Oxygen Stabilizes Photoluminescence of CdSe/CdS Core/Shell Quantum Dots via Deionization

Weakly Confined Semiconductor Nanocrystals Excel in Photochemical and Optoelectronic Properties: Evidence from Single-Dot Studies

Single-molecule spectroscopy offers state-resolved measurements on charge-transfer reactions of single semiconductor nanocrystals, leading to the discovery of up to six single-charge transfer reactions with seven transient states for single CdSe/CdS core/shell nanocrystals with water (or oxygen) as the hole (or electron) acceptors. Kinetic rates of three photoinduced single-hole transfer reactions decrease significantly upon increasing the number of excess electrons in a nanocrystal, mainly due to efficient Auger nonradiative recombination of the charged single excitons. Conversely, the kinetic rates of three single-electron transfer reactions of an unexcited nanocrystal increase proportionally to the number of excess electrons in it. Results here reveal that charge-transfer reactions of nanocrystals, at the center of nearly all their functions, could only be deciphered at a state-resolved level on a single nanocrystal. Size-dependent studies validate the weakly confined semiconductor nanocrystals, instead of strongly confined ones (quantum dots), as optimal candidates for photochemical and optoelectronic applications.

A closer look at the effects of oxygen on the photoluminescence properties of CdSe/CdS quantum dots

We present a detailed study on the effects of oxygen on the photoluminescence properties of CdSe/CdS quantum dots (QDs). We investigated the role of oxygen by performing confocal measurements on thin films as well as on single particles while rapidly exchanging the gaseous environment between oxygen and an inert gas atmosphere. We found that the deionization of negatively charged particles by oxygen depends on both the excitation power and the shell thickness of the QDs. For QDs with thin shells, which exhibit strong photoluminescence blinking, we observed that the presence of oxygen affects both band-edge carrier blinking and hot-carrier blinking.

Ultrasensitive Fluorescence Detection of Ascorbic Acid Using Silver Ion-Modulated High-Quality CdSe/CdS/ZnS Quantum Dots

Improving the sensitivity of the fluorescence method for the detection of bioactive molecules is crucial in biochemical analysis. In this work, an ultrasensitive sensing strategy was constructed for the detection of ascorbic acid (AA) using high-quality 3-mercaptopropionic acid-capped CdSe/CdS/ZnS quantum dots (MPA-CdSe/CdS/ZnS QDs) as the fluorescent probe. The prepared water-soluble QDs exhibited a high photoluminescence quantum yield (PL QY) of up to 96%. Further, the fluorescence intensity of the QDs was intensively quenched through the dynamic quenching of Ag+ ions due to an efficient photoinduced electron transfer progress. While the existence of AA before adding Ag+ ions, Ag+ ions were reduced. Thus, the interaction of the QDs and Ag+ ions was destroyed, which led to the fluorescence distinct recovery. The detection limit of AA could be as low as 0.2 nM using this sensing system. Additionally, most relevant small molecules and physiological ions had no influence on the analysis of AA. Satisfactory results were obtained in orange beverages, showing its great potential as a meaningful platform for highly sensitive and selective AA sensing for clinical analysis.

Fluctuations in the Photoluminescence Excitation Spectra of Individual Semiconductor Nanocrystals

Most single quantum emitters display non-steady emission properties. Models that explain this effect have primarily relied on photoluminescence measurements that reveal variations in intensity, wavelength, and excited-state lifetime. While photoluminescence excitation spectroscopy could provide complementary information, existing experimental methods cannot collect spectra before individual emitters change in intensity (blink) or wavelength (spectrally diffuse). Here, we present an experimental approach that circumvents such issues, allowing the collection of excitation spectra from individual emitters. Using rapid modulation of the excitation wavelength, we collect and classify excitation spectra from individual CdSe/CdS/ZnS core/shell/shell quantum dots. The spectra, along with simultaneous time-correlated single-photon counting, reveal two separate emission-reduction mechanisms caused by charging and trapping, respectively. During bright emission periods, we also observe a correlation between emission red-shifts and the increased oscillator strength of higher excited states. Quantum-mechanical modeling indicates that diffusion of charges in the vicinity of an emitter polarizes the exciton and transfers the oscillator strength to higher-energy transitions.

