Let there be light: how silver clusters in nano-cages produce light

24-06-2016

Future LEDs might have been partly brewed at the ESRF’s beamlines DUBBLE and LISA. An international team of researchers has shown that highly luminescent clusters of silver atoms can be assembled in the porous framework of minerals known as zeolites. The high efficiency of light emission from the materials, along with cheap and scalable synthesis makes them very attractive for next generation fluorescent lamps and LEDs or for biological imaging.

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Silver clusters are small ensembles of just a few silver atoms (<10), which have remarkable catalytic and optical properties. Current applications for silver clusters are limited due to a natural tendency to aggregate into larger particles which do not exhibit these enhanced properties. To overcome this limitation, researchers from the Université de Strasbourg & CNRS and KU Leuven (Belgium) assembled and stabilised the clusters in nano-scale confined spaces. In particular, they used the pores in carefully chosen minerals, called zeolites.

Zeolites can be found naturally or produced synthetically on an industrial scale. Owing to their rigid and well-defined framework made of molecular-scale channels and cavities, zeolites are already used for a wide range of domestic and industrial applications (e.g. laundry detergent, water purification, gas separation, catalysis).

In this study, the researchers investigated silver clusters assembled in four different types of zeolite. Silver ions were introduced into the zeolites by means of ion-exchange, leading to the partial or total replacement of the native sodium or potassium ions in the parent zeolites. Thermal treatment at elevated temperature allowed the controlled assembly of the silver ions into well-defined clusters within the confined space of the zeolite cavities.

The team used the Dutch-Belgian beamline BM26 and the Italian beamline BM08 at the ESRF to carry out an in-depth characterisation of these heat-treated silver-exchanged zeolites using X-ray absorption fine structure (EXAFS). Wim Bras, responsible for BM26, explains that "these results show that X-ray spectroscopy really is an unique tool and required for the characterization of technologically important new materials".   

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A comparison of the luminescence properties of the new "Zeo-Light" phosphors with conventional commercial materials.

                                                                        

The analysis provided unambiguous evidence of a strong influence of the zeolite host and degree of silver uptake on the structural, electronic and optical properties of the clusters. Didier Grandjean, one of the scientists in the team says that “thanks to the detailed EXAFS analysis carried out at the ESRF’s DUBBLE and BM08-LISA beamline we could unravel the structures of the luminescent silver clusters and develop rational design rules for assembling them in zeolites. This allowed us to synthesise materials with luminescence efficiencies close to 100%”.

These results can contribute to elucidate the structure–property relationships of small metal clusters and to ultimately develop highly luminescent materials with potential applications in optoelectronics (luminescent tags, fluorescent lamps, LEDs), bioimaging and catalysis.

Reference:

Fenwick, O. et al, Nature Materials (2016), DOI: 10.1038/nmat4652

Top image: Schematic representation of small luminescent silver clusters (green spheres) embedded in the pores of zeolite frameworks.