Atomic Defects in Two-Dimensional Materials: From Single-Atom Spectroscopy to Functionalities in Opto-/Electronics, Nanomagnetism, and Catalysis

Defect Engineering in Bi-Based Photo/Electrocatalysts for Nitrogen Reduction to Ammonia

Main group Bi-based materials have gained popularity as N2 reduction reaction (NRR) photo/electrocatalysts due to their ability to inhibit competitive H2 evolution reactions (HER) and the unique N2 adsorption activities. The introduction of defects in Bi-based catalysts represents a highly effective strategy for enhancing light absorption, promoting efficient separation of photogenerated carriers, optimizing the activity of free radicals, regulating electronic structure, and improving catalytic performance. In this review, we outline the various applications of state of the defect engineering in Bi-based catalysts and elucidate the impact of vacancies on NRR performance. In particular, the types of defects, methods of defects tailoring, advanced characterization techniques, as well as the Bi-based catalysts with abundant defects and their corresponding catalytic behavior in NRR were elucidated in detail. Finally, the main challenges and opportunities for future development of defective Bi-based NRR catalysts are discussed, which provides a comprehensive theoretical guidance for this field.

Design of Multifunctional Electrocatalysts for ORR/OER/HER/HOR: Janus Makes Difference

Multifunctional electrocatalysts for hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) have broad application prospects; However, realization of such kinds of materials remain difficulties because it requires the materials to have not only unique electronic properties, but multiple active centers to deal with different reactions. Here, employing density functional theory (DFT) computations, it is demonstrated that by decorating the Janus-type 2D transition metal dichalcogenide (TMD) of TaSSe with the single atoms, the materials can achieve multifunctionality to catalyze the ORR/OER/HER/HOR. Out of sixteen catalytic systems, Pt-VS (i.e., Pt atom embedded in the sulfur vacancy), Pd-VSe, and Pt-VSe@TaSSe are promising multifunctional catalysts with superior stability. Among them, the Pt-VS@TaSSe catalyst exhibits the highest activity with theoretical overpotentials ηORR = 0.40 V, ηOER = 0.39 V, and ηHER/HOR = 0.07 V, respectively, better than the traditional Pt (111), IrO2 (110). The interplays between the catalyst and the reaction intermediate over the course of the reaction are then systematically investigated. Generally, this study presents a viable approach for the design and development of advanced multifunctional electrocatalysts. It enriches the application of Janus, a new 2D material, in electrochemical energy storage and conversion technology.

Scanning Probe Microscopies for Characterizations of 2D Materials

2D materials are intriguing due to their remarkably thin and flat structure. This unique configuration allows the majority of their constituent atoms to be accessible on the surface, facilitating easier electron tunneling while generating weak surface forces. To decipher the subtle signals inherent in these materials, the application of techniques that offer atomic resolution (horizontal) and sub-Angstrom (z-height vertical) sensitivity is crucial. Scanning probe microscopy (SPM) emerges as the quintessential tool in this regard, owing to its atomic-level spatial precision, ability to detect unitary charges, responsiveness to pico-newton-scale forces, and capability to discern pico-ampere currents. Furthermore, the versatility of SPM to operate under varying environmental conditions, such as different temperatures and in the presence of various gases or liquids, opens up the possibility of studying the stability and reactivity of 2D materials in situ. The characteristic flatness, surface accessibility, ultra-thinness, and weak signal strengths of 2D materials align perfectly with the capabilities of SPM technologies, enabling researchers to uncover the nuanced behaviors and properties of these advanced materials at the nanoscale and even the atomic scale.

On-Surface Synthesis of Non-Benzenoid Nanographenes Embedding Azulene and Stone-Wales Topologies

The incorporation of non-benzenoid motifs in graphene nanostructures significantly impacts their properties, making them attractive for applications in carbon-based electronics. However, understanding how specific non-benzenoid structures influence their properties remains limited, and further investigations are needed to fully comprehend their implications. Here, we report an on-surface synthetic strategy toward fabricating non-benzenoid nanographenes containing different combinations of pentagonal and heptagonal rings. Their structure and electronic properties were investigated via scanning tunneling microscopy and spectroscopy, complemented by computational investigations. After thermal activation of the precursor P on the Au(111) surface, we detected two major nanographene products. Nanographene Aa-a embeds two azulene units formed through oxidative ring-closure of methyl substituents, while Aa-s contains one azulene unit and one Stone-Wales defect, formed by the combination of oxidative ring-closure and skeletal ring-rearrangement reactions. Aa-a exhibits an antiferromagnetic ground state with the highest magnetic exchange coupling reported up to date for a non-benzenoid containing nanographene, coexisting with side-products with closed shell configurations resulted from the combination of ring-closure and ring-rearragement reactions (Ba-a , Ba-s , Bs-a and Bs-s ). Our results provide insights into the single gold atom assisted synthesis of novel NGs containing non-benzenoid motifs and their tailored electronic/magnetic properties.

Characterization of emerging 2D materials after chemical functionalization

The chemical modification of 2D materials has proven a powerful tool to fine tune their properties. With this motivation, the development of new reactions has moved extremely fast. The need for speed, together with the intrinsic heterogeneity of the samples, has sometimes led to permissiveness in the purification and characterization protocols. In this review, we present the main tools available for the chemical characterization of functionalized 2D materials, and the information that can be derived from each of them. We then describe examples of chemical modification of 2D materials other than graphene, focusing on the chemical description of the products. We have intentionally selected examples where an above-average characterization effort has been carried out, yet we find some cases where further information would have been welcome. Our aim is to bring together the toolbox of techniques and practical examples on how to use them, to serve as guidelines for the full characterization of covalently modified 2D materials.

Atomistic Probing of Defect-Engineered 2H-MoTe2 Monolayers

Point defects dictate various physical, chemical, and optoelectronic properties of two-dimensional (2D) materials, and therefore, a rudimentary understanding of the formation and spatial distribution of point defects is a key to advancement in 2D material-based nanotechnology. In this work, we performed the demonstration to directly probe the point defects in 2H-MoTe2 monolayers that are tactically exposed to (i) 200 °C-vacuum-annealing and (ii) 532 nm-laser-illumination; and accordingly, we utilize a deep learning algorithm to classify and quantify the generated point defects. We discovered that tellurium-related defects are mainly generated in both 2H-MoTe2 samples; but interestingly, 200 °C-vacuum-annealing and 532 nm-laser-illumination modulate a strong n-type and strong p-type 2H-MoTe2, respectively. While 200 °C-vacuum-annealing generates tellurium vacancies or tellurium adatoms, 532 nm-laser-illumination prompts oxygen atoms to be adsorbed/chemisorbed at tellurium vacancies, giving rise to the p-type characteristic. This work significantly advances the current understanding of point defect engineering in 2H-MoTe2 monolayers and other 2D materials, which is critical for developing nanoscale devices with desired functionality.

