Intrinsic half-metallicity in modified graphene nanoribbons

A novel 2D intrinsic metal-free ferromagnetic semiconductor Si3C8 monolayer

Metal-free magnets, a special kind of ferromagnetic (FM) material, have evolved into an important branch of magnetic materials for spintronic applications. We herein propose a silicon carbide (Si3C8) monolayer and investigate its geometric, electronic, and magnetic properties by using first-principles calculations. The thermal and dynamical stability of the Si3C8 monolayer was confirmed by ab initio molecular dynamics and phonon dispersion simulations. Our results show that the Si3C8 monolayer is a FM semiconductor with a band gap of 1.76 eV in the spin-down channel and a Curie temperature of 22 K. We demonstrate that the intrinsic magnetism of the Si3C8 monolayer is derived from pz orbitals of C atoms via superexchange interactions. Furthermore, the half-metallic state in the FM Si3C8 monolayer can be induced by electron doping. Our work not only illustrates that carrier doping could manipulate the magnetic states of the FM Si3C8 monolayer but also provides an idea to design two-dimensional metal-free magnetic materials for spintronic applications.

Intrinsic half-metallicity in two-dimensional Cr2TeX2 (X = I, Br, Cl) monolayers

Two-dimensional (2D) materials with intrinsic half-metallicity at or above room temperature are important in spin nanodevices. Nevertheless, such 2D materials in experiment are still rarely realized. In this work, a new family of 2D Cr2TeX2 (X = I, Br, Cl) monolayers has been predicted using first-principles calculations. The monolayer is made of five atomic sublayers with ABCAB-type stacking along the perpendicular direction. It is found that the energies for all the ferromagnetic (FM) half-metallic states are the lowest. The phonon spectrum calculations and molecular dynamics simulations both demonstrate that the FM states are stable, indicating the possibility of experimentally obtaining the 2D Cr2TeX2 monolayers with half-metallicity. The Curie temperatures from Monte Carlo simulations are 486, 445, and 451 K for Cr2TeI2, Cr2TeBr2, and Cr2TeCl2 monolayers, respectively, and their half-metallic bandgaps are 1.72, 1.86 and 1.90 eV. The corresponding magnetocrystalline anisotropy energies (MAEs) are about 1185, 502, 899 μeV per Cr atom for Cr2TeX2 monolayers, in which the easy axes are along the plane for the Cr2TeBr2 and Cr2TeCl2 monolayers, but being out of the plane in the Cr2TeI2. Our study implies the potential application of the 2D Cr2TeX2 (X = I, Br, Cl) monolayers in spin nanodevices.

Effect of ZnO dimers on the thermoelectric performance of armchair graphene nanoribbons

Enhancing the thermoelectric performance in engineered graphene nanoribbons is used to produce thermoelectric nanodevices, which are important in many applications. By using a chemical doping method, armchair graphene nanoribbons (AGNRs) can have thermoelectric properties that are tunable. We predicted that changing the number and geometrical pattern of zinc oxide (ZnO) dimers in an AGNR can engineer thermoelectric properties, so we used density functional-based tight binding (DFTB) combined with the non-equilibrium Green's function (NEGF) to investigate the geometric, electronic, and thermoelectric properties of the AGNR with and without various dopants of ZnO dimers. With three forms of ZnO dimers, ortho, meta, and para dimers, different concentration ratios of Zn and O atoms are used. Our results indicate that the electronic features of AGNR are influenced not only by the concentrations of ZnO dimers but also by the geometrical pattern of ZnO dimers in the AGNR. These results are helpful in better understanding the effect of chemical doping on the transport properties of AGNRs and in motivating nanodevices to improve their thermoelectric performance.

Bipolar spin-filtering and giant magnetoresistance effect in spin-semiconducting zigzag graphene nanoribbons

Spintronics is one of the main topics in condensed matter physics, in which half-metallicity and giant magnetoresistance are two important objects to achieve. In this work, we study the spin dependent transport properties of zigzag graphene nanoribbons (ZGNR) with asymmetric edge hydrogenation and different magnetic configurations using the non-equilibrium Green's function method combined with density functional calculations. Our results show that when the magnetic configurations of the electrodes change from parallel to antiparallel, the currents in the tunnel junction change substantially, resulting in a high conductance state and a low conductance state, with the tunnel magnetoresistance (TMR) ratio larger than 1 × 10⁵% achieved. In addition, in the parallel magnetic configurations, an ideal bipolar spin filtering effect is observed, making it flexible to switch the spin polarity of current by reversing the bias direction. All these features originate from the spin semiconducting behavior of the asymmetrically hydrogenated ZGNRs. The findings suggest that asymmetric edge hydrogenation provides an important way to construct multi-functional spintronic devices with ZGNRs.

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.

Tailoring magnetism in silicon-doped zigzag graphene edges

Recently, the edges of single-layer graphene have been experimentally doped with silicon atoms by means of scanning transmission electron microscopy. In this work, density functional theory is applied to model and characterize a wide range of experimentally inspired silicon doped zigzag-type graphene edges. The thermodynamic stability is assessed and the electronic and magnetic properties of the most relevant edge configurations are unveiled. Importantly, we show that silicon doping of graphene edges can induce a reversion of the spin orientation on the adjacent carbon atoms, leading to novel magnetic properties with possible applications in the field of spintronics.

