Energy scientists unravel the mystery of gold’s glow

All metals glow to some extent; However, how this happens needs to be better understood. Its microscopic origins are intensely debated, and its potential for unraveling nanoscale carrier dynamics is largely untapped.

In a new study, EPFL scientists revealed quantum mechanical effects in the luminescence emanating from thin monocrystalline gold flakes. Specifically, they presented experimental evidence, supported by first-principles simulations, to demonstrate the origin of photoluminescence upon excitation in the interband regime.

Scientists have created the first thorough model of the quantum mechanical processes behind photoluminescence in thin gold plates; this finding could spur the creation of solar fuels and batteries.

Scientists have developed very high-quality thin gold films between 13 and 113 nanometers for this research. These films allowed them to clarify this process without the confounding factors of previous experiments.

The team focused laser beams on the films and then examined the resulting faint glow. Their experiment yields surprising and puzzling results. To interpret this data, they worked with quantum mechanics specialists from the Barcelona Institute of Science and Technology, the University of Southern Denmark and the Rensselaer Polytechnic Institute in the United States.

Working together, they were able to settle the disagreement over the type of luminescence the films emitted and determine that it was photoluminescence. The way holes, the opposite of electrons, respond to light determines this luminescence. They also created the first thorough and fully quantitative model of this phenomenon in gold, which they also developed to be applicable to other metals.

Giulia Tagliabue, head of the Laboratory of Nanoscience for Energy Technologies (LNET) at the School of Engineering, said: “Using a thin film of monocrystalline gold, produced with a new synthesis technique, the team studied the photoluminescence process as they made the metal thinner and thinner. We observed certain quantum mechanical effects in films of up to 40 nanometers, which was unexpected, because normally for a metal you only see such effects when you go well below 10 nm.”

These observations provided crucial spatial information about the precise location of the gold’s photoluminescence activity before the metal was used as a probe. Another unexpected result of the research was that gold’s photoluminescent signal (Stokes) could be used to measure the surface temperature of the material, which would be extremely useful for scientists at the nanoscale.

Tagliabue says: “For many chemical reactions at the surface of metals, there is great debate about why and under what conditions these reactions occur. Temperature is an important parameter, but measuring temperature at the nanoscale is extremely difficult because a thermometer can affect your measurement. So it is a huge advantage to investigate a material using the material itself as a probe.”

Scientists believe their findings will make it possible to use metals to gain unprecedented detailed insights into chemical reactions. The LNET’s next research will focus on metals such as copper and gold, which can start critical processes such as converting carbon dioxide (CO2) into carbon-based goods such as solar fuels, which use chemical bonds to store solar energy.

LNET postdoc Alan Bowman, the first author of the study, said: “To combat climate change, we need technologies to convert CO2 into other useful chemicals.”

“Using metals is one way to do that, but if we don’t understand how these reactions happen on their surfaces, we can’t optimize them. Luminescence offers a new way to understand what is happening in these metals.”

Magazine reference:

1. Bowman, A.R., Rodríguez Echarri, A., Kiani, F. et al. Quantum mechanical effects in photoluminescence of thin crystalline gold films. Light Sci Appl 13, 91 (2024). DOI: 10.1038/s41377-024-01408-2