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

Conversion of Photoluminescence Blinking Types in Single Colloidal Quantum Dots

Almost all colloidal quantum dots (QDs) exhibit undesired photoluminescence (PL) blinking, which poses a significant obstacle to their use in numerous luminescence applications. An in-depth study of the blinking behavior, along with the associated mechanisms, can provide critical opportunities for fabricating high-quality QDs for diverse applications. Here the blinking of a large series of colloidal QDs is investigated with different surface ligands, particle sizes, shell thicknesses, and compositions. It is found that the blinking behavior of single alloyed CdSe/ZnS QDs with a shell thickness of up to 2 nm undergoes an irreversible conversion from Auger-blinking to band-edge carrier blinking (BC-blinking). Contrastingly, single perovskite QDs with particle sizes smaller than their Bohr diameters exhibit reversible conversion between BC-blinking and more pronounced Auger-blinking. Changes in the effective trapping sites under different excitation conditions are found to be responsible for the blinking type conversions. Additionally, changes in shell thickness and particle size of QDs have a significant effect on the blinking type conversions due to altered wavefunction overlap between excitons and effective trapping sites. This study elucidates the discrepancies in the blinking behavior of various QD samples observed in previous reports and provides deeper understanding of the mechanisms underlying diverse types of blinking.

Game of Crowns: Na+ Is Coming! Red NIR-Emissive Hybrid Liquid Crystals Containing Discotic Crown Ethers and Na2Mo6X8iCl6 (Xi = Cl or Br)

Molecular or supramolecular materials that can self-organize into columns such as discotic liquid crystals are of interest for several applications in the field of optoelectronics. We show in this work that red near-infrared (NIR)-emissive metal cluster compounds of general formula Na2Mo6X8iCl6 (Xi = Cl or Br) can be readily complexed with discotic liquid crystals containing a crown ether. Three cavity sizes have been tested with crown ethers bearing 4, 5, or 6 oxygen atoms. In all cases, 1:1 complexes were formed, thanks to the well-known supramolecular interactions existing between the Na+ cations of the metal cluster salt and the crown ether derivatives. All obtained hybrids are homogeneous, emit in the red NIR region, and show liquid crystalline properties on a wider temperature range than their precursors. Charge transport properties have been investigated by using a space charge limited current device. Obtained results demonstrate that metal cluster compounds can enhance the charge carrier mobility by 5 orders of magnitude compared to the native discotic organic ligands. Considering that the presented organic crown ether derivatives are not the best candidates to design optoelectronic devices because of their inherently low conductivity, but that similar compounds were developed to design proton conductive porous framework, our results open promising perspectives for the use of metal cluster compounds in devices dedicated to such a field.

Charge and energy transfer dynamics in single colloidal quantum dots/monolayer MoS2 heterostructures

The charge and energy transfer dynamics in colloidal CdSeTe/ZnS quantum dots (QDs)/monolayer molybdenum disulfide (MoS₂) heterostructures have been investigated by time-resolved single-dot photoluminescence (PL) spectroscopy. A time-gated method is used to separate the PL photons of single QDs from the PL photons of monolayer MoS₂, which are impossible to be separated by the spectral filter due to their spectral overlap. It is found that the energy transfer from MoS₂ to single QDs increases the exciton generation of the QDs by 37.5% and the energy transfer from single QDs to MoS₂ decreases the PL quantum yield of the QDs by 66.9%. In addition, it is found that MoS₂ increases the discharging rate of single QDs by 59%, while the charging rate remains unchanged. This investigation not only provides valuable insight into the exciton generation and recombination at the single-dot level across such hybrid 0D-2D interfaces but also promotes the application of the hybrid system in various optoelectronic devices.

