First-principles study of defect-induced magnetism in carbon

The Case for a Defect Genome Initiative

The Materials Genome Initiative (MGI) has streamlined the materials discovery effort by leveraging generic traits of materials, with focus largely on perfect solids. Defects such as impurities and perturbations, however, drive many attractive functional properties of materials. The rich tapestry of charge, spin, and bonding states hosted by defects are not accessible to elements and perfect crystals, and defects can thus be viewed as another class of "elements" that lie beyond the periodic table. Accordingly, a Defect Genome Initiative (DGI) to accelerate functional defect discovery for energy, quantum information, and other applications is proposed. First, major advances made under the MGI are highlighted, followed by a delineation of pathways for accelerating the discovery and design of functional defects under the DGI. Near-term goals for the DGI are suggested. The construction of open defect platforms and design of data-driven functional defects, along with approaches for fabrication and characterization of defects, are discussed. The associated challenges and opportunities are considered and recent advances towards controlled introduction of functional defects at the atomic scale are reviewed. It is hoped this perspective will spur a community-wide interest in undertaking a DGI effort in recognition of the importance of defects in enabling unique functionalities in materials.

Arc-discharge synthesis of nitrogen-doped C embedded TiCN nanocubes with tunable dielectric/magnetic properties for electromagnetic absorbing applications

The development of novel composites consisting of ceramic and C materials to alleviate increasingly serious electromagnetic radiation is of great significance in the microwave absorption (MA) field, considering their superior anti-oxidation/corrosion performances and good mechanical strength as well as adjustable dielectric loss capabilities. However, it is still a great challenge to broaden their effective absorption bandwidth (reflection loss value ≤-10 dB) and strengthen the absorption intensity simultaneously, which is mostly attributed to the unreliable impedance matching degree at the absorber/air interface. Herein, a feasible strategy is adopted to synthesize TiCN@N-doped C nanocubes, whose low graphitization degree provides desirable impedance matching conditions. In the meantime, masses of core/shell hetero interfaces ensure strong microwave absorption capability. Experimental results reveal that the optimal effective absorption bandwidth of the prepared TiCN@N-doped C nanocubes can reach up to 5.44 GHz with a thickness of 1.88 mm. Our work demonstrates that the TiCN@N-doped C nanocubes have potential for electromagnetic absorbing applications.

Thermodynamic stability of magnetic states of monovacancy in graphene revealed by ab initio molecular dynamics simulations

The stability of magnetic states is essential for potential spintronic applications. Here we report on the thermal stability of magnetic states of monovacancy graphene using ab initio molecular dynamics simulations. At room temperature, thermal fluctuations of the graphene lattice induce a rapid magnetic switching between two states with a high and low magnetic moment, indicating that due to the instability of the atomic structure of the vacancy, the associated magnetic moment is thermodynamically unstable. Lowering the temperature can significantly reduce the rate of the switching process and enhance the resident time on the high magnetic state. It stabilizes in the high magnetic state at as low as 30 K. Analyzing the atomic trajectories and the instant electronic structures confirms that these two magnetic states in MD simulations correspond to the magnetic and nonmagnetic states reported in the literatures. Such fluctuations of local magnetic moments are associated with the vertical displacement of the carbon atoms with the unsaturated dangling bond. This study reveals the dynamical correlation between atomic movement and the magnetic switching, and a comprehensive picture of vacancy magnetism in graphene. It has implications in graphene based spintronic devices.

Enhanced paramagnetism of mesoscopic graphdiyne by doping with nitrogen

The new two-dimensional graphitic material, graphdiyne, has attracted great interest recently due to the superior intrinsic semiconductor properties. Here we investigate the magnetism of pure graphdiyne material and find it demonstrating a remarkable paramagnetic characteristic, which can be attributed to the appearance of special sp-hybridized carbon atoms. On this basis, we further introduce nitrogen with 5.29% N/C ratio into graphdiyne followed by simply annealing in a dopant source and realize a twofold enhancement of saturation moment at 2 K. Associate with the density of states calculation, we investigate the influence of the nitrogen atom doping sites on paramagnetism, and further reveal the important role of doped nitrogen atom on benzene ring in improving local magnetic moment. These results can not only help us deeply understand the intrinsic magnetism of graphdiyne, but also open an efficient way to improve magnetism of graphdiyne by hetero atom doping, like nitrogen doping, which may promote the potential application of graphdiyne in spintronics.

