Giant Enhancement of Field Emission from Selectively Edge Grown

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Giant Enhancement of Field Emission from Selectively Edge Grown ZnOCarbon Nanotube Heterostructure Arrays via Diminishing the Screen Effect Nishuang Liu, Guojia Fang,* Wei Zeng, Hao Long, and Xingzhong Zhao Key Laboratory of Artificial Micro- and Nano-structures of Ministry of Education of China, Department of Electronic Science and Technology, School of Physics and Technology, Wuhan University, Wuhan, 430072, P. R. China ABSTRACT: Novel selectively edge grown ZnOcarbon nanotube (CNT) heterostructure field emission (FE) arrays have been fabricated by a facile and repeatable two-step process. First, with the combined effect from a ZnO seed layer and a passivation layer for nanorod growth, ZnO nanorods could only grow on the edge of a 4 μm diameter circle. Meanwhile, the local morphology such as ZnO nanorod density, uniformity, and growth direction can be modulated by the thickness of ZnO seed layer. With this adjustment, the FE performance of the ZnO nanorod ring pattern arrays is significantly improved. Then CNTs were radically grown on the surface of the ZnO nanorod ring pattern arrays. As a consequence of all these optimizations, the lowest turn-on field of 2.1 V/μm, threshold field of 3.19 V/μm, and a current density of 7 mA/cm2 at a field of 4.95 V/μm were ultimately obtained. Via calculating the electrostatic field distribution, it was found that this giant enhancement of FE performance is due to the typical morphology which can significantly diminish the screen effect.

1. INTRODUCTION Field emitters with good performance are urgently needed in a wide range of field emission (FE) based devices such as flat-panel displays, FE microscopes, microwave amplifiers, X-ray source, etc. Owing to the unique geometries of high aspect ratios and small curvature radius, one-dimensional nanostructures and onedimensional heteronanostructures such as nanowires, nanorods, and nanotubes can exactly meet the requirements for their good FE performance.17 Until now, many nanostructures and nanoheterostructures such as carbons nanoubes (CNTs),1,8 zinc oxide (ZnO) nanowires,3,4 silicon (Si) nanowirs,9 CNTs grown on ZnO nanowires,10 ZnO nanowires grown on CNTs,11,12 CNTs grown on Si nanowires,1315 and CNTs grown on Si pyramid16 have been studied extensively over the past decade. However, nanostructures such as CNTs or ZnO nanowires are usually closely packed or bundled, resulting in a serious screen effect which could largely depresses their FE performance. In order to diminish the screen effect, a simple growth technique and a precision control of morphology, alignment, and position are required. So far, a controlled growth of well-aligned ZnO nanostructures has already been achieved using various techniques such as nanosphere lithography,17 self-assembled monolayers,18,19 electron-beam lithography,2022 and conventional photolithography.20 However, these methods, except the conventional photolithography, all require expensive masks, complex multistep processes, and costly equipment. Conventional photolithography is no doubt an easy and economical approach to achieve required patterns on substrates, but it could only produce micrometer scale pattern, which is absolutely not r 2011 American Chemical Society

enough for avoiding the screen effect in field emission. Meanwhile, the controlled aligned ZnO nanostructures are usually grown on crystallography matching substrates, such as GaN, using vapor phase deposition at a temperature higher than 500 °C. This method is often involved with the using of metal catalysts, which has a risk of unintended catalyst doping into NWs affecting their properties.2326 In addition, the high temperature raises the cost. Recently, Wang and his co-workers have achieved patterned vertical ZnO nanowire arrays via a hydrothermal technique, which is much simpler, with lower temperature, more cost-effective, and easier scalability to large areas.22,27 However, expensive electron beam lithography was used to prepattern the substrate.22 In the meantime, it has been reported that some materials can prevent the growth of ZnO nanorods in hydrothermal reaction.2833 Therefore, with this mechanism we can achieve more precision pattern of ZnO nanorod arrays than with conventional photolithography through hydrothermal approach. What’s more, through using these position-controllable ZnO nanorod arrays with large surface-to-volume ratio as templates to grow CNTs could further increase the emitter density, decrease the screen effect, achieve large emission current density, and thus enhance the FE performance.10,34 In this article, we report the selectively edge growth of submicrometer scale patterned ZnOCNT heterostructure Received: May 21, 2011 Revised: June 23, 2011 Published: June 24, 2011 14377

dx.doi.org/10.1021/jp204724t | J. Phys. Chem. C 2011, 115, 14377–14385

The Journal of Physical Chemistry C

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Figure 1. SEM images of ZnO nanorod ring pattern arrays with the ZnO seed layer of (a) 50, (b) 200, and (c) 400 nm. (d) SEM image of the sample shown in (b) with a large-scale area.

arrays on Si substrate with excellent FE performance via a facile and repeatable way. The selectively edge grown ZnO nanorod arrays were first fabricated on Si substrate, and then CNTs were radically grown on the surface of the ZnO nanorods. On the one hand, it has been found out that the thickness of the ZnO seed layer can determine the local morphology, which is very important for FE. After measured and compared their FE properties, we found the optimal condition for the selectively edge growth of ZnO nanorod field emitters. On the other hand, we also found the optimal CNT growth parameter for the ZnOCNT heterostructure array field emitters. Via these solutions, the screen effect can be diminished to the minimum with an appropriate overall density still enough for FE. Moreover, computer simulation was also utilized for the electrostatic calculation to verify the experiment results. We believe that the unique structure, morphology, as well as the optimal ZnOCNT heterostructure alignment and density are the reason for the enhanced FE performance.