Quantum Dot Fluorescent Imaging: Using Atomic Structure Correlation Studies to Improve Photophysical Properties

Efforts to study intricate, higher-order cellular functions have called for fluorescence imaging under physiologically relevant conditions such as tissue systems in simulated native buffers. This endeavor has presented novel challenges for fluorescent probes initially designed for use in simple buffers and monolayer cell culture. Among current fluorescent probes, semiconductor nanocrystals, or quantum dots (QDs), offer superior photophysical properties that are the products of their nanoscale architectures and chemical formulations. While their high brightness and photostability are ideal for these biological environments, even state of the art QDs can struggle under certain physiological conditions. A recent method correlating electron microscopy ultrastructure with single-QD fluorescence has begun to highlight subtle structural defects in QDs once believed to have no significant impact on photoluminescence (PL). Specific defects, such as exposed core facets, have been shown to quench QD PL in physiologically accurate conditions. For QD-based imaging in complex cellular systems to be fully realized, mechanistic insight and structural optimization of size and PL should be established. Insight from single QD resolution atomic structure and photophysical correlative studies provides a direct course to synthetically tune QDs to match these challenging environments.

Unraveling Mechanisms of Highly Efficient Yet Stable Electrochemiluminescence from Quantum Dots

With CdSe/CdS/ZnS core/shell/shell quantum dots (QDs) as the model system, time- and potential-resolved spectroelectrochemical measurements are successfully applied for studying the general mechanisms and kinetics of electrochemiluminescence (ECL) generation. The rate constant of electron injection from the cathode into a QD to form a negatively charged QD (QD-) increases monotonically from -0.88 V to -1.2 V (vs Ag/AgCl). Mainly due to the deep LUMO of the QDs, the resulting QD- as the key intermediate for ECL generation is structurally stable and possesses very slow spontaneous deionization channels. The latter (the main non-ECL channels) are usually 3-4 orders of magnitude slower than the rate constant of the successive hole injection from an active co-reactant into a QD-. The kinetic studies quantify the internal ECL quantum yield of ideal QD ECL emitters to be nearly identical to that of photoluminescence, which is near unity for the current system. Identification of the key intermediate, discovery of the related elementary steps, and determination of all rate constants not only establish a general framework for understanding ECL generation but also offer basic design rules for ECL emitters.

Anomalous efficiency elevation of quantum-dot light-emitting diodes induced by operational degradation

Quantum-dot light-emitting diodes promise a new generation of high-performance and solution-processed electroluminescent light sources. Understanding the operational degradation mechanisms of quantum-dot light-emitting diodes is crucial for their practical applications. Here, we show that quantum-dot light-emitting diodes may exhibit an anomalous degradation pattern characterized by a continuous increase in electroluminescent efficiency upon electrical stressing, which deviates from the typical decrease in electroluminescent efficiency observed in other light-emitting diodes. Various in-situ/operando characterizations were performed to investigate the evolutions of charge dynamics during the efficiency elevation, and the alterations in electric potential landscapes in the active devices. Furthermore, we carried out selective peel-off-and-rebuild experiments and depth-profiling analyses to pinpoint the critical degradation site and reveal the underlying microscopic mechanism. The results indicate that the operation-induced efficiency increase results from the degradation of electron-injection capability at the electron-transport layer/cathode interface, which in turn leads to gradually improved charge balance. Our work provides new insights into the degradation of red quantum-dot light-emitting diodes and has far-reaching implications for the design of charge-injection interfaces in solution-processed light-emitting diodes.

