By studying gold nanoparticles with highly uniform sizes and shapes, scientists
now understand how they lose energy, a key step towards producing nanoscale
detectors for weighing any single atom.
Such ultrasensitive measurements could ultimately be used in areas such as
medical research and diagnostics, enabling the detection of minuscule disease-causing
agents such as viruses and prions at the single molecule level.
Researchers are interested in nanosized materials because the smaller the components
of a detection device, the more sensitive it is.
In this study, the team from the University
of Melbourne, Argonne's Center for Nanoscale Materials in Illinois and the
University of Chicago synthesized and studied tiny gold rods with a width 5000
times smaller than the thickness of a human hair.
The work will be published online this week in Nature Nanotechnology.
Professor John Sader from the Department of Mathematics and Statistics, University
of Melbourne says that in the same way as a classroom ruler decreases its frequency
of vibration when an eraser is attached, nanomechanical mass sensors work by
measuring their change in vibration frequency as mass is added.
The sensitivity of such nanomechanical devices is intimately connected to how
much energy they displace. So researchers needed to understand how damping (loss
of energy) is transferred both to the fluid surroundings and within the nanostructures.
With the lower the damping, the purer the mechanical resonance and higher the
sensitivity.
It has not previously been possible to determine the rate at which vibrations
in metal nanoparticle systems are damped, because of significant variations
in the dimensions of the particles that have been studied – which masks
the vibrations.
However, by studying a system of bipyramid-shaped gold nanoparticles with highly
uniform sizes and shapes, the researchers overcame this limitation.
"Previous measurements of nanomechanical damping have primarily focused
on devices where only one- or two-dimensions are nanoscale, such as long nanowires.
Our measurements and calculations provide insight into how energy is dissipated
in devices that are truly nanoscale in all three-dimensions," says Professor
Sader.
Illuminating these bipyramidal nanoparticle systems with an ultra-fast laser
pulse, set them vibrating mechanically at microwave frequencies. These vibrations
were long-lived and for the first time damping in these nanoparticle systems
could be interrogated and characterized.
Moreover, the researchers separated out the portion of damping that is due
to the material itself and that surrounding liquid for which they developed
a parameter-free theoretical model that quantitatively explains this fluid damping.
Posted July 27th, 2009
