Photoluminescence Intermittency and Photo-Bleaching of Single Colloidal Quantum Dot

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.

Crystallographic and Photophysical Analysis on Facet-Controlled Defect-Free Blue-Emitting Quantum Dots

The burgeoning demand for commercializing self-luminescing quantum dot (QD) light-emitting diodes (LEDs) has stimulated extensive research into environmentally friendly and efficient QD materials. Hydrofluoric acid (HF) additive improves photoluminescence (PL) properties of blue-emitting ZnSeTe QDs, ultimately reaching a remarkable quantum yield (QY) of 97% with an ultranarrow peak width of 14 nm after sufficient HF addition. The improvement in optical properties of the QDs is accompanied by a morphology change of the particles, forming cubic-shaped defect-free ZnSeTe QDs characterized by a zinc blende (ZB) crystal structure. This treatment improves the QD-emitting properties by facilitating facet-specific growth, selectively exposing stabilized (100) facets, and reducing the lattice disorders. The facet-specific growth process gives rise to defect-free monodispersed cubic dots that exhibit remarkably narrow and homogeneous PL spectra. Meticulous time-resolved spectroscopic studies allow an understanding of the correlation between ZnSeTe QDs' particle shape and performance following HF addition. These investigations shed light on the intricacies of the growth mechanism and the factors influencing the PL efficiency of the resulting QDs. The findings significantly contribute to understanding the role of HF treatment in tailoring the optical properties of ZnSeTe QDs, thereby bringing it closer to the realization of highly efficient and bright QD-LEDs for various practical applications.

Multiphoton emission of single CdZnSe/ZnS quantum dots coupled with plasmonic Au nanoparticles

The fluorescence blinking and low multiphoton emission of quantum dots (QDs) have limited their application in lasing, light-emitting diodes, and so on. Coupling of single QDs to plasmonic nanostructures is an effective approach to control the photon properties. Here plasmon-exciton systems including Au nanoparticles and CdZnSe/ZnS QDs were investigated at the single particle level. With the modulation of the local electromagnetic field, the fluorescence intensity of single QDs is increased, accompanied by a significant suppression in blinking behavior, and the lifetime is shortened from 15 ns to 2 ns. Moreover, the second-order photon intensity correlation at zero lag time g2(0) of coupled single QDs is larger than 0.5, indicating an increased probability of multiphoton emission. The enhancement factors of radiative and nonradiative decay rates of QDs coupled with Au nanoparticles are calculated. The sharply increased radiative decay rate can be comparable to the nonradiative Auger rate, leading to dominated multiple exciton radiative recombination with PL intensity enhancement, suppressed blinking, lifetime shortening, and multiphoton emission. The results of the exciton decay dynamics and emission properties of single QDs in this work are helpful in exploring the mechanism of plasmon-exciton interaction and optoelectronic application of single QDs.

Bright Nonblinking Photoluminescence with Blinking Lifetime from a Nanocavity-Coupled Quantum Dot

Colloidal quantum dots (QDs) are excellent luminescent nanomaterials for many optoelectronic applications. However, photoluminescence blinking has limited their practical use. Coupling QDs to plasmonic nanostructures shows potential in suppressing blinking. However, the underlying mechanism remains unclear and debated, hampering the development of bright nonblinking dots. Here, by deterministically coupling a QD to a plasmonic nanocavity, we clarify the mechanism and demonstrate unprecedented single-QD brightness. In particular, we report for the first time that a blinking QD could obtain nonblinking photoluminescence with a blinking lifetime through coupling to the nanocavity. We show that the plasmon-enhanced radiative decay outcompetes the nonradiative Auger process, enabling similar quantum yields for charged and neutral excitons in the same dot. Meanwhile, we demonstrate a record photon detection rate of 17 MHz from a colloidal QD, indicating an experimental photon generation rate of more than 500 MHz. These findings pave the way for ultrabright nonblinking QDs, benefiting diverse QD-based applications.

Direct Optical Patterning of Quantum Dots: One Strategy, Different Chemical Processes

Patterning, stability, and dispersion of the semiconductor quantum dots (scQDs) are three issues strictly interconnected for successful device manufacturing. Recently, several authors adopted direct optical patterning (DOP) as a step forward in photolithography to position the scQDs in a selected area. However, the chemistry behind the stability, dispersion, and patterning has to be carefully integrated to obtain a functional commercial device. This review describes different chemical strategies suitable to stabilize the scQDs both at a single level and as an ensemble. Special attention is paid to those strategies compatible with direct optical patterning (DOP). With the same purpose, the scQDs' dispersion in a matrix was described in terms of the scQD surface ligands' interactions with the matrix itself. The chemical processes behind the DOP are illustrated and discussed for five different approaches, all together considering stability, dispersion, and the patterning itself of the scQDs.

