The Strain Effects and Interfacial Defects of Large ZnSe/ZnS Core/Shell Nanocrystals.

The shell growth of large ZnSe/ZnS nanocrystals( is of great importance in the pursuit of pure-blue emitters for display applications, however, suffers from the challenges of spectral blue-shifts and reduced photoluminescence quantum yields. In this work, the ZnS shell growth on different-sized ZnSe cores is investigated. By controlling the reactivity of Zn and S precursors, the ZnS shell growth can be tuned from defect-related strain-released to defect-free strained mode, corresponding to the blue- and red-shifts of resultant nanocrystals respectively. The shape of strain-released ZnSe/ZnS nanocrystals can be kept nearly spherical during the shell growth, while the shape of strained nanocrystals evolutes from spherical into island-like after the critical thickness. Furthermore, the strain between ZnSe core and ZnS shell can convert the band alignment from type-I into type-II core/shell structure, resulting in red-shifts and improved quantum yield. By correlating the strain effects with interfacial defects, a strain-released shell growth model is proposed to obtain large ZnSe/ZnS nanocrystals with isotropic shell morphology.

Brilliant quantum dots' photoluminescence from a dual-resonance plasmonic grating.

Semiconductor quantum dots (QDs) have recently caused a stir as a promising and powerful lighting material applied in real-time fluorescence detection, display, and imaging. Photonic nanostructures are well suited for enhancing photoluminescence (PL) due to their ability to tailor the electromagnetic field, which raises both radiative and nonradiative decay rate of QDs nearby. However, several proposed structures with a complicated manufacturing process or low PL enhancement hinder their application and commercialization. Here, we present two kinds of dual-resonance gratings to effectively improve PL enhancement and propose a facile fabrication method based on holographic lithography. A maximum of 220-fold PL enhancement from CdSe/CdS/ZnS QDs are realized on 1D Al-coated photoresist (PR) gratings, where dual resonance bands are excited to simultaneously overlap the absorption and emission bands of QDs, much larger than those of some reported structures. Giant PL enhancement realized by cost-effective method further suggests the potential of better developing the nanostructure to QD-based optical and optoelectronic devices.

Resolving the Emission Transition Dipole Moments of Single Doubly Excited Seeded Nanorods via Heralded Defocused Imaging.

Semiconductor nanocrystal emission polarization is a crucial probe of nanocrystal physics and an essential factor for nanocrystal-based technologies. While the transition dipole moment for the lowest excited state to ground state transition is well characterized, the dipole moment of higher multiexcitonic transitions is inaccessible via most spectroscopy techniques. Here, we realize direct characterization of the doubly excited-state relaxation transition dipole by heralded defocused imaging. Defocused imaging maps the dipole emission pattern onto a fast single-photon avalanche diode detector array, allowing the postselection of photon pairs emitted from the biexciton-exciton emission cascade and resolving the differences in transition dipole moments. Type-I1/2 seeded nanorods exhibit higher anisotropy of the biexciton-to-exciton transition compared to the exciton-to-ground state transition. In contrast, type-II seeded nanorods display a reduction of biexciton emission anisotropy. These findings are rationalized in terms of an interplay between the transient dynamics of the refractive index and the excitonic fine structure.

Higher-Order Photon Correlation as a Tool To Study Exciton Dynamics in Quasi-2D Nanoplatelets.

Colloidal semiconductor nanoplatelets, in which carriers are strongly confined only along one dimension, present fundamentally different excitonic properties than quantum dots, which support strong confinement in all three dimensions. In particular, multiple excitons strongly confined in just one dimension are free to rearrange in the lateral plane, reducing the probability for multibody collisions. Thus, while simultaneous multiple photon emission is typically quenched in quantum dots, in nanoplatelets its probability can be tuned according to size and shape. Here, we focus on analyzing multiexciton dynamics in individual CdSe/CdS nanoplatelets of various sizes through the measurement of second-, third-, and fourth-order photon correlations. For the first time, we can directly probe the dynamics of the two, three, and four exciton states at the single nanocrystal level. Remarkably, although higher orders of correlation vary substantially among the synthesis' products, they strongly correlate with the value of second order antibunching. The scaling of the higher-order moments with the degree of antibunching presents a small yet clear deviation from the accepted model of Auger recombination through binary collisions. Such a deviation suggests that many-body contributions are present already at the level of triexcitons. These findings highlight the benefit of high-order photon correlation spectroscopy as a technique to study multiexciton dynamics in colloidal semiconductor nanocrystals.

