Magnetic polaron on dangling-bond spins in CdSe colloidal nanocrystals

Coherent phenomena and dynamics of lead halide perovskite nanocrystals for quantum information technologies

Solution-processed colloidal nanocrystals of lead halide perovskites have been intensively investigated in recent years in the context of optoelectronic devices, during which time their quantum properties have also begun to attract attention. Their unmatched ease of synthetic tunability and unique structural, optical and electronic properties, in conjunction with the confinement of carriers in three dimensions, have motivated studies on observing and controlling coherent light-matter interaction in these materials for quantum information technologies. This Review outlines the recent efforts and achievements in this direction. Particularly notable examples are the observation of coherent single-photon emission, evidence for superfluorescence and the realization of room-temperature coherent spin manipulation for ensemble samples, which have not been achieved for prototypical colloidal CdSe nanocrystals that have been under investigation for decades. This Review aims to highlight these results, point out the challenges ahead towards realistic applications and bring together the efforts of multidisciplinary communities in this nascent field.

Induced Huge Optical Activity in Nanoplatelet Superlattice

Chiral superstructures with unique chiroptical properties that are not inherent in the individual units are essential in applications such as 3D displays, spintronic devices, biomedical sensors, and beyond. Generally, chiral superstructures are obtained by tedious procedures exploring various physical and chemical forces to break spatial symmetry during the self-assembly of discrete nanoparticles. In contrast, we herein present a simple and efficient approach to chiral superstructures by intercalating small chiral molecules into preformed achiral superstructures. As a model system, the chiral CdSe nanoplatelet (NPL) superlattice exhibits a giant and tunable optical activity with the highest g-factor reaching 3.09 × 10-2 to the excitonic transition of the NPL superlattice, nearly 2 orders of magnitude higher than that of the corresponding separated chiral NPLs. The theoretical analysis reveals that the chiral deformation in the NPL superlattice induced by the chiral perturbation of the small chiral molecules is critical to the observed huge optical activity. We anticipate that this research lays a foundation for understanding and applying chiral inorganic nanosystems.

Methods for Obtaining One Single Larmor Frequency, Either v1 or v2, in the Coherent Spin Dynamics of Colloidal Quantum Dots

The coexistence of two spin components with different Larmor frequencies in colloidal CdSe and CdS quantum dots (QDs) leads to the entanglement of spin signals, complicating the analysis of dynamic processes and hampering practical applications. Here, we explored several methods, including varying the types of hole acceptors, air or anaerobic atmosphere and laser repetition rates, in order to facilitate the obtention of one single Larmor frequency in the coherent spin dynamics using time-resolved ellipticity spectroscopy at room temperature. In an air or nitrogen atmosphere, manipulating the photocharging processes by applying different types of hole acceptors, e.g., Li[Et₃BH] and 1-octanethiol (OT), can lead to pure spin components with one single Larmor frequency. For as-grown QDs, low laser repetition rates favor the generation of the higher Larmor frequency spin component individually, while the lower Larmor frequency spin component can be enhanced by increasing the laser repetition rates. We hope that the explored methods can inspire further investigations of spin dynamics and related photophysical processes in colloidal nanostructures.

Enabling metallic behaviour in two-dimensional superlattice of semiconductor colloidal quantum dots

Semiconducting colloidal quantum dots and their assemblies exhibit superior optical properties owing to the quantum confinement effect. Thus, they are attracting tremendous interest from fundamental research to commercial applications. However, the electrical conducting properties remain detrimental predominantly due to the orientational disorder of quantum dots in the assembly. Here we report high conductivity and the consequent metallic behaviour of semiconducting colloidal quantum dots of lead sulphide. Precise facet orientation control to forming highly-ordered quasi-2-dimensional epitaxially-connected quantum dot superlattices is vital for high conductivity. The intrinsically high mobility over 10 cm² V^-1 s^-1 and temperature-independent behaviour proved the high potential of semiconductor quantum dots for electrical conducting properties. Furthermore, the continuously tunable subband filling will enable quantum dot superlattices to be a future platform for emerging physical properties investigations, such as strongly correlated and topological states, as demonstrated in the moiré superlattices of twisted bilayer graphene.

