Blinking suppression in CdSe/ZnS single quantum dots by TiO2 nanoparticles

Photoinduced interfacial electron transfer from perovskite quantum dots to molecular acceptors for solar cells

Bandgap-engineered inorganic and hybrid halide perovskite (HP) films, nanocrystals, and quantum dots (PQDs) are promising for solar cells. Fluctuations of photoinduced electron transfer (PET) rates affect the interfacial charge separation efficiencies of such solar cells. Electron donor- or acceptor-doped perovskite samples help analyze PET and harvest photogenerated charge carriers efficiently. Therefore, PET in perovskite-based donor-acceptor (D-A) systems has received considerable attention. We analyzed the fluctuations of interfacial PET from MAPbBr₃ or CsPbBr₃ PQDs to classical electron acceptors such as 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 1,2,4,5-tetracyanobenzene (TCNB) at single-particle and ensemble levels. The significantly negative Gibbs free energy changes of PET estimated from the donor-acceptor redox potentials, the donor-acceptor sizes, and the solvent dielectric properties help us clarify the PET in the above D-A systems. The dynamic nature of PET is apparent from the decrease in photoluminescence (PL) lifetimes and PL photocounts of PQDs with an increase in the acceptor concentrations. Also, the acceptor radical anion spectrum helps us characterize the charge-separated states. Furthermore, the PL blinking time and PET rate fluctuations (10⁸ to 10⁷ s^-1) provide us with single-molecule level information about interfacial PET in perovskites.

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.

Single Photon Source from a Nanoantenna-Trapped Single Quantum Dot

Single photon sources with high brightness and subnanosecond lifetimes are key components for quantum technologies. Optical nanoantennas can enhance the emission properties of single quantum emitters, but this approach requires accurate nanoscale positioning of the source at the plasmonic hotspot. Here, we use plasmonic nanoantennas to simultaneously trap single colloidal quantum dots and enhance their photoluminescence. The nano-optical trapping automatically locates the quantum emitter at the nanoantenna hotspot without further processing. Our dedicated nanoantenna design achieves a high trap stiffness of 0.6 (fN/nm)/mW for quantum dot trapping, together with a relatively low trapping power of 2 mW/μm². The emission from the nanoantenna-trapped single quantum dot shows 7× increased brightness, 50× reduced blinking, 2× shortened lifetime, and a clear antibunching below 0.5 demonstrating true single photon emission. Combining nano-optical tweezers with plasmonic enhancement is a promising route for quantum technologies and spectroscopy of single nano-objects.

Influence of Shape Anisotropy on the Emission of Low-Dimensional Semiconductors

The emergence of precise and scalable synthetic methods for producing anisotropic semiconductor nanostructures provides opportunities to tune the photophysical properties of these particles beyond their band gaps, and to incorporate them into higher-order structures with macroscopic anisotropic responses to electric and optical fields. This perspective article discusses some of these opportunities in the context of colloidal semiconductor nanoplatelets, with a focus on the influence of confinement anisotropy on processes that dictate the emission.

Real-Time Blinking Suppression of Perovskite Quantum Dots by Halide Vacancy Filling

Despite the excellent optoelectronic properties of halide perovskites, the ionic and electronic defects adversely affect the stability and durability of perovskites and their devices. These defects, intrinsic or produced by environmental factors such as oxygen, moisture, or light, not only cause chemical reactions that disintegrate the structure and properties of perovskites but also induce undesired photoluminescence blinking to perovskite quantum dots and nanocrystals. Blinking is also caused by the nonradiative Auger processes in the photocharged quantum dots or nanocrystals. Herein, we find real-time suppression of halide vacancy-assisted nonradiative exciton recombination and photoluminescence blinking in MAPbBr₃ and MAPbI₃ perovskite quantum dots by filling the vacancies using halide precursors (MABr and MAI). Also, halide vacancy filling increases the photoluminescence quantum efficiencies and lifetimes of the quantum dots. We estimate the rates of halide vacancy-assisted nonradiative recombination at 1 × 10⁸ s^-1 for MAPbBr₃ and 1.9 × 10⁹ s^-1 for MAPbI₃ quantum dots. The real-time blinking suppression using the halide precursors and statistical analysis of the ON/OFF blinking time reveal that the halide vacancies contribute to the type-A blinking through charging and discharging. Conversely, the blinking of the quantum dots after halide vacancy filling is dominated by the type-B mechanism.

