J. Phys. Chem. C 2007, 111, 9081-9085
9081
Zinc Oxide Nanostructures and Their Core-Shell Luminescence Properties Xing Liao and X. Zhang* Laboratory of AdVanced Materials, Department of Materials Science and Engineering, Tsinghua UniVersity, Beijing 100084, People’s Republic of China ReceiVed: September 26, 2006; In Final Form: February 28, 2007
Two ZnO nanostructures with multisegment structure have been synthesized by a chemical vapor deposition (CVD) method on a silicon substrate. The top size of the nanostructures can reduce to 10-30 nm, which is beneficial to field emission applications. Scanning and transmission electron microscope methods are applied to analyze the morphology and microstructure of the nanostructures. Cathodoluminescence study shows that the ZnO nanostructures have core-shell luminescent structure that will affect significantly the mechanical and electrical properties of the ZnO nanostructures.
1. Introduction Zinc oxide (ZnO) one-dimensional nanostructures have attracted great attention in recent years for their distinguished properties. ZnO is a wide band gap semiconductor with a band gap of 3.37 eV and an excitation banding energy of 60 meV, which is much larger than other wide band gap semiconductors, so that ZnO nanowires are thought to be a good candidate for ultraviolet lasers.1,2 Field emission properties of ZnO nanostructures are another point of interest. ZnO one-dimensional nanostructures with different morphologies have been prepared to enhance the field emission properties by reducing the top size of the nanostructures. Lee et al.3 studied the field emission of a ZnO nanowire array. Zhu et al.4 have done research on field emission properties of a ZnO nanoneedle array. ZnO nanopencil,5 nanonail,6 and nanopin7 structures were also fabricated to improve the field emission properties. Moreover, measurement of mechanical and electrical properties of ZnO nanostructures is another hotspot of ZnO nanostructure research. Many different methods were used to measure Young’s modulus of ZnO nanowires and nanobelts and the results are controversial.8-11 I-V characteristic of ZnO nanowires was also studied by using field effect transistor technique and interesting results are obtained.12-14 Here, we report our chemical vapor deposition (CVD) synthesis of two ZnO multisegment nanostructures with potential applications in field emission, We studied the luminescence of the ZnO nanostructures using cathodoluminescence (CL) spectra and found that the ZnO nanostructures have core-shell luminescence structures. We propose a growth mechanism of the ZnO nanostructures. 2. Experimental Section In our experiments, Zn powder (99.99% purity) was used as source material and the synthesis of ZnO nanostructure were performed in a horizontal tube furnace (Figure 1a). About 1.0 g of Zn powder was spread in an alumina boat placed at the center of the furnace tube. Si(100) substrate was placed about 8 cm downstream of the boat to receive the products. A unilateral flow of Ar gas [200 sccm (standard cubic centimeter mass)] was used as the carrier gas. In stage 1 of the synthesis, * Corresponding author: e-mail
[email protected].
Figure 1. Schematic diagram of synthesis of ZnO nanostructures. (a) In the first stage, one end of the quartz tube is sealed while the other end is open to the air. (b) After the temperature reaches 750 °C, both ends of the quartz tube are sealed.
Ar gas was brought in from one end of the tube, and the other end of the tube was open to the air. The temperature at the tube center increased from room temperature to reaction temperature (∼750 °C) at a constant rate of 25 °C/min. When the temperature at the tube center reached 750 °C, the tube was immediately isolated from the ambient environment and the temperature at the tube was preserved for 90 min (stage 2, Figure 1b). After stage 2, the furnace power was turned off and the products were cooled down in the furnace to about 100 °C. In the process of synthesis, the Zn powder was heated, vaporized, transported along the Ar flow and finally deposited on the substrates to form the final products through reaction. The synthesized products were characterized by scanning electron microscopy (SEM; Hitachi 5500) and transmission electron microscopy (TEM; JEOL JEM-2010F). Cathodoluminescence (CL) spectra were performed by mono-CL, which is an attachment on the Quanta 200F environment scanning
10.1021/jp0663208 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/01/2007
9082 J. Phys. Chem. C, Vol. 111, No. 26, 2007
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Figure 2. SEM pictures showing typical morphologies of the ZnO nanostructure: (a, d) low-magnification images; (b, c, e, f) high-magnification images showing details of the nanostructures.
