Relations of exciton dynamics in quantum dots to photoluminescence, lasing, and energy harvesting

Waveguide-Integrated Colloidal Nanocrystal Supraparticle Lasers

Supraparticle (SP) microlasers fabricated by the self-assembly of colloidal nanocrystals have great potential as coherent optical sources for integrated photonics. However, their deterministic placement for integration with other photonic elements remains an unsolved challenge. In this work, we demonstrate the manipulation and printing of individual SP microlasers, laying the foundation for their use in more complex photonic integrated circuits. We fabricate CdSxSe1-x/ZnS colloidal quantum dot (CQD) SPs with diameters from 4 to 20 μm and Q-factors of approximately 300 via an oil-in-water self-assembly process. Under a subnanosecond-pulse optical excitation at 532 nm, the laser threshold is reached at an average number of excitons per CQD of 2.6, with modes oscillating between 625 and 655 nm. Microtransfer printing is used to pick up individual CQD SPs from an initial substrate and move them to a different one without affecting their capability for lasing. As a proof of concept, a CQD SP is printed on the side of an SU-8 waveguide, and its modes are successfully coupled to the waveguide.

Nanomaterials for Fluorescence and Multimodal Bioimaging

Bioconjugated nanomaterials replace molecular probes in bioanalysis and bioimaging in vitro and in vivo. Nanoparticles of silica, metals, semiconductors, polymers, and supramolecular systems, conjugated with contrast agents and drugs for image-guided (MRI, fluorescence, PET, Raman, SPECT, photodynamic, photothermal, and photoacoustic) therapy infiltrate into preclinical and clinical settings. Small bioactive molecules like peptides, proteins, or DNA conjugated to the surfaces of drugs or probes help us to interface them with cells and tissues. Nevertheless, the toxicity and pharmacokinetics of nanodrugs, nanoprobes, and their components become the clinical barriers, underscoring the significance of developing biocompatible next-generation drugs and contrast agents. This account provides state-of-the-art advancements in the preparation and biological applications of bioconjugated nanomaterials and their molecular, cell, and in vivo applications. It focuses on the preparation, bioimaging, and bioanalytical applications of monomodal and multimodal nanoprobes composed of quantum dots, quantum clusters, iron oxide nanoparticles, and a few rare earth metal ion complexes.

Effectual Interface and Defect Engineering for Auger Recombination Suppression in Bright InP/ZnSeS/ZnS Quantum Dots

The main issue in developing a quantum dot light-emitting diode (QLED) display lies in successfully replacing heavy metals with environmentally benign materials while maintaining high-quality device performance. Nonradiative Auger recombination is one of the major limiting factors of QLED performance and should ideally be suppressed. This study scrutinizes the effects of the shell structure and composition on photoluminescence (PL) properties of InP/ZnSeS/ZnS quantum dots (QDs) through ensemble and single-dot spectroscopic analyses. Employing gradient shells is discovered to suppress Auger recombination to a high degree, allowing charged QDs to be luminescent comparatively with neutral QDs. The "lifetime blinking" phenomenon is observed as evidence of suppressed Auger recombination. Furthermore, single-QD measurements reveal that gradient shells in QDs reduce spectral diffusion and elevate the energy barrier for charge trapping. Shell composition dependency in the gradience effect is observed. An increase in the ZnS composition (ZnS >50%) in the gradient shell introduces lattice mismatch between the core and the shell and therefore rather reverses the effect and reduces the QD performance.

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.

Photoinduced photoluminescence enhancement in self-assembled clusters of formamidinium lead bromide perovskite nanocrystals

Nanocrystals of formamidinium lead bromide perovskite (FAPbBr3) self-assemble into clusters in powder and film samples and provide a prolonged photoluminescence lifetime, which is attributed to the diffusion of charge carriers through interparticle states formed among nanocrystals. Interestingly, the photoluminescence lifetime decreases and the emission intensity increases for the clusters, which is with the increase in the intensity of excitation light. By doping the nanocrystal clusters with C60, we successfully harvested the photogenerated charge carriers. Nonetheless, at high intensities of excitation, the rate of radiative recombination becomes comparable to that of the electron transfer to C60. Thus, the optimum rate of electron transfer to C60 is accomplished by minimally exciting the self-assembled nanocrystals.

Chemical Synthesis and Characterization of Poly(poly(ethylene glycol) methacrylate)-Grafted CdTe Nanocrystals via RAFT Polymerization for Covalent Immobilization of Adenosine

This paper describes the functionalization of poly(poly(ethylene glycol) methacrylate) (PPEGMA)-grafted CdTe (PPEGMA-g-CdTe) quantum dots (QDs) via surface-initiated reversible addition–fragmentation chain transfer (SI-RAFT) polymerization for immobilization of adenosine. Initially, the hydroxyl-coated CdTe QDs, synthesized using 2-mercaptoethanol (ME) as a capping agent, were coupled with a RAFT agent, S-benzyl S′-trimethoxysilylpropyltrithiocarbonate (BTPT), through a condensation reaction. Then, 2,2′-azobisisobutyronitrile (AIBN) was used to successfully initiate in situ RAFT polymerization to generate PPEGMA-g-CdTe nanocomposites. Adenosine-above-PPEGMA-grafted CdTe (Ado-i-PPEGMA-g-CdTe) hybrids were formed by the polymer shell, which had successfully undergone bioconjugation and postfunctionalization by adenosine (as a nucleoside). Fourier transform infrared (FT-IR) spectrophotometry, energy-dispersive X-ray (EDX) spectroscopy, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy results indicated that a robust covalent bond was created between the organic PPEGMA part, cadmium telluride (CdTe) QDs, and the adenosine conjugate. The optical properties of the PPEGMA-g-CdTe and Ado-i-PPEGMA-g-CdTe hybrids were investigated by photoluminescence (PL) spectroscopy, and the results suggest that they have a great potential for application as optimal materials in biomedicine.

Blinking Suppression in Highly Excited CdSe/ZnS Quantum Dots by Electron Transfer under Large Positive Gibbs (Free) Energy Change

Semiconductor quantum dots with stable photoluminescence are necessary for next generation optoelectronic and photovoltaic devices. Photoluminescence intensity fluctuations of cadmium and lead chalcogenide quantum dots have been extensively investigated since the first observation of blinking in CdSe nanocrystals in 1996. In a quantum dot, blinking originates from stochastic photocharging, nonradiative Auger recombination, and delayed neutralization. So far, blinking is suppressed by defect passivation, electron transfer, and shell preparation, but without any deep insight into free energy change of electron transfer. We report real-time detection of significant blinking suppression for CdSe/ZnS quantum dots exposed to N, N-dimethylaniline, which is accompanied by a considerable increase in the time-averaged photoluminescence intensity of quantum dots. Although the Gibbs (free) energy change (Δ G_et = +2.24 eV), which is estimated electrochemically and from density functional theory calculations, is unfavorable for electron transfer from N, N-dimethylaniline to a quantum dot in the minimally excited (band-edge) state, electron transfer is obvious when a quantum dot is highly excited. Nonetheless, Δ G_et crosses from the positive to negative scale as the solvent dielectric constant exceeds 5, favoring electron transfer from N, N-dimethylaniline to a quantum dot excited to the band-edge state. Based on single-molecule photoluminescence and ensemble electron transfer studies, we assign blinking suppression to the transfer of an electron from N, N-dimethylaniline to the hot hole state of a quantum dot. In addition to blinking suppression by electron transfer, complete removal of blinking is limited by short-living OFF states induced by the negative trion.