Two-Dimensional Growth and Field Emission Properties of ZnO

The two-dimensional growth mechanism of the microtowers was investigated. ... Field emission measurements demonstrated that the microtowers with sharp...
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J. Phys. Chem. C 2007, 111, 4099-4104

4099

Two-Dimensional Growth and Field Emission Properties of ZnO Microtowers Feng Xu, Ke Yu, Qiong Li, and Ziqiang Zhu* Department of Electronic Engineering, East China Normal UniVersity, Shanghai 200062, People’s Republic of China

Takafumi Yao Interdisciplinary Research Center, Tohoku UniVersity, Sendai 980-8578, Japan ReceiVed: NoVember 7, 2006; In Final Form: January 19, 2007

ZnO microtowers were synthesized on silicon substrate by the vapor-phase transport method at a low temperature of around 550 °C under atmospheric pressure. The microstructure and morphology of the ZnO microtowers were evaluated by using X-ray diffraction and scanning and transmission electron microscopies. The two-dimensional growth mechanism of the microtowers was investigated. Room temperature photoluminescence spectra of the microtowers showed a UV emission band centering at about 388 nm and wide green emissions around 500 nm. Field emission measurements demonstrated that the microtowers with sharp tips of several nanometers in curvature radius possessed good performance with a turn-on field of ∼1.8 V/µm and a threshold field of ∼4.8 V/µm.

Introduction Zinc oxide (ZnO), one of the most important functional semiconductor materials with a direct wide band gap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature, has high mechanical strength, thermal stability, and chemical stability. Recently, the interest in field emission applications of ZnO based one-dimensional nanostructures, such as nanowires,1,2 nanorods,3 nanoneedles,4,5 and nanopencils,6 has been increasing because of their high aspect ratio, as well as other excellent inherent properties. As a result, many demonstrations reveal that the nanostructures with sharp tips have high current emission at a relatively low field,4-8 which is most attractive for field emission applications. Therefore, significant efforts are being devoted to reduce the tip size. On the other hand, an increase of emitting sites of the nanostructures has been proved to be another effective way to greatly enhance the emission current.9,10 However, up to now, most of the fabricated ZnO nanostructures with sharp tips have smooth trunks and the electrons can only be emitted from the tips of the nanostructures, which may restrict the further improvement of the field emission properties. Thus, it is still a challenge to develop some structures possessing a number of active emitting centers for a given emitter. On the other hand, to realize functional nanosystems, the integration of nanoscale building blocks into two- and threedimensional superstructures or complex functional architectures is a crucial step.11 Even though many ZnO hierarchical nanostructures including nanocombs, nanopropellers, nanoflowers, and nanobridges have been synthesized by the vaporphase process, the growth conditions usually require high temperatures or very low pressures.12-15 Therefore, it is urgently needed to develop some simple methods to controllably synthesize novel self-organized architectures to meet the demand for exploring the potentials of ZnO. * Author to whom correspondence should be addressed. E-mail: [email protected].

In this paper, unique tower-like ZnO microstructures, which not only have nanotips in the end but also have many edges and corners periodically presented around the stems surfaces, were synthesized by using a simple vapor-phase transport method at 550 °C. A clear structural evolution with the distance of the region to the source material was also observed. The twodimensional growth mechanism of the microtowers was discussed in detail. Furthermore, the photoluminescence and field emission properties of the as-grown products were investigated. Experimental Section The growth was performed in a conventional horizontal tube furnace at atmospheric pressure. High-purity Zn powder was loaded into a quartz boat. A strip of clean silicon substrate was then placed over the quartz boat and the vertical distance between the Zn powder and the substrate was kept at 5 mm. It was then pushed into the center of a preheated tube furnace at 550 °C. The carrier gas, argon, was introduced at one end of the quartz tube at a flow rate of 1000 sccm (standard cubic centimeters per minute), and the other end of the quartz was open to air. After 50 min of reaction, the quartz boat was taken out from the furnace and cooled down to room temperature. A white semitransparent layer was found on the Si wafer. The morphology and structure of the products were characterized by field emission scanning electron microscopy (FESEM, JEOL-JSM-6700F), X-ray diffraction (XRD, D/MAX 2550V) with Cu KR radiation and transmission electron microscopy (TEM, Philips Tecnai 20U-TWIN). Photoluminescence (PL) measurements were performed at room temperature, using a He-Cd laser line of 325 nm as an excitation source. Field emission properties of the samples were measured by using a simple diode configuration in a vacuum chamber under a pressure of 5 × 10-5 Pa. The Si substrate with nanomaterial (as a cathode) was separated from an indium tin oxide (ITO)/ glass anode by two Teflon spacers with a thickness of 200 µm. The measured emission area was about 10 mm2.

