Silicon Carbon Nanotube Junctions

Si nanowire-carbon nanotube junction was fabricated in Lieber's group [11] by two approaches as shown in Figure 11. The first approach is growth of silicon nanowires

Si nanowires

Nanowire/nanotube b

Si nanowires

Nanowire/nanotube

Carbon nanotubes

Nanotube.'nanowire

Figure 11. Synthetic approaches to nanotube-silicon nanowires junctions. (a) Growth of Si nanowires, afterwards growth of nanotube from the same catalytic particles. (b) Growth of nanotube and attachment of Au nanoparticles, then growing nanowires. Reprinted with permission from [11], J. T. Hu et al., Nature 399, 48 (1999). © 1999, Macmillan Magazines Ltd.

Carbon nanotubes

Nanotube.'nanowire

Figure 11. Synthetic approaches to nanotube-silicon nanowires junctions. (a) Growth of Si nanowires, afterwards growth of nanotube from the same catalytic particles. (b) Growth of nanotube and attachment of Au nanoparticles, then growing nanowires. Reprinted with permission from [11], J. T. Hu et al., Nature 399, 48 (1999). © 1999, Macmillan Magazines Ltd.

from iron nanoparticles. Afterwards, nanotubes were grown from the same particles by chemical vapor deposition method. The second approach: catalyst nanoparticles are deposited on nanotube ends and then used to direct the growth of Si nanowires. The key point for the first approach is that both growth processes share the same catalyst particle. The interface between nanowire and nanotube for the first approach might not be clean, while the second approach yields clean interface, considering that base growth model can be applied to carbon nanotube growth and that tip growth model can be applied to silicon nanowire growth. For base growth, the catalyst particles responsible for nucleation and growth remain pinned on the support surface, while tip growth involves a metal catalyst nanoparticle at a nanowire or nanotube end that is carried away as the nanowire or nanotube grows which is responsible for supply feedstock for the growth.

It is established that Fe and Fe/Au are efficient catalysts for the growth of silicon nanowires [3]. Meanwhile, it is found that Fe is a good catalyst for the growth of carbon nanotubes. Lieber's group used a laser to ablate a Fe0 9Au0A target in 20-torr 10%/90% silane/He at 450 °C. After silicon nanowire growth, the nanotubes were grown from the same catalyst particles in 300-torr ethylene at 600 °C. A transmission electron microscope (TEM) image in Figure 12a shows that a silicon nanowire and multiwalled carbon nanotube junction were obtained. There is a common catalyst particle at the junction. Electron diffraction on silicon nanowires shows that the silicon nanowire maintains its crystalline structure after tube growth.

Lieber's group also used the second approach to fabricate nanojunctions. Gold cluster was deposited on the end of nanotubes by electrodeposition. Silicon nanowire was then grown from the attached gold cluster by using silane mixed with He for 15 min at 510 °C. A scanning electron microscope image of as-grown junction is shown in Figure 12b. The Au nanoparticle is sitting at the end of silicon nanowire. A clean interface was achieved.

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