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 SiO-AlO-BO-NaO-ZnF-CaF glasses and were identified by their characteristic luminescence bands: violet blue luminescence (Ag: 4d5s → 4d), green white molecular fluorescence (molecule-like [Ag], named ML-Ag) and orange molecular fluorescence ([Ag] pairs). Due to the relatively low aggregation degrees of [Ag] and [Ag], non-radiative transitions were highly suppressed, and the PL quantum yields (QYs) of ML-Ag and [Ag] pairs reached 73.7% and 89.7%, respectively. The substitution of 0.5BO-0.5NaO with SiO promoted the partial reduction of Ag to Ag and the subsequent aggregation of Ag and Ag to form [Ag] (ML-Ag). The absence of NaO also resulted in an increasing amount of Ag-Ag pairs with closing interionic distance to form [Ag] 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 [Ag] centers could be effectively produced by lowering the total amount of other competitive charge compensators, such as Na, or by introducing negatively charged [BO], [AlO], and [ZnO] tetrahedrons into the glass matrix.