Wafer-thin gold flakes emit a faint glimmer after being hit with a laser, and researchers are starting to understand why. Gold flakes are widely used in nanoscience, so the new knowledge should give scientists more control over chemical reactions useful for energy storage and catalysis.
Gold owes its dazzling lustre to the way it reflects light. But ultra-thin gold flakes can also generate and emit their own light in a process called photoluminescence. This begins when the gold is illuminated: some of the photons in the light are absorbed by the metal, and they then trigger the photoluminescence process that sees the gold kick out new photons.
But exactly how this happens is unclear, so at the Swiss Federal Institute of Technology in Lausanne and his colleagues decided to investigate.
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They took flakes of gold with that ranged in thickness between 13 and 113 nanometres and illuminated them with a powerful laser to make them photoluminesce. Then, they used a super-sensitive detector to collect and analyse the photons released by the weakly glowing gold. Bowman says that the equipment was so sensitive that it could gather information such as the wavelength of the gold-emitted photons even when there were as few as one of them for every billion photons from the laser.
Using their very precise measurements and computer simulations, the researchers put together a new and complete model for how photoluminescence occurs in gold flakes. They already knew the basic mechanics of the process: the energy from the incoming photon nudges an electron and a 鈥渉ole鈥� 鈥� a particle-like deficit of charge that behaves as the opposite of an electron 鈥� within the flake. This causes the electron and hole to combine and emit a photon. The new study revealed that there are multiple ways for this to happen.
Some of the holes that combine with electrons do so immediately, while some bounce around inside the gold flake for a while first, says Bowman. The team also found that, for flakes more than about 40 nanometres thick, some of the properties of holes and electrons 鈥� like their energy and momenta 鈥� become more susceptible to quantum effects than was previously known. This discovery will be useful for researchers who may want to use these electrons in devices like photovoltaic cells or light detectors.
at Texas A&M University says that understanding the photoluminescence process in gold flakes at this level of detail could advance the way tiny gold structures are used by researchers, both to concentrate and amplify energy from light to make better biosensors, and to speed up chemical reactions such as those used to produce fertilisers and fuels.
鈥淣ano structures made from gold are a workhorse platform in nanoscience, but we don鈥檛 always know how to analytically think about what happens in them. This study is trying to answer some major questions posed by researchers in this area,鈥� he says.
And the researchers very much have chemistry on their mind, says , also at the Swiss Federal Institute of Technology in Lausanne, who worked on the experiment. 鈥淓lectrons and holes in gold can pass lots of energy onto molecules, enhancing or altering some reactions, so we could both drive chemical transformations and monitor it by shooting lasers onto it,鈥� she says.
Her team is hoping to eventually use these particles to build novel light-rechargeable batteries.
arXiv
