Magnitude of the Förster Radius in Colloidal Quantum Dot Solids

Abstract

Excitonic energy transfer among colloidal nanocrystal quantum dots (QDs) is responsible for exciton transport in many QD optoelectronic devices. While Förster theory has successfully accounted for the distance scaling of energy transfer in many QD systems, the overall magnitude of the Förster radius in close-packed QD solids remains an open question. Here, we use spectrally resolved transient photoluminescence quenching to measure the magnitude of the Förster radius in blended donor–acceptor QD assemblies. For blends of CdSe/CdZnS core/shell QDs consisting of 4.0 nm diameter donors (λmax,em ≈ 550 nm) and 5.5 nm acceptors (λmax,abs ≈ 590 nm), we measure energy transfer rates per donor–acceptor pair that are 10–100 times faster than the predictions of Förster theory. These rates correspond to an effective Förster radius of 8–9 nm, compared to a theoretical Förster radius of 5–6 nm. We discuss possible sources for the discrepancy between theory and experimentincluding the magnitude of the absorption cross section, dipole orientation, and dipole–multipole couplingand suggest that several common assumptions should be considered carefully before applying Förster theory to solid QD films.

Mapping the Exciton Diffusion in Semiconductor Nanocrystal Solids

“…[41][42][43][44][45][46] Studying the motion of neutral excitons, however, is a far more challenging task. To date, the exciton diffusion in NC solids has been investigated via steady-state and time-resolved spectroscopy methods 36,44,[47][48][49][50][51][52] while their 4 diffraction limited spatial profile has been visualized through optical microcopy. 35 Despite these efforts, the exact nature of the exciton dissociation mechanism as well as exciton diffusion trajectories in nanocrystal solids still remain poorly understood.…”

Recombination dynamics in arrays of II-VI epitaxial quantum dots with Förster resonance energy transfer

“…For FRET, K ET is close to unity when r 0.5R 0 and decreases to zero when r 1.5R 0 [26]. The 18-nm barrier is much wider than R 0 = 4 ÷ 10 nm reported previously for II-VI nanocrystals [28,38,49], and FRET should be virtually suppressed at such a distance. We use two values (400 and 550 ps) of τ 0 D to show the influence of its variation.…”

Optical techniques for probing the excited state dynamics of quantum dot solids

“…Notably, the observed ET rate was about an order of magnitude greater than the one reported by Kagan et al 38 Recently, Tisdale group has reported the distance dependence of the energy transfer rate between the donor-and acceptor-type nanocrystals. 50 To this end, QD films were fabricated by depositing homogeneous blends of small and large diameter CdSe/CdZnS core/shell NCs, using either long-chain (octadecylphosphonic acid) or short-chain (benzylphosphonic acid) crosslinking ligands. Based on the observed modulation of the acceptor emission, it was concluded that the non-radiative energy transfer between donor and acceptor QDs deviates from the Förster type, 1/R 6 , scaling.…”

“…However, with recent advances in film fabrication techniques and surface passivation strategies, the use of PL-based techniques to studying the excited state dynamics in QD solids has witnessed a revival, with an ever-increasing number of publications appearing in recent years. [40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57] These days, the complexity of PL-based methods for probing the excitation dynamics of artificial solids continues to evolve. By offering both temporal and spatial profile of the energy flow in artificial solids, these techniques become increasingly valuable in gauging important charge transport and exciton diffusion characteristics.…”