Phenalenyl Radical: Smallest Polycyclic Odd Alternant Hydrocarbon Present in the Graphene Sheet

Phenalenyl, a zigzag-edged odd alternant hydrocarbon unit can be found in the graphene nanosheet. Hückel molecular orbital calculations indicate the presence of a nonbonding molecular orbital (NBMO), which originates from the linear combination of atomic orbitals (LCAO) arising from 13 carbon atoms of the phenalenyl molecule. Three redox states (cationic, neutral radical, and anionic) of the phenalenyl-based molecules were attributed to the presence of this NBMO. The cationic state can undergo two consecutive reductions to result in neutral radical and anionic states, stepwise, respectively. The phenalenyl-based radicals were found as crucial building blocks and attracted the attention of various research fields such as organic synthesis, material science, computation, and device physics. From 2012 onward, a strategy was devised using the cationic state of phenalenyl-based molecules and in situ generated phenalenyl radicals, which created a new domain of catalysis. The in situ generated phenalenyl radicals were utilized for the single electron transfer (SET) process resulting in redox catalysis. This emerging range of applications rejuvenates the more than six decades-old phenalenyl chemistry. This review captures such developments ranging from fundamental understanding to multidirectional applications of phenalenyl-based radicals.

An intrinsic room-temperature half-metallic ferromagnet in a metal-free PN2 monolayer

In spintronics, the embodiment of abundance availability, long spin relaxation time, complete spin-polarization and high Curie temperature (T_C) in intrinsic metal-free half-metallic ferromagnets (MFHMFs) are highly desirable and challenging. In this work, employing density functional theory, we first propose a dynamically, thermally, and mechanically stable two-dimensional (2D) intrinsic MFHMF, i.e. a MoS₂-like PN₂ monolayer, which possesses not only completely spin-polarized half-metallicity, but also an above-room-temperature T_C (385 K). The half-metallic gap is calculated to be 1.70 eV, which can effectively prevent the spin-flip transition caused by thermal agitation. The mechanism of magnetism in the PN₂ monolayer is mainly derived from the p electron direct exchange interaction that separates from usual d-state magnetic materials. Moreover, the robustness of the ferromagnetism and half-metallicity is observed against an external strain and carrier (electron or hole) doping. Surprisingly, electron doping can effectively increase the Curie temperature of the PN₂ monolayer. The proposed research work provides an insight that PN₂ can be a promising candidate for realistic room-temperature metal-free spintronic applications.

Computational design of double transition metal MXenes with intrinsic magnetic properties

Two-dimensional transition metal carbides (MXenes) have great potential to achieve intrinsic magnetism due to their available chemical and structural diversity. In this work, by spin-polarized density functional theory calculations, we designed and comprehensively investigated 50 double transition metal (DTM) MXenes MCr₂CT_x (T = H, O, F, OH, or bare) based on the chemical formula of M₂C (M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Mo, W). We highlight that ferromagnetic half-metallicity, antiferromagnetic semiconduction, as well as antiferromagnetic half-metallicity have been achieved in the DTM MXenes. Herein, ferromagnetic half-metallic ScCr₂C₂, ScCr₂C₂H₂, ScCr₂C₂F₂, and YCr₂C₂H₂ are characterized with wide band gaps and high Curie temperatures. Very interestingly, the ScCr₂C₂-based magnetic tunnel junction presents a tunnel magnetoresistance ratio as high as 176 000%. In addition, the antiferromagnetic semiconducting TiCr₂C₂, ZrCr₂C₂, and ZrCr₂C₂(OH)₂, possessing moderate band gaps and high Néel temperatures, have been predicted. Especially, the Néel temperature of ZrCr₂C₂(OH)₂ can reach 425 K. Moreover, the Dirac cone-like band structure feature is highlighted in antiferromagnetic half-metallic ZrCr₂C₂H₂. Our study provides a new potential strategy for designing MXenes in spintronics.

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.

Energy Gaps in BN/GNRs Planar Heterostructure

Using the tight-binding approach, we study the band gaps of boron nitride (BN)/ graphene nanoribbon (GNR) planar heterostructures, with GNRs embedded in a BN sheet. The width of BN has little effect on the band gap of a heterostructure. The band gap oscillates and decreases from 2.44 eV to 0.26 eV, as the width of armchair GNRs, nA, increases from 1 to 20, while the band gap gradually decreases from 3.13 eV to 0.09 eV, as the width of zigzag GNRs, nZ, increases from 1 to 80. For the planar heterojunctions with either armchair-shaped or zigzag-shaped edges, the band gaps can be manipulated by local potentials, leading to a phase transition from semiconductor to metal. In addition, the influence of lattice mismatch on the band gap is also investigated.