2. EXPERIMENTAL SECTION A (100) Si wafer was first dipped in HF solution to remove the SiO2 layer, and then the Si substrate was cleaned in sequence with acetone and ethanol. To prepare seed layer patterns, a conventional photolithography followed by lift-off techniques was used. Before growing the ZnO nanorods, ZnO seed layer with three different thickness (50, 200, and 400 nm) was deposited on the Si substrate using a radio frequency magnetron sputtering deposition system. Then we sputtered a Sn layer on the patterned ZnO seed layer for preventing the ZnO local growth. The nutrient solution for ZnO nanorods growth was an aqueous solution of 0.025 M zinc nitrate [Zn(NO3)2 3 6H2O] and hexamethylenetetramine. The reaction was kept at 90 °C for 2 h. The fabricated samples were removed from the solution, rinsed with distilled

water, and dried in air. Through this method we can get the selectively edge grown ZnO nanorod ring pattern arrays. At last, CNTs were grown on these position-controllable ZnO nanorod arrays with different growth time (1, 5, and 10 min) following the procedure reported previously.10 In addition, we have also fabricated CNTs on common patterned ZnO nanorod solid arrays to compare their FE performance. Furthermore, in order to show the superiority of our ZnOCNT heterostructure field emission arrays, we have fabricated patterned CNT arrays for compare, employing the same method as we previously reported.34 The morphology and structure of the as-grown field emitters were characterized by field emission scanning electron microscopy (FESEM, FEI XL-30), transmission electron microscopic (TEM, JEOL JEM 2010) and high-resolution TEM (HRTEM, JEOL JEM 2010FEF). The field emission characteristics were investigated with a two-parallel-plate configuration in a vacuum chamber with a base pressure of 2.0 105 Pa. The cathode area was 1 cm 1 cm. The anode and cathode were separated by a 210 μm Teflon spacer. The currentvoltage characteristics were measured by a Keithley 6485 picoammeter and a Keithley 248 high voltage supply. In order to get reliable emission current, we have performed electrical annealing on all our samples.

3. RESULTS AND DISCUSSION The basic fabrication procedure of the selectively edge growth of submicrometer scale patterned ZnO nanorod arrays is described below. At first, a photoresist mask of 4 μm diameter circles was made on Si substrate via conventional photolithography followed by lift-off techniques. This was used as a template. Subsequently, different thicknesses of the ZnO seed layer were deposited on the Si substrate using a radio frequency magnetron sputtering deposition system. Then a layer of Sn was 14378

dx.doi.org/10.1021/jp204724t |J. Phys. Chem. C 2011, 115, 14377–14385


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Figure 2. SEM images of the ZnOCNT heterostructure ring pattern arrays with CNT growth time of (a and b) 1 min, (c and d) 5 min, and (e and f) 10 min.

deposited on the patterned ZnO seed layer for preventing the ZnO local growth. After cleaning the photoresist mask, a hydrothermal approach was implemented to grow the ZnO nanorod arrays. Since the local morphology such as ZnO nanorod density and growth direction can be affected by the thickness of ZnO seed layer, the nanorod density and uniformity within the formed arrays can be correspondingly adjusted. This kind of adjustments gives us a chance to regulate the arrays, to minimize the screen effects and to keep enough emitting sites. As shown in Figure 1, ZnO nanorods of the ring pattern arrays only grew on the edge of the pattern. From the image, we can also observe that ZnO nanorods have a diameter of about 200 nm and a length of about 2 μm. Also the hexagonal cross section of nanorods implies that c axis of ZnO nanorod is along its length direction. Owing to the covered Sn layer, several ZnO nanorods tilting at various angles to the substrate were grown on the edge of the pattern. The position controlled growth of the nanorods implies that the nucleation leading to the growth of nanorods take place only at the open area exposed to the ZnO seed layer. In addition, owing to the advantages of hydrothermal approach, we can easily get uniform field emission arrays with large area, which can be seen from Figure 1d. Furthermore, we can observe that the

