High Defect Density Research Articles

High-Performance CsPbBr3 Quantum Dot/ZTO Heterojunction Phototransistor with Enhanced Stability and Responsivity.

Although all inorganic metal halide perovskite quantum dots (QDs) have shown great potential in photodetectors, many reported devices still suffer from low responsivity due to poor mobility and high defect densities. To overcome these challenges, heterojunction structures that separate electron and hole pathways have emerged as a promising approach to improve the responsivity by reducing radiative recombination. Two-dimensional materials such as graphene and MoS2 have been integrated with perovskites to enhance performance; however, these materials often lead to high dark-state currents, which hinder the on/off ratios in photodetectors. Hence, identifying alternative functional materials that can complement perovskite QDs while minimizing these drawbacks is critical. Amorphous metal oxide semiconductors, such as zinc tin oxide (ZTO), have attracted attention as high-performance channel materials in thin-film transistors (TFTs) due to their high field-effect mobility, thermal stability, and ability to be processed at low temperatures over large areas. However, ZTO suffers from persistent photoconductivity (PPC) caused by oxygen vacancies, which leads to slow response times and prolonged conductivity after light exposure, thereby limiting its effectiveness in optoelectronic devices. In this work, we demonstrate a high-performance phototransistor using a planar heterojunction structure composed of CsPbBr3 quantum dots and ZTO via a simple solution-processing method. This device exhibits a responsivity of over 103 A/W, a specific detectivity of 7.0 × 1014 Jones, and an on/off ratio of 5 × 104 under 390 nm light illumination with an intensity of 0.03 mW/cm2. The combination of ZTO and CsPbBr3 QDs offers significant improvements over devices that lack either layer, showcasing superior performance compared to that of most perovskite photodetectors.

Atomistic Origin of Microsecond Carrier Lifetimes at Perovskite Grain Boundaries: Machine Learning-Assisted Nonadiabatic Molecular Dynamics.

The polycrystalline nature of perovskites, stemming from their facile solution-based fabrication, leads to a high density of grain boundaries (GBs) and point defects. However, the impact of GBs on perovskite performance remains uncertain, with contradictory statements found in the literature. We developed a machine learning force field, sampled GB structures on a nanosecond time scale, and performed nonadiabatic (NA) molecular dynamics simulations of charge carrier trapping and recombination in stoichiometric and doped GBs. The simulations reveal long, microsecond carrier lifetimes, approaching experimental data, stemming from charge separation at the GBs and small NA coupling, 0.01-0.1 meV. Stoichiometric GBs exhibit transient trap states, which, however, are not particularly detrimental to the carrier lifetime. Halide dopants form interstitial defects in the bulk, but have a stabilizing influence on the GB structure by passivating undersaturated Pb atoms and reducing the transient trap state formation. On the contrary, excess Pb destabilizes GBs, allowing formation of persistent midgap states that trap charges. Still, the charge carrier lifetime reduces relatively little, because the midgap states decouple from the bands, and charges are more likely to escape back into bands upon a GB structural fluctuation. The atomistic study into the structural dynamics of perovskite GBs and its influence on charge carrier trapping and recombination provides valuable insights into the complex properties of perovskites and the intricate role of GBs in the material performance.

An Electrochemical Dopamine Assay with Cobalt Oxide Palatinose Carbon Dots.