Fluorescent Nanoparticles for Super-Resolution Imaging

Super-resolution imaging techniques that overcome the diffraction limit of light have gained wide popularity for visualizing cellular structures with nanometric resolution. Following the pace of hardware developments, the availability of new fluorescent probes with superior properties is becoming ever more important. In this context, fluorescent nanoparticles (NPs) have attracted increasing attention as bright and photostable probes that address many shortcomings of traditional fluorescent probes. The use of NPs for super-resolution imaging is a recent development and this provides the focus for the current review. We give an overview of different super-resolution methods and discuss their demands on the properties of fluorescent NPs. We then review in detail the features, strengths, and weaknesses of each NP class to support these applications and provide examples from their utilization in various biological systems. Moreover, we provide an outlook on the future of the field and opportunities in material science for the development of probes for multiplexed subcellular imaging with nanometric resolution.

Single-Molecule Blinking Fluorescence Enhancement by Surface Plasmon-Coupled Emission-Based Substrates for Single-Molecule Localization Imaging

Surface plasmon-coupled emission (SPCE) substrates to enhance the blinking fluorescence of spontaneously blinking fluorophores in single-molecule localization microscopy (SMLM) were fabricated to reduce the excitation power density requirement and reveal the distribution of fluorophore-labeled proteins on a plasma membrane with nanoscale-level resolution. The systemic investigation of the contribution of local field enhancement, modified quantum yield, and emission coupling yield through glass coverslip substrates coated with metal layers of different thicknesses revealed that the silver-layer substrate with a thickness of 44 nm produces the highest SPCE fluorescence in spontaneously blinking fluorophores, and it has a highly directional SPCE fluorescence, which helps improve the detection efficiency. Moreover, the uniform and surface-enhanced field created on the substrate surface is beneficial for fluorescence background reduction in single fluorophore detection and localization, as well as for revealing the real position of fluorophores. Consequently, compared with a glass coverslip substrate, the presented SPCE substrate demonstrated a fluorescence enhancement of 480% and an increase in blinking events from a single spontaneously blinking fluorophore; moreover, the required excitation power density for SMLM imaging was significantly reduced to 23 W cm^-2 for visualizing the distribution of epidermal growth factor receptors (EGFRs) on the basal plasma membrane of A549 lung cancer cells with a localization precision of 19 ± 7 nm. Finally, the fluorophore-labeled EGFRs on the basal plasma membrane in the presence of PIKfyve-specific inhibitor treatment were explored using SPCE-SMLM imaging; the results revealed a distinct reduction in the density of localization events because of a decrease in EGFR abundance at the plasma membranes of the cells.

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.

Cytosolic delivery of membrane-penetrating QDs into T cell lymphocytes: implications in immunotherapy and drug delivery

We report single-particle characterization of membrane-penetrating semiconductor quantum dots (QDs) in T cell lymphocytes. We functionalized water-soluble CdSe/CdZnS QDs with a cell-penetrating peptide composed of an Asp-Ser-Ser (DSS) repeat sequence. DSS and peptide-free control QDs displayed concentration-dependent internalization. Intensity profiles from single-particle imaging revealed a propensity of DSS-QDs to maintain a monomeric state in the T cell cytosol, whereas control QDs formed pronounced clusters. Single-particle tracking showed a direct correlation between individual QD clusters' mobility and aggregation state. A significant portion of control QDs colocalized with an endosome marker inside the T cells, while the percentage of DSS-QDs colocalized dropped to 9%. Endocytosis inhibition abrogated the internalization of control QDs, while DSS-QD internalization only mildly decreased, suggesting an alternative cell-entry mechanism. Using 3D single-particle tracking, we captured the rapid membrane-penetrating activity of a DSS-QD. The ability to characterize membrane penetrating activities in live T cells creates inroads for the optimization of gene therapy and drug delivery through the use of novel nanomaterials.

