Irradiation-induced magnetism in carbon nanostructures

Boron-doped graphene topological defects: unveiling high sensitivity to NO molecule for gas sensing applications

Global air quality has deteriorated significantly in recent years due to large emissions from the transformation industry and combustion vehicles. This issue requires the development of portable, highly sensitive, and selective gas sensors. Nanostructured materials, including defective graphene, have emerged as promising candidates for such applications. In this work, we investigated the B-doped topological line defect in graphene as a sensing material for various gas molecules (CO, CO2, NO, and NH3) based on a combination of density functional theory and the non-equilibrium Green's function method. The electronic transport calculations reveal that the electric current can be confined to the line defect region by gate voltage control, revealing highly reactive sites. The B-doped topological line defect is metallic, favoring the adsorption of NO and NH3 over CO and CO2 molecules. We notice changes in the conductance after gas molecule adsorption, producing a sensitivity of 50% (16%) for NO (NH3). In addition, the recovery time for nitride gases was calculated for different temperatures and radiation frequencies. At 300 K the ultraviolet (UV) has a fast recovery time compared to the visible (VIS) one by about two orders of magnitude. This study gives an understanding of how engineering transport properties at the microscopic level (by topological line defect and chemical B-doping) leads to promising nanosensors for detecting nitride gas.

Production, surface modification, physicochemical properties, biocompatibility, and bioimaging applications of nanodiamonds

Nanodiamonds (ND) are chemically inert and stable owing to their sp3 covalent bonding structure, but their surface sp2 graphitic carbons can be easily homogenized with diverse functional groups via oxidation, reduction, hydrogenation, amination, and halogenation. Further surface conjugation of NDs with hydrophilic ligands can boost their colloidal stability and functionality. In addition, NDs are non-toxic as they are made of carbons. They exhibit stable fluorescence without photobleaching. They also possess paramagnetic and ferromagnetic properties, making them suitable for use as a new type of fluorescence imaging (FI) and magnetic resonance imaging (MRI) probe. In this review, we focused on recently developed ND production methods, surface homogenization and functionalization methods, biocompatibilities, and biomedical imaging applications as FI and MRI probes. Finally, we discussed future perspectives.

Nanometers-Thick Ferromagnetic Surface Produced by Laser Cutting of Diamond

In this work, we demonstrate that cutting diamond crystals with a laser (532 nm wavelength, 0.5 mJ energy, 200 ns pulse duration at 15 kHz) produced a ≲20 nm thick surface layer with magnetic order at room temperature. We measured the magnetic moment of five natural and six CVD diamond crystals of different sizes, nitrogen contents and surface orientations with a SQUID magnetometer. A robust ferromagnetic response at 300 K was observed only for crystals that were cut with the laser along the (100) surface orientation. The magnetic signals were much weaker for the (110) and negligible for the (111) orientations. We attribute the magnetic order to the disordered graphite layer produced by the laser at the diamond surface. The ferromagnetic signal vanished after chemical etching or after moderate temperature annealing. The obtained results indicate that laser treatment of diamond may pave the way to create ferromagnetic spots at its surface.

Construction of Heptagon-Containing Molecular Nanocarbons

Molecular nanocarbons containing heptagonal rings have attracted increasing interest due to their dynamic behavior, electronic properties, aromaticity, and solid-state packing. Heptagon incorporation can not only induce negative curvature within nanocarbon scaffolds, but also confer significantly altered properties through interaction with adjacent non-hexagonal rings. Despite the disclosure of several beautiful examples in recent years, synthetic strategies toward heptagon-embedded molecular nanocarbons remain relatively limited due to the intrinsic challenges of heptagon formation and incorporation into polyarene frameworks. In this Review, recent advances in solution-mediated and surface-assisted synthesis of heptagon-containing molecular nanocarbons, as well as the intriguing properties of these frameworks, will be discussed.

