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General hypothesis and shell model for the synthesis of semiconductor nanotubes, including carbon nanotubes
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Image of FIG. 1.
FIG. 1.

Schematic diagrams showing nine different representative nanotubes (NT-1 to NT-9). The nanotube sidewalls are shown in deep green, the nanotube seeds are shown in red, and the FECA is shown in purple (violet). The FECA may lie at the nanotube base (see NT-1, NT-3, NT-5) or at the nanotube head (see NT-2, NT-3, NT-7, and NT-9). The nanotube may also be filled with FECA (see NT-4).

Image of FIG. 2.
FIG. 2.

The calculated variation in the size-dependent melting point depression (e.g., ) with the nanoparticle radius for Au, Ni, and Fe nanoparticles, respectively. is the melting point of the nanoparticle.

Image of FIG. 3.
FIG. 3.

The calculated solubility-dependent melting point depression for Au, Ni, Co, and Fe nanoparticles, respectively.

Image of FIG. 4.
FIG. 4.

The nanotube growth by the adsorption-induced process (e.g., by direct landing of the species on the droplet surface) and the diffusion-induced process (e.g., impingement of the species on the substrate surface). The numeral 1 corresponds to landing and adsorption of the species directly into the droplet surface; the numeral 2 to desorption of the species from the droplet surface; the numeral 3 to impingement of the species on location P of the substrate surface; the numeral 4 to diffusion of the species from the substrate surface location P to the nanotube base Q; the numeral 5 to diffusion of the species from the nanotube base to the droplet surface; and the numeral 6 to impingement of adatoms at or near the nanotube base. The nanotube length is , the external nanotube diameter is , and the internal nanotube diameter is . The center of the nanotube base is the origin of the coordinate system, and the distance between the points P and Q is .

Image of FIG. 5.
FIG. 5.

Schematic diagrams of three possible structures (e.g., NSA, NSB, and NSC) of nanotube droplets (e.g., molten/semimolten seeds).

Image of FIG. 6.
FIG. 6.

Schematic diagrams of narrow stripes (nanopores) formed in NSA (NSA-1 to NSA-8) seeds made of X or FECA/X material. The solid seeds are shown as red shells, the molten (semimolten) seed is shown as orange shell; and the accumulation of the species on the exterior surface of the seed is shown as patterned blue shell. The seed NSA-1 is entirely solid; the seed NSA-2 is entirely molten (semimolten); and the seeds NSA-3 to NSA-8 are solid but have disturbed and disordered regions. Narrow white stripes and tiny white spheres resulted from disturbance and disorder in the seed. The landing of the species directly from the vapor phase is shown by arrow.

Image of FIG. 7.
FIG. 7.

Schematic diagrams showing the creation of bamboo-shaped nanotubes. (a) Swelling (extension) of the inner wall of the NSA seed due to the accumulation of the X atoms; blue-green shell is the swollen X-rich segment of the droplet. (b) Formation of cone (or umbrella) shaped diaphragms and (c) almost horizontal diaphragms formed from NSA seed. (d) Formation of inverted cone (or inverted umbrella) shaped diaphragms formed from NSB and/or NSC seeds. The ejection of the accumulated X atoms to the side surface of the seed is shown by black arrows but the diffusion of the species through the droplet/nanotube interface is shown by blue arrows.

Image of FIG. 8.
FIG. 8.

Gallium seeds formed on (a) GaN matrix and (b) GaN clusters and hillocks during GaN nanotube growth by the SCG mechanism.

Image of FIG. 9.
FIG. 9.

GaN nanotubes grown from the seeds; (a) various structural forms (e.g., circular, hexagonal, pentagonal, irregular, etc.) of the nanotubes; (b) nanotubes with incomplete (broken) walls.


Generic image for table
Table I.

List of input data used for the calculations of the size-dependent and solubility-dependent melting temperature depression of some representative FECA/carbon (, etc.) materials (e.g., mixtures, solid solutions, or eutectic/noneutectic alloys). is the melting temperature, is the latent heat of fusion, is the molar volume, and is the surface energy.

Generic image for table
Table II.

List of growth temperatures, carbon sources, and FECAs used for the growths of some representative carbon nanotubes by the CVD.

Generic image for table
Table III.

List of material Layer structures created for the nanotube growth by various mechanisms.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: General hypothesis and shell model for the synthesis of semiconductor nanotubes, including carbon nanotubes