3.2 Experimenal
4.3.5 Field Emission Properties
Because of their novel morphology, field emission (FE) properties of the pagoda-topped nanopillar arrays were investigated. For a typical sample, Figure 4.6 shows that the current density (J) of the emitted electrons increases dramatically over an electric field (E) threshold, the turn-on E, E0. At E0 of 12.4 V µm−1, the J value is observed to be 10 µA cm−2. The inset shows a plot of ln(J/E2) versus 1/E. For 1/E between 0.06 and 0.15 µm V-1, a straight line is observed. This indicates that the field emission character follows the model described by Fowler–Nordheim (FN) equation J = A(β2E2/Φ)exp(-BΦ3/2/βE).[21] In the equation, in addition to J and E mentioned above, Φ is the work function of Cu (4.47 eV), and A and B are constants, 1.56 × 10-10 (A V-2 eV) and 6.83 × 103 (V eV-3/2 µm-1), respectively.[14] β is field enhancement factor, a general parameter describing emitter performance. β is dependent on geometry and morphology of the nanostructure, crystal structure of the material, and density of the emitting points. Here, β is calculated to be 713.[14,21] In contrast, a layer of cube-like Cu NPs deposited at 1 h, does not show
Figure 4.6 Emission current density as a function of applied electric field on Cu nanopillars (Pillar-3) and NPs (Pillar-4) (inset: corresponding F-N plots).
significant J within the field applied. With their high aspect ratios and unique pagoda-shaped tips, it is not a surprise that the nanopillars can extrude electrons much more efficiently than the NPs. In addition, β of the Cu nanopillars is superior to those of the other 1D Cu nanostructures reported previously, such as the NWs grown by hard-template assisted and CVD processes, 245 and 443 respectively.[13,15]
4.4 Conclusion
In conclusion, we have achieved a simple process to grow of arrays of pagoda-topped tetragonal Cu nanopillars with {100} side faces in this study. Growth time and DTAC concentration affect the product morphology significantly. FE measurement shows that the Cu nanopillars can emit electrons under relatively low electric field strength. We anticipate that this novel material could be employed for interesting nanodevice applications in the future.
4.5 References
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Li, T. J. Phys. Chem. B 2005, 109, 13857.
2. Nikoobakht, B.; El-Sayed, M. A. J. Phys. Chem. A 2003, 107, 3372.
3. Grand, J.; de la Chapelle, M. L.; Bijeon, J. L.; Adam, P. M.; Vial, A.; Royer, P. Phys. Rev.
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4. Hernandez, J.; Solla-Gullon, J.; Herrero, E.; Aldaz, A.; Feliu, J. M. J. Phys. Chem. B 2005, 109, 12651.
5. Langford, R. M.; Wang, T. X.; Thornton, M.; Heidelberg, A.; Sheridan, J. G.; Blau, W.;
Leahy, R. J. Vac. Sci. Technol. B 2006, 24, 2306.
6. Hsia, C.-H.; Yen, M.-Y.; Lin, C.-C.; Chiu, H.-T.; Lee, C.-Y. J. Am. Chem. Soc. 2003, 125, 9940.
7. Yen, M.-Y.; Chiu, C.-W.; Hsia, C.-H.; Chen, F.-R.; Kai, J.-J.; Lee, C.-Y.; Chiu, H.-T. Adv.
Mater. 2003, 15, 235.
8. Choi, J.; Sauer, G.; Goring, P.; Nielsch, K.; Wehrspohn, R. B.; Gosele, U. J. Mater. Chem.
2003, 13, 1100.
9. Dobrev, D.; Yao, H. J.; Sun, Y. M.; Hou, M. D.; Mo, D.; Wang, Z. G.; Neumann, R.
Nanotechnology 2006, 17, 1922.
10. Li, N.; Li, X.; Yin, X.; Wang, W.; Qiu, S. Solid State Commun. 2004, 132, 841.
11. Choi, J.; Sauer, G.; Nielsch, K.; Wehrspohn, R. B.; Gosele, U. Chem. Mater. 2003, 15, 776.
12. Dangwal, A.; Pandey, C. S.; Muller, G.; Karim, S.; Cornelius, T. W.; Trautmann, C. Appl.
Phys. Lett. 2008, 92, 3.
13. Maurer, F.; Dangwal, A.; Lysenkov, D.; Muller, G.; Toimil-Molares, M. E.; Trautmann, C.; Brotz, J.; Fuess, H. Nucl. Instrum. Meth. B 2006, 245, 337.
