First-principles prediction of metal-free magnetism and intrinsic half-metallicity in graphitic carbon nitride

Density functional theory-based modeling of the half-metallic g-C3N4/CoN4 heterojunction for photocatalytic water splitting reaction

Using density functional theory (DFT), we have investigated the structural, optical, electronic and magnetic properties of a graphitic carbon nitride (g-C3N4) and CoN4 composite to explore the effect of the heterojunction on the photocatalytic performance of g-C3N4. The structure of g-C3N4 is modified while complexing with CoN4 and the corresponding stabilization is confirmed through adhesion energy calculation. The phonon spectra analysis furthermore guaranteed the lattice-dynamic stability of the CoN4 bulk and the CoN4 slab. Pristine g-C3N4 is a wide band gap semiconductor, which becomes half metallic upon CoN4 inclusion. The metallicity in the g-C3N4/CoN4 composite originates from the spin down channel, keeping the spin up channel in a semiconducting state. The charge density analysis and work function calculation suggest a substantial amount of charge transfer from g-C3N4 to the CoN4 unit in the g-C3N4/CoN4 heterojunction. The model heterojunction of the g-C3N4/CoN4 composite can enhance the utilization ratio of visible light for the g-C3N4 photocatalyst. In g-C3N4/CoN4, the valence band maximum (VBM) has a more positive potential compared to O2/H2O (+1.23 V) on the normal hydrogen electrode (NHE) scale. However, the conduction band minimum (CBM) displays a more negative potential compared to H+/H2 (0 V) on the NHE scale. The details of the band structure, density of states and band edge position determining calculations confirm that the g-C3N4/CoN4 composite forms a type 1 heterojunction, making it a suitable photocatalyst for water splitting reaction. The practical application of the g-C3N4/CoN4 heterostructure as a photocatalyst was substantiated in the presence of polar solvent (water) by calculating the band gap, charge transfer interaction and charge density difference. There is a significant decrease of charge transfer and thereby charge density difference in the g-C3N4/CoN4 heterojunction in the presence of water; however, it still holds potential for use as a photocatalyst for water splitting reaction. The state-of-the-art theoretical modeling of the g-C3N4/CoN4 heterojunction is the first theoretical study incorporating the CoN4 crystal.

Prediction of two-dimensional C3N2 semiconductors with outstanding stability, moderate band gaps, and high carrier mobility

Two-dimensional (2D) semiconductors with suitable band gaps, high carrier mobility, and environmental stability are crucial for applications in the next generation of electronics and optoelectronics. However, current candidate materials each have one or more issues. In this work, two novel C3N2 monolayers, P-C3N2 and I-C3N2 are proposed by first-principles calculations. Both structures have demonstrated excellent dynamical and mechanical stability, with thermal stability approaching 3000 K. Importantly, P-C3N2 shows a distinct advantage in formation energy compared to currently synthesized 2D carbon nitride materials, indicating its potential for experimental synthesis. Electronic structure calculations reveal that both P-C3N2 and I-C3N2 are intrinsic semiconductors with moderate band gaps of 2.19 and 1.81 eV, respectively. Additionally, both C3N2 monolayers display high absorption coefficients up to 105 cm-1, with P-C3N2 showing significant absorption capabilities in the visible light region. Remarkably, P-C3N2 possesses an ultra-high carrier mobility of up to 104 cm2 V-1 s-1. These findings provide theoretical insights and candidates for future applications in the electronics and optoelectronics fields.

Comparative study of the photocatalytic activity of g-C3N4/MN4 (M = Mn, Fe, Co) for water splitting reaction: A theoretical study

In this study, nanocomposites of g-C3N4/MN4 (where M is Mn, Fe and Co) have been designed using advanced density functional theory (DFT) calculations. A comprehensive analysis was conducted on the geometry, electronic, optical properties, work function, charge transfer interaction and adhesion energy of the g-C3N4/MN4 heterostructures and concluded that g-C3N4/FeN4 and g-C3N4/CoN4 heterojunctions exhibit higher photocatalytic performance than individual units. The better photocatalytic activity can be attributed mainly by two facts; (i) the visible light absorption of both g-C3N4/FeN4 and g-C3N4/CoN4 interfaces are higher compared to its isolated analogs and (ii) a significant enhancement of band gap energy in g-C3N4/FeN4 and g-C3N4/CoN4 heterostructures limited the electron-hole recombination significantly. The potential of the g-C3N4/MN4 heterojunctions as a photocatalyst for the water splitting reaction was assessed by examining its band alignment for water splitting reaction. Importantly, while the electronic and magnetic properties of MN4 systems were studied, this is the first example of inclusion of MN4 on graphene-based material (g-C3N4) for studying the photocatalytic activity. The state of the art DFT calculations emphasis that g-C3N4/FeN4 and g-C3N4/CoN4 heterojunctions are half metallic photocatalysts, which is limited till date.

Unraveling the Activity and Mechanism of TM@g-C4N3 Electrocatalysts in the Oxygen Reduction Reaction

In this work, a high-throughput screening strategy and density functional theory (DFT) are jointly employed to identify high-performance TM@g-C4N3 (TM = 3d, 4d, 5d transition metals) single-atom catalysts (SACs) for the oxygen reduction reaction (ORR). Comprehensive studies demonstrated that Cu@, Zn@, and Ag@g-C4N3 show high ORR catalytic activities under both acidic and alkaline conditions with favorable overpotentials (ηORR) of 0.70, 0.89, and 0.89 V, respectively; among them, Cu@g-C4N3 is the best candidate. The ORR follows a four-electron mechanism with the final product H2O/OH-. Cu@, Zn@, and Ag@g-C4N3 catalysts also exhibit good thermal (500 K) and electrochemical (0.93-3.14 V) stabilities. Cu@, Zn@, and Ag@g-C4N3 demonstrate superior activities with low ηORR due to its moderate adsorption strength of *OH. The ηORR and the Gibbs free energy changes of *OH (ΔG4(acidic)/ΔG4(alkaline)) resemble a volcano-type relationship under acidic/alkaline conditions, respectively. Additionally, the O-O bond length in *OOH emerged as an effective structural descriptor for rapidly identifying the promising electrocatalysts. This research provides valuable insights into the origin of the ORR activity on TM@g-C4N3 and offers useful guidance for the efficient exploration of high-performance catalyst candidates.