Quantitative Predictions of the Thermal Conductivity in Transition Metal Dichalcogenides: Impact of Point Defects in MoS2 and WS2 Monolayers

Transition metal dichalcogenides are investigated for various applications at the nanoscale because of their unique combination of properties and dimensionality. For many of the anticipated applications, heat conduction plays an important role. At the same time, these materials often contain relatively large amounts of point defects. Here, we provide a systematic analysis of the impact of intrinsic and selected extrinsic defects on the lattice thermal conductivity of MoS2 and WS2 monolayers. We combine Boltzmann transport theory and Green's function-based T-matrix approach for the calculation of scattering rates. The force constants for the defect configurations are obtained from density functional theory calculations via a regression approach, which allows us to sample a rather large number of defects at a moderate computational cost and to systematically enforce both the translational and rotational acoustic sum rules. The calculated lattice thermal conductivity is in quantitative agreement with the experimental data for heat transport and defect concentrations for both MoS2 and WS2. Crucially, this demonstrates that the strong deviation from a 1/T temperature dependence of the lattice thermal conductivity observed experimentally can be fully explained by the presence of point defects. We furthermore predict the scattering strengths of the intrinsic defects to decrease in the sequence VMo ≈ V2S= > V2S⊥ > VS > Sad in both materials, while the scattering rates for the extrinsic (adatom) defects decrease with increasing mass such that Liad > Naad > Kad. Compared with earlier work, we find that both intrinsic and extrinsic adatoms are relatively weak scatterers. We attribute this difference to the treatment of the translational and rotational acoustic sum rules, which, if not enforced, can lead to spurious contributions in the zero-frequency limit.

Defect Emission and Its Dipole Orientation in Layered Ternary Znln2 S4 Semiconductor

Defect engineering is promising to tailor the physical properties of 2D semiconductors for function-oriented electronics and optoelectronics. Compared with the extensively studied 2D binary materials, the origin of defects and their influence on physical properties of 2D ternary semiconductors are not clarified. Here, the effect of defects on the electronic structure and optical properties of few-layer hexagonal Znln2 S4 is thoroughly studied via versatile spectroscopic tools in combination with theoretical calculations. It is demonstrated that the Zn-In antistructural defects induce the formation of a series of donor and acceptor energy levels and sulfur vacancies induce donor energy levels, leading to rich recombination paths for defect emission and extrinsic absorption. Impressively, the emission of donor-acceptor pair in Znln2 S4 can be significantly tailored by electrostatic gating due to efficient tunability of Fermi level (Ef ). Furthermore, the layer-dependent dipole orientation of defect emission in Znln2 S4 is directly revealed by back focal plane imagining, where it presents obviously in-plane dipole orientation within a dozen-layer thickness of Znln2 S4 . These unique features of defects in Znln2 S4 including extrinsic absorption, rich recombination paths, gate tunability, and in-plane dipole orientation are definitely a benefit to the advanced orientation-functional optoelectronic applications.

Defect engineered magnetism induction and electronic structure modulation in monolayer MoS2

The electronic, magnetic, and optical characteristics of a defective monolayer MoS2 were examined by employing density functional theory (DFT)-based first-principles calculations. The effects of several defects on the electrical, magnetic, and optical properties, including Mo vacancies, MoS3 vacancies, and the substitution of a single Mo atom by two S atoms were studied in this work. Our first-principles calculations revealed that different types of defects produced distinct energy states within the band gap, leading to a band gap reduction after the introduction of various types of defects, which caused a change from semiconducting to metallic behavior. The spin-up and spin-down states were separated in the case of MoS3 vacancy. The total magnetization was ∼ -0.83 μ B /cell, and the absolute magnetization was ∼ 1.23 μ B /cell. Moreover, spin-up states had a 0.45 eV band gap, whereas spin-down states were metallic. Consequently, it can be promising for spin filter applications. It was disclosed that the broadband part of the electromagnetic spectrum has a high absorption coefficient, which is necessary for applications including impurity detection, photodiodes, and solar cells. Designing spintronic and optoelectronic devices will benefit from the modification of the electrical, optical, and magnetic properties by defect engineering of MoS2 monolayers presented here.

Selective oxidation of p-phenylenediamine for blood glucose detection enabled by Se-vacancy-rich TiSe2-x@Au nanozyme

Nanozymes with enzyme-like characteristics have drawn wide interest but the catalytic activity and substrate selectivity of nanozymes still need improvement. Herein, Se-vacancy-rich TiSe2-x@Au nanocomposites are designed and demonstrated as nanozymes. The TiSe2-x@Au nanocomposites show excellent peroxidase-like activity and the chromogenic substrate p-phenylenediamine (PPD) can be selectively oxidized to compounds that exhibit an absorption peak at 413 nm that differs from that of self-oxidation or generally oxidized species, suggesting high catalytic activity and strong substrate selectivity. Theoretical calculations reveal that the PPD adsorption geometry at Se vacancies with an adsorption energy of -3.00 eV shows a unique spatial configuration and charge distribution, thereby inhibiting the free reaction and promoting both the activity and selectivity in PPD oxidation. The TiSe2-x@Au colorimetric system exhibits a wide linear range of 0.015 mM-0.6 mM and a low detection limit of 0.0037 mM in the detection of glucose. The blood glucose detection performance for human serum samples is comparable to that of a commercial glucose meter in the hospital (relative standard deviation < 6%). Our findings demonstrate a new strategy for rapid and accurate detection of blood glucose and our results provide insights into the future design of nanozymes.

Metallic CrP2 monolayer: potential applications in energy storage and conversion

Phosphorus-rich compounds have emerged as a promising class of energy storage and conversion materials due to their interesting structures and electrochemical properties. Herein, we propose that a metallic CrP2 monolayer, isomorphic to 1H-phase MoS2, is a good prospect as an anode for K-ion batteries and a catalyst for hydrogen evolution through first-principles calculations. The CrP2 monolayer demonstrates not only a desirable high K storage capacity (940 mA h g-1) but also a low K-ion diffusion barrier (0.10 eV) and average open circuit voltage (0.40 V). On the other hand, its Gibbs free energy (0.02 eV)/active site density is superior/comparable to that of commercial Pt, resulting from the contribution of the lone pair electrons of the P atom. Its high structural stability and intrinsic metallicity can ensure high safety and performance during the cyclic process. These interesting properties make the CrP2 monolayer a promising multifunctional material for energy storage and conversion devices.

