5.3 Result
5.3.1 SEM characterization
Sample Thorn was obtained by reduction of HAuCl4 by Sn in the presence of only NaNO3. After reaction, growth of a golden layer on the Si surface was observed. SEM images of Sample Thorn are shown in Figure 5.1. Figure 5.1A depicts that a high density of nanothorns with a length of 1-2 µm grow up on roughened Si. The width is about 0.5 µm near the base. A high magnification SEM image of nanothorns, as shown in Figure 5.1B, shows a pyramidal shape of a nanothorn.
Figure 5.1. (A) Side-view and (B) top-view SEM images of Au nanothorns grown on Si.
Sample Urchin-6h, Urchin-12h, and Urchin-18h were prepared in an aqueous solution of both CTAC and NaNO3 at three different growth time. Figure 5.2A shows SEM image of Urchin-6h at the early growth stage. The 2 µm diameter nuclei deposit dispersively on Si.
An enlarged view of a nucleus shows that many nanosized pins protruded on the nucleus, as shown in Figure 5.2B. Each pin has a length of less than 200 nm. Figure 5.2C shows SEM image of Urchin-12h grown at 12 h. The density and dimension of nuclei are similar to Urchin-6h. Especially, a detailed image, as shown in Figure 5.2D, points out that a rod-like nanostructure is evolved from a small pin in Figure 5.2B. The diameter and length is about 100 nm and up to 500 nm, respectively. Because they have urchin-like nanostructures, we name gold nanourchins. When elongating growth time to 18 h, SEM images of Urchin-18h show multiple- and long- needle nanourchins, as shown in Figure 5.2E. Figure 5.2F shows that the protruding needles are on top of a nuclear particle. Each needle is like a straight nanowire, has a diameter of about 100 nm and a length of up to 3 µm.
Figure 5.3 shows an apparent pentagonal cross-section of short Au nanowires protruded on a nanourchin. This agrees well with the penta-twinned NW structure characterized by TEM studies in our previous report.12
Figure 5.2. SEM images of Au nanourchins grown on pre-roughened Si at different growth time. (A)-(B) 6 h, (C)-(D) 12 h, and (E)-(F) 18 h.
Figure 5.3. High Magnification SEM images of Au nanowires on nanourchins showing a pentagonal cross-section and a step structure on the side wall.
5.3.2 XRD analysis
Figure 5.4 shows XRD pattern of Thorn. The peaks at 2θ = 38.1o, 44.3o, and 64.5o are assigned to Au (111), (200), and (220) reflections, respectively (JCPDF 89-3697). Lattice constant a is estimated to be 0.408 nm, close to the reported value, 0.4079 nm of Au (JCPDF 89-3697). This XRD study suggested that nanothorns consist of fcc Au.
Figure 5.4 XRD of Au nanothorns grown on Si
5.3.3 FE Properties
Because of their novel morphology and direct growth on Si, field emission (FE) properties of Au nanothorns and nanourchins were investigated. The FE properties and the corresponding Fowler-Nordheim (F-N) plot are illustrated in Figure 5.5A and 5.5B, respectively. The turn-on field E0 was designated as the interceptions of straight lines extrapolated from the low-field and high-field segments of the F-N plots. Thorn, Urchin-12h, and Urchin-18h show E0 of 13.3, 10.2, and 6.3 Vµm-1, respectively.
Figure 5.5 (A) Emission current density as a function of applied electric field (J-E plots) on Au nanostructures; (B) corresponding Fowler-Nordheim (F-N) plots.
Above this field strength, the emission current density increases dramatically. F-N plots of ln(J/E2) versus 1/E observe a straight line in the high field. This indicates that the field emission character follows the model described by Fowler–Nordheim (FN) equation J = A(β2E2/Φ)exp(-BΦ3/2/βE).13 In the equation, in addition to J and E mentioned above, Φ is the work function of Au (5.00 eV), A and B are constants, 1.56 × 10-10 (A V-2 eV) and 6.83 × 103 (V eV-3/2 µm-1), respectively.7 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 of Thorn, Urchin-12h, and Urchin-18h are calculated to be 410, 822, and 1150.
In contrast, Urchin-6h does not show significant J within the E applied. This suggests that nanothorn and nanourchins with nanorods and nanowires exhibit the FE performace.
Especially, nanowires with higher aspect ratio can extrude electrons much more efficiently.
In addition, β of the Au nanourchins grown at 18h is superior to those of the other 1D Au and Cu nanostructures reported previously, such as the NWs grown by hard-template assisted and CVD processes, 632 and 443 respectively.7, 10
5.4 Conclusion
In conclusion, a simple process to grow of Au nanothorns and nanourchins on Si is achieved in this study. Growth time and additives affect the product morphology significantly. FE measurement shows that the Au nanothorns and nanourchins display a FE properties. Especially, nanourchin grown at 18 h 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.
5.5 References
7. Dangwal, A.; Pandey, C. S.; Muller, G.; Karim, S.; Cornelius, T. W.; Trautmann, C., Appl. Phys. Lett. 2008, 92, 063115-3.
8. Maurer, F.; Dangwal, A.; Lysenkov, D.; Muller, G.; Toimil-Molares, M. E.; Trautmann, C.; Brotz, J.; Fuess, H., Nuclear Inst. and Methods in Physics Research, B 2006, 245, 337-341.
9. Davydov, D. N.; Sattari, P. A.; AlMawlawi, D.; Osika, A.; Haslett, T. L.; Moskovits, M., J. Appl. Phys. 1999, 86, 3983.
Chapter 6
Conclusions
In this thesis, we employed surfactant assisted electrochemical methods to grow Cu and Au nanostructures on hard substrates. Galvanic reduction of CuCl2(aq) by Al(s) and HAuCl4(aq)
by Sn(s) can grow Cu nanobelts, pagoda-topped tetragonal nanopillar arrays, Au nanothorns, and nanourchins. Cu NBs can also be grown on carbon electrode by electrochemical deposition in CuCl2(aq) aqueous solution. Kinetic control dominates formation of these novel nanocrystal. Diffusion limited condition formed in the electrochemical growth process is thought to cause anisotropic dendridic crystal. Surfactants, such as CTAC and DTAC play a role of capping reagent to confine and stabilize crystal growth to form the non-branching 1D nanostructures. Moreover, the environment of NO3- and Cl- may provide a selective etching ability on a certain crystal facets. These are all the key factors controlling evolution of 1D nanocrystal in the electrochemical system.
We have also explored the properties and application of these unique nanostructures. Cu NB electrodes exhibit a large surface area and high electro-catalytic ability of glucose. In amperometric sensing of glucose, Cu NB electrode possesses high sensitivity and low detection limit compared to the other electrodes reported previously. A array of free-standing Cu nanopillars grow upward on Au/glass. The characteristics of a straight structure, high aspect ratio, and pointed tip of nanopillars lead to an excellent FE property with high emission current and large β. Au nanothorns and nanourchins are composed of radially protruding 1D nanostructures on Si. The comparison of their FE measurements reveal that Au nanourchin grown at 18 h exhibits a superior FE performance. It suggests that
electron emitting efficiency is promoted as the aspect ratio increases.
We conclude that the surfactant assisted electrochemical method is an effective route to grow distinctive Cu and Au nanostructure. This method also provides a straightforward way to integrate nanomaterials into nanodevices. It can be anticipated that more unusual nanostructures and high-efficiency minimized devices will be developed by this method in future.