Progress of charge carrier dynamics and regulation strategies in 2D CxNy-based heterojunctions

Two-dimensional carbon nitrides (CxNy) have gained significant attention in various fields including hydrogen energy development, environmental remediation, optoelectronic devices, and energy storage owing to their extensive surface area, abundant raw materials, high chemical stability, and distinctive physical and chemical characteristics. One effective approach to address the challenges of limited visible light utilization and elevated carrier recombination rates is to establish heterojunctions for CxNy-based single materials (e.g. C2N3, g-C3N4, C3N4, C4N3, C2N, and C3N). The carrier generation, migration, and recombination of heterojunctions with different band alignments have been analyzed starting from the application of CxNy with metal oxides, transition metal sulfides (selenides), conductive carbon, and Cx'Ny' heterojunctions. Additionally, we have explored diverse strategies to enhance heterojunction performance from the perspective of carrier dynamics. In conclusion, we present some overarching observations and insights into the challenges and opportunities associated with the development of advanced CxNy-based heterojunctions.

Electrocatalytic Urea Synthesis via N2 Dimerization and Universal Descriptor

Electrocatalytic urea synthesis through N2 + CO2 coreduction and C-N coupling is a promising and sustainable alternative to harsh industrial processes. Despite considerable efforts, limited progress has been made due to the challenges of breaking inert N≡N bonds for C-N coupling, competing side reactions, and the absence of theoretical principles guiding catalyst design. In this study, we propose a mechanism for highly electrocatalytic urea synthesis using two adsorbed N2 molecules and CO as nitrogen and carbon sources, respectively. This mechanism circumvents the challenging step of N≡N bond breaking and selective CO2 to CO reduction, as the free CO molecule inserts into dimerized *N2 and binds concurrently with two N atoms, forming a specific urea precursor *NNCONN* with both thermodynamic and kinetic feasibility. Through the proposed mechanism, Ti2@C4N3 and V2@C4N3 are identified as highly active catalysts for electrocatalytic urea formation, exhibiting low onset potentials of -0.741 and -0.738 V, respectively. Importantly, taking transition metal atoms anchored on porous graphite-like carbonitride (TM2@C4N3) as prototypes, we introduce a simple descriptor, namely, effective d electron number (Φ), to quantitatively describe the structure-activity relationships for urea formation. This descriptor incorporates inherent atomic properties of the catalyst, such as the number of d electrons, the electronegativity of the metal atoms, and the generalized electronegativity of the substrate atoms, making it potentially applicable to other urea catalysts. Our work advances the comprehension of mechanisms and provides a universal guiding principle for catalyst design in urea electrochemical synthesis.

The regulatory function of the d-orbital structure in TM@g-t-C4N3 for bifunctional catalysis of the oxygen evolution/reduction reaction

Highly efficient catalysts for the oxygen evolution/reduction reaction (OER/ORR) have attracted great attention in research for energy devices with high conversion efficiency. Herein, systematic first-principles investigations are performed to explore the catalytic performance of graphitic C4N3 loaded with single transition metal atoms (TM@g-t-C4N3) for the OER/ORR. The results show that Fe, Co, Ni and Rh@g-t-C4N3 exhibit fascinating bifunctional catalytic activities for both the OER and ORR. Moreover, it is observed that better activities are easily achieved when the valence d orbitals of doped TM atoms are nearly fully occupied. Further analysis reveals the volcano relationship between the OER/ORR performance and the adsorption Gibbs free energy. The adsorption free energy of intermediates in the OER/ORR process is also found to highly correlate with the electronic structures of TM@g-t-C4N3, which are mainly characterized by two quantities, one is the descriptor φ related to the electronegativity and the number of valence electrons in d orbitals, and the other is the projected d band center. The results indicate that it is possible to predict the catalytic performance of TM@g-t-C4N3 by a detailed examination of the electronic properties of the doped TM atoms to some extent. This research not only provides several highly active g-t-C4N3-based single-atom catalysts (SACs) for the OER/ORR, but also reveals some potential regularities of SAC systems.

Stacking Engineering of Heterojunctions in Half-Metallic Carbon Nitride for Efficient CO2 Photoreduction

Enhancing charge separation in semiconductor photocatalysts is a major challenge for efficient artificial photosynthesis. Herein, a compact heterojunction is designed by embedding half-metallic C(CN)3 (hm-CN) hydrothermally in BiOBr (BOB) as the backbone. The interface between hm-CN and BOB is seamless and formed by covalent bonding to facilitate the transmission of photoinduced electrons from BOB to hm-CN. The transient photocurrents and electrochemical impedance spectra reveal that the modified composite catalyst exhibits a larger electron transfer rate. The photocatalytic activity of hm-CN/BOB increases significantly as indicated by a CO yield that is about four times higher than that of individual components. Density-functional theory calculations verify that the heterojunction improves electron transport and decreases the reaction energy barrier, thus promoting the overall photocatalytic CO2 conversion efficiency. The half-metal nitride coupled semiconductor heterojunctions might have large potential in artificial photosynthesis and related applications.

The metal atomic substitution induced half-metallic properties, metallic properties and semiconducting properties in X-N4 nanoribbons

Armchair X-N4 nanoribbons (X-AN4NRs) and zigzag X-N4 nanoribbons (X-ZN4NRs) were calculated using first-principles calculations. Ferromagnets (FM) were found to be the most stable among the initial magnetic structures. Furthermore, nanoribbons were found to be thermodynamically stable through molecular dynamics simulations. It can be found that when the temperature and total energy of X-AN4NRs and X-ZN4NRs change with time, they have a small oscillation range, which confirms the dynamic stability of X-AN4NRs and X-ZN4NRs under realistic experimental conditions. Subsequently, the magnetic moment analysis of the X-AN4NRs and X-ZN4NRs revealed that the magnetic moment of the X-AN4NRs is significantly smaller than that of X-ZN4NRs. The band structure and density of states (DOS) of the X-AN4NRs and X-ZN4NRs were also computed, which indicate different properties for different transition metal nanoribbons. The results suggest that different edge structures and transition metals can influence the electronic structure of the nanoribbons. Moreover, based on the band structure and DOS, it was found that Mn-AN4NRs and Fe-ZN4NRs exhibit half-metallic properties. They can generate 100% polarized currents at the Fermi level, providing valuable information for developing spintronic devices. Our study has a positive value for regulating the properties of the nanoribbons by metal atom substitution.

