Using experiment, simulation and theory, researchers at ETH Zurich have explained how and why vibrations at the surface of a nanomaterial (q) can interact strongly with electrons (k and k'). Image: Deniz Bozyigit/ETH Zurich.Materials are made up of atoms that vibrate, and these vibrations, or 'phonons', are responsible for how electric charge and heat is transported in the materials. Atomic vibrations in metals, semiconductors and insulators are well studied, but what happens to atomic vibrations when a material is nanosized is far less well understood.
In a recent paper in Nature, Vanessa Wood at ETH Zurich in Switzerland 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. Their paper shows that for nanomaterials smaller than about 10–20nm the vibrations of the outermost atomic layers at the surface of the nanomaterial are comparatively large and play an important role in how it 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 helps to explain why nanoparticle-based solar cells have until now not reached their full potential. The researchers showed using both experiment and theory that these 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," says Wood.
Wood's research group has worked for some time on a particular type of nanomaterial known as colloidal nanocrystals, which are semiconductors with a diameter of 2–10nm. These materials are interesting because their optical and electrical properties are dependent on their size, which can be easily changed during their synthesis.
Colloidal nanocrystals are already being used commercially as emitters of red and green light in LED-based TVs and are also being explored as possible materials for low cost, solution-processed solar cells. Researchers have noticed that placing certain atoms around the surface of the nanocrystals can improve the performance of the solar cells, but the reason why this works has not been understood. The work published in the Nature paper now provides the answer: a hard shell of atoms can suppress the surface vibrations and their interaction with electrons, producing a higher photocurrent and a higher efficiency solar cell.
The experiments were conducted in Wood's labs at ETH Zurich and at the Swiss Spallation Neutron Source at the Paul Scherrer Institute, also in Switzerland. By observing how neutrons scatter off atoms in a material, it is possible to quantify how the atoms in a material vibrate. To understand the neutron measurements, simulations of the atomic vibrations were run at the Swiss National Supercomputing Center in Lugano.
"Without access to these large facilities, this work would not have been possible," says Wood. "We are incredibly fortunate here in Switzerland to have these world-class facilities."
This story is adapted from material from ETH Zurich, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.