Universal Chiral-Plasmon-Induced Upward and Downward Transfer of Circular Dichroism to Achiral Molecules.

Electromagnetic chirality transfer represents an effective means of the nanoscale manipulation of optical chirality. While most of the previous reports have exclusively focused on the circular dichroism (CD) transfer from UV-responsive chiral molecules toward visible-resonant achiral colloidal nanoparticles, here we demonstrate a reverse process in which plasmonic chirality can be transferred to achiral molecules, either upward from visible to UV or downward from visible to near infrared (NIR). By hybridizing achiral UV- or NIR-responsive dye molecules with chiral metal nanoparticles in solution, we observe a chiral-plasmon-induced CD (CPICD) signal at the intrinsically achiral molecular absorption bands. Full-wave electromagnetic modeling reveals that both near-field Coulomb interaction and far-field radiative coupling contribute to the observed CPICD, indicating that the mechanism considered here is universal for different material systems and types of optical resonances. Our study provides a set of design guidelines for broadband nanophotonic chiral sensing from the UV to NIR spectral regime.

Trap-Mediated Sensitization Governs Near-Infrared Emission from Yb3+-Doped Mixed-Halide CsPbClxBr3-x Perovskite Nanocrystals.

Understanding the photosensitization mechanisms in Yb3+-doped perovskite nanocrystals is crucial for developing their anticipated photonic applications. Here, we address this question by investigating near-infrared photoluminescence of Yb3+-doped mixed-halide CsPbClxBr3-x nanocrystals as a function of temperature and revealing its strong dependence on the stoichiometry of the host perovskite matrix. To explain the observed experimental trends, we developed a theoretical model in which energy transfer from the perovskite matrix to Yb3+ ions occurs through intermediate trap states situated beneath the conduction band of the host. The developed model provides an excellent agreement with experimental results and is further validated through the measurements of emission saturation at high excitation powers and near-infrared photoluminescence quantum yield as a function of the anion composition. Our findings establish trap-mediated energy transfer as a dominant photosensitization mechanism in Yb3+-doped CsPbClxBr3-x nanocrystals and open up new ways of engineering their optical properties for light-emitting and light-harvesting applications.

Optical Properties Prediction for Red and Near-Infrared Emitting Carbon Dots Using Machine Learning.

Functional nanostructures build up a basis for the future materials and devices, providing a wide variety of functionalities, a possibility of designing bio-compatible nanoprobes, etc. However, development of new nanostructured materials via trial-and-error approach is obviously limited by laborious efforts on their syntheses, and the cost of materials and manpower. This is one of the reasons for an increasing interest in design and development of novel materials with required properties assisted by machine learning approaches. Here, the dataset on synthetic parameters and optical properties of one important class of light-emitting nanomaterials - carbon dots are collected, processed, and analyzed with optical transitions in the red and near-infrared spectral ranges. A model for prediction of spectral characteristics of these carbon dots based on multiple linear regression is established and verified by comparison of the predicted and experimentally observed optical properties of carbon dots synthesized in three different laboratories. Based on the analysis, the open-source code is provided to be used by researchers for the prediction of optical properties of carbon dots and their synthetic procedures.

Ultraviolet Circularly Polarized Luminescence in Chiral Perovskite Nanoplatelet-Molecular Hybrids: Direct Binding Versus Efficient Triplet Energy Transfer.

The development of ultraviolet circularly polarized light (UVCPL) sources has the potential to benefit plenty of practical applications but remains a challenge due to limitations in available material systems and a limited understanding of the excited state chirality transfer. Herein, by constructing hybrid structures of the chiral perovskite CsPbBr3 nanoplatelets and organic molecules, excited state chirality transfer is achieved, either via direct binding or triplet energy transfer, leading to efficient UVCPL emission. The underlying photophysical mechanisms of these two scenarios are clarified by comprehensive optical studies. Intriguingly, UVCPL realized via the triple energy transfer, followed by the triplet-triplet annihilation upconversion processes, demonstrates a 50-fold enhanced dissymmetry factor glum. Furthermore, stereoselective photopolymerization of diacetylene monomer is demonstrated by using such efficient UVCPL. This study provides both novel insights and a practical approach for realizing UVCPL, which can also be extended to other material systems and spectral regions, such as visible and near-infrared.

Red-Emitting CsPbI3/ZnSe Colloidal Nanoheterostructures with Enhanced Optical Properties and Stability.