14. Wang, J.-H.; Yang, T.-H.; Wu, W.-W.; Chen, L.-J.; Chen, C.-H.; Chu, C.-J.
Nanotechnology 2006, 17, 719.
15. Kim, C.; Gu, W.; Briceno, M.; Robertson, I. M.; Choi, H.; Kim, K. K. Adv. Mater. 2008, 20, 1859.
16. Huang, T. K.; Cheng, T. H.; Yen, M. Y.; Hsiao, W. H.; Wang, L. S.; Chen, F. R.; Kai, J. J.;
Lee, C. Y.; Chiu, H. T. Langmuir 2007, 23, 5722.
17. Huang, T.-K.; Chen, Y.-C.; Ko, H.-C.; Huang, H.-W.; Wang, C.-H.; Lin, H.-K.; Chen, F.-R.; Kai, J.-J.; Lee, C.-Y.; Chiu, H.-T. Langmuir 2008, 24, 5647.
18. Bard, A. J.; Faulkner, L. R., Electrochemical Methods: Fundamentals and Applications, John Wiley & Sons, New York 1980.
19. Fukami, K.; Nakanishi, S.; Yamasaki, H.; Tada, T.; Sonoda, K.; Kamikawa, N.; Tsuji, N.;
Sakaguchi, H.; Nakato, Y. J. Phys. Chem. C 2007, 111, 1150.
20. Walter, E. C.; Murray, B. J.; Favier, F.; Kaltenpoth, G.; Grunze, M.; Penner, R. M. J.
Phys. Chem. B 2002, 106, 11407.
21. Fowler, R. H.; Nordheim, L. W. Proc. R. Soc. A 1928, 119, 173.
Chapter 5
Field Emission Properties of Gold Nanostructures Grown Directly on Silicon
5.1 Introduction
The field emission of electrons is a quantum-mechanical phenomenon, as a tunneling process across the energy barrier between the vacuum and the emitter. One-dimensional (1D) nanomaterials have been demonstrated to be induced several orders of magnitude of an external applied field due to their characteristics of the small tip and high aspect ratio.1-5 In FE simulation, metal nanowires (NWs) are thought to be an excellent FE materials.6 However, there are much few related reports. The key issue is that fabrication of NW arrays is crucial. The template-assisted approach, such as anodic aluminum oxide (AAO) and ion track membrane, is a known method.7-9 Recently, chemical vapor deposition (CVD) is employed to grow free-standing Cu NWs for the FE device.10,11 Until to now, growth and FE properties of 1D Au nanostructures on hard substrates without hard templates are less reported. As mentioned in Chapter 2-4, the electrochemical methods provided a successful approach to grow Cu nanobelts and nanopillars on electrodes. In this article, we present that a facile galvanic reduction to grow directly Au nanothorns and nanourchins on Si. Their shape-depend FE performance will be explored.
5.2 Experimental
A summary of growth conditions of Au nanostructures is listed in Table 5.1. Growth of Au nanourchins by galvanic reduction of HAuCl4(aq) by Sn(s) in the presence of CTAC and NaNO3 (Urchin-18h) is described below as a typical example.
Table 5.1 A summary of growth conditions of Au nanostructures.
A n-type Si(100) wafer (TSR High Purity Si) was pre-roughened by a electroless etching process to enhance Au nucleation. After cleaned with acetone, the Si substrate, cut into 0.5 cm × 0.5 cm in size, was immersed into a Teflon bottle containing an etching solution, which was a mixture of 0.07 g AgNO3 (Fisher), 2 mL of 48 % w.t. HF (Merck) and 10 mL deionized water for 5 min. After the etching step, the substrate was washed with concentrated HNO3 (J. T. Baker) and followed by deionized water to remove Ag precipitates formed in the etching process. Finally, the pre-roughened Si was dried under a N2 stream.
5.2.2 Fabrication of Sn-pasted Silicon
A piece of Sn metal (99.998%, Aldrich), used as the reducing metal, was pasted on the backside of the pre-roughened Si substrate by Ag glue (Toyobo). Then, the combined piece was dried in the oven at 383 K for 1 h.
5.2.3 Growth of Au Nanourchins
In a glass vial, a mixed growth solution containing HAuCl4(aq) (5 mM, SHOWA), CTAC(aq)