Reversible Facet Reconstruction of CdSe/CdS Core/Shell Nanocrystals by Facet-Ligand Pairing

Synthesis of colloidal semiconductor nanocrystals with defined facet structures is challenging, though such nanocrystals are essential for fully realizing their size-dependent optical and optoelectronic properties. Here, for the mostly developed colloidal wurtzite CdSe/CdS core/shell nanocrystals, facet reconstruction is investigated under typical synthetic conditions, excluding nucleation, growth, and interparticle ripening. Within the reaction time window, two reproducible sets of facets─each with a specific group of low-index facets─can be reversibly reconstructed by switching the ligand system, indicating thermodynamic stability of each set. With a unique <0001> axis, atomic structures of the low-index facets of wurtzite nanocrystals are diverse. Experimental and theoretical studies reveal that each facet in a given set is paired with a common ligand in the solution, namely, either fatty amine and/or cadmium alkanoate. The robust bonding modes of ligands are found to be strongly facet-dependent and often unconventional, instead of following Green's classification. Results suggest that facet-controlled nanocrystals can be synthesized by optimal facet-ligand pairing either in synthesis or after-synthesis reconstruction, implying semiconductor nanocrystal formation with size-dependent properties down to an atomic level.

Deterministic Quantum Light Arrays from Giant Silica-Shelled Quantum Dots

Colloidal quantum dots (QDs) are promising candidates for single-photon sources with applications in photonic quantum information technologies. Developing practical photonic quantum devices with colloidal materials, however, requires scalable deterministic placement of stable single QD emitters. In this work, we describe a method to exploit QD size to facilitate deterministic positioning of single QDs into large arrays while maintaining their photostability and single-photon emission properties. CdSe/CdS core/shell QDs were encapsulated in silica to both increase their physical size without perturbing their quantum-confined emission and enhance their photostability. These giant QDs were then precisely positioned into ordered arrays using template-assisted self-assembly with a 75% yield for single QDs. We show that the QDs before and after assembly exhibit antibunching behavior at room temperature and their optical properties are retained after an extended period of time. Together, this bottom-up synthetic approach via silica shelling and the robust template-assisted self-assembly offer a unique strategy to produce scalable quantum photonics platforms using colloidal QDs as single-photon emitters.

Single-Particle Photoluminescence and Dark-Field Scattering during Charge Density Tuning

Single-particle spectroelectrochemistry provides optical insight into understanding physical and chemical changes occurring on the nanoscale. While changes in dark-field scattering during electrochemical charging are well understood, changes to the photoluminescence of plasmonic nanoparticles under similar conditions are less studied. Here, we use correlated single-particle photoluminescence and dark-field scattering to compare their plasmon modulation at applied potentials. We find that changes in the emission of a single gold nanorod during charge density tuning of intraband photoluminescence can be attributed to changes in the Purcell factor and absorption cross section. Finally, modulation of interband photoluminescence provides an additional constructive observable, giving promise for establishing dual channel sensing in spectroelectrochemical measurements.

Exciton Dynamics in Colloidal CdS Quantum Dots with Intense and Stokes Shifted Photoluminescence in a Single Decay Channel

CdS quantum dots (QDs), synthesized by a sol-gel method, exhibit significantly Stokes shifted bright photoluminescence (PL), predominantly from the trap states. Surprisingly, the PL decay at the emission maximum is single-exponential. This is an unusual observation for as-prepared QDs and indicates a narrow distribution in the nature of trap states. A closer look reveals an additional fast component for the decays at shorter emission wavelengths, presumably due to the band edge emission, which remains elusive in the steady-state spectra. Indeed, a significantly narrower and blue-shifted emission band is observed in the decay-associated spectra. The contribution of this component to the steady-state PL intensity is shown to be overwhelmed by that of the significantly stronger trap emission. Exciton dynamics in the quantum dots is elucidated using transient absorption spectra, in which the stimulated emission is observed even at low pump power.