Biexciton Dynamics in Single Colloidal CdSe Quantum Dots

The investigation of biexciton dynamics in single colloidal quantum dots (QDs) is critical to biexciton-based applications. Generally, a biexciton exhibits an extremely low photoluminescence (PL) quantum yield as well as very fast PL decay due to strong nonradiative Auger recombination, making it difficult to investigate the biexciton dynamics. Here, we develop a quantitative method based on intensity- and time-resolved photon statistics to investigate the biexciton dynamics in single colloidal QDs. This robust method can be used under high-excitation conditions to determine the absolute radiative and Auger recombination rates of both neutral and charged biexciton states in a single QD level, and the corresponding ratios between the two states agree with the theoretical predictions of the asymmetric band structures of CdSe-based QDs. Furthermore, the surface traps are found to provide additional nonradiative recombination pathways for the biexcitons, and their contributions are quantified by the method.

Hot electron transfer in Zn-Ag-In-Te nanocrystal-methyl viologen complexes enhanced with higher-energy photon excitation

The dynamics of hot electron transfer from Zn-Ag-In-Te (ZAITe) nanocrystals (NCs) to adsorbed methyl viologen (MV2+) were investigated by transient absorption spectroscopy. The bleaching of the exciton peak in the ZAITe NC-MV2+ complexes evolved faster than that of ZAITe NCs. The hot electron transfer efficiency increased from 45% to 72% with increasing excitation photon energy.

Gold nanoparticle on semiconductor quantum dot: Do surface ligands influence Fermi level equilibration

Semiconductor-metal heterojunction nanostructures possess an ability to store electrons upon photoexcitation through Fermi level equilibration. The unique role of capping ligands in modulating the equilibration of Fermi level in CdSe-Au heteronanostructures is explored by taking alkyl thiols and alkyl amines as examples. Alkyl thiol having its highest occupied molecular orbital (HOMO) above the valence band of the heterojunction nanostructure inhibits the exciton recombination by scavenging the photogenerated hole. This leads to the elevation in the Fermi level of Au and equilibration with the conduction band of CdSe. The Fermi level equilibrated electrons are further transferred to an acceptor molecule such as methyl viologen, demonstrating the potential of heterojunction nanostructures capped with hole accepting ligands for charge transport application in photovoltaics. In contrast, alkyl amine being a non-hole acceptor ligand with its HOMO placed below its valence band promotes rapid Au mediated exciton recombination, limiting its usefulness in charge transport application. Thus, the energetics of ligands on heterojunction nanostructures plays a decisive role in Fermi level equilibration.

Insights on heterogeneity in blinking mechanisms and non-ergodicity using sub-ensemble statistical analysis of single quantum-dots

Photo-luminescence (P-L) intermittency (or blinking) in semiconductor nanocrystals (NCs), a phenomenon ubiquitous to single-emitters, is generally considered to be temporally random intensity fluctuations between "bright" ("On") and "dark" ("Off") states. However, individual quantum-dots (QDs) rarely exhibit such telegraphic signals, and yet, a vast majority of single-NC blinking data are analyzed using a single fixed threshold which generates binary trajectories. Furthermore, while blinking dynamics can vary dramatically over NCs in the ensemble, the extent of diversity in the exponents (m_On/Off) of single-particle On-/Off-time distributions (P(t_On/Off)), often used to validate mechanistic models of blinking, remains unclear due to a lack of statistically relevant data sets. Here, we subclassify an ensemble of QDs based on the emissivity of each emitter and subsequently compare the (sub)ensembles' behaviors. To achieve this, we analyzed a large number (>1000) of blinking trajectories for a model system, Mn⁺² doped ZnCdS QDs, which exhibits diverse blinking dynamics. An intensity histogram dependent thresholding method allowed us to construct distributions of relevant blinking parameters (such as m_On/Off). Interestingly, we find that single QD P(t_On/Off)s follow either truncated power law or power law, and their relative proportion varies over subpopulations. Our results reveal a remarkable variation in m_On/Off amongst as well as within subensembles, which implies multiple blinking mechanisms being operational amongst various QDs. We further show that the m_On/Off obtained via cumulative single-particle P(t_On/Off) is distinct from the weighted mean value of all single-particle m_On/Off, evidence for the lack of ergodicity. Thus, investigation and analyses of a large number of QDs, albeit for a limited time span of a few decades, are crucial to characterize the spatial heterogeneity in possible blinking mechanisms.