Superhard bulk C4N3 compounds with metal-free magnetism assembled from two-dimensional C4N3: a first-principles study

Enriching the electronic properties of superhard materials is very important to extend their applications, and some superhard materials with metallic or superconducting characteristics have been designed via theoretical or experimental methods. However, their magnetic features have scarcely been studied, since most of them are limited to nonmagnetic ordering. Here, with the help of first-principles calculations, a series of C4N3 compounds are designed by stacking C4N3 sheets with different sequences. As expected, some of them exhibit both magnetic and superhard characteristics. Notably, all these compounds exhibit dynamic and mechanical stabilities, indicating that their dynamic and mechanical stabilities are independent of the stacking sequence. Among them, the ABC-stacked one is energetically favorable, and it exhibits antiferromagnetic ordering and has a hardness of ∼54.0 GPa, and the electronic calculations show that it is a semiconductor with a direct band gap of ∼1.20 eV. Besides, the magnetism of all magnetic C4N3 compounds is caused by the lower coordinated atoms, and the magnetic moments are located on three-fold C or two-fold coordinated N atoms. Additionally, the magnetic property is deeply dependent on the external pressure. This work opens a potential way to design magnetic superhard materials and can arouse their applications in the spintronic field.

Templated Synthesis of Exfoliated Porous Carbon with Dominant Graphitic Nitrogen

We present here a new approach for the synthesis of nitrogen-doped porous graphitic carbon (g-NC) with a stoichiometry of C6.3H3.6N1.0O1.2, using layered silicate as a hard sacrificial template. Autogenous exfoliation is achieved due to the heterostacking of 2D silicate and nitrogen-doped carbon layers. Micro- and meso-porosity is induced by melamine and cetyltrimethylammonium (C16TMA). Our density functional calculations and X-ray photoelectron spectroscopy (XPS) observations confirm that the most dominant nitrogen configuration in g-CN is graphitic, while pyridinic and pyrrolic nitrogens are thermodynamically less favored. Our large-scale lattice dynamics calculations show that surface termination with H and OH groups at pores accounts for the observed H and O in the composition of the synthesized g-NC. We further evaluate the electrocatalytic and the supercapacitance activities of g-NC. Interestingly, this material exhibits a specific capacitance of ca. 202 F g-1 at 1 A g-1, retaining 90% of its initial capacitance after 10,000 cycles.

Strong ferromagnetism of g-C3N4 achieved by atomic manipulation

Two-dimensional (2D) metal-free ferromagnetic materials are ideal candidates to fabricate next-generation memory and logic devices, but optimization of their ferromagnetism at atomic-scale remains challenging. Theoretically, optimization of ferromagnetism could be achieved by inducing long-range magnetic sequence, which requires short-range exchange interactions. In this work, we propose a strategy to enhance the ferromagnetism of 2D graphite carbon nitride (g-C₃N₄), which is facilitating the short-range exchange interaction by introducing in-planar boron bridges. As expected, the ferromagnetism of g-C₃N₄ was significantly enhanced after the introduction of boron bridges, consistent with theoretical calculations. Overall, boosting ferromagnetism of 2D materials by introducing bridging groups is emphasized, which could be applied to manipulate the magnetism of other materials.

Integrating Photoluminescence and Ferromagnetism in Carbon Quantum Dot/ZnO by Interfacial Orbital Hybridization for Multifunctional Bioprobes

Integrating ferromagnetism (FM) and photoluminescence (PL) into one particular nanostructure as biological probe plays an irreplaceable role in accurate clinical diagnosis combining magnetic resonance and photoluminescence imaging technology. However, magnetic emergence generally needs a spin polarization at Fermi level to display a half-metallic electronic feature, which is not beneficial for preserving radiation recombination ability of photo-excited electron-hole carriers. To overcome this intrinsic difficulty, we propose a feasible atomic-hybridization strategy to anchor carbon quantum dots (CQDs) onto ZnO microsphere surface via breakage of C=O bonds at CQDs and subsequent Zn-3d and C-2p orbital hybridization, which not only ensures the carrier recombination but also leads to a room-temperature magnetism. Herein, the photoluminescence and magnetism coexist in this multifunctional heterojunction with outstanding biocompatibility. This work suggests that integration of magnetism and photoluminescence could be accomplished by particular interfacial orbital hybridization.

Tailored modifications of the electronic properties of g-C3N4/C2N-h2D nanoribbons by first-principles calculations

The electronic structure of g-C₃N₄/C₂N-h2D nanoribbons was investigated by first-principles calculations. As a splice structure, we first computed the three magnetic coupled states of g-C₃N₄/C₂N-h2D nanoribbons. After self-consistent calculations, both the antiferromagnetic and paramagnetic coupling states become ferromagnetic coupling states. It was proved that the ferromagnetic coupling state is the most stable state. Thermodynamic stability was subsequently verified based on the ferromagnetic coupling state. It had a steady electron spin polarization, with a magnetic moment of 1 μ_B for each primitive cell. It changed from a direct band-gap semiconductor to an indirect band-gap semiconductor and exhibited the properties of a narrow band gap semiconductor through the analysis of the energy band and charge density. To transform the electronic structure, we adsorbed different transition metals in g-C₃N₄/C₂N-h2D nanoribbons. We investigated the electronic structure of g-C₃N₄/C₂N-h2D nanoribbons adsorbed by different transition metals. It was shown that the electronic structure of g-C₃N₄/C₂N-h2D nanoribbons could be regulated by the adsorption of different transition metal atoms. Moreover, the adsorption of Fe and Ni can generate a 100% polarized current in the Fermi surface, which will provide more application potential for spintronics devices.

