Enhancement of the Er3+ luminescence in Er-doped silica by few-atom metal aggregates

The mechanisms of the Er3+ photoluminescence enhancement induced by ultrasmall Au nanoclusters (made by less than 20 atoms) incorporated by low-fluence ion implantation into Er-doped silica are discussed. We show that the energy-transfer process from moleculelike Au nanoclusters to the rare-earth ions critically depends on the evolution of the early stages of the Au metal clustering process starting from dispersed or oxidized atoms up to the formation of metal nanometric clusters. This feature has been investigated in detail combining extended x-ray-absorption fine-structure and photoluminescence spectroscopies and using different annealing atmospheres (neutral or reducing) in order to better evidence the chemical with respect to the thermal effect on Au nucleation and growth. We experimentally showed that the enhancement mechanism is improved when the optically active Er ions are coupled to Au clusters composed by 5-10 atoms and it remarkably decreases when the Au cluster size is above 20-30 atoms.