Cascading plasmonic and nonradiative energy transfer interactions by plasmon-coupling only donor or only acceptor quantum dots of the energy transfer pairs

摘要

Nonradiative energy transfer mechanism finds important applications in nanophotonics and nanobiology including nanoscale optical waveguiding and biological nanosensors.^1,2 Various fluorophores can take part in such energy transfer interactions and their emission kinetics can thus be strongly modified. For example, colloidal semiconductor quantum dots, also known as nanocrystals (NCs), have widely been shown to serve as donors and acceptors among themselves or with other fluorescent species to transfer excitation energy nonradiatively in close proximity when the spectral conditions are set right. Emission characteristics of such fluorophores can be altered also when coupled with plasmonic structures, e.g., metal nanoparticles (MNPs), in their close proximity via strong plasmon-exciton interactions. One favored result of this plasmonic coupling mechanism is the spontaneous emission enhancement of NCs. Recently plasmon-mediated Förster-type resonance energy transfer (FRET) has been demonstrated and FRET rate has been reported to be increased between acceptor-donor pairs of different species (including NCs) that are both plasmon-coupled.^3–5 In these previous reports, however, either donors and acceptors are blended and these blends are plasmon-coupled, or metal layer is placed between the donor and acceptor thin films to plasmon-couple both at the same time. In all of these prior works, the resulting plasmonexciton interactions are not controlled to take place either at the donor site or the acceptor site but at both of the sites. Therefore it has not been possible to identify the coupled interactions. Here we present the first proposition and demonstration of cascaded plasmonic and nonradiative energy transfer interactions that are controlled by selectively plasmon-coupling either only donor NCs or only acceptor NCs of the energy transfer pairs. This scheme uniquely allows for the ability to spatially control plasmon-exciton intera--ctions to take place either at the “start” site (donors) or “finish” site (acceptors) of the energy transfer. This control is achieved by placing the plasmonic layer in the right proximity of the donors (for strong donor-exciton plasmon-coupling) while sufficiently being far away from the acceptors (for weak acceptor-exciton plasmon-coupling), or vice versa. Here we comparatively study and analyze consequent modifications of NC emission kinetics in response to both cases of plasmon-coupling to only the donor NCs and to only the acceptor NCs through steady-state and time-resolved photoluminescence measurements, along with their lifetime and rate calculations.