NANOSTRUCTURAL ZnO FABRICATION BY VAPOR-PHASE TRANSPORT IN AIR

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1 International Journal of Modern Physics B Vol. 18, No. 0 (2004) 1 8 c World Scientific Publishing Company NANOSTRUCTURAL ZnO FABRICATION BY VAPOR-PHASE TRANSPORT IN AIR C. X. XU, X. W. SUN, B. J. CHEN, C. Q. SUN and B. K. TAY School of Electrical and Electronic Engineering, Nanyang Technological University, Nanyang Avenue, Singapore exwsun@ntu.edu.sg Received Nanostructural zinc oxide has been successfully fabricated by heating the mixture of ZnO and graphite powders in air. The growth of these zinc oxide nanostructures with respect to the growing time and temperature has been studied. The morphologies and the crystal structures have been characterized by scanning electronic microscopy and the X-ray diffraction. The results indicated that ZnO nanostructure formed mainly along the crystal orientation (002) on silicon substrate at moderate temperatures. The crystallization was improved by prolonging growth time and the morphologies mainly depended on the distribution of the growth temperature. The growth process was attributed to vapor-liquid-solid mechanism. Keywords: ZnO nanostructure; single orientation; vapor transport. 1. Introduction Zinc oxide, a direct wide bandgap semiconductor, has been intensively investigated in recent years because of its promising versatile applications 1 6 such as ultra-violet (UV) light emitting diodes (LED), UV laser diodes (LD) and sensors. Nanosized materials have attracted much attention due to their great potential for foundational research as well as for applications in nanodevice and technology. ZnO nanostructures, including nanodots, nanowires, nanorods, nanoribbons, nanotubes and nanowhiskers, have been prepared through many kinds of methods such as catalyst assisted vapor-phase transport, 7 metal organic vapor-phase epitexy, 8 aqueous thermal decomposition 9 and porous templates. 10 Some nanodevices have been demonstrated with these materials in the form of biological and chemical sensing 11 and one dimensional light-emitting. 12 Optimizing the fabrication process is very important for these potential applications. Up to now, vapor-phase transport method has been widely employed to prepare nanostructural ZnO. However, in general, catalyst such as gold and carrier gas such as the mixture of oxygen Corresponding author. 1

2 2 C. X. Xu et al. and argon are introduced into growth system. In this paper, a very simple method has been used to fabricate ZnO nanostructure with mainly orientated crystalline structure along (002) direction on silicon substrate. No any carrier gas and catalyst were used during the growth of the ZnO nanostructure. The fabrication process has been investigated by controlling the growth temperature and time. 2. Experimental ZnO structures have been prepared by a simple vapor-phase transport method in air. High purity ZnO (99.999%) and graphite powders were mixed thoroughly and then placed at the end of a slender one-end sealed quartz tube. Strips of silicon with (100) orientation were used as substrates, which were put into the quartz tube to collect the nanostructures. There is no metal catalyst in our experiment. Then the small quartz tube was placed into a bigger quartz tube with both sides open. The bigger quartz tube was placed in a tube furnace with controllable temperature such that the ZnO and graphite powder mixture was at the center of the furnace where the thermal couple is located. The ZnO and graphite powder mixture was heated at 1100 C for 5, 10, 20, 30 and 45 minutes respectively. The temperature of the silicon strip was varied from 1000 C to 650 C depending on the distance from the powder source due to the temperature gradient. After growth for desired time, the samples were taken out from the furnace into air for them to cool down rapidly. A layer of white sponger-like products was observed at the middle of the silicon strip and a grey layer extended from the middle range to both ends of the silicon strip. The morphologies of the products were characterized by JEOL JSM-5910LV scanning electron microscopy (SEM) at low and medium magnifications, respectively. The samples were cut into several pieces corresponding to different growing temperatures in order to carry out the X-ray diffraction (XRD) measurement. A Seimens D5005 XRD diffractometer was applied for the XRD measurement. The X-ray was generated from Cu K α1 line under the accelerating voltage of 40 kv and the current of 40 ma. 3. Results Figure 1 shows the SEM pictures of the white sponger-like middle part of the sample sintered for 45 minutes. The growth temperature gradually decreases from 880 C, 850 C to 800 C that correspond to Figs. 1a, 1b and 1c respectively. It can be seen from Figs. 1a and 1c that, the diameter of the disordered ZnO rods mainly varies from 80 nm to 200 nm, and the length of which is in the order of several tenfold microns, although a few thinner rods are about 20 to 30 nm in diameter and extend several hundred microns in length. Figure 1b shows the radially grown nanorods like cauliflowers which size is about 80 to 300 nm in diameter and several hundred microns in length. Figures 2a and 2b show the radial cauliflower-like rods grown at 850 C for 5 minutes and 10 minutes respectively. The radial cauliflower-like structures, as

