Illustration representing topography of a gold nanoring where a new method of sensing has been shown based on the damping of acoustic vibrations probed by transient absorption spectroscopy Copyright : A*STAR |
Metal
nanoparticles could play a key role in next-generation light detectors,
optical circuits, and cancer therapies. For these future technologies
to be realized, it is important to understand what happens when
nanoparticles are caused to undergo vibrations, and the consequent
scattering of light that can occur due to oscillations, or surface
plasmons, in their free electron cloud. However, little is known about
exactly how these vibrations are affected by the nanoparticle’s
immediate surroundings—in particular, how the environment affects the
dissipation of energy from a nanoparticle when it vibrates.
Sudhiranjan
Tripathy at the A*STAR Institute of Materials Research and Engineering
and co-workers, collaborating with Arnaud Arbouet and colleagues from
the National Center of Scientific Research (CNRS) in France, have now
analyzed the effect of different environments on individual gold
nanoparticles, their acoustic vibrations and associated energy
dissipation.
The
researchers examined individual nanorings made of gold using transient
absorption spectroscopy, which involves exciting the sample with a pulse
of laser light before measuring the absorbance of light at various
wavelengths. They measured both the vibration period and damping
time—the rate at which the nanoring loses its energy to its
surroundings.
“When
a metallic system is downsized to nanometric dimensions, its vibration
modes can become very different in comparison to its bulk form,”
explains Tripathy. “For example, the damping of the acoustic vibrations
is strongly affected by the elastic properties of the environment and
the interface between the nanoparticle and its environment.”
Previous
spectroscopy studies have experimented with large groups of
nanoparticles, but the collective approach has its limits because
nanoparticles of different sizes may have different vibration periods.
The researchers overcame the problem by working with individual
nanorings, but the workaround did have its own difficulties.
The
first challenge was the nanofabrication of perfectly controlled and
characterized nano-objects. Secondly, there was the issue of detecting
and monitoring the acoustic vibrations of one single metal nano-object.
This meant that the researchers had to measure relative changes on the
order of one in 10 million.
The
researchers studied individual nanorings that were surrounded by either
air or glycerol, and focused on how the different environments affected
the damping time of the vibrations. This provided valuable insight into
how energy dissipated from the nanorings to their environment. Most
tellingly, the damping times were significantly shorter in the highly
viscose glycerol.
“Our
work opens up exciting perspectives including the use of metal
nanoparticles as mass sensors, or as nanosized probes of the elastic
properties of their local environments,” says Tripathy.
Damping of the acoustic vibrations of individual gold nanoparticles