Highly Sensitive Photoelectrochemical Immunosensor Based on Organic Multielectron Donor Nanocomposite as Signal Probe

Organic photoelectric materials with conjugated electron-rich structures and good biocompatibilities have broad application prospects in biosensors. Herein, we report a promising organic photoelectric multielectron donor nanocomposite for highly sensitive PEC immunoassays. Specifically, the organic multielectron donor nanocomposite (DA-ZnTCPP-g-C₃N₄) was prepared from dopamine (DA, polyphenol hydroxyl structure substance), zinc tetracarboxylate porphyrin (ZnTCPP, large p-π conjugated heterocyclic compound), and two-dimensional graphene-like nitrogen carbide (g-C₃N₄) via an amidation reaction. With a multielectron donor structure and photoelectricity, this nanocomposite can achieve sensitization by self-structure without the addition of an electron donor in the test solution. It was utilized to label the carcinoembryonic detection antibody as a immuno-probe (Ab2-DA-ZnTCPP-g-C₃N₄). Meanwhile, the glassy carbon electrode electrodeposited with gold nanoparticles anchoring the capture antibody was used as a PEC immunomatrix (Ab1/DpAu/GCE). The enhanced PEC current, "signal on", was confirmed by the immunosensor via sandwich immunorecognition of a carcinoembryonic antigen (CEA). Under optimal conditions, the as-prepared sensing platform displayed high sensitivity for CEA with a dynamic linear response range from 10 fg·mL^-1 to 1 mg·mL^-1 and a lower detection limit of 3.6 fg·mL^-1. This organic nanocomposite showed good sensitivity and stability in an immunosensing system with a low background. This strategy affords a promising approach for biological applications of organic photoelectric materials.

Prediction of Two-Dimensional Group IV Nitrides AxNy (A = Sn, Ge, or Si): Diverse Stoichiometric Ratios, Ferromagnetism, and Auxetic Mechanical Property

In this work, we unveiled a new class of two-dimensional (2D) group IV nitride A_xN_y (A = Sn, Ge, or Si) prototypes, C2/m A₄N, P3̅m1 A₃N, P3m1 A₂N, P3̅m1 A₃N₂, P6̅m2 AN, P3̅m1 AN, P6̅2m A₃N₄, P3m1 A₂N₃, P4̅2₁m AN₂, and P3̅m1 AN₃, by using evolutionary algorithms combined with first-principles calculations. Using HSE06 functional calculations, a wide range of band gaps from metal to semiconductor (0.405-5.050 eV) and ultrahigh carrier mobilities (1-24 × 10³ cm² V^-1 s^-1) were evidenced in these 2D structures. We found that 2D P3m1 Sn₂N₃, Ge₂N₃, and Si₂N₃ are intrinsic ferromagnetic semiconductors with gaps of 0.677, 1.285, and 2.321 eV, respectively. The lattice symmetry and Si-to-N₂ charge transfer upon strain lead to large anisotropic negative Poisson's ratios (-0.281 to -0.146) along whole in-plane directions in 2D P4̅2₁m SiN₂. Our findings not only enrich the family of 2D nitrides but also highlight the promising optoelectronic and nanoauxetic applications of 2D group IV nitrides.

Selective catalytic oxidation of formaldehyde on single V- and Cr-atom decorated magnetic C4N3 substrate: A first principles study

Formaldehyde (HCHO) is the most common indoor hazardous pollutant and has attracted great concern because its long-term exposure has adverse health effects on humans. Retention and catalytic oxidation of highly hazardous HCHO is an efficient and environmentally friendly method to use for air remediation, but a major obstacle to this procedure is the lack of an appropriate catalyst. Herein, two-dimensional magnetic C₄N₃ material with a 3d-transition metal as activate sites was systemically investigated in HCHO oxidation using density functional theory calculations. The results show that V-C₄N₃ and Cr-C₄N₃ have high structural stability and shallow activation barriers for O₂ decomposition; these characteristics provide the necessary precursors for the subsequent oxidation reaction. Moreover, the V-C₄N₃ and Cr-C₄N₃ catalysts have unique selective adsorption and catalysis toward HCHO in a mixture of some typical in-door volatile organic compounds (VOCs) and air. The corresponding dynamic barrier for each reaction step was investigated and the mechanism involved in HCHO oxidation was revealed in detail. Aggregation of metal atoms in the V-C₄N₃ and Cr-C₄N₃ catalysts is prevented by enormous diffusion resistance, and this is further confirmed by AIMD simulations. These results provide insightful guidance for developing advanced magnetic catalysts for HCHO oxidation to improve the remediation of air contaminants.

Structural, Electronic, and Magnetic Characteristics of Graphitic Carbon Nitride Nanoribbons and Their Applications in Spintronics

The development of quantum information and quantum computing technology requires special materials to design and manufacture nanosized spintronic devices. Possessing remarkable structural, electronic, and magnetic characteristics, graphitic carbon nitride (g-C₃N₄) can be a promising candidate as a building block of futuristic nanoelectronics and spintronic systems. Here, using first-principles calculations, we perform a comprehensive study on the structural stability as well as electronic and magnetic properties of triazine-based g-C₃N₄ nanoribbons (gt-CNRs). Our calculations show that gt-CNRs with different edge conformation exhibit distinct electronic and magnetic characteristics, which can be tuned by the edge H-passivation rate. By investigating gt-CNRs with various possible edge configurations and H-termination rates, we show that while the ferromagnetic (FM) ordering of gt-CNRs stays preserved for all of the studied configurations, half metallicity can only be achieved in nanoribbons with specific edge structure under full H-passivation rate. For spintronic application purposes, we also study spin-transport properties of half-metal gt-CNRs. By determining the suitable gt-CNR configuration, we show the possibility of developing a perfect gt-CNR-based spin filter with a spin filter efficiency (SFE) of 100%. Considering the above-mentioned notable electronic and magnetic characteristics as well as its high thermal stability, we show that gt-CNR would be a remarkable material to fabricate multifunctional spintronic devices.