Underestimated effect of the polymer encapsulation process on the photoluminescence of perovskite revealed by in situ single-particle detection

Photostability has always been an important issue that limits the performance of organo-metal halide perovskites in optoelectronic devices. Although the photostability can be partially improved by polymer coating/encapsulation, one rising question that needs to be considered is whether the improvement of photostability is accessed at the expense of intangible loss in photoluminescence (PL) properties. By in situ analyzing the evolution of PL properties of individual perovskite crystals during the polymer encapsulation procedure, we demonstrate here that poly(methyl methacrylate), a common polymeric encapsulant, would passivate the surface defects of perovskite crystals, leading to the suppress of PL blinking. However, somewhat counterintuitive, the toluene solvent will induce the PL decline of individual perovskite crystals via accumulation of the number of quenchers that, most probably, are related to the ion migration in perovskite. The findings at the single-particle level emphasize the often-neglected role of the polymer matrix and the solvent in the optical properties of perovskite material during the polymer encapsulation process, and will guide the further design of more stable and high-performance devices based on perovskite.

Photoluminescence Blinking and Biexciton Auger Recombination in Single Colloidal Quantum Dots with Sharp and Smooth Core/Shell Interfaces

There is an inconsistence on whether a smooth core/shell interface can reduce Auger recombination and suppress photoluminescence (PL) blinking in single colloidal quantum dots (QDs). Here, we investigate the influence of a core/shell interface on PL blinking and biexciton Auger recombination by comparing the single-dot PL spectra of Cd_xZn_1-xSe_yS_1-y/ZnS core/shell QDs with sharp and smooth interfaces. The inconsistence can be clarified when considering different PL blinking mechanisms. For the single QDs showing Auger blinking, a smooth core/shell interface potential can suppress PL blinking through reducing the Auger recombination. In contrast, we find slightly reduced biexciton Auger recombination rates but increased PL blinking activities in the band-edge carrier (BC)-blinking QDs with the smooth core/shell interface. This is because the smooth interface potential cannot reduce the PL blinking caused by the transfer of electrons to the surface states; however, there is potential to increase electron wave function delocalization for reducing the biexciton Auger recombination rate.

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.

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.

Construction of 3D-rendering imaging of an ischemic rat brain model using the planar FMMD technique

Occlusion of the major cerebral artery usually results in brain hypoxic-ischemic injury, which evokes neuroinflammation and microglial activation. Activated microglia are considered a source of multiple neurotoxic factors, such as reactive oxygen species (ROS), in the central nervous system (CNS). We herein present a 3D-rendering brain imaging technique in an experimental rodent model of cerebral ischemia based on 2D magnetic images of superparamagnetic iron oxide nanoparticles (SPIONs) using the planar frequency mixing magnetic detection (p-FMMD) technique. A rat model of cerebral ischemia was established by unilateral middle cerebral artery occlusion with reperfusion (MCAO/R) injury. 2,3,5-Triphenyltetrazolium chloride (TTC) staining was performed to demonstrate the irreversibly damaged ischemic brain tissues, and double immunofluorescent labeling of OX6 (activated microglial marker) and ethidium (ROS marker) was conducted to confirm ROS generation in the activated microglia in the infarcted brain region. The ischemic brain sections treated with OX6-conjugated SPIONs were scanned using our p-FMMD system, yielding 2D images on the basis of the nonlinear magnetic characteristics inherent in SPIONs. The p-FMMD signal images representing microglia activation show an infarct ratio of 44.6 ± 7.1% compared to the contralateral counterpart, which is smaller than observed by TTC (60.9 ± 4.9%) or magnetic resonance imaging (MRI, 65.7 ± 2.7%). Furthermore, we developed a 3D-rendering brain imaging process based on the 2D p-FMMD signal images. The 3D reconstructed model showed a decreased ratio of coincidence of the ischemic regions compared with MRI models. In this study, we successfully conducted a feasibility test on whether our p-FMMD technology, a technique for signaling and imaging based on the nonlinearity of SPIONs, can be used to visualize the ischemic brain region in real time by detecting activated microglia in an MCAO/R animal model. Therefore, our method might allow for a different approach to analyze the pathophysiology of ischemic stroke through molecular imaging. Furthermore, we propose that this magnetic particle imaging (MPI) technique that detects the nonlinear magnetization properties of SPIONs could be applied not only to a stroke model but also to various types of pathophysiological studies as a new bioimaging tool.