First-principles search for half-metallic ferromagnetism in CsCrZ2 (Z = O, S, Se or Te) Heusler alloys

Exploring highly spin-polarized materials is crucial for the development of spin-based devices. In this paper, we investigate the atomic structure, electronic, half-metallic and magnetic properties of the CsCrZ₂ (Z = O, S, Se or Te) Heusler alloys, by performing first-principles calculations based on density functional theory (DFT). The geometry optimization process shows that the alloys are more stable in Cu₂MnAl type structure than in Hg₂CuTi one. To find a magnetic ground state, the total energy of the alloys is calculated in the non-magnetic (NM), ferromagnetic (FM) and anti-ferromagnetic (AFM) ordering. We find that, the FM ordering yields the lowest energy, thereby confirming that the alloys are FM in the ground state. On computing the cohesive and formation energy in the ground state, it is found that alloys are chemically and thermodynamically stable, respectively. Spin polarized band structures and density of states (DOS) demonstrate that the CsCrO₂, CsCrS₂ and CsCrSe₂ alloys are true half-metals with 100% spin-polarization at the Fermi level, while the CsCrTe₂ alloy is predicted as highly spin polarized material. Furthermore, the CsCrO₂, CsCrS₂ and CsCrSe₂ alloys possess 2.529, 2.250 and 2.050 eV gaps in the spin down band structure, respectively. The calculated total magnetic moments reveal that half-metallic alloys have an integral total magnetic moment of 3.000 μ_B, which satisfies the Slater-Pauling rule M_t = Z_t-16. The main contribution to the total magnetic moment comes from the Cr atoms (about 3.9 μ_B). Furthermore, the Curie temperature T_C calculated within classical Heisenberg model is estimated to be about 822 K (CsCrO₂), 685 K (CsCrS₂), 753 K (CsCrSe₂) and 636 K (CsCrTe₂). The obtained high spin polarization and above room temperature FM ordering make the materials as promising materials to be used in spintronic technology.

First-principles study of structural, electronic, magnetic, thermodynamic and mechanical properties of ferromagnetic Mn2MoAl Heuslar alloy

The Mn-based Mn₂MoAl type ternary Heusler alloy is of particular interest due to its potential ferromagnetic properties and high spin-polarization. The present paper aims to explore the physical properties of this new alloy for its possible technological applications and also provide theoretical basis to the future experiments. The electronic structure, magnetism, and elastic properties of the alloy are studied by using first-principles calculations based on density functional theory (DFT), along with calculation of thermodynamic properties within quasi-harmonic approximation. The structural analysis predicts the optimized lattice constant of 5.90 Å and the ferromagnetic stable state. The electronic band structure calculations at GGA-level reveal that present alloy is nearly half-metal with spin-polarization up to 94% at the Fermi level, whereas, GGA + U calculations reveal Mn₂MoAl alloy to be a quite spin polarized metal. The calculated total magnetic moment M_t of the unit-cell is found to be 1.04 μ_B, which nearly obeys M_t = │Z_t-24│formula. The individual Mn, Mo and Al atoms carry magnetic moment of 0.70 μ_B, -0.28 μ_B, -0.01μ_B, respectively. Employing quasi-harmonic approximation, interesting thermodynamic properties of the alloy have been investigated. Furthermore, the elastic and mechanical properties have been investigated. The results confirm that Mn₂MoAl alloy is mechanically stable, ductile, anisotropic, with metallic inter-atomic bonding and is expected to be beneficial for the spintronic technology.

Realization of Ambient-Stable Room-Temperature Ferromagnetism by Low-Temperature Annealing of Graphene Oxide Nanoribbons

Graphene oxide nanoribbons (GONRs) annealed at the low temperature of 400 °C (aGONRs-400) are developed as an excellent room-temperature (RT) ferromagnet. The saturated magnetization ( M_s) of aGONRs-400 is high, up to 0.39 emu/g at room temperature, and the RT ferromagnetism (FM) exhibits excellent ambient stability with M_s preserved for over half a year. The preferential distribution of the magnetic phenolic hydroxyl toward the edges, which contributes to the long-range ferromagnetic couplings, was confirmed by X-ray photoemission spectroscopy measurement and gradient annealing analysis. The approach of low-temperature annealing is proved to be efficient both to remove the prominent nonmagnetic epoxy groups on the basal plane of GONRs or transform them to magnetic hydroxyl groups and to preserve the magnetic phenolic hydroxyl at the edges to realize a strong and ambient stable FM.