electron microscope. The photoluminescence (PL) of the products was measured on a Hitachi 850 fluorescence spectrophotometer with a Xe lamp at room temperature. 3. Results and Discussion The morphologies of ZnO nanostructures we synthesized are shown in Figure 2. About 7 cm downstream of the source materials, we obtained large amount of nanostructures that look like candles on the silicon substrate (Figure 2a). The structure of the body of the nanocandle is simple and can be seen as a nanorod with a diameter of 150-200 nm and a length of several micrometers. The wick part of the nanocandle is more complex. It is composed of several conjoined segments: a segment with continuously decreasing diameter follows with a segment with constant diameter. This cycle is repeated 2-4 times and finally comes out as a tiny rod with a flat top of about 30 nm in diameter (Figure 2b,c). A similar result was reported by Wang et al.,5 who named their nanostructure as a nanopencil. However, their nanopencil does not have the multisegment structure like our nanocandle, and the top of their nanopencil is sharp instead of flat. Therefore the growth mechanism of the two nanostructures is different and will be discussed later. At a place 2-3 mm downstream of the place we obtained nanocandles, another ZnO nanostructure was found, which we called nanomultitip. Instead of a single wick on the nanorod in the nanocandle, many small tips are found. The diameter of the small tips is about 10 nm, and the length ranges from 100 to 500 nm (Figure 2d,e). Figure 2f shows that at the top the tips are slightly twisted together. This ZnO nanostructure has not been reported yet. Many results4,15 have demonstrated that electrons are more easily emitted from ZnO nanostructures with sharp tips. Both nanostructures we synthesized have small size on the top so that these nanostructures will probably have potential application in field emission. Especially, the nanocandles have uniform diameter and are formed like an array, which will have advantages for the field emission properties.
Figure 3. TEM micrographs of the ZnO nanocandle: TEM image (a) and corresponding SAD (inset) and HRTEM image obtained at the joint part (b) and the top part (c).
To study the microstructure of the nanostructures, ZnO nanocandles and nanomultitips are characterized by TEM. Figure 3a shows detail of the multisegment structure of the nanocandle. From the SAD pattern (Figure 3a, inset) and high-resolution TEM (HRTEM) images (Figure 3b,c) obtained at the joint part and the top part, we found that the growth direction of the ZnO nanostructure is along the [0001] direction of the ZnO crystal and the whole structure is a single crystal. Comparison of the two HRTEM images reveals that the top part has more defects than the joint part. Figure 4a shows a low-magnification image of the upper part of the ZnO nanomultitip. The SAD pattern (Figure 4b) shows that there are several series of diffraction patterns, each with the same structure but with a small rotation from other patterns.
Cathodoluminescence of ZnO Nanostructures
Figure 4. TEM micrographs of the ZnO nanomultitips: TEM image (a) and corresponding SAD (b) and HRTEM images obtained from the different parts of tips (c and d).
J. Phys. Chem. C, Vol. 111, No. 26, 2007 9083
Figure 6. EDS spectrum of ZnO nanostructure.
Figure 7. Photoluminescence (PL) spectrum of ZnO nanostructure. Figure 5. Cathodoluminescence (CL) spectrum of ZnO nanostructure.
This is because all the tips have the same structure with the growth direction [0001] (Figure 4c,d). However, the tips slightly twist together at the top so that the orientations of the tips have a tiny perturbation. The luminescence structures of the two ZnO nanostructures were carefully investigated by the CL method (Figure 5). Two peaks were found: one at 377 nm corresponds to the recombination of free excitons between conductive band and valence band and is called near-band-edge emission. Another at 518 nm corresponds to the deep level emission. Deep level emission can have many origins such as single ionized oxygen vacancies,16 antisite oxygen,17 donor-acceptor complexes,18,19 and so on. For our nanostructures, we use energy-dispersed spectroscopy (EDS), which is equipped on the TEM to study the element ratio of the tip of the nanostructures. Figure 6 is the EDS spectrum of the nanostructures, which shows that they have only two elements: Zn and O. The inset of Figure 6 shows the weight and atom percentages of both elements. We find that the element ratio of Zn:O is 1:0.552, which means that many oxygen vacancies exist in the crystal. Associated with the EDS results, we think that in our experiment single ionized oxygen vacancies may be the most possible origin of the visible light luminescence. The photoluminescence (PL) spectrum of ZnO nanostructures shows similar features as the CL
spectrum, that is, a comparatively small, sharp peak at around 380 nm and a comparatively large, wide peak at around 500 nm (Figure 7) However, because the PL spectrum provides average information not only from nanostructures, like the CL spectrum, but also from substrate and other matters in a large area, the peak in the PL spectrum is not so sharp as that in the CL spectrum and the positions of peaks also have small changes. CL imaging was used to investigate the origin of the luminescence. Using panchromatic mode to image the ZnO nanostructures, we found that all the nanostructures are luminescent, and the core part of the nanostructure is a little darker than the outer part of the nanostructure (Figure 8a,d). Using monochromatic mode by selection of the energy windows around 375 and 515 nm, respectively, we obtained the monochromatic CL image at 375 and 515 nm. Figure 8b,e are the CL monochromatic images under 375 nm light, and they show that only the core of the nanostructure emits light, while the shell part of the nanorod and the tips emit very little light that can be ignored. However, when we used 515 nm light to image the same region (Figure 8c,f), we found that the core part is dark while the shell part and the tips are bright, indicating that the nanostructure is a core-shell structure. The core part has few oxygen vacancies so that near-band-edge emission is dominant. As the shell part and the tips have a large number of oxygen vacancies, deep level emission is the main luminescence factor. The shell part is estimated to be about 40 nm in width.