Visualized and Nondestructive Quality Identification of Two-Dimensional MoS2 Based on Principal Component Analysis

To date, the common quality characterizations for MoS2 are inefficient or cause irreversible damage to the samples, which have limited scalability and low throughput. Here, we propose a visualized and nondestructive approach to evaluate the quality of MoS2 based on the PCA machine learning method. Through PCA processing of PL mapping, the CVD grown MoS2 with different edge defect densities can be well distinguished. Furthermore, six twin GBs along the sulfur zigzag direction of the six pointed MoS2 stars are also successfully identified. To verify the correctness of the identification results, we measured the lifetime mapping and thermal expansion coefficient of the synthesized MoS2 samples. It is found that the high quality MoS2 samples have a shorter carrier lifetime (∼0.291 ns) and lower thermal expansion coefficient (∼2.03 × 10-5K-1). Therefore, our work offers a new approach to evaluate the quality of MoS2 to drive their practical application.

Phase Engineering and Synchrotron-Based Study on Two-Dimensional Energy Nanomaterials

In recent years, there has been significant interest in the development of two-dimensional (2D) nanomaterials with unique physicochemical properties for various energy applications. These properties are often derived from the phase structures established through a range of physical and chemical design strategies. A concrete analysis of the phase structures and real reaction mechanisms of 2D energy nanomaterials requires advanced characterization methods that offer valuable information as much as possible. Here, we present a comprehensive review on the phase engineering of typical 2D nanomaterials with the focus of synchrotron radiation characterizations. In particular, the intrinsic defects, atomic doping, intercalation, and heterogeneous interfaces on 2D nanomaterials are introduced, together with their applications in energy-related fields. Among them, synchrotron-based multiple spectroscopic techniques are emphasized to reveal their intrinsic phases and structures. More importantly, various in situ methods are employed to provide deep insights into their structural evolutions under working conditions or reaction processes of 2D energy nanomaterials. Finally, conclusions and research perspectives on the future outlook for the further development of 2D energy nanomaterials and synchrotron radiation light sources and integrated techniques are discussed.

Lattice-Strain Engineering for Heterogenous Electrocatalytic Oxygen Evolution Reaction

The energy efficiency of metal-air batteries and water-splitting techniques is severely constrained by multiple electronic transfers in the heterogenous oxygen evolution reaction (OER), and the high overpotential induced by the sluggish kinetics has become an uppermost scientific challenge. Numerous attempts are devoted to enabling high activity, selectivity, and stability via tailoring the surface physicochemical properties of nanocatalysts. Lattice-strain engineering as a cutting-edge method for tuning the electronic and geometric configuration of metal sites plays a pivotal role in regulating the interaction of catalytic surfaces with adsorbate molecules. By defining the d-band center as a descriptor of the structure-activity relationship, the individual contribution of strain effects within state-of-the-art electrocatalysts can be systematically elucidated in the OER optimization mechanism. In this review, the fundamentals of the OER and the advancements of strain-catalysts are showcased and the innovative trigger strategies are enumerated, with particular emphasis on the feedback mechanism between the precise regulation of lattice-strain and optimal activity. Subsequently, the modulation of electrocatalysts with various attributes is categorized and the impediments encountered in the practicalization of strained effect are discussed, ending with an outlook on future research directions for this burgeoning field.

Supercritical CO2 Directional-Assisted Synthesis of Low-Dimensional Materials for Functional Applications

Supercritical CO2 (SC CO2 ), as one of the unique fluids that possess fascinating properties of gas and liquid, holds great promise in chemical reactions and fabrication of materials. Building special nanostructures via SC CO2 for functional applications has been the focus of intense research for the past two decades, with facile regulated reaction conditions and a particular reaction field to operate compared to the more widely used solvent systems. In this review, the significance of SC CO2 on fabricating various functional materials including modification of 1D carbon nanotubes, 2D materials, and 2D heterostructures is stated. The fundamental aspects involving building special nanostructures via SC CO2 are explored: how their structure, morphology, and chemical composition be affected by the SC CO2 . Various optimization strategies are outlined to improve their performances, and recent advances are combined to present a coherent understanding of the mechanism of SC CO2 acting on these functional nanostructures. The wide applications of these special nanostructures in catalysis, biosensing, optoelectronics, microelectronics, and energy transformation are discussed. Moreover, the current status of SC CO2 research, the existing scientific issues, and application challenges, as well as the possible future directions to advance this fertile field are proposed in this review.

Effect of vacancy defect and strain on the structural, electronic and magnetic properties of carbon nitride 2D monolayers by DFTB method

We investigate the electronic and magnetic properties of CnNm(C₆N₆, C₂N, C₃N and C₃N₄) using density functional tight-binding (DFTB) method. We find that these compounds are dynamically stable and their electronic band gaps are in the range of 0.59-3.28 eV. We show that the electronic structure is modulated by strain and the semiconducting behavior is well preserved except for C₃N at +5% biaxial strain, where a transition from semiconductor to metal was observed. Under +3% biaxial strain, C₃N₄undergoes a transition from an indirect (K-Γ) to a direct (Γ-Γ) band gap. Moreover, band gap of C₂N transforms from direct (Γ-Γ) to indirect (M-Γ) under +4% biaxial strain. However, no change in the nature of the band gap of C₆N₆. Further, when the studied materials under uniaxial tensile strain, their bandgaps reduce. Our theoretical study showed that an indirect-to-direct nature transition may occur for C₆N₆and for C₃N₄, which broadens their applications. On the other hand, magnetism is observed in all N-vacancy defected CnNm, which encourages its application in spintronic. Moreover, calculations of formation energies indicate that N-vacancy is energetically more favorable than C-vacancy in both C₂N and C₃N₄. Opposite behavior found for C₆N₆and C₃N.

Machine learning and DFT investigation of CO, CO2 and CH4 adsorption on pristine and defective two-dimensional magnesene

Adsorption study of environmentally toxic small gas molecules on two-dimensional (2D) materials plays a significant role in analyzing the performance of sensors. In this work, density functional theory (DFT) and machine learning (ML) techniques have been employed to systematically study the adsorption properties of CO, CO₂, and CH₄ gas molecules on the pristine and defective planar magnesium monolayer, known as magnesene (2D-Mg). The DFT analysis showed that mechanically robust 2D-Mg retains its metallicity in the presence of both mono and di-vacancy defects. Our observations have shown that 2D-Mg, whether defective or pristine, exhibits distinct adsorption behaviors towards CO, CO₂, and CH₄ gas molecules, including varying chemisorption and physisorption, charge transfer, and distance from the gas molecules. When analyzing the recovery time of gas molecules at room temperature, it is clear that adsorption energy has a direct correlation with the adsorption-desorption cycles, and CH₄ possesses an ultra-low recovery time (15.27 ps) compared to CO₂ (1.04 ns) and CO (0.90 μs) molecules. The analysis showed that defects do not have a significant impact on the work function of 2D-Mg. However, the work function decreased upon adsorption of CH₄, resulting in improved sensitivity due to changes in the electronic properties. Additionally, we explored supervised ML regression models to evaluate their ability to act as a surrogate for the DFT-based adsorption energy calculation. Using both system statistics and smooth overlap of atomic position (SOAP)-based featurization, we observed that adsorption energies can be predicted with a mean absolute error of 0.10 eV.

