Field Emission from Injector-Like ZnO Nanostructure and Its

J. Phys. Chem. C 2008, 112, 13447–13449

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Field Emission from Injector-Like ZnO Nanostructure and Its Simulation Chen Li, Yunsong Di, Wei Lei,* Qin Yin,† Xiaobing Zhang, and Zhiwei Zhao School of Electronic Science and Engineering, Southeast UniVersity, Nanjing 210096, China ReceiVed: March 23, 2008; ReVised Manuscript ReceiVed: May 14, 2008

Injector-like ZnO nanostructures with different areal density and morphology (one or two needles) have been synthesized by the vapor-phase transport method. The cone-shape base of the injector-like ZnO nanostructure plays an important role in field emission. It holds up the needle to obtain a higher aspect ratio, separates the emission needles from each other to minimize the screening effect, and also reduces the risk of reaching an excessive temperature which is considered to be a cause of nanostructure breakdown. The sample with a long, sharp needle and medium areal density has the best FE performance due to the above three reasons. Moreover, the simulation of emission current density is in relatively good agreement with our experimental results. It shows that the highest current density is obtained when the spacing between the neighboring nanostructures is similar to the height for the single needle nanostructures. The nanostructures with double needles even reach higher emission current density when the space is two times the nanostructure’s height. Injector-like ZnO nanostructures exhibit a good field emission property and have great potential for applications in field emission devices. 1. Introduction One-dimensional (1D) nanostructures have received increased attention because of their excellent electrical and optical properties. Especially, the nanostructures with a good field emission (FE) property stimulated considerable interest in their potential applications in electron devices such as electron emitters of field emission display (FED),1 as a probe in a scanning electron microscope (SEM)2 and as a microwave power amplifier. 3 In the past few years, field emitters composed of carbon-based materials have been studied intensively.4,5 Recently, various kinds of zinc oxide (ZnO) nanostructures6–8 fabricated by vapor-phase transport9 and hydrothermal methods10 have been reported. In previous work, researchers have also attempted to improve the FE performance of ZnO nanostructure, such as growing aligned, ultralong ZnO nanobelts to increase the aspect ratio,11 doping gallium in ZnO to lower the working function and reduce the resistivity of the field emitters,12 and also depositing the ZnO nanostructures on carbon films to combine the high field enhancement factor of the emitters and the tunneling effect through a CNT/ZnO heterojunction.13 Besides the above attempts, minimizing the screening effect is also an important method to enhance FE performance. Some previous work involves the screening effect influence on FE performance of CNTs. Nilsson et al.’s14 simulation predicted that an intertube distance of about 2 times the height of CNTs optimizes the emission current density. However, Suh et al.’s15 experiment showed that the FE was optimal when the intertube distance was similar to the tube height. Consequently, Bonard et al.16 reported that 3 µm in height and 2 µm in length is an ideal scale for the CNTs to avoid a screening effect and ensure homogeneous emission. So, an agreement on what is the best intertube distance to height ratio of the CNTs still hasn’t been achieved. * To whom all correspondence should be addressed: Tel/Fax: 86-2583363222, e-mail: [email protected]. † Current address: Spansion LLC, Suzhou Design Center, China.

The screening effect was also an important factor which influences the FE performance of the ZnO nanostructures. In this study, we fabricated the injector-like ZnO nanostructures with different areal density and morphology (one or two needles). We tried to determine the optimal nanostructure spacing to height ratio to minimize the screening effect of the ZnO nanostructures. Computer simulation was also utilized for the electrostatic calculation to verify the experimental results. 2. Experimental Section Injector-like ZnO nanostructures were grown on a Si substrate by the vapor-phase transport method. A mixture of high purity ZnO and Zn powders was used as the source material which was loaded in a ceramic boat and placed in a tube furnace. The substrates used were n-type (111) Si wafers coated with a 2 nm Au layer by sputtering. In the synthesis, the growth temperature varied from 530 to 600 °C. A stream of Ar and O2 was blown into the furnace. A bluish-white product was obtained on the surface of the substrate after the experiment. The morphology of the samples was characterized by scanning electron microscopy (SEM, JEOL JSM-5910) and X-ray diffraction (XRD, Siemens D5005). High-resolution transmission microscopy (HRTEM, JEOL JEM-2010F) was employed to detect the lattice structure of the as-prepared samples. The FE measurement was carried out with a simple diode configuration in a vacuum chamber. The pressure in the chamber was maintained with 2.0 × 10-6 Torr at room temperature. A Teflon film with an aperture of 4 mm2 was taken as a spacer and inserted between the anode and cathode. The emission current was recorded by varying the anode applied voltage with a Keithley I-V meter. 3. Results and Discussion SEM images of the as-prepared ZnO nanostructure samples A-D are shown in Figure 1a-d, respectively, and the corresponding insets show the enlarged SEM images. High (sample A and B, 4 × 108/cm2), medium (sample C, 2.5 × 107/cm2),