Spin current distribution in antiferromagnetic zigzag graphene nanoribbons under transverse electric fields

The spin current transmission properties of narrow zigzag graphene nanoribbons (zGNRs) have been the focus of much computational research, investigating the potential application of zGNRs in spintronic devices. Doping, fuctionalization, edge modification, and external electric fields have been studied as methods for spin current control, and the performance of zGNRs initialized in both ferromagnetic and antiferromagnetic spin states has been modeled. Recent work has shown that precise fabrication of narrow zGNRs is possible, and has addressed long debated questions on their magnetic order and stability. This work has revived interest in the application of antiferromagnetic zGNR configurations in spintronics. A general ab initio analysis of narrow antiferromagnetic zGNR performance under a combination of bias voltage and transverse electric field loading shows that their current transmission characteristics differ sharply from those of their ferromagnetic counterparts. At relatively modest field strengths, both majority and minority spin currents react strongly to the applied field. Analysis of band gaps and current transmission pathways explains the presence of negative differential resistance effects and the development of spatially periodic electron transport structures in these nanoribbons.

Realizing stable half-metallicity in zigzag silicene nanoribbons with edge dihydrogenation and chemical doping

Although many schemes have been proposed to obtain full half-metallicity in zigzag silicene nanoribbons with edge monohydrogenation (H-H ZSiNRs) by chemical modification, the resulted negligible energy difference between the antiferromagnetic (AFM) and ferromagnetic (FM) configurations makes the half-metallicity hardly observable practically. In this work, based on density functional calculations, we find that the ZSiNRs with edge dihydrogenation (H2-H2 ZSiNRs) can be tuned to be half-metallic by replacing the central two zigzag Si chains with two zigzag Al-P chains, and more importantly, the FM-AFM energy difference is significantly increased compared with the H-H cases. The obtained half-metallicity originates from the different potential between two edges of the ribbon after doping, which results in the edge states of two spin channels shifting oppositely in energy. This mechanism is so robust that the half-metallicity can always be achieved, irrespective of the ribbon width. Our finding provides a fantastic way for achieving stable half-metallicity in ZSiNRs.

Antiferromagnetic spin ordering in two-dimensional honeycomb lattice of SiP3

Magnetism in low-dimensional materials has been of sustained interest due to its intriguing quantum mechanical origin and promising device applications. Here, we propose a buckled honeycomb lattice of stoichiometry SiP₃, a two-dimensional binary group-IV and V material that exhibits an antiferromagnetic ground state with itinerant electrons. Here we perform elemental Si substitution in pristine blue phosphorene to downshift the Fermi energy and induce the Fermi instability that results in a spin polarized ground state. Within first-principles calculations, we observe antiferromagnetic spin alignment between adjacent ferromagnetic triangular domains where each Si atom is coupled with three neighboring P atoms with a ferromagnetic interaction. Such unique spin structure and resulting magnetic ground state are unprecedented in defect-free two-dimensional materials made of only p-block elements. This metal-free magnetism can be exploited for advanced spintronic and memory storage applications.

On-surface synthesis of graphene nanostructures with π-magnetism

Graphene nanostructures (GNs) including graphene nanoribbons and nanoflakes have attracted tremendous interest in the field of chemistry and materials science due to their fascinating electronic, optical and magnetic properties. Among them, zigzag-edged GNs (ZGNs) with precisely-tunable π-magnetism hold great potential for applications in spintronics and quantum devices. To improve the stability and processability of ZGNs, substitutional groups are often introduced to protect the reactive edges in organic synthesis, which renders the study of their intrinsic properties difficult. In contrast to the conventional wet-chemistry method, on-surface bottom-up synthesis presents a promising approach for the fabrication of both unsubstituted ZGNs and functionalized ZGNs with atomic precision via surface-catalyzed transformation of rationally-designed precursors. The structural and spin-polarized electronic properties of these ZGNs can then be characterized with sub-molecular resolution by means of scanning probe microscopy techniques. This review aims to highlight recent advances in the on-surface synthesis and characterization of a diversity of ZGNs with π-magnetism. We also discuss the important role of precursor design and reaction stimuli in the on-surface synthesis of ZGNs and their π-magnetism origin. Finally, we will highlight the existing challenges and future perspective surrounding the synthesis of novel open-shell ZGNs towards next-generation quantum technology.

Half metallicity and ferromagnetism of vanadium nitride nanoribbons: a first-principles study

Half metallic materials with intrinsic ferromagnetism are identified as the pillar of next generation spintronic devices. In search of new low-dimensional materials with these excellent properties, herein we systematically study the electronic and magnetic properties of edge dependent (armchair (ac) and zigzag (zz)) vanadium nitride nanoribbons (VNNRs) using density functional theory (DFT) based calculations. Both the ac and zz VNNRs show robust ferromagnetism and extensive half-metallicity with large band gaps (3.9-4.3 eV for ac and 2.5-3.0 eV for zz VNNRs) for the down spin channel. Interestingly, even with the application of uniaxial strain (both tensile and compressive) along the axis of the ribbons, VNNRs retain their extensive half metallicity with a large spin band gap and robust ferromagnetic behavior. Spin dependent electronic transport reveals the 100% spin filtering efficiency of nanoribbons, in both the free state and under applied strain, which support the robust half metallicity of VNNRs. Our study of VNNRs on a MoS2 substrate also shows half metallicity along with high stability, indicating the usage of MoS2 as a substrate for the synthesis of VNNRs. All these results guide the potential application of VNNRs in spintronic devices.

