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Site-controlled Growth and Field Emission Properties of ZnO Nanorod Arrays Yang Zhang*,† and Ching-Ting Lee*,‡ Institute of Microelectronics, Department of Electrical Engineering, National Cheng Kung UniVersity, Tainan, Taiwan, and Department of Physics, Henan UniVersity, Kaifeng, Henan 475004, China ReceiVed: June 01, 2008; ReVised Manuscript ReceiVed: February 17, 2009
ZnO nanorod arrays with different structures were grown on Si substrates at different temperatures by using a chemical vapor transport process. The growth sites and population of ZnO nanorods were controlled by the primarily grown micropyramids. The density and size of these nanorods were greatly affected by the growth temperature. A growth mechanism is proposed which attributes the overgrowth of ZnO nanorods to the orientation adhesion and to the difference in the saturation pressure at different temperatures. The field emission (FE) measurements show that the high density arrays of small nanorods exhibit excellent FE properties. 1. Introduction One-dimensional (1-D) nanostructures have attracted considerable attention due to their novel physical properties1,2 as well as potential applications in various nanometer-scale devices.3,4 Among these novel properties, the field emission (FE) behavior of 1-D nanostructures with high aspect ratio is of particular interest due to their highly enhanced efficiency.5,6 Vertically well-aligned 1-D nanostructured arrays could be applied in flat displays, vacuum microwave amplifiers, and electron emitters.7,8 FE properties of numerous 1-D structured arrays such as 1-D carbon nanotubes,9,10 AlN nanorods,11 LaB6 nanowires,12 and ZnO nanorods13 have been studied extensively. In recent years, ZnO materials as electron emitters have attracted intensive research interests due to their easy preparation, high melting temperature and chemical stability.14 FE properties of various 1-D ZnO nanostructures including nanoneedle arrays,15 ultralong nanobelts,16 nanowire arrays,13 and nanorod arrays17,18 have been investigated in order to obtain efficient field emitter arrays.19 The density and tip size of nanorods play critical roles in the FE properties. For low-density arrays the emission current does not satisfy the application requirement due to the smaller population of emitters. However, highly dense arrays can greatly reduce the local field enhancement at the emitter tips due to the field screening effect. Therefore, it is important to control the array density of ZnO nanorods to obtain efficient field emission. The work on controlled growth of ZnO nanowires was pioneered by P. Yang.20 Recently, Wang et al.21 developed the density control of ZnO nanowire arrays by varying the thickness of the gold catalyst. Kumar et al.22 demonstrated that the density of ZnO nanowire arrays could be controlled by using different temperature ramp rates in vapor phase transport. Although these ZnO nanostructures and their FE properties have been investigated, controlled growth of antennalike structure of nanorods on pyramids and their FE properties have rarely been reported. In particular, antennalike structure of nanorod on pyramid helps the electron emission more than other structures. Therefore, it is necessary to successfully grow such an antennalike structure for electron emission. In this work, we reported the site* To whom correspondence should be addressed. E-mail: yzhang@ henu.edu.cn (Y.Z.);
[email protected] (C.T.L.). † Henan University. ‡ National Cheng Kung University.