High-Frequency EPR and ENDOR Spectroscopy of Mn2+ Ions in CdSe/CdMnS Nanoplatelets

Semiconductor colloidal nanoplatelets based of CdSe have excellent optical properties. Their magneto-optical and spin-dependent properties can be greatly modified by implementing magnetic Mn²⁺ ions, using concepts well established for diluted magnetic semiconductors. A variety of magnetic resonance techniques based on high-frequency (94 GHz) electron paramagnetic resonance in continuous wave and pulsed mode were used to get detailed information on the spin structure and spin dynamics of Mn²⁺ ions in core/shell CdSe/(Cd,Mn)S nanoplatelets. We observed two sets of resonances assigned to the Mn²⁺ ions inside the shell and at the nanoplatelet surface. The surface Mn demonstrates a considerably longer spin dynamics than the inner Mn due to lower amount of surrounding Mn²⁺ ions. The interaction between surface Mn²⁺ ions and ¹H nuclei belonging to oleic acid ligands is measured by means of electron nuclear double resonance. This allowed us to estimate the distances between the Mn²⁺ ions and ¹H nuclei, which equal to 0.31 ± 0.04, 0.44 ± 0.09, and more than 0.53 nm. This study shows that the Mn²⁺ ions can serve as atomic-size probes for studying the ligand attachment to the nanoplatelet surface.

2D II-VI Semiconductor Nanoplatelets: From Material Synthesis to Optoelectronic Integration

The field of colloidal synthesis of semiconductors emerged 40 years ago and has reached a certain level of maturity thanks to the use of nanocrystals as phosphors in commercial displays. In particular, II-VI semiconductors based on cadmium, zinc, or mercury chalcogenides can now be synthesized with tailored shapes, composition by alloying, and even as nanocrystal heterostructures. Fifteen years ago, II-VI semiconductor nanoplatelets injected new ideas into this field. Indeed, despite the emergence of other promising semiconductors such as halide perovskites or 2D transition metal dichalcogenides, colloidal II-VI semiconductor nanoplatelets remain among the narrowest room-temperature emitters that can be synthesized over a wide spectral range, and they exhibit good material stability over time. Such nanoplatelets are scientifically and technologically interesting because they exhibit optical features and production advantages at the intersection of those expected from colloidal quantum dots and epitaxial quantum wells. In organic solvents, gram-scale syntheses can produce nanoparticles with the same thicknesses and optical properties without inhomogeneous broadening. In such nanoplatelets, quantum confinement is limited to one dimension, defined at the atomic scale, which allows them to be treated as quantum wells. In this review, we discuss the synthetic developments, spectroscopic properties, and applications of such nanoplatelets. Covering growth mechanisms, we explain how a thorough understanding of nanoplatelet growth has enabled the development of nanoplatelets and heterostructured nanoplatelets with multiple emission colors, spatially localized excitations, narrow emission, and high quantum yields over a wide spectral range. Moreover, nanoplatelets, with their large lateral extension and their thin short axis and low dielectric surroundings, can support one or several electron-hole pairs with large exciton binding energies. Thus, we also discuss how the relaxation processes and lifetime of the carriers and excitons are modified in nanoplatelets compared to both spherical quantum dots and epitaxial quantum wells. Finally, we explore how nanoplatelets, with their strong and narrow emission, can be considered as ideal candidates for pure-color light emitting diodes (LEDs), strong gain media for lasers, or for use in luminescent light concentrators.