10.1021/jp802515d CCC: $40.75  2008 American Chemical Society Published on Web 08/07/2008

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Li et al.

Figure 3. Field emission current density as a function of applied field, and the inserted logarithmic Fowler-Nordheim plot.

Figure 1. The SEM images of samples A-D, respectively, and the corresponding enlarged SEM images of the injector-like ZnO nanostructures as the insets.

Figure 4. Calculated average current density for single and double needle nanostructures as a function of distance between the nanostructures. The schematic drawing of the computation model is shown in the inset. The applied electric field is fixed at 10 V/µm.

Figure 2. (a) XRD pattern of sample C. (b) HRTEM image of the needle of the injector-like nanostructure of sample C. The inset shows the corresponding SAED pattern. The zone axis is [2110].

and low (sample D, 6.25 × 106/cm2) areal density ZnO nanostructures were obtained by controlling the reaction temperature and source material weight. Most of the nanostructures were vertically aligned on the substrate, and they were composed of two parts: a cone-shape base and single, sharp needle (samples A, C, and D) or two sharp needles (sample B). The diameters of the cone-shape base which determined the growth site of the nanostructures range from approximately 300 nm (samples A, B) to 2 µm (samples C, D). The length of the needle ranges from 400 nm (samples A, B) to 2 µm (samples C, D). The spacing between the neighboring nanostructures is a critical parameter considering the electric field screening effect on the FE performance. So it is worth noting that the spacing is about 0.5, 0.5, 2, and 4 µm which is also 0.5, 0.5, 1, and 2 times the nanostructure’s total height for samples A-D, respectively. Figure 2a shows the XRD pattern of the sample C nanostructures. All diffraction peaks exhibit the typical wurtzite hexagonal structure like bulk ZnO with unit cell constants of a ) 0.325 nm and c ) 0.521 nm. The strongest peak at 2θ ) 34.4° corresponding to (002) is observed. It indicates that the ZnO nanostructures are primarily grown along the [001] direction. Figure 2b shows a representative HRTEM image of the needle from sample C and its corresponding SAED pattern (inset), displaying the needle of the injector-like nanostructure in a single crystal growing along the c axis of ZnO. The growth process of injector-like ZnO nanostructures follows the vapor-liquid-solid (VLS) mechanism.17 Zn powder was evaporated at the source region and carried by the Ar and O2 gas to the Si substrate. Then the vapor was condensed into liquid nanodroplets on the substrate surface. The droplets acted as reaction nuclei and recombined with oxygen to form ZnO nanostructures after the nuclei were saturated. The different areal