Unusual spin-polarized electron state in fullerene induced by carbon adatom defect

First-principles calculations show that a carbon adatom defect at the Def[5, 6] site on the surface of C₆₀ can produce a more stable spin-polarized singlet electronic state instead of a magnetic triplet state. This is clearly different from the cases of graphene and nanotubes. The mechanism results from the electron population of the adatom, which produces antiferromagnetic coupling around the C₆₀ cage and the adatom itself. Our calculations show the same phenomenon occurs in other IPR fullerenes, such as C₇₀ and C₈₀. These findings extend the understanding of the magnetic origin of pure carbon structures and are valuable for research related to the spin polarization of carbon systems.

Strain controlled ferromagnetic-ferrimagnetic transition and vacancy formation energy of defective graphene

Single vacancy (SV)-induced magnetism in graphene has attracted much attention motivated by its potential in achieving new functionalities. However, a much higher vacancy formation energy limits its direct application in electronic devices and the dependency of spin interaction on the strain is unclear. Here, through first-principles density-functional theory calculations, we investigate the possibility of strain engineering towards lowering vacancy formation energy and inducing new magnetic states in defective graphene. It is found that the SV-graphene undergoes a phase transition from an initial ferromagnetic state to a ferrimagnetic state under a biaxial tensile strain. At the same time, the biaxial tensile strain significantly lowers the vacancy formation energy. The charge density, density of states and band theory successfully identify the origin and underlying physics of the transition. The predicted magnetic phase transition is attributed to the strain driven spin flipping at the C-atoms nearest to the SV-site. The magnetic semiconducting graphene induced by defect and strain engineering suggests an effective way to modulate both spin and electronic degrees of freedom in future spintronic devices.

The possible formation of a magnetic FeS2 phase in the two-dimensional MoS2 matrix

The possibility of a FeS₂ phase formation in the 2D MoS₂ structure was investigated by an ab initio DFT approach. Various concentrations of FeS₂ in MoS₂ have been analyzed, and it is shown that the energy favorable structures of the Mo_1-xFe_xS₂ composition are in-plane hybrid phases, FeS₂ and MoS₂ domains. After increasing the Fe/Mo concentration ratio up to 0.68, a complete transformation of the whole structure is predicted. We have found that the introduction of only a small amount of Fe atoms leads to a change in the electronic and magnetic properties of the film. An increase of the FeS₂ nucleus size leads to the nearly monotonous increase of the magnetic moment governed by the exponential law.

Tunable magnetism in metal adsorbed fluorinated nanoporous graphene

Developing nanostructures with tunable magnetic states is crucial for designing novel data storage and quantum information devices. Using density functional theory, we investigate the thermodynamic stability and magnetic properties of tungsten adsorbed tri-vacancy fluorinated (TVF) graphene. We demonstrate a strong structure-property relationship and its response to external stimuli via defect engineering in graphene-based materials. Complex interplay between defect states and the chemisorbed atom results in a large magnetic moment of 7 μB along with high in-plane magneto-crystalline anisotropy energy (MAE) of 17 meV. Under the influence of electric field, spin crossover effect accompanied by a change in the MAE is observed. The ascribed change in spin-configuration is caused by the modification of exchange coupling between defect states and a change in the occupation of d-orbitals of the metal complex. Our predictions open a promising way towards controlling the magnetic properties in graphene based spintronic and non-volatile memory devices.

Plasma modification of the electronic and magnetic properties of vertically aligned bi-/tri-layered graphene nanoflakes

Saturation magnetization of vertically aligned bi/tri-layers is further enhanced by hydrogen, nitrogen plasma modification while organo-silane treatment reduces magnetization.

Determination of the hyperfine magnetic field in magnetic carbon-based materials: DFT calculations and NMR experiments

The prospect of carbon-based magnetic materials is of immense fundamental and practical importance, and information on atomic-scale features is required for a better understanding of the mechanisms leading to carbon magnetism. Here we report the first direct detection of the microscopic magnetic field produced at (13)C nuclei in a ferromagnetic carbon material by zero-field nuclear magnetic resonance (NMR). Electronic structure calculations carried out in nanosized model systems with different classes of structural defects show a similar range of magnetic field values (18-21 T) for all investigated systems, in agreement with the NMR experiments. Our results are strong evidence of the intrinsic nature of defect-induced magnetism in magnetic carbons and establish the magnitude of the hyperfine magnetic field created in the neighbourhood of the defects that lead to magnetic order in these materials.

