Apr 22 2024Reviewed by Lexie Corner
In a recent paper published in the journal Light: Science & Applications, EPFL researchers created the first thorough model of the quantum-mechanical processes underlying photoluminescence in thin gold films.
For centuries, it has been known that semiconductor materials like silicon can undergo luminescence, which is the emission of photons by a substance exposed to light. Electrons are frequently used as probes to characterize electronic processes, such as those in solar cells, because their behavior at the nanoscale when they absorb and re-emit light can reveal a great deal about the characteristics of semiconductors to researchers.
All metals glow to some extent, as scientists discovered in 1969, but the years that followed produced no conclusive evidence for why this happens. The debate over the origins of this light emission has been rekindled due to its renewed interest in nanoscale temperature mapping and photochemistry applications. However, up until this point, the response remained ambiguous.
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.
Giulia Tagliabue, Head, School of Engineering, Laboratory of Nanoscience for Energy Technologies
Tagliabue and the LNET team focused laser beams on the incredibly thin gold films, between 13 and 113 nm, and examined the faint glow. They worked with theorists at the University of Southern Denmark, the Barcelona Institute of Science and Technology, and the Rensselaer Polytechnic Institute (USA) to rework and apply quantum mechanical modeling methods because the data generated from their precise experiments was so detailed and unexpected.
The type of luminescence coming from the films is called photoluminescence, which is determined by how electrons and their oppositely charged counterparts (holes) behave in response to light. The researchers were able to resolve this debate by their thorough approach. It additionally enabled them to create the first comprehensive, entirely quantitative model of this phenomenon in gold, which is transferable to any other metal.
Unexpected Quantum Effects
According to Tagliabue, the group examined the photoluminescence process as they progressively thinned the metal using a thin film of monocrystalline gold made using a unique synthesis method.
Tagliabue said, “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.”
A necessary condition for using gold as a probe is that these observations gave important spatial details about the precise location of the photoluminescence process in the metal. One other unexpected finding from the study was that the gold's photoluminescent (Stokes) signal could be used to measure the material's surface temperature, which would be very helpful for nanoscale scientists.
For many chemical reactions on the surface of metals, there is a big debate about why and under what conditions these reactions occur. 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.
Giulia Tagliabue, Head, School of Engineering, Laboratory of Nanoscience for Energy Technologies
A Gold Standard for Solar Fuel Development
According to the researchers, their discoveries will make it possible to use metals to get previously unattainable levels of detail into chemical reactions, particularly those related to energy research. The LNET's next research focus will be on metals like copper and gold, which can start some important processes, like the reduction of carbon dioxide (CO2) back into carbon-based products like solar fuels, which use chemical bonds to store solar energy.
To combat climate change, we are going to need technologies to convert CO2 into other useful chemicals one way or another. 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.
Alan R. Bowman, Study First Author and Postdoc, Laboratory of Nanoscience for Energy Technologies
Journal Reference:
Bowman, R. A., et al. (2024) Quantum-mechanical effects in photoluminescence from thin crystalline gold films. Light: Science & Applications. doi.org/10.1038/s41377-024-01408-2
Source: https://www.epfl.ch/en/