Microstructural Effect on the Enhancement of Field Electron Emission Properties of Nanocrystalline Diamond Films by Li-Ion Implantation and Annealing Processes

The impact of lithium-ion implantation and postannealing processes on improving the electrical conductivity and field electron emission (FEE) characteristics of nitrogen-doped nanocrystalline diamond (nNCD) films was observed to be distinctly different from those of undoped NCD (uNCD) films. A high-dose Li-ion implantation induced the formation of electron trap centers inside the diamond grains and amorphous carbon (a-C) phases in grain boundaries for both types of NCD films. Postannealing at 1000 °C healed the defects, eliminated the electron trap centers, and converted the a-C into nanographitic phases. The abundant nanographitic phases in the grain boundaries of the nNCD films as compared to the uNCD films made an interconnected path for effectual electron transport and consequently enhanced the FEE characteristics of nNCD films.

Multifunctional Carbon-Based Nanomaterials: Applications in Biomolecular Imaging and Therapy

Molecular imaging has been widely used not only as an important detection technology in the field of medical imaging for cancer diagnosis but also as a theranostic approach for cancer in recent years. Multifunctional carbon-based nanomaterials (MCBNs), characterized by unparalleled optical, electronic, and thermal properties, have attracted increasing interest and demonstrably hold the greatest promise in biomolecular imaging and therapy. As such, it should come as no surprise that MCBNs have already revealed a great deal of potential applications in biomedical areas, such as bioimaging, drug delivery, and tumor therapy. Carbon nanomaterials can be categorized as graphene, single-walled carbon nanotubes, mesoporous carbon, nanodiamonds, fullerenes, or carbon dots on the basis of their morphologies. In this article, reports of the use of MCBNs in various chemical conjugation/functionalization strategies, focusing on their applications in cancer molecular imaging and imaging-guided therapy, will be comprehensively summarized. MCBNs show the possibility to serve as optimal candidates for precise cancer biotheranostics.

Effect of defects controlled by preparation condition and heat treatment on the ferromagnetic properties of few-layer graphene

Magnetism in graphene has stimulated extensive studies to search for novel metal-free magnetic device. In this paper, we use a synthesis method far from equilibrium state named self-propagating high temperature synthesis (SHS) to produce few-layer graphene with different defect contents and then use a heat treatment process (vacuum-annealing and air-cooling) to further control the defects in graphene. We find that the type and content of defects in graphene can be controlled by adjusting the mole ratio of reactants (Mg: CaCO₃) for SHS reaction and the temperature of the subsequent heat treatment. The deviation of the ratio of reactants from stoichiometric ratio benefits the production of graphene with higher concentration of defects. It is indicated that the temperature of the heat treatment has remarkable influences on the structure of graphene, Raman-sensitive defects can be recovered partly by heat treatment while IR-sensitive defects are closely related with the oxidation and decomposition of the oxygen-containing groups at elevated temperature. This work indicates that SHS is a promising method to produce graphene with special magnetism, and the heat treatment is an effective way to further adjust the magnetism of graphene. This work sheds light on the study to develop carbon materials with controlled ferromagnetism.

Superconducting Ferromagnetic Nanodiamond

Superconductivity and ferromagnetism are two mutually antagonistic states in condensed matter. Research on the interplay between these two competing orderings sheds light not only on the cause of various quantum phenomena in strongly correlated systems but also on the general mechanism of superconductivity. Here we report on the observation of the electronic entanglement between superconducting and ferromagnetic states in hydrogenated boron-doped nanodiamond films, which have a superconducting transition temperature T_c ∼ 3 K and a Curie temperature T_Curie > 400 K. In spite of the high T_Curie, our nanodiamond films demonstrate a decrease in the temperature dependence of magnetization below 100 K, in correspondence to an increase in the temperature dependence of resistivity. These anomalous magnetic and electrical transport properties reveal the presence of an intriguing precursor phase, in which spin fluctuations intervene as a result of the interplay between the two antagonistic states. Furthermore, the observations of high-temperature ferromagnetism, giant positive magnetoresistance, and anomalous Hall effect bring attention to the potential applications of our superconducting ferromagnetic nanodiamond films in magnetoelectronics, spintronics, and magnetic field sensing.

