Plasmon-Induced Energy Transfer: When the Game Is Worth the Candle

The superior optical extinction characteristics of noble metal nanoparticles have long been considered for enhancing the solar energy absorption in light-harvesting devices. The energy captured through a plasmon resonance mechanism can potentially be transferred to a surrounding semiconductor matrix in the form of excitons or charge carriers, offering a promising light-sensitization strategy. Of particular interest is the plasmon near-field energy conversion, which is predicted to yield substantial gains in the photocarrier generation. Such a short-range interaction, however, is often inhibited by processes of backward electron and energy transfer, which obscure its net benefit. Here, we employ sample-transmitted excitation photoluminescence spectroscopy to determine the quantum efficiency for the plasmon-induced energy transfer (ET) in assemblies of Au nanoparticles and CdSe nanocrystals. The present technique distinguishes the Au-to-CdSe ET contribution from metal-induced quenching processes, thus enabling accurate estimates of the photon-to-exciton conversion efficiency. We show that in the case of 9.1 nm Au nanoparticles only 1-2% of the Au absorbed radiation is converted to excitons in the surrounding CdSe nanocrystal matrix. For larger, 21.0 nm Au, the photon-to-exciton conversion efficiency increases to 29.5%. The results of the present measurements were used to develop an empirical model for estimating the maximum gain in plasmon-induced carriers versus the mass fraction of Au in a film.