9084 J. Phys. Chem. C, Vol. 111, No. 26, 2007
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Figure 8. Mapping of the CL properties of ZnO nanostructures: (a, d) mapping with lights of all wavelengths; (b, e) mapping with lights of 372 nm only; (c, f) mappings with lights of 515 nm only.
A luminescence spectrum similar to that shown here has been reported.20,21 It was found that ZnO nanostructures have PL or CL spectra that have a wide and large deep level emission peak around 500 nm. Deficiency of oxygen is quite common in the process of fabricating ZnO nanostructure by CVD, especially in the last stage of synthesis. Moreover, the shell part will probably lose oxygen in the ambient environments. Thus, we think that this core-shell luminescence structure may also exist in many ZnO nanostructures like nanorods, nanoneedles, etc. This core-shell luminescence structure will affect significantly the mechanical and electrical properties of ZnO nanostructures. Large numbers of oxygen vacancies will weaken the crystal structure of ZnO so that the elastic modulus of the shell part is smaller than that of the core part. In mechanical properties of ZnO nanostructures, the shell part is the place that will endure a majority of stresses and strains so that the measurement results of intrinsic Young’s modulus of ZnO nanostructures will have a large error if the core-shell luminescence structure is not taken into account. The conductivity of ZnO with large amounts of oxygen vacancies will be larger than that of ZnO with few oxygen vacancies, so that the core-shell model will be helpful in the study of electrical properties of ZnO nanostructure. Furthermore, the ratio of the core part to the shell part will reduce with the reduction of diameter of ZnO nanostructures. When the diameter is reduced to about several tens of nanometers, the core part disappears and the whole structure can be regarded as the shell, so that the tip part shows only deep level emission. This can have a great effect on the measurement of mechanical and electrical properties of ZnO nanowires with small diameters. Several groups have reported different results of elastic modulus of ZnO nanowires. For example, Song et al.10 reported an elastic modulus about 26 GPa for a nanowire of 45 nm diameter. However, Chen et al.11 reported an elastic module of more than 170 GPa for nanowires with diameter of about 50 nm. We think that our model could be an explanation to the controversy. When the elastic modulus of ZnO nanowire is calculated, the diameter of the ZnO core, where there are few defects, is a crucial parameter. Two ZnO nanowires with similar diameters but different core-shell structures will have different calculated elastic moduli. For example, two ZnO nanowires may have similar diameterd of about 50 nm but different core-shell structures (one has a ZnO core diameter
Figure 9. Schematic diagram of the growth mechanism of ZnO nanostructures.
