Active Defects Research Articles

Quantum sensing with optically accessible spin defects in van der Waals layered materials

Quantum sensing has emerged as a powerful technique to detect and measure physical and chemical parameters with exceptional precision. One of the methods is to use optically active spin defects within solid-state materials. These defects act as sensors and have made significant progress in recent years, particularly in the realm of two-dimensional (2D) spin defects. In this article, we focus on the latest trends in quantum sensing that use spin defects in van der Waals (vdW) materials. We discuss the benefits of combining optically addressable spin defects with 2D vdW materials while highlighting the challenges and opportunities to use these defects. To make quantum sensing practical and applicable, the article identifies some areas worth further exploration. These include identifying spin defects with properties suitable for quantum sensing, generating quantum defects on demand with control of their spatial localization, understanding the impact of layer thickness and interface on quantum sensing, and integrating spin defects with photonic structures for new functionalities and higher emission rates. The article explores the potential applications of quantum sensing in several fields, such as superconductivity, ferromagnetism, 2D nanoelectronics, and biology. For instance, combining nanoscale microfluidic technology with nanopore and quantum sensing may lead to a new platform for DNA sequencing. As materials technology continues to evolve, and with the advancement of defect engineering techniques, 2D spin defects are expected to play a vital role in quantum sensing.

Degradation Analysis of Exciplex-Based Organic Light-Emitting Devices Using Carbazole-Based Materials.

A spectral shift and new emission bands in the green and red regions have been observed in deep blue exciplex-based organic light-emitting diodes (OLEDs) using carbazole-based materials, namely, tris(4-carbazoyl-9-ylphenyl)amine (TCTA). To deeply understand the origin of these new bands, single-layer and bilayer TCTA-based OLEDs subjected to electrical and optical (ultraviolet (UV)) stresses were investigated by using various optical, electrical, morphological, and chemical measurements. The results showed that the stress-induced emission bands primarily originate from morphological changes rather than chemical changes. The accumulation of excitons in the TCTA layer induces molecular aggregation, leading to the formation of electrically active electronic states, namely, electroplexes and electromers, which lead to the appearance of additional emission bands in green and red regions. Impedance spectroscopy measurements on single-layer OLEDs complemented this study. The results showed that TCTA degradation affects charge injection and transport. It was concluded that the stress-induced emission bands are caused by aggregate domain formation and are closely linked to the formation of electrically active defects, which act as trap states for charge carriers in the TCTA band gap.

Surface Defects Control Bulk Carrier Densities in Polycrystalline Pb-Halide Perovskites.

The (opto)electronic behavior of semiconductors depends on their (quasi-)free electronic carrier densities. These are regulated by semiconductor doping, i.e., controlled "electronic contamination". For metal halide perovskites (HaPs), the functional materials in several device types, which already challenge some of the understanding of semiconductor properties, this study shows that doping type, density and properties derived from these, are to a first approximation controlled via their surfaces. This effect, relevant to all semiconductors, and already found for some, is very evident for lead (Pb)-HaPs because of their intrinsically low electrically active bulk and surface defect densities. Volume carrier densities for most polycrystalline Pb-HaP films (<1µm grain diameter) are below those resulting from even < 0.1% of surface sites being electrically active defects. This implies and is consistent with interfacial defects controlling HaP devices in multi-layered structures with most of the action at the two HaP interfaces. Surface and interface passivation effects on bulk electrical properties are relevant to all semiconductors and are crucial for developing those used today. However, because bulk dopant introduction in HaPs at controlled ppm levels for electronic-relevant carrier densities is so difficult, passivation effects are vastly more critical and dominate, to first approximation, their optoelectronic characteristics in devices.

Quantum Sensing of Spin Dynamics Using Boron-Vacancy Centers in Hexagonal Boron Nitride.

Spin defects embedded in solid-state systems are appealing for quantum sensing of materials and for quantum science and engineering. The spin-sensitive photoluminescence of optically active spin defects in Van der Waals based materials, such as the boron-vacancy (V_{B}^{-}) center in hexagonal boron nitride, enables its application as a quantum sensor to detect weak, spatially localized magnetic static and dynamic fields. However, the utility of V_{B}^{-} centers to probe spin dynamics in magnetic systems has yet to be demonstrated; this is essential to establish the V_{B}^{-} as a modular sensing platform that can be seamlessly integrated with emergent quantum materials to probe a wide range of static and dynamic phenomena. Here, we use V_{B}^{-} centers to experimentally probe uniform mode magnon dynamics and optically perform ferromagnetic resonance spectroscopy on a thin magnetic film.

