Researchers at EPFL, led by Giulia Tagliabue, have developed a comprehensive model of the quantum-mechanical effects behind photoluminescence in thin gold films. This discovery could advance the development of solar fuels and batteries. The team used high-quality gold films and laser beams to study the photoluminescence process, observing unexpected quantum mechanical effects in films up to about 40 nanometers. The study, published in Light: Science and Applications, was conducted in collaboration with the Barcelona Institute of Science and Technology, the University of Southern Denmark, and the Rensselaer Polytechnic Institute.
Unraveling the Mystery of Gold’s Luminescence
Researchers from the École Polytechnique Fédérale de Lausanne (EPFL) have developed the first comprehensive model of the quantum-mechanical effects behind photoluminescence in thin gold films. This discovery could potentially drive the development of solar fuels and batteries.
Photoluminescence, the emission of photons by a substance exposed to light, has been known to occur in semiconductor materials like silicon for centuries. The behavior of electrons at the nanoscale as they absorb and then re-emit light can provide valuable insights into the properties of semiconductors. This is why they are often used as probes to characterize electronic processes, such as those occurring inside solar cells.
In 1969, scientists discovered that all metals luminesce to some degree. However, the intervening years failed to yield a clear understanding of how this occurs. Renewed interest in this light emission, driven by nanoscale temperature mapping and photochemistry applications, has reignited the debate surrounding its origins.
A New Approach to Understanding Luminescence in Metals
The EPFL team developed high-quality metal gold films, which allowed them to elucidate this process without the confounding factors of previous experiments. In a recent study published in Light: Science and Applications, the team focused laser beams at the extremely thin gold films, between 13 and 113 nanometers, and then analyzed the resulting faint glow.
The data generated from their precise experiments was so detailed and unexpected that they collaborated with theoreticians at the Barcelona Institute of Science and Technology, the University of Southern Denmark, and the Rensselaer Polytechnic Institute (USA) to rework and apply quantum mechanical modelling methods.
Unveiling Unexpected Quantum Effects
The researchers’ comprehensive approach allowed them to settle the debate surrounding the type of luminescence emanating from the films – photoluminescence – which is defined by the specific way electrons and their oppositely charged counterparts (holes) behave in response to light.
Using a thin film of monocrystalline gold produced with a novel synthesis technique, the team studied the photoluminescence process as they made the metal thinner and thinner. They observed certain quantum mechanical effects emerging in films of up to about 40 nanometers, which was unexpected, because normally for a metal, such effects are not seen until you go well below 10 nm.
Gold as a Probe and Temperature Indicator
These observations provided key spatial information about exactly where the photoluminescence process occurred in the gold, which is a prerequisite for the metal’s use as a probe. Another unexpected outcome of the study was the discovery that the gold’s photoluminescent (Stokes) signal could be used to probe the material’s own surface temperature – a boon for scientists working at the nanoscale.
Measuring temperature at the nanoscale is extremely difficult, because a thermometer can influence your measurement. Therefore, it’s a huge advantage to be able to probe a material using the material itself as the probe.
Implications for Solar Fuel Development
The researchers believe their findings will allow metals to be used to obtain unprecedentedly detailed insights into chemical reactions, especially those involved in energy research. Metals like gold and copper can trigger certain key reactions, like the reduction of carbon dioxide (CO2) back into carbon-based products like solar fuels, which store solar energy in chemical bonds.
To combat climate change, technologies to convert CO2 into other useful chemicals are needed. Using metals is one way to do that, but without a good understanding of how these reactions happen on their surfaces, optimization is impossible. Luminescence offers a new way to understand what is happening in these metals.
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