Elevated dopamine (DA) levels in urine denote neuroblastoma, a pediatric cancer. Saccharide-derived carbon dots (CDs) were applied to assay DA detection in simulated urine (SU) while delineating the effects of graphene defect density on electrocatalytic activity. CDs were hydrothermally synthesized to vary graphene defect densities using sucrose, raffinose, and palatinose, depositing them onto glassy carbon electrodes (GCEs). Co3O4 nanoparticles (NPs) were encapsulated by the CDs. Cyclic (CV) and linear sweep (LSV) voltammetry measurements were obtained, drop-casting the CDs onto GCEs and measuring DA in a phosphate-buffer solution (pH = 7). DA had an oxidation peak at +0.2 V with SucCDs, with the highest current correlating with the highest defect density. PalCD-Co3O4 exhibited the largest signal for DA detection in simulated urine (SU) using the oxidation peak at +0.5 V; the composite had a lower defect density compared to SucCD-Co3O4. The Co3O4-PalCDs had a DA detection range of 1 to 90 µM with an LOD of 0.88 μM in SU. SEM-EDX analysis of the electrode surface revealed semi-spherical structures with an average particle diameter of 80 ± 19 nm (n = 347) with PalCDs decorating the Co3O4 NPs. XRD characterization showed the incorporation of PalCD and Co3O4 within the composite. XPS showed electron density donation from the PalCD to Co3O4.

Revealing the Principles of Confining Electroplated Lithium beneath the CVD Grown Single Layer 2D Materials.

Owing to the nanoscale thickness, excellent mechanical and chemical stabilities, 2D materials including graphene and hexagonal boron nitride have emerged as promising artificial solid electrolyte interphase (SEI) candidates for lithium metal batteries. However, whether the implementation of 2D materials is beneficial to electrochemical performance remains controversial, and the key to confining the electroplated Li beneath the 2D materials remains elusive. Here, a nanocrystalline graphene (NG) film is synthesized on high-carbon Cu and the Li plating/stripping behavior on Cu grown with different 2D materials is investigated. Interestingly, in contrast to the commonly obtained Li particles on other substrates during nucleation, a smooth Li layer is obtained on NG/Cu, leading to a compact Li layer with more stable electrochemical performance. The finite element method simulations validate that the low electrical conductivity and the high density of defects on NG film drive the fast entry of electrolytes into the NG/Cu interface and promote homogenous Li nucleation. This work reveals the principles of confining electroplated Li beneath the 2D materials, and paves the way for the applications of 2D materials in artificial SEIs and anode-free lithium metal batteries.

Two-dimensional Czochralski growth of single-crystal MoS2.

Batch production of single-crystal two-dimensional (2D) transition metal dichalcogenides is one prerequisite for the fabrication of next-generation integrated circuits. Contemporary strategies for the wafer-scale high-quality crystallinity of 2D materials centre on merging unidirectionally aligned, differently sized domains. However, an imperfectly merged area with a translational lattice brings about a high defect density and low device uniformity, which restricts the application of the 2D materials. Here we establish a liquid-to-solid crystallization in 2D space that can rapidly grow a centimetre-scale single-crystal MoS2 domain with no grain boundaries. The large MoS2 single crystal obtained shows superb uniformity and high quality with an ultra-low defect density. A statistical analysis of field effect transistors fabricated from the MoS2 reveals a high device yield and minimal variation in mobility, positioning this FET as an advanced standard monolayer MoS2 device. This 2D Czochralski method has implications for fabricating high-quality and scalable 2D semiconductor materials and devices.

Scalable and Low-Energy Synthesis of Metal-Organic Frameworks by a Seed-Mediated Approach.

The synthesis of metal-organic frameworks (MOFs) by low energy input has been a long-term target for practical applications yet remains a great challenge. Herein, we developed a low-energy MOF growth strategy at a temperature down to 50 °C by simply introducing seeds into the reaction system. The MOFs are continuously grown on the surface of the seeds at a growth rate dozens of times higher than that of conventional solvothermal synthesis at low temperature, while the resulting MOFs possess high crystallinity, porosity, and stability similar to solvothermal seeds. Remarkably, the obtained MOFs feature high-density structural defects with Lewis acidity, thereby displaying more than one order of magnitude higher activity than the MOFs obtained by the conventional solvothermal method in the iodination reaction of substituted arenes. This low-energy synthetic approach is readily scaled up, which would be a significant step forward in the dream of the MOF industry.

Designing Carbon-Foam Composites via Molten-State Reduction for Multifunctional Electromagnetic Interference Shielding.