controlled growth of ZnO nanorods by the epitaxial overgrowth of secondary nanorods on the primarily grown pyramids. The FE measurements of different ZnO nanorod arrays showed that the dense nanorods with small size exhibited excellent FE properties. 2. Experimental Section ZnO nanorod arrays with different densities were grown on Si substrates placed in different temperature regions by a vapor transport process. Zn powder (purity 99.5%), the source material, was placed at the sealed end of quartz tube in a furnace. The system was maintained at a pressure of about 30 Torr. Argon gas was used as the carrier gas with a constant flow rate of 200 sccm. Oxygen gas with 30 sccm flow rate was introduced into the furnace at 450 °C. Samples A, B, C, and D were placed at ∼13, 18, 23, and 28 cm from the center of furnace with their temperatures being ∼640, 630, 560, and 470 °C, respectively. The temperatures at different positions from the center of furnace tube were determined by the measured temperatures in an empty tube at an atmosphere, which can be used to approximate those of the vacuum system. The Zn source was maintained at 650 °C for 30 min. Then the furnace was cooled down to room temperature naturally. The morphologies of the samples were examined by field-emission scanning electron microscopy (FESEM, JEOL JSM-7000F). The crystal structure of the samples was studied by X-ray diffraction (XRD). FE measurements were carried out with a two-parallel-plate configuration at room temperature in a vacuum chamber with a pressure of less than 5 × 10-6 Torr. Indium tin oxide glass was used as an anode, and ZnO samples served as a cathode. The distance between the electrodes was 130 µm. 3. Results and Discussion Figure 1 shows SEM images of the samples of A, B, C, and D prepared at ∼640, 630, 560, and 470 °C, respectively. Sample A consists of some micropyramids on the Si substrate. Only a few short nanorods formed on the tips of those micropyramids, as shown in Figure 1a. Sample B shown in Figure 1b consists of micropyramids with thin nanorods on their tips. Sample C shows high-density ZnO nanorod arrays on the tips of small micropyramids. However, the size and structure of sample D grown at ∼470 °C nanorods differ from the other types of samples. The tip size of the overgrown nanorods is larger than
10.1021/jp8100927 CCC: $40.75 2009 American Chemical Society Published on Web 03/24/2009
Site-Controlled Growth and FE of ZnO Nanorod Arrays
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Figure 1. SEM images of samples A, B, C, and D grown at (a) 640, (b) 630, (c) 560, and (d) 470 °C, respectively. The magnifications of parts a, b, and d are 5000. The magnification of part c is 10000.
SCHEME 1: Schematic Diagram of ZnO Crystal Evolution of Morphology: Nucleation in Vapor, the Primary Growth of Hexagonal Prisms and Pyramids, and the Secondary Growth of Orientation Adhesion Growth
Figure 2. XRD patterns of samples A, B, C, and D grown at (a) 640, (b) 630, (c) 560, and (d) 470 °C, respectively.
the root size. In a word, the nanorods were almost overgrown on the tips of micropyramids. The sites and population of nanorods were almost controlled by the primarily grown micropyramids. The crystal structures of all samples in Figure 1 were characterized by XRD. The diffraction patterns of all samples A, B, C, and D are displayed in Figure 2, which shows all samples are polycrystalline with a hexagonal wurtzite structure. It can also be seen that there are clearly preferential orientations for all samples in the (002) plane of ZnO crystal. The (002) orientation becomes weak with decreasing growth temperature, which suggests that the crystal quality and growth orientation grown at a high temperature is better than those grown at a low temperature. As seen from Figure 1, the growth process of ZnO nanorods on the tips of micropyramids can be divided into two stages: the primary growth of pyramids and secondary growth of nanorods, as shown in Scheme 1. The ZnO grains were first grown into hexagonal prisms and then grown into pyramids. According to the periodic bond chain
(PBC) theory,23 in the epitaxial growth process, the growth rate of ZnO crystal planes in positive c axis is V(101j0) > V(101j1) > V(0001). As a result, the possible exposed crystal planes are hexagonal prism planes {101j0}, hexagonal single pyramid planes {101j1}, and top hexagonal planes (0001). This prediction is in agreement with the observation in the SEM images in Figure 1. Moreover, it is found that the nanorod densities of samples depend on the sizes of the pyramids. The nanorod density of sample C is larger than those of samples A, B, and D, suggesting that at 560 °C the ZnO nucleation rate is larger than the growth rate. Therefore, the density of micropyramids grown at 560 °C is larger than those grown at higher or lower temperatures. At the secondary growth stage, ZnO nanorods were overgrown on the tips of micropyramids by the orientation adhesion, and the density and size of nanorods were heavily affected by the growth temperature. On one hand, the effective exchange interaction of the energies between solid and vapor at the interface is the most crucial factor since it influences directly the kinetics of crystal. The highest-energy facet (0001) of ZnO crystals obtains Zn atoms in vapor at the highest force than other facets.24 Therefore, the coaxially shaped ZnO nanorods will grow along the [0001] direction, which results in the overgrowth of ZnO nanorods on the top surface (0001) of ZnO micropyramids. On the other hand, the density and size of
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Zhang and Lee
Figure 3. Saturation vapor pressure of zinc as a function of temperature.