Competition mechanism of exciton decay channels in the stacked multilayer tungsten sulfide

The competition mechanism of exciton decay channels in the multilayer TMDs remains poorly understood. Here, the exciton dynamics in the stacked WS₂ was studied. The exciton decay processes are divided into the fast and slow decay processes, which are dominated by the exciton-exciton annihilation (EEA) and defect-assisted recombination (DAR), respectively. The lifetime of EEA is on the order of hundreds of femtoseconds (400∼1100 fs). It is decreased initially, followed by an increase with adding layer thickness, which can be attributed to the competition between phonon-assisted effect and defect effect. The lifetime of DAR is on the timescale of hundreds of picoseconds (200∼800 ps), which is determined by the defect density especially in a high injected carrier density.

2D Organic Materials: Status and Challenges

In the past few decades, 2D layer materials have gradually become a central focus in materials science owing to their uniquely layered structural qualities and good optoelectronic properties. However, in the development of 2D materials, several disadvantages, such as limited types of materials and the inability to synthesize large-scale materials, severely confine their application. Therefore, further exploration of new materials and preparation methods is necessary to meet technological developmental needs. Organic molecular materials have the advantage of being customizable. Therefore, if organic molecular and 2D materials are combined, the resulting 2D organic materials would have excellent optical and electrical properties. In addition, through this combination, the free design and large-scale synthesis of 2D materials can be realized in principle. Furthermore, 2D organic materials exhibit excellent properties and unique functionalities along with great potential for developing sensors, biomedicine, and electronics. In this review, 2D organic materials are divided into five categories. The preparation methods and material properties of each class of materials are also described in detail. Notably, to comprehensively understand each material's advantages, the latest research applications for each material are presented in detail and summarized. Finally, the future development and application prospects of 2D organic materials are briefly discussed.

Magnetic Amplification at Yb3+ "Designer Defects" in the van der Waals Ferromagnet CrI3

The two-dimensional (2D) van der Waals ferromagnet CrI₃ has been doped with the magnetic optical impurity Yb³⁺ to yield materials that display sharp multiline Yb³⁺ photoluminescence (PL) controlled by the magnetism of CrI₃. Magneto-PL shows that Yb³⁺ magnetization is pinned to the magnetization of CrI₃. An effective internal field of ∼10 T at Yb³⁺ is estimated, attributed to strong in-plane Yb³⁺-Cr³⁺ superexchange coupling. The anomalously low energy of Yb³⁺ PL in CrI₃ reflects relatively high Yb³⁺-I^- covalency, contributing to Yb³⁺-Cr³⁺ superexchange coupling. The Yb³⁺ PL energy and line width both reveal the effects of spontaneous zero-field CrI₃ magnetic ordering within 2D layers below T_C, despite the absence of net magnetization in multilayer samples. These results illustrate the use of optical impurities as "designer defects" to introduce unique functionality to 2D magnets.

A theoretical study on the line defects in β12-borophene: enhanced direct-current and alternating-current conductances

Using density functional theory and the non-equilibrium Green's function method, we theoretically investigated the structures, stabilities, electronic properties, and the direct-current (DC) and alternating-current (AC) transport properties of the line defects in two-dimensional material β₁₂-borophene. Our results suggest that there exist six line defects that can enhance the stability of β₁₂-borophene and the line defects have profound influences on the electronic structure of β₁₂-borophene. Along the zigzag direction, the line defects can change the atomic orbital components of the Dirac cones in perfect β₁₂-borophene, but the line defects along the armchair direction have complicated influences on the Dirac cones. In the case of DC transport, some of the line defects lead to the constant DC phenomenon and the negative differential resistance effect, and enhance the DC conductances since the line defects exhibit typical one-dimensional characteristics. In the case of AC transport, some of the line defects enhance the AC conductances in the medium-frequency and high-frequency ranges through the photon-assisted tunneling effect. The microscopic mechanisms of the enhanced DC and AC conductances are different. In addition, for a low-frequency range, the equivalent circuits of β₁₂-borophene and the line defects were also suggested, which will be beneficial for designing borophene-based functional nanodevices.

Layer-Structured Anisotropic Metal Chalcogenides: Recent Advances in Synthesis, Modulation, and Applications

The unique electronic and catalytic properties emerging from low symmetry anisotropic (1D and 2D) metal chalcogenides (MCs) have generated tremendous interest for use in next generation electronics, optoelectronics, electrochemical energy storage devices, and chemical sensing devices. Despite many proof-of-concept demonstrations so far, the full potential of anisotropic chalcogenides has yet to be investigated. This article provides a comprehensive overview of the recent progress made in the synthesis, mechanistic understanding, property modulation strategies, and applications of the anisotropic chalcogenides. It begins with an introduction to the basic crystal structures, and then the unique physical and chemical properties of 1D and 2D MCs. Controlled synthetic routes for anisotropic MC crystals are summarized with example advances in the solution-phase synthesis, vapor-phase synthesis, and exfoliation. Several important approaches to modulate dimensions, phases, compositions, defects, and heterostructures of anisotropic MCs are discussed. Recent significant advances in applications are highlighted for electronics, optoelectronic devices, catalysts, batteries, supercapacitors, sensing platforms, and thermoelectric devices. The article ends with prospects for future opportunities and challenges to be addressed in the academic research and practical engineering of anisotropic MCs.

Oxoiron(IV)-dominated Heterogeneous Fenton-like Mechanism of Fe-Doped MoS2

Oxoiron(IV) species are a critical intermediate in the Fe-based Fenton-like process at circumneutral pH, and its oxidative reactivity is closely related to the ligands. An optional inorganic host material, MoS₂ , is selected to construct a highly reactive sulfur ligand coordinated Fe species in this work. The Fe species doped in MoS₂ is presented as the Fe^II centre and triggers the transformation of the 2H phase to the octahedral 1T phase MoS₂ . The role of the interaction between doped Fe and the MoS₂ host lattice on the formation of oxoiron(IV) is studied. A significant Fenton-like reactivity and a remarkable accumulation of oxoiron(IV) species were observed for Fe-MoS₂ . The quenching experiment was implemented to disclose the predominant role of oxoiron(IV) species in the Fe-MoS₂ /H₂ O₂ Fenton-like system. Furthermore, oxoiron(IV) species could transform into the ⋅O₂ ^- and ¹ O₂ , which further expedites the Fenton-like reaction.