EPR spin trapping of nucleophilic and radical reactions at colloidal metal chalcogenide quantum dot surfaces

The participation of the surfaces of colloidal semiconductor nanocrystal quantum dots (QDs) in QD-mediated photocatalytic reactions is an important factor that distinguishes QDs from other photosensitizers (e.g. transition metal complexes or organic dyes). Here, we probe nucleophilic and radical reactivity of surface sulfides and selenides of metal chalcogenide (CdSe, CdS, ZnSe, and PbS) QDs using chemical reactions and NMR spectroscopy. Additionally, the high sensitivity of EPR spectroscopy is adapted to study these surface-centered reactions through the use of spin traps like 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) under photoexcitation and thermal conditions. We demonstrate that DMPO likely adds to CdSe QD surfaces under thermal conditions by a nucleophilic mechanism in which the surface chalcogenides add to the double bond, followed by further oxidation of the surface-bound product. In contrast, CdS QDs more readily form surface sulfur-centered radicals that can perform reactions including alkene isomerization. These results indicate that QD surfaces should be an important consideration for the design of photocatalysis beyond simply tuning QD semiconductor band gaps.

Band-Edge Energy Levels of Dynamic Excitons in Cube-Shaped CdSe/CdS Core/Shell Nanocrystals

An electron-hole pair in a cube-shaped CdSe/CdS core/shell nanocrystal exists in the form of dynamic excitons across the strongly and weakly confined regimes under ambient temperatures. Photochemical doping is applied to distinguish the band-edge electron and hole levels, confirming an effective mass model with universal constants. Reduction of the optical bandgap upon epitaxy of the CdS shells is caused by lowering the band-edge electron level and barely affecting the band-edge hole level. Similar shifts of the electron levels, yet retaining the hole levels, can switch the order in energy of the three lowest-energy transitions. Thermal distribution of 1-4 electrons among the two thermally accessible electron levels follows number-counting statistics, instead of Fermi-Dirac distribution.

Quantum Dot Biomimetic for SARS-CoV-2 to Interrogate Blood-Brain Barrier Damage Relevant to NeuroCOVID Brain Inflammation

Despite limited evidence for infection of SARS-CoV-2 in the central nervous system, cognitive impairment is a common complication reported in "recovered" COVID-19 patients. Identification of the origins of these neurological impairments is essential to inform therapeutic designs against them. However, such studies are limited, in part, by the current status of high-fidelity probes to visually investigate the effects of SARS-CoV-2 on the system of blood vessels and nerve cells in the brain, called the neurovascular unit. Here, we report that nanocrystal quantum dot micelles decorated with spike protein (COVID-QDs) are able to interrogate neurological damage due to SARS-CoV-2. In a transwell co-culture model of the neurovascular unit, exposure of brain endothelial cells to COVID-QDs elicited an inflammatory response in neurons and astrocytes without direct interaction with the COVID-QDs. These results provide compelling evidence of an inflammatory response without direct exposure to SARS-CoV-2-like nanoparticles. Additionally, we found that pretreatment with a neuro-protective molecule prevented endothelial cell damage resulting in substantial neurological protection. These results will accelerate studies into the mechanisms by which SARS-CoV-2 mediates neurologic dysfunction.

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.

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

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

Deep Blue and Highly Emissive ZnS-Passivated InP QDs: Facile Synthesis, Characterization, and Deciphering of Their Ultrafast-to-Slow Photodynamics

InP-based quantum dots (QDs) are an environment-friendly alternative to their heavy metal-ion-based counterparts. Herein we report a simple procedure for synthesizing blue emissive InP QDs using oleic acid and oleylamine as surface ligands, yielding ultrasmall QDs with average sizes of 1.74 and 1.81 nm, respectively. Consecutive thin coating with ZnS increased the size of these QDs to 4.11 and 4.15 nm, respectively, alongside a significant enhancement of their emission intensities centered at ∼410 nm and ∼430 nm, respectively. Pure phase synthesis of these deep-blue emissive QDs is confirmed by powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM). Armed with femtosecond to millisecond time-resolved spectroscopic techniques, we decipher the energy pathways, reflecting the effect of successive ZnS passivation on the charge carrier (electrons and holes) dynamics in the deep-blue emissive InP, InP/ZnS, and InP/ZnS/ZnS QDs. Successive coating of the InP QDs increases the intraband relaxation times from 200 to 700 fs and the lifetime of the hot electrons from 2 to 8 ps. The lifetime of the cold holes also increase from 1 to 4 ps, and remarkably, the Auger recombination escalates from 15 to 165 ps. The coating also drastically decreases the quenching by the molecular oxygen of the trapped charge carriers at the surfaces of the QDs. Our results provide clues to push further the emission of InP QDs into more energetically spectral regions and to increase the fluorescence quantum yield, targeting the construction of efficient UV-emissive light-emitting devices (LEDs).