Subtraction-based DNA Origami Cryptography by using Structural Defects for Information Encryption.

Conventional cryptographic methods rely on increased computational complexity to counteract the threat posed by growing computing power for sustainable protection. DNA cryptography circumvents this threat by leveraging complex DNA recognition to maintain information security. Specifically, DNA origami has been repurposed for cryptography, using programmable folding of the long scaffold strand carrying additional tagged strands for information encryption. Herein, a subtraction-based cryptographic strategy is presented that uses structural defects on DNA origami to contain encrypted information. Designated staple strands are removed from the staple pool with "hook" strands to create active defect sites on DNA origami for information encryption. These defects can be filled by incubating the structures with the intact pool of biotinylated staple strands, resulting in biotin patterns that can be used for protein-binding steganography. The yields of individual protein pixels reached over 91%, and self-correction codes are implemented to aid the information recovery. Furthermore, the encrypted organization of defective DNA origami structures is investigated to explore the potential of this method for scalable information storage. This method uses DNA origami to encrypt information in hidden structural features, utilizing subtraction for robust cryptography while ensuring the safety and recovery of data.

Tailoring optical and electrical properties of PVP-I2 complexes through gamma irradiation-induced structural defect Engineering for optoelectronic applications

This study investigates the complexation of iodine with polyvinylpyrrolidone (PVP) subjected to gamma irradiation at doses of 0, 10, 20, and 30 kGy. The complex formation involves I2 acting as a Lewis acid with electron-rich PVP as a Lewis base through a charge transfer mechanism. UV–visible spectroscopy revealed a red shift of the absorption peak at 337 nm for irradiated PVP-I2 samples, indicating stronger interactions between the triiodide ion (I3¯) and irradiated PVP. FTIR analysis confirmed the formation of new C≡N moieties arising from PVP-I2 complexation. The FT-Raman analysis revealed a new C≡N peak at 2360 cm−1 in irradiated PVP-I2 samples, with increasing intensity correlating to higher irradiation doses, indicating enhanced electron delocalization due to complex formation. XRD analysis showed that PVP-I2 samples irradiated at 0 and 10 kGy exhibited higher crystallinity than those irradiated at 20 and 30 kGy, suggesting that high-dose irradiation causes PVP radiolysis and ring opening. Dielectric measurements demonstrated enhanced polarizability and ionic conductivity of irradiated PVP-I2 films, related to structural disorder promoting charge dissociation and ion mobility. The direct bandgap decreased from 2.75 eV for unirradiated samples to 2.55 eV for samples irradiated at 30 kGy, indicating the creation of new optically active defect states. Urbach energy increased from 0.85 eV to 1.55 eV with increasing irradiation dose, confirming increased disorder in the PVP structure. These findings reveal that gamma irradiation significantly alters the optical and electrical properties of PVP-I2 films by creating defects and disrupting structure, offering new insights for tailoring these materials for optoelectronic applications.

Optically driven ultraviolet-C glowing from an in situ trapping-detrapping approach.

In recent years, the world has witnessed rapid progress in research on ultraviolet luminescent materials, ranging from high-level anticounterfeiting and solar-blind optical tagging to antibacterial applications. In particular, a background-signal free solar-blind surveillance of ultraviolet-C photons provides an opportunity in bright indoor and outdoor environments. However, ambient daylight or inevitable external photostimulation is always eliminated or underestimated in the research of persistent phosphors. Herein, an in situ trapping-detrapping experimental procedure is employed to reveal more information on the total trap energy and trap modulations after photostimulation. Our findings reveal the presence of optically active trapping defects with photostimulated detrapping and retrapping behavior. This work provides a fundamental advance in revealing the trap distribution and trap reshuffling during glowing-in-the-daylight events, offering what we believe to be new insights into manipulating traps.

