Energy transfer in solution-based clusters of CdTe nanocrystals electrostatically bound by calcium ions

Divalent positively charged Ca2+ ions acting as electrostatic chelating linkers were used to induce clustering of water-soluble CdTe nanocrystals through coordination to the negatively charged carboxylic acid groups of their ligands. An efficient Forster resonant energy transfer (FRET) was responsible for changes in the emission spectra and the luminescence decay times of CdTe nanocrystal clusters composed of particles of two and three different sizes. FRET was also observable in nominally single-sized samples with intrinsically broadened size distributions. Excitation energy transfer toward acceptor nanocrystals in binary clusters resulted in a 77% increase of the acceptor emission intensity at room temperature. Time-resolved fluorescence measurements revealed that at least part of the energy transfer involves long-lived intermediate trap states and the recycling of this trapped excitation energy by transfer to red-emitting nanocrystals.