Vibrations of
atoms in materials, the "phonons", are responsible for how
electric charge
and heat is transported in materials. (Graphics: Deniz Bozyigit / ETH Zurich)
(March 10, 2016) Researchers
at ETH have shown for the first time what happens to atomic vibrations when
materials are nanosized and how this knowledge can be used to systematically
engineer nanomaterials for different applications.
All materials are made up of atoms, which vibrate. These
vibrations, or "phonons", are responsible, for example, for how
electric charge and heat is transported in materials. Vibrations of metals,
semiconductors, and insulators in are well studied; however, now materials are
being nanosized to bring better performance to applications such as displays,
sensors, batteries, and catalytic membranes. What happens to vibrations when a
material is nanosized has until now not been understood.
Soft Surfaces Vibrate Strongly
In a recent publication in Nature, ETH Professor Vanessa
Wood and her colleagues explain what happens to atomic vibrations when
materials are nanosized and how this knowledge can be used to systematically
engineer nanomaterials for different applications.
The paper shows that when materials are made smaller than
about 10 to 20 nanometers - that is, 5,000 times thinner than a human air – the
vibrations of the outermost atomic layers on surface of the nanoparticle are
large and play an important role in how this material behaves.
“For some applications, like catalysis, thermoelectrics, or
superconductivity, these large vibrations may be good, but for other
applications like LEDs or solar cells, these vibrations are undesirable,”
explains Wood.
Indeed, the paper explains why nanoparticle-based solar
cells have until now not met their full promise. The researchers showed using both experiment
and theory that surface vibrations interact with electrons to reduce the
photocurrent in solar cells.
“Now that we have proven that surface vibrations are
important, we can systematically design materials to suppress or enhance these
vibrations,” say Wood.