Producing heterostructures of cesium lead halide perovskites and metal-chalcogenides in the form of colloidal nanocrystals can improve their optical features and stability, and also govern the recombination of charge carriers. Herein, the synthesis of red-emitting CsPbI3/ZnSe nanoheterostructures is reported via an in situ hot injection method, which provides the crystallization conditions for both components, subsequently leading to heteroepitaxial growth. Steady-state absorption and photoluminescence studies alongside X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy analysis evidence on a type-I band alignment for CsPbI3/ZnSe nanoheterostructures, which exhibit photoluminescence quantum yield of 96% due to the effective passivation of surface defects, and an enhancement in carrier lifetime. Furthermore, the heterostructure growth of ZnSe domains leads to significant improvement in the stability of the CsPbI3 nanocrystals under ambient conditions and against thermal and UV irradiation stress.

Fine-Tuning Crystal Structures of Lead Bromide Perovskite Nanocrystals through Trace Cadmium(II) Doping for Efficient Color-Saturated Green LEDs.

Decreasing perovskite nanocrystal size increases radiative recombination due to the quantum confinement effect, but also increases the Auger recombination rate which leads to carrier imbalance in the emitting layers of electroluminescent devices. Here, we overcome this trade-off by increasing the exciton effective mass without affecting the size, which is realized through the trace Cd2+ doping of formamidinium lead bromide perovskite nanocrystals. We observe an ~2.7 times increase in the exciton binding energy benefiting from a slight distortion of the [BX6]4- octahedra caused by doping in the case of that the Auger recombination rate is almost unchanged. As a result, bright color-saturated green emitting perovskite nanocrystals with a photoluminescence quantum yield of 96 % are obtained. Cd2+ doping also shifts up the energy levels of the nanocrystals, relative to the Fermi level so that heavily n-doped emitters convert into only slightly n-doped ones; this boosts the charge injection efficiency of the corresponding light-emitting diodes. The light-emitting devices based on those nanocrystals reached a high external quantum efficiency of 29.4 % corresponding to a current efficiency of 123 cd A-1, and showed dramatically improved device lifetime, with a narrow bandwidth of 22 nm and Commission Internationale de I'Eclairage coordinates of (0.20, 0.76) for color-saturated green emission for the electroluminescence peak centered at 534 nm, thus being fully compliant with the latest standard for wide color gamut displays.

Exploring CsPbX3 (X = Cl, Br, I) Perovskite Nanocrystals in Amorphous Oxide Glasses: Innovations in Fabrication and Applications.

Metal halide perovskites with excellent optical and electronic properties have become a trending material in the current research. However, their limited stability under ambient conditions degrades quality and threatens their potential commercialization as optoelectronic devices. Various approaches are adopted to improve the stability of perovskite nanocrystals (PeNC) while maintaining their advantageous optical properties, particularly strong luminescence. Among different possible improvement strategies, encapsulation of PeNCs within the amorphous glass matrices of inorganic oxides has drawn widespread attention because it ensures high resistance against chemical corrosion and high temperature, thus enhancing their chemical, thermal, and mechanical stability with improved light-emission characteristics. In this article, two types of materials, namely all-inorganic metal halide PeNCs and amorphous oxide glasses are briefly introduced, and then the methods are reviewed to fabricate and improve the quality of PeNC@glass composites. These methods are classified into three universal categories: compositional modification, structural modification, and dual encapsulation. In the final part of this review paper, examples of applications of PeNCs@glass composites in light-emitting devices and displays, data storage and anti-counterfeiting, lasing, photodetectors and X-ray detectors, photocatalysis, optical filters, solar concentrators, and batteries are provided.

Assignment of Core and Surface States in Multicolor-Emissive Carbon Dots.

It is important to reveal the luminescence mechanisms of carbon dots (CDs). Herein, CDs with two types of optical centers are synthesized from citric acid in formamide by a solvothermal method, and show high photoluminescence quantum yield reaching 42%. Their green/yellow emission exhibits pronounced vibrational structure and high resistance toward photobleaching, while broad red photoluminescence is sensitive to solvents, temperature, and UV-IR. Under UV-IR, the red emission is gradually bleached due to the photoinduced dehydration of the deprotonated surface of CDs in dimethyl sulfoxide, while this process is hindered in water. From the analysis of steady-state and time-resolved photoluminescence and transient absorption data together with density functional theory calculations, the green/ yellow emission is assigned to conjugated sp2 -domains (core state) similar to organic dye derivatives stacked within disk-shaped CDs; and the broad red emission-to oxygen-containing groups bound to sp2 -domains (surface state), whereas energy transfer from the core to the surface state can happen.

