Atomic vibrations in nanomaterials

March 9, 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. Using both experiment, simulation, and theory, they explain how and why vibriations at the surface of a nanomaterial (q) can interact strongly with electrons (k and k'). Credit: Deniz Bozyigit / ETH Zurich

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 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.

Improving Solar Cells

Wood's research group has worked for a long time on a particular type of nanomaterial—colloidal nanocrystals—semiconductors with a diameter of 2 to 10 nanometers. These materials are interesting because their optical and electrical properties are dependent on their size, which can be easily changed during their synthesis.

These materials are now used commercially as red- and green-light emitters in LED-based TVs and are being explored as possible for low cost, solution-processed solar cells. Researchers have noticed that placing certain atoms around the surface of the nanocrystal can improve the performance of . The reason why this worked had not been understood. The work published in the Nature paper now gives the answer: a hard shell of atoms can suppress the vibrations and their interaction with electrons. This means a higher photocurrent and a higher efficiency solar cell.

Big Science to Study the Nanoscale

Experiments were conducted in Professor Wood's labs at ETH Zurich and at the Swiss Spallation Neutron Source at the Paul Scherrer Institute. By observing how neutrons scatter off atoms in a material, it is possible to quantify how atoms in a material vibrate. To understand the neutron measurements, simulations of the were run at the Swiss National Supercomputing Center (CSCS) in Lugano. Wood says, "without access to these large facilities, this work would not have been possible. We are incredibly fortunate here in Switzerland to have these world class facilities."

Explore further: Clarifying the role of magnetism in high-temperature superconductors

More information: Deniz Bozyigit et al. Soft surfaces of nanomaterials enable strong phonon interactions, Nature (2016). DOI: 10.1038/nature16977

Related Stories

Demystifying nanocrystal solar cells

January 28, 2015

ETH researchers have developed a comprehensive model to explain how electrons flow inside new types of solar cells made of tiny crystals. The model allows for a better understanding of such cells and may help to increase ...

Controlling acoustic transport in hypersonic crystals

October 27, 2014

Center for Nanoscale Materials users from Toyota Research Institute of North America, working with CNM's Nanophotonics Group, have determined that bulk coherent acoustic vibrations are heavily damped by scattering from radially ...

A quantum leap for the next generation of superconductors

February 26, 2016

Quantum materials – materials designed at the sub-atomic level – can be finely-tuned to achieve extremely useful properties that are often not found in nature. These include superconductivity, the ability to conduct electricity ...

Recommended for you

Nano-decoy lures human influenza A virus to its doom

October 25, 2016

To infect its victims, influenza A heads for the lungs, where it latches onto sialic acid on the surface of cells. So researchers created the perfect decoy: A carefully constructed spherical nanoparticle coated in sialic ...

New method increases energy density in lithium batteries

October 24, 2016

Yuan Yang, assistant professor of materials science and engineering at Columbia Engineering, has developed a new method to increase the energy density of lithium (Li-ion) batteries. He has built a trilayer structure that ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.