Surface plasmon enhanced Förster resonant energy transfer (FRET) between CdTe nanocrystal quantum dots (QDs) has been observed in a multilayer acceptor QD-gold nanoparticle-donor QD sandwich structure. Compared to a donor-acceptor QD bilayer structure without gold nanoparticles, the FRET rate is enhanced by a factor of 80 and the Förster radius increases by 103%. Furthermore, a strong impact of the donor QD properties on the surface plasmon mediated FRET is reported.

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The influences of donor and acceptor concentrations on Förster resonant energy transfer (FRET) in a separated donor-acceptor quantum dot bilayer structure have been investigated. Donor intra-ensemble energy transfer is shown to have an impact on the donor-acceptor FRET efficiency in the bilayer structure. At high donor concentrations the FRET distance dependence and the acceptor concentration dependence in the separated donor-acceptor layer structure agree well with theories developed for FRET between randomly distributed, homogeneous donor and acceptor ensembles. However, discrepancies between measurement and theory are found at low donor concentrations. A donor concentration study shows that the FRET efficiency decreases with increasing donor concentration even though a donor concentration-independent FRET efficiency is predicted by standard theory. The observed dependence of the FRET efficiency on the donor concentration can be explained within the FRET rate model, for a constant, donor concentration independent FRET rate, by taking into account the concentration dependent donor reference lifetime arising from intra-donor ensemble FRET. This shows that the decrease in the FRET efficiency with increasing donor concentration is not a signature of a change in the donor-acceptor FRET rate, but due to the competition of the donor-acceptor and donor-donor energy transfer for the higher energy donors. As the intra-donor ensemble FRET represents another decay mechanism, the donor quantum yield for the higher energy donors decreases with increasing donor quantum dot (QD) concentration, as can also be seen from the redshift of the donor emission spectrum. Using this concentration dependent donor quantum yield in the calculation of the Förster radius, the FRET theory for homogeneous donor and acceptor ensembles can be modified to include the effect of the donor intra-ensemble transfer and to correctly describe the trends and absolute values of the measured FRET efficiencies as a function of the donor and the acceptor concentrations. These results show that in QD systems where intra-donor ensemble FRET is as important as the radiative and nonradiative donor decay mechanisms, the FRET rate rather than the FRET efficiency more appropriately characterizes the donor-acceptor FRET. By fitting with the rate model, FRET rates as high as (1.2 ns) −1 have been determined for the structures presented here.

The distance dependence of localized surface plasmon (LSP) coupled Förster resonance energy transfer (FRET) is experimentally and theoretically investigated using a trilayer structure composed of separated monolayers of donor and acceptor quantum dots with an intermediate Au nanoparticle layer. The dependence of the energy transfer efficiency, rate, and characteristic distance, as well as the enhancement of the acceptor emission, on the separations between the three constituent layers is examined. A d –4 dependence of the energy transfer rate is observed for LSP-coupled FRET between the donor and acceptor planes with the increased energy transfer range described by an enhanced Förster radius. The conventional FRET rate also follows a d –4 dependence in this geometry. The conditions under which this distance dependence is valid for LSP-coupled FRET are theoretically investigated. The influence of the placement of the intermediate Au NP is investigated, and it is shown that donor–plasmon coupling has a greater influence on the characteristic energy transfer range in this LSP-coupled FRET system. The LSP-enhanced Förster radius is dependent on the Au nanoparticle concentration. The potential to tune the characteristic energy transfer distance has implications for applications in nanophotonic devices or sensors.

The quantum dot ͑QD͒ concentration dependence of the optical properties of QD monolayers is shown to be dominated by Förster resonant energy transfer ͑FRET͒ from smaller to larger QDs in the ensemble. With increasing QD concentration a redshift of the peak emission wavelength, a shortening of the photoluminescence lifetime of the QDs on the high-energy side of the ensemble emission spectrum as well as increased difference in the lifetimes on the high-and low-energy sides are observed in the layer-by-layer deposited QD monolayers. There is also evidence of an increased rise time in the time-resolved photoluminescence decays on the low-energy side of the QD emission for two of the three samples presented in most detail. A theory of FRET in two dimensions is applied to explain the lifetime decrease on the high-energy side of the ensemble emission and confirms that the impact of the QD concentration on the optical properties is primarily due to FRET from the smaller to larger QDs in the ensemble. The concentration effects are stronger in QD samples which have a broader emission peak compared to the Stokes shift. Based on good agreement with FRET theory, the QD concentration and the overlap of the QD emission and absorption peaks can both be used to control the efficiency of the FRET process in monodispersed QD layers.