A Comparative Study of the Band-Edge Exciton Fine Structure in Zinc Blende and Wurtzite CdSe Nanocrystals

In this paper, we studied the role of the crystal structure in spheroidal CdSe nanocrystals on the band-edge exciton fine structure. Ensembles of zinc blende and wurtzite CdSe nanocrystals are investigated experimentally by two optical techniques: fluorescence line narrowing (FLN) and time-resolved photoluminescence. We argue that the zero-phonon line evaluated by the FLN technique gives the ensemble-averaged energy splitting between the lowest bright and dark exciton states, while the activation energy from the temperature-dependent photoluminescence decay is smaller and corresponds to the energy of an acoustic phonon. The energy splittings between the bright and dark exciton states determined using the FLN technique are found to be the same for zinc blende and wurtzite CdSe nanocrystals. Within the effective mass approximation, we develop a theoretical model considering the following factors: (i) influence of the nanocrystal shape on the bright-dark exciton splitting and the oscillator strength of the bright exciton, and (ii) shape dispersion in the ensemble of the nanocrystals. We show that these two factors result in similar calculated zero-phonon lines in zinc blende and wurtzite CdSe nanocrystals. The account of the nanocrystals shape dispersion allows us to evaluate the linewidth of the zero-phonon line.

Electron Spin Coherence in CdSe Nanocrystals in a Glass Matrix

The coherent spin dynamics of electrons in CdSe nanocrystals embedded in a glass matrix with diameters from 3.3 up to 6.1 nm are investigated by time-resolved Faraday ellipticity at room and cryogenic temperatures. Only one Larmor precession frequency is detected, which corresponds to the larger of the two precession frequencies and thus g-factor values found in the typical signal from solution-grown colloidal CdSe nanocrystals. We identify this frequency accordingly as associated with the spin precession of resident electrons localized in the nanocrystals in the vicinity of the surface. We provide a detailed theoretical analysis of the exciton level spin structure in the magnetic field and model the spin dynamics in CdSe nanocrystals of different symmetries. This allows us to exclude the exciton as the origin of the experimentally observed oscillating signal. At a cryogenic temperature of 6 K, an additional nonoscillating component emerges in the spin dynamics. We consider several possible origins of this signal and conclude that it is related to the hole spin polarization.

Thickness-Dependent Dark-Bright Exciton Splitting and Phonon Bottleneck in CsPbBr3-Based Nanoplatelets Revealed via Magneto-Optical Spectroscopy

The optimized exploitation of perovskite nanocrystals and nanoplatelets as highly efficient light sources requires a detailed understanding of the energy spacing within the exciton manifold. Dark exciton states are particularly relevant because they represent a channel that reduces radiative efficiency. Here, we apply large in-plane magnetic fields to brighten optically inactive states of CsPbBr3-based nanoplatelets for the first time. This approach allows us to access the dark states and directly determine the dark-bright splitting, which reaches 22 meV for the thinnest nanoplatelets. The splitting is significantly less for thicker nanoplatelets due to reduced exciton confinement. Additionally, the form of the magneto-PL spectrum suggests that dark and bright state populations are nonthermalized, which is indicative of a phonon bottleneck in the exciton relaxation process.

Exchange Spin Coupling in Optically Excited States

In optically excited states in molecules and materials, coupling between local electron spins plays an important role for their photoemission properties and is interesting for potential applications in quantum information processing. Recently, it was experimentally demonstrated that the photogenerated local spins in donor-acceptor metal complexes can interact with the spin of an attached radical, resulting in a spin-coupling-dependent mixing of excited doublet states, which controls the local spin density distributions on donor, acceptor, and radical subunits in optically excited states. In this work, we propose an energy-difference scheme to evaluate spin coupling in optically excited states, using unrestricted and spin-flip simplified time-dependent density functional theory. We apply it to three platinum complexes which have been studied experimentally to validate our methodology. We find that all computed coupling constants are in excellent agreement with the experimental data. In addition, we show that the spin coupling between donor and acceptor in the optically excited state can be fine-tuned by replacing platinum with palladium and zinc in the structure. Besides the two previously discussed excited doublet states (one bright and one dark), our calculations reveal a third, bright excited doublet state which was not considered previously. This third state possesses the inverse spin polarization on donor and acceptor with respect to the previously studied bright doublet state and is by an order of magnitude brighter, which might be interesting for optically controlling local spin polarizations with potential applications in spin-only information transfer and manipulation of connected qubits.

