Influence of colloidal-gold films on the luminescence of Eu(TTFA)3 in PMMA

The effects of colloidal-gold layers on the luminescent properties of thin films of Eu(TTFA)₃(TTFA=thenoyltrifluoroacetonate) in PMMA (PMMA=poly(methyl methacrylate)) were investigated. Layers of spherical gold nanoparticles (12 nm) were formed by self-assembly on the surface of amino-derivatized glass slides. Eu(TTFA)₃-PMMA films were then spin-coated either directly onto the Au metal surfaces or onto spacer layers covering the gold. The luminescence properties were characterized both as a function of the density of Au particles in the colloidal layer, and as a function of the distance between the Au layer and the luminescent film. The distance between the metal and luminescent layers was controlled using polyelectrolyte spacer layers deposited on the colloidal-gold films by a spin-assisted, layer-by-layer (SA-LBL) method. It was found that the colloidal gold layer has a net quenching effect on Eu(TTFA)₃ luminescence under all conditions considered in this study. The luminescence intensities and lifetimes decrease with increasing density of Au nanoparticles and with decreasing separation (d) between the luminescent film and the gold layer. The measured luminescence intensity drops more quickly with decreasing distance than one would predict based solely on lifetime data, if one assumes a constant radiative relaxation rate. Fits of the luminescence decay kinetics to a model for non-radiative energy-transfer from Eu(TTFA)₃ to the gold layer yields a 1/d² dependence, where d is the distance from the gold layer to the nearest face of the luminescent film. It is suggested that there is no reasonable physical interpretation of this result within the constraints of the model and, therefore, the interaction between the luminescent and gold layer cannot be explained solely in terms of non-radiative energy transfer.