Anomalous Emission Shift of CdSe/CdS/ZnS Quantum Dots at Cryogenic Temperatures

The band-gap energy of most bulk semiconductors tends to increase as the temperature decreases. However, non-monotonic temperature dependence of the emission energy has been observed in semiconductor quantum dots (QDs) at cryogenic temperatures. Here, using stable and highly efficient CdSe/CdS/ZnS QDs as the model system, we quantitatively reveal the origins of the anomalous emission red-shift (∼8 meV) below 40 K by correlating ensemble and single QD spectroscopy measurements. About one-quarter of the anomalous red-shift (∼2.2 meV) is caused by the temperature-dependent population of the band-edge exciton fine levels. The enhancement of electron-optical phonon coupling caused by the increasing population of dark excitons with temperature decreases contributes an ∼3.4 meV red-shift. The remaining ∼2.4 meV red-shift is attributed to temperature-dependent electron-acoustic phonon coupling.

Unraveling the Effect of Surface Ligands on the Auger Process in an Inorganic Perovskite Quantum-Dot System

This work systematically scrutinizes the role of surface-ligand modification in affecting the Auger process in a porotype perovskite system of CsPbBr₃-octanoic acid (OcA) and CsPbBr₃-oleic acid (OA) quantum dots (QDs), by means of steady-state/time-resolved/temperature-dependent photoluminescence spectroscopy and ultrafast transient absorption spectroscopy. The difference in the ligand chain length (i.e., C₈ and C₁₈ alkyl chains for OcA and OA, respectively) is found to significantly affect Auger recombination and hot-carrier cooling processes. More importantly, we provide fresh insight into the involved carrier dynamics; i.e., the modification of CsPbBr₃ QDs with short-chain (long-chain) ligand leads to the formation of trapped (free) carriers, which causes a pronounced difference in the ability to suppress the detrimental Auger process. In addition, a careful analysis of spectral evolution reveals that the Auger suppression is related to the carrier population of a certain transition state. The valuable mechanistic information gleaned from the exciton/carrier dynamics perspective would assist in surface engineering through a facile ligand-modification strategy toward rational design and optimization of QD-based photoelectrochemical applications.

Understanding the Synergistic Oxidation in Dichalcogenides through Electrochemiluminescence Blinking at Millisecond Resolution

The oxidation of transition metal dichalcogenides (TMDCs) has been extensively studied and applied in electronics, optics, and energy sources because of its tunable structure and performance. However, due to the lack of appropriate technology, dynamically observe the oxidation process remains an arduous task. Herein, the synergistic oxidation between edge and basal plane in molybdenum disulfide (MoS₂ ) is observed through electrogenerated chemiluminescence (ECL) blinking with a millisecond resolution. In addition, the ECL method provides a simple, convenient, and quick way to judge structural changes. The transient elevation of the ECL intensity proved the intermittent doping of oxygen at MoS₂ , which generates O-atom active sites. High ECL intensity enhanced from the produced hydroperoxide intermediates eases the monitoring of MoS₂ particles. Further study shows that the formation of sulfur vacancies at MoS₂ , by the edge activation of hydrogen peroxide and the migration of oxygen to the basal plane, is more conducive to oxygen doping that favors the formation of MoOMo as new active sites to induce bursts. The revealing of sulfur vacancy-governed blinking from MoS₂ indicates a complex interaction between oxygen and MoS₂ . The same phenomenon is observed on tungsten disulfide (WS₂ ), which provides new information about the oxidation feature of 2D dichalcogenides.

Water Effects on Colloidal Semiconductor Nanocrystals: Correlation of Photophysics and Photochemistry

With high-quality CdSe/CdS core/shell nanocrystals as the main model system and under a controlled atmosphere, responses of photoexcited semiconductor nanocrystals to two active species (water and/or oxygen) in an ambient environment are studied systematically. Under photoexcitation, although high-quality semiconductor nanocrystals in either thin solid films or various solutions have a near-unity photoluminescence quantum yield, there is still a small probability (∼10^-5 per photon absorbed) to be photoreduced by the water molecules efficiently accumulated in the highly hydrophilic nanocrystal-ligands interface. The resulting negatively charged nanocrystals are the starting point of most photophysical variations, and the hydroxyl radical─key photo-oxidation product of water─plays the main role for initiating various photochemical processes. Depending on the supplementation of water to the interface, accessibility to oxygen, photoirradiation power, type of matrices, type of measurement schemes, and solubility of nanocrystals in the solution, various photophysical/photochemical phenomena─either reported or not reported in the literature─are reproducibly observed. Results confirm that photophysical properties and photochemical reactions can be well-correlated, offering a unified and unique basis for fundamental studies and the design of processing techniques in industry.