Nanophotonic Approach to Study Excited-State Dynamics in Semiconductor Nanocrystals

In semiconductor nanocrystals, excited electrons relax through multiple radiative and nonradiative pathways. This complexity complicates characterization of their decay processes with standard time- and temperature-dependent photoluminescence studies. Here, we exploit a simple nanophotonic approach to augment such measurements and to address open questions related to nanocrystal emission. We place nanocrystals at different distances from a gold reflector to affect radiative rates through variations in the local density of optical states. We apply this approach to spherical CdSe-based nanocrystals to probe the radiative efficiency and polarization properties of the lowest dark and bright excitons by analyzing temperature-dependent emission dynamics. For CdSe-based nanoplatelets, we identify the charge-carrier trapping mechanism responsible for strongly delayed emission. Our method, when combined with careful modeling of the influence of the nanophotonic environment on the relaxation dynamics, offers a versatile strategy to disentangle the complex excited-state decay pathways present in fluorescent nanocrystals as well as other emitters.

Enhanced Emission from Bright Excitons in Asymmetrically Strained Colloidal CdSe/CdxZn1-xSe Quantum Dots

Colloidal CdSe quantum dots (QDs) designed with a high degree of asymmetric internal strain have recently been shown to host a number of desirable optical properties including subthermal room-temperature line widths, suppressed spectral diffusion, and high photoluminescence (PL) quantum yields. It remains an open question, however, whether they are well-suited for applications requiring emission of identical single photons. Here we measure the low-temperature PL dynamics and the polarization-resolved fluorescence line narrowing spectra from ensembles of these strained QDs. Our spectroscopy reveals the radiative recombination rates of bright and dark excitons, the relaxation rate between the two, and the energy spectra of the quantized acoustic phonons in the QDs that can contribute to relaxation processes. In comparison to conventional colloidal CdSe/ZnS core/shell QDs, we find that in asymmetrically strained CdSe QDs over six times more light is emitted directly by the bright exciton. These results are therefore encouraging for the prospects of chemically synthesized colloidal QDs as emitters of single indistinguishable photons.

Revealing the Exciton Fine Structure in Lead Halide Perovskite Nanocrystals

Lead-halide perovskite nanocrystals (NCs) are attractive nano-building blocks for photovoltaics and optoelectronic devices as well as quantum light sources. Such developments require a better knowledge of the fundamental electronic and optical properties of the band-edge exciton, whose fine structure has long been debated. In this review, we give an overview of recent magneto-optical spectroscopic studies revealing the entire excitonic fine structure and relaxation mechanisms in these materials, using a single-NC approach to get rid of their inhomogeneities in morphology and crystal structure. We highlight the prominent role of the electron-hole exchange interaction in the order and splitting of the bright triplet and dark singlet exciton sublevels and discuss the effects of size, shape anisotropy and dielectric screening on the fine structure. The spectral and temporal manifestations of thermal mixing between bright and dark excitons allows extracting the specific nature and strength of the exciton–phonon coupling, which provides an explanation for their remarkably bright photoluminescence at low temperature although the ground exciton state is optically inactive. We also decipher the spectroscopic characteristics of other charge complexes whose recombination contributes to photoluminescence. With the rich knowledge gained from these experiments, we provide some perspectives on perovskite NCs as quantum light sources.

Hole-Acceptor-Manipulated Electron Spin Dynamics in CdSe Colloidal Quantum Dots

Electron spin dynamics in CdSe quantum dots with hole acceptors are investigated by time-resolved ellipticity spectroscopy. Two types of hole acceptors, Li[Et₃BH] and 1-octanethiol, result in distinctly different electron spin dynamics. The differences include electron g factors, spin dephasing/relaxation times, and mechanisms. In CdSe quantum dots with Li[Et₃BH], the electron spin dephasing and relaxation are dominated by electron-nuclear hyperfine interactions in zero and weak magnetic fields. In contrast, hyperfine interactions, electron carrier lifetimes, and exchange interactions between electrons and holes or surface dangling bond spins control the electron spin dynamics in CdSe quantum dots with 1-octanethiol. Inhomogeneous dephasing limits the spin coherence time in larger transverse magnetic fields for both hole acceptor cases, but with distinct different g-factor inhomogeneity. These findings manifest that surface conditions play an important role in the spin dynamics and that thereby the surface and its surroundings can be exploited to control the spin in colloidal nanostructures.