Strain Investigation on Spin-Dependent Transport Properties of γ-Graphyne Nanoribbon Between Gold Electrodes

Strain engineering has become one of the effective methods to tune the electronic structures of materials, which can be introduced into the molecular junction to induce some unique physical effects. The various γ-graphyne nanoribbons (γ-GYNRs) embedded between gold (Au) electrodes with strain controlling have been designed, involving the calculation of the spin-dependent transport properties by employing the density functional theory. Our calculated results exhibit that the presence of strain has a great effect on transport properties of molecular junctions, which can obviously enhance the coupling between the γ-GYNR and Au electrodes. We find that the current flowing through the strained nanojunction is larger than that of the unstrained one. What is more, the length and strained shape of the γ-GYNR serves as the important factors which affect the transport properties of molecular junctions. Simultaneously, the phenomenon of spin-splitting occurs after introducing strain into nanojunction, implying that strain engineering may be a new means to regulate the electron spin. Our work can provide theoretical basis for designing of high performance graphyne-based devices in the future.

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.

Switchable metal-to-half-metal transition at the semi-hydrogenated graphene/ferroelectric interface

Tuning the half-metallicity of low-dimensional materials using an electric field is particularly appealing for spintronic applications but typically requires an ultra-high field, hampering practical applications. Interface engineering has been suggested as an alternative practical means to overcome this limitation and control the metal-to-half-metal transition. Here, we show from first-principles calculations that the polarization switching at the interface of semi-hydrogenated graphene (i.e., graphone) and a ferroelectric PbTiO3 layer can reversibly tune a metal to half-metal transition in graphone. Using a simple Hubbard model, this is rationalized using interface atomic orbital hybridization, which also reveals the origin of the high-quality screening of metallic graphone, preserving bulk-like stable ferroelectric polarization in the PbTiO3 film down to a thickness of two unit cells. These findings do not only open a new perspective on engineering half-metallicity at the interface of two-dimensional materials and ferroelectrics, but also identify graphone as a powerful atomically thin electrode, which holds great promise for the design of ultrafast and high integration density information-storage devices.

Modulating Electronic Structures of Armchair GaN Nanoribbons by Chemical Functionalization under an Electric Field Effect

The electronic and magnetic properties of oxygen- and sulfur-passivated one-dimensional armchair GaN nanoribbons (A-GaNNRs) are revealed using both first-principles density-functional theory and ab initio molecular dynamics simulations. We explore that an applied external electric field can further modulate the electronic properties of both pristine and passivated A-GaNNRs, thus changing their properties (semiconducting-metallic-half-metallic). A-GaNNRs of 0.9-3.1 nm width are subjected to further investigations, which reveal that sulfur termination transforms pristine A-GaNNRs from direct into indirect band gap semiconductors, without affecting their nonmagnetic nature. On the other hand, oxygen passivation introduces spin-polarized behavior with a finite magnetic moment. Magnetism characteristics in both bare and sulfur-passivated A-GaNNRs are induced by applying a critical electric field along the direction of NR width. The passivated A-GaNNRs are more stable compared to bare ones, while sulfur-passivated A-GaNNRs exhibit higher stability at higher temperatures (>500 °C). Thus, our results suggest that A-GaNNRs can be used in a broad range of electronic, optoelectronic, and spintronic applications.

Generating pure spin current with spin-dependent Seebeck effect in ferromagnetic zigzag graphene nanoribbons

Pure spin current is of great importance in spintronics and may be achieved by spin dependent Seebeck effect (SDSE) in magnetic systems. Zigzag-edged graphene nanoribbons (ZGNRs) are very well-known 2D magnetic nanostructures. However, perfect and pristine ZGNRs either in the anti-ferromagnetic ground state or in the ferromagnetic (FM) state are not capable of producing pure spin current by SDSE at low temperature. In this work, by density functional theory calculations, we propose a scheme for generating pure spin current using SDSE in FM-ZGNRs by introducing antidots. Specifically, by creating a hexagonal antidot with either armchair edges or zigzag edges in the scattering region, we can get finite Seebeck thermopower for both spin channels with opposite signs, leading to the opposite flow directions of the two spin channels. The mechanism is well explained by the cooperation of the varying localization features of states around the Fermi level and the antidot induced scattering potential. By slightly tuning the chemical potential, pure spin current can be achieved. The size and edge shape effects have also been systematically studied. The findings indicate a novel scheme for thermally generating pure spin current in zigzag graphene nanoribbons and may find important application in graphene based spintronics.

Spin polarization in graphene nanoribbons functionalized with nitroxide

Fine-tuning of magnetic states via an understanding of spin injection on the edge of graphene nanoribbons should allow for greater flexibility of the design of graphene-based spintronics. On the basis of calculations, we predict that coupling constants of the exchange interaction in the series of nitroxide-functionalized ribbon compounds are antiferromagnetic across the ribbons with values 0.2-0.4 cm^-1 and ferromagnetic along the ribbon with absolute values from 0.05 to 0.07 cm^-1. Such interacting nitroxide groups induce spin polarization of the edge states of stable graphene nanoribbons. Graphical abstract Exchange coupling constants inducing spin polarization in graphene nanoribbons functionalized with nitroxides.