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.

Real-time three-dimensional tracking of single synaptic vesicles reveals that synaptic vesicles undergoing kiss-and-run fusion remain close to their original fusion site before reuse

The release of neurotransmitters via the fusion between synaptic vesicles and the presynaptic membrane is an essential step in synaptic transmission. Synaptic vesicles generally undergo two distinct modes of exocytosis called full-collapse fusion and kiss-and-run fusion. In kiss-and-run fusion, the fusion pore of the synaptic vesicle opens transiently without the vesicle collapsing fully into the plasma membrane; thus, each synaptic vesicle can be used multiple times to release neurotransmitters. Despite considerable research, the detailed mechanisms that underlie kiss-and-run fusion remain elusive, particularly the location of synaptic vesicles after kiss-and-run events. To address this question, we performed real-time three-dimensional tracking of single synaptic vesicles labeled with a single quantum dot in the presynaptic terminal of cultured hippocampal neurons and analyzed the three-dimensional trajectories of these vesicles undergoing kiss-and-run fusion. We found that the majority of these synaptic vesicles underwent another exocytosis event within 120 nm of their original fusion site and underwent a second exocytosis event within 10 s of the first fusion event. These results indicate that after kiss-and-run fusion, synaptic vesicles remain relatively close to their original fusion site and can release repeatedly at brief intervals, allowing neurons to maintain neurotransmitter release during bursting activity.

On the degradation mechanisms of quantum-dot light-emitting diodes

The operating lifetime of blue quantum-dot light-emitting diodes (QLED) is currently a short slab for this emerging display technology. To pinpoint the origin of device degradation, here we apply multiple techniques to monitor the electric-field distribution and space-charge accumulation across the multilayered structure before and after lifetime tests. Evident by charge-modulated electro-absorption and capacitance-voltage characteristics, the excited electrons in blue quantum dots (QD) are prone to cross the type II junction between the QD emission layer and the electron-transporting layer (ETL) due to the offset of conduction band minimum, leading to space-charge accumulation and operating-voltage rise in the ETL. Therefore, unlike those very stable red devices, of which the lifetime is primarily limited by the slow degradation of hole-transporting layer, the poor lifetime of blue QLED originates from the fast degradation at the QD-ETL junction. Materials engineering for efficient electron injection is prerequisite for the boost of operating lifetime.

Wide application of quantum dot light emitting diodes (QLED) in display technology is hindered by the poor lifetime of the blue QLEDs. Here, the degradation mechanism is shown to originate from space charge accumulation in the electron-transporting layer enabling improvements in blue QLED lifetimes.

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.

Suppressing the photobleaching and photoluminescence intermittency of single near-infrared CdSeTe/ZnS quantum dots with p-phenylenediamine

Intrinsic photobleaching and photoluminescence (PL) intermittency of single quantum dots (QDs), originating from photo-oxidation and photo-ionization respectively, are roadblocks for most single-dot applications. Here, we effectively suppress the photobleaching and the PL intermittency of single near-infrared emitting QDs with p-phenylenediamine (PPD). The PPD cannot only be used as a high-efficient reducing agent to remove reactive oxygen species around QDs to suppress the photo-oxidation, but can also bond with the surface defect sites of single QDs to reduce electron trap states to suppress the photo-ionization. It is shown that the survival time of single QDs, the on-state probability of PL intensity traces, and the total number of emitted photons are significantly increased for single QDs in PPD compared with that on glass coverslip.