3 Nanostructural ZnO Fabrication by Vapor-Phase Transport in Air 3 (a) 15kV µm SEI (b) 15kV µm SEI (c) 15kV µm SEI Fig. 1. SEM pictures of the ZnO nanostructures grown on silicon at various temperatures of (a) 880 C, (b) 850 C and (c) 800 C and for 45 minutes. shown in Fig. 2, appear close to those samples grown for 20 or 45 minutes in diameter and length. This indicates that the cauliflower-like rods grew very rapidly. It is amazing to note that a long ZnO rod could be formed in less then 5 minutes. However, it was hard to observe the disordered nanostructural ZnO rods on the sample grown for short time. This is probably related to the growth process of the nanostructures. We noticed that the growing area of ZnO nanostructure extended from the middle region ( 850 C) of silicon strips to both ends with prolonged growth time. This indicates the cauliflower-like ZnO rods rapidly grew at the medium temperature of about 850 C. On both higher and lower temperature regions, the morphologies were

4 4 C. X. Xu et al. (a) (b) Fig. 2. SEM pictures of the radial cauliflower-like rods grown at 850 C for (a) 5 minutes and (b) 10 minutes. different from the middle part and the growth speed was slow so the nanostructure was sparse. After 45 minutes growth, short nanorods with several microns long and nanodots extended outward from the cauliflower-like rods region to higher and lower temperature regions, and radially grown long rods kept on growing in the middle area as shown Fig. 1c and Fig. 2. Figure 3 shows the SEM pictures captured at the two ends of the sample corresponding to high and low temperature regions of 950 C (Fig. 3a), 900 C (Fig. 3b), 750 C (Fig. 3c) and 650 C (Fig. 3d) for 20 minutes growth. The diverse nanostructural features can be observed. In the region of low temperature (Figs. 3c and 3d), the nanostructures are so sparse that we can collect only several nanodots or short rods. As shown in Fig. 3a, the nanodots with about 100 nm size grows up along the vertical direction. The nanodots might act as nucleation sites for nanorods. Figure 3c shows several collective rods that might become the center of cauliflower-like rods for longer growth time. XRD were employed to investigate the crystalline structures of the as-grown ZnO nanostructures. Figure 4 shows the XRD patterns of the middle region of the sample obtained after 45 minutes growth. The growth temperature decreases from 880 C, 850 C to 830 C which correspond to Figs. 4a, 4b and 4c respectively. The XRD spectra shown in Figs. 4a 4c correspond to the morphologies in Figs. 1a 1c,

5 Nanostructural ZnO Fabrication by Vapor-Phase Transport in Air 5 (a) (b) 10kV µm SEI (c) (d) 10kV 2, µm SEI Fig. 3. Typical SEM images of ZnO nanostructures grown at various temperatures of (a) 950 C, (b) 900 C, (c) 750 C and (d) 650 C for 20 minutes. respectively. From Fig. 4, it can be seen clearly that the diffraction peaks matched with the typical hexagonal structure of ZnO except a strong peak corresponding to Si (311) and it grows mainly along the crystal direction (002). It is easily understood that the nanostructure grown at the lowest temperature area exhibits the weakest diffraction intensity as illustrated in Fig. 4c. The growth temperature of the cauliflower-like rods was intervenient between the disorder rod areas corresponding to higher and lower temperature regions respectively, but the XRD pattern presented by the cauliflower region is the strongest in intensity and the narrowest in spectral width for the (002) peak. This result indicates that the cauliflower-like rods have better crystallinity. So there is probably an optimal temperature for the best crystalline quality nanostructures. At the same temperature region of different samples, the appearances of ZnO structures were almost independence on the growth time except the width of the middle rods region increased with the increase of growing time. Similar to the sample obtained for 20 minutes growth, the XRD patterns of the samples grown for 5, 10 and 45 minutes also demonstrated the similar nanocrystal structures. These SEM and XRD measurements indicate that ZnO nanostructures are mainly influenced by the growth temperature rather than time. The growth time did not obviously affect the morphologies and orientation but not the crystal quality. Figure 5 shows

6 6 C. X. Xu et al. Si(311) (002) intensity (a.u.) (100) (101) (102) (110) (103) (112)(201) (b) (a) 0 (c) θ (degree) Fig. 4. XRD patterns of ZnO nanostructures grown at various temperatures of (a) 880 C with disordered nanorods feature shown in Fig. 1a, (b) 850 C with cauliflower-like feature shown in Fig. 1b, and (c) 800 C with disordered nanorods feature shown in Fig. 1c for 45 minutes. intensity (a.u.) min 10 min 20 min 45 min θ (degree) Fig. 5. The normalized XRD data for (002) peak of ZnO caulifower-like structures obtained at 850 C for (a) 5 minutes, (b) 10 minutes and (c) 45 minutes. the normalized XRD (002) peaks of the middle cauliflower nanorod regions grown for 5 min, 10 min, 20 min and 45 min respectively. It can be seen from Fig. 5 that, the full width at half maximum (FWHM) of the peaks decreases with prolonged growth. Meanwhile, the peak intensity enhances from 4024, 5764, to

