Why Silver Clusters Emit Light

Only small clusters of four silver atoms in the form of a tetrahedron and surrounded by water molecules emit light. (Source: KU Leuven)

Zeolites have a very rigid and well-known structure containing numerous small channels and voids. In chemistry, they are used to stimu­late certain reactions. Molecules that are caught in the voids of the zeolites lose their mobi­lity and start behaving dif­ferently. This research allowed the scien­tists to discover why and how silver clusters emit light when they are enclosed in the cages of a specific type of zeolite.

“We irradiated a mixture of silver clusters with synchro­tron radiation at the European Synchro­tron Radia­tion Facility in Grenoble”, explains researcher Didier Grandjean. “What is good about this is that it provides us with a lot of infor­mation on the structure and proper­ties of the material. However, as we speci­fically wanted to look at the optical properties, we used a new method that deli­berately only measured the emitted light. This way, we were sure that we were only looking at the specific particles responsible for the light.” The research provided con­clusive evidence that only small clusters of four silver atoms in the form of a tetra­hedron and surrounded by water molecules emit light.

“The tetra­hedra form a unit in which two electrons can move freely. This forms a super atom: a structure composed of several atoms, but behaving very much like a single atom”, says Peter Lievens. “The optical proper­ties of the clusters are caused by the two free electrons. These decay from a higher to a lower energy level, resulting in a certain shade of green light. In turn, the energy levels are determined by the chemical proper­ties of the super atom.” This new observation can be attri­buted to an intense colla­boration between researchers in chemistry and physics. “Further­more, our experi­mental obser­vations are confirmed by advanced theo­retical calcu­lations”, says Peter Lievens.

This new-found know­ledge allows the researchers to start modi­fying the properties of silver clusters or find new materials with the desired optical proper­ties. The results of this work are important for the study of light and its appli­cations, among other things. (Source: KU Leuven)

Reference: D. Grandjean et al.: Origin of the bright photoluminescence of few-atom silver clusters confined in LTA zeolites, Science 361, 686 (2018); DOI: 10.1126/science.aaq1308

Link: Laboratory of Solid State Physics and Magnetism, KU Leuven, Leuven, Belgium

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