Polarized emission of CdSe nanocrystals in magnetic field: the role of phonon-assisted recombination of the dark exciton

The recombination dynamics and spin polarization of excitons in CdSe nanocrystals synthesized in a glass matrix are investigated using polarized photoluminescence in high magnetic fields up to 30 Tesla. The dynamics are accelerated by increasing temperature and magnetic field, confirming the dark exciton nature of low-temperature photoluminescence (PL). The circularly polarized PL in magnetic fields reveals several unusual appearances: (i) a spectral dependence of the polarization degree, (ii) its low saturation value, and (iii) a stronger intensity of the Zeeman component which is higher in energy. The latter feature is the most surprising being in contradiction with the thermal population of the exciton spin sublevels. The same contradiction was previously observed in the ensemble of wet-chemically synthesized CdSe nanocrystals but was not understood. We present a theory which explains all the observed features and shows that the inverted ordering of the circularly polarized PL maxima from the ensemble of nanocrystals is a result of competition between the zero phonon (ZPL) and one optical phonon-assisted (1PL) emission of the dark excitons. The essential aspects of the theoretical model are different polarization properties of the dark exciton emission via ZPL and 1PL recombination channels and the inhomogeneous broadening of the PL spectrum from the ensemble of nanocrystals exceeding the optical phonon energy.

Optical and Electronic Properties of Colloidal CdSe Quantum Rings

Luminescent colloidal CdSe nanorings are a recently developed type of semiconductor structure that have attracted interest due to the potential for rich physics arising from their nontrivial toroidal shape. However, the exciton properties and dynamics of these materials with complex topology are not yet well understood. Here, we use a combination of femtosecond vibrational spectroscopy, temperature-resolved photoluminescence (PL), and single-particle measurements to study these materials. We find that on transformation of CdSe nanoplatelets to nanorings, by perforating the center of platelets, the emission lifetime decreases and the emission spectrum broadens due to ensemble variations in the ring size and thickness. The reduced PL quantum yield of nanorings (∼10%) compared to platelets (∼30%) is attributed to an enhanced coupling between (i) excitons and CdSe LO-phonons at 200 cm^-1 and (ii) negatively charged selenium-rich traps, which give nanorings a high surface charge (∼-50 mV). Population of these weakly emissive trap sites dominates the emission properties with an increased trap emission at low temperatures relative to excitonic emission. Our results provide a detailed picture of the nature of excitons in nanorings and the influence of phonons and surface charge in explaining the broad shape of the PL spectrum and the origin of PL quantum yield losses. Furthermore, they suggest that the excitonic properties of nanorings are not solely a consequence of the toroidal shape but also a result of traps introduced by puncturing the platelet center.

Complex diffusion-based kinetics of photoluminescence in semiconductor nanoplatelets

We present a diffusion-based simulation and theoretical models for explanation of the photoluminescence (PL) emission intensity in semiconductor nanoplatelets. It is shown that the shape of the PL intensity curves can be reproduced by the interplay of recombination, diffusion and trapping of excitons. The emission intensity at short times is purely exponential and is defined by recombination. At long times, it is governed by the release of excitons from surface traps and is characterized by a power-law tail. We show that the crossover from one limit to another is controlled by diffusion properties. This intermediate region exhibits a rich behaviour depending on the value of diffusivity. The proposed approach reproduces all the features of experimental curves measured for different nanoplatelet systems.