nanorods are determined by the growth temperatures. After the growth of ZnO micropyramids at the first stage, the degree of supersaturation of the Zn vapor gets reduced due to the timedependent comsumption of Zn source. Meanwhile, the temperature at different positions from the center of the furnace also affects the zinc concentration due to the difference in the Zn saturation vapor pressure at different temperatures. At a higher temperature, the saturation vapor pressure of Zn is larger than that at a lower temperature. The relation between the saturation vapor pressure of liquid zinc and the absolute temperature (T) is given by25
(1)
Figure 4. (a) Current density vs applied electric field for ZnO nanorod arrayswithdifferentstructuresand(b)thecorrespondingFowler-Nordheim (F-N) plots.
where p is the pressure in atm and T is the absolute temperature. The saturation vapor pressure of Zn as a function of temperature is shown in Figure 3. As can be seen, decrease in temperature leads to dramatic decrease in the saturation pressure of Zn. An amount of Zn are nucleated from vapor. At the high temperature region, the Zn in vapor is partially carried by the carrying gas due to its relatively high saturation concentration. The as-grown nanorods are smaller and fewer, which is in agreement with Figure 1a. The saturation concentration of Zn in the low temperature region is less than in the high temperature region. Moreover, Zn vapor at the high temperature region which is closer to the center of furnace is partially carried to the low temperature region which is farther away by the carrying gas. Therefore, we could obtain denser and larger secondary nanorods on the pyramidal tips in the low temperature region farther away from the center of furnace. These results were proved by the observation of parts b-d of Figure 1. Similarly, Shen et al.26 reported that the formation of CdS nanostructures varied with the sample positions from the source material and demonstrated the effects of the vapor pressure and concentration of the source material on the nanostructures. Obviously, the concentration and growth rate of Zn are different at different stages of the growth of ZnO nanorods on the pyramidal tips. At the first stage, ZnO was rapidly grown at high concentration. The morphology was determined by the growth rate of ZnO crystal planes. However, at the second stage, the overgrowth of nanorods varied with the sample positions from the source material at low Zn concentration. The growth of the nanorods was attributed to the orientation adhesion due to the surface energy. Moreover, the growth temperatures at
different positions from the source greatly influence the density and size of nanorods. However, the tip size of the nanorods grown at 470 °C is larger than their root size as shown in Figure 1d. This can also be possibly affected by the distance from the source and the growing temperature. As the reaction proceeds, the supply of Zn vapor decreases and the rate of producing ZnO decreases accordingly. Meanwhile, the growing temperature cannot provide enough energy to satisfy the requirement of the growth of unstable (0001) plane. The growth of more stable {101j0} planes dominates at this stage, and the capping {101j0} planes begin to appear. Thus, the tip size of nanorods grown at 470 °C is larger than the root size. Vertical nanorod arrays are ideal FE emitters for applications. FE properties of ZnO nanorod arrays with different structures were measured at room temperature. Figure 4a shows the FE current density of the different ZnO nanorod arrays vs the applied electric field. (Because of the smaller quantity of nanorods of smaple A, the FE data are