Silver and cheap minerals make LED phosphors
Used these highly luminescent clusters of silver atoms, the researchers hope the new phosphor could make next-generation fluorescent and LED lighting even cheaper and more efficient.
Silver clusters were already known for their optical properties but their applications so far were limited due to their natural aggregation into larger particles, losing their luminescence.
Professor Hofkens and his team from the Molecular Imaging and Photonics Unit have now found a way to keep the silver clusters apart by inserting them into the porous framework of zeolites. Zeolites are minerals that are either found in nature or produced synthetically on an industrial scale. The minerals have a very rigid and well-defined framework of small molecular-sized channels, pores, and cages.
“Zeolites contain sodium or potassium ions. We used ion exchange to replace these ions with silver ions. To obtain the clusters we wanted, we heated up the zeolites with the silver ions, so that the silver ions self-assembled into clusters” explained Professor Maarten Roeffaers from the Centre for Surface Chemistry and Catalysis.
In collaboration with Professor Peter Lievens’s Laboratory for Solid State Physics and Magnetism, the researchers examined the properties of these heat-treated ‘silver zeolites’. Using advanced techniques, they found that the structural, electronic, and optical properties of the zeolites were strongly influenced by the silver clusters. That’s how they discovered that the shape of the silver clusters is essential to obtain the right fluorescence properties.
“Clusters of silver atoms can assemble into different shapes, including a line or a pyramid. This pyramid shape is what we need to obtain the best fluorescence properties. Heating up the silver ions in the zeolite framework makes them adopt this shape. Because they are ‘trapped’, as it were, in the cages of the zeolites, they can only form a pyramid with up to four silver atoms. That is exactly the shape and size in which the silver cluster emits the largest amount of fluorescent light, with an efficiency close to 100%” notes Hofkens.
This study was funded by the European Union: EU FP7-NMP-2012 SACS (Self-Assembly in Confined Space; GA-310651).
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