Highly Polarized Single Photons from Strain-Induced Quasi-1D Localized Excitons in WSe2

Single photon emission from localized excitons in two-dimensional (2D) materials has been extensively investigated because of its relevance for quantum information applications. Prerequisites are the availability of photons with high purity polarization and controllable polarization orientation that can be integrated with optical cavities. Here, deformation strain along edges of prepatterned square-shaped substrate protrusions is exploited to induce quasi-one-dimensional (1D) localized excitons in WSe₂ monolayers as an elegant way to get photons that fulfill these requirements. At zero magnetic field, the emission is linearly polarized with 95% purity because exciton states are valley hybridized with equal shares of both valleys and predominant emission from excitons with a dipole moment along the elongated direction. In a strong field, one valley is favored and the linear polarization is converted to high-purity circular polarization. This deterministic control over polarization purity and orientation is a valuable asset in the context of integrated quantum photonics.

Light emission from plasmonic nanostructures

The mechanism of light emission from metallic nanoparticles has been a subject of debate in recent years. Photoluminescence and electronic Raman scattering mechanisms have both been proposed to explain the observed emission from plasmonic nanostructures. Recent results from Stokes and anti-Stokes emission spectroscopy of single gold nanorods using continuous wave laser excitation carried out in our laboratory are summarized here. We show that varying excitation wavelength and power change the energy distribution of hot carriers and impact the emission spectral lineshape. We then examine the role of interband and intraband transitions in the emission lineshape by varying the particle size. We establish a relationship between the single particle emission quantum yield and its corresponding plasmonic resonance quality factor, which we also tune through nanorod crystallinity. Finally, based on anti-Stokes emission, we extract electron temperatures that further suggest a hot carrier based mechanism. The central role of hot carriers in our systematic study on gold nanorods as a model system supports a Purcell effect enhanced hot carrier photoluminescence mechanism. We end with a discussion on the impact of understanding the light emission mechanism on fields utilizing hot carrier distributions, such as photocatalysis and nanothermometry.

Enhancing Dielectric Screening for Auger Suppression in CdSe/CdS Quantum Dots by Epitaxial Growth of ZnS Shell

Auger recombination is the main nonradiative process in multicarrier states of high-quality quantum dots (QDs). For the most-studied CdSe/CdS core/shell QDs, we effectively reduce the biexciton Auger rate by enhancing dielectric screening of band-edge carriers via epitaxial growth of additional ZnS shells. Super volume scaling of negative-trion Auger lifetime for CdSe/CdS core/shell QDs is achieved with the outermost ZnS shells. The volume of CdSe/CdS/ZnS QDs can be less than half that of CdSe/CdS QDs with the same negative-trion Auger lifetime. Auger suppression by the ZnS shells is more pronounced for QDs with wave functions of band-edge carriers spreading close to the inorganic-organic interface, such as CdSe/CdS QDs with small cores. A maximum drop of biexciton Auger rate of ∼50% and a maximum enhancement of biexciton emission quantum yield of 75% are achieved. Auger engineering by dielectric screening opens up new opportunities to improve the emission properties of multicarrier states in QDs.

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.

Delocalized Surface Electronic States on Polar Facets of Semiconductor Nanocrystals

Wurtzite CdSe@CdS dot@platelet nanocrystals with (001) and (00-1) polar facets as the basal planes and (100) family of nonpolar facets as the side planes are applied for studying surface defects on semiconductor nanocrystals. When they are terminated with cadmium ions coordinated with carboxylate ligands, a single set of absorption features and band-edge photoluminescence (PL) with near unity PL quantum yield and monoexponential PL decay dynamics (lifetime ∼28 ns) are observed. In addition to these spectral signatures, when the surface is converted to sulfur-terminated, a second set of sharp absorption features with decent extinction coefficients and a secondary band-edge PL with low PL quantum yield and long-lifetime (>78 ns) PL decay dynamics are reproducibly recorded. Photochemical analysis confirms that the secondary UV-vis and PL spectral features are quantitatively correlated with each other. Chemical analysis and X-ray photoelectron spectroscopy measurements confirm that such secondary spectral features are well correlated with the sulfide (such as -SH) and disulfide (such as -S-S-) surface sites of a basal plane, which likely form surface hole electronic states delocalized on the entire basal plane. Results suggest that, for studying surface defects on semiconductor nanocrystals, it is essential to prepare a nearly monodisperse surface structure in terms of facets and surface chemical bonding.