not shown in Figure 4.) It is found that the turn-on fields of three types of ZnO nanorod arrays are clearly different, which were defined as the applied field at the emission current density of 0.1 µA/cm2. The turnon fields of samples B, C, and D were about 5.22, 4.99, and 5.66 V/µm, respectively. It should be noted that the turn-on field of sample C with high density is lower than that of sample B with low density. Additionally, sample B with low density array fluctuated with the applied field below the turn-on field. The lower turn-on field of sample C would be attributed to the larger number of emitters compared to sample B. The current fluctuation for sample B may arise from some damaged longer
ln(p) )
-15250 - 1.255 ln(T) + 21.79 T
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J. Phys. Chem. C, Vol. 113, No. 15, 2009 5923
and/or thinner nanorods under higher local field, which caused the decrease in the emission current of ZnO nanorods. With the increase of the applied field, those relative thin nanorods can be damaged at relative high local field, leading to the decrease of current. As a result, a majority of nanorods together emitted electrons, and emission current did not fluctuate. For sample C with high-density nanorods, the local field near the nanorods was lower due to the stronger field screening, which prevented the damage of nanorods. Consequently, the fluctuation in current did not occur. The nanorod size also affects the FE properties. Because the tip size of sample D is much larger than those of sample B and C, the turn-on field of sample D is the highest among all samples. Therefore, the dense array with small nanorods has excellent FE properties. On the basis of the F-N model, the local field enhancement can be evaluated by the field enhancement factor (β), which can be estimated by the slope of the F-N plot. The F-N relation is given by27
J)A
( ) (
β2E2 Bφ3/2 exp φ βE
)
(2)
where J is the current density (A/m2), A and B are the constants with values of 1.54 × 10-6 AV-2 eV and 6.83 × 109 V eV-3/2 m-1, respectively, E is the electric field (V/m), and φ is the work function. The corresponding F-N plots are shown in Figure 4b. The value of β can be obtained from the slope of F-N plots, ln(J/E2) vs 1/E, taking the work function as 5.3 eV for ZnO.28 The values of β for samples B, C, and D are about 2334, 1494, and 1256, respectively. The local field strength Elocal at the tip of ZnO nanorod can be obtained from the equation, Elocal ) βV/d (V, the applied field, and d, the distance between two electrodes). This suggests that sample B with low density of ZnO nanorods has a stronge local electric field. With the increase of the applied field, the local field in the low density arrays will increase more quickly. This higher field can damage those relative thin nanorods, which leads to the current fluctuation. This is in good agreement with the current density change of FE as a function of the applied electric field, as shown in Figure 4a. By contrast, the local field near large nanorods in sample D is the lowest. Therefore, the dense arrays consisting of small ZnO nanorods have potential applications in the efficient FE field. 4. Conclusions ZnO nanorod arrays with different densities and sizes were fabricated by the site-controlled growth. The population of ZnO nanorods in an array was controlled by the secondary growth of nanorods on the tips of primarily grown pyramids. The growth process consists of the rapid growth in high concentration and the orientation adhesion from the surface energy. The hexagonal prisms and consequent pyramids are attributed to the difference in the growth rate of different crystal planes. The overgrowth of ZnO nanorods on the micropyramidal tips is due to the orientation adhesion. The density and size of these nanorods
