciton emission that become sharper with the external electric field treatment in deposition process, where configuration B showed the highest UV peak emission. A much stronger UV peak emission at 383 nm as donor bound exciton (DBE) emission were detected in the sample prepared with configuration B. A broad UV emission at 390 nm (3.18 ev) which can be attributed to free exciton-free exciton (biexciton) is observed from sample prepared with configuration A. As the external electric field used, we observed a increased in intensity of blue-green peak emission. Figure 7 PL spectra of ZnO nanorods on ITO glass substrate at 100 mm precursor concentration, deposition time of 8 hours with and without external electric field at room temperature. Conclusions In summary, the increasing of precursor concentration during the growth of ZnO crystal in the chemiscal solution deposition methods can increase the diameter of ZnO rods, improve its crystallinity and optical properties, which was showed in the XRD diffraction pattern, SEM images and photoluminescence at UV region (360 and 381 nm). The external electric field applied during the growth can also increase crystal orientation and homogeneity which was showed the domination of (002) reflection peak in the XRD pattern. The applied electric field with configuration B (positive electrode on the top of substrate), in particular, give better results and this were indicated by sharp distribution of the diameter of ZnO crystal, high intensity of excitonic emission in the photoluminescence spectra and smooth surface seen from the SEM images. References Wave Length(nm) Fonoberov, V. A., Alim, K. A., & Balandin, A. A Photoluminescence investigation of the carrier recombination processes in ZnO quantum dots and nanosrystals. Phys. Rev. B. 73:

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