Controlling amplified spontaneous emission of quantum dots by polymerized nanostructure interfaces

We report a new polymer/colloidal-quantum-dot (CQD) film with a nanostructured interface, which is fabricated through a template-assisted photopolymerization method, toward the use of amplified spontaneous emission. It is experimentally demonstrated that the amplified spontaneous emission of CQDs is able to be manipulated by changing the nanostructured polymeric interface with a weak scattering ability. The dependences of emission wavelength and threshold on the size of the nanostructure and CQD layer thickness are investigated.

Kinetics and Thermodynamics of Killing a Quantum Dot

Light-emitting nanocrystal quantum dots (QDs) are of high interest for use as down-conversion phosphors and direct emission sources in bulk solid-state devices and as reliable sources of single photons in quantum information science. However, these materials are prone to photooxidation that reduces the emission quantum yield over time. Current commercial applications use device architectures to prevent oxidation without addressing the underlying degradation reactions at the nanocrystal level. To instead prevent loss of functionality by better synthetic engineering of the nanoscale emitters themselves, the underlying properties of these reactions must be understood and readily accessible. Here, we use solid-state spectroscopy to obtain kinetic and thermodynamic parameters of photothermal degradation in single QDs by systematically varying the ambient temperature and photon pump fluence. We describe the resulting degradation in emission with a modified form of the Arrhenius equation and show that this reaction proceeds via pseudo-zero-order reaction kinetics by a surface-assisted process with an activation energy of 60 kJ/mol. We note that the rate of degradation is ∼12 orders of magnitude slower than the rate of excitonic processes, indicating that the reaction rate is not determined by electron or hole trapping. In the search for new robust light-emitting nanocrystals, the reported analysis method will enable direct comparisons between differently engineered nanomaterials.

Suppressed Blinking under Normal Air Atmosphere in Toxic-Metal-Free, Small Sized, InP-Based Core/Alloy-Shell/Shell Quantum Dots

Suppressed blinking has been reported in large (diameter ∼14.1 nm) core/shell InP quantum dots (QDs) under reduced air environment. We report here suppressed blinking with approximately four times smaller (diameter ∼3.6 nm) core/alloy-shell/shell InP QDs under ambient air atmosphere. The ⟨ON fraction⟩ has been obtained to be 0.65. Approximately 26% of the single QDs exhibit ON fraction >80%. The smaller ON exponent (1.19) magnitude in comparison to the OFF exponent (1.45) indicates longer ON events are interrupted by smaller OFF events. ON event truncation time is ∼1.5 times that of the OFF event, signifying the detrapping rate is much higher than the trapping rate. Interestingly, the detrapping rate/trapping rate (single-particle level property) could be directly correlated to the photoluminescence quantum yield (ensemble level property). An additional exponential term required to fit the probability density distribution of the ON event duration could be correlated with hole trapping, leading to extended ON times (>60 s).

Optical tuning of plasmon-enhanced photoluminescence

Photoluminescence (PL) can be enhanced and tuned with the assistance of surface plasmons. Traditional methods of generating tunable fluorophores are either of low efficiency or complicated with poor controllability. Here, we propose the optical tuning and enhancement of PL by modifying the plasmon modes, which shows unique advantages of generality, simplicity, and on-demand controllability. We adopted gold nanoparticles on a mirror as the plasmonic resonator, and responsive polymers as the spacers as well as the emitters in the nanogaps. By controlling the coating thickness of polymers, we can achieve tunable plasmon resonances as well as PL peaks with an enhancement factor up to ∼7000. Furthermore, we show the optical tuning of the plasmon resonances via laser irradiation which modifies the nanogaps. Thus, the PL peaks of the polymers shift accordingly with plasmon resonances. This tuning mechanism is based on plasmon-enhanced PL, which selectively enhances PL signals at different wavelengths. Such a plasmon-assisted PL selection by the optical tuning method provides new opportunities for photonic devices with applications of multiplex sensing and full colour displays.