density and growth aligned angle can be modulated by the thickness of ZnO seed layer. When the ZnO seed layer is 50 nm (as shown in Figure 1a), we can get more upward ZnO nanorods with less uniformity and less density. Additionally, when the ZnO seed layer is 400 nm (as shown in Figure 1c), we can observe that the uniform ZnO nanorods are all grown with a angle of about 45° but not perpendicular to the substrate, which is obviously not good for field emission. Finally, when the ZnO seed layer is 200 nm (as shown in Figure 1b), we can get ZnO nanorods with good uniformity, moderate density, and relatively good perpendicularity, which is very important for field emitter. We suppose the reason for the difference in the growth direction for the three samples must be the growth competition among the adjacent ZnO nanorods. When the seed layer is thinner, the ZnO nanorod amount is smaller because less seed layer is exposed to the reaction solution, and the growth competition among the adjacent ZnO nanorods is much less. Then the thinner seed layer can give a more straight growth. When the seed layer is thicker, the ZnO nanorod amount is larger because more seed layer is exposed to the reaction solution, and the growth competition among the adjacent ZnO nanorods is much more intense. Then ZnO nanorods 14379



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Figure 3. (a) TEM image of CNTs grown on ZnO nanorods. (b) HRTEM image of a typical pristine ZnO nanorod without growing CNTs (the SAED pattern of the ZnO nanorod is shown in the inset). (c) HRTEM image of a typical CNT grown on a ZnO nanorod.

Figure 4. (a and b) SEM micrographs of ZnO nanorod solid arrays. SEM images of ZnOCNT heterostructure solid arrays with CNT growth time of (c and d) 1 min, (e) 5 min, and (f) 10 min.

tend to deviate from the vertical growth direction. Furthermore, according to the practical FE experiment results which will be shown in the latter part of this paper, the sample with 200 nm ZnO seed layer actually show the best FE performance. Therefore, we used this kind of samples as template to fabricate ZnOCNT heterostructure field emission arrays.

Figure 2, panels a and b, shows typical SEM images of circle shaped ZnOCNT heterostructure ring pattern arrays grown selectively on a Si substrate, with CNTs growth time of 1 min. ZnOCNT heterostructure emitter ring pattern arrays were distributed uniformly on the substrate. It shows the CNT bundles cover the ZnO nanorod arrays, whereas no CNTs grew 14380



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Figure 5. (a and b) SEM micrographs of CNT pillar arrays grown on Si substrate.

on the space regions separating the arrays due to the strong adhesion between Ni film and the Si substrate. In addition, the latter four figures show the morphologies of the as-grown ZnOCNT heterostructure emitter ring pattern arrays with different CNTs growth times of 5 min (Figure 2, panels c and d) and 10 min (Figure 2, panels e and f). From Figure 2, we can also observed that CNTs were grown with an entangled shape not only on the top but also on the side surface of the ZnO nanorods, which formed a unique 3D ZnOCNT heterostructure. Furthermore, it can be seen that the hollow area decreases resulted from longer CNTs and longer growth time, which presents even more emission sites. To get more information about the structure of the as-grown ZnOCNT heterostructure, TEM investigations were implemented. TEM image in Figure 3a shows that the nanotube bundles form an outer shell of each ZnO nanorod without changing its rod morphology. The diameters of the nanotubes are in the range of 2030 nm. The HRTEM image (Figure 3b) and the select area electron diffraction (SAED) pattern from the body of the ZnO nanorod (the inset of Figure 3b) indicate that the nanorod is a single crystal with growth direction along the c axis. That is favorable for electron transport. Figure 3c shows the HRTEM image of a typical CNT, which reveals a kind of “herringbone structure” with multiple ordered graphite layers. Some lattice fringes are curved and not well graphitized, especially at the outer layers, which may be caused by the restricted property of the synthesis method. For comparison, we have also fabricated CNTs on common patterned ZnO nanorod solid arrays. Figure 4, panels a and b, is the FESEM images of the common patterned ZnO nanorod solid arrays which were fabricated using the same photolithograph mask. From the image, we can find out that the diameter and length of ZnO nanorods in the ring pattern arrays are much larger than solid arrays. Liu et al. believe that the consumption of Zn2+ may be higher at the area with dense nanorods. Thus, the local Zn2+ concentration may be lower. Therefore, the growth rate of the high-density nanorods is slower than that at the low-density area.35 Three different growth times of 1, 5, and 10 min have been used to grow ZnOCNT heterostructure emitter arrays, which are shown in panels c (and d), e, and f of Figure 4, respectively. Moreover, we have fabricated patterned CNT arrays using the same photolithograph mask for comparison. Figure 5 shows the FESEM images of circle shaped CNT arrays grown selectively on a Si substrate. Maybe due to the relatively smaller pattern, the asgrown CNT arrays have a poorer vertical alignment than the larger pattern which has been reported previously by us using the

Figure 6. FE current density vs electric field plot for the ZnO nanrod solid arrays and ring pattern arrays.

