April 21, 2005 —
Taking the a-plane: Nanotube
(mesh) on representation of appropriate crystal surface; below:
nanotubes growing on actual sapphire surface.
USC researchers have found that sapphire surfaces
spontaneously arrange carbon nanotubes into useful patterns — but only
the right surfaces.
Nanotubes are one-atom thick sheets of carbon rolled into seamless
cylinders. They can be used to work as chemical sensors and
transistors, like devices made from carbon's close chemical cousin,
As a substrate for the creation of single wall nanotube transistor (SWNT)
devices, sapphire has a critical advantage, says Chongwu Zhou of the
USC Viterbi School of Engineering's department of electrical
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.
Or more accurately, it sometimes happens automatically. With an
elegant experiment, Zhou 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.
In a paper accepted by the Journal of the American Chemical Society
(V127, P5294, 2005), Zhou says the understanding "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.
According to Zhou, nanotube transistor devices now have to be
painstakingly positioned and aligned using methods such as flow
alignment and electrical-field-assisted alignment and then individually
connected. Experimental techniques can create some more extensive
groups of tubes but "it remains difficult to produce planar nanotube
arrays over large areas with sufficiently high density and order," Zhou
Zhou believes exploitation of the properties of sapphire his team
investigated may allow production of the right kinds of dense, ordered
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 Planes Truth: the c-plane does't guide nanotube growth; the a-and r-planes
The crystal is six-sided, rising from a flat base, (see diagram, right)
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.
The self-guiding phenomenon was first reported last year
by a research team at the Weizman Institute in Israel: Zhou's team systematically
vertical slices, particularly the a- and r-planes, exhibit the
self-guiding nanotube behavior. The c-plane, parallel to the base did
According to Zhou, 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
To eliminate step edges as a possibility, Zhou's 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. Zhou's team is now investigating how the exact mechanisms at
work, in order to further control the process.
Zhou and his team have also, worked with quartz substrates for nanotube synthesis,
which did not exhibit any guided growth.
Zhou worked with Xiaolei Liu and Song Han on the research, which was
supported by an NSF career Award, an NSF-CENS grant, and an SRC MARCO/
Different strains on different planes. Clockwise from top left, growth
on the r, m, c, and a-planes. Note the resulaties on the r- and a-plnaes.
(upper left). On others, they grow at random. Zhou's research has
elucidated understanding and control of the self-guiding property.