Template-assembly and spectroscopic study of colloidal quantum dot molecules

Block copolymer template-assembly of quantum dots and plasmonic nanostructures is developed to provide a well-controlled platform to study the electronic coupling and Förster resonance energy transfer (FRET) between quantum dots (QDs), as well as the influence of surface plasmons on energy transfer. By fine-tuning the aspect ratio of the geometric features of PS-b-PMMA copolymer template, QDs are assembled into an array of QD clusters within nanoscopic holes on the template using capillary force assembly. Coupled QD clusters, termed quantum dot molecules (QDMs), are assembled in an array to probe the local coupling within QD molecules when the native insulating ligands are exchanged with shorter ligands. From absorption measurement of 1st exciton peak position of PbSe QDMs upon ligand exchange, a larger red-shift is found for QDMs than for a close packed film of PbSe QDs with the same ligand exchange, demonstrating localized electronic coupling of these QD molecules. Template-assembly of nanoparticles is further generalized to uniformly couple QDs clusters with plasmonic nanodisks of noble metals. Using PS nanospheres as reactive ion etch mask, Au nanodisks are fabricated on CdSe/ZnS core shell QD clusters, separated by a tunable space layer of PMMA. This highly controllable surface plasmon-coupled QD system minimizes the uncertainty in interfacial homogeneity, characterized by cross sectional scanning electron microscopy (SEM). Photoluminescence (PL) peak ratio of donor to acceptor emission and donor lifetime measurements show strong evidence of surface plasmon coupled energy transfer between donor-acceptor QDs, which depends on the position of the surface plasmon peaks as well as the separation between plasmonic structure and FRET QD clusters. The result suggests that a larger overlap of surface plasmon peak with the emission peak of acceptor leads to greater decrease in PL lifetime of donor. Donor lifetime decreases dramatically in the presence of both acceptor and surface plasmon compared to just in the presence of surface plasmon. Coupling between plasmonic nanodisks and QD clusters also decreases and results in longer donor lifetime as the thickness of PMMA separation layer increases.