same fabrication method.34 Jeong et al. believe that smaller CNT pillar arrays are easier to be affected by the fluctuation of the conditions during the growth, which is exactly similar to our situation.36 Anyway, since only a few CNT pillars have poor vertical alignment, we believe that it is good enough for comparing. Generally, the FowlerNordheim (FN) theory37 is used to describe field emission behavior of metals. The theory is expressed by the following equation: I ¼ AðE2 =ϕÞ expð Bϕ3=2 =EÞ where I is field emission current, E = βE0 is the local electrical field, E0 is the mean field between the cathode and anode, β is the field enhancement factor, ϕ is the work function, A and B are constants (B = 6.83 103 V eV3/2 μm1). The FN plot, ln(I/E02) vs 1/E0, is expected to be a straight line according to the theory. Basically, one-dimensional ZnO nanostructures and CNTs are potentially good field emitters for their favorable aspect ratios and suitable work functions. However, in practice, the FE properties are affected by many factors, for example, the radius of curvature of a single emitter, the uniformity and density of the emitters, and so on.3842 These factors could be controlled at the 14381



Figure 7. FN plots of the field emission JE characteristic curves.

same time by adjusting the alignment of nanostructures for ultimate improvements in field emission. At first, the FE current density versus electric field (JE0) characteristics of the as-grown ZnO nanorod arrays are illustrated in Figure 6. The applied electric field (E0) was defined as (V RI)/D, where V is the anode voltage; R is resistance of the resistor in series; D is the cathode-anode distance. To evaluate the FE current density, we simply employ the overall area as emission area, since it is difficult to calculate the exact effective emission area. The turn-on field (Eto) was defined as the electric field at 10 μA/cm2 of the FE current density, while the threshold field (Eth) was defined as the electric field when J reaches 1 mA/cm2 (the minimum required to produce a luminescence of 300 cd/m2 for a video graphics array field emission display with a typical high voltage phosphor screen efficiency of 9 lm/W). As shown in the figure, the ZnO nanorods solid arrays perform poorly as a field emitter, which is generally attributed to a screening effect.30 Also, for the ZnO nanorod ring pattern arrays, the threshold field first decreases from 6.56 to 5.09 V/μm with the ZnO seed layer thickness increasing from 50 to 200 nm and then slightly increases with further thickness increase. When the ZnO seed layer is 50 nm, the whole ZnO nanorod density is relatively low, and the empty area in the circle is relatively small, which depresses the FE performance. When the ZnO seed layer is increased to 200 nm, the empty area in the circle is increased, and the overall ZnO nanorod density increases. In addition, the uniformity and alignment are both improved in the meantime. We believe that it is the reason why the FE performance comes to a maximum when the ZnO seed layer is 200 nm. However, when the ZnO seed layer is further increased to 400 nm, the uniform ZnO nanorods are all aligned with a angle of about 45° but not perpendicular to the substrate, which spoils the emission efficiency. As shown in Figure 7, a two-stage slope behavior is observed in the FN curves for the ring pattern arrays. Such twostage slope characteristics are also reported in many other types of field emitters. However, the mechanism of the multistage slop FE phenomena is not clear yet, and there are many suppose to explain these phenomena, such as energy band, adsorbates, and defects.43,44 In general, for ZnO nanostructure, the slope shows a small value at a low applied voltage and a large value at a high voltage. However, in the present case, what we observed is just opposite to the reports about ZnO mentioned above. In fact, this two-slope behavior is similar to the FE from CNT emitters,

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Figure 8. FE current density vs electric field plot for the ZnOCNT heterostructure solid arrays, ring pattern arrays, and CNT pillar arrays.

which is considered to originate in a saturation of the emission current caused by space charge effect.45 Considering the fact that all our samples were produced by the same solution method, we believe that the reason for the two-stage slope behavior is probably due to amount of the emission sites. A smaller number of emission sites (compare to common ZnO solid nanorod arrays) lead to the saturation of the emission current at higher applied voltage. This is because the fewer number of emission sites increases the emission current per one ZnO nanorod on the sample under comparison at the same current density and makes this kind of saturation easier to happen. In addition, it is also the reason why the slop of the third sample turns at a higher applied voltage. Via growing CNTs on the ZnO nanorod arrays, the FE performance has been further improved, as shown in Figure 8. Clearly, the ZnOCNT heterostructure field emission arrays have better FE performance compared with those of the asgrown ZnO nanorod arrays. We can observe that the FE performance comes to a maximum when the growth time of CNTs is 5 min for the ZnO nanorod ring pattern arrays. It turns on at an electric field of 2.1 V/μm, and the threshold field is 3.19 V/μm. Additionally, the emitting current density can be as high as 7 mA/cm2 at an electric field of 4.95 V/μm. As displayed in the inset of Figure 8, the brightness of the ZnOCNT field emission ring pattern arrays is relative uniform. Generally, electrons transport along the nanorods. However, electrons can also emit from protrusive regions on the outer surface of the nanorods in which there exists a higher enhancement factor. Then, after the growth of CNT, the surface of each ZnO nanorod is sheathed with CNTs, of which the tips have a smaller curvature radius than that of the ZnO nanorod. And these tips become the prominent emission sites. Moreover, since ZnO nanorod and CNT (work function of 5.3 and 5.0 eV) can form ohmic contact,46 no Schottky barrier was established at the ZnO/ CNT junction. When the electrons are injected from the substrate into the ZnO nanorods, they can be easily transported from the ZnO nanorods into the conduction band of CNTs. Finally, the large density of CNTs with intrinsic low work function and high aspect ratio grown on the surface of the ZnO nanorods leads to the improvement of FE performance. With the growth time of CNTs increase from 1 to 5 min, the threshold field decreases 14382