Modifying spin current filtering and magnetoresistance in a molecular spintronic device

The zigzag edged graphene nanoribbon (ZGNR) is excellent for spintronics devices, and many efforts have been made to investigate its properties such as spin filtering, rectification and magnetoresistance. Here we propose a molecular spintronic transport device based on two ZGNR electrodes connected with a dibenzo[a,c]dibenzo[5,6:7,8]quinoxalino[2,3-i]phenazine (DDQP) molecule. By performing first-principles electron transport computations, we found an enhanced spin polarized current–voltage curve, giant spin filter efficiency, magnetoresistance and rectification ratio properties of the device compared to its all-carbon molecular analogue. Our systematic investigation suggests the vital role played in spin polarized electron transport by nitrogen atoms in DDQP, the ZGNR probe's width and terminal geometry, especially the increased spin filter efficiency with higher ZGNR width.

Three general factors of the molecule device were investigated to enhance its spin filtering efficiency.

Phagraphene nanoribbons: half-metallicity and magnetic phase transition by functional groups and electric field

Magnetic nanomaterials with the desirable nature are the basis for developing future spintronic devices, and research for them is of fundamental interest. Here, we explore the realization of half-metallicity and magnetic phase transition for phagraphene nanoribbons in virtue of functional groups (OH and CN) with different coverage fractions and external electric fields. The first-principles calculations show that a single-edge CN functionalization only makes a intrinsic spin-degenerate semiconducting ribbon converted to a quasi-metal or metal, while a single-edge OH modification leads to an occurrence of the half-semiconducting nature regardless of the coverage fraction of groups. Interestingly, the half-metal behavior for the CN and OH double-edge modified ribbons can be achieved either in the zero-electric-field intrinsic state for most of functionalized systems or at a very low electric field, 0.1 V Å^-1. More importantly, the observed critical electric field for the transition from ferromagnetic to nonmagnetic phase is lowered significantly almost for all systems, this benefits to design a low electric-field-controlling magnetic switch which can reversibly work between both magnetic and nonmagnetic states. The calculated Gibbs free energy confirms that the group-modified ribbons generally hold a more favorable energy stability in most of the cases, facilitating likely experimental realization.

Emerging chemical strategies for imprinting magnetism in graphene and related 2D materials for spintronic and biomedical applications

Graphene, a single two-dimensional sheet of carbon atoms with an arrangement mimicking the honeycomb hexagonal architecture, has captured immense interest of the scientific community since its isolation in 2004. Besides its extraordinarily high electrical conductivity and surface area, graphene shows a long spin lifetime and limited hyperfine interactions, which favors its potential exploitation in spintronic and biomedical applications, provided it can be made magnetic. However, pristine graphene is diamagnetic in nature due to solely sp2 hybridization. Thus, various attempts have been proposed to imprint magnetic features into graphene. The present review focuses on a systematic classification and physicochemical description of approaches leading to equip graphene with magnetic properties. These include introduction of point and line defects into graphene lattices, spatial confinement and edge engineering, doping of graphene lattice with foreign atoms, and sp3 functionalization. Each magnetism-imprinting strategy is discussed in detail including identification of roles of various internal and external parameters in the induced magnetic regimes, with assessment of their robustness. Moreover, emergence of magnetism in graphene analogues and related 2D materials such as transition metal dichalcogenides, metal halides, metal dinitrides, MXenes, hexagonal boron nitride, and other organic compounds is also reviewed. Since the magnetic features of graphene can be readily masked by the presence of magnetic residues from synthesis itself or sample handling, the issue of magnetic impurities and correct data interpretations is also addressed. Finally, current problems and challenges in magnetism of graphene and related 2D materials and future potential applications are also highlighted.

Tunable electronic properties of partially edge-hydrogenated armchair boron-nitrogen-carbon nanoribbons

We employ a first-principles calculations based density-functional-theory (DFT) approach to study the electronic properties of partially and fully edge-hydrogenated armchair boron-nitrogen-carbon (BNC) nanoribbons (ABNCNRs), with widths between 0.85 nm to 2.3 nm. Due to the partial passivation of edges, the electrons, which do not participate in the bonding, form new energy states located near the Fermi-level. Because of these additional bands, some ABNCNRs exhibit metallic behavior, which is quite uncommon in armchair nanoribbons. Our calculations reveal that metallic behavior is observed for the following passivation patterns: (i) when the B atom from one edge and the N atom from another edge are unpassivated. (ii) when the N atoms from both the edges are unpassivated. (iii) when the C atom from one edge and the N atom from another edge are unpassivated. Furthermore, spin-polarization is also observed for certain passivation schemes, which is also quite uncommon for armchair nanoribbons. Thus, our results suggest that the ABNCNRs exhibit a wide range of electronic and magnetic properties in that the fully edge-hydrogenated ABNCNRs are direct band gap semiconductors, while the partially edge-hydrogenated ones are either semiconducting, or metallic, while simultaneously exhibiting spin polarization, based on the nature of passivation. We also find that the ribbons with larger widths are more stable as compared to the narrower ones.

