Colloidal quantum dots (QDs) are promising materials for use in solar cells, light-emitting diodes, lasers, and photodetectors, but the mechanism and length of exciton transport in QD materials is not well understood. We use time-resolved optical microscopy to spatially visualize exciton transport in CdSe/ZnCdS core/shell QD assemblies. We find that the exciton diffusion length, which exceeds 30 nm in some cases, can be tuned by adjusting the inorganic shell thickness and organic ligand length, offering a powerful strategy for controlling exciton movement. Moreover, we show experimentally and through kinetic Monte Carlo simulations that exciton diffusion in QD solids does not occur by a random-walk process; instead, energetic disorder within the inhomogeneously broadened ensemble causes the exciton diffusivity to decrease over time. These findings reveal new insights into exciton dynamics in disordered systems and demonstrate the flexibility of QD materials for photonic and optoelectronic applications.

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.

A thin‐film transistor: An n‐type polymer semiconductor, poly(2,3‐bis(perfluorohexyl)thieno[3,4‐b]pyrazine), was synthesized through a Pd‐catalyzed polycondensation employing a perfluorinated multiphase solvent system. This is the first example of an n‐type polymer semiconductor with exclusive solubility in fluorinated solvents. The fabrication of organic field effect transistors containing this new n‐type polymer semiconductor is shown (see picture).

Recent experimental and theoretical results have highlighted the surprisingly dominant role of acoustic phonons in regulating dynamic processes in nanocrystals. While it has been known for many years that acoustic phonon frequencies in nanocrystals depend on their size, strategies for tuning acoustic phonon energy at a given fixed size were not available. Here, we show that acoustic phonon frequencies in colloidal quantum dots (QDs) can be tuned through choice of the surface ligand. Using low-frequency Raman spectroscopy, we explore the dependence of the ℓ = 0 acoustic phonon resonance in CdSe QDs on ligand size, molecular weight, and chemical functionality. Based on these aggregated observations, we conclude that the primary mechanism for this effect is mass loading of the QD surface, and that interactions between ligands and with the surrounding environment play a comparatively minor yet non-negligible role.

The measured low frequency vibrational energies of some quantum dots (QDs) deviate from the predictions of traditional elastic continuum models. Recent experiments have revealed that these deviations can be tuned by changing the ligands that passivate the QD surface. This observation has led to speculation that these deviations are due to a mass-loading effect of the surface ligands. In this article, we address this speculation by formulating a continuum elastic theory that includes the dynamical loading by elastic surface ligands. We demonstrate that this model is capable of accurately reproducing the l = 0 phonon energy across a variety of different QD samples, including cores with different ligand identities and epitaxially grown CdSe/CdS core/shell heterostructures. We highlight that our model performs well even in the small QD regime, where traditional elastic continuum models are especially prone to failure. Furthermore, we show that our model combined with Raman measurements can be used to infer the elastic properties of surface bound ligands, such as sound velocities and elastic moduli, that are otherwise challenging to measure.