Advanced electromagnetic interference (EMI) shielding materials are in great demand because of the severe electromagnetic population problem caused by the explosive growth of advanced electronics. Besides superior EMI shielding properties, the mechanical strength of the shielding materials is also critical for some specific application scenarios (e.g., shielding cases and shielding frames). Although most reported EMI shielding materials possess good shielding properties and lightweight characteristics, they usually exhibit a poor mechanical strength. Concurrently, multifunctionality is also essential for the application of the EMI shielding material. This study develops a molten-state-based in situ reduction strategy to fabricate an efficient EMI shielding composite, enabling the uniform dispersion of Co-nanoparticles on the carbon form matrix while featuring a high density of defects. This ensures the high mechanical strength of the composite due to the presence of a huge interface and significantly enhances the EMI shielding performance. The composite achieves an optimal shielding effectiveness of 32.6 dB and compressive strength of 38.31 MPa, respectively, improved by 65.4 and 123.4% compared to the pristine carbon foam. Simultaneously, the composite also exhibits desirable electrochemical and photothermal conversion properties. This research offers insights into the design of composites that excel in electromagnetic interference shielding, mechanical robustness, and multifunctionality.

Template-free synthesis of single-crystal SrTiO3 nanocages for photocatalytic overall water splitting.

In this study, we present a novel approach to achieve the template-free fabrication of nanocage-shaped SrTiO3 (N-STO) single crystals via molten salt flux treatment. Systematic characterizations demonstrate the high crystallinity and low defect density of N-STO. The N-STO single crystals enable overall water splitting (OWS) with hydrogen and oxygen evolution rates of 100.86 μmol h-1 g-1 and 44.2 μmol h-1 g-1, respectively, which is 5.7-fold higher than the porous SrTiO3 (P-STO) control.

Engineered high-density carbon defects enable accelerated sulfur conversion for kinetics-boosted room-temperature sodium-sulfur batteries

Engineered high-density carbon defects enable accelerated sulfur conversion for kinetics-boosted room-temperature sodium-sulfur batteries

InGaN/GaN edge emitting laser diodes using an epitaxial lateral overgrowth with a low-defect density area of more than 75%

Abstract We have successfully demonstrated InGaN/GaN edge-emitting laser diodes (EELDs) on a fully coalesced epitaxial lateral overgrown film from a c-plane GaN substrate. We achieve a high aspect ratio, and low defect density wing region covering 75%–88% of the substrates’ surface, which is amongst the largest reported areas in the literature. The devices at the wing region exhibit a threshold current density of 3.63 kA cm−2 at 408 nm, and an improved laser performance compared to the high defect density region is confirmed. Based on this work, it is promising that high performance, cost-efficient c-plane InGaN/GaN EELDs can be realized.

Utilizing Indonesian Empty Palm Fruit Bunches: Biochar Synthesis via Temperatures Dependent Pyrolysis.

Biomass, though a major energy source, remains underutilized. Biochar from biomass pyrolysis, with its high porosity and surface area, is especially useful as catalyst support, enhancing catalytic activity and reducing electron recombination in photocatalysis. Indonesia, the world's top palm oil producer, generated around 12 million tons of empty fruit bunches (EFBs) in 2023, making EFBs a promising biochar source. This study synthesizes biochar from leftover EFB fibers at 500, 800, and 1000 °C, analyzing structural changes via infrared and Raman spectroscopy, along with particle size and surface area analysis, laying the groundwork for future biochar research. The smallest particle size and highest surface area gained was 71.1 nm and 10.6 × 102 m2/g. Spectroscopic analysis indicates that biochar produced at 1000 °C has produced nano-crystalline graphite with a crystallite size of approximately 5.47 nm. This provides higher defect density, although with lower conductivity. Other studies indicate that our biochar can be used as catalyst support for various green energy-related applications, i.e., counter electrodes, electrocatalysts, and photocatalysts.