Mechanistic Investigation of the Defect Activity Contributing to the Photoluminescence Blinking of CsPbBr3 Perovskite Nanocrystals

Exploration of the full potential of the perovskite nanocrystals (NCs) for different applications requires a thorough understanding of the pathways of recombination of the photogenerated charge carriers and associated dynamics. In this work, we have tracked the recombination routes of the charge carriers by probing photoluminescence (PL) intermittency of the immobilized and freely diffusing single CsPbBr₃ NCs employing a time-tagged-time-resolved method. The immobilized single CsPbBr₃ NCs show a complex PL time-trace, a careful analysis of which reveals that nonradiative band-edge recombination through trap states, trion recombination, and trapping of the hot carriers contribute to the blinking behavior of any given NC. A drastically suppressed PL blinking observed for the NCs treated with a tetrafluoroborate salt indicates elimination of most of the undesired recombination processes. A fluorescence correlation spectroscopy (FCS) study on the freely diffusing single NCs shows that enhanced PL and suppressed blinking of the treated particles are the outcome of an increase in per-particle brightness, not due to any increase in the number of particles undergoing "off"-"on" transition in the observation volume. The mechanistic details obtained from this study on the origin of blinking in CsPbBr₃ NCs provide deep insight into the radiative and nonradiative charge carrier recombination pathways in these important materials, and this knowledge is expected to be useful for better design and development of bright photoluminescent samples of this class for optoelectronic applications.

In situ manipulation of fluorescence resonance energy transfer between quantum dots and monolayer graphene oxide by laser irradiation

The unique optical properties of solution-processable colloidal semiconductor quantum dots (QDs) highlight their promising applications in the next generation of optoelectronic and biomedical technologies. In order to optimize these applications, the tunability of QDs' optical properties is always highly desired. Although the tuning during synthesis stages has been intensively investigated, the in situ alteration after device fabrication is still limited. Here we report the tuning of the optical properties of CdSeTe/ZnS QDs through an in situ manipulation of fluorescence resonance energy transfer (FRET) between QDs and monolayer graphene oxide (GO). By increasing the acceptor's absorption ability of GO through laser irradiation, the efficiency of FRET between QDs and GO has been substantially improved from 29.7% to 70.0%. The corresponding energy transfer rate is enhanced by 5.5 times. These results can be well explored by a spectral overlap between the fluorescence emission of QDs and the absorption of original or reduced GO. Our scheme, with the features of in situ manipulation, high spatial resolution and wireless steering, enables the potential functionality of such hybrid structures in optoelectronic applications.

Excitons and Biexciton Dynamics in Single CsPbBr3 Perovskite Quantum Dots

Colloidal lead halide perovskite quantum dots, due to their optical versatility and facile solution processability, have been recently recognized as components of various optoelectronic devices. Detailed understanding of their exciton recombination dynamics at the single-particle level is necessary for utilizing their full potential. We conducted spectroscopic studies of the excitons and biexciton dynamics in single CsPbBr₃ perovskite quantum dots. It was found that while the rates of radiative recombination remain essentially constant, the overall relaxation process is dominated by nonradiative recombination of single excitons and biexcitons. The radiative lifetime scaling is determined to be ∼1.0 for single exciton and ∼4.4 for biexcitons. A linear dependence of fluorescence lifetime vs intensity distribution agrees well with the prediction of the model of multiple recombination centers. The blinking mechanism of CsPbBr₃ quantum dots is addressed by considering the trion states under higher excitation powers.