Adsorbing the magnetic superhalogen MnCl3 to realize intriguing half-metallic and spin-gapless-semiconducting behavior in zigzag or armchair SiC nanoribbon

By means of first-principles computations, we first propose a new and effective strategy through adsorbing the magnetic superhalogen MnCl3 to modulate the electronic and magnetic properties of zigzag- and armchair-edged SiC nanoribbons (zSiCNR and aSiCNR, respectively). In view of its large intrinsic magnetic moment and strong electron-withdrawing ability, the adsorption of magnetic superhalogen MnCl3 can introduce magnetism in the substrate SiCNR, and simultaneously induce the electron transfer process from SiCNR to MnCl3, resulting in the evident increase of electrostatic potential in the ribbon plane, like applying an electric field. As a result, the magnetic degeneracy of pristine zSiCNR can be broken and a robust ferromagnetic half-metallicity or metallicity can be observed in the modified zSiCNR systems, while a robust ferromagnetic half-metallic or spin-gapless-semiconducting behavior can be obtained in the modified aSiCNR systems. Note that both the appealing half-metallicity and spin-gapless-semiconductor behavior are key features which hold promise for future spintronic applications. Moreover, all of these new superhalogen-SiC nanosystems can possess considerably high structural stabilities. These intriguing findings will be advantageous for promoting excellent SiC-based nanomaterials in the applications of spintronics and multifunctional nanodevices in the near future.

The unique Raman fingerprint of boron nitride substitution patterns in graphene

Boron nitride-substituted graphene (BNsG) two-dimensional structures are new materials of wide technological interest due to the rich variety of electronic structures and properties they can exploit. The ability to accurately characterize them is key to their future success. Here we show, by means of ab initio simulations, that the vibrational Raman spectra of such compounds are extremely sensitive to substitution motifs and concentration, and that each structure has unique and distinct features. This result can be useful as a guide for the optimization of production processes.

Strained zigzag graphene nanoribbon devices with vacancies as perfect spin filters

The transport properties of zigzag graphene nanoribbons (zGNRs) were studied by density functional theory (DFT) in conjunction with Green's function analysis. In particular, spin transport through a zGNR (12,0) device was investigated under the constraint of ferromagnetic coordination of the ribbon edges. Several configurations with two vacant sites in the edge and the bulk region of the zGNR device were derived from this system. For all structures, magnetocurrent ratios (MCRs) were recorded as a function of the bias as well as the amount of strain applied longitudinally to the devices. ZGNR devices with vacancies in the edge regime turn out to exhibit perfect spin-filter activity for well-defined choices of the strain and the bias, carrying completely polarized minority spin currents. In the alternative structure, characterized by vacancies in the bulk regime, spin currents with majority orientation prevail. With respect to both the sign and the size, the MCR is seen to depend sensitively on the device parameters, i.e., the vacancy locations, the bias, and the amount of strain. These results are interpreted in terms of density-of-states distributions, transmission spectra, and transmission operator eigenstates.

Ferromagnetism and Half-Metallicity in a High-Band-Gap Hexagonal Boron Nitride System

Metal-free half-metallicity is the subject of intense research in the field of spintronics devices. Using density functional theoretical calculations, atom-thin hexagonal boron nitride (h-BN)-based systems are studied for possible spintronics applications. Ferromagnetism is observed in patterned C-doped h-BN systems. Interestingly, such a patterned C-doped h-BN exhibits half-metallicity with a Curie temperature of approximately 324 K at a particular C-doping concentration. It shows half-metallicity more than metal-free systems studied to date. Thus, such a BN-based system can be used to achieve a 100 % spin-polarised current at the Fermi level. Furthermore, this C-doped system shows excellent dynamical, thermal, and mechanical properties. Therefore, a stable metal-free planar ferromagnetic half-metallic h-BN-based system is proposed for use in room-temperature spintronics devices.

Sequential BN-doping induced tuning of electronic properties in zigzag-edged graphene nanoribbons: a computational approach

We employed first-principles methods to elaborate doping induced electronic and magnetic perturbations in one-dimensional zigzag graphene nanoribbon (ZGNR) superlattices. Consequently, the incorporation of alternate boron and nitrogen (hole–electron) centers into the hexagonal network instituted substantial modulations to electronic and magnetic properties of ZGNR. Our theoretical analysis manifested some controlled changes to electronic and magnetic properties of the ZGNR by tuning the positions (array) of impurity centers in the carbon network. Subsequent DFT based calculations also suggested that the site-specific alternate electron–hole (B/N) doping could regulate the band-gaps of the superlattices within a broad range of energy. The consequence of variation in the width of ZGNR in the electronic environment of the system was also tested. The systematic analysis of various parameters such as the structural orientations, spin-arrangements, the density of states (DOS), band structures, and local density of states envisioned a basis for the band-gap engineering in ZGNR and attributed to its feasible applications in next generation electronic device fabrication.

Incorporation of an alternate impurity array in the ZGNR don't break the spin-degeneracy, providing the freedom to tune the electronic behaviors without affecting the spin-dependent properties.

Photogalvanic effect induced fully spin polarized current and pure spin current in zigzag SiC nanoribbons

Using nonequilibrium Green's function combined with density functional theory, we investigate the spin-related current generated by the photogalvanic effect (PGE) in monolayer zigzag SiC nanoribbons (ZSiCNRs) by first-principles calculations.