Spin-thermoelectric properties and giant tunneling magnetoresistance of boron-substituted graphene nanoribbon: a first principle study

In this study we have designed a spin caloritronic device based on boron doped armchair graphene nanoribbons (B2-7AGNR). In presence of ferromagnetic (FM) graphitic-carbon nitride (g-C₄N₃) electrodes the spin-thermoelectric features of the device, both for FM and antiferromagnetic (AFM) states, are studied using first principle calculations. The spin polarized transmission peaks and the presence of density of states near the Fermi level indicate that the system have large spin-thermoelectric figure of merit. In addition, it is observed that the system has a large tunneling magnetoresistance due to the difference in total current between FM and AFM configurations. Further studies reveal that the spin component of the Seebeck coefficient of the device is much higher than the other zigzag and armchair nanoribbons. When the spin magnetic moments of the electrodes are aligned in parallel manner, spin-thermoelectric figure of merit of the system becomes significantly high. It has also been found that on decreasing temperature the efficiency of the device increases. As a whole, the numerical results show thatg-C₄N₃-B2-7AGNR-g-C₄N₃system in FM configuration is an efficient low temperature thermoelectric device.

Presence and absence of intrinsic magnetism in graphitic carbon nitrides designed through C-N-H building blocks

We use the first principle calculation to investigate the intrinsic magnetism of graphitic carbon nitrides (GCNs). By preserving three-fold symmetry, the GCN building blocks have been built out of different combinations between 6 components which are C atom, N atom, s-triazine, heptazine, heptazine with C atom at the center, and benzimidazole-like component. That results in 20 phases where 11 phases have been previously reported, and 9 phases are newly derived. The partial density of states and charge density have been analyzed through 20 phases to understand the origin of the presence and absence of intrinsic magnetism in GCNs. The intrinsic magnetism will be present not only because the GCNs comprising of radical components but also the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\pi$$\end{document}π-conjugated states are not the valence maximum to break the delocalization of unpaired electrons. The building blocks are also employed to study alloys between g-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {C}_3\hbox {N}_4$$\end{document}C3N4 and g-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {C}_4\hbox {N}_3$$\end{document}C4N3. The magnetization of the alloys has been found to be linearly dependent on a number of C atoms in the unit cell and some magnetic alloys are energetically favorable. Moreover, the intrinsic magnetism in GCNs can be promoted or demoted by passivating with a H atom depending on the passivated positions.

Transition Metal-Free Half-Metallicity in Two-Dimensional Gallium Nitride with a Quasi-Flat Band

Two-dimensional half-metallicity without a transition metal is an attractive attribute for spintronics applications. On the basis of first-principles calculation, we revealed that a two-dimensional gallium nitride (2D-GaN), which was recently synthesized between graphene and SiC or wurtzite GaN substrate, exhibits half-metallicity due to its half-filled quasi-flat band. We found that graphene plays a crucial role in stabilizing a local octahedral structure, whose unusually high density of states due to a flat band leads to a spontaneous phase transition to its half-metallic phase from normal metal. It was also found that its half-metallicity is strongly correlated to the in-plane lattice constants and thus subjected to substrate modification. To investigate the magnetic property, we simplified its magnetic structure with a two-dimensional Heisenberg model and performed Monte Carlo simulation. Our simulation estimated its Curie temperature (T_C) to be ∼165 K under a weak external magnetic field, suggesting that transition metal-free 2D-GaN exhibiting p orbital-based half-metallicity can be utilized in future spintronics.

Metallic C5N monolayer as an efficient catalyst for accelerating redox kinetics of sulfur in lithium-sulfur batteries

Lithium-sulfur battery is one of the most promising applicants for the next generation of energy storage devices whose commercial applications are impeded by the key issue of the shuttle effect. To overcome this obstacle, various two-dimensional (2D) carbon-based metal-free compounds have been proposed to serve as anchoring materials for immobilizing soluble lithium polysulfides (LiPs), which however suffer from low electronic conductivity implying unsatisfactory performance for catalyzing sulfur redox. Therefore, we have predicted metallic C₅N monolayers, possessing hexagonal (H) and orthorhombic (O) phases, exhibiting excellent performance for suppressing the shuttle effect. First-principles simulations demonstrate that O-C₅N could serve as a bifunctional anchoring material due to its strong adsorption capability to LiPs and excellent catalytic performance for sulfur redox with active sites from both basal plane and zigzag edges. Furthermore, the rate of Li₂S oxidation over O-C₅N is fast due to the low energy barrier of 0.93 eV for Li₂S decomposition. While for H-C₅N, only N atoms located at the armchair edges can efficiently trap LiPs and boost the formation and dissociation of Li₂S during discharge and charge processes, respectively. The current work opens an avenue of designing 2D metallic carbon-based anchoring materials for lithium-sulfur batteries, which deserves further experimental research efforts.

From half-metallic to magnetic semiconducting triazine g-C4N3: computational designs and insight

We have given, for the first time, physicochemical insight into the electronic structure routes from half-metallic to magnetic semiconducting triazine g-C4N3. To this end, three material designs have been proposed using density functional calculations. In one design, this half-metal is first made semiconducting via hydrogenation, then tailored with B and N atomic species, which gives a new prototype of the antiferromagnetic semiconductor monolayer HC4N3BN. In the others, it can be rendered spin gapless semiconducting with H and B or C, followed by F or O tailoring, which eventually leads to the two new bipolar ferromagnetic semiconductors HC4N3BF and HC4N3CO. These monolayers are considered to be novel materials in spintronics.