Cu/CuOx@C Composite as a High-Efficiency Electrocatalyst for Oxygen Reduction Reactions

Among clean energy transformation devices, fuel cells have gained special attention over the past years; however, advancing appropriate non-valuable metal impetuses to halfway supplant the customary Pt/C impetus is still in progress. In this paper, we propose a specific electrocatalyst in the formula of highly-active Cu species, associated with coated carbon (Cu@C-800), for oxygen reduction reaction (ORR) through post-treatment of a self-assembled precursor. The optimized catalyst Cu@C-800 showed excellent ORR performance (i.e., the onset potential was 1.00 V vs. RHE, and half-wave potential of 0.81 V vs. RHE), high stability, resistance to methanol, and high four-electron selectivity. The enhancement is attributed to the synergy between the carbon matrix and a high explicit surface region and rich Cu nano-species.

Computational Insight into Defective Boron Nitride Supported Double-Atom Catalysts for Electrochemical Nitrogen Reduction

Designing highly selective and efficient double-atom electrocatalysts (DACs) is essential for achieving a superior nitrogen-reduction reaction (NRR) performance. Herein, we explored the defective boron nitride–supported cage-like double-atom catalysts to rummage the qualified NRR catalysts. Based on a systematic evaluation of the stability, N2 adsorption, NRR selectivity and activity of 10 DACs of TM1-TM2@VB-BN, we predicted Ru-Ti@VB-BN to be the NRR candidate with a limiting potential of −0.40 V. Compared to the corresponding single-atom catalysts, the introduction of Ti/Mo modulates the d-band center of the active metal atom, which improves the NRR performance. Moreover, the magnetic Ru-Ti dimer can facilitate the transfer of charge to molecular N2, ensuring a significant activation of the inert N≡N bond. This research not only opens up new avenues for designing boron nitride–supported DACs for NRR, but also deepens the understanding of DACs in N2 activation.

Competitive Coordination-Pairing between Ru Clusters and Single-Atoms for Efficient Hydrogen Evolution Reaction in Alkaline Seawater

The coordination environment of Ru centers determines their catalytic performance, however, much less attention is focused on cluster-induced charge transfer in a Ru single-atom system. Herein, by density functional theory (DFT) calculations, a competitive coordination-pairing between Ru clusters (RuRu bond) and single-atoms (RuO bond) is revealed leading to the charge redistribution between Ru and O atoms in ZnFe₂ O₄ units which share more free electrons to participate in the hydrogen desorption process, optimizing the proton adsorption and hydrogen desorption. Thus, a clicking confinement strategy for building a competitive coordination-pairing between Ru clusters and single-atoms anchored on ZnFe₂ O_x nanosheets over carbon via RuO ligand (Ru_{1, n} -ZnFe₂ O_x -C) is proposed. Benefiting from the optimized coordination effect and the electronic synergy between Ru clusters and single-atoms, such a catalyst demonstrates the excellent activity and excellent stability in alkaline and seawater media, which has exceptional hydrogen evolution reaction activity with overpotentials as low as 10.1 and 15.9 mV to reach the current density of 10 mA cm^-2 in alkaline and seawater media, respectively, higher than that of commercial Pt/C catalysts as a benchmark. Furthermore, it owns remarkably outstanding mass activity, approximately 2 and 8 times higher than that of Pt catalysts in alkaline and seawater media, respectively.

Magnetically tunable and stable deep-ultraviolet birefringent optics using two-dimensional hexagonal boron nitride

Birefringence is a fundamental optical property that can induce phase retardation of polarized light. Tuning the birefringence of liquid crystals is a core technology for light manipulation in current applications in the visible and infrared spectral regions. Due to the strong absorption or instability of conventional liquid crystals in deep-ultraviolet light, tunable birefringence remains elusive in this region, notwithstanding its significance in diverse applications. Here we show a stable and birefringence-tunable deep-ultraviolet modulator based on two-dimensional hexagonal boron nitride. It has an extremely large optical anisotropy factor of 6.5 × 10^-12 C² J^-1 m^-1 that gives rise to a specific magneto-optical Cotton-Mouton coefficient of 8.0 × 10⁶ T^-2 m^-1, which is about five orders of magnitude higher than other potential deep-ultraviolet-transparent media. The large coefficient, high stability (retention rate of 99.7% after 270 cycles) and wide bandgap of boron nitride collectively enable the fabrication of stable deep-ultraviolet modulators with magnetically tunable birefringence.

Efficient Charge Transfers in Highly Conductive Copper Selenide Quantum Dot-Confined Catalysts for Robust Oxygen Evolution Reaction

Defective quantum dots (QDs) are the emerging materials for catalysis by virtue of their atomic-scale size, high monodispersity, and ultra-high specific surface area. However, the dispersed nature of QDs fundamentally prohibits the efficient charge transfer in various catalytic processes. Here, we report efficient and robust electrocatalytic oxygen evolution based on defective and highly conductive copper selenide (CuSe) QDs confined in an amorphous carbon matrix with good uniformity (average diameter 4.25 nm) and a high areal density (1.8 × 10¹² cm^-2). The CuSe QD-confined catalysts with abundant selenide vacancies were prepared by using a pulsed laser deposition system benefitted by high substrate temperature and ultrahigh vacuum growth conditions, as evidenced by electron paramagnetic resonance characterizations. An ultra-low charge transfer resistance (about 7 Ω) determined by electrochemical impedance spectroscopy measurement indicates the efficient charge transfer of CuSe quantum-confined catalysts, which is guaranteed by its high conductivity (with a low resistivity of 2.33 μΩ·m), as revealed by electrical transport measurements. Our work provides a universal design scheme of the dispersed QD-based composite catalysts and demonstrates the CuSe QD-confined catalysts as an efficient and robust electrocatalyst for oxygen evolution reaction.