Spectroelectrochemistry of CdSe/CdxZn1-xS Nanoplatelets

We report an unexpected enhancement of photoluminescence (PL) in CdSe-based core/shell nanoplatelets (NPLs) upon electrochemical hole injection. Moderate hole doping densities induce an enhancement of more than 50% in PL intensity. This is accompanied by a narrowing and blue-shift of the PL spectrum. Simultaneous, time-resolved PL experiments reveal a slower luminescence decay. Such hole-induced PL brightening in NPLs is in stark contrast to the usual observation of PL quenching of CdSe-based quantum dots following hole injection. We propose that hole injection removes surface traps responsible for the formation of negative trions, thereby blocking nonradiative Auger processes. Continuous photoexcitation causes the enhanced PL intensity to decrease back to its initial level, indicating that photocharging is a key step leading to loss of PL luminescence during normal aging. Modulating the potential can be used to reversibly enhance or quench the PL, which enables electro-optical switching.

CdTe quantum dot-based self-supporting films with enhanced stability for flexible light-emitting devices

The enhancement of photoluminescence (PL) stability of colloidal quantum dots (CQDs) is of great significance in light-emitting devices. In this work, the PL stability of CdTe CQDs under time storage, strong light irradiation, acid and alkali corrosion and low temperature freezing is analyzed, and the PL quenching mechanism in a harsh environment is analyzed. Furthermore, the PL stability is extremely improved by core-shell coating, film deposition and polymer encapsulation. This solves the problem of rapid dropping of the PL intensity at the initial illumination stage and improves the corrosion resistance in an acidic environment and long-term storage stability of film devices. CQD polymer films have an interesting phenomenon of fluorescence enhancement under illumination due to the light-soaking effect. Biocompatible coating and encapsulation materials expand the application of CQD devices in the field of biological tissue imaging and sensing. Through the PL regulation of CQD solutions and the simple superposition of self-supporting films, a panchromatic light-emitting device with broadband adjustable chromaticity is realized. The solid-state stable whispering-gallery-mode (WGM) laser is realized by monodisperse SiO₂ microspheres embedded in the film. This work is of great significance for the application of CQDs in flexible light-emitting devices.

Binary temporary photo-response of ZnSe:Mn/ZnS quantum dots for visible time-domain anti-counterfeiting

The development of multi-level anti-counterfeiting techniques is of great significance for economics and security issues, particularly the newly emerged temporal-domain techniques based on lifetime coding. However, the intricate reading methods required to obtain temporal-level information are inevitably cumbersome and expensive, which greatly limits the practical applications of these techniques. Herein, we report a novel, unclonable time-domain anti-counterfeiting strategy for the first time, which is achieved using photo-responsive ZnSe:Mn/ZnS quantum dots (QDs) with dynamic luminescence and can be authenticated by the naked eye. Through introducing electron traps and constructing cascade electron channels in the QDs, the binary temporary photo-response is tailored and manifested as distinctive response rates between the band-edge and Mn ⁴T₁-⁶A₁ transition emissions. Impressively, the generated photo-response is instantaneous, is capable of delayed recovery, and can be visibly detected under UV irradiation. The prospective use of colorless, nontoxic aqueous-phase ZnSe:Mn/ZnS QDs provides a new idea and important guidance for developing the next generation of multi-level anti-counterfeiting techniques without the need for complex time-gated decoding instrumentation.

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.

