can change the structures being grown by rapidly
changing the temperature," explained Samuel Graham,
also an assistant professor in Georgia Tech's School
of Mechanical Engineering. "We can also change the
kinetics of growth, which is something that is difficult
to do using conventional technology."
By demonstrating that carbon nanotubes can be growth
on an AFM cantilever, the technique also provides
a new way to integrate nanometer-scale structures
The research was supported in part by the National
Science Foundation's CAREER award, and has been reported
in the journal Applied Physics Letters.
King, Graham and collaborators Erik O. Sunden, Jungchul
Lee and Tanya L. Wright began with an AFM cantilever
fabricated in their Georgia Tech lab. The cantilever
had an integrated electric-resistance heater whose
output temperature could be controlled by varying
the current. Actual heater temperatures were measured
to within four degrees Celsius using Laser Raman
Calibration of the cantilevers over a large temperature
range using Raman spectroscopy was a key aspect of
the success of this research, allowing the first
detailed temperature maps of these devices, Graham
The researchers used electron beam evaporation to
deposit a 10 nanometer iron catalyst film onto the
cantilever. After heating, the iron film formed islands
that provided catalytic sites for growing nanotubes.
cantilever was then placed into a quartz tube,
which was purged of contaminants with argon gas.
The cantilever heating was then turned on and the
temperature held at approximately 800 degrees Celsius
for 15 minutes. A combination of methane, hydrogen
and acetylene – precursors for carbon nanotubes – was
then flowed into the chamber. Only the cantilever
tip and the reaction gas immediately around it were
heated, leaving the remainder of the experimental
set-up at room temperature.
After removal from the tube, the cantilever was
examined using a scanning electron microscope, which
showed vertically aligned carbon nanotubes growing
from the cantilever heater region. The nanotubes
ranged in length from five to 10 microns, and were
10 to 30 nanometers in diameter. Although the entire
cantilever was coated with the iron catalyst, the
nanotubes grew only on the heated area. A temperature
gradient on the heater created differences in the
types of nanotubes grown.
Both before and after the growth, the cantilever
was vibrated so its resonance frequency could be
measured. Those measurements showed a frequency decline
from 119.10 to 118.23 kHz after the nanotubes were
grown on the cantilever. After the resonance measurements
were made, the cantilever was heated beyond 900 degrees
Celsius in air to burn off the nanotubes. When the
resonance frequency was measured again, it had changed
to 119.09 kHz, showing that the frequency drop had
been due to the mass of the nanotubes.
From the change in the resonance frequency, the
researchers were able to calculate the mass of the
carbon nanotubes they had grown as approximately
four picograms (4 x 10-14) kg.
"We are working on integrating the growing and weighing
of the nanotubes so we can do both of them at the
same time," said King. "That would allow us to monitor
the materials growth as it happens."
Once the two processes are integrated, the researchers
expect to increase the number of cantilevers operating
simultaneously. Cantilever arrays could allow many
different growth temperatures and conditions to be
measured in parallel, accelerating the task of charting
the growth kinetics to determine the optimal settings.
"This is a platform for materials discovery, so
we could test tens or even thousands of different
chemistry or growth conditions in a very short period
of time," King said. "With a thousand cantilevers,
we could do in a single day experiments that would
take years using conventional growth techniques.
Once the right conditions were found, the production
process could be scaled up."
Technical Contacts: Bill King (404-385-4224); E-mail:
( firstname.lastname@example.org )
or Samuel Graham (404-894-2264); E-mail: ( email@example.com ).