Core/Shell ZnO/ZnS Nanoparticle Electron Transport Layers Enable Efficient All-Solution-Processed Perovskite Light-Emitting Diodes.

Solution-processed perovskite-based light-emitting diodes (PeLEDs) are promising candidates for low-cost, large-area displays, while severe deterioration of the perovskite light-emitting layer occurs during deposition of electron transport layers from solution in an issue. Herein, core/shell ZnO/ZnS nanoparticles as a solution-processed electron transport layer in PeLED based on quasi-2D PEA2 Csn-1 Pbn Br3n+1 (PEA = phenylethylammonium) perovskite are employed. The deposition of ZnS shell mitigates trap states on ZnO core by anchoring sulfur to oxygen vacancies, and at the same time removes residual hydroxyl groups, which helps to suppress the interfacial trap-assisted non-radiative recombination and the deprotonation reaction between the perovskite layer and ZnO. The core/shell ZnO/ZnS nanoparticles show comparably high electron mobility to pristine ZnO nanoparticles, combined with the reduced energy barrier between the electron transport layer and the perovskite layer, improving the charge injection balance in PeLEDs. As a result, the optimized PeLEDs employing core/shell ZnO/ZnS nanoparticles as a solution-processed electron transport layer exhibit high peak luminance reaching 32 400 cd m-2 , external quantum efficiency of 10.3%, and 20-fold extended longevity as compared to the devices utilizing ZnO nanoparticles, which represents one of the highest overall performances for solution-processed PeLEDs.

White Light Afterglow in Carbon Dots Achieved via Synergy between the Room-Temperature Phosphorescence and the Delayed Fluorescence.

Carbon dot (CD) based long-lived afterglow emission materials have attracted attention in recent years, but demonstration of white-light room-temperature afterglow remains challenging, due to the difficulty of simultaneous generation of multiple long-lived excited states with distinct chromatic emission. In this work, a white-light room-temperature long-lived afterglow emission from a CD powder with a high efficiency of 5.8% and Commission International de l'Eclairage (CIE) coordinates of (0.396, 0.409) is realized. The afterglow of the CDs originates from a synergy between the phosphorescence of the carbon core and the delayed fluorescence associated with the surface CN moieties, which is accomplished by matching the singlet state of the surface groups of the CDs with the long-lived triplet state of the carbon core, resulting in an efficient energy transfer. It is demonstrated how the long-lived afterglow emission of CDs can be utilized for fabrication of white light emitting devices and in anticounterfeiting applications.

A Flexible Plasmonic-Membrane-Enhanced Broadband Tin-Based Perovskite Photodetector.

Lead-free perovskite quantum dots (QDs) have been widely investigated for optoelectronic devices because of their excellent electrical and optical properties. However, optoelectronic devices based on such lead-free perovskites still have much lower performance than those made of Pb-based counterparts. Herein, we developed a lead-free photodetector with an enhanced broadband spectral response ranging from 300 to 630 nm. By balancing plasmonic near-field enhancement and surface energy quenching through precisely controlling the thickness of Al2O3 spacer between the CsSnBr3 QDs and silver nanoparticle membrane, the photodetector with 5 nm thick Al2O3 experiences a maximum photocurrent enhancement of 6.5-fold at 410 nm, with a responsivity of 62.3 mA/W and detectivity of 4.27 × 1011 Jones. Moreover, its photocurrent shows a negligible decrease after 100 cycles of bending, which is ascribed to the tension-offset induced by the self-assembled nanoparticle membrane. The proposed plasmonic membrane enhancement provides a great potential for high-performance perovskite optoelectronic devices.

Multiexcitonic Emission in Zero-Dimensional Cs2ZrCl6:Sb3+ Perovskite Crystals.

Metal halide perovskites are highly attractive for lighting applications, but the multiexcitonic emission processes in these crystals are largely unexplored. This study presents an investigation of Sb3+-doped Cs2ZrCl6 perovskite crystals that display double luminescence due to the intrinsic host self-trapped excitons (denoted as host STEs) and dopant-induced extrinsic self-trapped excitons (denoted as dopant STEs), respectively. Steady-state and transient-state spectroscopy reveal that the host and dopant STEs can be independently charged at specific energies. Density functional theory calculations confirm that the multiexcitonic emission stems from minimal interactions between the host and dopant STEs in the zero-dimensional crystal lattice. By selective excitation of different STEs through precise control of excitation wavelength, we further demonstrate dynamic color tuning in the Cs2ZrCl6:Sb3+ crystals. The color kinetic feature offers exciting opportunities for constructing multicolor light-emitting devices and encrypting multilevel optical codes.