Quenching Efficiency of Quantum Dots Conjugated to Lipid Bilayers on Graphene Oxide Evaluated by Fluorescence Single Particle Tracking

A single particle observation of quantum dots (QDs) was performed on lipid bilayers formed on graphene oxide (GO). The long-range fluorescence quenching of GO has been applied to biosensing for various biomolecules. We demonstrated the single particle observation of a QD on supported lipid bilayers in this study, aiming to detect the quenching efficiency of lipid and protein molecules in a lipid bilayer by fluorescence single particle tacking (SPT). A single lipid bilayer or double lipid bilayers were formed on GO flakes deposited on a thermally oxidized silicon substrate by the vesicle fusion method. The QDs were conjugated on the lipid bilayers, and single particle images of the QDs were obtained under the quenching effect of GO. The quenching efficiency of a single QD was evaluated from the fluorescence intensities on the regions with and without GO. The quenching efficiency reflecting the layer numbers of the lipid bilayers was obtained.

Nonblinking Colloidal Quantum Dots via Efficient Multiexciton Emission

Nonblinking colloidal quantum dots (QDs) are significant to their applications as single-photon sources or light-emitting materials. Herein, a simple heat-up method was developed to synthesize high-qualityWZ-CdSe/CdS core-shell colloidal QDs, which achieved a near-unity photoluminescence quantum yield (PLQY). It was found that the blinking behavior of such QDs was completely suppressed at high excitation intensities, and ultra-stable PL emission was observed. For this reason, a systematic investigation was conducted, revealing that the complete blinking suppression was attributed mainly to the efficient multiexciton emission at high excitation intensities. Such high-quality QDs with nonblinking behaviors and nearly ideal PL properties at high excitation intensities have massive potential applications in various robust conditions, including QD display screens, single-particle tracks, and single-photon sources.

High-efficiency visible-light photocatalytic H2O2 production using CdSe-based core/shell quantum dots

Quantum dots are demonstrated as photocatalysts for high-efficiency photocatalytic production of H2O2 in a designed oil/water two-phase system.

Construction of Photoelectrochemical DNA Biosensors Based on TiO2@Carbon Dots@Black Phosphorous Quantum Dots

In this work, carbon dots (CDs) and black phosphorus quantum dots (BPQDs) were used to decorate titanium dioxide to enhance the photoelectrochemical (PEC) properties of the nanocomposites (TiO₂@CDs@BPQDs), and the modified nanocomposites were used to sensitively detect DNA. We used the hydrothermal method and citric acid as a raw material to prepare CDs with good dispersion and strong fluorescence properties. BPQDs with a uniform particle size were prepared from black phosphorus crystals. The nanocomposites were characterized by fluorescence spectroscopy, UV-Vis absorption spectroscopy, Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM). The preparation method of the working electrode was explored, the detection conditions were optimized, and the sensitive detection of target DNA was achieved. The results demonstrate that CDs and BPQDs with good optical properties were successfully prepared, and they were successfully combined with TiO₂ to improve the PEC performance of TiO₂@CDs@BPQDs. The TiO₂-based PEC DNA detection method was constructed with a detection limit of 8.39 nM. The constructed detection method has many advantages, including good sensitivity, a wide detection range, and good specificity. This work provides a promising PEC strategy for the detection of other biomolecules.

II-VI core/shell quantum dots and doping with transition metal ions as a means of tuning the magnetoelectronic properties of CdS/ZnS core/shell QDs: A DFT study