Edge morphology induced rectifier diode effect in C3N nanoribbon

We find that edge morphology induces interesting electronic transport properties in step-like heterojunction devices composed of width-variable zigzag C₃N nanoribbons.

h-BN/graphene van der Waals vertical heterostructure: a fully spin-polarized photocurrent generator

By constructing transport junctions using graphene-based van der Waals (vdW) heterostructures in which a zigzag-edged graphene nanoribbon (ZGNR) is sandwiched between two hexagonal boron-nitride sheets, we computationally demonstrate a new scheme for generating perfect spin-polarized quantum transport in ZGNRs by light irradiation. The mechanism lies in the lift of spin degeneracy of ZGNR induced by the stagger potential it receives from the BN sheets and the subsequent possibility of single spin excitation of electrons from the valence band to the conduction band by properly tuning the photon energy. This scheme is rather robust in that we always achieve desirable results irrespective of whether we decrease or increase the interlayer distance by applying compressive or tensile strain vertically to the sheets or shift the BN sheets in-plane relative to the graphene nanoribbons. More importantly, this scheme overcomes the long-standing difficulties in traditional ways of using solely electrical field or chemical modification for obtaining half-metallic transport in ZGNRs and thus paves a more feasible way for their application in spintronics.

Two-Dimensional Intrinsic Half-Metals With Large Spin Gaps

Through a systematic search of all layered bulk compounds combined with density functional calculations employing hybrid exchange-correlation functionals, we predict a family of three magnetic two-dimensional (2D) materials with half-metallic band structures. The 2D materials, FeCl₂, FeBr₂, and FeI₂, are all sufficiently stable to be exfoliated from bulk layered compounds. The Fe²⁺ ions in these materials are in a high-spin octahedral d⁶ configuration leading to a large magnetic moment of 4 μ_B. Calculations of the magnetic anisotropy show an easy-plane for the magnetic moment. A classical XY model with nearest neighbor coupling estimates critical temperatures, T_c, for the Berezinskii-Kosterlitz-Thouless transition ranging from 122 K for FeI₂ to 210 K for FeBr₂. The quantum confinement of these 2D materials results in unusually large spin gaps, ranging from 4.0 eV for FeI₂ to 6.4 eV for FeCl₂, which should defend against spin current leakage even at small device length scales. Their purely spin-polarized currents and dispersive interlayer interactions should make these materials useful for 2D spin valves and other spintronic applications.

Adsorbing the 3d-transition metal atoms to effectively modulate the electronic and magnetic behaviors of zigzag SiC nanoribbons

On the basis of first-principles computations, we propose a simple and effective strategy through surface-adsorbing 3d-transition metal (TM) atoms, including Ti, Cr, Mn, Fe and Co, to modulate the electronic and magnetic behaviors of zigzag SiC nanoribbons (zSiCNRs), in view of the unique d electronic structures and intrinsic magnetic moments of TM atoms. It is revealed that like applying an electric field, the adsorption of these transition metal atoms can induce an evident change in the electrostatic potential of the substrate zSiCNRs owing to the electron transfer from the TM atom to the substrate. This can break the magnetic degeneracy of zSiCNRs and solely ferromagnetic (FM) or antiferromagnetic (AFM) metallicity and even intriguing FM or AFM half-metallicity can be observed in the TM-modified zSiCNR systems. Moreover, all these modified systems can exhibit considerably large adsorption energies ranging from -0.872 eV to -4.304 eV, indicating their considerably high structural stabilities. These intriguing findings will be advantageous for promoting excellent SiC-based nanomaterials in the practical application of spintronics and multifunctional nanodevices in the near future.

Current and future directions in electron transfer chemistry of graphene

The participation of graphene in electron transfer chemistry, where an electron is transferred between graphene and other species, encompasses many important processes that have shown versatility and potential for use in important applications.

Asymmetric passivation of edges: a route to make magnetic graphene nanoribbons

Zigzag graphene nanoribbons (ZGNRs) are known to carry interesting properties beyond graphene, such as finite band gaps and magnetic properties.

Uniform and perfectly linear current–voltage characteristics of nitrogen-doped armchair graphene nanoribbons for nanowires

Edge nitrogen-doping induces uniform and perfectly linearI–Vcharacteristics in AGNRs for nanowire applications in molectronics.

Role of a singlet diradical character in carbon nanomaterials: a novel hot spot for efficient nonlinear optical materials