Dynamics of Active Defects on the Anisotropic Surface of an Ellipsoidal Droplet

We investigate the steady state of an ellipsoidal active nematic shell using experiments and numerical simulations. We create the shells by coating microsized ellipsoidal droplets with a protein-based active cytoskeletal gel, thus obtaining ellipsoidal core-shell structures. This system provides the appropriate conditions of confinement and geometry to investigate the impact of nonuniform curvature on an orderly active nematic fluid that features the minimum number of defects required by topology. We identify new time-dependent states where topological defects periodically oscillate between translational and rotational regimes, resulting in the spontaneous emergence of chirality. Our simulations of active nematohydrodynamics demonstrate that, beyond topology and activity, the dynamics of the active material are profoundly influenced by the local curvature and viscous anisotropy of the underlying droplet, as well as by external hydrodynamic forces stemming from the self-sustained rotational motion of defects. These results illustrate how the incorporation of curvature gradients into active nematic shells orchestrates remarkable spatiotemporal patterns, offering new insights into biological processes and providing compelling prospects for designing bioinspired micromachines. Published by the American Physical Society 2024

Optical and electrical studies on the TS defect in 4H-SiC

Abstract When annealing a 4H silicon carbide (SiC) crystal, a sequence of optically active defect centers&amp;#xD;occurs among which the TS center is a prominent example. Here, we present low-temperature photoluminescence&amp;#xD;analyses on the single defect level. They reveal that the three occurring spectral&amp;#xD;signatures TS1, TS2 and TS3 originate from one single defect. Their polarization dependences expose&amp;#xD;three different crystallographic orientations in the basal plane, which relate to the projections of&amp;#xD;the nearest neighbor directions. Accordingly, we find a three-fold level-splitting in ensemble studies,&amp;#xD;when applying mechanical strain. This dependency is quantitatively calibrated. A complementary&amp;#xD;electrical measurement, deep level transient spectroscopy, reveals a charge transition level of the TS&amp;#xD;defect at 0.6 eV above the valence band. For a future identification, this accurate characterization&amp;#xD;of its optical and electronic properties along with their response to mechanical strain is a milestone.

Construction of atom-scale Co/Fe/Ni M-N-C active sites within nitrogen-doped carbon spheres for high-performance bifunctional oxygen catalysts in rechargeable zinc-air batteries

Developing carbon-based catalysts with metal‑nitrogen‑carbon (M-N-C) active sites as efficient and low-cost bifunctional catalysts to replace precious metal catalysts for rechargeable zinc-air batteries devices has garnered significant attention. Herein, nitrogen-doped submicron carbon-based spherical bifunctional electrocatalysts (CNPD-CoFeNi) with abundant atom-scale Co/Fe/Ni M-N-C active sites and defects were synthesized. The introduction of massive amounts of Zn species increases the spatial distance of Co/Fe/Ni metal sites to prevent aggregation during pyrolysis, and the evaporation of Zn at high temperatures creates defects synergizing with M-N-C. Consequently, CNPD-CoFeNi possesses a stable carbon-based spherical structure, high electrical conductivity, rich nitrogen content, abundant atom-scale Co/Fe/Ni M-N-C active sites, and defects, displaying outstanding durability, oxygen reduction reaction (ORR) activity with a half-wave potential of 0.87 V, and satisfactory OER performance, surpassing or comparable with the state-of-art Pt/C and RuO2, respectively. Notably, liquid Zn-air batteries with CNPD-CoFeNi catalyst exhibit an exceptional high peak power density of 146.5 mW cm−2 and stable charge-discharge cycling over 270 h, outperforming Pt/C-RuO2 based devices. Additionally, the all-solid-state Zn-air battery also displays satisfactory practicability and stability. The synthesis strategy and its remarkable results provide valuable guidance and inspiration for the future advancement of precious metal substitutes in ORR/OER and rechargeable zinc-air batteries.

