A Composite Electrodynamic Mechanism to Reconcile Spatiotemporally Resolved Exciton Transport in Quantum Dot Superlattices

Quantum dot (QD) solids are promising optoelectronic materials; further advancing their device functionality depends on understanding their energy transport mechanisms. The commonly invoked near-field Förster resonance energy transfer (FRET) theory often underestimates the exciton hopping rate in QD solids, yet no consensus exists on the underlying cause [1–3].<br/>We elucidate a mixed near-field FRET and far-field emission/reabsorption mechanism excitonic energy transport by combining time-dependent exciton energy and exciton diffusivity measurements in a heterogeneous QD superlattice (QDSL) [4]. We first quantitatively characterize the heterogeneous energetic landscape of CdSe:Te/CdS QDSL monolayers by extracting inhomogeneous and intrinsic spectral components of QDs by single-particle emission spectroscopy [5]. We next track the time-dependent decay of the mean exciton energy by time-resolved emission spectra (TRES) upon QDSL photoexcitation. The enhanced dynamic redshift occurring in the QDSLs relative to the solution phase QDs immediately following excitation indicates excitons, on average, sample progressively lower-energy QDs as they explore the spatioenergetic landscape. Finally, we measure exciton transport by monitoring the spatiotemporal expansion of an exciton population after a local photoexcitation in QDSL monolayers with time-resolved ultrafast stimulated emission depletion (TRUSTED) microscopy [6]. The result is that the data may be fitted to a simple model of the TRUSTED protocol [6] to determine migration parameters, such as diffusivity.<br/>We use all three types of experimental results to constrain parameters in a kinetic Monte Carlo simulation, and we find that FRET theory is incompatible with the TRUSTED and TRES results simultaneously. This approach is unique because we connect the exciton energy and diffusion dynamics directly, which puts stringent test on the existing FRET theory. We show that only by introducing far-field emission/reabsorption terms that allows hops well beyond the nearest neighbors, can the model reproduce both all three experimental results. Our inclusion of far-field coupling was inspired by Andrews <i>et al.</i> [7]<i>,</i> who originally wrote down how the energy transfer rate between donor and acceptor TDM can most generally be written as a dipole-dipole coupling expansion, , where represents the FRET rate and dominates in the near-field. The far-field term (scales with <i>r</i>^–2) dominates when <i>r</i> is much greater than λ/2π.<br/>The explanation of our own multimodal study also addresses a longstanding paradox in exciton transport within QD solids, i.e., the typically-employed FRET near-field model has repeatedly fallen short of explaining experimental results despite being perpetuated as a standard. We furthermore showed how our model reconciles the original report of this paradox from Tisdale and co-workers [1]. Overall, this work yields a much-needed unified framework in which to characterize transport in QD solids and new principles for device design.<br/><br/><b>References</b><br/>[1] G. M. Akselrod, <i>et al.,</i> <i>Subdiffusive Exciton Transport in Quantum Dot Solids</i>, Nano Lett. <b>14</b>, 3556 (2014).<br/>[2] A. J. Mork,<i> et al</i>., <i>Magnitude of the Förster Radius in Colloidal Quantum Dot Solids</i>, J. Phys. Chem. C <b>118</b>, 13920 (2014).<br/>[3] K. Zheng, <i>et al.</i>, <i>Directed Energy Transfer in Films of CdSe Quantum Dots: Beyond the Point Dipole Approximation</i>, J. Am. Chem. Soc. <b>136</b>, 6259 (2014).<br/>[4] R. Yuan et al., <i>A Composite Electrodynamic Mechanism to Reconcile Spatiotemporally Resolved Exciton Transport in Quantum Dot Superlattices</i>, Sci. Adv. <b>9</b>, eadh2410 (2023).<br/>[5] Z. Zhang, <i>et al</i>., <i>Ultrahigh-Throughput Single-Molecule Spectroscopy and Spectrally Resolved Super-Resolution Microscopy</i>, Nature Methods <b>12</b>, 935 (2015).<br/>[6] S. B. Penwell, <i>et al.,</i> <i>Resolving Ultrafast Exciton Migration in Organic Solids at the Nanoscale</i>, Nat. Mater. <b>16</b>, 1136 (2017).<br/>[7] D. L. Andrews, <i>A Unified Theory of Radiative and Radiationless Molecular Energy Transfer</i>, Chem. Phys. <b>135</b>, 195 (1989).