density and morphology of the nanostructures were obviously influenced by the temperature and the quantity of source materials. Higher reaction temperature could cause a faster growth speed of the nanostructures. Therefore samples C and D have a larger cone-shape base, which separates the needles atop it. The diameter of the needles decreased dramatically because of the substantial decrease in vapor pressure. The quantity of source materials for sample B was larger than the other samples. The Zn vapor continued to condense to nanodroplets on the cone-shape base while it was growing. So, most of the injector-like ZnO nanostructures in sample B have two needles. Figure 3 shows the measured field emission (FE) current density as a function of electric field (J-E curve) for the prepared four samples with an inset of the Fowler-Nordheim plot. The threshold electric fields (current density of 1 mA/ cm2) are 6.1, 8.5, 4.2, and 4.8 V/µm for samples A-D, respectively. The best FE performance of sample C is attributed to its long, sharp needles which obtain a high aspect ratio and medium areal density with a minimized screening effect. Sample D with low areal density gives poor emission since the nanostructures are bent and not aligned as that of the other samples. The relatively poor emission performance of samples A and B is explained by a stronger screening effect invoked by the neighboring nanostructure and the needles on the same nanostructure. It is well-known that FE performance is largely determined by the electric field at the tip of the emitters, which is given by E ) βE0, where β is the field enhancement factor and E0 is the applied electric field. The field enhancement β was also considered to be affected by the internal factors βin and external factor βext.18 Therefore, the electric field at the tip could also be given by E ) βinβextE0. βin correlated with the morphology of the emitter and was determined internally by the emitter itself. Approximately, it could be expressed as βin ) l/r, where l is length of the emitter and r is the radius of the tip. βext was derived externally from the electric field screening condition.

Field Emission from Injector-Like ZnO Nanostructure

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It manifested its effect in an array of emitters. As the spacing between the emitters decreases, the βext decreases exponentially because of the screening effect from the neighboring emitters. As a result, under the above conditions the nanostructures of sample C resulted in a high βin and βext, so it is not surprising to reach high emission current density at a lower electric field. Also the cone-shape base had another function during the emission: the temperature of the nanostructure could rise as high as a few thousand Kelvin because of the Joule heating.19 An expression of T ) 1/r4 could be used to estimate the temperature caused by Joule heating, where r is the diameter of the nanostructure. So the bulky cone-shape base of the nanostructure reduces the risk of reaching an excessive temperature which is considered to be a cause of nanostructure breakdown. In order to verify the experimental results, we calculated the electrostatic field distribution of the nanostructure with single and double needles. The FE current density was estimated by the Fowler-Nordheim (F-N) equation20 described as

J ) A(β2E2/φt(y)2)exp(-BV(y)φ3⁄2/E)

(1)

where J is the current density and the Fowler-Nordheim constants A and B are 1.56 × 10-10 (A · V-2 · eV) and 6.83 × 109 (V · eV-3/2 · µm-1), respectively. The work function φ of ZnO is set to be 5.3 eV, and the function t(y) and V(y) are approximated by t(y)2 ) 1.1, V(y) ) 0.95 - y2 with the Schottky lowering of the work function barrier y ≈ 3.79 × 10-5 × E1/ 2/φ. The average current density Jav is defined as n

Jav )

∑ J/s

(2)

i)1

where J is the derived from eq 1, n is the number of emitters, and s is the whole area of the emitter array. The schematic drawing and parameters of the nanostructures in the simulation were extracted from SEM images of samples A (single needle) and B (double needle), as shown in the inset of Figure 4. The cone-shape base holds up the needle to reach a higher aspect ratio by increasing the total height of the nanostructures. The calculated average current density Jav is investigated as a function of distance between the neighboring nanostructures in Figure 4.The current density increases exponentially for both kinds of nanostructures when the space is smaller than the nanostructure’s height, which indicates that the screening effect plays a dominant role in this distance region. When the space increases, but smaller than two times of the nanostructure’s height, the area of emitting tips per unit area becomes more important as the screening effect on the local electric field becomes slowly ineffective. For larger spacing, the number of nanostructures decreases dramatically and the screening effect decreases steadily. So the current density decreases because of the lack of emitting area. As a result, the optimal spacing for single and double needle nanostructures is one and two times their height, respectively. This simulation result is in relatively good agreement with our experimental results. Compromise between the screening effect and emitting area is essential to obtain high current density. 4. Conclusion In summary, the vapor-phase transport method is employed to synthesize injector-like ZnO nanostructures with different