Increased interlayer spacing N-doping Ti3C2Tx MXene-based mezzanine heterostructure for enhanced performance lithium/sodium storage

Structure of the electrode materials play a critical role in electrochemical performance for both lithium-ion batteries and sodium ion batteries. Herein, MXene modified by element doping and compositing metal–organic frameworks (MOF)-based Fe2O3@Carbon nanofibers (CNFs) with a mezzanine heterostructure, is constructed to achieve low interfacial impedance, high specific capacity stability in lithium/sodium storage processes. The prepared mezzanine heterostructure Fe2O3@C@N-Ti3C2Tx composite is highly conductive and mesoporous. During the nitrogen-doping thermal treatment process, the interlayer spacing of Ti3C2Tx MXene was increased, providing wider ion transport channels. Additionally, inert carbon atoms were transformed into electrochemically active nitrogen atoms, leading to the construction of more active centers and surface defects. The spatial confinement of N-Ti3C2Tx MXene during the electrochemical process, effectively mitigating volume expansion and maintaining structural stability upon applied as electrode materials. Benefitting from the unique 3D conductive network, the mezzanine heterostructure Fe2O3@C@N-Ti3C2Tx composite nanofibers exhibits excellent Li/Na storage performance. In LIBs, it delivers a capacity of 226 mAh/g at 10 A/g and retains 314 mAh/g after 2800 ultra-long cycles at 5 A/g. For SIBs, it exhibits a capacity of 135 mAh/g at 5 A/g and maintains 209 mAh/g after 3000 cycles at 2 A/g.

Synthesis and characterization of fluorescent carbon dots obtained from citrus x sinensis by an eco-friendly method

The synthesis and characterization of fluorescent carbon dots (CDs) obtained by an environmentally friendly method through carbonization, fragmentation and centrifugation of orange peel (citrus x sinensis) are reported. By thermogravimetry, 4 degradation regions associated with hemicellulose, lignin and organic matter were found. The SEM micrograph shows a rough surface morphology with flakes and granulation due to the carbonization process. X-ray diffraction (XRD) shows a hexagonal crystalline structure associated with graphitic carbon, which was confirmed by Raman scatterings, Raman spectroscopy that presented two main vibrational bands, D and G, associated with active defects in the graphitic carbon crystalline structure. The Transmission electron microscopy (TEM) micrograph showed CDs of size 33.65 ± 0.68n, and with High-Resolution TEM (HRTEM) an interplanar distance of 0.213 nm in the direction of the crystal planes (100) of graphitic carbon was obtained, which were confirmed by XRD and Raman spectroscopy. X-ray photoelectron spectroscopy revealed the relative atomic composition and identified the strongest photoelectron transitions of carbon, sp2 (286 eV) and sp3 (285 eV), in graphitic carbon. Ultraviolet–visible (UV–Vis) spectroscopy of the CDs obtained from fragmentation graphitic carbon showed a band at 205 nm associated with CC bonds π-π* of CC and n-π*, as well as two bands at 276 and 328 nm associated with bond transitions. The Fourier Transform Infrared (FTIR) spectrum showed bands associated with CHs bonds as well as CC, CN bonds of the CDs. Fluorescence spectroscopy showed three emission bands at 2.43, 2.54 and 2.87 eV. The CDs showed the emission of the characteristic blue colour.

Saturation degree in dopant monolayers as modulator of Al-doping of ZnO by the Atomic Layer Deposition-supercycle approach

Transparent conductive oxides are trending topic in science and technology due to counterintuitive properties: optical transparency and high electrical conductivity. ZnO is a promising example with optoelectronic properties enhanced by supervalent doping. Al-doped ZnO (AZO) has emerged as leading candidate to replace environmentally hazardous In-doped SnO2. However, an ongoing debate exists regarding whether Al-doping improves its optoelectronic properties by substitutional doping or by promoting active defects. Here, we focused on Al-doping of ZnO by atomic layer deposition (ALD) applying the supercycle approach, showing that, decreasing Zn precursor dosing during dopant cycles, the decreasing saturation degree of Zn-species monolayers leads to morphological and microstructural changes that negatively impact optoelectronic properties, whereas Al content remains invariant. Our results demonstrate that unsaturated surfaces after decreasing Zn precursor dosing play a crucial role in Al incorporation, suggesting that, to maximize the effect of doping, complete oxide substitution reactions rather than those of conventional ALD must control growth, while crystallinity must remain. These findings could impact strategy designing for optimization of optoelectronic properties of AZO films deposited by ALD by inclining the debate towards the hypothesis that electrical properties are determined by Al substitutional doping together with active defects formed due to substitutional doping itself.

Deepening the Understanding of Carbon Active Sites for ORR Using Electrochemical and Spectrochemical Techniques.