Enhancing the blinking fluorescence of single-molecule localization imaging by using a surface-plasmon-polariton-enhanced substrate

Super-resolution imaging based on single-molecule localization microscopy combined with the surface plasmon polariton (SPP)-enhanced fluorescence of spontaneously blinking fluorophores was demonstrated to visualize the nanoscale-level positioning information of cell-adhesion-associated proteins. Glass substrates with a deposited silver layer were utilized to induce a SPP-enhanced field on the silver surface and significantly strengthen the fluorescence signals of the fluorophores by more than 300%. The illumination power density for localization imaging at a spatial resolution of 25 ± 11 nm was 31.6 W cm-2. This low illumination power density will facilitate the reduction of phototoxicity of the biospecimens for single-molecule localization imaging. The proposed strategy provides a uniform distribution of the SPP-enhanced field on the silver surface, enabling visualization of the spatial distribution of labeled proteins without interference caused by the enhanced field distribution.

A White Light-Emitting Quantum Dot Complex for Single Particle Level Interaction with Dopamine Leading to Changes in Color and Blinking Profile

The interaction of the neurotransmitter dopamine is reported with a single particle white light-emitting (WLE) quantum dot complex (QDC). The QDC is composed of yellow emitting ZnO quantum dots (Qdots) and blue emitting Zn(MSA)₂ complex (MSA = N-methylsalicylaldimine) synthesized on their surfaces. Sensing is achieved by the combined changes in the visual luminescence color from white to blue, chromaticity color coordinates from (0.31, 0.33) to (0.24, 0.23) and the ratio of the exponents (α_on /α_off ) of on/off probability distribution (from 0.24 to 3.21) in the blinking statistics of WLE QDC. The selectivity of dopamine toward ZnO Qdots, present in WLE QDC, helps detect ≈13 dopamine molecules per Qdot. Additionally, the WLE QDC exhibits high sensitivity, with a limit of detection of 3.3 × 10^-9 m (in the linear range of 1-100 × 10^-9 m) and high selectivity in presence of interfering biological species. Moreover, the single particle on-off bilking statistics based detection strategy may provide an innovative way for ultrasensitive detection of analytes.

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.

Fast recognition of single quantum dots from high multi-exciton emission and clustering effects

Recognition of single quantum dots (QDs) from high multi-exciton emission and clustering effects is challenging using the conventional second-order correlation function method. Here we demonstrate a method for fast recognizing single QDs based on the probabilities of detecting single- and two-photon events. The time-tagged, time-resolved and time-correlated single-photon counting technique is applied to effectively remove multi-exciton emission and low-counting background. By this way, single QDs can be fastly recognized by the spatial coincidence-counting model. In addition, the fast recognition of single QDs by using the collected photons during the confocal scanning imaging process has been achieved synchronously.

Quantum Light in Curved Low Dimensional Hexagonal Boron Nitride Systems

Low-dimensional wide bandgap semiconductors open a new playing field in quantum optics using sub-bandgap excitation. In this field, hexagonal boron nitride (h-BN) has been reported to host single quantum emitters (QEs), linking QE density to perimeters. Furthermore, curvature/perimeters in transition metal dichalcogenides (TMDCs) have demonstrated a key role in QE formation. We investigate a curvature-abundant BN system - quasi one-dimensional BN nanotubes (BNNTs) fabricated via a catalyst-free method. We find that non-treated BNNT is an abundant source of stable QEs and analyze their emission features down to single nanotubes, comparing dispersed/suspended material. Combining high spatial resolution of a scanning electron microscope, we categorize and pin-point emission origin to a scale of less than 20 nm, giving us a one-to-one validation of emission source with dimensions smaller than the laser excitation wavelength, elucidating nano-antenna effects. Two emission origins emerge: hybrid/entwined BNNT. By artificially curving h-BN flakes, similar QE spectral features are observed. The impact on emission of solvents used in commercial products and curved regions is also demonstrated. The 'out of the box' availability of QEs in BNNT, lacking processing contamination, is a milestone for unraveling their atomic features. These findings open possibilities for precision engineering of QEs, puts h-BN under a similar 'umbrella' of TMDC's QEs and provides a model explaining QEs spatial localization/formation using electron/ion irradiation and chemical etching.