Atomically thin layers of B-N-C-O with tunable composition

In recent times, atomically thin alloys of boron, nitrogen, and carbon have generated significant excitement as a composition-tunable two-dimensional (2D) material that demonstrates rich physics as well as application potentials. The possibility of tunably incorporating oxygen, a group VI element, into the honeycomb sp(2)-type 2D-BNC lattice is an intriguing idea from both fundamental and applied perspectives. We present the first report on an atomically thin quaternary alloy of boron, nitrogen, carbon, and oxygen (2D-BNCO). Our experiments suggest, and density functional theory (DFT) calculations corroborate, stable configurations of a honeycomb 2D-BNCO lattice. We observe micrometer-scale 2D-BNCO domains within a graphene-rich 2D-BNC matrix, and are able to control the area coverage and relative composition of these domains by varying the oxygen content in the growth setup. Macroscopic samples comprising 2D-BNCO domains in a graphene-rich 2D-BNC matrix show graphene-like gate-modulated electronic transport with mobility exceeding 500 cm(2) V(-1) s(-1), and Arrhenius-like activated temperature dependence. Spin-polarized DFT calculations for nanoscale 2D-BNCO patches predict magnetic ground states originating from the B atoms closest to the O atoms and sizable (0.6 eV < E g < 0.8 eV) band gaps in their density of states. These results suggest that 2D-BNCO with novel electronic and magnetic properties have great potential for nanoelectronics and spintronic applications in an atomically thin platform.

Slippage in stacking of graphene nanofragments induced by spin polarization

Spin polarization and stacking are interesting effects in complex molecular systems and are both presented in graphene-based materials. Their possible combination may provide a new perspective in understanding the intermolecular force. The nanoscale graphene structures with zigzag edges could possess spin-polarized ground states. However, the mechanical effect of spin polarization in stacking of graphene nanofragments is not clear. Here we demonstrate the displacement between two stacked rhombic graphene nanofragments induced by spin polarization, using first-principles density-functional methods. We found that, in stacking of two rhombic graphene nanofragments, a spin-polarized stacked conformation with zero total spin is energetically more favorable than the closed-shell stacking. The spin-polarized conformation gives a further horizontal interlayer displacement within 1 angstrom compared with the closed-shell structure. This result highlights that, besides the well-known phenomenologically interpreted van der Waals forces, a specific mechanism dependent on the monomeric spin polarization may lead to obvious mechanical effects in some intermolecular interactions.

Atomic structure and dynamics of metal dopant pairs in graphene

We present an atomic resolution structural study of covalently bonded dopant pairs in the lattice of monolayer graphene. Two iron (Fe) metal atoms that are covalently bonded within the graphene lattice are observed and their interaction with each other is investigated. The two metal atom dopants can form small paired clusters of varied geometry within graphene vacancy defects. The two Fe atoms are created within a 10 nm diameter predefined location in graphene by manipulating a focused electron beam (80 kV) on the surface of graphene containing an intentionally deposited Fe precursor reservoir. Aberration-corrected transmission electron microscopy at 80 kV has been used to investigate the atomic structure and real time dynamics of Fe dimers embedded in graphene vacancies. Four different stable structures have been observed; two variants of an Fe dimer in a graphene trivacancy, an Fe dimer embedded in two adjacent monovacancies and an Fe dimer trapped by a quadvacancy. According to spin-sensitive DFT calculations, these dimer structures all possess magnetic moments of either 2.00 or 4.00 μB. The dimer structures were found to evolve from an initial single Fe atom dopant trapped in a graphene vacancy.

Realization of ferromagnetic graphene oxide with high magnetization by doping graphene oxide with nitrogen

The long spin diffusion length makes graphene very attractive for novel spintronic devices, and thus has triggered a quest for integrating the charge and spin degrees of freedom. However, ideal graphene is intrinsic non-magnetic, due to a delocalized π bonding network. Therefore, synthesis of ferromagnetic graphene or its derivatives with high magnetization is urgent due to both fundamental and technological importance. Here we report that N-doping can be an effective route to obtain a very high magnetization of ca. 1.66 emu/g, and can make graphene oxide (GO) to be ferromagnetism with a Curie-temperature of 100.2 K. Clearly, our findings can offer the easy realization of ferromagnetic GO with high magnetization, therefore, push the way for potential applications in spintronic devices.

Intrinsic magnetism of grain boundaries in two-dimensional metal dichalcogenides

Grain boundaries (GBs) are structural imperfections that typically degrade the performance of materials. Here we show that dislocations and GBs in two-dimensional (2D) metal dichalcogenides MX2 (M = Mo, W; X = S, Se) can actually improve the material by giving it a qualitatively new physical property: magnetism. The dislocations studied all display a substantial magnetic moment of ∼1 Bohr magneton. In contrast, dislocations in other well-studied 2D materials are typically nonmagnetic. GBs composed of pentagon-heptagon pairs interact ferromagnetically and transition from semiconductor to half-metal or metal as a function of tilt angle and/or doping level. When the tilt angle exceeds 47°, the structural energetics favor square-octagon pairs and the GB becomes an antiferromagnetic semiconductor. These exceptional magnetic properties arise from interplay of dislocation-induced localized states, doping, and locally unbalanced stoichiometry. Purposeful engineering of topological GBs may be able to convert MX2 into a promising 2D magnetic semiconductor.