Two-dimensional Dirac half-metal in porous carbon nitride C6N7monolayer via atomic doping

Motivated by the recent experimental discovery of C₆N₇monolayer (Zhaoet al2021Science Bulletin66, 1764), we show that C₆N₇monolayer co-doped with C atom is a Dirac half-metal by employing first-principle density functional theory calculations. The structural, mechanical, electronic and magnetic properties of the co-doped C₆N₇are investigated by both the PBE and HSE06 functionals. Pristine C₆N₇monolayer is a semiconductor with almost isotropic electronic dispersion around the Γ point. As the doping of the C₆N₇takes place, the substitution of an N atom with a C atom transforms the monolayer into a dilute magnetic semiconductor, with the spin-up channel showing a band gap of 2.3 eV, while the spin-down channel exhibits a semimetallic phase with multiple Dirac points. The thermodynamic stability of the system is also checked out via AIMD simulations, showing the monolayer to be free of distortion at 500 K. The emergence of Dirac half-metal in carbon nitride monolayer via atomic doping reveals an exciting material platform for designing novel nanoelectronics and spintronics devices.

Spin-Gapless States in Two-Dimensional Molecular Ferromagnet Fe2(TCNQ)2

Two-dimensional van der Waals magnetic atomic crystals have provided unprecedented access to magnetic ground states due to a quantum confinement effect. Here, using first-principles calculations, we demonstrate a spin-gapless molecular ferromagnet, namely, Fe₂(TCNQ)₂, with superior mechanical stability and a remarkable linear Dirac cone, which can be exfoliated from its already-synthesized van der Waals crystal. Especially, Young's modulus has values of 175.28 GPa·nm along the x- and y-directions with a Poisson's ratio of 0.29, while the Curie temperature within the Ising model is considerably higher than room temperature. Furthermore, spin-orbit coupling can open a band gap at the Dirac point, leading to topologically nontrivial electronic states characterized by an integer value of the Chern number and the edge states of its nanoribbon. Our results offer versatile platforms for achieving plastic spin filtering or a quantum anomalous Hall effect with promising applications in spintronics devices.

Manipulation of the Magnetic Properties of Janus WSSe Monolayer by the Adsorption of Transition Metal Atoms

Two-dimensional Janus materials have great potential for the applications in spintronic devices due to their particular structures and novel characteristics. However, they are usually non-magnetic in nature. Here, different transition metals (TMs: Co, Fe, Mn, Cr, and V) adsorbed WSSe frameworks are constructed, and their structures and magnetic properties are comprehensively investigated by first-principles calculations. The results show that the top of W atom is the most stable absorption site for all the TM atoms, and all the systems exhibit magnetism. Moreover, their magnetic properties significantly depend on the adsorbed elements and the adsorbent chalcogens. A maximal total magnetic moment of 6 μB is obtained in the Cr-adsorbed system. The induced magnetism from S-surface-adsorption is always stronger than that for the Se-surface-adsorption due to its larger electrostatic potential. Interestingly, the easy magnetization axis in the Fe-adsorbed system switches from the in-plane to the out-of-plane when the adsorption surface changes from Se to S surface. The mechanism is analyzed in detail by Fe-3d orbital-decomposed density of states. This work provides a guidance for the modification of magnetism in low-dimensional systems.

All-boron planar ferromagnetic structures: from clusters to monolayers

Ferromagnetism in all-boron planar clusters is revealed based on high-throughput first-principles calculations. Magnetic boron clusters induced from p electrons have been confirmed with large spins, e.g., S = 3 in a B34 cluster, which can be assembled to construct all-boron ferromagnetic monolayers. Notably, the ferromagnetic semiconductors of boron monolayers can be designed with the hybridization of a nonmagnetic B36 cluster in experimental synthesis. The ferromagnetism-paramagnetism transition and semiconductor-metal transition in these boron nanostructures will occur around 500 K according to ab initio molecular dynamics simulation, indicating the potential applications in nano-devices at room temperature. The coexisting ferromagnetic and semiconducting properties in boron monolayers are attributed to the unique multicenter bonds together with the modulation of structural symmetry, which might be worth experimental attempts in the future.

Metal cyclopropenylidene sandwich cluster and nanowire: structural, electronic, and magnetic properties

Organometallic sandwich clusters and nanowires can offer prototypes for molecular ferromagnet and nanoscale spintronic devices due to the strong coupling of local magnetic moments in the nanowires direction and experimental feasibility. Here, on the basis of first-principles calculations, we reportTM_n(c-C₃H₂)_n+1(TM= Ti, Mn;n= 1-4) sandwich clusters and 1D [TM(c-C₃H₂)]_∞sandwich nanowires building from transitional metal and the smallest aromatic carbene of cyclopropenylidene (c-C₃H₂). Based on the results of lattice dynamic and thermodynamic studies, we show that the magnetic moment of Mn_n(c-C₃H₂)_n+1clusters increases linearly with the number ofn, and 1D [Mn(c-C₃H₂)]_∞nanowire is a stable ferromagnetic semiconductor, which can be converted into half metal with carrier doping. In contrary, both Ti_n(c-C₃H₂)_n+1and 1D [Ti(c-C₃H₂)]_∞nanowire are nonmagnetic materials. This study reveals the potential application of the [TM(c-C₃H₂)]_∞nanowire in spintronics.

Tunable electron property induced by B-doping in g-C3N4

Graphitic carbon nitrides are a research hotspot of two-dimensional (2D) materials, which attract more and more attention from researchers. Topological properties are a focus in graphitic carbon nitrides materials. Using first-principles calculations, we modified the g-C₃N₄ (formed by tri-s-triazine) by B atoms, proposing a novel two-dimensional monolayer, g-C₆N₇B, which showed excellent stability verified by positive phono modes, molecular dynamic simulations and mechanical criteria. The valence band and conduction band touch at the Γ point. Interestingly, g-C₆N₇B is topologically nontrivial, because the valance and conduction band can be gapped by the spin–orbit coupling (SOC) effect associated with robust gapless edge states. Additionally, molecular dynamic simulations indicate that g-C₆N₇B will still maintain good geometry structure when the temperature is as high as 1500 K. The flexibility of g-C₆N₇B is confirmed by its elastic constants and Young's moduli. This work opens an avenue for graphitic carbon nitride materials with topological properties.