10.1021/jp067362z CCC: $37.00 © 2007 American Chemical Society Published on Web 02/27/2007

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Figure 1. (a) Low, (b) medium, and (c) high magnification SEM images of the ZnO microtowers. (d) XRD pattern of the as-grown products.

Results and Discussion Figure 1a shows a typical SEM image of the tower-like microstructures. It can be seen that there are a great many microtowers with sharp tips covering the surface of the substrate uniformly and compactly. Interestingly, every tens of microtowers are jointed together at the bottom but radially oriented on the top, forming a perfectly flower-like unit, as shown in Figure 1b. The high-magnification SEM image reveals that the microtowers become thinner along the length through a stepped growth until a spire with a sharp tip was formed. Apparently, each layer of the microtowers has six side surfaces and undergoes a gradual transformation from the hexagram shaped bottom to the hexagonal top. The lengths of the ZnO microtowers were more than 20 µm while the diameters ranged from several nanometers at the top to 5 µm at the base. An enlarged SEM image of the tip is inserted in Figure 1c to highlight the sharpness of the tip, from which the trace of stepped growth can still be clearly seen. Figure 1d illustrates the XRD pattern of the as-grown products. All the peaks are indexed to the typical wurtzite hexagonal phase of ZnO with lattice constants of a ) 3.250 Å and c ) 5.206 Å. No peaks for Zn or other impurities were detected in the spectrum, revealing the phase purity of the products. Figure 2a presents a TEM image of the spire of a single microtower, clearly demonstrating that the stem of the tower decreases to a sharp tip step by step, which is in agreement with the SEM images. A typical high-resolution TEM (HRTEM) image of the tip is shown in the inset of Figure 2a, revealing that the curvature radius of the tip emitter is as small as 6 nm. The sharp tip of the microtower is believed to be an ideal configuration for field emission application. Figure 2b is a

HRTEM image of the arrow marked part in Figure 2a. It gives a lattice fringe of about 0.52 nm and confirms the single-crystal nature of the ZnO microtowers growing along the [0001] direction. The corresponding selected area electron diffraction (SAED) pattern is consistent with the HRTEM results, as displayed in the inset of Figure 2b. Further investigation found that the morphology of the microstructures was very sensitive to its distance from the Zn source along the downstream side of the flowing argon. As the distance between the Zn source and the substrates increases, the morphology of the structures is also changed in an interesting manner. Figure 3 shows the SEM images of a series of ZnO microstructures with different morphologies along the carrier gas flow direction. As can be seen from Figure 3a, hexagonal heads were grown on the top of the microtowers. The heads are comprised of two layers of coaxial hexagonal nanounits, the top of which are smaller than the bottom one (Figure 3b). By comparing the SEM images of various microstructures, it was found that stems of all the structures have the same morphology as the microtowers while the heads gradually grow bigger with the increase of the distance to the source. Apparently, the diameter of the top head increases faster than that of the bottom one (Figure 3c,d). Figure 3e shows the transitional structures whose top and bottom heads have the same size of about 1.5-2 µm in diameters. Continually, the top heads grow larger than the bottom ones, and become so large that they almost cover the whole surface, as shown in Figure 3f. The inset of Figure 3f is a TEM image of a representative head broken off the stem, from which double-layer structure can be clearly seen. The top layer with the side length of about 5 µm is bigger than the bottom one, which is in good agreement with

ZnO Microtowers

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Figure 2. (a) TEM image of the spire of a single ZnO microtower. The inset is the HRTEM image of the tip. (b) HRTEM image of the part marked with an arrow in panel a. The inset is the corresponding SAED pattern.