Excitation and Emission Transition Dipoles of Type-II Semiconductor Nanorods.

The mechanisms of exciton generation and recombination in semiconductor nanocrystals are crucial to the understanding of their photophysics and for their application in nearly all fields. While many studies have been focused on type-I heterojunction nanocrystals, the photophysics of type-II nanorods, where the hole is located in the core and the electron is located in the shell of the nanorod, remain largely unexplored. In this work, by scanning single nanorods through the focal spot of radially and azimuthally polarized laser beams and by comparing the measured excitation patterns with a theoretical model, we determine the dimensionality of the excitation transition dipole of single type-II nanorods. Additionally, by recording defocused patterns of the emission of the same particles, we measure their emission transition dipoles. The combination of these techniques allows us to unambiguously deduce the dimensionality and orientation of both excitation and emission transition dipoles of single type-II semiconductor nanorods. The results show that in contrast to previously studied quantum emitters, the particles possess a 3D degenerate excitation and a fixed linear emission transition dipole.

One-step synthesis of nail-like Mn-doped CdS/CdBr2 hetero-nanostructures for potential lasing application.

Nanoscale heterostructures, which incorporate two or more materials such as core-shell nanocrystals, core-crown nanoplates, or seeded nanorods, allow better control of the optical, electrical and magnetic properties that are inaccessible in single component nanostructure, yet their variety and controlled growth are still challenging. Here, a nail-like Mn-doped CdS/CdBr2 hetero-nanostructure, which has a hexagonal plate on top of a nanowire, is firstly fabricated by a simple one-step thermal evaporation process. According to the characterization results, its growth mechanism could be obtained, in which the manganese bromide precursor plays a critical role in the formation of such nail morphology. The amplified spontaneous emission of the 'nanonail' is achieved at a low threshold at room temperature, which come from the local and dense exciton scattering due to their interactions excited by fs pulse. These interesting nail-like heterostructures may provide promising templates for constructing high-performance optoelectronic devices.

Brightly luminescent and color-tunable green-violet-emitting halide perovskite CH3NH3PbBr3 colloidal quantum dots: an alternative to lighting and display technology.

Organic-inorganic hybrid perovskite (CH3NH3PbX3, X = Cl, Br, or I) quantum dots have become one of the most promising materials for optoelectronic applications. We controllably synthesized CH3NH3PbBr3 quantum dots with a tunable spectrum with the emission peaks covering the range from green (523.6 nm), blue and eventually to deep violet (409.4 nm), which is wider than that of quantum dots obtained without changing the halide component. The mechanism of the blueshift was investigated. The purified quantum dots have allowed the fabrication of efficient electroluminescence devices having a simple glass/ITO/PEDOT:PSS/TFB/CH3NH3PbBr3 quantum dot/TPBi/LiF/Al structure. CH3NH3PbBr3 quantum dots with 5-30 µL n-octylamine showed an ideal color-saturated green emission with Commission Internationale de l'Eclairage color coordinates of (0.123, 0.744) and a narrow full width at half-maximum of 19-24 nm. The photoluminescence quantum yield was up to 90.2%. In addition, it is also worth noting that the chromaticity coordinates (x, y) of CH3NH3PbBr3 quantum dots with 50-100 µL n-octylamine are (0.300, 0.344), (0.305, 0.314) and (0.323, 0.318) in the white region. All these properties indicate that these MAPbBr3 quantum dots can provide effective data support for the application of white LEDs, and may potentially be used as single-component multicolor-emitting materials, which can be applied to lighting and display technology.

In situ aggregation of ZnSe nanoparticles into supraparticles: shape control and doping effects.

The ability to tune the size, shape, and properties of supraparticles is of great importance for fundamental study as well as their promising applications. We previously developed a method to synthesize monodisperse ZnSe supraparticles via "in situ aggregation" of ZnSe nanoparticles through a simple hot-injection method. In the present work, we show that the "in situ aggregation" strategy can be extended to tune the shapes of ZnSe supraparticles, and introduce novel functional magnetic and luminescence properties. Shape control is manipulated with oleic acid as ligands, which balances the attractive interparticles van der Waals forces and steric repulsive forces from the ligands. With the increase of oleic acid concentration, a morphology change from microspheres to asymmetrical multimer and three-dimensional nanoflowers was observed. "Doping" preformed Fe(3)O(4) nanoparticles into ZnSe supraparticles endow them with magnetic properties. The magnetism of these Fe(3)O(4)@ZnSe supraparticles depends on the dosage of dopant. Doping of preformed CdS nanocrystals was also studied, resulting in emissive hybrid CdS@ZnSe supraparticles with diameters of 50-100 nm. It is noted that the doping of Fe(3)O(4) and CdS nanoparticles show differing morphologies. The differences can be explained by variance in the lattice mismatches which leads to differing potentials for crystal growth.

