Förster resonant energy transfer in quantum dot layers

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Abstract

Förster resonant energy transfer (FRET) in quantum dot (QD) layer structures has been analyzed. Small and large colloidal CdTe QDs were used as donors and acceptors, respectively. A FRET theory for random donor/acceptor distributions in two dimensions, taking into account exclusion zones around the donors, was applied to characterize FRET in a mixed monolayer. The exclusion zones provide a possibility to include the QD size in the FRET analysis and to determine its impact on the FRET efficiency. The acceptor concentration dependence of the FRET efficiency can also be described within this theory. In a separate donor/acceptor layer structure the distance dependence of the FRET efficiency as well as the acceptor enhancement was investigated. Both were found to agree well with the model of FRET between donor and acceptor layers.

Introduction

Förster resonant energy transfer (FRET) is a nanoscale mechanism by which energy is transferred from donors to acceptors via dipole–dipole interactions [1]. FRET can be engineered in artificial structures to create energy funneling structures [2] or nanosensors [3]. Due to their unique optical properties, semiconductor quantum dots (QDs) represent valuable building blocks for these structures. Compared to the commonly used molecular dyes, QDs are especially interesting because of their tunable emission wavelength and high photostability. However, most FRET theories are mainly focusing on small molecules, which can be treated as identical point dipoles. The size of the QDs is of the order of a few nanometers and lies therefore in the range of the characteristic distance over which FRET can take place. This is also the case for polymers, that are better modeled within a line dipole approach rather than as point dipoles [4]. Additionally, the QDs in one ensemble are not identical but show an inhomogeneous broadening due to their size distribution. These two main differences are not taken into account by molecular theories and thus it is not clear whether these theories can be easily applied to QD FRET structures [5]. Therefore, it is important to investigate FRET between QD donors and acceptors in order to understand how these two properties can influence the energy transfer process and how the performance of QD FRET structures can be optimized.

We report on FRET in two different QD structures, a mixed QD monolayer and a separate donor/acceptor layer structure. A theory of FRET in two dimensions is applied to characterize FRET in the mixed layer structure and highlights the influence of the QD size, which can be included in this theory in the form of an exclusion zone radius [6]. The theory not only allows for the extraction of all the important FRET parameters, such as the Förster radius, the FRET efficiency and rate, from time-resolved donor photoluminescence measurements alone, but it also explains the dependence of the FRET efficiency on the acceptor concentration [7]. In the separate donor/acceptor layer structure the distance dependence of the FRET process is studied [8].

Section snippets

Experimental methods

Negatively charged colloidal CdTe quantum dots, stabilized by thioglycolic acid (TGA), were used to prepare mixed donor/acceptor monolayers and separate donor/acceptor layer structures. The QDs acting as energy donors had a diameter of 1.9 nm and emitted at 524 nm. Larger QDs, with a diameter of 3.5 nm and emitting at 617 nm, were used as acceptors.

The FRET structures were deposited on quartz substrates by a layer-by-layer deposition technique based on the electrostatic assembly of charged

Results and discussion

After presentation of the properties of pure QD monolayers, the FRET process and its acceptor concentration dependence in the mixed QD monolayer will be analyzed. A discussion of the distance dependence of FRET in the separate donor/acceptor layer structure will follow.

In Fig. 1a the photoluminescence (PL) as well as the absorption spectra of pure donor and acceptor monolayers are shown. The characteristic peaks of the acceptor spectra are red-shifted, due to the larger size of the acceptor

Conclusion

FRET between QDs in a mixed monolayer and a separate donor/acceptor layer structure has been investigated. For the analysis of FRET in the mixed monolayer a theory of FRET in two dimensions that takes into consideration the effect of the QD size on the FRET process in the form of exclusion zones was applied. The extracted values of the exclusion zone radius and the Förster radius were found to be in good agreement with those determined by the more common spectral analysis. The acceptor

Acknowledgements

We thank Robert Gunning for the thickness measurements of the polyelectrolyte layers using X-ray diffraction. This work was financially supported by Science Foundation Ireland 05/PICA/1797.

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