The contribution made by lattice vacancies to the Wigner effect in radiation-damaged graphite

Models for radiation damage in graphite are reviewed and compared, leading to a re-examination of the contribution made by vacancies to annealing processes. A method based on density functional theory, using large supercells with orthorhombic and hexagonal symmetry, is employed to calculate the properties and behaviour of lattice vacancies and displacement defects. It is concluded that annihilation of intimate Frenkel defects marks the onset of recovery in electrical resistivity, which occurs when the temperature exceeds about 160 K. The migration of isolated monovacancies is estimated to have an activation energy of E(a) ≈ 1.1 eV. Coalescence into divacancy defects occurs in several stages, with different barriers at each stage, depending on the path. The formation of pairs ultimately yields up to 8.9 eV energy, which is nearly 1.0 eV more than the formation energy for an isolated monovacancy. Processes resulting in vacancy coalescence and annihilation appear to be responsible for the main Wigner energy release peak in radiation-damaged graphite, occurring at about 475 K.

Super-stable ultrafine beta-tungsten nanocrystals with metastable phase and related magnetism

Ultrafine tungsten nanocrystals (average size of 3 nm) with a metastable phase (beta-tungsten with A15 structure, β-W) have been prepared by laser ablation of tungsten in liquid nitrogen. The as-prepared metastable nanocrystals exhibited super-stablity, and can keep the same metastable structure over a period of 6 months at room temperature. This super-stability is attributed to the nanosized confinement effect of ultrafine nanocrystals. The magnetism measurements showed that the β-W nanocrystals have weak ferromagnetic properties at 2 K, which may arise from surface defects and unpaired electrons on the surface of the ultrafine nanocrystals. These findings provided useful information for the application of ultrafine β-W nanocrystals in microelectronics and spintronics.

Dimerization of boron dopant in diamond (100) epitaxy induced by strong pair correlation on the surface

Experiments have shown that boron incorporation in diamond epitaxies is orientation dependent. Our first-principles calculations reveal that at a (100) surface, the formation of the boron dimer is more favored than that of the monomer, indicating a high density of ineffective boron formed under heavy doping. The reconstructed surface layer of carbon dimers in which the electrons are strongly pair correlated provides the mechanism. Hydrogen adsorption affects the correlation and thus the favorability of boron dimer formation, while at a (111) surface, the formation of boron monomer is more favored due to the less correlated surface electrons and hydrogen adsorption has no effect on the favorability.

Strain-induced magnetic transitions in half-fluorinated single layers of BN, GaN and graphene

Recently, extensive experimental and theoretical studies on single layers of BN, GaN and graphene have stimulated enormous interest in exploring the properties of these sheets by decorating their surfaces. In the present work we discuss half-fluorinated single layers of BN, GaN and graphene, in the context of intercoupling between strain and magnetic property. First-principles calculations reveal that the energy difference between ferromagnetic and antiferromagnetic couplings increases significantly with strain increasing for half-fluorinated BN, GaN and graphene sheets. More surprisingly, the half-fluorinated BN and GaN sheets exhibit intriguing magnetic transitions between ferromagnetism and antiferromagnetism by applying strain, even giving rise to half-metal when the sheets are under compression of 6%. It is found that the magnetic coupling as well as the strain-dependent magnetic transition behavior arise from the combined effects of both through-bond and p-p direct interactions. Our work offers a new avenue to facilitate the design of controllable and tunable spin devices.

Sustained ferromagnetism induced by H-vacancies in graphane

The electronic and magnetic properties of graphane flakes with H-vacancies were investigated using quantum-chemistry methods. The hybridization of the edges is found to be absolutely crucial in defining the size of the HOMO-LUMO gap, which is increased from 3.04 to 7.51 eV when the hybridization is changed from the sp(2) to the sp(3) type. The H-vacancy defects also influence the size of the gap, which depends on the number of defects and their distribution between the two sides of the graphane plane. Further, the H-vacancy defects induced on one side of the graphane plane and placed on the neighboring carbon atoms are found to be the source of ferromagnetism which is distinguished by the high stability of the state with a large spin number in comparison to that of the singlet state and is expected to persist even at room temperatures. However, the ferromagnetic ordering of the spins is found to be limited by the concentration of H-vacancy defects and ordering would be preserved if number of defects does not exceed eight.