Direct in situ photolithography of perovskite quantum dots based on photocatalysis of lead bromide complexes.

Photolithography has shown great potential in patterning solution-processed nanomaterials for integration into advanced optoelectronic devices. However, photolithography of perovskite quantum dots (PQDs) has so far been hindered by the incompatibility of perovskite with traditional optical lithography processes where lots of solvents and high-energy ultraviolet (UV) light exposure are required. Herein, we report a direct in situ photolithography technique to pattern PQDs based on the photopolymerization catalyzed by lead bromide complexes. By combining direct photolithography with in situ fabrication of PQDs, this method allows to directly photolithograph perovskite precursors, avoiding the complicated lift-off processes and the destruction of PQDs by solvents or high-energy UV light, as PQDs are produced after lithography exposure. We further demonstrate that the thiol-ene free-radical photopolymerization is catalyzed by lead bromide complexes in the perovskite precursor solution, while no external initiators or catalysts are needed. Using direct in situ photolithography, PQD patterns with high resolution up to 2450 pixels per inch (PPI), excellent fluorescence uniformity, and good stability, are successfully demonstrated. This work opens an avenue for non-destructive direct photolithography of high-efficiency light-emitting PQDs, and potentially expands their application in various integrated optoelectronic devices.

Chiral FA Conjugated CdTe/CdS Quantum Dots for Selective Cancer Ablation.

Inducing apoptosis in cancer cells is considered a potential therapeutic mechanism underlying cancers. Here, chiral folic acid (FA) conjugated Cys-CdTe/CdS quantum dots (QDs) conjugated with a cancer-targeting ligand were fabricated to induce apoptosis in vivo. Ligand-induced chirality mechanism for FA-Cys-CdTe/CdS QDs was discussed, which is verified by density functional theory (DFT) simulation. Interestingly, we found that the circular dichroism (CD) signals of chiral QDs can effectively distinguish breast cancer cells from normal cells, where a sharp decrease in CD signal and absorption intensity can be seen. Notably, chiral FA-Cys-CdTe/CdS QDs showed significant apoptosis-inducing ability after the release of mitochondrial apoptotic factors. Furthermore, in vivo experiments showed that chiral FA-Cys-CdTe/CdS QDs provide an efficient cancer ablation through the apoptosis process with negligible toxicity, demonstrating their great potential utility in targeted anticancer agent for future clinic application.

Temperature Dependence of Excitonic and Biexcitonic Decay Rates in Colloidal Nanoplatelets by Time-Gated Photon Correlation.

Excitons in colloidal semiconductor nanoplatelets (NPLs) are weakly confined in the lateral dimensions. This results in significantly smaller Auger rates and, consequently, larger biexciton quantum yields, when compared to spherical quantum dots (QDs). Here we report a study of the temperature dependence of the biexciton Auger rate in individual CdSe/CdS core-shell NPLs, through the measurement of time-gated second-order photon correlations in the photoluminescence. We also utilize this method to directly estimate the single-exciton radiative rate. We find that whereas the radiative lifetime of NPLs increases with temperature, the Auger lifetime is almost temperature-independent. Our findings suggest that Auger recombination in NPLs is qualitatively similar to that of semiconductor quantum wells. Time-gated photon correlation measurements offer the unique ability to study multiphoton emission events, while excluding effects of competing fast processes, and can provide significant insight into the photophysics of a variety of nanocrystal multiphoton emitters.

Fluorescence and Optical Activity of Chiral CdTe Quantum Dots in Their Interaction with Amino Acids.

