Researchers at Rensselaer
Polytechnic Institute have discovered the origins of nanorod diameter, demonstrating
that the competition and collaboration among various mechanisms of atomic transport
hold the key to nanorod size. The researchers say it is the first study to identify
the fundamental reasons why nearly all nanorods have a diameter on the order
of 100 nanometers.
“Scientists have been fabricating nanorods for decades, but no one has
ever answered the question, ‘Why is that possible?’” said
Hanchen Huang, professor in Rensselaer’s Department of Mechanical, Aerospace,
and Nuclear Engineering, who led the study. “We have used computer modeling
to identify, for the first time, the fundamental reasons behind nanorod diameter.
With this new understanding, we should be able to better control nanorods, and
therefore design better devices.”
Results of the study, titled “A characteristic length scale of nanorods
diameter during growth,” were recently published in the journal Physical
Review Letters.
When fabricating nanorods, atoms are released at an oblique angle onto a surface,
and the atoms accumulate and grow into nanorods about 100 nanometers in diameter.
A nanometer is one billionth of a meter in length.
The accumulating atoms form small layers. After being deposited onto a layer,
it takes varying amounts of energy for atoms to travel or “step”
downward to a lower layer, depending on the step height. In a previous study,
Huang and colleagues calculated and identified these precise energy requirements.
As a result, the researchers discovered the fundamental reason nanorods grow
tall: as atoms are unable to step down to the next lowest layer, they begin
to stack up and grow higher.
It is the cooperation and competition of atoms in this process of multi-layer
diffusion that accounts for the fundamental diameter of nanorods, Huang shows
in the new study. The rate at which atoms are being deposited onto the surface,
as well as the temperature of the surface, also factor into the equation.
“Surface steps are effective in slowing down the mass transport of surface
atoms, and aggregated surface steps are even more effective,” Huang said.
“This extra effectiveness makes the diameter of nanorods around 100 nanometers;
without it the diameter would go up to 10 microns.”
Beyond advancing scientific theory, Huang said the discovery could have implications
for developing photonic materials and fuel cell catalysts.
Huang co-authored the paper with Rensselaer Research Scientist Longguang Zhou.
Funding for this research was provided by the U.S. Department of Energy Office
of Basic Energy Science.
Posted March 19th, 2009
