Stabilization of ultra-small [Ag2]2+ and [Agm]n+ nano-clusters through negatively charged tetrahedrons in oxyfluoride glass networks: To largely enhance the luminescence quantum yields

Abstract

Herein, three different silver species were stably formed in SiO₂–Al₂O₃–B₂O₃–Na₂O–ZnF₂–CaF₂ glasses and were identified by their characteristic luminescence bands: violet blue luminescence (Ag⁺: 4d⁹5s¹ → 4d¹⁰), green white molecular fluorescence (molecule-like [Ag_m]ⁿ⁺, named ML-Ag) and orange molecular fluorescence ([Ag₂]²⁺ pairs). Due to the relatively low aggregation degrees of [Ag_m]ⁿ⁺ and [Ag₂]²⁺, non-radiative transitions were highly suppressed, and the PL quantum yields (QYs) of ML-Ag and [Ag₂]²⁺ pairs reached 73.7% and 89.7%, respectively. The substitution of 0.5B₂O₃−0.5Na₂O with SiO₂ promoted the partial reduction of Ag⁺ to Ag⁰ and the subsequent aggregation of Ag⁺ and Ag⁰ to form [Ag_m]ⁿ⁺ (ML-Ag). The absence of Na₂O also resulted in an increasing amount of Ag⁺–Ag⁺ pairs with closing interionic distance to form [Ag₂]²⁺ in glass. According to the X-ray photoelectron spectra (XPS) and magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra, a solubility strategy and a charge compensation model were proposed to describe the transformations between different silver species. The formation of ML-Ag was further controlled via the solubility of Ag⁺ in glass, whereas [Ag₂]²⁺ centers could be effectively produced by lowering the total amount of other competitive charge compensators, such as Na⁺, or by introducing negatively charged [BO₄]⁻, [AlO₄]⁻, and [ZnO₄]²⁻ tetrahedrons into the glass matrix.