Two-dimensional Honeycomb Lattice Research Articles

Spin-Dependent Bandgap Structures and Spin Filtering in Graphene With Multiple Ferromagnetic Barriers

Motivated by the experimental isolation of graphene [1], intense investigations have been paid the physical properties of the two-dimensional (2D) honeycomb lattice. Graphene is regarded as a promising material for spintronic applications due to its weak spin-orbit coupling and long spin lifetimes [2]. Up to now, several groups have studied the spin transport properties in graphene with magnetic and electrostatic modulation potentials [3]. How to get efficient spin filtering is one of the research focuses for the application of spintronics. In this work, we investigate spin-dependent bandgap structures and spin filtering in graphene with multiple ferromagnetic barriers. It is shown that the spin-up and spin-down electrons possess different bandgap structures in the case of oblique incidence. As a result, full spin polarization can be achieved within several separated energy intervals. The width of the energy interval with full spin polarization can be effectively manipulated by adjusting the strength of the exchange field. Correspondingly, conductance steps with full spin polarization are observed in the systems. Our investigations may have potential applications in the design of carbon-based spin filters.

High-temperature superconductivity in heavily N- or B-doped graphene

A two-dimensional honeycomb lattice of graphene, if heavily doped with electrons or holes, has been predicted to possess a wealth of fascinating properties including high-temperature superconductivity. Although such a material is possible with high concentration of N or B substitution, its experimental realization has been hindered due to its dynamic instability. Using density functional theory combined with a global structural search and phonon dispersion calculations, we show that an ordered $50%$ N- or B-doped graphene can be made energetically and dynamically stable by simultaneous doping carriers and applying biaxial tensile strain; carrier doping moves the system toward aromaticity while tensile strain reduces adverse effects associated with electrostatic interaction. Electron-phonon coupling calculations show that the N- or B-doped graphene is superconducting with critical temperature reaching above the melting point of nitrogen in the case of $50%$ N-doped graphene. In addition, the ideal strength of N-doped graphene is even higher than that of pure graphene.

Coriolis force induced topological order for classical mechanical vibrations

We show that topological order and vibrational edge modes can exist in a classical mechanical system consisting of a two-dimensional honeycomb lattice of masses and springs. The band structure shows the existence of Dirac cones and unconventional edge states that are similar to the vibrational modes in graphene. Interestingly, as the system is placed on a constantly rotational coordinate system, the Coriolis force resulting from the non-inertial reference frame introduces time-reversal symmetry breaking and leads to topologically nontrivial band gaps. The nontrivial topological orders are further verified by the calculation of Chern numbers for corresponding bands.

Dirac cones in transition metal doped boron nitride

The transition metal (TM) doped zinc blende boron nitride (c-BN) is studied by using the first principle calculation. TM atoms fill in the interstitials in c-BN and form two-dimensional honeycomb lattice. The generalized gradient approximation and projector augmented wave method are used. The calculated density of states and band structures show that d electrons of TM atoms form impurity bands in the gap of c-BN. When the TM-BN system is in ferromagnetic or non-magnetic state, Dirac cones emerge at the K point in Brillouin zone. When TM is Ti and Co, the Dirac cones are spin polarized and very close to the Fermi level, which makes them promising candidates of Dirac half-metal [H. Ishizuka and Y. Motome, Phys. Rev. Lett. 109, 237207 (2012)]. While TM is Ni and Cu, the system is non-magnetic and Dirac cones located above the Fermi level.

Topological properties of the bond-modulated honeycomb lattice

We study the combined effects of lattice deformation, e-e interaction and spin-orbit coupling in a two-dimensional (2D) honeycomb lattice. We adopt different kinds of hopping modulation--generalized dimerization and a Kekule distortion--and calculate topological invariants for the non-interacting system and for the interacting system. We identify the parameter range (Hubbard U, hopping modulation, spin-orbit coupling) where the 2D system behaves as a trivial insulator or Quantum Spin Hall Insulator.

Design, synthesis, and characterization of graphene-nanoparticle hybrid materials for bioapplications.