Optical Properties of Carbon Dots in the Deep-Red to Near-Infrared Region Are Attractive for Biomedical Applications.

Carbon dots (CDs) represent a recently emerged class of luminescent materials with a great potential for biomedical theranostics, and there are a lot of efforts to shift their absorption and emission toward deep-red (DR) to near-infrared (NIR) region falling in the biological transparency window. This review offers comprehensive insights into the synthesis strategies aimed to achieve this goal, and the current approaches of modulating the optical properties of CDs over the DR to NIR region. The underlying mechanisms of their absorption, photoluminescence, and chemiluminescence, as well as the related photophysical processes of photothermal conversion and formation of reactive oxygen species are considered. The already available biomedical applications of CDs, such as in the photoacoustic imaging and photothermal therapy, photodynamic therapy, and their use as bioimaging agents and drug carriers are then shortly summarized.

Bright CsPbI3 Perovskite Quantum Dot Light-Emitting Diodes with Top-Emitting Structure and a Low Efficiency Roll-Off Realized by Applying Zirconium Acetylacetonate Surface Modification.

Zirconium acetylacetonate used as a co-precursor in the synthesis of CsPbI3 quantum dots (QDs) increased their photoluminescence quantum efficiency to values over 90%. The top-emitting device structure on a Si substrate with high thermal conductivity (to better dissipate Joule heat generated at high current density) was designed to improve the light extraction efficiency making use of a strong microcavity resonance between the bottom and top electrodes. As a result of these improvements, light-emitting diodes (LEDs) utilizing Zr-modified CsPbI3 QDs with an electroluminescence at 686 nm showed external quantum efficiency (EQE) of 13.7% at a current density of 108 mA cm-2, which was combined with low efficiency roll-off (maintaining an EQE of 12.5% at a high current density of 500 mA cm-2) and a high luminance of 14 725 cd m-2, and the stability of the devices being repeatedly lit (cycled on and off at high drive current density) has been greatly enhanced.

Stable, Strongly Emitting Cesium Lead Bromide Perovskite Nanorods with High Optical Gain Enabled by an Intermediate Monomer Reservoir Synthetic Strategy.

One-dimensional (1D) semiconductor nanorods are important for numerous applications ranging from optics and electronics to biology, yet the direct synthesis of high-quality metal halide perovskite nanorods remains a challenge. Here, we develop an intermediate monomer reservoir synthetic strategy to realize the controllable growth of uniform and low-defect CsPbBr3 perovskite nanorods. Intermediates composed of CsPb2Br5 and Cs3In2Br9 are obtained through the substitution of Pb2+ with In3+ cations in the template of CsPbBr3 nanocubes and act as a precursor reservoir to gradually release monomers, ensuring both the slow growth rate and low defects of nanorods. We have used branched tris(diethylamino)phosphine as a ligand, which not only has unequal binding energies with different crystal faces to promote the orientation growth but also provides strong steric hindrance to shield the nanorods in solution. Because of minor amount of defects and an effective ligand passivation, in addition to significantly enhanced stability, the perovskite nanorods show a high photoluminescence quantum yield of up to 90% and exhibit a net mode gain of 980 cm-1, the latter being a record value among all the perovskite materials. An extremely low amplified spontaneous emission threshold of 7.5 µJ cm-2 is obtained under excitation by a nanosecond laser, which is comparable to that obtained using femtosecond lasers in other recent studies.

Zn-Alloyed CsPbI3 Nanocrystals for Highly Efficient Perovskite Light-Emitting Devices.

We alloyed Zn2+ into CsPbI3 perovskite nanocrystals by partial substitution of Pb2+ with Zn2+, which does not change their crystalline phase. The resulting alloyed CsPb0.64Zn0.36I3 nanocrystals exhibited an improved, close-to-unity photoluminescence quantum yield of 98.5% due to the increased radiative decay rate and the decreased non-radiative decay rate. They also showed an enhanced stability, which correlated with improved effective Goldschmidt tolerance factors, by the incorporation of Zn2+ ions with a smaller radius than the Pb2+ ions. Simultaneously, the nanocrystals switched from n-type (for CsPbI3) to nearly ambipolar for the alloyed nanoparticles. The hole injection barrier of electroluminescent LEDs was effectively eliminated by using alloyed CsPb0.64Zn0.36I3 nanocrystals, and a high peak external quantum efficiency of 15.1% has been achieved.