KO2: realization of orbital ordering in a p-orbital system

KO2 is a molecular solid consisting of oxygen dimers. K present in the lattice donates an electron which goes on to occupy the O p levels. As the basic electronic structure is similar to that of an oxygen molecule, except for broadening due to solid state effects, KO2 represents the realization of the doping of oxygen molecules arranged in a lattice. These considerations alone result in magnetism with high ordering temperatures as our calculations reveal. However, we find that the high temperature structure is unstable to an orbital ordering (OO) transition. The microscopic considerations driving the OO transition, however, are electrostatic interactions instead of the often encountered superexchange driven ordering within the Kugel-Khomskii model often used to describe the OO. This OO transition is also found to preclude any possibility of high magnetic ordering temperatures, which otherwise seemed possible.

Magnetism of solids resulting from spin polarization of p orbitals

Magnetism in systems that do not contain transition metal or rare earth ions was recently observed or predicted to exist in a wide variety of systems. We summarize both experimental and theoretical results obtained for ideal bulk II-V and II-IV compounds, molecular crystals containing O(2) or N(2) molecules as structural units, as well as for carbon-based materials such as graphite and graphene nanoribbons. Magnetism can be an intrinsic property of a perfect crystal, or it can be induced by non-magnetic dopants or defects. In the case of vacancies, spin polarization is local and results in their high spin states. The non-vanishing spin polarization is shown to originate in the strong spin polarization of the 2p shell of light atoms from the second row of the periodic table.

The role of sp-hybridized atoms in carbon ferromagnetism: a spin-polarized density functional theory calculation

We address the room-temperature (RT) carbon ferromagnetism by considering the magnetic states of low-dimensional carbons linked by sp-hybridized carbon atoms. Based on the spin-polarized density functional theory calculations, we find that the sp(*) orbitals of carbon atoms can bring magnetic moments into different carbon allotropes which may eventually give rise to the long-range ferromagnetic ordering at room temperature through an indirect carrier-mediated coupling mechanism. The fact that this indirect coupling is Fermi-level-dependent predicts that the individual magnetism of diverse carbon materials is governed by their chemical environments. This mechanism may help to illuminate the RT magnetic properties of carbon-based materials and to explore the new magnetic applications of carbon materials.

Itinerant flat-band magnetism in hydrogenated carbon nanotubes

We investigate the electronic and magnetic properties of hydrogenated carbon nanotubes using ab initio spin-polarized calculations within both the local density approximation (LDA) and the generalized gradient approximation (GGA). We find that the combination of charge transfer and carbon network distortion makes the spin-polarized flat-band appear in the tube's energy gap. Various spin-dependent ground state properties are predicted with the changes of the radii, the chiralities of the tubes, and the concentration of hydrogen. It is found that strain or external electric field can effectively modulate the flat-band spin-splitting and even induce an insulator-metal transition.

Magnetism in graphene due to single-atom defects: dependence on the concentration and packing geometry of defects

The magnetism in graphene due to single-atom defects is examined by using spin-polarized density functional theory. The magnetic moment per defect due to substitutional atoms and vacancy defects is dependent on the density of defects, while that due to adatom defects is independent of the density of defects. It reduces to zero with decrease in the density of substitutional atoms. However, it increases with decrease in density of vacancies. The graphene sheet with B adatoms is nonmagnetic, but with C and N adatoms it is magnetic. The adatom defects distort the graphene sheet near the defect perpendicular to the sheet. The distortion in graphene due to C and N adatoms is significant, while the distortion due to B adatoms is very small. The vacancy and substitutional atom (B, N) defects in graphene are planar in the sense that there is in-plane displacement of C atoms near the vacancy and substitutional defects. Upon relaxation the displacement of C atoms and the formation of pentagons near the vacancy site due to Jahn-Teller distortion depends upon the density and packing geometry of vacancies.

Defect-Induced intrinsic magnetism in wide-gap III nitrides

Cation-vacancy induced intrinsic magnetism in GaN and BN is investigated by employing density-functional theory based electronic structure methods. The strong localization of defect states favors spontaneous spin polarization and local moment formation. A neutral cation vacancy in GaN or BN leads to the formation of a net moment of 3 muB with a spin-polarization energy of about 0.5 eV at the low density limit. The extended tails of defect wave functions, on the other hand, mediate surprisingly long-range magnetic interactions between the defect-induced moments. This duality of defect states suggests the existence of defect-induced or mediated collective magnetism in these otherwise nonmagnetic sp systems.