Graphene is composed of single-atom thick sheets of sp2 bonded carbon atoms that are arranged in a perfect two-dimensional (2D) honeycomb lattice. Because of this structure, graphene is characterized by a number of unique and exceptional structural, optical, and electronic properties.1 Specifically, these extraordinary properties include, but are not limited to, a high planar surface area that is calculated to be 2630 m2 g−1,2 superior mechanical strength with a Young’s modulus of 1100 GPa,3 unparalleled thermal conductivity (5000 W m−1 K−1),4 remarkable electronic properties (e.g., high carrier mobility [10 000 cm2 V−1 s−1] and capacity),5 and alluring optical characteristics (e.g., high opacity [~97.7%] and the ability to quench fluorescence).6 As such, it should come as no surprise that graphene is currently, without any doubt, the most intensively studied material for a wide range of applications that include electronic, energy, and sensing outlets.1c Moreover, because of these unique chemical and physical properties, graphene and graphene-based nanomaterials have attracted increasing interest, and, arguably, hold the greatest promise for implementation into a wide array of bioapplications.7 In the last several years, numerous studies have utilized graphene in bioapplications ranging from the delivery of chemotherapeutics for the treatment of cancer8 to biosensing applications for a host of medical conditions9 and even for the differentiation and imaging of stem cells.10 While promising and exciting, recent reports have demonstrated that the combination of graphene with nanomaterials such as nanoparticles, thereby forming graphene–nanoparticle hybrid structures, offers a number of additional unique physicochemical properties and functions that are both highly desirable and markedly advantageous for bioapplications when compared to the use of either material alone (Figure 1).11 These graphene–nanoparticle hybrid structures are especially alluring because not only do they display the individual properties of the nanoparticles, which can already possess beneficial optical, electronic, magnetic, and structural properties that are unavailable in bulk materials, and of graphene, but they also exhibit additional advantageous and often synergistic properties that greatly augment their potential for bioapplications. Open in a separate window Figure 1 Graphene nanoparticle hybrids exist in two forms, as graphene–nanoparticle composites and graphene-encapsulated nanoparticles, and can be used for various bioapplications including biosensors, photothermal therapies, stem cell/tissue engineering, drug/gene delivery, and bioimaging. Panel (A) reprinted with permission from ref 110. Copyright 2012 Wiley. Panel (B) reprinted with permission from ref 211. Copyright 2013 Elsevier. Panel (C) reprinted with permission from ref 244. Copyright 2013 Wiley.

Progress in the materials science of silicene

In its freestanding, yet hypothetical form, the Si counterpart of graphene called silicene is predicted to possess massless Dirac fermions and to exhibit an experimentally accessible quantum spin Hall effect. Such interesting electronic properties are not realized in two-dimensional (2D) Si honeycomb lattices prepared recently on metallic substrates where the crystal and hybrid electronic structures of these ‘epitaxial silicene’ phases are strongly influenced by the substrate, and thus different from those predicted for isolated 2D structures. While the realization of such low-dimensional Si π materials has hardly been imagined previously, it is evident that the materials science behind silicene remains challenging. In this contribution, we will review our recent results that lead to an enhanced understanding of epitaxial silicene formed on diboride thin films, and discuss the remaining challenges that must be addressed in order to turn Si 2D nanostructures into technologically interesting nanoelectronic materials.

High lithium anodic performance of highly nitrogen-doped porous carbon prepared from a metal-organic framework.

Theoretical and experimental results have revealed that the lithium-ion storage capacity for nitrogen-doped graphene largely depends on the nitrogen-doping level. However, most nitrogen-doped carbon materials used for lithium-ion batteries are reported to have a nitrogen content of approximately 10 wt% because a higher number of nitrogen atoms in the two-dimensional honeycomb lattice can result in structural instability. Here we report nitrogen-doped graphene particle analogues with a nitrogen content of up to 17.72 wt% that are prepared by the pyrolysis of a nitrogen-containing zeolitic imidazolate framework at 800 °C under a nitrogen atmosphere. As an anode material for lithium-ion batteries, these particles retain a capacity of 2,132 mA h g(-1) after 50 cycles at a current density of 100 mA g(-1), and 785 mAh g(-1) after 1,000 cycles at 5 A g(-1). The remarkable performance results from the graphene analogous particles doped with nitrogen within the hexagonal lattice and edges.

Candidate quantum spin liquid due to dimensional reduction of a two-dimensional honeycomb lattice.