A novel graphitic carbon nitride material: g-C₆N₇B, with topological properties.

A new 2D auxetic CN2 nanostructure with high energy density and mechanical strength

The existence of a new two dimensional CN2 structure was predicted using ab initio molecular dynamics (AIMD) and density-functional theory calculations. It consists of tetragonal and hexagonal rings with C-N and N-N bonds arranged in a buckling plane, isostructural to the tetrahex-carbon allotrope. It is thermodynamically and kinetically stable suggested by its phonon spectrum and AIMD. This nanosheet has a high concentration of N and contains N-N single bonds with an energy density of 6.3 kJ g-1, indicating its potential applications as a high energy density material. It possesses exotic mechanical properties with a negative Poisson's ratio and an anisotropic Young's modulus. The modulus in the zigzag direction is predicted to be 340 N m-1, stiffer than those of h-BN and penta-CN2 sheets and comparable to that of graphene. Its ideal strength of 28.8 N m-1 outperforms that of penta-graphene. The material maintains phonon stability upon the application of uniaxial strain up to 10% (13%) in the zigzag (armchair) direction or biaxial strain up to 5%. It possesses a wide indirect HSE band gap of 4.57 eV, which is tunable between 3.37-4.57 eV through strain. Double-layered structures are also explored. Such unique properties may facilitate its potential applications as a high energy density material and in nanomechanics and electronics.

A high performance N-doped graphene nanoribbon based spintronic device applicable with a wide range of adatoms

Designing and fabricating nanosize spintronic devices is a crucial task to develop information technology of the future. However, most of the introduced spin filters suffer from several limitations including difficulty in manipulating the spin current, incapability in utilizing a wide range of dopants to provide magnetism, or obstacles in their experimental realization. Here, by employing first principles calculations, we introduce a structurally simple and functionally efficient spin filter device composed of a zigzag graphene nanoribbon (ZGNR) with an embedded nitrogenated divacancy. We show that the proposed system, possessing a robust ferromagnetic (FM) ordering, exhibits perfect half metallic behavior in the absence of frequently used transition metals (TMs). Our calculations also show that the suggested system is compatible with a wide range of adatoms including basic metals, metalloids, and TMs. It means that besides d electron magnetism originating from TMs, p electrons of incorporated elements of the main group can also cause half metallicity in the electronic structure of the introduced system. Our system exploiting the robustness of doping-induced FM ordering would be beneficial for promising multifunctional spin filter devices.

Spin Polarization-Induced Facile Dioxygen Activation in Boron-Doped Graphitic Carbon Nitride

Dioxygen (O₂) activation is a vital step in many oxidation reactions, and a graphitic carbon nitride (g-C₃N₄) sheet is known as a famous semiconductor catalytic material. Here, we report that the atomic boron (B)-doped g-C₃N₄ (B/g-C₃N₄) can be used as a highly efficient catalyst for O₂ activation. Our first-principles results show that O₂ can be easily chemisorbed at the B site and thus can be highly activated, featured by an elongated O-O bond (∼1.52 Å). Interestingly, the O-O cleavage is almost barrier free at room temperatures, independent of the doping concentration. It is revealed that the B atom can induce considerable spin polarization on B/g-C₃N₄, which accounts for O₂ activation. The doping concentration determines the coupling configuration of net-spin and thus the magnitude of the magnetism. However, the distribution of net-spin at the active site is independent of the doping concentration, giving rise to the doping concentration-independent catalytic capacity. The unique monolayer geometry and the existing multiple active sites may facilitate the adsorption and activation of O₂ from two sides, and the newly generated surface oxygen-containing groups can catalyze the oxidation coupling of methane to ethane. The present findings pave a new way to design g-C₃N₄-based metal-free catalysts for oxidation reactions.

Porphyrin nanoribbon-based spin filtering devices

Advancement in molecular electronics opens up another new domain with a new possibility of realizing its spin-polarized version, which is called molecular spintronics. This novel domain has a range of applications such as high-capacity storage devices and quantum computers. Several contemporary researchers have considered porphyrin molecules and their derivatives as potential candidates for molecular devices. Herein, using the first-principles calculations, we propose a porphyrin nanoribbon-based system for spin-filtering applications. Such a system shows robust half-metallicity and also exhibits itinerant magnetism. Our calculated spin transport properties exhibit that our device can give 100% spin-polarizing efficiency, which is very promising for next-generation spin-filtering applications.

Investigation of strain and doping on the electronic properties of single layers of C6N6 and C6N8: a first principles study

In this work, by performing first-principles calculations, we explore the effects of various atom impurities on the electronic and magnetic properties of single layers of C6N6 and C6N8. Our results indicate that atom doping may significantly modify the electronic properties. Surprisingly, doping Cr into a holey site of C6N6 monolayer was found to exhibit a narrow band gap of 125 meV upon compression strain, considering the spin-orbit coupling effect. Also, a C atom doped in C6N8 monolayer shows semi-metal nature under compression strains larger than -2%. Our results propose that Mg or Ca doped into strained C6N6 may exhibit small band gaps in the range of 10-30 meV. In addition, a magnetic-to-nonmagnetic phase transition can occur under large tensile strains in the Ca doped C6N8 monolayer. Our results highlight the electronic properties and magnetism of C6N6 and C6N8 monolayers. Our results show that the electronic properties can be effectively modified by atom doping and mechanical strain, thereby offering new possibilities to tailor the electronic and magnetic properties of C6N6 and C6N8 carbon nitride monolayers.

Unconventional deformation potential and half-metallicity in zigzag nanoribbons of 2D-Xenes

Realization of half-metallicity (HM) in low dimensional materials is a fundamental challenge for nano spintronics and a critical component for developing alternative generations of information technology. Using first-principles calculations, we reveal an unconventional deformation potential for zigzag nanoribbons (NRs) of 2D-Xenes. Both the conduction band minimum (CBM) and valence band maximum (VBM) of the edge states have negative deformation potentials. This unique property, combined with the localization and spin-polarization of the edge states, enables us to induce spin-splitting and HM using an inhomogeneous strain pattern, such as simple in-plane bending. Indeed, our calculation using the generalized Bloch theorem reveals the predicted HM in bent zigzag silicene NRs. Furthermore, the magnetic stability of the long range magnetic order for the spin-polarized edge states is maintained well against the bending deformation. These aspects indicate that it is a promising approach to realize HM in low dimensions with the zigzag 2D-Xene NRs.