Room-temperature Magnetism in Carbon Dots and Enhanced Ferromagnetism in Carbon Dots-Polyaniline Nanocomposite

Room temperature magnetic ordering is reported for very small carbon dots (CDs), mat-like polyaniline nanofibers (Mat-PANI) and a composite of CDs@Mat-PANI containing 0.315 wt% CDs. We have found saturation magnetization (M S ) of CDs, Mat-PANI and CDs@Mat-PANI at 5 (20/300) K equals to 0.0079 (0.0048/0.0019), 0.0116 (0.0065/0.0055) and 0.0349 (0.0085/0.0077) emu/g, respectively. The M S enhancement in CDs@Mat-PANI (200% and 40% at 5 K and 300 K, respectively) is attributed to electron transfer from Mat-PANI imine N-atoms to the encapsulated CDs. Changes in M S values reveal that 0.81 (0.08) electron/CD is transferred at 5 (300) K, which is supported by observation of CDs photoluminescence (PL) redshift while in CDs@Mat-PANI. Band-bending and bandgap-renormalization calculations are used to predict a redshift of 117 meV at 300 K as a result of the electron transfer, in excellent agreement with the PL data (110 meV). Raman, X-ray diffraction and X-ray photoelectron spectroscopy data are used to confirm the electron transfer process as well as the strong interaction of CDs with PANI within CDs@Mat-PANI, which increases the crystalline domain size of Mat-PANI from about 4.8 nm to 9.2 nm while reducing the tensile strain from about 6.2% to 1.8%.

Origins of sp(3)C peaks in C1s X-ray Photoelectron Spectra of Carbon Materials

X-ray photoelectron spectroscopy (XPS) is among the most powerful techniques to analyze defective structures of carbon materials such as graphene and activated carbon. However, reported assignments of defects, especially sp(3)C and sp(2)C, are questionable. Most reports assign sp(3)C peaks to be higher than sp(2)C peaks, whereas a few reports assign sp(3)C peaks to be lower than sp(2)C peaks. Our group previously reported that calculated binding energies of sp(3)C were basically lower than those of sp(2)C. This work clarified that one of the reasons for the prevailing ambiguous assignments of sp(3)C peaks is charging effects of diamond.

Science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems

We present the science and technology roadmap for graphene, related two-dimensional crystals, and hybrid systems, targeting an evolution in technology, that might lead to impacts and benefits reaching into most areas of society. This roadmap was developed within the framework of the European Graphene Flagship and outlines the main targets and research areas as best understood at the start of this ambitious project. We provide an overview of the key aspects of graphene and related materials (GRMs), ranging from fundamental research challenges to a variety of applications in a large number of sectors, highlighting the steps necessary to take GRMs from a state of raw potential to a point where they might revolutionize multiple industries. We also define an extensive list of acronyms in an effort to standardize the nomenclature in this emerging field.

Nanosized graphene crystallite induced strong magnetism in pure carbon films

We report strong magnetism in pure carbon films grown by electron irradiation assisted physical vapor deposition in electron cyclotron resonance plasma. The development of graphene nanocrystallites in the amorphous film matrix, and the dependence of the magnetic behavior on amorphous, nanocrystallite and graphite-like structures were investigated. Results were that the amorphous structure shows weak paramagnetism, graphene nanocrystallites lead to strong magnetization, and graphite-like structures corresponded with a lower magnetization. At a room temperature of 300 K, the highest saturation magnetization of 0.37 emu g(-1) was found in the nanosized graphene nanocrystallite structure. The origin of strong magnetism in nanocrystallites was ascribed to the spin magnetic moment at the graphene layer edges.

Field-induced transition from room-temperature ferromagnetism to diamagnetism in proton-irradiated fullerene

Room-temperature ferromagnetism in proton-irradiated C60 fullerene is demonstrated. The ferromagnetism turns into diamagnetism intrinsic to the fullerene as the magnetic field increases above a critical field.