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simulated electric-field distributions of the two models ranged from the one side to the other side radially (with the origin representing the center) reveal that the ring pattern structure exhibits enhanced electric fields at both the inner and outer edges. In addition, for the ring pattern structure, there are two emission edges contributing to field emission in comparison to only one outer edge for the solid structure. Furthermore, the ring pattern structure exhibits comparable field for the inner edge and the outer edge, which is different than the other report.47 This difference must come from our more optimal alignment parameter. Anyway, the simulation results imply that the electric field is predominantly concentrated in the ZnOCNT nanostructure at the edge of the array, which act as major emission sites. Therefore, the more efficient field emission from the ring pattern arrays can be attributed to the reduction of the screen effect.

Figure 9. Simulated electric-field vs radical distance of two models.

from 4.81 to 3.19 V/μm. It is reasonable that longer CNTs and higher aspect ratio leads to better FE performance. However, when the growth time of CNTs is increased from 5 to 10 min, the threshold field increases from 3.19 to 3.96 V/μm. Considering the fact that the hollow area diminishes resulted from longer CNTs and longer growth time, we believe that the depression of FE performance must come from the screen effect. For the ZnO nanorod solid arrays, the FE performance has also been improved via the growth of CNTs. But different to the ring pattern arrays, the threshold field decrease from 7.93 to 5.98 V/μm monotonously. This difference must come from the unique morphology of the selective edge grown ZnOCNT heterostructure field emission arrays, which is also the reason why the ZnOCNT field emission ring pattern arrays have a better FE performance than the solid arrays. Furthermore, our ZnOCNT field emission ring pattern arrays shows a better FE property than CNT arrays, as shown in Figure 8. Maybe ring pattern CNT pillars with the same pattern have a better FE property than our ZnOCNT field emission ring pattern arrays, but we should recognize the fact that it is not so easy to get such ring pattern CNT pillars with the same precision pattern to our ZnOCNT ring pattern arrays. In fact, according to the newest reports about ring pattern CNT pillars, the patterns are all much larger than us restricted by the limition of conventional photolithography.47 In addition, the field emission stability of the best behaviored sample was tested at a constant electric field of 3.38 V/μm, and no obvious degradation of FE current was observed. The field emission current fluctuation was with 3.3% during 4 h while J was kept at about 1.5 mA/cm2, which reflected its high field emission stability. To investigate the origin of the efficient field emission from the ZnOCNT ring pattern arrays, we simulated the electrostatic field based on the finite element method with ANSYS v10.0 software.48 Because the as-grown ZnOCNT heterostructures are as dense as bulk material, we just used column and cylinder as the models for the solid arrays and ring pattern arrays to make the simulation simple. For investigating the trends of variation of electric field on the top surfaces of samples, two paths are established. The two paths are all initiated from the one side to the other side radially in the model. An anode to cathode separation of 210 μm was used, and a positive potential of 2500 V was applied to the anode. As shown in Figure 9, the

4. CONCLUSIONS In conclusion, we demonstrate a facile and repeated way to fabricate ZnOCNT heterostructure arrays on Si substrate with excellent FE performance. The growth process involves two steps. Also, the first step is the selectively edge growth of ZnO nanorod arrays. We found that the thickness of ZnO seed layer can adjust the local morphology such as ZnO nanorod density, uniformity, and growth direction, which can affect the FE performance in turn. Then CNTs were grown radically on the surface of the ZnO nanorod ring pattern arrays with the best FE property, and the effect of growth time of CNTs on the FE performance has also been investigated. Via these solutions, the screen effect can be diminished to the minimum with an appropriate overall density still enough for FE. Meanwhile, the CNTs sheathing also enhanced the FE property. As a consequence of all these optimization, the FE performance of the ZnOCNT arrays was substantially improved. The lowest turnon field of 2.1 V/μm, threshold field of 3.19 V/μm, and a current density of 7 mA/cm2 (at a field of 4.95 V/μm) were ultimately obtained. Our results indicate that ZnOCNT heterostructure arrays are promising candidates for the applications in flat panel displays and high brightness electron sources. ’ AUTHOR INFORMATION Corresponding Author

*Tel: +86 (0)27 68752147 (Lab). Fax: +86 (0)27 68752569. E-mail: [email protected].

