Spectrally Resolved Dynamics of Energy Transfer in Quantum-Dot Assemblies: Towards Engineered Energy Flows in Artificial Materials

We report on the dynamics of resonant energy transfer in monodisperse, mixed-size, and energy-gradient (layered) assemblies of CdSe nanocrystal quantum dots. Time-resolved and spectrally resolved photoluminescence directly reveals the energy-dependent transfer rate of excitons from smaller to larger dots via electrostatic coupling. The data show a rapid (0.7–1.9 ns) energy transfer directly across a large tens-of-meV energy gap (i.e., between dots of disparate size), and suggest that interdot energy transfer can approach picosecond time scales in structurally optimized systems.

Figure 1

(a) PL decays from a dense film of monodisperse 12.4 Å $\mathrm{C}\mathrm{d}\mathrm{S}\mathrm{e}/\mathrm{Z}\mathrm{n}\mathrm{S}$ NQDs at the energies specified in the inset. Inset: cw PL spectra from film (solid) and original solution (dashed). (b) Dynamic redshift of the peak emission. Inset: PL spectra at the specified times.

Figure 2

(a) cw PL from films of 17, 18, and 21 Å CdSe NQDs, and from an equal mixture of the three (squares). (b) Corresponding PL lifetimes vs energy.

Figure 3

(a) Calculated decay rate vs energy with no gap (dashed) and 55 meV gap (solid) between donor and acceptor. Points are measured data. (b) “Resonant” ET; interpenetrating TOPO caps give $\sim 11\AA$ dot spacing (not to scale). (c) Typical Förster parameters for resonant NQDs, and for B800-B850 transfer in LH2 complex of purple bacteria. NQD values based on measured radiative lifetime (22 ns), assumption of 20 meV homogeneous linewidths at 300 K, and calculated oscillator strengths from Ref. 10, using $n=1$ and ${\kappa }^2=2/3$.

Figure 4

(a) Schematic of NQD energy-gradient bilayer for light harvesting—13 Å dots on 20.5 Å dots. (b) “Instantaneous” PL spectra at 500 ps intervals (from 0 to 5 ns), showing rapid collapse of emission from 13 Å dots.