Microstructure change of ZnO nanowire induced by energetic x-ray radiation and its effect on the field emission properties

The changes in the microstructure of ZnO nanowire were studied after exposure to different radiation doses of energetic x-rays. Detailed structural, composition and optical analyses were carried out. It was found that the surface composition changed and defects were formed in the irradiated ZnO nanowires. The structural change of ZnO nanowires after thermal treatment was also studied and similar defects were observed. It is proposed that phonon-induced localized heating is the main reason for the observed changes in microstructure. Finally, the field emission properties of ZnO nanowires before and after x-ray radiation were studied. It was found that the increase of work function and change in morphology induced by the irradiation were the reasons for the observed change in field emission properties.

Structure and magnetism of disordered carbon

Motivated by the observation of ferromagnetism in carbon foams, we perform a massive search for (meta)stable disordered structures of elemental carbon by a generate and test approach. We use the density functional based program SIESTA to optimize the structures and calculate the electronic spectra and spin densities. About 1% of the 24,000 optimized structures present magnetic moments, a necessary but not sufficient condition for intrinsic magnetic order. We analyse the results using elements of graph theory. Although the relation between the structure and the occurrence of magnetic moments is not yet fully clarified, we give some minimal requirements for this possibility, such as the existence of three-fold coordinated atoms surrounded by four-fold coordinated atoms. We discuss in detail the most promising structures.

Cancer cell labeling and tracking using fluorescent and magnetic nanodiamond

Nanodiamond, a promising carbon nanomaterial, develops for biomedical applications such as cancer cell labeling and detection. Here, we establish the nanodiamond-bearing cancer cell lines using the fluorescent and magnetic nanodiamond (FMND). Treatment with FMND particles did not significantly induce cytotoxicity and growth inhibition in HFL-1 normal lung fibroblasts and A549 lung cancer cells. The fluorescence intensities and particle complexities were increased in a time- and concentration-dependent manner by treatment with FMND particles in lung cancer cells; however, the existence of FMND particles inside the cells did not alter cellular size distribution. The FMND-bearing lung cancer cells could be separated by the fluorescent and magnetic properties of FMNDs using the flow cytometer and magnetic device, respectively. The FMND-bearing cancer cells were identified by the existence of FMNDs using flow cytometer and confocal microscope analysis. More importantly, the cell morphology, viability, growth ability and total protein expression profiles in the FMND-bearing cells were similar to those of the parental cells. The separated FMND-bearing cells with various generations were cryopreservation for further applications. After re-thawing the FMND-bearing cancer cell lines, the cells still retained the cell survival and growth ability. Additionally, a variety of human cancer types including colon (RKO), breast (MCF-7), cervical (HeLa), and bladder (BFTC905) cancer cells could be used the same strategy to prepare the FMND-bearing cancer cells. These results show that the FMND-bearing cancer cell lines, which reserve the parental cell functions, can be applied for specific cancer cell labeling and tracking.

DEFECT EVOLUTION AND STRUCTURAL IMPROVEMENT IN LOW ENERGY ION IRRADIATED CARBON NANOTUBES: MICROSCOPIC AND SPECTROSCOPIC STUDIES

The capability of graphitic networks to reorganize their structures under irradiation of energetic particles provides the tool for nano-engineering of carbon nanotubes (CNTs). We have studied the effect of 30 keV N + ion irradiation with three different fluences 1012, 1013, and 1014 ions/cm2 on the structural and spectroscopic properties of single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs). Irradiation-induced structural defects and coalescence of the nanotubes are studied using high-resolution transmission electron microscopy (HRTEM). Upon irradiation, some of the radial breathing modes in Raman spectra disappear due to conversion from single-walled to multiwalled structure. We observed a systematic change in intensity of the intermediate frequency mode (IFM) with increasing dose of ion-irradiation and these IFM modes are attributed to structural defects of SWNTs. Dramatic improvement in the intensity ratio G-band to D-band (at ~1335 cm-1) for ion fluence of 1013 ions/cm2 indicates improved graphitic structure as a result of reconstruction. Similarly, X-ray Photoelectron spectroscopy studies show improvement in the amount of sp 2 carbon upon 1013 ions/cm2 N + ion irradiation dose. At a higher dose (1014 N + ions/cm 2), vacancy and bent structures having Stone–Wales defects were observed in HRTEM, whereas MWNTs show formation of surface hillock like protrusions leading to formation of fullerene-like structures.