’ ACKNOWLEDGMENT This work was partially supported by the Scholarship of Excellent Doctoral Student granted by the Ministry of Education of China, the National Natural Science Foundation of China (11074194), the Natural Science Foundation of Hubei province (2010CDA016), the National High Technology Research, Development Program of China (2009AA03Z219), and the National Basic Research Program (2011CB933300) of China. ’ REFERENCES (1) de Heer, W. A.; Châtelain, A.; Ugarte, D. A Carbon Nanotube Field-Emission Electron Source. Science 1995, 270, 1179–1180. (2) Banerjee, D.; Jo, S. H.; Ren, Z. F. Enhanced Field Emission of ZnO Nanowires. Adv. Mater. 2004, 16, 2028–2032. 14383


The Journal of Physical Chemistry C (3) Fang, X.; Bando, Y.; Gautam, U. K.; Ye, C.; Golberg, D. Inorganic semiconductor nanostructures and their field-emission applications. J. Mater. Chem. 2008, 18, 509–522. (4) Chen, C. H.; Chang, S. J.; Chang, S. P.; Tsai, Y. C.; Chen, I. C.; Hsueh, T. J.; Hsu, C. L. Enhanced field emission of well-aligned ZnO nanowire arrays illuminated by UV. Chem. Phys. Lett. 2010, 490, 176–179. (5) Fang, X.; Zhai, T.; Gautam, U. K.; Li, L.; Wu, L.; Bando, Y.; Golberg, D. ZnS nanostructures: From synthesis to applications. Prog. Mater. Sci. 2011, 56, 175–287. (6) Fang, X. S.; Bando, Y.; Shen, G. Z.; Ye, C. H.; Gautam, U. K.; Costa, P. M. F. J.; Zhi, C. Y.; Tang, C. C.; Golberg, D. Ultrafine ZnS Nanobelts as Field Emitters. Adv. Mater. 2007, 19, 2593–2596. (7) Fang, X. S.; Bando, Y.; Ye, C. H.; Golberg, D. Crystal orientationordered ZnS nanobelt quasi-arrays and their enhanced field-emission. Chem. Commun. 2007, 29, 3048–3050. (8) Calderon-Colon, X.; Geng, H. Z.; Gao, B.; An, L.; Cao, G. H.; Zhou, O. A carbon nanotube field emission cathode with high current density and long-term stability. Nanotechnology 2009, 20, 325707. (9) Tzeng, Y. F.; Wu, H. C.; Sheng, P. S.; Tai, N. H.; Chiu, H. T.; Lee, C. Y.; Lin, I. N. Stacked Silicon Nanowires with Improved Field Enhancement Factor. ACS Appl. Mater. Interfaces 2010, 2, 331–334. (10) Li, C.; Fang, G. J.; Yuan, L. Y.; Liu, N. S.; Ai, L.; Xiang, Q.; Zhao, D. S.; Pan, C. X.; Zhao, X. Z. Field emission from carbon nanotube bundle arrays grown on self-aligned ZnO nanorods. Nanotechnology 2007, 18, 155702. (11) Ok, J. G.; Tawfick, S. H.; Juggernauth, K. A.; Sun, K.; Zhang, Y.; Hart, A. J. Electro-Optical Materials: Electrically Addressable Hybrid Architectures of Zinc Oxide Nanowires Grown on Aligned Carbon Nanotubes. Adv. Funct. Mater. 2010, 20, 2470–2480. (12) Yan, X.-b.; Tay, B.-k.; Yang, Y.; Po, W. Y. K. Fabrication of Three-Dimensional ZnOCarbon Nanotube (CNT) Hybrids Using Self-Assembled CNT Micropatterns as Framework. J. Phys. Chem. C 2007, 111, 17254–17259. (13) Liu, Y.; Fan, S. Field emission properties of carbon nanotubes grown on silicon nanowire arrays. Solid State Commun. 2005, 133, 131–134. (14) Li, X. J.; Jiang, W. F. Enhanced field emission from a nest array of multi-walled carbon nanotubes grown on a silicon nanoporous pillar array. Nanotechnology 2007, 18, 065203. (15) Wu, H. C.; Tsai, T. Y.; Chu, F. H.; Tai, N. H.; Lin, H. N.; Chiu, H. T.; Lee, C. Y. Electron Field Emission Properties of Nanomaterials on Rough Silicon Rods. J. Phys. Chem. C 2010, 114, 130–133. (16) Edgeworth, J. P.; Burt, D. P.; Dobson, P. S.; Weaver, J. M. R.; Macpherson, J. V. Growth and morphology control of carbon nanotubes at the apexes of pyramidal silicon tips. Nanotechnology 2010, 21, 105605. (17) Fan, H. J.; Fuhrmann, B.; Scholz, R.; Syrowatka, F.; Dadgar, A.; Krost, A.; Zacharias, M. Well-ordered ZnO nanowire arrays on GaN substrate fabricated via nanosphere lithography. J. Cryst. Growth 2006, 287, 34–38. (18) Liu, D. F.; Xiang, Y. J.; Wu, X. C.; Zhang, Z. X.; Liu, L. F.; Song, L.; Zhao, X. W.; Luo, S. D.; Ma, W. J.; Shen, J.; Zhou, W. Y.; Wang, G.; Wang, C. Y.; Xie, S. S. Periodic ZnO nanorod arrays defined by polystyrene microsphere self-assembled monolayers. Nano Lett. 2006, 6, 2375–2378. (19) Masuda, Y.; Kinoshita, N.; Sato, F.; Koumoto, K. Site-selective deposition and morphology control of UV- and visible-light-emitting ZnO crystals. Cryst. Growth Des. 2006, 6, 75–78. (20) Ahsanulhaq, Q.; Kim, J. H.; Hahn, Y. B. Controlled selective growth of ZnO nanorod arrays and their field emission properties. Nanotechnology 2007, 18, 485307. (21) Kim, Y. J.; Yoo, J.; Kwon, B. H.; Hong, Y. J.; Lee, C. H.; Yi, G. C. Position-controlled ZnO nanoflower arrays grown on glass substrates for electron emitter application. Nanotechnology 2008, 19, 315202. (22) Xu, S.; Wei, Y.; Kirkham, M.; Liu, J.; Mai, W.; Davidovic, D.; Snyder, R. L.; Wang, Z. L. Patterned Growth of Vertically Aligned ZnO Nanowire Arrays on Inorganic Substrates at Low Temperature without Catalyst. J. Am. Chem. Soc. 2008, 130, 14958–14959.

