Energy Scientists Say They Have Solved the Centuries-Old Mystery of Why Ultra-Thin Sheets of Gold Glow - The Debrief

When announcing the breakthrough discovery, the energy scientists from Ecole Polytechnique Federale De Lausanne (EPFL) explained that the presence of photoluminescence in semiconductors has been observed "for hundreds of years." This not only includes the most common semiconductors such as silicon, but also includes gold. However, the exact mechanism behind what makes gold glow was never completely understood.

Some progress was reported in 1969 when researchers determined that all metals absorb and emit photons. Still, how this was happening remained a mystery. Now, the EPFL team says their unusual collaboration with some of Europe's top theoretical physicists has revealed the answer.

Quantum Mechanical Theorists Help Explain What Makes Gold Glow

To make the discovery the energy scientists created extremely thin gold films, ranging from 13 to 113 nanometers in thickness. Creating gold sheets this thin is always difficult, but the researchers say the relatively new ability to create gold at this level of thinness and purity was the key to unlocking the magic behind what makes gold glow without the stumbling blocks encountered by previously unsuccessful efforts.

"We developed very high-quality metal gold films, which put us in a unique position to elucidate this process without the confounding factors of previous experiments," explained Giulia Tagliabue, the head of the EFPL's Laboratory of Nanoscience for Energy Technologies (LNET) in the School of Engineering.

After blasting these ultra-thin gold sheets with a powerful laser, the researchers conducted a detailed examination of the photoluminescent glow radiating off of them. While the glow was detected, the team says its results were so precise and so "unexpected" that they left them scratching their heads.

"We observed certain quantum mechanical effects emerging in films of up to about 40 nanometers, which was unexpected because normally, for a metal, you don't see such effects until you go well below 10 nm," Tagliabue explained.

Suspecting that a quantum mechanical process may be at work, the team reached out to physics theoreticians from the Barcelona Institute of Science and Technology, the University of Southern Denmark, and the Rensselaer Polytechnic Institute. Working in tandem, the team says the two groups were able to join their knowledge to figure out exactly how the electrons in the metals were interacting with their oppositely charged counterparts, called "holes."

This work revealed "key spatial information" within this nearly invisible energy exchange occurring at the quantum level, which was making these gold sheets glow. The team said that this work, which is published in the journal Light Science & Applications, has resulted in the "first complete, fully quantitative model of this phenomenon in gold, which can be applied to any metal."

Glowing Gold Could Help Revolutionize Energy Generation

Beyond some potentially novel applications, the team says that their work means scientists could theoretically use a material like gold in advanced sensing methods that support energy generation. That's because some of the most precise sensors can affect their own readings, whereas a gold-based sensor's "Stokes" signal, a.k.a. glow, can reveal the precise temperature of the probe's surface.

"For many chemical reactions on the surface of metals, there is a big debate about why and under what conditions these reactions occur," said Tagliabue. "Temperature is a key parameter, but measuring temperature at the nanoscale is extremely difficult because a thermometer can influence your measurement. So, it's a huge advantage to be able to probe a material using the material itself as the probe."

This unique ability the scientists explain, can offer "unprecedentedly" detailed insights into chemical reactions, "especially those involved in energy research." In fact, the team believes that their explanation of what makes gold glow, and similar work they are exploring with copper, could help reduce carbon emissions in the generation of solar fuels, which store energy in chemical bonds.

"To combat climate change, we are going to need technologies to convert CO2 into other useful chemicals one way or another," says LNET postdoc Alan Bowman, the study's first author. "Using metals is one way to do that, but if we don't have a good understanding of how these reactions happen on their surfaces, then we can't optimize them. Luminescence offers a new way to understand what is happening in these metals."

Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.