Porous gold metamaterial tunes electronic behaviour

Gold plays a crucial role in modern advanced technology thanks to its unique properties.

New research now demonstrates that changing the material’s physical structure — its morphology — can fundamentally enhance both its electronic behaviour and its ability to interact with light.

“This might make it possible to improve the efficiency of chemical reactions such as those used in hydrogen production or carbon capture,” said Tlek Tapani, one of the leading researchers behind the study and a doctoral student at Umeå University’s Department of Physics.

The researchers worked with nanoporous gold, a so-called metamaterial produced in a laboratory. Thanks to its sponge-like structure, nanoporous gold has even better properties for technical applications than ordinary solid gold.

The researchers observed that a thin film of nanoporous gold interacts with light in ways that solid gold cannot. By exposing the ‘gold sponge’ to ultra-short laser pulses, they found that the porous structure allows the material to absorb more light energy over a wider spectrum.

As a result, the electrons become considerably more energetic. The electronic temperature was estimated to reach about 3200 K (~2900°C) in the nanoporous film, compared with just 800 K (~500°C) in the unstructured gold film used as a reference, under the same conditions. It also takes longer for the ‘hot’ electrons to cool down and return to their initial state at room temperature.

“Such elevated electronic temperatures enable light-induced transitions that would otherwise be nearly impossible,” said Nicolò Maccaferri, senior author of the article. “Interestingly, using advanced electron microscopy and X-ray photoelectron spectroscopy experiments (XPS) here at Umeå University, we were able to confirm that these unique behaviours are driven solely by the material’s physical shape and not by changes to the electronic structure of gold itself.”

The experiments suggest that nanoporous structure can be used as a new design parameter to engineer materials used in advanced technologies. By systematically varying the filling factor (the ratio of gold to air in the ‘sponge’), researchers can tune the electronic behaviour of not only gold but also other metals in a controllable way, which could improve the efficiency of chemical reactions.

This is a modified version of a news item published by Umea University. The original version of the news item can be accessed here.

Top image caption: In the laser laboratory, Tlek Tapani and Nicolò Maccaferri are testing how porous structures enable gold to absorb more light energy than ordinary gold. Image credit: Mattias Pettersson.