Unlocking Near 100% PLQY: The Impact of Post‐Treatment on Emission Efficiency of Mn2+: CsPbCl3 Nanocrystals

AbstractMn2+ doped CsPbCl3 nanocrystals (Mn2+:CsPbCl3 NCs) show immense promise in lighting and display technologies owing to their outstanding optical characteristics. Nonetheless, high defect density, inefficient Mn2+ doping, and poor structural robustness pose considerable challenges in the synthesis of Mn2+:CsPbCl3 NCs with high photoluminescence quantum yield (PLQY) and long‐term stability. This study introduces an eco‐friendly room‐temperature post‐treatment method utilizing MgCl2/oleylamine solution, boosting the total PLQY of the NCs from 65% to 98%, including a 30% increase for Mn2+ emission. Additionally, the post‐treatment significantly improves the NCs’ resilience against UV light, retaining 80% of their initial PL intensity after 120 hours of UV exposure. Comprehensive analyses including density functional theory simulation and temperature‐dependent PL spectra suggested that the PLQY enhancement is due to the improved Mn2+ doping efficiency and the effective passivation of native vacancy defects. The stability of the NCs is enhanced by replacing Pb2+ with Mg2+ and Mn2+, mitigating lattice distortions within the [PbCl6]4− octahedral framework. The color rendering index of white light‐emitting diodes fabricated with the post‐treated Mn2+:CsPbCl3 NCs achieves an exceptional value of 98. This work offers novel insights into the fabrication of high‐PLQY Mn2+:CsPbCl3 NCs, advancing the practical deployment of perovskite nanomaterials in various industries.

Intrinsic defects in α-Al2O3: structural and electronic properties

Abstract Due to their high dielectric constants and wide band gaps, Al 2 O 3 has become popular in semiconductor applications, but with high-density interface defect structures. Understanding the structural distortions of intrinsic defects and their impact on electronic properties, is of particular importance for the applications. Herein, we study the impacts of four intrinsic defects on the structural and electronic properties of α- Al 2 O 3 using first-principles calculations. The four intrinsic defects cause structural distortions at their respective defect sites and defect states within the band gap, with O i inducing the most significant distortions and exhibiting the most complex defect states accordingly. When the Fermi level is 3.05 eV under O-poor conditions, VO is the most favorable defects based on the formation energy calculations. Additionally, the intrinsic defects in α- Al 2 O 3 can effectively modulate band edges, and thus modify the leakage currents. In summary, our results provide significant theoretical insights for α- Al 2 O 3 -based semiconductor applications.

2D/Quasi‐2D/3D CsPbIxBr3–x Vertical Heterostructures for High‐Performance Infrared‐Blind Visible‐Light Photodetectors Toward Imaging Application

AbstractIntrinsic near‐infrared response of silicon‐based photodetectors can cause undesirable noise and thus undermine performance in visible‐light applications. All‐inorganic perovskite CsPbIxBr3–x with the absorption cutoff at ≈720 nm shows significant potential in visible‐light photodetection. However, its poor phase stability and high defect density on surfaces of polycrystalline films significantly impairs its performance. Constructing low‐dimension/3D heterostructure atop CsPbIxBr3–x is an effective way to simultaneously address stability and defect challenges. Herein, a facile and universal strategy is demonstrated to construct 2D PEA2PbI2Br2/quasi‐2D PEA2CsPb2I5Br2/3D CsPbIxBr3–x heterostructures by two‐step sequential surface treatment with phenylethylammonium bromide (PEABr). The PEABr pre‐treatment in the first step favors the volatilization of dimethylammonium iodide to produce higher phase stability. With the addition of strong‐polarity methanol in the solution for the second treatment, PEABr can easily react with the PbIxBr2–x rich surface to form low‐dimensional/3D perovskites heterostructure. Resultant heterostructure‐based photodetectors achieve superior performance in photodiode‐type photodetectors and enhanced stability, specifically a champion external quantum efficiency of 84% and detectivity of 3.1 × 1012 Jones at 690 nm, and linear dynamic range of 163 dB. Finally, the visible‐light imaging is demonstrated. This work offers a universal approach to realize CsPbIxBr3–x heterostructures and stimulates infrared‐blind visible‐light applications.

