All materials are composed of atoms, and the atoms vibrate all the time. These vibrations become phonons and are responsible for charging the material and transferring heat. Research on the vibration of metals, semiconductors and insulators is also underway; now nano-scale materials can bring better performance to applications such as displays, sensors, batteries, and catalytic membranes. However, when the material is on the nanometer scale, it is unclear how the material vibrates.
Soft surface vibrates strongly
In a recent article published in "Science", Vanessa Wood, a professor at ETH University and her colleagues explained how atoms vibrate and how to apply this knowledge to different nanomaterial system engineering when the material is at the nanoscale. The paper shows that when the prepared material is less than 10-20nm-that is, 5000 times thinner than human hair-the outermost atoms on the surface of the nanoparticles vibrate greatly and have a great influence on the performance of the material. "For some applications such as catalysis, thermoelectric technology or superconductivity, these large vibrations may be good, but for other applications such as LEDs or solar cells, these vibrations are not good," Wood explained. In fact, the paper explains why nano solar cells have not been fully realized until now. The researchers used experiments and theories to show that the interaction of surface vibration and electrons reduces the photocurrent of solar cells. "Now that we have demonstrated that surface vibration is very important, we can systematically design materials to suppress or enhance these vibrations," Wood said.
Improve solar cell performance
Wood's research team has studied a special semiconductor nanomaterial, colloidal nanocrystal, for a long time, which is a semiconductor with a diameter of 2-10 nm. Because of the size dependence of their optical and electrical properties, these materials are meaningful. The material size can be easily changed during synthesis. Now commercially, these materials are used in red and green light emitters for LED TVs, and are also being explored for low-cost, solution-processed solar cells. The researchers noticed that placing certain atoms on the surface of nanocrystals can improve the performance of solar cells. However, its working principle has not been clarified yet. The paper now published in "Science" gives the answer: the hard shell of atoms can suppress vibration and its interaction with electrons. This means higher photocurrent and efficiency of solar cells.
Studying big science at the nanoscale
The experiment was conducted in Professor Wood's ETH University Laboratory and the Swiss Neutron Source Paul Scherrer Institute. By observing how neutrons scatter atoms in a substance, the vibration of atoms in the material can be quantified. Neutron measurement and atomic vibration simulation were performed at the Swiss National Supercomputing Center (CSCS). Wood said: "Without these large-scale equipment, this work is impossible. We are very lucky to have these world-class facilities in Switzerland