Researchers at EPFL have created the first detailed model explaining the quantum mechanical effects that cause photoluminescence in thin gold films; This is a breakthrough that could lead to the development of solar fuels and batteries.
Luminescence, the process by which substances emit photons when exposed to light, has long been observed in semiconductor materials such as silicon. This phenomenon consists in the fact that nanoscale electrons absorb light and then re-emit it. This behavior gives researchers valuable information about the properties of semiconductors, making them useful tools for studying electronic processes such as those in solar cells.
In 1969, scientists discovered that all metals luminescent to some degree, but in recent years a clear understanding of how this happens has been elusive. The resurgence of interest in this light emission, driven by applications of nanoscale temperature mapping and photochemistry, has reignited the debate about its origin. But so far the answer has been unclear.
“We have developed very high-quality metallic gold films that give us a unique opportunity to elucidate this process without the confounding factors of previous experiments,” says Julia Tagliabue, head of the school’s Energy Technologies Nanoscience Laboratory (LNET). engineering.
In a recently published study Light: Science and Applications Tagliabue and the LNET team focused laser beams onto extremely thin (13 to 113 nanometers) gold films and then analyzed the resulting faint glow. The data from their definitive experiments were so detailed and so unexpected that they collaborated with theorists from the Barcelona Institute of Science and Technology, the University of Southern Denmark and Rensselaer Polytechnic Institute (USA) to re-study and apply quantum mechanical simulations. methods.
The researchers’ sophisticated approach allowed them to resolve the debate around the type of luminescence emitted from the films (photoluminescence), which is determined by the specific behavior of electrons and their oppositely charged counterparts (holes) in response to light. This also allowed them to create the first complete, fully quantitative model of this phenomenon on gold that could be applied to any metal.
Unexpected quantum effects
Tagliabu explains that the team investigated the photoluminescence process by making the metal even thinner, using a thin single-crystal gold film made using a new synthesis technique. “We saw some quantum mechanical effects occurring in the films down to about 40 nanometers, which was unexpected because normally for a metal you don’t see these effects until they get below 10 nanometers,” he says.
These observations provided important spatial information about exactly where the photoluminescence process occurs within the gold, a prerequisite for using the metal as a probe. Another surprising result of the research was the discovery that gold’s photoluminescence (Stokes) signal could be used to measure the surface temperature of the material itself; This is a boon for scientists working at the nanoscale.
“For many chemical reactions on the surface of metals, there is a great deal of 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 interfere with your measurement. So you can measure a material by using the material itself as a probe.” Being able to probe is a huge advantage,” says Tagliabue.
The gold standard of solar fuel development
The researchers believe their findings will enable metals to be used to gain an unprecedentedly detailed understanding of chemical reactions, especially those used in energy research. Metals such as gold and copper, the next target of LNET research, can trigger some important reactions, such as recycling carbon dioxide (CO2) into carbon products such as solar fuel, which store solar energy in chemical bonds.
“To combat climate change, we will need technologies to convert CO2 into other useful chemicals in one way or another,” says LNET postdoctoral researcher Alan Bowman, first author of the study.
“Using metals is one way to do this, but unless we have a good understanding of how these reactions occur on their surfaces, we can’t optimize them. Luminescence offers a new way to understand what’s happening in these metals.”
Source: Port Altele
