Metal cluster consists of only few metal atoms directly bonded together showing peculiar and unique properties that strongly differ from their bulk counterpart. In fact, they show molecule-like energy transitions, strong photoluminescence and high (photo-)catalytic activity. These properties arise from their discrete energy levels. Therefore, a promising application for their remarkable luminescent properties is as phosphors for LED applications where they can surpass the performance of current state-of-the-art rare earth and quantum dot based phosphors, while offering comparable efficiencies and stabilities.
However, metal clusters have the strong tendency to aggregate in bigger particle loosing the aforementioned properties. Therefore, our research group is focusing on the application of microporous frameworks as confining agents for metal clusters. In fact, these frameworks have suitable physicochemical properties such as ion exchange capacity, cages/channels and thermal-chemical stability to confine and stabilize metal clusters.
Schematic representation of the metal cluster synthesis
The Hofkens lab is mainly working on the synthesis and characterization of confined metal clusters in microporotus frameworks. The synthesis are carried out with simple and straightforward methods such as hydrothermal techniques, ion exchange reactions and calcination. The materials are then characterized in our lab with steady state (diffuse reflectance and fluorescence spectroscopy) and time resolved spectroscopy (covering the range from femtosecond up to seconds), powder X-ray diffraction and Raman (micro-)spectroscopy. While, the cluster structure is determined through X-ray absorption spectroscopy such as EXAFS and XEOL-EXAFS in synchrotron facilities. In fact, our research group obtained several times the beamtime at the European Synchrotron Radiation Facility (ESRF) to carry out these experiments that lead to publication in Science  and Nature .
By Duduer Grandejean, Eduardo Coutiño-Gonzalez, Ngo Tuan Cuong, Eduard Fron, Wouter Baekelant, Saleh Aghakhani, Philomena Schlexer, Francesco D'Acapito, Dipanjan Banerjee, Maarten B.J. Roefaers, Minh Tho Nguyen, Johan Hofkens, Peter Lievens, Science 17 aug 2018: 686-690. DOI: 10.1126/science.aaq1308
 Fenwick, O., Coutiño-Gonzalez, E., Grandjean, D. et al. Tuning the energetics and tailoring the optical properties of silver clusters confined in zeolites. Nature Mater 15, 1017–1022 (2016). https://doi.org/10.1038/nmat4652