areal density and morphology (one or two needles). The coneshape base of the nanostructure plays an important role in the FE. It holds up the needle to obtain a higher aspect ratio, separates the emission needles with each other to minimize the screening effect, and also reduces the risk of reaching high temperature of the nanostructures. Undoubtedly, sample C has the best FE performance due to the above three reasons. Moreover, the calculation of the current density is in relatively good agreement with our experimental results. It shows that the highest current density is obtained when the spacing between the neighboring nanostructures is similar to the height for the single needle nanostructures. The nanostructures with double needles reach even higher emission current density when the space is two times the nanostructure’s height. Acknowledgment. The authors acknowledge financial support from National Key Basic Research Program 973 (2003CB314702, 2003CB314706), the Chinese 111 project (B07027), Program for New Century Excellent Talents in University/NCET-04-0473 NCET-05-0466), and Outstanding Young Teacher on Teaching/Science Research Award from Southeast University. We thank Dr. Daniel den Engelsen for his fruitful discussion and help on this work. References and Notes (1) Chen; Jun; Dai, Y. Y.; Luo, J.; Li, Z. L.; Deng, S. Z.; She, J. C.; Xu, N. S. Appl. Phys. Lett. 2007, 90, 253105. (2) Neo, Y.; Mimura, H.; Matsumoto, T. Appl. Phys. Lett. 2006, 88, 073511. (3) Han, J. H.; Lee, T. Y.; Kim, D. Y.; Yoo, J. B.; Park, C. Y.; Choi, J. J.; Jung, T.; Han, I. T.; Jung, J. E.; Kim, J. M.; Vac, J. Sci. Technol. B 2004, 22, 4. (4) Smith, R. C.; Cox, D. C.; Silva, S. R. P. Appl. Phys. Lett. 2005, 87, 103112. (5) Sveningsson, M.; Morjan, R. E.; Nerushev, O. A.; Campbell, Eleanor, E. B.; Malsch, D.; Schaefer, J. A. Appl. Phys. Lett. 2004, 85, 4487. (6) Wen, J. G.; Lao, J. Y.; Wang, D. Z.; Kyaw, T. M.; Foo, Y. L.; Ren, Z. F. Chem. Phys. Lett. 2003, 372, 717. (7) Kan, S.; Mokari, T.; Rothenberg, E.; Banin, U. Nat. Mater. 2003, 2, 155. (8) Goldberger, J.; He, R.; Zhang, Y.; Lee, S.; Yan, H.; Choi, H. J.; Yang, P. Nature 2003, 422, 599. (9) Zhang, Y. S.; Yu, K.; Ouyang, S. X.; Zhu, Z. Q. Mater. Lett. 2006, 60, 4. (10) Wei, A.; Sun, X. W.; Xu, C. X.; Dong, Z. L.; Yu, M. B.; Huang, W. Appl. Phys. Lett. 2006, 88, 213102. (11) Wang, W. Z.; Zeng, B. Q.; Yang, J.; Poudel, B.; Huang, J. Y.; Naughton, M. J.; Ren, Z. F. AdV. Mater. 2006, 18, 3275. (12) Xu, C. X.; Sun, X. W. Appl. Phys. Lett. 2003, 83, 3806. (13) Yu, K.; Zhang, Y. S.; Xu, F.; Li, Q.; Zhua, Z. Q.; Wan, Q. Appl. Phys. Lett. 2006, 88, 153123. (14) Nilsson, L.; Groening, O.; Emmenegger, C.; Kuettel, O.; Schaller, E.; Kind, L.; Schlapbach, H.; Bonard, J.-M.; Kern, K. Appl. Phys. Lett. 2000, 76, 15. (15) Suh, J.-S.; Jeong, K.; Seok, L.; Jin, S.; Han, I. Appl. Phys. Lett. 2002, 80, 13. (16) Bonard, J. M.; Weiss, N.; Kind, H.; Stockli, T.; Forro, L.; Kern, K.; Chatelain, A. AdV. Mater. 2001, 13, 3. (17) Wagner, R. S.; Ellis, W. C. Appl. Phys. Lett. 1964, 4, 5. (18) Spindt, C. A.; Brodie, I.; Hunphrey, L.; Westerberg, E. R. J. Appl. Phys. 1976, 47, 5248. (19) Vincent, P.; Purcell, S. T.; Journet, C.; Binh, V. T. Phys. ReV. B 2002, 66, 075406. (20) Kim, D.; Bouree, J. E.; Kim, S. Y. Appl. Phys. 2006, 83, 111.

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