Mystery of Gold’s Glow: Energy Scientists Unveil the Quantum-Mechanical Effects of Photoluminescence From Crystalline Metal Films

All metals glow to some degree, but how this occurs needs to be understood better. Until now, its microscopic origin has been intensely debated, and its potential for unraveling nanoscale carrier dynamics has been largely unexploited.

(Photo: Wikimedia Commons/ Tim Denholm)

Luminescence From Metals

Luminescence from semiconductors following steady-state photoexcitation has been known since ancient times. Today, this property is widely utilized as a non-invasive probe of diverse phenomena, such as chemical reaction monitoring, assessment of solar cell efficiencies, and rock dating.

In 1969, scientists discovered that all metals exhibit luminescence to some degree. In recent years, photon emission from metals has received increased attention in the context of plasmonic nanostructures. These materials promise to revolutionize the energy, healthcare, and sensing industries. This can be made possible by the ability of plasmon-generated hot carriers to dramatically increase local electronic temperatures, increase solar cell absorption, and improve weak luminescence processes from molecules.

Despite its usefulness, there are still uncertainties surrounding the origin of emitted light, mainly whether it is due to the recombination of electrons and holes (photoluminescence) or other forms of inelastic light scattering. This mystery has been debated among theoretical and experimental studies over the last 50 years.

Understanding Gold's Glow

In a recent study, Swiss Federal Institute of Technology experts in Lausanne (EPFL) explored the quantum-mechanical effects in the luminescence that emanate from thin monocrystalline gold flakes. The details of their investigation are described in the paper "Quantum-mechanical effects in photoluminescence from thin crystalline gold films."

Led by Alan R. Bowman, the research team presented experimental evidence, supported by first-principles simulations, to demonstrate the photoluminescence origin of gold when exciting in the interband regime. This was possible by creating the first thorough model of the quantum-mechanical processes behind photoluminescence in thin gold sheets.

The researchers developed high-quality, skinny gold films between 13 and 113 nanometers for this study. These films enabled them to elucidate the process without the confounding factors of previous experiments.

Bowman and colleagues directed laser beams at the gold films and examined the resulting faint glow. The experiment provided surprising and puzzling data, which were interpreted in collaboration with the quantum mechanic specialists from the Rensselaer Polytechnic Institute in the US, the University of Southern Denmark, and the Barcelona Institute of Science and Technology.

The team worked together to settle the disagreement involving the kind of luminescence emitted by the films, establishing that it was photoluminescence. This luminescence was determined by how the holes, opposite of electrons, respond to light. The study authors also created the first thorough and entirely quantitative model of this phenomenon in gold, which is also applicable to other metals.

The findings could help scientists gain new insights into the creation of solar fuels and batteries. The observations made by the scientists gave crucial spatial information about the precise location of the gold's photoluminescence activity before using the metal as a probe. Another unexpected result was that the photoluminescent signal of gold could be utilized to measure the material's surface temperature, a parameter that would be extremely useful to nanoscale scientists.