Radiative Recombination Research Articles

Cathodoluminescence from interlayer excitons in a 2D semiconductor heterobilayer.

Photoluminescence has widely been used to study excitons in semiconducting transition metal dichalcogenide (MX$_2$) monolayers, demonstrating strong light matter interactions and locked spin and valley degrees of freedom. In heterobilayers composed of overlapping monolayers of two different MX$_2$, an interlayer exciton can form, with the hole localised in one layer and the electron in the other. These interlayer excitons are long-lived, field-tunable, and can be trapped by moir'e patterns formed at small twist angles between the layers. Here we demonstrate that emission from radiative recombination of interlayer excitons can be observed by cathodoluminescence from a WSe$_2$/MoSe$_2$ heterobilayer encapsulated in hexagonal boron nitride. The higher spatial resolution of cathodoluminescence, compared to photoluminescence, allows detailed analysis of sample heterogeneity at the 100s of nm lengthscales over which twist angles tend to vary in dry-transfer fabricated heterostructures.

Variation of carrier lifetime with optical excitation power density in micro-LEDs

GaN-based micro-LEDs are applied to visible light communication due to their high modulation bandwidth with reduced chip size. It requires a deep understanding of recombination processes and their impact on the bandwidth, which is mainly determined by the carrier lifetime. We employed confocal time-resolved photoluminescence (TRPL) to characterize the variation of carrier lifetime with optical excitation power density on micro-LEDs. We observed an initial increase followed by a sudden decrease within the power density range of 96.7 kW/cm2 to 546 kW/cm2 on a blue micro-LED with a chip size of 80 µm. We attribute this phenomenon to increased optical excitation power, gradually saturating the defect-dominated non-radiative recombination centers, with radiative recombination processes gradually taking over. We compared the power density at the inflection point for different regions on the sample and the samples with different sizes and sidewall structures. The power density for the lifetime inflection point at the center of the sample is smaller than that at the edge. We also find that the value is smaller for the sample with a chip size of 40 µm which prompts fewer total defects. The power density for sudden lifetime drop on samples with inclined sidewall structures is also smaller than those with vertical sidewall structures. Furthermore, we find that the excitation power density corresponding to the highest luminous efficiency is higher than that corresponding to the sudden drop at the beginning of the lifetime. This opens up possibilities for simultaneously achieving high modulation bandwidth and high efficiency between the two inflection points.

PyArc: A python package for computing absorption and radiative coefficients from first principles

Light absorption and radiative recombination are two critical processes in optoelectronic materials that characterize the energy conversion efficiency. The absorption and radiative coefficients are thus key properties for device optimization and design. Here, we develop a python package named pyArc that allows rigorous computation of absorption and radiative coefficients from first principles. By integrating several interpolation strategies to augment k-point sampling in reciprocal space, our code is accurate yet highly efficient. In addition to evaluation of the coefficients, our code is capable of intuitive analysis of carrier distribution, facilitating a deeper understanding of the microscopic mechanisms underlying the radiative coefficients. Utilizing GaAs as a prototypical example, we demonstrate how to employ our package to compute absorption and radiative coefficients and to investigate the key features in the electronic structure that give rise to these coefficients. Program SummaryProgram title: PyArcCPC Library link to program files:https://doi.org/10.17632/5x9g9bvhcv.1Licensing provisions: MIT licenseProgramming language: Python 3Nature of problem: Light absorption and radiative recombination processes in semiconductors critically impact the energy conversion efficiency of optoelectronic devices. Developing a method to calculate coefficients of the two processes based on first-principles theory is essential, which not only can help to obtain the key properties of those semiconductor materials and guide the device design, but also can unveil the underlying microscopic mechanisms.Solution method: PyArc, written in the Python language, implements first-principles methodologies for the computation of absorption and radiative coefficients of semiconductors based on Fermi's golden rule. This package takes the electronic eigenvalues and dipole matrix elements of a material computed from first-principles codes such as VASP as input. Dense k-point sampling for the Brillouin zone is achieved through efficient interpolation schemes implemented in our code to acquire well converged results. The functionality of cross-sectional visualization of carrier distribution in our code provides intuitive insights into the fundamental mechanism beneath the charge-carrier radiative recombination process.