Defect-containing carbon nanotube materials were prepared by subjecting two commercial multiwalled carbon nanotubes (MWCNTs) of different purities to purification (HCl) and oxidative conditions (HNO3) and further heat treatment to remove surface oxygen groups. The as-prepared carbon materials were physicochemically characterized to observe changes in their properties after the different treatments. TEM microscopy shows morphological modifications in the MWCNTs after the treatments such as broken walls and carbon defects including topological defects. This leads to both higher surface areas and active sites. The carbon defects were analysed by Raman spectroscopy, but the active surface area (ASA) and the electrochemical active surface area (EASA) values showed that not all the defects are equally active for oxygen reduction reactions (ORRs). This suggests the importance of calculating either ASA or EASA in carbon materials with different structures to determine the activity of these defects. The as-prepared defect-containing multiwalled carbon nanotubes exhibit good catalytic performance due to the formation of carbon defects active for ORR such as edge sites and topological defects. Moreover, they exhibit good stability and methanol tolerances. The as-prepared MWCNTs sample with the highest purity is a promising defective carbon material for ORR because its activity is only related to high concentrations of active carbon defects including edge sites and topological defects.

Charge Carrier Capture by Prominent Defect Centers in 4H-SiC

The knowledge of capture properties of electrically active defects is of primary importance as it helps to understand which deep states are effective in controlling the excess free carriers’ lifetime. Combining DLTS capture experiments with thermal emission measurements enables an overall thermodynamic description of deep states, thus making it possible to characterize recombination centers in semiconductor-based devices. In the present study, junction DLTS capture rate measurements were employed to extract the true capture cross-sections (inversely proportional to the carrier lifetime) and capture energy barriers for the main lifetime limiting defects in 4H-SiC (silicon carbide). A peculiar forward bias dependence of the capture parameters was observed for the shallow boron (B) hole trap. Capture rate measurements on the deep boron (D-center) trap also evidenced the presence of two capture mechanisms, thus allowing discrimination of D1 and D2 deep states within the D-center DLTS peak. The results were combined with activation energies and apparent capture cross-sections to obtain the free energy (ΔG) of electronic activation for the analysed deep states.

Practical Improvement of Noncontact Production Monitoring of Doping in SiC Wafers with Extended Epilayer Defects

We discuss two defect related practical improvements in the corona noncontact CV metrology, (CnCV) for SiC. The improvements are introduced in response to requests from industrial tool users. The first improvement quantifies mapping of electrically active defects with the QUAD technique (Quality, Uniformity, and Defects). It provides the capability of user selectable die grids directly comparable with Near UV-PL and optical defect mapping. This shall enhance understanding of the device killer defects and help to correlate epilayer defects and device yield. The second improvement introduces auto-remeasurement of outliers appearing in doping measurements on defective sites. This procedure is analogous to that used in the Hg probe technique and it provides a means for correcting defect related distortions in SPC doping monitoring charts.

Near zero-field magnetoresistance and defects in gallium nitride pn junctions

As gallium nitride (GaN) grows in importance, an atomic scale understanding of device performance is of significant relevance. In this paper, we present the first observations of near zero-field magnetoresistance (NZFMR) detecting electrically active defects in GaN-based devices. Our observations involve recombination current in GaN pn junction diodes. We attribute the NZFMR response to gallium vacancies.

Pt nanoparticles in cooperation with Mn-P composite drive base-free selective oxidation of 5-hydroxymethylfurfural

Selective oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) is a key approach to sustainable upgrading of furfural based biomass resources. In this work, small Pt nanoparticles (about 2.1 nm) loaded on MnPnOx composite were shown to effectively drive the base-free aerobic oxidation of HMF to FDCA in water. Regulating the P content in carrier enabled to tune HMF conversion and product distribution. The optimal Pt/MnP0.5Ox catalyst can reach 98% yield of FDCA at 110 °C under 10 bar of O2 after 24 h. It also showed the highest initial conversion rate of HMF (13 mmol molPt−1 s−1) and productivity of FDCA (4.1 mmol molPt−1 h−1) among all the Pt/MnPnOx catalysts. The carrier effect of P addition on promoting Pt oxidation catalysis was elucidated by using rigorous kinetic investigations and comprehensive characterizations. The initial conversion rate, rate constant and apparent activation energy were independently measured on oxidation of HMF and its derived intermediates. ICP-MS, XRD, TEM, H2-TPR, O2-TPD and XPS were used to acquire catalytic properties. It was demonstrated that P addition led to changes in carrier structure and metal-support interaction, which eventually promoted the formation of highly active electron-rich Pt0 sites and mobile reactive defect oxygen species. These features allowed improving the selective oxidation catalysis of supported-Pt nanoparticles for the base-free conversion of HMF to FDCA.