Figure 3. (a-f) SEM images of four different morphologies of ZnO nail-like structures along the gas flow direction. The inset of panel f is the typical TEM image and SAED pattern of the head of the nail-like structures.

the SEM observation. The corresponding electron diffraction pattern is shown in the inset and the result also confirms the formation of a single-crystalline ZnO nanostructure with the growth direction along the [0001] direction.

Since no catalyst was used, the growth process of the obtained ZnO microstructures can be interpreted by vapor-phase mechanism.16 First, Zn is evaporated and oxidized by the oxygen in the reaction system to form ZnOx gases. There was so sufficient

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Figure 4. (a-c) Typical SEM images of ZnO microtowers in various growth stages.

vapor pressure in the initial stage that relatively big ZnO clusters are formed on the substrate in a short time. Then, further evaporation, oxidization, and nucleation cause homogeneous epitaxial growth of the microstructures from the ZnO clusters in various directions to form flower-like structures. To elucidate the growth mechanism of the single microtower, a series of SEM images of ZnO microtowers in various stages of their growth are shown in Figure 4a-c. It was observed that the layers of microtowers gradually increase with time, and a small single island was always found to be grown on the top surfaces of the structures at each stage. Thus, a two-dimensional growth mechanism is proposed for the microtowers, which will be discussed as follows. In our work, the observed island, which is believed to be formed due to the existence of an EhrlichSchwoebel barrier for diffusion of adspecies on the (0001) ZnO surface,17-19 is considered as playing an important role as centers of growth in promoting the formation of microtowers. Island slopes are selected such that the diffusion current on the side faces is zero, with this slope corresponding to the vicinal {011h1} planes in our case. Then the island grows by the continuous addition of atoms to the slopes, leaving behind a new section of c-face at the top of the island. When the area of this c-face becomes larger than the critical size for island nucleation, a new island can nucleate and the process will repeat itself.20 At last, each island grows along the six directions of 〈011h0〉 to form a hexagonal nanounit bounded with the six crystallographic, lower surface energy facets of {011h0} surfaces and capped with {011h1} planes, as shown in Figure 4a. Due to the existence of the {011h1} planes, the two-dimensional growth of the later formed nanounit may stop when it meets the top edges of the sloping faces of the base nanounit.21 Such a process is repeated again and again, resulting in a regular, hexagonal, tower-like morphology. However, if the Ehrlich-Schwoebel barrier is negligibly small, or does not exist, then the atomic layer completes before the next one nucleates so that the growth is layer by layer,21 yielding a smooth surface, as in the report about the growth of coaxial-shaped ZnO nanocolumns.22 Whether the Ehrlich-Schwoebel barrier dominates in the growth of microstructures or not may be affected by many experimental parameters, including reaction temperature, vapor dynamics, oxidation rate, etc.21 So far, nanonails of ZnO have been reported extensively with use of the vapor transport process.23-28 Lao et al.23 and Shen et al.24 attribute the growth of ZnO nanonails to the fact that the nanorod cap had a better chance to absorb ZnOx vapor than the bottom, which results in a gradual reduction in diameter from the cap to the bottom. They also reported that the increase of the nanonail in its length was promoted by the ZnOx vapor absorbed on the bottom. As can be seen in our results, however, the shaft of the nail-like structures has the same construction as the microtowers, whose diameters gradually become smaller

Figure 5. Photoluminescence spectra measured at room temperature from the ZnO microtowers and the heads of the nail-like structures shown in Figure 3f.