Regulating ligand length to improve the PL QY and stability of CsPbCl0.9Br2.1 nanocrystals for LEDs and photoelectric applications

Perovskite luminescent materials have been extensively investigated due to their comprehensive applications for full-color display, lighting, and communication. However, the development of blue-emissive perovskites has seriously lagged behind that of green and red counterparts, primarily because of their low photoluminescence quantum yield (PL QY) and poor stability. Here, we prepared blue-emissive CsPbCl0.9Br2.1 perovskite nanocrystals (PeNCs) and enhanced their QY from 61.3 % to 90.4 % by regulating the chain lengths of ligands. Post-treated PeNCs also exhibit good stability, their PL intensity is maintained at about 90 % after storage at ambient conditions for 10 days. The exciton dynamics results obtained from time-resolved fluorescence spectra and transient absorption spectra show that dodecyldimethylammonium bromide (DDAB), featuring double 12-hydrocarbon chains, is more capable of passivating surface defects and inhibiting non-radiative composites than the other double 8- or double 16-carbon chain ligands, which greatly improved the probability and rate of radiative recombination. Subsequently, the mechanism of ligand chain length regulation on the PL properties and stability of PeNCs was analyzed in terms of ligand-NC surface interaction, ligand polarity, hydrophobicity, and spatial effects. Furthermore, the device based on DDAB-CsPbCl0.9Br2.1 demonstrated stable EL and long operation lifetime, indicating its significant potential as an efficient blue emitter for backlighting in display technologies.

Single CsPbBr3 Perovskite Microcrystals: From Microcubes to Microrods with Improved Crystallinity and Green Emission.

All-inorganic perovskite materials are promising in optoelectronics, but their morphology is a key parameter for achieving high device efficiency. We prepared CsPbBr3 perovskite microcrystals with different shapes grown directly on planar substrate by conventional drop casting. We observed the formation of CsPbBr3 microcubes on bare indium tin oxide (ITO)-coated glass. Interestingly, with the same technique, CsPbBr3 microrods were obtained on (3-Aminopropyl) triethoxysilane (APTES)-modified ITO-glass, which we ascribe to the modification of formation kinetics. The obtained microcrystals exhibit an orthorhombic structure. A green photoluminescence (PL) emission is revealed from the CsPbBr3 microrods. Contact angle measurements, Fourier-transform infrared and PL spectroscopies confirmed that APTES linked successfully to the ITO-glass substrate. We propose a qualitative mechanism to explain the anisotropic growth. The microrods exhibited improved PL and a slower PL lifetime compared to the microcubes, likely due to the diminished occurrence of defects. This work demonstrates the importance of the substrate surface to control the growth of perovskite single crystals and to boost the radiative recombination in view of high-performance optoelectronic devices.

Towards Large-Area Black Phosphorous Midwave-IR Optoelectronics

High-efficiency mid-wavelength infrared (mid-IR, 3–5 µm) optoelectronics, including light emitting diodes (LEDs) and photodetectors, are of high demand for emerging applications in spectroscopy, imaging, and gas sensing. Black phosphorus (bP) has emerged as a unique optoelectronic material for mid-IR applications with performances surpassing those of conventional III-V and II-VI semiconductors of similar bandgap. In this talk, I will present recent advancements on understanding and controlling the radiative and non-radiative recombination rates as a function of bP thickness. We observe higher photoluminescence quantum yields in bP as compared to conventional III-V and II-VI semiconductors of similar bandgaps due to the smaller Auger recombination rate. As a result, bP mid-IR LEDs with external quantum efficiency of >4% are reported, outperforming the state-of-the-art in this wavelength range. Furthermore, device encapsulation and packaging technologies are explored with extrapolated half lifetime of ~15,000 hours based on accelerated lifetime measurements. Finally, I will present strategies for large-area device fabrication and processing.