Anisotropic Magnetic Resonance in Random Nanocrystal Quantum Dot Ensembles

Magnetic anisotropy critically determines the utility of magnetic nanocrystals (NCs) in new nanomagnetism technologies. Using angular-dependent electron magnetic resonance (EMR), we observe magnetic anisotropy in isotropically arranged NCs of a nonmagnetic material. We show that the shape of the EMR angular variation can be well described by a simple model that considers magnetic dipole-dipole interactions between dipoles randomly located in the NCs, most likely due to surface dangling bonds. The magnetic anisotropy results from the fact that the energy term arising from the magnetic dipole-dipole interactions between all magnetic moments in the system is dominated by only a few dipole pairs, which always have an anisotropic geometric arrangement. Our work shows that magnetic anisotropy may be a general feature of NC systems containing randomly distributed magnetic dipoles.

From low to high-index facets of noble metal nanocrystals: a way forward to enhance the performance of electrochemical CO2 reduction

To date, noble metal nanoparticles, mainly the gold (Au) and silver (Ag) nanoparticles, are the most active and selective heterogeneous catalysts that have revealed a tendency to form CO and directly synthesize syngas as a result of the electrochemical CO2 reduction reaction (CO2RR). The CO2RR activity and selectivity are influenced by a wide range of factors, such as morphology, surface structure, shape, composition, type of electrolyte used, and pH. Most of these issues have been reviewed and evaluated critically. Herein, the CO2RR activity and selectivity to CO formation on the low and high-index facets of Au, Ag, and Pt NCs were evaluated with a greater motive to provide new insights into the field. The author refers to different experimental approaches and the corresponding theoretical methods that have been employed to study the product formation activity and selectivity on low and high-index facet noble metal NCs for the CO2RR. In conclusion, some perspectives have been provided on the future research of the low and high-index facets of noble metal NCs for CO2 reduction.

Surface spin magnetism controls the polarized exciton emission from CdSe nanoplatelets

The surface of nominally diamagnetic colloidal CdSe nanoplatelets can demonstrate paramagnetic behaviour owing to the uncompensated spins of dangling bonds, as we reveal here by optical spectroscopy in high magnetic fields up to 15 T using the exciton spin as a probe of the surface magnetism. The strongly nonlinear magnetic field dependence of the circular polarization of the exciton emission is determined by the magnetization of the dangling-bond spins (DBSs), the exciton spin polarization as well as the spin-dependent recombination of dark excitons. The sign of the exciton-DBS exchange interaction depends on the nanoplatelet growth conditions.

Hyperfine Interactions and Slow Spin Dynamics in Quasi-isotropic InP-based Core/Shell Colloidal Nanocrystals

Colloidal InP core nanocrystals are taking over CdSe-based nanocrystals, notably in optoelectronic applications. Despite their use in commercial devices, such as display screens, the optical properties of InP nanocrystals and especially their relation to the exciton fine structures remain poorly understood. In this work, we show that the ensemble magneto-optical properties of InP-based core/shell nanocrystals investigated in strong magnetic fields up to 30 T are strikingly different from other colloidal nanostructures. Notably, the mixing of the lowest spin-forbidden dark exciton state with the nearest spin-allowed bright state does not occur up to the highest magnetic fields applied. This lack of mixing in an ensemble of nanocrystals suggests an anisotropy tolerance of InP nanocrystals. This striking property allowed us to unveil the slow spin dynamics between Zeeman sublevels (up to 400 ns at 15 T). Furthermore, we show that the unexpected magnetic-field-induced lengthening of the dark exciton lifetime results from the hyperfine interaction between the spin of the electron in the dark exciton with the nuclear magnetic moments. Our results demonstrate the richness of the spin physics in InP quantum dots and stress the large potential of InP nanostructures for spin-based applications.

Origin of Two Larmor Frequencies in the Coherent Spin Dynamics of Colloidal CdSe Quantum Dots Revealed by Controlled Charging

Coherent spin dynamics in colloidal CdSe quantum dots (QDs) typically show two spin components with different Larmor frequencies, whose origin is an open question. We exploit the photocharging approach to identify their origin and find that surface states play a key role in the appearance of the spin signals. By controlling the photocharging with electron or hole acceptors, we show that the specific spin component can be enhanced by the choice of acceptor type. In core/shell CdSe/ZnS QDs, the spin signals are significantly weaker. Our results exclude the neutral exciton as the spin origin and suggest that both Larmor frequencies are related to the coherent spin precession of electrons in photocharged QDs. The lower frequency is due to the electron confined in the middle of the QD, and the higher frequency to the electron additionally localized in the vicinity of the surface.