This paper examines the alterations in the properties of II-VI Quantum Dots (QDs) when these are coated with a shell made of another material of the same family and investigates the structural, electronic and magnetic properties of doped CdS/ZnS core/shell QDs. The core/shell QDs have been constructed by building the shell over the bare core QD and it is found that this construction of a shell over the bare QD can bring about dramatic changes in its optical properties. On changing the shell by varying either the cation or the anion, substantial variations are brought about in the band gap and electrophilicity. The trend of Fermi energies is more negative for core/shell QDs than for the QDs without a shell, and the value is almost the same for core/shell QDs with the same core. Swapping of the core and the shell materials brings greater stability in the case of shells of the wider band gap materials. Binding energy data demonstrates that the CdS/ZnS, CdSe/ZnSe, CdSe/CdS core/shell systems are more stable than ZnS/CdS, ZnSe/CdSe, CdS/CdSe core/shell systems, respectively. An augmentation in the properties is found on doping the QD with transition metal ions. The binding energies are found to be functions of the kind of dopant as well as the spin multiplicity and account for the stability of one spin state over the other at a specific site of the QD. The most fascinating property that plays a decisive role in the extant work is the introduction of magnetism in core/shell QDs as a result of the entry of unpaired electrons within the CdS/ZnS QDs on doping with transition metal ions. The deviation of the observed magnetic moments from the expected values increases as the dopant is varied from Mn²⁺ to Fe²⁺ to Co²⁺ to Ni²⁺ to Cu²⁺. Hirshfeld charge analysis shows that the doped ion accepts negative charge from the sulfide ions in the core, with the smallest charge transfer seen in the case of Hg²⁺ ions. As we move from Mn²⁺ to Hg²⁺, the trend followed for the Hirshfeld charges indicates that the overall charge on the core is lower and that on the shell is higher for all the doped cases in comparison to the undoped CdS/ZnS core/shell QD. The band gap values reveal that the Fe²⁺ doped CdS/ZnS core/shell structures have the smallest band gaps. Hence, we expect that this paper will help researchers to develop a strategy to produce QDs of the anticipated properties for various applications, and transition metal ions can be successfully employed for modification of various magnetoelectronic properties of the host semiconductor for future applications in nanotechnology.

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.

Optical-Gain-based Sensing Using Inorganic-Ligand-Passivated Colloidal Quantum Dots

Thanks to their extremely large surface-to-volume ratio, colloidal quantum dots are potential high-performance sensing materials. However, previous sensing works using their spontaneous emission suffer from low sensitivities. The absence of an amplification process and the presence of the steric hindrance of long-chain organic ligands are two possible causations. Herein we propose that these two issues can be circumvented by using the amplified spontaneous emission of colloidal quantum dots capped by short-chain inorganic ligands. To exemplify this concept, we performed humidity sensing and observed a ∼31 times enhancement in sensitivity. Meanwhile, we found that the amplified spontaneous emission threshold power was reduced by 34% in a high humidity environment. On the basis of our transient absorption measurements, we attribute these observations to the mitigation of ultrafast subpicosecond trapping processes, which are enabled by the absorption of water molecules.

Exciton-Related Raman Scattering, Interband Absorption and Photoluminescence in Colloidal CdSe/CdS Core/Shell Quantum Dots Ensemble

By using the numerical discretization method within the effective-mass approximation, we have theoretically investigated the exciton-related Raman scattering, interband absorption and photoluminescence in colloidal CdSe/CdS core/shell quantum dots ensemble. The interband optical absorption and photoluminescence spectra have been revealed for CdSe/CdS quantum dots, taking into account the size dispersion of the ensemble. Numerical calculation of the differential cross section has been presented for the exciton-related Stokes–Raman scattering in CdSe/CdS quantum dots ensemble with different mean sizes.

Nanoscale Encapsulation of Perovskite Nanocrystal Luminescent Films via Plasma-Enhanced SiO2 Atomic Layer Deposition