(Invited) III-Nitride Ultraviolet LEDs and Lasers for Applications in Biology and Medicine

Ultraviolet (UV) light emitters with large-energy photons are being developed for emerging biology and medicine applications such as sterilization, water/air purification, and medical treatments, as a replacement for mercury vapor lamps. Solid-state UV light-emitting diodes (LEDs) and lasers based on wide bandgap III-Nitrides offer compact sizes, wavelength tuning, long lifetimes, and sustainability over mercury vapor lamps. While visible LEDs and lasers with longer emission wavelengths find widespread commercial uses for solid-state lighting and display, UV emitters with AlGaN heterostructures struggle with poor external quantum efficiencies of typically less than 10%, which drop further when wavelengths get shorter. Therefore, it is extremely challenging to realize high-efficiency UV emitters despite the demand for compact UV light source, especially for disinfection and sterilization from the pandemic as those high-energy photons can break DNA/RNA bonds and deactivate virus/bacteria. Here, I will discuss our recent developments toward realization of high-efficiency UV LEDs and lasers.To enhance the radiative recombination rate (R sp) from the AlGaN quantum well (QW) active region, we had pursued novel QW design to overcome issues from the existence of valence subbands crossover. Specifically, the arrangement of the three energy-degenerate valence subbands - heavy hole (HH), light hole (LH), and the crystal-field split-off hole (CH) bands for high Al-content and low Al-content AlGaN QWs are completely different. The ordering of the valence bands strongly affects the optical matrix element via the oscillator strength, as well as the direction of the photon emission. Our work has proposed promising solution on the use of delta-QW design to address the valence subbands crossover issue, in order to significantly boost up the R sp. In addition, to enhance the extraction efficiency of UV emitters, we demonstrated the realization and characterization of top-down fabricated, electrically driven UV micropillar array LEDs emitting at 286 nm with a narrow, 9 nm linewidth and output power densities up to 35 mW/cm2. Top-down fabrication allows for formation of perfectly uniform arrays of micropillars with easily tunable diameter and pitch, which cannot be achieved with epitaxially grown nanowires without the use of selective area epitaxy. Dry etch damage induced surface losses were avoided through the use of a two-step etch process combining a Cl dry etch and a hydroxyl-based wet etch to form the micropillar arrays. We also revealed that the unique inverse-taper profile of our micropillars could produce significant enhancements in the extraction efficiency with the increased taper angle. Lastly, our developed III-Nitride wet etch by the use of hydroxyl-based chemicals such as potassium hydroxide (KOH) investigated the effects of temperature, concentration, and the damage recovery aspects of hydroxyl etching of III-Nitride nano and micro structures, leading to important smooth surface profiles for micropillar/nanowire LEDs and optimized mirror facets for ridge-emitting UV lasers.

Chiral Perovskite Heterostructure Films of CsPbBr3 Quantum Dots and 2D Chiral Perovskite with Circularly Polarized Luminescence Performance and Energy Transfer.

This work reports the synthesis of chiral perovskite heterostructure films by combining a two-dimensional (2D) chiral (R-/S-MBA)2PbI4 perovskite with CsPbBr3 quantum dots (QDs). The as-synthesized chiral heterostructure films exhibit obvious circularly polarized luminescence (CPL) properties, even though pure 2D chiral perovskite cannot present photoluminescence. It indicates that the chirality of the excited state of the QDs originates from the 2D chiral perovskite. The circular polarization-resolved transient absorption (TA) spectra further demonstrate that the CPL response of heterostructure films originates from the energy transfer between the chiral perovskite layer and QDs layer and the suppression of spin relaxation, which induces the imbalance of the spin population of excited states in QDs layer. In addition, the photoluminescence (PL), circular dichroism (CD), and CPL spectra of these heterostructure films can be controlled by varying the thickness and component of the chiral perovskite layer, which demonstrates that the anion exchange between chiral perovskite and CsPbBr3 QDs can tune the chemical composition and optoelectronic properties due to the low bonding energy difference between them and decrease the strain within the QDs layer to reduce the radiative recombination lifetime. This work provides guidance for the synthesis of chiral perovskites with a strong CPL response and further provides insight into the origination of CPL.