Engineering Auger recombination in colloidal quantum dots via dielectric screening

Auger recombination is the main non-radiative decay pathway for multi-carrier states of colloidal quantum dots, which affects performance of most of their optical and optoelectronic applications. Outstanding single-exciton properties of CdSe/CdS core/shell quantum dots enable us to simultaneously study the two basic types of Auger recombination channels-negative trion and positive trion channels. Though Auger rates of positive trion are regarded to be much faster than that of negative trion for II-VI quantum dots in literature, our experiments find the two rates can be inverted for certain core/shell geometries. This is confirmed by theoretical calculations as a result of geometry-dependent dielectric screening. By varying the core/shell geometry, both types of Auger rates can be independently tuned for ~ 1 order of magnitude. Experimental and theoretical findings shed new light on designing quantum dots with necessary Auger recombination characteristics for high-power light-emitting-diodes, lasers, single-molecular tracking, super-resolution microscope, and advanced quantum light sources.

Control of the compositions and morphologies of uranium oxide nanocrystals in the solution phase: multi-monomer growth and self-catalysis

The presence of mixed products and impurities, which always confuse researchers, are common during synthesizing nanomaterials. Even though many studies have been conducted with an objective to control the synthesis of nanomaterials, very few studies have investigated a mechanism to control the composition of nanomaterials. Various products include UO₃·H₂O, U₃O₈, UO₂, and U₄O₉ were produced by simply adjusting the pH with ammonia. The morphology of UO₂ and U₃O₈ are tunable. In this study, we suggest two mechanisms that can be used to control the nanomaterial composition. Various experiments have been conducted to understand the mechanism that controls the composition of nanomaterials. We indicate that a multi-monomer growth model can be used to control the uranium oxide composition. We have developed a new oxidation-reduction system using acetone, and this system is capable of controlling both the morphology and composition of uranium oxide micro/nanomaterials. Further, the presence of the self-catalysis mechanism can be used to regulate processes that control the monomer transformation. Thus, the results of this study can be applied to help in the construction of mixed-valence metal oxides.

Photodegradation of Organometal Hybrid Perovskite Nanocrystals: Clarifying the Role of Oxygen by Single-Dot Photoluminescence

Photostability has been a major issue for perovskite materials. Understanding the photodegradation mechanism and suppressing it are of central importance for applications. By investigating single-dot photoluminescence spectra and the lifetime of MAPbX₃ (MA = CH₃NH₃⁺, X = Br, I) nanocrystals with quantum confinement under different conditions, we identified two separate pathways in the photodegradation process. The first is the oxygen-assisted light-induced etching process (photochemistry). The second is the light-driven slow charge-trapping process (photophysics), taking place even in oxygen-free environment. We clarified the role of oxygen in the photodegradation process and show how the photoinduced etching can be successfully suppressed by OSTE polymer, preventing an oxygen-assisted reaction.

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.

One-Step Synthesis of Ultrasmall and Ultrabright Organosilica Nanodots with 100% Photoluminescence Quantum Yield: Long-Term Lysosome Imaging in Living, Fixed, and Permeabilized Cells

Water-dispersible nanomaterials with superbright photoluminescence (PL) emissions and narrow PL bandwidths are urgently desired for various imaging applications. Herein, for the first time, we prepared ultrasmall organosilica nanodots (OSiNDs) with an average size of ∼2.0 nm and ∼100% green-emitting PL quantum efficiency via a one-step hydrothermal treatment of two commercial reagents (a silane molecule and rose bengal). In particular, the structural reorganization and halide loss of rose bengal during the hydrothermal treatment contribute to the ultrahigh quantum yield and low phototoxicity of OSiNDs. Owing to their low pH-induced precipitation/aggregation property, the as-prepared OSiNDs can be used as excellent lysosomal trackers with many advantages: (1) They have superior lysosomal targeting ability with a Pearson's coefficient of 0.98; (2) The lysosomal monitoring time of OSiNDs is up to 48 h, which is much longer than those of commercial lysosomal trackers (<2 h); (3) They do not disturb the pH environment of lysosomes and can be used to visualize lysosomes in living, fixed, and permeabilized cells; (4) They exhibit intrinsic lysosomal tracking ability without the introduction of lysosome-targeting ligands (such as morpholine) and superior photostability; (5) The easy, cost-effective, and scalable synthetic method further ensures that these OSiNDs can be readily used as exceptional lysosomal trackers. We expect that the ultrasmall OSiNDs with superior fluorescence properties and easily modifiable surfaces could be applied as fluorescent nanoprobes, light-emitting diode phosphor, and anticounterfeiting material, which should be able to promote the preparation and application of silicon-containing nanomaterials.