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(23) Fan, H. J.; Lee, W.; Hauschild, R.; Alexe, M.; Le Rhun, G.; Scholz, R.; Dadgar, A.; Nielsch, K.; Kalt, H.; Krost, A.; Zacharias, M.; Gosele, U. Template-assisted large-scale ordered arrays of ZnO pillars for optical and piezoelectric applications. Small 2006, 2, 561–568. (24) Zhang, G.; Wang, D. Y.; Mohwald, H. Decoration of microspheres with gold nanodots-giving colloidal spheres valences. Angew. Chem., Int. Ed. 2005, 44, 7767–7770. (25) Xu, H.; Goedel, W. A. Preparation of ultrathin free-standing porous metal films. Small 2005, 1, 808–812. (26) Kosiorek, A.; Kandulski, W.; Glaczynska, H.; Giersig, M. Fabrication of nanoscale rings, dots, and rods by combining shadow nanosphere lithography and annealed polystyrene nanosphere masks. Small 2005, 1, 439–444. (27) Ahsanulhaq, Q.; Umar, A.; Hahn, Y. B. Growth of aligned ZnO nanorods and nanopencils on ZnO/Si in aqueous solution: growth mechanism and structural and optical properties. Nanotechnology 2007, 18, 115603. (28) Qin, Y.; Yang, R. S.; Wang, Z. L. Growth of Horizonatal ZnO Nanowire Arrays on Any Substrate. J. Phys. Chem. C 2008, 112, 18734–18736. (29) Liu, N. S.; Fang, G. J.; Zeng, W.; Long, H.; Fan, X.; Yuan, L. Y.; Zou, X.; Liu, Y. P.; Zhao, X. Z. Strong Effect of Interelectrode Distance on the Performance of a Novel ZnO Nanorod Lateral Field Emission Device Fabricated by a Single-Step Hydrothermal Approach. J. Phys. Chem. C 2010, 114, 8575–8580. (30) Liu, N. S.; Fang, G. J.; Zeng, W.; Long, H.; Yuan, L. Y.; Zhao, X. Z. Diminish the screen effect in field emission via patterned and selective edge growth of ZnO nanorod arrays. Appl. Phys. Lett. 2009, 95, 153505. (31) Liu, N. S.; Fang, G. J.; Zeng, W.; Long, H.; Yuan, L. Y.; Zhao, X. Z. Novel ZnO Nanorod Flexible Strain Sensor and Strain Driving Transistor with an Ultrahigh 107 Scale “On”-“Off ” Ratio Fabricated by a Single-Step Hydrothermal Reaction. J. Phys. Chem. C 2011, 115, 570–575. (32) Liu, N. S.; Fang, G. J.; Zeng, W.; Zhou, H.; Cheng, F.; Zheng, Q. A.; Yuan, L. Y.; Zou, X. A.; Zhao, X. Z. Direct Growth of Lateral ZnO Nanorod UV Photodetectors with Schottky Contact by a Single-Step Hydrothermal Reaction. ACS Appl. Mater. Interfaces 2010, 2, 1973–1979. (33) Liu, N. S.; Fang, G. J.; Zeng, W.; Zhou, H.; Long, H.; Zou, X. A.; Liu, Y. P.; Zhao, X. Z. High performance ZnO nanorod strain driving transistor based complementary metal-oxide-semiconductor logic gates. Appl. Phys. Lett. 2010, 97, 243504. (34) Liu, N. S.; Fang, G. J.; Yang, X. X.; Zeng, W.; Li, C.; Wang, M. J.; Li, J.; Zhao, X. Z. Synthesis of patterned carbon nanotube arrays for field emission using a two layer Sn/Ni catalyst in an ethanol flame. Diam. Relat. Mater. 2009, 18, 1375–1380. (35) Liu, J.; She, J. C.; Deng, S. Z.; Chen, J.; Xu, N. S. Ultrathin seedlayer for tuning density of ZnO nanowire arrays and their field emission characteristics. J. Phys. Chem. C 2008, 112, 11685–11690. (36) Jeong, G. H.; Olofsson, N.; Falk, L. K. L.; Campbell, E. E. B. Effect of catalyst pattern geometry on the growth of vertically aligned carbon nanotube arrays. Carbon 2009, 47, 696–704. (37) Fowler, R. H.; Nordheim, L. Electron Emission in Intense Electric Fields. Proc. R. Soc. London, Ser. A 1928, 119, 173–181. (38) Wang, W. Z.; Zeng, B. Q.; Yang, J.; Poudel, B.; Huang, J. Y.; Naughton, M. J.; Ren, Z. F. Aligned ultralong ZnO nanobelts and their enhanced field emission. Adv. Mater. 2006, 18, 3275–3278. (39) Wang, X. D.; Zhou, J.; Lao, C. S.; Song, J. H.; Xu, N. S.; Wang, Z. L. In situ field emission of density-controlled ZnO nanowire arrays. Adv. Mater. 2007, 19, 1627–1631. (40) Shen, G. Z.; Bando, Y.; Liu, B. D.; Golberg, D.; Lee, C. J. Characterization and field-emission properties of vertically aligned ZnO nanonails and nanopencils fabricated by a modified thermal-evaporation process. Adv. Funct. Mater. 2006, 16, 410–416. (41) Li, L.; Fang, X. S.; Chew, H. G.; Zheng, F.; Liew, T. H.; Xu, X. J.; Zhang, Y. X.; Pan, S. S.; Li, G. H.; Zhang, L. D. Crystallinity-controlled germanium nanowire arrays: Potential field emitters. Adv. Funct. Mater. 2008, 18, 1080–1088. 14384