Tailoring Niox/Perovskite Interface via a Multifunctional Self‐Assembled Molecule for High‐Performance Blue Perovskite Light‐Emitting Diodes

Nickel oxide (NiOx) serves as one of the most promising hole transport materials for perovskite light‐emitting diodes (PeLEDs). However, only moderate PeLED performances have been reported on the pristine NiOx layer due to insufficient hole injection, interfacial exciton quenching, and poor perovskite quality. Herein, a multifunctional molecule of 3‐mercapto‐1‐propanesulfonate (MPS) is demonstrated to successfully tailor the NiOx–perovskite heterogenous interface by addressing the above issues. In detail, the large binding energy between mercapto sulfur and nickel induces preferential self‐assembly of the mercapto group on the NiOx surface, which simultaneously enlarges the NiOx work function by the formation of interfacial dipole and suppresses the trap‐assisted exciton quenching by the passivation of the oxygen vacancies. Meanwhile, the self‐assembled MPS on NiOx also favors high‐quality perovskite films with good morphology, high crystallinity, and reduced defects for efficient carrier radiative recombination. As a result, blue PeLEDs show a remarkable efficiency of 10.4%, representing one of the highest efficiencies for NiOx‐based blue PeLEDs, as well as a very low turn‐on voltage of 2.8 V. Consequently, this work contributes to an efficient approach to tailor the NiOx–perovskite interface for highly efficient blue PeLEDs.

General algorithm for characterization of donor-acceptor pair recombination processes in solid-state materials

Radiative recombination processes can occur in solid-state systems through the pairing of donor and acceptor defects of the lattice. Recently, donor-acceptor pairs (DAP) have been proposed as promising candidates for quantum applications, and their signature has been observed in emerging low-dimensional materials. Therefore, the identification of such processes is gaining interest and requires methods to efficiently and reliably characterize them. Here, we introduce a general algorithm to identify DAP processes starting from the experimental photoluminescence (PL) emission spectrum and basic material parameters, including the lattice structure and dielectric constant. The algorithm recognizes possible DAP transitions from the emission pattern in the spectrum and returns the characteristic energy of the DAP transition and the separation between the donor and acceptor sites. By testing the algorithm on the photoluminescence spectrum of hexagonal boron nitride (hBN), we show that our method is robust against experimental errors and adds new capabilities to the investigation toolbox of semiconductors and their optical properties.

Pressure‐Tuning Localized Excitons Toward Enhanced Emission, Photocurrent Enhancement and Piezochromism in Unconventional ACI‐Type 2D Hybrid Perovskites

Simultaneous enhancement of free excitons (FEs) emission and self‐trapped excitons (STEs) emission remains greatly challenging because of the radiative pathway competition. Here, a significant fluorescence improvement, associated with the radiative recombination of both FEs and STEs is firstly achieved in an unconventional ACI‐type hybrid perovskite, (ACA)(MA)PbI4 (ACA=acetamidinium) crystals with {PbI6} octahedron units, through hydrostatic pressure processing. Note that (ACA)(MA)PbI4 exhibits a 91.5‐fold emission enhancement and considerable piezochromism from green to red in a mild pressure interval of 1 atm to 2.5 GPa. The substantial distortion of both individual halide octahedron and the Pb‐I‐Pb angles between two halide octahedra under high pressure indeed determines the pressure‐tuning localized excitons behavior. Upon higher pressure, photocurrent enhancement is also observed, which is attributed to the promoted electronic connectivity in (ACA)(MA)PbI4. The anisotropic compaction reduces the distance between neighboring organic molecules and {PbI6} octahedra, leading to the enhancement of hydrogen bonding interactions. This work not only offers a deep understanding of the structure‐optical relationships of ACI‐type perovskites, but also presents insights into breaking the limits of luminescent efficiency by pressure‐suppressed nonradiative recombination.

Pressure-Tuning Localized Excitons Toward Enhanced Emission, Photocurrent Enhancement and Piezochromism in Unconventional ACI-Type 2D Hybrid Perovskites.