Two-dimensional graphitic carbon nitride (g-C4N3) for superior selectivity of multiple toxic gases (CO, NO2, and NH3)

Using first principles calculations, we investigated the possibility of selecting multiple toxic gases using one substrate material. Here, we explored the transport property of H₂, N₂, O₂, CO, CO₂, NO_2, and NH₃ gas molecules on two-dimensional graphitic carbon nitride (2D g-C₄N₃). The homonuclear molecule such as H₂, N₂, and O₂ has very weak adsorption energy (equal to or less than 0.1 eV) and also CO₂ has an adsorption energy of 0.23 eV. In the typical toxic gas molecule adsorption systems, we found an appreciable charge transfer. In CO and NH₃ adsorption systems, the charge transfer of 0.397 and 0.418 electrons from the molecule to the substrate was found, while the NO₂ molecule gained 0.124 electrons from the substrate. Due to this large amount of charge transfer, we obtained large adsorption energies of 4.57, 1.29, and 1.93 eV in CO, NO_2, and NH₃ systems. Moreover, through the I-V curve calculations, we found large difference in the current. The calculated current was 21, 13.11, and 16.16 μA for CO, NO_2, and NH₃ systems at the bias voltage of 0.5 V. Our results imply that the 2D g-C₄N₃ can be a superior substrate material for sensing of multiple toxic gases.

A systematic investigation of the catalytic performances of monolayer carbon nitride nanosheets C1-xNx

Graphitic carbon nitrides (CNs) are potential candidate materials for the electro-catalytic industry due to their unique physical and chemical properties. However, to date, a full understanding of the electro-catalytic properties of CNs is still lacking. Herein, by using density functional theory calculations, we systematically investigate the catalytic performances in the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), N2 reduction reaction (NRR), and CO2 reduction reaction (CO2RR) of monolayer graphitic carbon nitrides (C1-xNx), C3N (x = 1/4), C2N (x = 1/3), and g-C3N4 (x = 4/7). We also evaluated the NRR activity of B doped C1-xNx, and the CO2RR activity of Cu and Pd modified C1-xNx. The cohesive energy and ab initio molecular dynamics (AIMD) results show that C3N, C2N, and g-C3N4 are stable at room temperature. The C3N-C1 site is predicted to deliver the best HER catalytic performance with a reaction Gibbs free energy (ΔGH*) of -0.03 eV (close to the ideal value (0 eV)). Among the studied C1-xNx materials, the C3N-C2 site is predicted to possess a favorable ηOER of 0.82 V for OER. Pure C3N, C2N, and g-C3N4 are not suitable for NRR and CO2RR. Due to the strong hybridization between the N 2p orbital and the B 2p orbital, the NRR performances of B doped BN-C2N, BN-C3N, and BN-g-C3N4 are greatly enhanced, with corresponding overpotential ηNRR of 0.57 V, 0.70 V, and 0.72 V, respectively. The transition metals Cu and Pd can enhance the CO2RR activity of C3N, C2N, and g-C3N4. The limiting potentials UL of pure C3N, C2N, and g-C3N4 are 0.96 V, 0.86 V, and 2.37 V, respectively, while these values are 0.63 V, 0.68 V, and 0.77 V with Cu or Pd modification. This work provides deep understanding of the catalytic properties of monolayer C1-xNx and guidance for synthesizing higher activity catalysts in the future.

Control of C3N4 and C4N3 carbon nitride nanosheets’ electronic and magnetic properties through embedded atoms

In the present work, the effect of various embedded atom impurities on tuning electronic and magnetic properties of C₃N₄ and C₄N₃ nanosheets have been studied using first-principles calculations.

Embedding of atoms into the nanopore sites of the C6N6 and C6N8 porous carbon nitride monolayers with tunable electronic properties

Using first-principles calculations, we study the effect of embedding various atoms into the nanopore sites of both C₆N₆ and C₆N₈ monolayers.

Defect driven spin state transition and the existence of half-metallicity in CoO

We unveil the native defect induced high spin to low spin state transition in [Formula: see text] and half-metallicity in CoO. First principles calculations unravel that, defect density holds a key role in dictating the spin-state transition in [Formula: see text] ion in CoO, and introducing the half-metallicity. Charge transfer in the vicinity of vacancy plane favors the stabilization and coexistence of bivalent [Formula: see text] and trivalent [Formula: see text] ion in CoO. We propose that defect engineering could serve as a route to design the half metallicity in transition metal mono-oxides.

Spin-Crossover Assisted Spin-Switching and Rectification Action in Half-Metallic Graphitic Carbon Nitride(g-C4 N3 )

Herein, we report on the potential multifunctional spintronic action of half-metallic graphitic carbon nitride (g-C₄ N₃ ). We observed electrostatic spin-crossover action at an applied electric field of -0.77 V nm^-1 , which eventually leads to spin-switching action and change in sign of bias dependent spin injection coefficient. The system also acts as a spin polarized charge current rectifier with rectification ratio of 65.41 in spin-up channel only. This electric field-controlled spin switching action and simultaneous existence of rectification action makes graphitic carbon nitride a perfect multifunctional spintronic system-an ideal material for quantum logic gate design. Results obtained have been substantiated through transmission spectra and transmission pathways analyses. An analysis of projected device density of states of the system and molecular projected self consistent Hamiltonian states analysis reveals that the electron flow of the system is mainly facilitated by 2p orbitals of C and N atoms.

Superoctahedral two-dimensional metallic boron with peculiar magnetic properties

A novel two-dimensional ferromagnetic stable boron material has been predicted and exhaustively studied with DFT methods. Its magnetism can be described by the presence of two unpaired delocalized bonding elements inside each distorted octahedron.