of 30 nm and the other has a ZnO core diameter of 40 nm). Their calculated elastic moduli will be different because they have different ZnO core diameters. The difference between the results of Song et al.10 and Chen et al.11 may be not so large if the core-shell structure is taken into account. On the basis of the core-shell model, we propose a threestage growth mechanism (Figure 9) for the nanocandle and nanomultitip. For the nanocandle, in the beginning of the synthesis there is plenty of oxygen gas in the tube because of isolation from the ambient environment. When the temperature is kept as a constant, the core part of the nanocandle with few
Cathodoluminescence of ZnO Nanostructures oxygen vacancies grows along the dominant direction of [0001]. This is just like the growth process of other ZnO nanowires and nanorods that follows the vapor-solid mechanism (Figure 9a). Oxygen is continuously consumed, and when the oxygen partial pressure drops to a critical value, the newly formed part, the shell part, of the nanorod will have a large number of oxygen vacancies (Figure 9b). When the furnace power is turned off and the temperature starts to drop, the diameter of the nanostructure begins to decrease. At the beginning of the cooling process, the cooling speed is large so that the decrease of diameter is also very quick. When the temperature at the center of the furnace reaches about 400 °C, the cooling speed slows down and the growth of ZnO reaches another equilibrium. The reduction of diameter will stop and ZnO nanostructures grown in this part of the process will have constant diameters. When the temperature drops further, the diameter of the ZnO nanocandle will decrease again. These processes will repeat several times. That is why we see the multisegments in the ZnO nanocandle. When the temperature reaches the lower threshold for ZnO nanorod growth, the growth of ZnO nanocandle is stopped. Compared with the two-stage growth mechanism of nanopencil in ref 5, there are several differences. One is that with continually provided oxygen gas to the reaction environment, like in ref 5, an outer layer of nanostructure with a large number of oxygen vacancies will not be synthesized. Another is that the cooling speed reported in ref 5 is slower than ours, so that they obtain a needlelike structure with gradually decreasing diameter rather than the multisegment structure on the top of the nanocandle. The growth mechanism of ZnO nanomultitips at the first and second growth stage is similar to that of ZnO nanocandles (Figure 9d,e). However, there are some differences at the moment when the temperature starts to decrease. Because the place the nanomultitip grows is downstream of the place the nanocandle grows, the growth temperature of nanomultitips is lower than that of nanocandles. Many small nuclei will form and grow into many small tips (Figure 9f), instead of the decrease in the diameter. 4. Conclusion Two ZnO nanostructures, ZnO nanocandle and ZnO nanomultitip, are synthesized by use of CVD. Their morphologies
J. Phys. Chem. C, Vol. 111, No. 26, 2007 9085 and microstructures are studied. The luminescent properties of the nanostructures are investigated by CL methods, and it is found that the nanostructures have a core-shell structure in which the shell part of the structure has many more oxygen vacancies than the core part. These nanostructures are thought to exist in many ZnO nanostructures and will have an effect on the properties of ZnO nanostructures. The growth mechanism of the nanostructures is discussed on the basis of the coreshell mode. These ZnO nanostructures are expected to have potential applications in field emission and gas sensing. References and Notes (1) Wong, E. M.; Searson, P. C. Appl. Phys. Lett. 1999, 74, 2939. (2) Huang, M. H.; Mao, S.; Feick, H.; Yan, H. Q.; Wu, Y.; Kind, H.; Weber, E.; Eusso, R.; Yang, P. Science 2001, 292, 1897. (3) Lee, C. J.; Lee, T. J.; Lyu, S. C. Appl. Phys. Lett. 2002, 81 (19), 3648. (4) 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 (1), 144. (5) Wang, R. C.; Liu, C. P.; Huang, J. L. Appl. Phys. Lett. 2005, 87 (1), 013110. (6) Shen, G. Z.; Bando, Y.; Liu, B. D.; Golberg, D.; Lee, C. J. AdV. Funct. Mater. 2006, 16, 410-416. (7) Xu, C. X.; Sun, X. W. Appl. Phys. Lett. 2003, 83 (18), 3806. (8) Bai, X. D.; Gao, P. X.; Wang, Z. L.; Wang, E. G. Appl. Phys. Lett. 2003, 82, 4806. (9) Yum, K.; Wang, Z.; Suryavanshi, A. P.; Yu, M. F. J. Appl. Phys. 2004, 96, 3933. (10) Song, J.; Wang, X.; Riedo, E.; Wang, Z. L. Nano Lett. 2005, 5, 1954. (11) Chen, C. Q.; Shi, Y.; Zhang, Y. S.; Zhu, J.; Yan, Y. J. Phys. ReV. Lett. 2006, 96, 075505. (12) Liu, C. H.; Yiu, W. C.; Au, F. C. K. Appl. Phys. Lett. 2003, 83 (15), 3168-3170. (13) Wan, Q.; Li, Q. H.; Chen, Y. J. Appl. Phys. Lett. 2004, 84, 3654. (14) Park, W.; Kim, J. S.; Yi, G. Appl. Phys. Lett. 2004, 85, 5052. (15) Wan, Q.; Yu, K.; Wang, T. H.; Lin, C. L. Appl. Phys. Lett. 2003, 83, 2253. (16) Vanheusden, K.; Warren, W. L.; Seager, C. H. J. Appl. Phys. 1996, 79, 7983. (17) Lin, B.; Fu, Z.; Jia, Y. Appl. Phys. Lett. 2001, 79, 943. (18) Studenikin, S. A.; Cocivera, M. J. Appl. Phys. 2002, 91, 5060. (19) Reynolds, D. C.; Look, D. C.; Jogai, B. J. Appl. Phys. 2001, 89, 6189. (20) Zhang, Z.; Yuan, H.; Zhou, J. J. Phys. Chem. B 2006, 110, 85668569. (21) Li, D.; Leung, Y. H.; Djurisˇiæ, A. B. Appl. Phys. Lett. 2004, 85 (9), 1601.