Simultaneous enhancement of free excitons (FEs) emission and self-trapped excitons (STEs) emission remains greatly challenging because of the radiative pathway competition. Here, a significant fluorescence improvement, associated with the radiative recombination of both FEs and STEs is firstly achieved in an unconventional ACI-type hybrid perovskite, (ACA)(MA)PbI4 (ACA=acetamidinium) crystals with {PbI6} octahedron units, through hydrostatic pressure processing. Note that (ACA)(MA)PbI4 exhibits a 91.5-fold emission enhancement and considerable piezochromism from green to red in a mild pressure interval of 1 atm to 2.5 GPa. The substantial distortion of both individual halide octahedron and the Pb-I-Pb angles between two halide octahedra under high pressure indeed determines the pressure-tuning localized excitons behavior. Upon higher pressure, photocurrent enhancement is also observed, which is attributed to the promoted electronic connectivity in (ACA)(MA)PbI4. The anisotropic compaction reduces the distance between neighboring organic molecules and {PbI6} octahedra, leading to the enhancement of hydrogen bonding interactions. This work not only offers a deep understanding of the structure-optical relationships of ACI-type perovskites, but also presents insights into breaking the limits of luminescent efficiency by pressure-suppressed nonradiative recombination.

Enhancing exciton-to-Mn2+ energy transfer and emission efficiency in Mn2+-doped CsPbCl3 perovskite nanocrystals via CaCl2 post-treatment

Mn2+-doped CsPbCl3 perovskite nanocrystals (NCs) exhibit a significant Stokes shift and emit bright orange-red light, making them promising candidates for optoelectronic devices. However, these NCs are prone to surface defects during multiple purification steps, which hinder the transfer of energy from excitons to Mn2+ and reduce luminescence efficiency. Herein, we introduce a post-treatment method using a CaCl2 ethanol solution to passivate surface defects in Mn2+-doped CsPbCl3 NCs. This treatment enhances the energy transfer from excitons to Mn2+ and boosts luminescence performance, achieving a photoluminescence quantum yield of up to 96 % and maintaining stability through several washing cycles. In this process, Ca2+ ions, as a Lewis acid, replace oleic acid molecules on NC surfaces, leading to stronger binding. The Cl− from the CaCl2 solution fill vacancies on NC surfaces, effectively passivating surface defects. This reduces non-radiative recombination centers, increases energy transfer efficiency from 83 % to 87 %, and accelerates the radiative recombination rates of excitons and Mn2+. The post-treated NCs also retain their PL against continuous heat and ultraviolet irradiation. The passivation of surface defects on perovskite NCs achieved through this post-processing strategy enhances the energy transfer from excitons to doped ions, providing guidance for achieving efficient and stable doped perovskite NCs.

Photon energy loss in ternary polymer solar cells based on nonfullerene acceptor as a third component

AbstractUnderstanding photon energy loss caused by the charge recombination in ternary blend polymer solar cells based on nonfullerene acceptors (NFAs) is crucial for achieving further improvements in their device performance. In such a ternary system, however, the two types of donor/acceptor interface coexist, making it more difficult to analyze the photon energy loss. Here, we have focused on the origin of the voltage loss behind a high open‐circuit voltage (VOC) in ternary blend devices based on one donor polymer (poly(2,5‐bis(3‐(2‐butyloctyl)thiophen‐2‐yl)‐thiazolo[5,4‐d]thiazole) [PTzBT]) and two acceptors, including a fullerene derivative ([6,6]‐phenyl‐C61‐butyric acid methyl ester [PCBM]) and an NFA ((2,2′‐((2Z,2′Z)‐(((4,4,9,9‐tetrakis(4‐hexylphenyl)‐4,9‐dihydro‐sindaceno[1,2‐b:5,6‐b′]dithiophene‐2,7‐diyl)bis(4‐((2‐ethylhexyl)oxy)thiophene‐5,2‐diyl))bis(methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile) [IEICO‐4F]), which exhibit VOC similar to that of fullerene‐based PTzBT/PCBM binary devices. From the temperature‐dependent VOC, we found that the effective interfacial bandgap is the same between them: the PTzBT/PCBM/IEICO‐4F ternary blend device is the same as the PTzBT/PCBM fullerene‐based binary device rather than the PTzBT/IEICO‐4F nonfullerene‐based binary device. This means that the recombination center of the ternary blend device is still the interface of PTzBT/PCBM regardless of the incorporation of a small amount of NFA. On the basis of detailed balance theory, we found that the radiative and nonradiative recombination voltage losses for PTzBT/PCBM/IEICO‐4F ternary devices significantly reduced compared to those of fullerene‐based PTzBT/PCBM binary counterparts. This is ascribed to the disappearance of charge transfer absorption due to overlap with the absorption of NFA and the reduction of energetic disorder due to the incorporation of NFA. Through this study, the role of NFAs in voltage loss is once again emphasized, and a ternary system capable of achieving high VOC resulting from significantly reduced voltage loss in ternary blend solar cells is proposed. Therefore, we believe that this research proposes the guidelines that can further enhance the power conversion efficiency of polymer solar cells.

