  The distance-dependent interaction of an emitter with a plasmonic
nanoparticle or surface forms the basis of the field of plexitonics.
Semiconductor quantum dots (QDs) are robust emitters due to their
photostability, and offer the possibility of understanding the fundamental
photophysics between one emitter and one metal nanoparticle. A key enabling
challenge is the formation of systems containing both QDs and plasmonic
nanoparticles in high purity. We present the translation of DNA-based
self-assembly techniques to assemble metal and semiconductor nanocrystals into
discrete hybrid structures, including dimers, of high purity. This method gives
control over the interparticle separation, geometry, and ratio of QD:metal
nanoparticle, as well as the spectral properties of the metal/QD components in
the assembly to allow investigation of plasmon-exciton interaction. The hybrid
assemblies show the expected enhancement in steady-state photoluminescence
accompanied by an increase in the QD emission rate for assemblies with a strong
overlap between the QD emission and localised surface plasmon resonance. In
contrast, lengthening of the QD emission lifetime (a reduction of the emission
rate) of up to 1.7-fold, along with an enhancement in steady-state PL of 15-75%
is observed upon detuning of the QD and metal nanoparticle spectral properties.
This understood in terms of the Purcell effect, where the gold nanoparticle
acts as a damped, nanoscale cavity. Considering the metal nanoparticle using
generalised nonlocal optical response theory (GNOR) and the QD as an open
quantum system, the response is driven by the interference experienced by the
emitter for parallel and perpendicular field orientations. This provides a
mechanism for control of the emission rate of a QD by a metal nanoparticle
across a much wider range of lifetimes than previously understood.