Quantum Dots Compete at the Acme of MXene Family for the Optimal Catalysis

It is well known that two-dimensional (2D) MXene-derived quantum dots (MQDs) inherit the excellent physicochemical properties of the parental MXenes, as a Chinese proverb says, "Indigo blue is extracted from the indigo plant, but is bluer than the plant it comes from." Therefore, 0D QDs harvest larger surface-to-volume ratio, outstanding optical properties, and vigorous quantum confinement effect. Currently, MQDs trigger enormous research enthusiasm as an emerging star of functional materials applied to physics, chemistry, biology, energy conversion, and storage. Since the surface properties of small-sized MQDs include the type of surface functional groups, the functionalized surface directly determines their performance. As the Nobel Laureate Wolfgang Pauli says, "God made the bulk, but the surface was invented by the devil," and it is just on the basis of the abundant surface functional groups, there is lots of space to be thereof excavated from MQDs. We are witnessing such excellence and even more promising to be expected. Nowadays, MQDs have been widely applied to catalysis, whereas the related reviews are rarely reported. Herein, we provide a state-of-the-art overview of MQDs in catalysis over the past five years, ranging from the origin and development of MQDs, synthetic routes of MQDs, and functionalized MQDs to advanced characterization techniques. To explore the diversity of catalytic application and perspectives of MQDs, our review will stimulate more efforts toward the synthesis of optimal MQDs and thereof designing high-performance MQDs-based catalysts.

Rational coordination regulation in carbon-based single-metal-atom catalysts for electrocatalytic oxygen reduction reaction

Single-metal-atom catalysts (SMACs) have garnered extensive attention for various electrocatalytic applications, owing to their maximum atom-utilization efficiency, tunable electronic structure, and remarkable catalytic performance. In particular, carbon-based SMACs exhibit optimal electrocatalytic activity for the oxygen reduction reaction (ORR) which is of paramount importance for several sustainable energy conversion and generation technologies, such as fuel cells and metal-air batteries. Despite continuous endeavors in developing various advanced carbon-based SMACs for electrocatalytic ORR, the rational regulation of coordination structure and thus the electronic structure of carbon-based SMACs remains challenging. In this review, we critically examine the role of coordination structure, including local coordination structure (i.e., metal atomic centers and the first coordination shell) and extended local coordination structure (i.e., the second and higher coordination shells), on the rational design of carbon-based SMACs for high-efficiency electrocatalytic ORR. Insights into the relevance between coordination structures and their intrinsic ORR activities are emphatically exemplified and discussed. Finally, we also propose the major challenges and future perspectives in the rational design of advanced carbon-based SMACs for electrocatalytic ORR. This review aims to emphasize the significance of coordination structure and deepen the insightful understanding of structure-performance relationships.

Modulation doping and charge density wave transition in layered PbSe-VSe2 ferecrystal heterostructures

Controlling charge carrier concentrations remains a major challenge in the application of quasi-two-dimensional materials. A promising approach is the modulation doping of transport channels via charge transfer from neighboring layers in stacked heterostructures. Ferecrystals, which are metastable layered structures created from artificial elemental precursors, are a perfect model system to investigate modulation doping, as they offer unparalleled freedom in the combination of different constituents and variable layering sequences. In this work, differently stacked combinations of rock-salt structured PbSe and VSe₂ were investigated using X-ray photoelectron spectroscopy. The PbSe layers act as electron donors in all heterostructures, with about 0.1 to 0.3 donated electrons per VSe₂ unit cell. While they initially retain their inherent semiconducting behavior, they themselves become metallic when combined with a larger number of VSe₂ layers, as evidenced by a change of the XPS core level lineshape. Additional analysis of the valence band structure was performed for selected stacking orders at different sample temperatures to investigate a predicted charge density wave (CDW) transition. While there appear to be hints of a gap opening, the data so far is inconclusive and the application of spatially resolved techniques such as scanning tunneling microscopy is encouraged for further studies.

Resolving the Heterogeneous Adsorption of Antibody Fragment on a 2D Layered Molybdenum Disulfide by Super-Resolution Imaging

The development of nanomaterials such as two-dimensional (2D) layered materials advanced applications in many fields, including biosensors format based on field-effect transistors. The unique physical and chemical properties of 2D layered materials enable the detection limit of biomolecules as low as ∼1 pg/mL. The majority of 2D layered materials contain different structural features and defects introduced in chemical synthesis and fabrication processing. These structural features have different physicochemical properties, causing heterogeneous adsorption of bioreceptors like antibodies, enzymes, etc. Understanding the correlation between the adsorption of bioreceptors and properties of structural features is essential for building highly efficient, sensitive biosensors based on 2D layered materials. Here, we utilize a single-molecule localization-based super-resolved fluorescence imaging method to unveil the inhomogeneous adsorption of antibody fragments on 2D layered molybdenum disulfide (MoS₂). The surface coverage of antibody fragments on MoS₂ thin flakes is quantitatively measured and compared at different structural features and different layer thicknesses. The methodology in the current work can be extended to study bioreceptor adsorption on other types of 2D layered materials and pave a way to improve biosensors' sensitivity based on defect engineering 2D layered materials.

Partially delocalized charge in crystalline Co-S-Se/NiOx nanocomposites for boosting electrocatalytic oxygen evolution

Although reasonably specified and adjustable preparation of nanostructures with specific morphologies, ordered chemical compositions and electronic structures involving advanced energy chemistries is an important research area, high-efficiency, stable, and low-cost electrocatalysts for water splitting are greatly desirable and challenging. In this study, partially delocalized Co-S-Se ultra-thin nanosheets are obtained via a liquid-liquid interface-mediated strategy at an oil-water interface. These Co-S-Se ultra-thin nanosheets exhibit different-sized lamellar structures and have an average thickness of 0.83 nm. The ternary ultra-thin Co_0.45S_0.38Se_0.17 nanosheets demonstrate excellent performance for the OER, accompanied by an overpotential of 290 mV (1.52 V vs. RHE) at 10 mA cm^-2, and a Tafel slope of 74.5 mV dec^-1. In the meantime, the catalyst recombined with a stoichiometry NiO_x catalyst to form a composite interface, which also exhibited a good OER performance, with an overpotential of 260 mV at 10 mA cm^-2 and a smaller Tafel slope of 53.9 mV dec^-1. The nanosheets can rearrange the electronic density near the metal catalytic centers and increase the electron transfer. DFT calculations indicate that the partially delocalized charges can improve electrocatalytic performances, demonstrating modulated electroreduction properties. Due to the special atomic and electronic structure of the ternary transition metal alloy chalcogenide, the compound has great potential for energy storage, which will help in the rational design and synthesis of high-efficiency electrocatalysts.