Revisiting the statistical equilibrium of H- in stellar atmospheres

The negative hydrogen ion is, almost without exception, treated in local thermodynamic equilibrium (LTE) in the modelling of F, G, and K stars, where it is the dominant opacity source in the visual spectral region. This assumption rests in practice on a study from the 1960s. Since that work, knowledge of relevant atomic processes and theoretical calculations of stellar atmospheres and their spectra have advanced significantly, but this question has not been reexamined. We present calculations based on a slightly modified analytical model that includes H and together with modern atomic data and a grid of 1D LTE theoretical stellar atmosphere models with stellar parameters ranging from T$_ eff = 4000$ to 7000 K, $ g = 1$ to 5 cm/s$^2$, and Fe/H $=-3$ to 0. We find direct non-LTE effects on populations in spectrum-forming regions, continua, and spectral lines of about 1--2<!PCT!> in stars with higher T$_ eff $ and/or lower $ g$. Effects in models for solar parameters are smaller by a factor of 10, about 0.1--0.2<!PCT!>, and are practically absent in models with lower T$_ eff $ and/or higher $ g$. These departures from LTE found in our calculations originate from the radiative recombination of electrons with hydrogen to form exceeding photodetachment, that is, overrecombination. Modern atomic data are not a source of significant differences compared to the previous work, although detailed data for processes on resolved with vibrational and rotational states provide a more complete and complex picture of the role of in the equilibrium of In the context of modern studies of stellar spectra at the percent level, our results suggest that this question requires further attention, including a more extensive reaction network, and indirect effects due to non-LTE electron populations.

Recombination lifetimes and mechanisms of In0.75Ga0.25As and In0.53Ga0.47As as a function of doping density

The minority carrier lifetimes in InGaAs ternary alloys are investigated both experimentally and theoretically. Two groups of InGaAs/InP photodetector samples are designed for experiments, one with In0.75Ga0.25As and the other with In0.53Ga0.47As as the absorption layers. The lifetimes are obtained to be about 100–200 ns in In0.75Ga0.25As and 600 ns-1 µs in In0.53Ga0.47As by microwave photoconductivity decay technique. The calculated lifetimes are also found to be consistent with the experimental ones. Besides, for both InGaAs materials, the Shockley-Read-Hall (SRH), radiative and Auger recombination mechanisms are identified in calculations to be the dominant mechanisms at low, intermediate and high doping densities at room temperature, respectively. Furthermore, the effects of density, capture cross-sections and energy levels (Ets) of recombination centers, and bandgaps (Egs) on the SRH lifetimes are determined and compared for the two InGaAs materials. For an n-type material, when Et is near the mid-bandgap (Eg/2), the SRH lifetime is only inversely proportional to the product of the density and the capture cross-section of the recombination centers. When Et is not near the mid-bandgap, the magnitude of Eg also plays an important role in the SRH lifetime. By increasing the In-content from 53 % in In0.53Ga0.47As to 75 % in In0.75Ga0.25As, where the bandgap narrows from 0.75 to 0.52 eV, the SRH lifetime in In0.75Ga0.25As is shortened by a factor of about 1–42 for Et < Eg/2 and 1–65 for Et > Eg/2 within the bandgap, respectively.