Carbon atoms have the potential to produce a variety of fascinating all-carbon structures with amazing electronic and mechanical properties. Over the last few decades, several efforts have been made using experimental and computational techniques to functionalize graphene, carbon nanotubes and fullerenes for potential use in modern hi-tech electronic, medicinal, optical and nonlinear optical (NLO) applications. Since photons replaced electrons as a carrier of information, the field of NLO material design has drawn immense interest in contemporary materials science. There have been several reports of bridging the gap between the exciting fields of carbon nanomaterials and NLO materials by functionalizing carbon nanomaterials for potential NLO applications. In contrast to previous reports of the design of third-order NLO materials using conventional closed-shell materials, a novel strategy using open-shell diradical molecular systems has recently been proposed. Quantum chemically, diradical character is explained in terms of the instability of the chemical bonds in open-shell molecular systems. Interestingly, several carbon nanomaterials, which naturally possess open-shell singlet configurations, have recently gained momentum in the design of these classes of open-shell NLO materials with excellent NLO properties, stability and diversity. The present review establishes a systematic sequence of different studies (spanning over two decades of intense research efforts), starting from the simplest theoretical two-site diradical model, continuing to its experimental and theoretical realization in actual chemical systems, and finally applying the abovementioned model/rule to novel carbon nanomaterials to tune their NLO properties, particularly their second hyperpolarizability (γ). In the present report, we highlight several recent efforts to functionalize carbon nanomaterials by exploiting their open-shell diradical character to achieve efficient third-order NLO properties. Several issues and opportunities are discussed, including the inherited disadvantages of both experimental (the high reactivity and short life of diradical compounds) and quantum (need for multi-reference methodology) techniques when dealing with carbon nanomaterials. A comparative analysis of several quantum chemical investigations, along with contemporary experimental results, will be performed to emphasize the core issues and opportunities related to carbon nanomaterials with singlet open-shell diradical character. Thus, the present review will highlight carbon nanomaterials with diradical/biradical character for their prospective applications in the NLO field.

Atomically Thin B doped g-C3N4 Nanosheets: High-Temperature Ferromagnetism and calculated Half-Metallicity

Since the graphitic carbon nitride (g-C₄N₃), which can be seen as C-doped graphitic-C₃N₄ (g-C₃N₄), was reported to display ferromagnetic ground state and intrinsic half-metallicity (Du et al., PRL,108,197207,2012), it has attracted numerous research interest to tune the electronic structure and magnetic properties of g-C₃N₄ due to their potential applications in spintronic devices. In this paper, we reported the experimentally achieving of high temperature ferromagnetism in metal-free ultrathin g-C₃N₄ nanosheets by introducing of B atoms. Further, first-principles calculation results revealed that the current flow in such a system was fully spin-polarized and the magnetic moment was mainly attributed to the p orbital of N atoms in B doped g-C₃N₄ monolayer, giving the theoretic evidence of the ferromagnetism and half-metallicity. Our finding provided a new perspective for B doped g-C₃N₄ spintronic devices in future.

Tunable electronic and magnetic properties of two-dimensional materials and their one-dimensional derivatives

Low‐dimensional materials exhibit many exceptional properties and functionalities which can be efficiently tuned by externally applied force or fields. Here we review the current status of research on tuning the electronic and magnetic properties of low‐dimensional carbon, boron nitride, metal‐dichalcogenides, phosphorene nanomaterials by applied engineering strain, external electric field and interaction with substrates, etc, with particular focus on the progress of computational methods and studies. We highlight the similarities and differences of the property modulation among one‐ and two‐dimensional nanomaterials. Recent breakthroughs in experimental demonstration of the tunable functionalities in typical nanostructures are also presented. Finally, prospective and challenges for applying the tunable properties into functional devices are discussed. WIREs Comput Mol Sci 2016, 6:324–350. doi: 10.1002/wcms.1251

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Conflict of interest: The authors have declared no conflicts of interest for this article.

Realizing diverse electronic and magnetic properties in hybrid zigzag BNC nanoribbons via hydrogenation

By means of first-principles DFT computations, we systematically investigate the geometries, stabilities, electronic and magnetic properties of fully and partially hydrogenated zigzag BNC nanoribbons (fH-zBNCNRs and pH-zBNCNRs) with interfacial N-C or B-C connections. It is revealed that in the lowest-lying configuration of hybrid fH-zBNCNRs, the constituent C and BN segments can possess respective chair and boat conformations and both of them are connected by the chair mode, independent of the N-C/B-C interface. Changing the ribbon width and the ratio of BN to C can endow these fH-zBNCNR systems with abundant electronic and magnetic properties involving nonmagnetic (NM) semiconductivity, ferromagnetic (FM) metallicity, antiferromagnetic (AFM) metallicity as well as AFM half-metallicity. Besides, manipulating the hydrogenation pattern and ratio can also result in rich electronic and magnetic behaviors in pH-zBNCNRs, where NM semiconductivity, AFM semiconductivity, AFM metallicity and even AFM spin gapless semiconductor are observed. Additionally, the origin of the magnetism in these hydrogenated zBNCNRs is analyzed in detail. Finally, all of these hydrogenated BNC structures can possess a favorable formation energy, large binding energy per hydrogen atom and high thermal stability, indicating the great possibility of their experimental realization by hydrogenating pristine zBNCNRs. These valuable insights can be advantageous for promoting hybrid BNC-based nanomaterials in the applications of spintronics and multifunctional nanodevices.

Possibility of spin-polarized transport in edge fluorinated armchair boron nitride nanoribbons

Calculated DOS for edge-fluorinated. ABNNRs; featuring half-metallicity.

Vacancy-induced spin polarization in graphene and B–N nanoribbon heterojunctions

By using nonequilibrium Green's functions (NEGF) and density functional theory (DFT), we investigate the spin-separated electronic transport properties in heterojunctions constructed by zigzag graphene and boron nitride nanoribbons.