Nearly suppressed photoluminescence blinking of small-sized, blue–green–orange–red emitting single CdSe-based core/gradient alloy shell/shell quantum dots: correlation between truncation time and photoluminescence quantum yield

Nearly suppressed PL blinking of small sized CdSe based CGASS QDs.

Biexciton and trion energy transfer from CdSe/CdS giant nanocrystals to Si substrates

Observation of energy transfer (ET) from multiexcitonic (MX) complexes in nanocrystal quantum dots (NQDs) has been severely restricted due to efficient nonradiative Auger recombination leading to very low MX emission quantum yields. Here we employed "giant" CdSe/CdS NQDs with suppressed Auger recombination to study ET of biexcitons (BX) and charged excitons (trions) into Si substrate. Photoluminescence (PL) measurements of (sub)monolayers of gNQDs controllably assembled on various interacting surfaces and augmented by single gNQD's imaging reveal appearance of BX spectral signatures and progressive acceleration of PL lifetimes of all excitonic species on Si substrates. From statistical analysis of a large number of PL lifetime traces, representative exciton, trion and BX ET efficiencies are measured as ∼75%, 55% and 45% respectively. Detailed analysis of the MX's radiative rates demonstrate the crucial role of the radiative (waveguide) ET in maintaining high overall transfer efficiency despite the prevalent Auger recombination. Our observations point towards practical utilization of MX-bearing nanocrystals in future optoelectronics architectures.

Deciphering Charging Status, Absolute Quantum Efficiency, and Absorption Cross Section of Multicarrier States in Single Colloidal Quantum Dots

Upon photo- or electrical-excitation, colloidal quantum dots (QDs) are often found in multicarrier states due to multiphoton absorption, photocharging, or imbalanced carrier injection of the QDs. While many of these multicarrier states are observed in single-dot spectroscopy, their properties are not well studied due to random charging/discharging, emission intensity intermittency, and uncontrolled surface defects of single QDs. Here we report in situ deciphering of the charging status, precisely assessing the absorption cross section, and determining the absolute emission quantum yield of monoexciton and biexciton states for neutral, positively charged, and negatively charged single core/shell CdSe/CdS QDs. We uncover very different photon statistics of the three charge states in single QDs and unambiguously identify their charge signs together with the information on their photoluminescence decay dynamics. We then show their distinct photoluminescence saturation behaviors and evaluate the absolute values of absorption cross sections and quantum efficiencies of monoexcitons and biexcitons. We demonstrate that the addition of an extra hole or electron in a QD not only changes its emission properties but also varies its absorption cross section.

Role of Nonradiative Defects and Environmental Oxygen on Exciton Recombination Processes in CsPbBr3 Perovskite Nanocrystals

Lead halide perovskite nanocrystals (NCs) are emerging as optically active materials for solution-processed optoelectronic devices. Despite the technological relevance of tracing rational guidelines for optimizing their performances and stability beyond their intrinsic resilience to structural imperfections, no in-depth study of the role of selective carrier trapping and environmental conditions on their exciton dynamics has been reported to date. Here we conduct spectro-electrochemical (SEC) experiments, side-by-side to oxygen sensing measurements on CsPbBr3 NCs for the first time. We show that the application of EC potentials controls the emission intensity by altering the occupancy of defect states without degrading the NCs. Reductive potentials lead to strong (60%) emission quenching by trapping of photogenerated holes, whereas the concomitant suppression of electron trapping is nearly inconsequential to the emission efficiency. Consistently, oxidizing conditions result in minor (5%) brightening due to suppressed hole trapping, confirming that electron traps play a minor role in nonradiative decay. This behavior is rationalized through a model that links the occupancy of trap sites with the position of the NC Fermi level controlled by the EC potential. Photoluminescence measurements in controlled atmosphere reveal strong quenching by collisional interactions with O2, which is in contrast to the photobrightening effect observed in films and single crystals. This indicates that O2 acts as a scavenger of photoexcited electrons without mediation by structural defects and, together with the asymmetrical SEC response, suggests that electron-rich defects are likely less abundant in nanostructured perovskites than in the bulk, leading to an emission response dominated by direct interaction with the environment.