Energy Level Gradients from Surface to Bulk in Hybrid Metal-Halide Perovskite Thin Films

Variations in local strain, defect densities, and composition of hybrid metal-halide perovskites have been reported to create heterogeneous energy landscapes in thin films, which impact charge-carrier diffusion and recombination dynamics. Here, we employ one- and two-photon transient absorption spectroscopy to selectively probe the dynamics of charge carriers from surface and bulk regions of methylammonium lead bromide thin films. Differences in the transient absorption spectra indicate that an energy gradient of approximately 100 meV is formed between the higher band-gap surface and lower band-gap bulk regions. Thus, during their lifetime, photoexcited carriers move away from the surface to recombine in the bulk, where our experiments detect long-lived charge populations despite the significant band splitting that has conventionally been assumed to inhibit efficient radiative recombination. Supported by first-principles calculations, we demonstrate that bright emission can still arise from the bulk with states that occupy a wide range of momenta in the vicinity of the band extrema, which show strong dipole transitions. Our results report that photoexcitations in the hybrid perovskites avoid defect-rich surface regions, and that particularly strong emission is generated from accumulated excitation populations in the bulk. Published by the American Physical Society 2024

Positive impact of surface defects on Maxwell's displacement current-driven nano-LEDs: The application of TENG technology

As the core component of a nanopixel light-emitting display, the GaN-based nanoscale light-emitting diode (nLED) faces the problem of low electroluminescence efficiency resulting from the introduction of surface defects when its lateral size is reduced to the nanometer scale. Thus, reducing the surface defect density is an important direction in nLED-related research. This study, with the triboelectric nanogenerator-driven LED as its inspiration, reveals that surface defects have a positive impact on the performance of nLEDs driven by Maxwell’s displacement current, and we call the related driving mode the noncarrier injection mode. Through finite element simulations, we studied the dynamic variations of the carrier concentration, the energy band, and the light emission rate to analyze the impact of the behavior of surface defect excitation on device performance. We found that surface defects can act as electron pumps under the combined effect of the reverse electric field and the built-in electric field and can generate carriers through surface defect excitation to increase the intensity of noncarrier injection luminescence, which is completely different from the traditional understanding of surface defects. In addition, we propose a tapered structure to further increase the light emission rate by regulating the behaviors of radiation recombination and surface defect excitation. The results of this work open a new perspective on the impacts of surface defects on nLEDs and provide significant information for additional applications of Maxwell's displacement current.

Efficient C(sp3)-H Bond Oxidation on Perovskite Quantum Dots Based on Ce-Oxygen Affinity.

Perovskite quantum dots (QDs) have shown attractive prospects in the field of visible photocatalysis, especially in the synthesis of high value-added chemicals. However, under aerobic conditions, the stable operation of QD catalysts has been limited by the reactive oxygen species (ROS) generated by photoexcitation, especially superoxide species O2⋅-. Here, we propose a strategy of Ce3+ doping in perovskite QDs to guide superoxide species for photocatalytic oxidation reactions. In C(sp3)-H bond oxidation of hydrocarbons, superoxide species were rapidly generated and efficiently utilized on the surface of perovskite QDs, which achieves the stable operation of the catalytic system and obtains a high product conversion rate (15.3 mmol/g/h for benzaldehydes). The mechanism studies show that the strong Ce-oxygen affinity accelerates the relaxation process of photoinduced exciton transfer to superoxide species and inhibits the radiative recombination pathway. This work provides a new idea of utilizing oxygen species on perovskite surface and broadens the design strategy of high-performance QD photocatalysts.

Efficient C(sp3)−H Bond Oxidation on Perovskite Quantum Dots Based on Ce‐Oxygen Affinity

AbstractPerovskite quantum dots (QDs) have shown attractive prospects in the field of visible photocatalysis, especially in the synthesis of high value‐added chemicals. However, under aerobic conditions, the stable operation of QD catalysts has been limited by the reactive oxygen species (ROS) generated by photoexcitation, especially superoxide species O2⋅−. Here, we propose a strategy of Ce3+ doping in perovskite QDs to guide superoxide species for photocatalytic oxidation reactions. In C(sp3)−H bond oxidation of hydrocarbons, superoxide species were rapidly generated and efficiently utilized on the surface of perovskite QDs, which achieves the stable operation of the catalytic system and obtains a high product conversion rate (15.3 mmol/g/h for benzaldehydes). The mechanism studies show that the strong Ce‐oxygen affinity accelerates the relaxation process of photoinduced exciton transfer to superoxide species and inhibits the radiative recombination pathway. This work provides a new idea of utilizing oxygen species on perovskite surface and broadens the design strategy of high‐performance QD photocatalysts.