Nanotube Nanotube Junction

Nanotubes with different diameters can form junctions. Because bandgap of a nanotube is related with its diameter, semiconductor-semiconductor and semiconductor-metal junction can be formed between nanotubes. It can be defined as physical heterojunction.

Li et al. [158] used nanostructured alumina template channels to grow individual Y-junction carbon nanotube het-erostructures by the pyrolysis of acetylene with cobalt catalysis. Multiwalled carbon nanotube Y-junctions were obtained as shown in Figure 16. Ho et al. [159] applied high electric fields on multiwalled carbon nanotube arrays to create nanotube junctions. The high electric field as well as thermal effects result in carbon-carbon bond breaking and redeposition leading to nanojunction formation.

Satishkumar and colleagues [160] used a simple pyrol-ysis method to prepare multiwalled carbon nanotube Y-junctions. Organometallic precursor of nickellocene, together with thiophane, was decomposed at 1273 K. Two-temperature zone furnace was used for the synthesis; nick-ellocene was placed at the first zone with temperature of 623 K and thiophane was placed at the second zone with temperature of 1273 K. Argon and hydrogen are used as carrier gases. Y-junction structure was verified with HRTEM.

X- and Y-junctions of single-walled carbon nanotubes were fabricated by electron beam exposure at high temperatures [161]. The experiments were carried out in a highvoltage TEM with accelerating voltage of 1.25 MV Carbon nanotubes were heated to 800 °C by using a Gatan heating stage in the TEM chamber. It was noted that high temperature is necessary to induce the formation of junctions because electron irradiation at room temperature would rapidly lead to heavy radiation damage of the tubes. The ready-formed X-junctions can be manipulated in order to create Y- and T-like molecular connections by using careful conditions of irradiation of electron beam. Single-walled carbon nanotube Y-junctions were also synthesized by chemical vapor deposition of methane and hydrogen on Si3N4 membrane at 950 °C [162].

Chiu et al. [163] used a chemical approach to fabricate nanotube junctions. The single-walled carbon nano-tubes were sonicated in HNO3 for 20 min to produce shortened and open-ended nanotubes, meanwhile to create defects on side walls. During the treatment, oxygen-containing groups are attached to opened ends of tube and side wall of tubes. Subsequently Chiu et al. converted these oxygen-containing groups to corresponding acid chloride by reacting with SOCl2 at room temperature for 24 hr. Then the solution was decanted and the deposit was rinsed with toluene followed by immersion in tripropylentetramine.

Figure 16. A TEM image of Y-junction nanotube with 50 nm scar bar. Reprinted with permission from [158], J. Li et al., Nature 402, 253 (1999). © 1999, Macmillan Magazines Ltd.

It was supposed that the bifunctionalized amine interconnects two nanotubes via formation of acidic amide bonding. Figure 17 shows the schematic representative junction structure. They found a lot of junctions with atomic force microscope after the chemical modifications.

Single-walled carbon nanotube p-n junctions were fabricated by Zhou et al. [164] by using modulated chemical doping method. A 340-nm-thick polymethylmethacrylate layer covers a half of the nanotube, leaving the other half exposed which was doped by potassium doping in vacuum by electrical heating of a potassium source. As-grown semiconducting tubes are p-type [165, 166]. After doping with potassium, the other half of tube becomes n-type; thus p-n junction formed within the same nanotube. One disadvantage of this method is that potassium-doped tube is unstable in air. Later the same group found a way to functionalize carbon nanotube with polyethyleneimine, which makes nanotube n-type and stable in air [167].

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