Electronic properties of nanodiamond decorated graphene

The electronic properties of graphene sheets decorated with nanodiamond (ND) particles have been investigated. The chemical fusion of ND to the graphene lattice creates pockets of local defects with robust interfacial bonding. At the ND-bonded regions, the atoms of graphene lattice follow sp(3)-like bonding, and such regions play the role of conduction bottlenecks for the percolating sp(2) graphene network. The low-temperature charge transport reveals an insulating behavior for the disordered system associated with Anderson localization for the charge carriers in graphene. A large negative magnetoresistance is observed in this insulating regime, and its origin is discussed in the context of magnetic correlations of the localized charge carriers with local magnetic domains and extrinsic metal impurities associated with the ND.

Room-temperature ferromagnetism in graphitic petal arrays

We report room-temperature ferromagnetism of graphitic petal arrays grown on Si substrates by microwave plasma chemical vapor deposition without catalyst. The samples have been characterized by Raman and X-ray photoelectron spectroscopy to confirm the absence of possible ferromagnetic impurities. The petals exhibit ferromagnetic hysteresis with saturation magnetization of ∼4.67 emu cm(-3) and coercivity of ∼105 Oe at 300 K, comparable to the reported behavior of few-layer graphene. Upon O2 annealing the saturation magnetization and coercivity decreased to 2.1 emu cm(-3) and ∼75 Oe respectively. The origin of ferromagnetism is believed to arise from the edge defects and vacancies in the petals.

Structural defects in graphene

Graphene is one of the most promising materials in nanotechnology. The electronic and mechanical properties of graphene samples with high perfection of the atomic lattice are outstanding, but structural defects, which may appear during growth or processing, deteriorate the performance of graphene-based devices. However, deviations from perfection can be useful in some applications, as they make it possible to tailor the local properties of graphene and to achieve new functionalities. In this article, the present knowledge about point and line defects in graphene are reviewed. Particular emphasis is put on the unique ability of graphene to reconstruct its lattice around intrinsic defects, leading to interesting effects and potential applications. Extrinsic defects such as foreign atoms which are of equally high importance for designing graphene-based devices with dedicated properties are also discussed.

Limits on intrinsic magnetism in graphene

We have studied magnetization of graphene nanocrystals obtained by sonic exfoliation of graphite. No ferromagnetism is detected at any temperature down to 2 K. Neither do we find strong paramagnetism expected due to the massive amount of edge defects. Rather, graphene is strongly diamagnetic, similar to graphite. Our nanocrystals exhibit only a weak paramagnetic contribution noticeable below 50 K. The measurements yield a single species of defects responsible for the paramagnetism, with approximately one magnetic moment per typical graphene crystallite.

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 strange magnetism of oxides and carbons

Magnetism is not usually expected in simple sp oxides like MgO or in carbons like graphite. Yet basic intrinsic defects in these systems can be magnetic in ways that seem to be shared by more complex oxides. A second puzzle comes from reports of possible room temperature ferromagnetism in simple oxides, where experiments are not always in agreement. This paper discusses what determines whether point defects like cation vacancies in oxides have magnetic or non-magnetic ground states. It also discusses the possible connections between point defect ground states and oxide ferromagnetism. The connectivity issue raises questions about possible diffuse states in nanocrystalline oxides, several possibilities being outlined. These ideas raise the further possibility that the magnetism might be written in these oxides at the nanoscale, perhaps using atomic force microscopy.

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.