are influenced by the difference of saturation pressure at different temperatures and the transport of source material by the carrying gas. The FE measurements indicated excellent FE properties for the dense arrays with small nanorods. Acknowledgment. This work was supported by the Department of Science and Technology of Henan Province (No. 084300510054), the National Science Council of Taiwan, and the Center for Micro/Nano Science and Technology of the National Cheng Kung University. References and Notes (1) Bannani, A.; Bobisch, C.; Mo¨ller, R. Science 2007, 315, 1824. (2) Wang, J.; Gudiksen, M. S.; Duan, X.; Cui, Y.; Lieber, C. M. Science 2001, 293, 1455. (3) Wang, X.; Song, J.; Liu, J.; Wang, Z. L. Science 2007, 316, 102. (4) Soci, C.; Zhang, A.; Xiang, B.; Dayeh, S. A.; Aplin, D. P. R.; Park, J.; Bao, X. Y.; Lo, Y. H.; Wang, D. Nano Lett. 2007, 7, 1003. (5) Yamashita, T.; Hasegawa, S.; Nishida, S.; Ishimaru, M.; Hirotsu, Y.; Asahi, H. Appl. Phys. Lett. 2005, 86, 82109. (6) Fan, S.; Chapline, M. G.; Franklin, N. R.; Tombler, T. W.; Cassell, A. M.; Dai, H. Science 1999, 283, 512. (7) Edgcombe, C. J. Phys. ReV. B 2005, 72, 45420. (8) Lysenkov, D.; Engstler, J.; Dangwal, A.; Popp, A.; Mu¨ller, G.; Schneider, J. J.; Janardhanan, V. M.; Deutschmann, O.; Strauch, P.; Ebert, V.; Wolfrum, J. Small 2007, 3, 974. (9) Kim, K.; Lee, S. H.; Yi, W.; Kim, J.; Choi, J. W.; Park, Y.; Jin, J. I. AdV. Mater. 2003, 15, 1681. (10) Cha, S. I.; Kim, K. T.; Arshad, S. N.; Mo, C. B.; Lee, K. H.; Hong, S. H. AdV. Mater. 2006, 18, 553. (11) He, J. H.; Yang, R. S.; Chueh, Y. L.; Chou, L. J.; Chen, L. J.; He, J. H.; Wang, Z. L. AdV. Mater. 2006, 18, 650. (12) Zhang, H.; Tang, J.; Zhang, Q.; Zhao, G.; Yang, G.; Zhang, J.; Zhou, O.; Qin, L.-C. AdV. Mater. 2006, 18, 87. (13) Cui, J. B.; Daghlian, C. P.; Gibson, U. J.; Pu¨sche, R.; Geithner, P.; Ley, L. J. Appl. Phys. 2005, 97, 44315. (14) Tseng, Y.-K.; Huang, C.-J.; Cheng, H.-M.; Lin, I.-N.; Liu, K.-S.; Chen, I.-C. AdV. Funct. Mater. 2003, 13, 811. (15) Zhu, Y. W.; Zhang, H. Z.; Sun, X. C.; Feng, S. Q.; Xu, J.; Zhao, Q.; Xiang, B.; Wang, R. M.; Yu, D. P. Appl. Phys. Lett. 2003, 83, 144. (16) Wang, W. Z.; Zeng, B. Q.; Yang, J.; Poudel, B.; Huang, J. Y.; Naughton, M. J.; Ren, Z. F. AdV. Mater. 2006, 18, 3275. (17) Li, C.; Fang, G.; Yuan, L.; Liu, N.; Li, J.; Li, D.; Zhao, X. Appl. Surf. Sci. 2007, 253, 8478. (18) Dev, A.; Kar, S.; Chakrabarti, S.; Chaudhuri, S. Nanotechnology 2006, 17, 1533. (19) Zhao, Q.; Zhang, H. Z.; Zhu, Y. W.; Feng, S. Q.; Sun, X. C.; Xu, J.; Yu, D. P. Appl. Phys. Lett. 2005, 86, 203115. (20) Yang, P.; Yan, H.; Mao, S.; Russo, R.; Johnson, J.; Saykally, R.; Morris, N.; Pham, J.; He, R.; Choi, H.-J. AdV. Funct. Mater. 2002, 12, 323. (21) Wang, X. D.; Zhou, J.; Lao, C. S.; Song, J. H.; Xu, N. S.; Wang, Z. L. AdV. Mater. 2007, 19, 1627. (22) Kumar, R. T. R.; McGlynn, E.; McLoughlin, C.; Chakrabarti, S.; Smith, R. C.; Carey, J. D.; Mosnier, J. P.; Henry, M. O. Nanotechnology 2007, 18, 215704. (23) Hartman, P.; Perdok, W. G. Acta Crystallogr. 1955, 8, 49. (24) Wang, G. Z.; Wang, Y.; Yau, M. Y.; To, C. Y.; Deng, C. J.; Ng, D. H. L. Mater. Lett. 2005, 59, 3870. (25) Gaskell, D. R. Introduction to the Thermodynamics of Materials, 3rd ed.; Taylor and Francis, London, 1995. (26) Shen, G.; Lee, C. J. Cryst. Growth Des. 2005, 5, 1085. (27) Zhirnov, V. V.; Lizzul-Rinne, C.; Wojak, G. J.; Sanwald, R. C.; Hren, J. J. J. Vac. Sci. Technol. B 2001, 19, 87. (28) Bai, X.; Wang, E. G.; Gao, P.; Wang, Z. L. Nano Lett. 2003, 3, 1147.
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