Near-Quantitative Yield for Transfer of Near-Infrared Excitons within Solution-Phase Assemblies of PbS Quantum Dots

Covalent coupling of colloidal PbS quantum dots (QDs) of two different sizes through amide bond formation yielded assemblies that mediate near-infrared excitonic energy transfer from smaller QDs to larger QDs with 90% quantum yield, in chloroform solution. The energy transfer lifetimes, determined by fitting the kinetic data with a model that accounts for multiple hopping steps and different sizes of QD aggregates, were 113 +/- 26 ns and 850 +/- 330 ns, which compete favorably with intrinsic exciton decay in 600 ns to 2.5 mu s. The high yield of energy transfer was accomplished by optimizing the sizes of the donor and acceptor QDs to maximize spectral overlap, the ratio of donor QDs to acceptor QDs, the coverage of "functional" ligand (8-amino-1-octanethiol on the donor QDs and 8-mercapto-1-octanoic acid on the acceptor QDs) on the QD surfaces, the amount of ethyl-dimethylaminopropylcarbodiimide (EDC) and N-hydroxy-succinimide (NHS) coupling reagents, the degree of steric hindrance for the amide coupling reaction, and the lengths of all involved ligands to maximize the solubility of small QD aggregates. Transmission electron microscopy images show coupling of donor and acceptor QDs into well-mixed heteroassemblies (dimers, trimers, and higher oligomers). The quantum yield for energy transfer was determined by comparing the enhancement of the photoluminescence intensity of acceptor PbS QDs with that of PbS QDs within energy transfer-inactive PbS QD-CdS QD "control" assemblies, which underwent the same chemical treatment as the PbS QD PbS QD assemblies but do not have an available energy transfer pathway.