Suppressing the Fluorescence Blinking of Single Quantum Dots Encased in N-type Semiconductor Nanoparticles

N-type semiconductor indium tin oxide (ITO) nanoparticles are used to effectively suppress the fluorescence blinking of single near-infrared-emitting CdSeTe/ZnS core/shell quantum dots (QDs), where the ITO could block the electron transfer from excited QDs to trap states and facilitate more rapid regeneration of neutral QDs by back electron transfer. The average blinking rate of QDs is significantly reduced by more than an order of magnitude and the largest proportion of on-state is 98%, while the lifetime is not considerably reduced. Furthermore, an external electron transfer model is proposed to analyze the possible effect of radiative, nonradiative, and electron transfer pathways on fluorescence blinking. Theoretical analysis based on the model combined with measured results gives a quantitative insight into the blinking mechanism.

Simultaneous Spectroscopic and Topographic Imaging of Single-Molecule Interfacial Electron-Transfer Reactivity and Local Nanoscale Environment

The fundamental information related to the energy flow between molecules and substrate surfaces as a function of surface site geometry and molecular structure is critical for understanding interfacial electron-transfer (ET) dynamics. The inhomogeneous nanoscale molecule-surface and molecule-molecule interactions are presumably the origins of the complexity in interfacial ET dynamics; thus, identifying the environment of molecules at nanoscale is crucial. We have developed atomic force microscopy (AFM) correlated single-molecule fluorescence intensity/lifetime imaging microscopy (AFM-SMFLIM) capable of identifying and characterizing individual molecules distributed across the heterogeneous surface at the nanometer length scale. Using the novel AFM-SMFLIM imaging, we are able to obtain nanoscale morphology and interfacial ET dynamics at a single-molecule level. Moreover, the observed blinking behavior and lifetime of each molecule in combination with the topography of the environment at nanoscale provide the location of each molecule on the surface (TiO2 vs cover glass) at nanoscale and the coupling strength of each molecule with TiO2 nanoparticles.

Microscopic evidence of "necklace and bead"-like morphology of polymer-surfactant complexes: a comparative study on poly(vinylpyrrolidone)-sodium dodecyl sulfate and poly(diallyldimethylammonium chloride)-sodium dodecyl sulfate systems

Here, we report the microscopic evidence of "necklace and bead"-like morphology, which has long been the most widely accepted model for polymer-surfactant complexes. The lack of microscopic evidence of the initial complexation between surfactant and polymer has resulted in many contradictory reports in the literature. In this paper, we visualized these initial complexes formed between negatively charged surfactant sodium dodecyl sulfate (SDS) with neutral poly(vinylpyrrolidone) (PVP) and cationic poly(diallyldimethylammonium chloride) (PDADMAC) polymer through photoluminescence (PL) microscopy and atomic force microscopy (AFM) using silicon quantum dot (Si QD) as an external PL marker. It is observed that, for the PVP-SDS system, SDS molecules bind at the hydrophobic sites on the random-coiled PVP chain through their hydrocarbon tails, while for the PDADMAC-SDS system, SDS head groups are associated with the positively charged nitrogen centers of the polymer, where the polymer chain wraps around the surfactant head groups.

Probing spatially dependent photoinduced charge transfer dynamics to TiO2 nanoparticles using single quantum dot modified atomic force microscopy tips

Using single CdSe/CdS quantum dot (QD) functionalized atomic force microscopy (AFM) tips, we demonstrate that the spatial dependence of photoinduced electron transfer dynamics from the single QD to TiO2 nanoparticles can be controlled and probed with high spatial (subdiffraction-limited) and temporal (limited by fluorescence microscopy) resolutions. This finding suggests the feasibility of using electron donor or acceptor modified AFM tips for simultaneous high resolution imaging of morphology and photoinduced charge transfer dynamics in nanomaterials.

