Surface Charges on CdSe-Dot/CdS-Rod Nanocrystals: Measuring and Modeling the Diffusion of Exciton-Fluorescence Rates and Energies

Complete Mapping of Interacting Charging States in Single Coupled Colloidal Quantum Dot Molecules

Colloidal quantum dots (CQDs), major building blocks in modern optoelectronic devices, have so far been synthesized with only one emission center where the exciton resides. Recent development of coupled colloidal quantum dots molecules (CQDM), where two core-shell CQDs are fused to form two emission centers in close proximity, allows exploration of how charge carriers in one CQD affect the charge carriers in the other CQD. Cryogenic single particle spectroscopy reveals that while CQD monomers manifest a simple emission spectrum comprising a main emission peak with well-defined phonon sidebands, CQDMs exhibit a complex spectrum with multiple peaks that are not all spaced according to the known phonon frequencies. Based on complementary emission polarization and time-resolved analysis, this is assigned to fluorescence of the two coupled emission centers. Moreover, the complex peak structure shows correlated spectral diffusion indicative of the coupling between the two emission centers. Utilizing Schrödinger-Poisson self-consistent calculations, we directly map the spectral behavior, alternating between neutral and charged states of the CQDM. Spectral shifts related to electrostatic interaction between a charged emission center and the second emission center are thus fully mapped. Furthermore, effects of moving surface charges are identified, whereby the emission center proximal to the charge shows larger shifts. Instances where the two emission centers are negatively charged simultaneously are also identified. Such detailed mapping of charging states is enabled by the coupling within the CQDM and its anisotropic structure. This understanding of the coupling interactions is progress toward quantum technology and sensing applications based on CQDMs.

Blinking Mechanisms and Intrinsic Quantum-Confined Stark Effect in Single Methylammonium Lead Bromide Perovskite Quantum Dots

Lead halide perovskite quantum dots (QDs) are promising materials for next-generation photoelectric devices because of their low preparation costs and excellent optoelectronic properties. In this study, the blinking mechanisms and the intrinsic quantum-confined Stark effect (IQCSE) in single organic-inorganic hybrid CH₃ NH₃ PbBr₃ perovskite QDs using single-dot photoluminescence (PL) spectroscopy is investigated. The PL quantum yield-recombination rates distribution map allows the identification of different PL blinking mechanisms and their respective contributions to the PL emission behavior. A strong correlation between the excitation power and the blinking mechanisms is reported. Most single QDs exhibit band-edge carrier blinking under a low excitation photon fluence. While under a high excitation photon fluence, different proportions of Auger-blinking emerge in their PL intensity trajectories. In particular, significant IQCSEs in the QDs that exhibit more pronounced Auger-blinking are observed. Based on these findings, an Auger-induced IQCSE model to explain the observed IQCSE phenomena is observed.

Observation of Electron Shakeup in CdSe/CdS Core/Shell Nanoplatelets

While ensembles of CdSe nanoplatelets (NPLs) show remarkably narrow photoluminescence line widths at room temperature, adding a CdS shell to increase their fluorescence efficiency and photostability causes line width broadening. Moreover, ensemble emission spectra of CdSe/CdS core/shell NPLs become strongly asymmetric at cryogenic temperatures. If the origin of these effects were understood, this could potentially lead to stable core/shell NPLs with narrower emission, which would be advantageous for applications. To move in this direction, we report time-resolved emission spectra of individual CdSe/CdS core/shell NPLs at 4 K. We observe surprisingly complex emission spectra that contain multiple spectrally narrow emission features that change during the experiment. With machine-learning algorithms, we can extract characteristic peak energy differences in these spectra. We show that they are consistent with electron "shakeup lines" from negatively charged trions. In this process, an electron-hole pair recombines radiatively but gives part of its energy to the remaining electron by exciting it into a higher single-electron level. This "shakeup" mechanism is enabled in our NPLs due to strong exciton binding and weak lateral confinement of the charge carriers. Time-resolved single-photon-counting measurements and numerical calculations suggest that spectral jumps in the emission features originate from fluctuations in the confinement potential caused by microscopic structural changes on the NPL surface (e.g., due to mobile surface charges). Our results provide valuable insights into line width broadening mechanisms in colloidal NPLs.