Photoluminescence perovskite nanocrystals (NCs) have shown significant potential in optoelectronic applications in view of their narrow band emission with high photoluminescence quantum yields and color tunability. The main obstacle for practical applications is to obtain high durability against an external environment. In this work, a low temperature (50 °C) plasma-enhanced atomic layer deposition (PE-ALD) protection strategy was developed to stabilize CsPbBr₃ NCs. Silica was employed as the encapsulation layer because of its excellent light transmission performance and water corrosion resistance. The growth mechanism of inorganic SiO₂ via PE-ALD was investigated in detail. The Si precursor bis(diethylamino)silane (BDEAS) reacted with the hydroxyl groups (-OH) and thereby initiated the subsequent silica growth while having minimal influence to the organic ligands and did not cause PL quenching. Subsequently, O₂ plasma with high reactivity was used to oxidize the amine ligands of the BDEAS precursor while did not etch the NCs. The obtained CsPbBr₃ NCs/SiO₂ film exhibited exceptional stability in water, light, and heat as compared to the pristine NC film. Based on this method, a white light-emitting diode with improved operational stability was successfully fabricated, which exhibited a wide color gamut (∼126% of the National Television Standard Committee). Our work successfully demonstrates an efficient protection scheme via the PE-ALD method, which extends the applied range of other materials for stabilization of perovskite NCs through this approach.

Quantum Dots for Display Applications

This article offers a materials-chemistry perspective for colloidal quantum dots (QDs) in the field of display, including QD-enhanced liquid-crystal-display (QD-LCD) and QD-based light-emitting-diodes (QLEDs) display. The rapid successes of QDs for display in the past five years are not accidental but have a deep root in both maturity of their synthetic chemistry and their unique chemical, optical, and optoelectronic properties. This article intends to discuss the natural match of QD emitters for display and chemical means to eventually bring about their full potential.

Giant Enhancement of Fluorescence Emission by Fluorination of Porous Graphene with High Defect Density and Subsequent Application as Fe3+ Ion Sensors

Defect-mediated nonradiative recombination in traditional semiconductors, such as porous graphene, tremendously lowers the fluorescence emission, thus greatly restricting their applications in more extensive fields. Here, we report that the fluorescence emission of porous graphene with a high defect density has a giant enhancement (about two orders of magnitude) by a direct and simple fluorination strategy, showing a fine defect-tolerance characteristic. Meanwhile, the corresponding fluorocarbon bonds with excellent thermostability (over 500 °C in N₂ even air) also bring about good stability. The photophysical origins during the whole photoluminescence evolution are further investigated. In the excitation process, the coexistence of fluorine and aromatic regions in fluorinated porous graphene (FPG) contributes to producing a new electronic band gap structure to match the maximum excitation wavelength, then numerous excitons generate, which is a precondition for strong fluorescence emission. In the emission process, weak electron-phonon interactions, large rigidity, and constrained electron at the defects in FPG greatly reduce nonradiative recombination loss. Moreover, fluorine at the defects also reduces interlayer interactions among FPG nanosheets and resists the influence of absorbed impurities, thereby further restricting nonradiative recombination pathway. Highly fluorescent FPG has been utilized as a fascinating tool to achieve sensitive and naked-eye detection of Fe³⁺ ions with a high selectivity. The fluorescence quenching efficiency reaches 24% even with an ultralow concentration of Fe³⁺ (0.06 μM), and that increases to 84% when the concentration of Fe³⁺ is 396 μM.

In Situ Growth of 3D/2D (CsPbBr3/CsPb2Br5) Perovskite Heterojunctions toward Optoelectronic Devices

Two-dimensional (2D) CsPb₂Br₅ exhibits intriguing functions in enhancing the performance of optoelectronic devices in terms of environmental stability and luminescence properties when composited with other perovskites in different dimensionalities. We built a type I three-dimensional (3D) CsPbBr₃/2D CsPb₂Br₅ heterojunction through phase transition where CsPbBr₃ quantum dots in situ grew into 2D CsPb₂Br₅. A thorough growth mechanism study in combination with excited state dynamic investigations via femtosecond spectroscopy and first-principles calculations revealed that the type I hierarchy enhanced the stability of the heterojunction and spurred its luminous quantum yield by prolonging the lifetime of photogenerated carriers. Mixing the heterojunction with other phosphors yielded white-light-emitting diodes with a color rendering index of 94%. The work thus not only offered one new avenue for building heterojunctions by using the "soft crystal" nature of perovskites but also disentangled the enhanced luminescence mechanism of the heterojunction that can be harnessed for promising applications in the luminescence and display fields.