7 Nanostructural ZnO Fabrication by Vapor-Phase Transport in Air corresponding to the growth time changing from 5 min, 10 min, 20 min to 45 min. This indicates that the crystallization of the nanostructures improves with the increase of growing time. 4. Discussion The formation process of the nanostructural ZnO can be interpreted by means of so-called vapor liquid solid (VLS) mechanism. 12,13 The growth speed and the resultant yield were obviously influenced by the quantity of the graphite powder. ZnO nanostructure could not be obtained by heating ZnO powders without carbon at the same experimental conditions owing to the high melting point of ZnO. We got a few ZnO nanorods when a small quantity of carbon powders were mixed into ZnO powder. When excess graphite powders were add into the source, the growth speed became faster and the bulk quantity of nanostructural ZnO were produced. Carbon and its reaction resultant with oxygen CO acted as reduction agents during formation ZnO nanostructures. ZnO powders were reduced into Zn or its sub-oxide with low melting point. The Zn and ZnO x vapor transfers to the low temperature region and recombined with oxygen to form nano-zno on the substrate. The vapor pressure of Zn, ZnO x and CO gradually decreased from the powder source to the open end of the quartz tube. The CO vapor suppressed ZnO formation due to its reduction at the high temperature region of the substrate. At another end of the substrate farther from the source, the vapor of Zn and ZnO x was too low for growing ZnO. This is why ZnO nanostructure with high density and high crystal orientation appeared at the middle of the substrates. The VLS mechanism was also confirmed by another experimental phenomenon. A uniform metal-like thin film with light black color was deposited on the wall of the large quartz tube near the open end of the tube when the same mixture was heated at the same temperature by exhausting the large quartz tube to about 2 Torr pressure. This indicated that Zn and ZnO x reduced from ZnO did not reform ZnO due to the lack of oxygen in low vacuum. The VLS growth process of nanostructural ZnO is based on zinc vapor transport, therefore, the growth process is influenced by many factors, such as source temperature, growth temperature, vapor pressure, ingredient of the source materials and growth time. In general, catalyst such as Au 7,14 16 is employed as catalyst, the mixture of O2 and Ar/N2 is introduced as carrier gas 7,12,15,16 to control growth process, and the experiment is conducted in sealed environment. In the present case, the ZnO nanowires were simply fabricated by sintering the mixture of ZnO and C in air. No catalyst and carrier gas were utilized. The growth process was only influenced by the ratio of ZnO and carbon powder, the total amount of the mixture, temperatures at source and substrate regions, and the growth time. So there are less process parameters to manipulate compared to the well-controlled VLS process. Obviously, the advantage of our method is simple. But the transport of zinc vapor from the source to the substrate is by diffusion, which could not be

8 8 C. X. Xu et al. controlled. Because of the zinc vapor gradient and temperature gradient at the substrate, uniform growth in large area is difficult. 5. Conclusion Nanometer-sized ZnO nanostructures have been prepared by a very simple vapor transport method in atmospheric pressure. The mixture of zinc oxide and graphite powders were sintered in air for different time and the nanostructural ZnO formed in different temperature region. XRD patterns demonstrated that the nanostructures grew mainly along (002) orientation. The morphologies mainly depended on the growth temperature. The cauliflower-like nanorods grew quickly in the middle temperature region. The growth area extended from middle to both ends of the substrate and exhibited disorder nanorods with increased growing time. The growth process has been explained by VSL mechanism. Acknowledgment This project is supported by Agency for Science, Technology and Research (A*STAR) of Singapore. References 1. D. Wiersma, Nature 406, 132 (2000). 2. H. Cao, J. Y. Xu, E. W. Seelig and H. P. R. Chang, Microlaser made of disordered media, Appl. Phys. Lett. 76, 2997 (2000). 3. D. C. Look, Mat. Sci. Eng. B80, 383 (2001). 4. P. Yang and C. M. Liber, Nature 419, 553 (2002). 5. R. F. Xiao, H. B. Liao, N. Cue, X. W. Sun and H. S. Kwok, J. Appl. Phys. 80, 4226 (1996). 6. X. W. Sun and H. S. Kwok, J. Appl. Phys. 86, 408 (1999). 7. M. H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber and P. Yang, Adv. Mater. 13, 113 (2001). 8. W. I. Park, D. H. Kim, S. W. Jung and G. C. Yi, Appl. Phys. Lett. 80, 4232 (2002). 9. L. Vayssieres, K. Keis, S.-E. Lindquist and A. Hagfeldt, J. Phys. Chem. B105, 3350 (2001). 10. Y. Li, G. W. Meng, L. D. Zhang and F. Phillipp, Appl. Phys. Lett. 76, 2011 (2000). 11. Y. Cui, Q. Wei, H. Park and C. M. Lieber, Science 293, 1289 (2001). 12. M. H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo and P. Yang, Science 292, 1897 (2001). 13. X. Duan and C. M. Lieber, Adv. Mater. 12, 298 (2000). 14. C. X. Xu and X. W. Sun, Appl. Phys. Lett. to appear. 15. S. C. Lyu, Y. Zhang, H. Ruh, H. J. Lee, H. W. Shim, E. K. Suh and C. J. Lee, Chem. Phys. Lett (2002). 16. H. Yan, R. He, J. Johnson, M. Law, R. J. Saykally and P. Yang, J. Am. Chem. Soc. 125, 4728 (2003).