Effects of electrostatic environment on the electrically triggered production of entangled photon pairs from droplet epitaxial quantum dots

Entangled photon pair generation is a crucial task for development of quantum information based technologies, and production of entangled pairs by biexciton cascade decays in semiconductor quantum dots is so far one of the most advanced techniques to achieve it. However, its scalability toward massive implementation requires further understanding and better tuning mechanisms to suppress the fine structure splitting between polarized exciton states, which persists as a major obstacle for entanglement generation from most quantum dot samples. In this work, the influence of electrostatic environment arising from electrically biased electrodes and/or charged impurities on the fine structure splitting of GaAs/AlGaAs droplet epitaxial quantum dots is studied, by means of numerical simulations considering a realistic quantum dot confining potential and electron-hole exchange interaction within a multiband k · p framework. We find that reduction of the fine structure splitting can be substantially optimized by tilting the field and seeding impurities along the droplet elongation axis. Furthermore, our results provide evidence of how the presence of charged impurities and in-plane bias components, may account for different degrees of splitting manipulation in dots with similar shape, size and growth conditions.

In Situ Visualization of Electrocatalytic Reaction Activity at Quantum Dots for Water Oxidation

Exploring electrocatalytic reactions on the nanomaterial surface can give crucial information for the development of robust catalysts. Here, electrocatalytic reaction activity at single quantum dots (QDs) loaded silica microparticle involved in water oxidation is visualized using electrochemiluminescence (ECL) microscopy. Under positive potential, the active redox centers at QDs induce the generation of hydroperoxide surface intermediates as coreactants to remarkably enhance ECL emission from luminol derivative molecules for imaging. For the first time, in situ visualization of the catalytic activity of water oxidation with QDs catalyst was achieved, supported by a linear relation between ECL intensity and turn over frequency. A very slight diffusion trend attributed to only the luminol species proved in situ capture of hydroperoxide surface intermediates at catalytic active sites of QDs. This work provides tremendous potential in online imaging of electrocatalytic reactions and visual evaluation of catalyst performance.

Polarons Explain Luminescence Behavior of Colloidal Quantum Dots at Low Temperature

Luminescence properties of colloidal quantum dots have found applications in imaging, displays, light-emitting diodes and lasers, and single photon sources. Despite wide interest, several experimental observations in low-temperature photoluminescence of these quantum dots, such as the short lifetime on the scale of microseconds and a zero-longitudinal optical phonon line in spectrum, both attributed to a dark exciton in literature, remain unexplained by existing models. Here we propose a theoretical model including the effect of solid-state environment on luminescence. The model captures both coherent and incoherent interactions of band-edge exciton with phonon modes. Our model predicts formation of dressed states by coupling of the exciton with a confined acoustic phonon mode, and explains the short lifetime and the presence of the zero-longitudinal optical phonon line in the spectrum. Accounting for the interaction of the exciton with bulk phonon modes, the model also explains the experimentally observed temperature-dependence of the photoluminescence decay dynamics and temperature-dependence of the photoluminescence spectrum.

Addressing the exciton fine structure in colloidal nanocrystals: the case of CdSe nanoplatelets

We study the band-edge exciton fine structure and in particular its bright-dark splitting in colloidal semiconductor nanocrystals by four different optical methods based on fluorescence line narrowing and time-resolved measurements at various temperatures down to 2 K. We demonstrate that all these methods provide consistent splitting values and discuss their advances and limitations. Colloidal CdSe nanoplatelets with thicknesses of 3, 4 and 5 monolayers are chosen for experimental demonstrations. The bright-dark splitting of excitons varies from 3.2 to 6.0 meV and is inversely proportional to the nanoplatelet thickness. Good agreement between experimental and theoretically calculated size dependence of the bright-dark exciton splitting is achieved. The recombination rates of the bright and dark excitons and the bright to dark relaxation rate are measured by time-resolved techniques.