along the growth direction, and the heads were grown on the top of the sharp tips. Thus, it is different from the case as reported, indicating a different growth mechanism. In fact, we can conclude that the heads were grown on the stems during the last phase of the growth process from the SEM observation, which was most possibly caused by the abrupt change of the Zn-O concentration.25,28 If there are no such changes, they will grow along the same process to form sharp tips. When the growth frontier of the microtowers reaches the point where the Zn-O concentration is in favor of the faster lateral growth along the six directions of 〈011h0〉, the heads start to be formed on the top of the microtowers. The different size of the heads may be due to the different growth rate, corresponding to the spatial concentration distribution of the gas phases relative to the source materials. Photoluminescence of the ZnO microtowers in Figure 1a and the heads of the microstructures in Figure 3f were also investigated at room temperature and the result is shown in Figure 5. Both spectra exhibit UV emission centered at about 388 nm and wide green emissions at around 500 nm. The former is in good agreement with the typically reported free exciton peak position,29 and could be explained by the near-band edge emission while the latter is attributed to the singly ionized oxygen vacancy in the ZnO microstructures, and results from the radiative recombination of a photogenerated hole with an electron occupying the oxygen vacancy.30 It should also be noted that the relative intensity of the green emission to UV emission of the heads is higher than that of the microtowers. It was reported that the oxygen vacancies responsible for the green emission are located at the surface.31 As compared to the heads, a higher surface area-to-volume ratio for the microtowers due to their smaller size and rougher surfaces favors a higher level

ZnO Microtowers

J. Phys. Chem. C, Vol. 111, No. 11, 2007 4103 and is closely related to the geometrical factor of the emitters. The decreasing value of β is consistent with the change of geometry of the tip structures, as can be seen from the SEM observation. In our case, the radially grown characteristic with almost all tips of the microtowers facing toward the anode is another important factor that promotes the excellent field emission of the microtowers. Moreover, many corners and edges distributed along the main trunks were assumed to make a great contribution as additional emitting sites. Therefore, the asfabricated ZnO microtowers offer a promising candidate in future device applications such as flat panel displays and high brightness electron sources. Conclusions

Figure 6. J-E plots of the field emission from the selected three kinds of ZnO microstructures; the corresponding F-N plot is shown in the inset.

of surface oxygen vacancies, which can result in the fact that the microtowers have a higher ratio of green to UV emission than the heads. Figure 6 plots the field emission current density as a function of the applied electric field for the microtowers, and two typical nail-like microstructures with small and big heads, as shown in panels c and f of Figure 3. It is visible that the microtowers have the best field emission properties with the lowest turn-on field (defined as the field required to detect a current density of 0.1 µA/cm2) of ∼1.8 V/µm, and the lowest threshold field (defined as the field where the current density reaches 1 mA/ cm2) of ∼4.8 V/µm. At the same time, the turn-on field of 3.8 V/µm and the threshold field of 9.6 V/µm are obtained for the ZnO nail-like structures in Figure 3c. When the heads of the nail-like structures grow as big as shown in Figure 3f, the turnon field of those structures is found to be as high as 5.9 V/µm. Generally, it is well agreed that field emission mainly depends on the tip morphology and the density of the 1D nanostructures. Since the microtowers have much sharper tips compared to the nail-like structures, it is reasonable that electrons can be more easily emitted from the microtowers than from the nail-like structures based on the fact that nearly all kinds of the structures in our case have the same density. The above results indicate that the field emission properties of microtowers can be compared to those of the reported nanostructures with sharp tips, such as nanopins, nanopencils, and nanoneedle arrays.4,6,8 The extremely low turn-on field and threshold field for the ZnO microtowers may result from the nanoscale sharp tips. To further analyze the emission properties of the ZnO microstructures, the F-N law was employed to describe the exponential dependence between the emission current and the applied field. It can be expressed as J ) (Aβ2E2/Φ) exp(-BΦ3/2/ βE), where A and B are constants with the values of 1.56 × 10-10A V-2 eV and 6.83 × 103 V eV-3/2 µm-1, β is the field enhancement factor, and Φ is the work function of the emitter, which is 5.3 eV for ZnO. The inset of Figure 6 corresponds to the F-N plots of the three kinds of ZnO microstructures, showing that the field mission behaviors from the measured samples can be well described by the F-N law. From the averaged slope of the ln(J/E2)-1/E plots, the field enhancement factor β was estimated to be about 3105, 1480, and 731 for the ZnO microtowers and nail-like structures with small and big heads, respectively. It represents the true value of the electric field at the emitter compared to its average macroscopic value