Deep level transient spectroscopy: Tracing interface and bulk trap‐induced degradation in AlGaN/GaN‐heterostructure based devices

AbstractThe exceptional physical properties of gallium nitride (GaN) position GaN‐based power devices as leading candidates for next‐generation high‐efficiency smart power conversion systems. However, GaN's multi‐component nature results in a high density of epitaxial defects, whereas the introduction of dielectric layers further contributes to severe interface states and dielectric traps. These factors collectively impair reliability, manifesting as threshold voltage instability and current collapse, which pose significant barriers to the advancement of GaN‐based electronics. Establishing the intrinsic relationship between device reliability and defects is crucial for understanding and addressing reliability degradation issue. Deep level transient spectroscopy (DLTS) offers valuable insights by revealing defect‐induced changes in electrical parameters during the capture and emission processes under varying biases, thereby elucidating the influence of defects from GaN buffer layers, AlGaN barriers, dielectric layer, and even at dielectric/(Al)GaN interfaces. This research aims to provide a foundational understanding of reliability degradation whereas further enabling enhancements in device performance from the perspectives of epitaxial growth and process preparation, ultimately striving to improve the reliability of GaN‐based devices and unlock their full potential for practical applications.

Nondestructive halide exchange via SN2-like mechanism for efficient blue perovskite light-emitting diodes

Blue perovskite light-emitting diodes (PeLEDs) still remain poorly developed due to the big challenge of achieving high-quality mixed-halide perovskites with wide optical bandgaps. Halide exchange is an effective scheme to tune the emission color of PeLEDs, while making perovskites susceptible to high defect density due to solvent erosion. Herein, we propose a versatile strategy for nondestructive in-situ halide exchange to obtain high-quality blue perovskites with low trap density and tunable bandgaps through long alkyl chain chloride incorporated chloroform post-treatment. In comparison with conventional halide exchange method, the ionic exchange mechanism of the present strategy is similar to a bimolecular nucleophilic substitution process, which simultaneously modulates perovskite bandgaps and inhibits new halogen vacancy generation. Consequently, efficient PeLEDs across blue spectral regions are obtained, exhibiting external quantum efficiencies of 23.6% (sky-blue emission at 488 nm), 20.9% (pure-blue emission at 478 nm), and 15.0% (deep-blue emission at 468 nm), respectively.

Modeling the effects of electron irradiation on graphene drums using the local activation model

We study the effects of electron irradiation on suspended graphene monolayers and graphene supported on SiO2 substrates in the range 5.0 × 1015–4.3 × 1017 electrons/cm2. The suspended graphene monolayers are exfoliated over SiO2 substrates containing micrometer-sized holes, with graphene completely covering the hole, and are referred to as graphene drums. The irradiation was performed using a scanning electron microscope at 20–25 keV electron energy. We observe a two-stage behavior for the ID/IG, ID′/IG, and ID/ID′ ratios as a function of the average distance between defects, LD, where ID, IG, and ID′ are the intensities of the Raman D, G, and D′ peaks, respectively. Good fits to the dependence of the ratios on LD are obtained using the local activation model equation. The fits are used to characterize the defects at high defect densities. We also carried out annealing studies of samples irradiated to the first stage and used an Arrhenius plot to measure activation energies for defect healing, Ea. We measured Ea = 0.90 eV for the graphene drums, consistent with the hydroxyl groups; for supported graphene, we measured Ea = 0.36 eV, consistent with hydrogen adsorbates. We also studied the surface of the drums using atomic force microscopy and found no observable holes after irradiation and annealing. Our results show that the local activation model is useful in characterizing the defects in graphene drums.