Ligand-induced chirality in semiconducting nanocrystals has been the subject of extensive study in the past few years and shows potential applications in optics and biology. Yet, the origin of the chiroptical effect in semiconductor nanoparticles is still not fully understood. Here, we examine the effect of the interaction with amino acids on both the fluorescence and the optical activity of chiral semiconductor quantum dots (QDs). A significant fluorescence enhancement is observed for l/d-Cys-CdTe QDs upon interaction with all the tested amino acids, indicating suppression of nonradiative pathways as well as the passivation of surface trap sites brought via the interaction of the amino group with the CdTe QDs' surface. Heterochiral amino acids are shown to weaken the circular dichroism (CD) signal, which may be attributed to a different binding configuration of cysteine molecules on the QDs' surface. Furthermore, a red shift of both CD and fluorescence signals in l/d-Cys-CdTe QDs is only observed upon adding cysteine, while other tested amino acids do not exhibit such an effect. We speculate that the thiol group induces orbital hybridization of the highest occupied molecular orbital (HOMOs) of cysteine and the valence band of CdTe QDs, leading to the decrease of the energy band gap and a concomitant red shift of CD and fluorescence spectra. This is further verified by density functional theory calculations. Both the experimental and theoretical findings indicate that the addition of ligands that do not "directly" interact with the valence band (VB) of the QD (noncysteine moieties) changes the QD photophysical properties, as it probably modifies the way cysteine is bound to the surface. Hence, we conclude that it is not only the chemistry of the amino acid ligand that affects both CD and PL but also the exact geometry of binding that modifies these properties. Understanding the relationship between the QD's surface and chiral amino acid thus provides an additional perspective on the fundamental origin of induced chiroptical effects in semiconductor nanoparticles, potentially enabling us to optimize the design of chiral semiconductor QDs for chiroptic applications.

Facile synthesis of hollow mesoporous MgO spheres via spray-drying with improved adsorption capacity for Pb(II) and Cd(II).

Spherical-like MgO nanostructures have been synthesized efficiently via spray-drying combined with calcination using magnesium acetate as magnesium source. The products were characterized by means of X-ray powder diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and the specific surface areas were calculated using the Brunauer-Emmett-Teller (BET) method. The obtained spherical-like MgO nanostructures exhibit uniform pore sizes (7.7 nm) and high specific surface areas (180 m2 g-1). The adsorption kinetics and isotherm data agree well with pseudo-second-order model and Langmuir model, indicating the monolayer chemisorption of heavy metal ions. The spherical-like MgO nanostructures exhibited high adsorption performance for Pb(II) and Cd(II) ions, and the maximum adsorption capacities were up to 5214 mg g-1 and 4187 mg g-1, respectively. These values are much higher than those reported MgO-based adsorbents. Moreover, in less than 10 min, Pb(II) and Cd(II) ions in solution can be almost removed, which means that the spherical-like MgO possesses a high adsorption rate. XRD and FTIR analysis revealed the adsorption mechanism of Pb(II) and Cd(II) ions on MgO, which was mainly due to hydroxyl functional groups and ion exchange between Mg and heavy metal ions on the surface of MgO. These favorable performances recommend that the synthesized spherical-like MgO nanostructures would be a potential adsorbent for rapid removal of heavy metal ions from wastewater.

Rapid Voltage Sensing with Single Nanorods via the Quantum Confined Stark Effect.

Properly designed colloidal semiconductor quantum dots (QDs) have already been shown to exhibit high sensitivity to external electric fields via the quantum confined Stark effect (QCSE). Yet, detection of the characteristic spectral shifts associated with the effect of the QCSE has traditionally been painstakingly slow, dramatically limiting the sensitivity of these QD sensors to fast transients. We experimentally demonstrate a new detection scheme designed to achieve shot-noise-limited sensitivity to emission wavelength shifts in QDs, showing feasibility for their use as local electric field sensors on the millisecond time scale. This regime of operation is already potentially suitable for detection of single action potentials in neurons at a high spatial resolution.

Template-Free Synthesis of High-Yield Fe-Doped Cesium Lead Halide Perovskite Ultralong Microwires with Enhanced Two-Photon Absorption.

Doping in perovskite is challenging and competitive due to the inherently fast growth mechanism of perovskite structure. Here, we demonstrate successful synthesis of high-yield Fe-doped cesium lead halide perovskite ultralong microwires (MWs) that have diameters up to ∼5 µm and lengths up to millimeters via an antisolvent vapor-assisted template-free method. Microstructure characterization confirms the uniformly doped Fe in the high-quality crystal perovskite MWs. Significantly, doping the Fe(III) concentration can affect both the MW morphology and photoluminescence (PL). The band edge emission of the MW at variable excitation has been accounted for by the superposition and combination of optical transitions of nearby singlet, triplet, and magnetic polaronic excitons. High-quality two-photon PL emission and the enhanced nonlinear absorption coefficient of Fe-doped MWs have been experimentally demonstrated. This superhigh nonlinear absorption coefficient and high-quality optical properties endow it with promising applications in spin-related optical switching and optical limiting devices.