Synthesis of Anisotropic ZnSe Nanorods with Zinc Blende Crystal Structure.

Compared with the well-explored cadmium-based one-dimensional nanorods (NRs), it is still a challenge to produce heavy-metal-free II-VI semiconductor analogues with a controlled size, shape, and crystal structure. Herein, a synthetic strategy towards ZnSe NRs with a zinc blende crystal structure is presented, where use of the anisotropic nuclei produced via a high-temperature selenium injection favors anisotropic growth. Elongated ZnSe NRs were produced from anisotropic ZnSe nuclei, while spherical ZnSe nanocrystals were obtained starting from isotropic nuclei. The different free energy at (111) and (220) planes in anisotropic ZnSe nuclei induces the anisotropic growth of (111) plane for ZnSe NRs. Proper choice of the capping ligand (1-dodecanethiol) has an important implication for the formation of anisotropic ZnSe nuclei and also allows the control of the diameter of the final ZnSe NRs by limiting the growth of the (220) crystal plane of anisotropic ZnSe nuclei.

Two-Step Oxidation Synthesis of Sulfur with a Red Aggregation-Induced Emission.

Sulfur is not normally considered a light-emitting material, even though there have been reports of a dim luminescence of this compound in the blue-to-green spectral region. Now, it is shown how to make red-emissive sulfur by a two-step oxidation approach using elemental sulfur and Na2 S as starting materials, with a high photoluminescence quantum yield of 7.2 %. Polysulfide is formed first and is partially transformed into Na2 S2 O3 in the first step, and then turns back to elemental S in the second step. The elevated temperature and relatively oxygen-deficient environment during the second step transforms Na2 S2 O3 into Na2 SO3 incorporated with oxygen vacancies, thus resulting in the formation of a solid-state powder consisting of elemental S embedded in Na2 SO3 . It shows aggregation-induced emission properties, attributed to the influence of oxygen vacancies on the emission dynamics of sulfur by providing additional lower energy states that facilitate the radiative relaxation of excitons.

Synthesis and Optical Properties of Cubic Chalcopyrite/Hexagonal Wurtzite Core/Shell Copper Indium Sulfide Nanocrystals.

We report a synthetic method to produce heavy-metal-free heterophase core/shell CuInS2 nanocrystals, which comprise differently sized cores with a cubic chalcopyrite (CP) structure overgrown with hexagonal wurtzite (WZ) shells. For a given core size, CP/WZ core/shell CuInS2 nanocrystals experience shifts in both absorption and photoluminescence spectra toward the near-infrared with increasing shell thickness. Studies of the photoluminescence dynamics of these heterophase CuInS2 nanocrystals reveals behavior similar to that of the typical type II nanostructures constructed from different semiconductor materials of the same crystal structure. The interface between the CP core and the WZ shell has an important effect on the photoluminescence quantum yield.

Oxalic Acid Enabled Emission Enhancement and Continuous Extraction of Chloride from Cesium Lead Chloride/Bromide Perovskite Nanocrystals.

All-inorganic cesium lead halide perovskite nanocrystals (NCs) have demonstrated excellent optical properties and an encouraging potential for optoelectronic applications; however, mixed-halide perovskites, especially CsPb(Cl/Br)3 NCs, still show lower photoluminescence quantum yields (PL QY) than the corresponding single-halide materials. Herein, anhydrous oxalic acid is used to post-treat CsPb(Cl/Br)3 NCs in order to initially remove surface defects and halide vacancies, and thus, to improve their PL QY from 11% to 89% for the emission of 451 nm. Furthermore, due to the continuous chelating reaction with the oxalate ion, chloride anions from the mixed-halide CsPb(Cl/Br)3 perovskite NCs could be extracted, and green emitting CsPbBr3 NCs with PL QY of 85% at 511 nm emission are obtained. Besides being useful to improve the emission of CsPb(Cl/Br)3 NCs, the oxalic acid treatment strategy introduced here provides a further tool to adjust the distribution of halide anions in mixed-halide perovskites without using any halide additives.