Advances in Single‐Halogen Wide‐Bandgap Perovskite Solar Cells

AbstractWide‐bandgap (WBG) (Eg ≥ 1.65 eV) perovskite solar cells (PSCs) made from mixed‐halide strategy experience severe photo‐induced halide segregation, leading to detrimental effects on the long‐term operational stability. Developing single‐halogen WBG perovskites can be the fundamental solution to prevent halide segregation. In this review, the recent advances in single‐halogen WBG PSCs, focusing on the cesium (Cs)‐based pure‐iodide (I) perovskite and all the pure‐bromine (Br) perovskite species is summarized. A detailed discussion is conducted on the crystallization dynamics of different perovskite systems. The key challenge for all single‐halogen WBG PSCs is the huge energy loss due to inferior interfacial energy level alignment and high defect density in perovskite films, which greatly hinders efficiency improvement. To this end, it is systematically discuss optimization strategies, including regulating crystallization, passivating defects, achieving aligned energy levels, and eliminating interfacial microstrain, to enhance the photovoltaic performance of solar cells. Furthermore, it is highlighted that Cs‐based pure‐I WBG perovskites encounter significant stability issue due to their low structural tolerance factor, warranting substantial attention. Finally, perspectives are outlined to suggest ways to further advance the development and application of single‐halogen WBG PSCs.

Sn-Pb Perovskite with Strong Light and Oxygen Stability for All-Perovskite Tandem Solar Cells.

Research on mixed Sn-Pb perovskite solar cells (PSCs) is gaining significant attention due to their potential for high efficiency in all-perovskite tandem solar cells. However, Sn2+ in Sn-Pb perovskite is susceptible to oxidation, leading to a high defect density. The oxidation primarily occurs through two pathways: one involving a reaction with oxygen, and the other related to iodine defects, which generate I2 and further accelerate the oxidation of Sn2⁺, greatly reducing stability. First, to tackle the photo-stability issues caused by iodine defects, amber acid (AA) is screened as the additive. The Carboxyl group on AA can strongly coordinate with Sn2+, reinforcing the Sn─I bond and electrostatically interacting with negatively charged defects. This interaction inhibits the photoinduced formation of I2 and the subsequent oxidation of Sn2+, thereby enhancing the stability of Sn─Pb PSCs under continuous illumination. Building on the foundation of AA, a reductive sulfhydryl group is introduced to synthesize thiomalic acid (TA). It inhibits the formation of Sn4+ in both the perovskite precursor and the perovskite film, thereby improving air stability while maintaining strong photostability. Consequently, single PSCs achieved a champion efficiency of 22.7%. The best-performing two-terminal all-perovskite tandem solar cell achieved a power conversion efficiency of 28.6% with improved operational stability.

Minimizing vacancy defects with Eu passivation strategy to enable highly efficient perovskite photovoltaics

The intrinsic ionic nature of metal halide perovskites facilitates the formation of abundant defects at grain boundaries (GBs) and surfaces, consequently diminishing the stability and photovoltaic performance of perovskite solar cells (PSCs). Due to the low formation energies and instability of Pb-I bonds, iodine (I) and lead (Pb) vacancies are prevalent high-density point defects. In this study, we explore the use of europium (Eu) as a novel passivator to concurrently address I and Pb vacancies through first-principles calculations. Our results demonstrate that Eu significantly lowers the defect formation energies and adjusts the bandgap, enlarged by vacancy defects, to align with the ideal Shockley-Queisser limit, indicating a strong potential for achieving high power conversion efficiency (PCE). Furthermore, Eu passivation markedly increases the activation energy barriers for ion migration, resulting in substantially reduced migration rates. This suggests a minimal likelihood of I and Pb ion migration on defected surfaces, underscoring Eu’s potential to mitigate hysteresis effects and enhance stability. This work advances our understanding of the passivation effects of rare earth elements and establishes a foundation for designing highly effective passivation strategies to achieve stable and efficient PSCs.