Impairments of cells and genomic DNA by environmentally transformed engineered nanomaterials

Enormous increase in the production of nanomaterials and their growing applications in the device technology, biotechnology and biomedical areas suggest the need for developing models for predicting the environmental health and safety (EHS) risks posed by such nanomaterials. We hypothesize that CdSe quantum dots (QDs) and ZnO nanoparticles (NPs) encompassed in liposomes or not and transformed by simulated solar UV light can be model systems for studying the environmental toxicity of engineered nanomaterials. In this study, human lung epithelial adenocarcinoma cells (H1650) are exposed to photoirradiated CdSe QDs or ZnO nanopowder included or not in liposomes. The release of cadmium and zinc ions from the nanomaterials exposed to solar simulated UV radiation is detected and quantified by measuring the steady-state and time resolved fluorescence of the metal ion sensor tetracarboxyphenylporphyrin (TCPP) or the commercial Measure iT Pd/Cd sensor. Viability of cells treated with nanomaterials exposed to solar simulated UV radiation for different durations is measured by MTT assay. Enhanced etching of the nanoparticles exposed to solar simulated UV radiation results in the release of toxic levels of heavy metal ions, which considerably lower the viability of H1650 cells is due to the deactivation of DNA repair enzymes as evidenced by the pinching off of nuclear DNA in comet assays and DNA samples in electrophoresis. Results from this study highlight the need to obtain not only quantitative information about the environmental risks posed by engineered nanomaterials but also environment friendly nanomaterials for practical applications.

Assessing the stochastic intermittency of single quantum dot luminescence for robust quantification of biomolecules

Single molecule detection schemes promise that one has the ability to reach the ultimate limit of detection: one molecule. In this paper, we use the stochastic luminescence of single semiconductor nanocrystals (quantum dots, QDs) to detect and localize particles as digital counts. These digital counts can be correlated to the concentration of analytes in solution. Here, we use total internal reflection fluorescence (TIRF) microscopy to probe individual QDs immobilized on a functionalized substrate. QDs have found their niche in the bioanalytical community due to their remarkable brightness and stability. Despite their numerous outstanding photophysical properties, QDs at the single particle level display a pronounced intermittent luminescence, posing a challenge for the detection of individual particles. In this paper, we demonstrate a reliable method for detecting QDs that takes advantage of these signal fluctuations by comparing the variations in the QD's fluorescence signals against variations of the background signal. The quantitative methodology developed here results in signal-to-background ratios up to 90:1, which is at least 8-times higher than the ratios obtained using methodologies relying solely on signal integration. This enhanced signal-to-background ratio facilitates a robust thresholding process and results in femtomolar limits of detection.

Heterostructuring Nanocrystal Quantum Dots Toward Intentional Suppression of Blinking and Auger Recombination

At the level of a single particle, nanocrystal quantum dots (NQDs) are observed to fluoresce intermittently or "blink." They are also characterized by an efficient non-radiative recombination process known as Auger Recombination (AR). Recently, new approaches to NQD heterostructuring have been developed that directly impact both blinking and AR, resulting in dramatic suppression of these unwanted processes. The three successful hetero-NQD motifs are reviewed here: (1) interfacial alloying, (2) thick or "giant" shells, and (3) specific type-II electronic structures. These approaches, which rely on modifying or tuning internal NQD core/shell structures, are compared with alternative strategies for blinking suppression that rely, instead, on surface modifications or surface-mediated interactions. Finally, in each case, the unique synthetic approaches or challenges addressed that have driven the realization of novel and important functionality are discussed, along with the implications for development of a comprehensive 'materials design' strategy for blinking and AR-suppressed heterostructured NQDs.

