Sapphire Surfaces Spontaneously Arrange Carbon Nanotubes

University of Southern California (USC) researchers have found that sapphire surfaces spontaneously arrange carbon nanotubes into useful patterns — but only the right surfaces.

Single walled carbon nanotubes will grow along certain crystalline orientations on sapphire. No template has to be provided to guide this structuring: it takes place automatically. As a substrate for the creation of single wall nanotube transistor (SWNT) devices, sapphire has a critical advantage.

The experiment conducted at USC has resolved how and why this occurs. The process is potentially predictable and controllable, opening the door for systematic exploration of sapphire as a SWNT medium.

According to the researchers, understanding this process may allow registration-free fabrication and integration of nanotube devices by simply patterning source/ drain electrodes at desired locations, as the active material (i.e., nanotubes) is all over the substrate, to build such devices as sensors and integrated circuits for various uses.

Sapphire is aluminum oxide, also known as the mineral alumina, the abrasive corundum, and when colored by small quantities of iron, ruby. It is readily available as a cheap synthetic. The crystal is six-sided, rising from a flat base, and has four natural planes on which it can be split to form thin, smooth slices: one parallel to the base, and three other vertical ones.

Certain vertical slices, particularly the a- and r-planes, exhibit the self-guiding nanotube behavior. The c-plane, parallel to the base did not. Two possibilities might explain the difference. One would be the arrangement of the atoms in the matrix; the other, differences in the "step edge" properties of the surfaces (step edges are nanoscopic surface irregularities, minute rises from the suface level).

To eliminate step edges as a possibility, the research group annealed (treated with high, long-lasting heat) samples of both forms, and then tested. Annealing emphasizes step edges, and would accordingly emphasize the arrangement effect, if the effect was dependent on the edges; it did not.

The basal, horizontal slices remained unable to self-guide nanotubes. The two of the vertical slices continued to do so. The behavior seems to be due to the varied arrangement of aluminum and oxygen atoms on the surface. The research team is now investigating how the exact mechanisms work, in order to further control the process.


Taking the a-plane: Nanotube (mesh) on representation of appropriate crystal surface; below: nanotubes growing on actual sapphire surface.

Visit http://viterbi.usc.edu/news/news/2005/2005_04_13_sapphire.htm