Molecular dynamics study of damage production in single-walled carbon nanotubes irradiated by various ion species

The irradiation-induced damage production in single-walled carbon nanotubes (CNTs) by several types of ions is investigated using the molecular dynamics method with analytical potentials. We found that, in the incident energy range 25-1000 eV, the bonding action or the chemical effect of the ions could significantly enhance their damage capabilities to CNTs relative to that of non-bonding ions, and the dependence of damage yield on the ion mass is no longer monotonic. This is contrary to the previous viewpoint that the chemical aspect of the interaction is of no importance to the ion-induced defect production mechanism in CNTs. The bonding interaction of ions with CNTs also increases their implantation probabilities into CNTs. The chemical erosion effect of incident ions remarkably intensifies the sideward recoil from CNTs under irradiation while the downward recoil is still governed by the physical collision effect.

Magneto-mechanical coupling behavior of defective single-walled carbon nanotubes

Spin-polarized density functional theory is employed to investigate the mechanical and magnetic properties of a (5, 5) carbon nanotube (CN) with a mono-vacancy. It is found that the magnetic properties of a defective CN can be modified by applied uniaxial tensile strain. The effects of uniaxial strains on the magnetic properties are related to the stretched bonds in the defective CN, and the effect of a mixture of sp(2) and sp(3) hybridization on the magnetism is investigated. Furthermore, because of the strong magneto-mechanical coupling between the defective CN and metal, the mechanical properties of the defective (5, 5) CN are found to be significantly alternated by the substitution of metal atoms to its vacancy sites. Such strong coupling among magnetic moment, spin-dependent transport, and mechanical deformation in a defective CN could be beneficial in some potential applications of CNs in spintronic devices and metal matrix composites.

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.

Electric field effects on spin transport in defective metallic carbon nanotubes

On the basis of first-principles calculations, we investigate transport properties of spin-polarized electrons in defective metallic single-wall carbon nanotubes (SWCNTs) under homogeneous transverse electric fields. Either vacancies or carbon adatoms are introduced in (10,10) SWCNT and are shown to play a role of quasi-localized magnetic impurities. The applied transverse electric fields change the relative position of the energy levels of the defects with respect to the Fermi energy so that the spin-polarized conductances are shown to be tunable. For some impurities, the orientation of the majority spin electrons in conducting channels at the Fermi energy can be switched to the opposite spin by an experimentally attainable electric field. Our results suggest that pure carbon or organic nanomagnets could be realized in SWCNTs, and their spin transport properties are controllable by transverse electric fields.

Engineering of nanostructured carbon materials with electron or ion beams

Irradiating solids with energetic particles is usually thought to introduce disorder, normally an undesirable phenomenon. But recent experiments on electron or ion irradiation of various nanostructures demonstrate that it can have beneficial effects and that electron or ion beams may be used to tailor the structure and properties of nanosystems with high precision. Moreover, in many cases irradiation can lead to self-organization or self-assembly in nanostructures. In this review we survey recent advances in the rapidly evolving area of irradiation effects in nanostructured materials, with particular emphasis on carbon systems because of their technological importance and the unique ability of graphitic networks to reconstruct under irradiation. We dwell not only on the physics behind irradiation of nanostructures but also on the technical applicability of irradiation for nanoengineering of carbon and other systems.

Room-temperature ferromagnetism in carbon-doped ZnO

We report ferromagnetism in carbon-doped ZnO. Our first-principles calculations based on density functional theory predicted a magnetic moment of 2.02 mu(B) per carbon when carbon substitutes oxygen in ZnO, and an ferromagnetic coupling among magnetic moments of the carbon dopants. The theoretical prediction was confirmed experimentally. C-doped ZnO films deposited by pulsed-laser deposition showed ferromagnetism with Curie temperatures higher than 400 K. The measured magnetic moment based on the content of carbide in the films [(1.5-3.0) mu(B) per carbon] was in agreement with the theoretical prediction. The magnetism is due to the Zn-C system in the ZnO environment.

Role of electronic excitations in ion collisions with carbon nanostructures

By combining ab initio time-dependent density functional calculations for electrons with molecular dynamics simulations for ions in real time, we investigate the microscopic mechanism of collisions between energetic protons and graphitic carbon nanostructures. We identify not only the amount of energy lost by the projectile, but also the electronic and ionic degrees of freedom of the target that accommodate this energy as a function of the impact parameter and projectile energy. Our results establish validity limits for the Born-Oppenheimer approximation and the threshold energy for defect formation in carbon nanostructures.