Luminescent behavior of cadmium sulfide quantum dots for gallic acid estimation

Thioglycolic acid capped cadmium sulfide (CdS/T) quantum dots have been synthesized using wet chemistry and their optical behavior has been investigated using UV-visible absorption and fluorescence spectroscopy. The role of the capping agent, sulfide source concentration, pH and temperature has been studied and discussed. Studies showed that alkaline pH leads to a decrease in the size of quantum dots and reflux temperature above 70 °C resulted in red-shift of emission spectra which is due to narrowing of the bandgap. Further, to reduce the toxicity and photochemical instability of quantum dots, the quantum dots have been functionalized with polyethylene glycol (PEG), which resulted in a 20% enhancement of the fluorescence intensity. The application potential of CdS/T-PEG quantum dots was further studied using gallic acid as a model compound. The sensing is based on fluorescence quenching of quantum dots in the presence of gallic acid, and this study showed linearity in the range from 1.3 × 10(-8) to 46.5 × 10(-8) mM, with a detection limit of 3.6 × 10(-8) mM.

Suppressed blinking and auger recombination in near-infrared type-II InP/CdS nanocrystal quantum dots

Nonblinking excitonic emission from near-infrared and type-II nanocrystal quantum dots (NQDs) is reported for the first time. To realize this unusual degree of stability at the single-dot level, novel InP/CdS core/shell NQDs were synthesized for a range of shell thicknesses (~1-11 monolayers of CdS). Ensemble spectroscopy measurements (photoluminescence peak position and radiative lifetimes) and electronic structure calculations established the transition from type-I to type-II band alignment in these heterostructured NQDs. More significantly, single-NQD studies revealed clear evidence for blinking suppression that was not strongly shell-thickness dependent, while photobleaching and biexciton lifetimes trended explicitly with extent of shelling. Specifically, very long biexciton lifetimes-up to >7 ns-were obtained for the thickest-shell structures, indicating dramatic suppression of nonradiative Auger recombination. This new system demonstrates that electronic structure and shell thickness can be employed together to effect control over key single-dot and ensemble NQD photophysical properties.

Auger recombination suppression in nanocrystals with asymmetric electron-hole confinement

Type II and quasi-type II nanocrystals with thick shells exhibit reduced blinking. However, after a number of monolayers, the influence of the shell thickness is found to vanish. Using a two-band Kane Hamiltonian, it is shown that this behavior is a consequence of interband coupling and asymmetric confinement of electrons and holes. Interface alloying provides an additional, order-of-magnitude contribution to the Auger suppression, in agreement with recent experiments. The existence is predicted of critical shell thicknesses that strongly quench Auger processes for any core size.

Photofabrication of fullerene-shelled quantum dots supramolecular nanoparticles for solar energy harvesting

Quantum dots-based electron donor-acceptor systems play a rising role in the design of renewable and carbon-free energy harvesting technologies. In this article, we discuss the photofabrication of fullerene-shelled quantum dots supramolecular nanoparticles, in which the fullerene shell acts as not only a well-defined electron acceptor but also a robust protecting layer against the photocorrosion of the quantum dot core. We evaluate the ensemble and single-molecule electron transfer from the core to the shell in the nanoparticles and the photocurrent response of a photoelectrochemical cell constructed using the nanoparticles. The supramolecular nanoparticle has been prepared by the covalent tethering of a fullerene-thiol monolayer to the quantum dot followed by the photochemical reactions of free fullerene-thiol to the tethered monolayer. The nanoparticles are characterized using scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. Correlated single-photon emission and the two-state ON-OFF photoluminescence show that single quantum dots are included in the supramolecular nanoparticles. The fullerene-shells suppress the blinking of single quantum dots by acting as well-defined electron traps, without allowing the transfer of Auger electrons to unknown traps. Electron transfer from the quantum dot-core to the fullerene-shell is apparent from the short ON and OFF durations in the photoluminescence intensity trajectories of single quantum dots, quenching of the photoluminescence intensity and lifetime of quantum dots at the ensemble level, and the characteristic transient absorption band of the anion radical of fullerene. We next construct a photoelectrochemical cell using the supramolecular nanoparticles, and the transferred electron is externally driven in the cell to generate ∼400 μA/cm(2) photocurrent. Electron transfer from the highly stable quantum dots to the protecting fullerene-shells places the supramolecular nanoparticles among the most promising antenna systems for the construction of cost-effective and stable next generation solar energy harvesting systems.