In Situ PL Probes the Effect of 2D SnSe Nanosheets on the Crystallization Process of CsPbI2Br Perovskite Solar Cells.

Reducing defects in the active layer is important for improving the crystalline quality of all-inorganic perovskite solar cells (PSCs). Exploring novel additives is one of the most promising approaches to minimize active layer defects. In this work, two-dimensional (2D) SnSe nanosheets with excellent optoelectronic properties are prepared using an ultrasonic exfoliation method. The prepared 2D SnSe nanosheets are introduced into a CsPbI2Br precursor, which reduces the defect formation at grain boundaries and enhances the crystallinity of CsPbI2Br perovskites. We use the in situ photoluminescence (PL) technique to investigate the role of 2D materials in the crystallization process. The results show that SnSe nanosheets primarily shorten the grain boundary merging time and reduce the defect generation during the grain boundary merging stage, thereby regulating the crystallization of perovskite. In addition, SnSe nanosheets passivate uncoordinated Pb atoms at grain boundaries by Se atoms, further reducing the defect density in perovskite. As a result, PSCs exhibit a higher power conversion efficiency (PCE) of 14.24% and a Voc of 1.22 V. This study highlights the role of 2D materials in enhancing the crystalline quality and PCE of PSCs.

Ag/Nico LDH Heterogeneous Structure for Efficient Alkaline Oxygen Evolution

Hydrogen is regarded as an ideal energy carrier owing to the highest energy density, presenting the possibility of substituting for traditional fossil fuels. Nowadays, water splitting is significant in producing large-scale hydrogen. However, the sluggish reaction kinetics during anodic OER, caused by the four-electron transfer, has hindered this energy conversion strategy. Although IrO2 and RuO2 are currently the representative OER electrocatalysts, their high cost and unsatisfactory performance limit their commercial application.Numerous electrocatalysts, including oxides, hydroxides, high-entropy alloy, metal organic frameworks, and their hybrids, have been developed to address this issue. Among them, layered double hydroxides (LDH) have demonstrated outstanding performance due to their highly adjustable structure, powerful host-guest interaction, and single-atomic-like active sites. NiCo LDH has attracted significant attention because of its appropriate atomic and electronic structure, intrinsic active sites and defects, compared with single-type Ni-based or Co-based catalysts. However, to meet the requirements of industrial production, a simple and scalable method to synthesize electrocatalysts with favorable OER activity and stability is imperative.We propose a facile and scalable strategy to synthesize Ag-decorated NiCo LDH (Ag/NiCo LDH) by electrodeposition and subsequent spontaneous redox reaction. A spontaneous redox reaction driven by the difference in standard redox potential of reactants can be applied to prepare heterogeneous catalysts in consequence of the facile process, tight combination between substrate and decorated materials, and no supplementary energy input. Silver metal, with its appropriate standard redox potential, good conductivity, and low cost compared to Iridium and Ruthenium, serves as a suitable decorated material. This strategy provides several advantages, for example, Ⅰ) optimizing the electronic structure of Ni and Co by the charge transfer from Ni, Co to Ag; Ⅱ) exposing more active surface area because of the heterogeneous structure, Ⅲ) improving the charge transfer capacity owing to the excellent conductivity of Ag nanoparticles, Ⅳ) increasing the stability of NiCo LDH due to the tight combination of Ag and NiCo LDH, Ⅴ) lowering the catalysts' price compared with Iridium and Ruthenium.The synthesized Ag/NiCo LDH exhibits enhanced OER performance in 1M KOH electrolyte, achieving an overpotential of 440 mV at a current density of 1000 mA cm-2 geo, surpassing that of NiCo LDH (722 mV at 1000 mA cm-2 geo). Surprisingly, Ag/NiCo LDH demonstrates exceptional durability, lasting over 425 h at 1000 mA cm-2 geo, an improvement of nearly eight-fold compared to NiCo LDH (50 h). This study provides a promising approach for developing efficient and stable electrocatalysts for alkaline oxygen evolution.