Picosecond Energy Transfer in Quantum Dot Langmuir−Blodgett Nanoassemblies

Abstract

We study spectrally resolved dynamics of Förster energy transfer in single monolayers and bilayers of semiconductor nanocrystal quantum dots assembled using Langmuir-Blodgett (LB) techniques. For a single monolayer, we observe a distribution of transfer times from ~50 ps to ~10 ns, which can be quantitatively modeled assuming that the energy transfer is dominated by interactions of a donor nanocrystal with acceptor nanocrystals from the first three "shells" surrounding the donor. We also detect an effective enhancement of the absorption cross section (up to a factor of 4) for larger nanocrystals on the "red" side of the size distribution, which results from strong, inter-dot electrostatic coupling in the LB film (the light-harvesting antenna effect). By assembling bilayers of nanocrystals of two different sizes, we are able to improve the donor-acceptor spectral overlap for engineered transfer in a specific ("vertical") direction. These bilayers show a fast, unidirectional energy flow with a time constant of ~120 ps.

Influence of quantum dot concentration on Förster resonant energy transfer in monodispersed nanocrystal quantum dot monolayers

“…Therefore, the peak wavelength shift of all three samples is compared over a smaller concentration range be- redshift of the PL emission peak is a signature of energy transfer from the blue, high-energy side of the emission spectrum of the QD ensemble ͑small QDs͒ to the red, low-energy side ͑larger QDs͒. 16,18,20,23 In order to investigate this more closely, time-resolved PL measurements have been carried out on all QD layers. The PL decays for QD layers with a concentration c QD of approximately 2.4ϫ 10 17 m −2 are shown over the first 5 ns in the three panels of Fig.…”

“…15 Numerous reports have been published on energy transfer in QD structures formed using these techniques among others. [16][17][18][19][20][21][22][23] To date there have been only a few reports investigating the influence of the QD concentration on the optical properties and/or energy transfer in monodispersed QD layers, 16 even though they are the fundamental building blocks of many of the systems and devices highlighted above. It is important to consider the QD concentration when comparing the results reported in different publications; its impact on the signal levels, outcome of measurements and performance of devices has to be determined and taken into account.…”

“…18 We have previously reported on Förster resonant energy transfer ͑FRET͒ in mixed QD monolayers consisting of QD donors and acceptors with two different sizes, 44 and FRET in ͑mostly three-dimensional͒ monodispersed QD assemblies has already been observed before. 16,18,20,23 Here, we present a detailed analysis of FRET in monodispersed QD monolayers giving rise to concentration dependent optical properties.…”

Approaches for biological and biomimetic energy conversion

“…Noncovalent self-assembly techniques are being developed for synthetic photoactive devices (48,50,(81)(82)(83). Although some of the materials are self-organized through chemical interactions to produce larger scale arrays, others are organized on surfaces by using techniques such as Langmuir-Blodgett films or synthesized by assembling encapsulated materials into or onto host-guest inorganic matrices.…”

Nonradiative exciton energy transfer in hybrid organic-inorganic heterostructures

“…Following the direct observation of the transfer to the organic overlayer, the extra rate, Theoretically the energy transfer rate scales linearly with the spectral overlap between donor and acceptor [19]. We therefore expect here the ratio of the transfer rate of the two hybrid structures, k A /k B =1.18±0.21, to be equal to the ratio of the corresponding spectral overlaps of the two quantum wells (samples A and B) with the organic dye absorption spectrum.…”