Photoluminenscence blinking dynamics of colloidal quantum dots in the presence of controlled external electron traps

The effect of the external charge trap on the photoluminescence blinking dynamics of individual colloidal quantum dots is investigated with a series of colloidal quantum dot-bridge-fullerene dimers with varying bridge lengths, where the fullerene moiety acts as a well-defined, well-positioned external charge trap. It is found that charge transfer followed by charge recombination is an important mechanism in determining the blinking behavior of quantum dots when the external trap is properly coupled with the excited state of the quantum dot, leading to a quasi-continuous distribution of 'on' states and an early fall-off from a power-law distribution for both 'on' and 'off' times associated with quantum dot photoluminescence blinking.

Single-molecule photochemical reactions of Auger-ionized quantum dots

Photoinduced electron transfer in donor-acceptor systems composed of quantum dots (QDs) and electron donors or acceptors is a subject of considerable recent research interest due to the potential applications of such systems in both solar energy harvesting and degradation of organic pollutants. Herein, we employed single-molecule imaging and spectroscopy techniques for the detection of photochemical reactions between 1,4-diaminobutane (DAB) and CdSe/ZnS single QDs. We investigated the reactions by analyzing photoluminescence (PL) intensity and lifetime of QDs at ensemble and single-molecule levels. While DAB was applied to single QDs tethered on a cover slip or QDs dispersed in a solution, PL intensity of QD continuously decreased with a concomitant increase in the PL lifetime. Interestingly, these changes in the PL properties of QD were predominant under high-intensity photoactivation. We hypothesize that the above changes in the PL properties surface due to the transfer of an electron from DAB to Auger-ionized QD followed by elimination of a proton from DAB and the formation of a QD-DAB adduct. Thus, a continuous decrease in the PL intensity of QDs under high-intensity photoactivation is attributed to continuous photochemical reactions of DAB with single QDs and the formation of QD-(DAB)(n) adducts. We believe that detection and analysis of such photochemical reactions of single QDs with amines will be of considerable broad interest due to the significant impact of photoinduced electron transfer reactions in energy management and environmental remediation.

Sized controlled synthesis, purification, and cell studies with silicon quantum dots

This article describes the size control synthesis of silicon quantum dots with simple microemulsion techniques. The silicon nanocrystals are small enough to be in the strong confinement regime and photoluminesce in the blue region of the visible spectrum and the emission can be tuned by changing the nanocrystal size. The silicon quantum dots were capped with allylamine either a platinum catalyst or UV-radiation. An extensive purification protocol is reported and assessed using (1)H NMR to produce ultra pure silicon quantum dots suitable for biological studies. The highly pure quantum dots were used in cellular uptake experiments and monitored using confocal microscopy. The results showed that the amine terminated silicon nanocrystals accumulated in lysosome but not in nuclei and could be used as bio-markers to monitor cancer cells over long timescales.

Probing and controlling fluorescence blinking of single semiconductor nanoparticles

In this review we present an overview of the experimental and theoretical development on fluorescence intermittency (blinking) and the roles of electron transfer in semiconductor crystalline nanoparticles. Blinking is a very interesting phenomenon commonly observed in single molecule/particle experiments. Under continuous laser illumination, the fluorescence time trace of these single nanoparticles exhibit random light and dark periods. Since its first observation in the mid-1990s, this intriguing phenomenon has attracted wide attention among researchers from many disciplines. We will first present the historical background of the discovery and the observation of unusual inverse power-law dependence for the waiting time distributions of light and dark periods. Then, we will describe our theoretical modeling efforts to elucidate the causes for the power-law behavior, to probe the roles of electron transfer in blinking, and eventually to control blinking and to achieve complete suppression of the blinking, which is an annoying feature in many applications of quantum dots as light sources and fluorescence labels for biomedical imaging.