Lanthanide Ions Sensitization by Small Noble Metal Nanoclusters

Rare-earth ions sensitization is, nowadays, a relevant topic in modern technologies. Noble metal nanoclusters can effectively sensitize lanthanide photoluminecence (PL) via excitation energy transfer (EET). Recent experimental works reported how this process strongly depends on the nanoclusters size and composition, however, a comprehensive understanding of this phenomenon is still lacking. Inspired by the current paradigm on the lanthanide-antenna complexes, where light is absorbed by the organic ligand, which then converts to a triplet and transfers the excitation to the lanthanide, we propose it also applies to sensitization by metal clusters. To prove this, we studied the optoelectronic features of several MN nanoclusters (M = Au, Ag, Au/Ag mix; N = 12, 20, and 58) at the Time Dependent Density Functional Theory (TDDFT) level, including, via simplified models, the silica matrix and its possible defects, and make considerations on the role these features can have on the EET toward Er3+ ions. Our analysis suggests that PL enhancement is generally more effective when N = 12 and M = Ag or Au/Ag mix, while the worst cases are obtained when M = Au and N = 58. These findings are coherent with prior experimental data and with novel measures that are here presented for the first time. Notably, we recover that the matrix defects can actively take part in the EET and, in some cases, could be (counterintuitively) beneficial for the process efficiency. Globally, this theoretical framework gives a comprensive rationale that can guide the design of new effective rare-earth ion sensitizers based on metal clusters.