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Origin of the bright photoluminescence of few-atom silver clusters confined in LTA zeolites

D. Grandjean,E. Coutiño-Gonzalez,N. T. Cuong,E. Fron,W. Baekelant,S. Aghakhani,P. Schlexer,F. d’Acapito,D. Banerjee,M. Roeffaers,M. Nguyen,J. Hofkens,P. Lievens

Published 2018 in Science

ABSTRACT

Unmasking the glow of silver clusters Small silver clusters stabilized by organic materials or inorganic surfaces can exhibit bright photoluminescence, but the origin of this effect has been difficult to establish, in part because the materials are heterogeneous and contain many larger but inactive clusters. Grandjean et al. studied silver clusters in zeolites, using x-ray excited optical luminescence to monitor only the emissive structures (see the Perspective by Quintanilla and Liz-Marzán). Aided by theoretical calculations, they identified the electronic states of four-atom silver clusters bound with water molecules that produce bright green emission—thus identifying candidate materials for application in lighting, imaging, and therapeutics. Science, this issue p. 686; see also p. 645 The bright luminescence of Ag-LTA zeolites originates from long-lived triplet states in Ag4(H2O)2 and Ag4(H2O)4 clusters. Silver (Ag) clusters confined in matrices possess remarkable luminescence properties, but little is known about their structural and electronic properties. We characterized the bright green luminescence of Ag clusters confined in partially exchanged Ag–Linde Type A (LTA) zeolites by means of a combination of x-ray excited optical luminescence-extended x-ray absorption fine structure, time-dependent–density functional theory calculations, and time-resolved spectroscopy. A mixture of tetrahedral Ag4(H2O)x2+ (x = 2 and x = 4) clusters occupies the center of a fraction of the sodalite cages. Their optical properties originate from a confined two-electron superatom quantum system with hybridized Ag and water O orbitals delocalized over the cluster. Upon excitation, one electron of the s-type highest occupied molecular orbital is promoted to the p-type lowest unoccupied molecular orbitals and relaxes through enhanced intersystem crossing into long-lived triplet states.

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