ARTICLE

(42) Gautam, U. K.; Panchakarla, L. S.; Dierre, B.; Fang, X. S.; Bando, Y.; Sekiguchi, T.; Govindaraj, A.; Golberg, D.; Rao, C. N. R. Solvothermal Synthesis, Cathodoluminescence, and Field-Emission Properties of Pure and N-Doped ZnO Nanobullets. Adv. Funct. Mater. 2009, 19, 131–140. (43) Ramgir, N. S.; Late, D. J.; Bhise, A. B.; Mulla, I. S.; More, M. A.; Joag, D. S.; Pillai, V. K. Field emission studies of novel ZnO nanostructures in high and low field regions. Nanotechnology 2006, 17, 2730–2735. (44) Xu, C. X.; Sun, X. W.; Fang, S. N.; Yang, X. H.; Yu, M. B.; Zhu, G. P.; Cui, Y. P. Electrochemically deposited zinc oxide arrays for field emission. Appl. Phys. Lett. 2006, 88, 161921. (45) Choi, Y. C.; Shin, Y. M.; Bae, D. J.; Lim, S. C.; Lee, Y. H.; Lee, B. S. Patterned growth and field emission properties of vertically aligned carbon nanotubes. Diam. Relat. Mater. 2001, 10, 1457–1464. (46) Min, Y.-S.; Bae, E. J.; Kim, U. J.; Park, W.; Hwang, C. S. Direct growth of single-walled carbon nanotubes on conducting ZnO films and its field emission properties. Appl. Phys. Lett. 2006, 89, 113116. (47) Silan, J. L.; Niemann, D. L.; Ribaya, B. P.; Rahman, M.; Meyyappan, M.; Nguyen, C. V. Carbon nanotube pillar arrays for achieving high emission current densities. Appl. Phys. Lett. 2009, 95, 133111. (48) Zeng, W.; Fang, G. J.; Liu, N. S.; Yuan, L. Y.; Yang, X. X.; Guo, S. S.; Wang, D. Y.; Liu, Z. Q.; Zhao, X. Z. Numerical calculations of field enhancement and field amplification factors for a vertical carbon nanotube in parallel-plate geometry. Diam. Relat. Mater. 2009, 18, 1381–1386.

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