In summary, ZnO microtowers have been successfully fabricated on Si substrate via the vapor-phase transport method at 550 °C. The two-dimensional growth mechanism was observed. A series of nail-like structures can be obtained along the downstream side of the gas flow, indicating that the morphology of the structures can be controlled by adjusting the position of the substrate from the source materials. Photoluminescence spectra of the obtained ZnO structures exhibit UV emission centered at about 388 nm and wide green emissions at around 500 nm. Field emission measurements of the microstructures demonstrated that the microtowers possessed excellent field emission performance, which provide us a promising future for using ZnO structures as a competitive cathode material in field emission microelectronic devices. Acknowledgment. The authors acknowledge the financial support from the Chinese National Key Basic Research Special Found (Grant No. 2006CB921700), the NSF of China (Grant No. 60476004), the Cultivation Fund of the Key Scientific and Technical Innovation Project, Ministry of Education (Grant No. 704022), Specialized Research Fund for the Doctoral Program of Higher Education (Grant No. 20050269004), and the Key Project of Scientific and Technology Committee of Shanghai. References and Notes (1) Lee, C. J.; Lee, T. J.; Lyu, S. C.; Zhang, Y. Appl. Phys. Lett. 2002, 81, 3648. (2) Jo, H.; Lao, J. Y.; Ren, Z. F. Appl. Phys. Lett. 2003, 83, 4821. (3) Zhao, Q.; Xu, X. Y.; Song, X. F.; Zhang, X. Z.; Yu, D. P.; Li, C. P.; Guo, L. Appl. Phys. Lett. 2006, 88, 033102. (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, 144. (5) Li, Y. B.; Bando, Y.; Golberg, D. Appl. Phys. Lett. 2004, 84, 3603. (6) Wang, R. C.; Liu, C. P.; Huang, J. L.; Chen, S.-J.; Tseng, Y.-K.; Kung, S.-C. Appl. Phys. Lett. 2005, 87, 013110. (7) Wan, Q.; Yu, K.; Wang, T. H.; Lin, C. L. Appl. Phys. Lett. 2003, 83, 2253. (8) Xu, C. X.; Sun, X. W. Appl. Phys. Lett. 2003, 83, 3806. (9) Yin, L. W.; Bando, Y.; Zhu, Y. C.; Li, M. S.; Li, Y. B.; Golberg, D. AdV. Mater. 2005, 17, 110. (10) He, J. H.; Yang, R.; Chueh, Y. L.; Chou, L. J.; Chen, L. J.; Wang, Z. L. AdV. Mater. 2006, 18, 650. (11) Shen, G. Z.; Bando Y.; Lee, C.-J. J. Phys. Chem. B 2005, 109, 10779. (12) Lyu, S. C.; Zhang, Y.; Lee, C. J.; Ruh, H.; Lee, H. J. Chem. Mater. 2003, 15, 3294. (13) Gao, P. X.; Wang, Z. L. Appl. Phys. Lett. 2004, 84, 2883. (14) Sun, X. H.; Lam, S.; Sham, T. K.; Heigl, F.; Ju¨rgensen, A.; Wong, N. B. J. Phys. Chem. B 2005, 109, 3120. (15) Lao, J. Y.; Huang, J. Y.; Wang, D. Z.; Ren, Z. F. Nano. Lett. 2003, 3, 235. (16) Brenner, S. S.; Sears, G. W. Acta. Metall. Mater. 1956, 4, 268. (17) Umar, A.; Hahn, Y. B. Appl. Phys. Lett. 2006, 88, 173120. (18) Baxter, J. B.; Wu, F.; Aydila, E. S. Appl. Phys. Lett. 2003, 83, 3797.

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