Ultrathin Ce-doped La2O3nanofilm electrocatalysts for efficient oxygen evolution reactions

It is still highly desired to develop efficient, resource-abundant and inexpensive electrocatalysts to improve the sluggish kinetics of oxygen evolution reaction (OER) in electrochemical water splitting systems. In this work, the large-area ultrathin (2.52 nm thick) Ce-doped La₂O₃nanofilms were developed via a facile and reliable ionic layer epitaxy method with different Ce content. The ultrathin Ce-doped La₂O₃nanofilm with optimum composition of La_1.22Ce_0.78O₃exhibited an excellent OER performance with a very low overpotential of 221 mV at 10 mA cm^-2and a small Tafel slope of 33.7 mV dec^-1. A remarkable high mass activity of 6263.2 A g^-1was also obtained from ultrathin La_1.22Ce_0.78O₃nanofilm at the overpotential of 221 mV. Such a high mass activity was three orders of magnitude higher than state-of-the-art commercial IrO₂powders (3.8 A g^-1) and more than 30 times higher than La₂O₃nanofilm (196.7 A g^-1) without Ce doping at the same overpotential. This high mass activity was even significantly higher than other recently reported typical OER catalysts. The substantial OER performance gain by the Ce doping was attributed to the improved conductivity and electrochemical active surface areas of nanofilms as a result of favorable tuning on the charge transfer and electronic structures. This work provides a promising approach to develop high-performance two-dimensional (2D) electrocatalysts by effective heteroatom doping strategy.

Emerging 2D Materials for Electrocatalytic Applications: Synthesis, Multifaceted Nanostructures, and Catalytic Center Design

Currently, the development of advanced 2D nanomaterials has become an interdisciplinary subject with extensive studies due to their extraordinary physicochemical performances. Beyond graphene, the emerging 2D-material-derived electrocatalysts (2D-ECs) have aroused great attention as one of the best candidates for heterogeneous electrocatalysis. The tunable physicochemical compositions and characteristics of 2D-ECs enable rational structural engineering at the molecular/atomic levels to meet the requirements of different catalytic applications. Due to the lack of instructive and comprehensive reviews, here, the most recent advances in the nanostructure and catalytic center design and the corresponding structure-function relationships of emerging 2D-ECs are systematically summarized. First, the synthetic pathways and state-of-the-art strategies in the multifaceted structural engineering and catalytic center design of 2D-ECs to promote their electrocatalytic activities, such as size and thickness, phase and strain engineering, heterojunctions, heteroatom doping, and defect engineering, are emphasized. Then, the representative applications of 2D-ECs in electrocatalytic fields are depicted and summarized in detail. Finally, the current breakthroughs and primary challenges are highlighted and future directions to guide the perspectives for developing 2D-ECs as highly efficient electrocatalytic nanoplatforms are clarified. This review provides a comprehensive understanding to engineer 2D-ECs and may inspire many novel attempts and new catalytic applications across broad fields.

Selective Antisite Defect Formation in WS2 Monolayers via Reactive Growth on Dilute W-Au Alloy Substrates

Defects are ubiquitous in 2D materials and can affect the structure and properties of the materials and also introduce new functionalities. Methods to adjust the structure and density of defects during bottom-up synthesis are required to control the growth of 2D materials with tailored optical and electronic properties. Here, the authors present an Au-assisted chemical vapor deposition approach to selectively form S_W and S2_W antisite defects, whereby one or two sulfur atoms substitute for a tungsten atom in WS₂ monolayers. Guided by first-principles calculations, they describe a new method that can maintain tungsten-poor growth conditions relative to sulfur via the low solubility of W atoms in a gold/W alloy, thereby significantly reducing the formation energy of the antisite defects during the growth of WS₂ . The atomic structure and composition of the antisite defects are unambiguously identified by Z-contrast scanning transmission electron microscopy and electron energy-loss spectroscopy, and their total concentration is statistically determined, with levels up to ≈5.0%. Scanning tunneling microscopy/spectroscopy measurements and first-principles calculations further verified these antisite defects and revealed the localized defect states in the bandgap of WS₂ monolayers. This bottom-up synthesis method to form antisite defects should apply in the synthesis of other 2D materials.

Electrochemiluminescence detection of oxygen vacancies in layered double hydroxides

A novel electrochemiluminescence (ECL) platform was established to screen oxygen vacancies in layered double hydroxides (LDHs) by fabricating graphitic carbon nitride/LDH nanocomposites. The oxygen vacancy concentrations determined by the developed ECL platform were in good agreement with those obtained by XPS.

Defect Engineering of Out-of-Plane Charge Transport in van der Waals Heterostructures for Bi-Direction Photoresponse

Defects are ubiquitous in two-dimensional (2D) transition-metal dichalcogenides (TMDs), generated by the initial growth- or the postprocessing. However, the defects may play negative roles in the photoelectronic properties of TMDs due to the reduction of in-plane transport of carriers. In this work, we demonstrate that the Se-vacancy defects in MoSe₂ side of the van der Waal heterostructure is able to switch direction of out-of-plane charge transport. Photoresponse spectra showed defect density enable modified surface potential of MoSe_2-x, leading to the barrier reverse between graphene and MoSe_2-x and switches of the photoresponse from the negative to the positive. This unexpected property stemmed from appearance of midgap states by defects at heterostructure, as demonstrated by the density functional theory calculation and scanning tunneling microscope results. MoSe_2-0.2/graphene heterostructure has a broadband response ranging from 450 to 1064 nm and exhibits comparable or higher positive responsivity (5.4 × 10³ A/W to -15.3 × 10³ A/W at 632.8 and 5.7 × 10³ A/W to -1.2 × 10³ A/W at 1064 nm) to the negative one of the pristine MoSe₂/graphene. Based on defect-engineered heterostructures, we construct optoelectronic OR and AND logic devices with a broadband operation. Our work elucidates an alternative avenue to tailor the out-of-plane charge transport in TMD-based heterostructure through defects, and potentially invokes applicable utilization for 2D photodetectors and optoelectronic logic gates.

Defect engineering in thermoelectric materials: what have we learned?

Thermoelectric energy conversion is an all solid-state technology that relies on exceptional semiconductor materials that are generally optimized through sophisticated strategies involving the engineering of defects in their structure. In this review, we summarize the recent advances of defect engineering to improve the thermoelectric (TE) performance and mechanical properties of inorganic materials. First, we introduce the various types of defects categorized by dimensionality, i.e. point defects (vacancies, interstitials, and antisites), dislocations, planar defects (twin boundaries, stacking faults and grain boundaries), and volume defects (precipitation and voids). Next, we discuss the advanced methods for characterizing defects in TE materials. Subsequently, we elaborate on the influences of defect engineering on the electrical and thermal transport properties as well as mechanical performance of TE materials. In the end, we discuss the outlook for the future development of defect engineering to further advance the TE field.

