Concentration dependence of Förster resonant energy transfer between donor and acceptor nanocrystal quantum dot layers: Effect of donor-donor interactions

Abstract

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.

Experimental and Theoretical Investigation of the Distance Dependence of Localized Surface Plasmon Coupled Förster Resonance Energy Transfer

“…The measured quantum yield of the donor QD monolayer is 3%. 15,35 A correction term has been added to the dielectric permittivity of Au NPs to account for finite-size effects.…”

“…PE spacer layers have been previously used to investigate the distance dependence of FRET between QDs and QD-plasmon coupling effects. [33][34][35][36][37] The negatively charged CdTe QDs, stabilized by thioglycolic acid, were synthesized in water according to standard procedures. 38,39 The sizes of the QDs and the QD concentrations in the multilayer structures were determined from the absorption spectra.…”

“…The theoretical dependencies were calculated using the measured QY of the QDs in monolayers. 35 The colloidal Au NPs were stabilized by 4-dimethylaminopyridine (DMAP) and had an average diameter of 5.5 nm. 40 per pixel were used.…”

Increase in exciton decay rate due to plane-to-plane interaction between cyanine thin films

“…30,33 Moreover, in bilayer structures, the rate because of the plane-to-plane interaction is known to be proportional to R −4 . 32,35 Therefore, the energy transfer is not the major factor for the fast response.…”

“…The resonant energy transfer was focused on as a method for increasing the decay rate, [30][31][32][33][34][35][36] in particular, on the cyanine molecule thin films. 37,38 The energy transfer causes ultrafast relaxation of the lowest excited states with a decrease in the distance between the energy donor and the acceptor molecules, which enables high repetition operation without the pattern effect.…”