Doping regulation in transition metal compounds for electrocatalysis

In electrocatalysis, doping regulation has been considered as an effective method to modulate the active sites of catalysts, providing a powerful means for creating a large variety of highly efficient catalysts for various reactions. Of particular interest, there has been growing research concerning the doping of two-dimensional transition-metal compounds (TMCs) to optimize their electrocatalytic performance. Despite the previous achievements, mechanistic insights of doping regulation in TMCs for electrocatalysis are still lacking. Herein, we provide a systematic overview of doping regulation in TMCs in terms of background, preparation, impacts on physicochemical properties, and typical applications including the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, CO₂ reduction reaction, and N₂ reduction reaction. Notably, we bridge the understanding between the doping regulation of catalysts and their catalytic activities via focusing on the physicochemical properties of catalysts from the aspects of vacancy concentrations, phase transformation, surface wettability, electrical conductivity, electronic band structure, local charge distribution, tunable adsorption strength, and multiple adsorption configurations. We also discuss the existing challenges and future perspectives in this promising field.

The potential impact of Octopus cyanea extracts to improve eggplant resistance against Fusarium-wilt disease: in vivo and in vitro studies

The novelty of the present research is conducting a new method in the systemic resistance of plant diseases by using distinct marine extracts. The ability of two octopus extracts to reduce the wilt disease caused by Fusarium oxysporum was observed. The applied methods are soaked roots (SR) and foliar shoots (FS). The antioxidant enzyme activities, percent disease index (PDI), and growth parameters were measured. In vitro antifungal potential of the octopus extracts against F. oxysporum was examined. The obtained result shows that SR extracts reduced PDI. Additionally, all the tested treatments promoted the growth and photosynthetic pigments of the infected plants. SR (in ethanolic extracts) was the most prominent inducer which offered a high advancement in the total soluble protein contents. Also, SR (in methanolic extracts) was the most suitable inducer which provided a very necessary development not only in the total phenol but also in the peroxidase (POD) and polyphenol oxidase (PPO) activities. GC-MS investigation of the octopus extracts exhibited that the compounds which possess antifungal activity were furoscrobiculin B and/or eugenol. They demonstrated a notable antifungal potential against F. oxysporum with a maximum activity of 38.5 and 12.7 mm ZOI after the treatment with the ethanolic and methanolic extract, respectively. FTIR results illustrated the functional group of the compound responsible for the antifungal activity. Additionally, an atomic absorption result reveals that there are traces of metals detected such as Pb, Ag, Cu, Zn, and Mg. The antifungal activity was decreased as the concentrations were reduced. Accordingly, the present extracts may be used as the vital agents in the agricultural field to restrain the plant pathogenic fungi, especially F. oxysporum from a proliferation.

Energy-Level Alignment at Interfaces between Transition-Metal Dichalcogenide Monolayers and Metal Electrodes Studied with Kelvin Probe Force Microscopy

We studied the energy-level alignment at interfaces between various transition-metal dichalcogenide (TMD) monolayers, MoS₂, MoSe₂, WS₂, and WSe₂, and metal electrodes with different work functions (WFs). TMDs were deposited on SiO₂/silicon wafers by chemical vapor deposition and transferred to Al and Au substrates, with significantly different WFs to identify the metal-semiconductor junction behavior: oxide-terminated Al (natural oxidation) and Au (UV-ozone oxidation) with a WF difference of 0.8 eV. Kelvin probe force microscopy was employed for this study, based on which electronic band diagrams for each case were determined. We observed the Fermi-level pinning for MoS₂, while WSe₂/metal junctions behaved according to the Schottky-Mott limit. WS₂ and MoSe₂ exhibited intermediate behavior.

Bandgap Engineering and Near-Infrared-II Optical Properties of Monolayer MoS2: A First-Principle Study

The fluorescence-based optical imaging in the second near-infrared region (NIR-II, 1,000-1,700 nm) has broad applications in the biomedical field, but it is still difficult to find new NIR-II fluorescence materials in the two dimension. As a crucial characteristic of the electronic structure, the band structure determines the fundamental properties of two-dimensional materials, such as their optical excitations and electronic transportation. Therefore, we calculated the electronic structures and optical properties of different crystalline phases (1T phase and 2H phase) of pure monolayer MoS2 films and found that the 1T phase has better absorption and thus better fluorescence in the NIR-II window. However, its poor stability makes the 1T-phase MoS2 less useful in vivo bioimaging. By introducing vacancy defects and doping with foreign atoms, we successfully tuned the bandgap of the monolayer 2H-MoS2 and activated it in the NIR-II. Our results show that by engineering the vacancy defects, the bandgap of the 2H phase can be tailored to around 1 eV, and there are three candidates of vacancy structures that exhibit strong absorption in the NIR-II.

B5N3 and B7N5 Monolayers with High Carrier Mobility and Excellent Optical Performance

An ab initio evolutionary search algorithm was combined with density functional theory (DFT) calculations to predict a series of 2-D B_xN_y (1 < x/y ≤ 2). Particularly, B₅N₃ and B₇N₅ monolayers have sufficiently low formation enthalpy and excellent dynamic stability that make them promising for synthesis in experiments. Electronic structure calculations reveal that B₅N₃ and B₇N₅ monolayers possess an indirect band gap of 1.99 eV and a direct band gap of 2.40 eV, respectively. The calculated absorption coefficients for B₅N₃ and B₇N₅ monolayers are significantly improved in the low end of the visible region compared with that of 2-D h-BN. Moreover, our calculations reveal that both B₅N₃ and B₇N₅ monolayers have high electron carrier mobilities. The narrow band gaps, high carrier mobilities, strong near-ultraviolet absorption, and high synthesis possibility of B₅N₃ and B₇N₅ monolayers render them promising new materials for application in novel electronics and environmentally benign solar energy conversion.

Steric Hindrance- and Work Function-Promoted High Performance for Electrochemical CO Methanation on Antisite Defects of MoS2 and WS2

CO methanation from electrochemical CO reduction reaction (CORR) is significant for sustainable environment and energy, but electrocatalysts with excellent selectivity and activity are still lacking. Selectivity is sensitive to the structure of active sites, and activity can be tailored by work function. Moreover, intrinsic active sites usually possess relatively high concentration compared to artificial ones. Here, antisite defects Mo_S2 and W_S2 , intrinsic atomic defects of MoS₂ and WS₂ with a transition metal atom substituting a S₂ column, were investigated for CORR by density functional theory calculations. The steric hindrance from the special bowl structure of Mo_S2 and W_S2 ensured good selectivity towards CO methanation. Coordination environment variation of the active sites, the under-coordinated Mo or W atoms, effectively lowered the work function, making Mo_S2 and W_S2 highly active for CO methanation with the required potential of -0.47 and -0.49 V vs. reversible hydrogen electrode, respectively. Moreover, high concentration of active sites and minimal structural deformation during the catalytic process of Mo_S2 and W_S2 enhanced their attraction for future commercial application.