As with quantum spin liquids based on two-dimensional triangular and kagome lattices, the two-dimensional honeycomb lattice with either a strong spin-orbital coupling or a frustrating second-nearest-neighbor coupling is expected to be a source of candidate quantum spin liquids. An ammonium salt [(C3H7)3NH]2[Cu2(C2O4)3](H2O)2.2 containing hexagonal layers of Cu2+ was obtained from solution. No structural transition or long-range magnetic ordering was observed from 290 K to 2 K from single crystal X-ray diffraction, specific heat and susceptibility measurements. The anionic layers are separated by sheets of ammonium and H2O with distance of 3.5 Å and no significant interaction between anionic layers. The two-dimensional honeycomb lattice is constructed from Jahn-Teller distorted Cu2+ and oxalate anions, showing a strong antiferromagnetic interaction between S = 1/2 metal atoms with θ = −120 (1) K. Orbital analysis of the Cu2+ interactions through the oxalate-bridges suggests a stripe mode pattern of coupling with weak ferromagnetic interaction along the b axis, and strong antiferromagnetic interaction along the a axis. Analysis of the magnetic susceptibility shows that it is dominated by a quasi-one-dimensional contribution with spin chains that are at least as well isolated as those of well-known quasi-one-dimensional spin liquids.

Experimental percolation studies of two-dimensional honeycomb lattice: Li2Mn1–xTixO3

Li2MnO3 with a S = 3/2 two-dimensional Mn honeycomb lattice has a Neel-type antiferromagnetic transition at TN = 36 K with a broad maximum in magnetic susceptibility at TM = 48 K. We have investigated site percolation effects by replacing Mn with nonmagnetic Ti, and completed a full phase diagram of Li2Mn1–xTixO3 solid solution systems to find that antiferromagnetic transition is continuously suppressed without a clear sign of changes in the Neel-type antiferromagnetic structure. The magnetic ordering eventually disappears at a critical concentration of xc = 0.7. This experimental observation is consistent with percolation theories for a honeycomb lattice when one considers up to 3rd nearest-neighbor interactions. This study highlights the importance of interaction beyond nearest neighbors even for the Mn element with relative localized 3d electrons in the honeycomb lattice.

Progress in biological safety of graphene

Graphene, a carbon nanomaterial with sp2-hybridized carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice. Owing to the excellent optical, electrical, and mechanical properties, graphene has emerged as a revolutionary carbon material with great potential in materials science, micro-nanomachining, energy, biomedicine, and drug deliver. With the mass production and the widespread use of grapheme, the biological safety issues of graphene have caused a lot of concern. As similar to other nano particles, the biotoxicity of graphene and its derivative depends on their physicochemical property, and also depends on their concentration. But the biotoxicity of graphene and its derivative is complicatied. The same material may have different effect on the different organisms. So we must do further studies to clarify the biotoxicity of graphene. This review summarizes the current biotoxicity studies of graphene and its derivative. We want to provide a reference for biological safety assessment, and to provide a scientific guidance in the application of biological and medical domain.

Thermal conductivity of silicene calculated using an optimized Stillinger-Weber potential

Silicene, the silicon-based counterpart of graphene with a two-dimensional honeycomb lattice, has attracted tremendous interest both theoretically and experimentally due to its significant potential industrial applications. From the aspect of theoretical study, the widely used classical molecular dynamics simulation is an appropriate way to investigate the transport phenomena and mechanisms in nanostructures such as silicene. Unfortunately, no available interatomic potential can precisely characterize the unique features of silicene. Here, we optimized the Stillinger-Weber potential parameters specifically for a single-layer Si sheet, which can accurately reproduce the low buckling structure of silicene and the full phonon dispersion curves obtained from ab initio calculations. By performing equilibrium and nonequilibrium molecular dynamics simulations and anharmonic lattice dynamics calculations with the new potential, we reveal that the three methods consistently yield an extremely low thermal conductivity of silicene and a short phonon mean-free path, suggesting silicene as a potential candidate for high-efficiency thermoelectric materials. Moreover, by qualifying the relative contributions of lattice vibrations in different directions, we found that the longitudinal phonon modes dominate the thermal transport in silicene, which is fundamentally different from graphene, despite the similarity of their two-dimensional honeycomb lattices.

Lithium Storage Characteristics and Possible Applications of Graphene Materials

Graphene materials are materials with a flat mono/few layer of carbon atoms tightly packed to a two-dimensional honeycomb lattice. Graphene materials are expected to be applied in lithium ion batteries due to their unique structural, mechanical and electrical properties. As an anode material, the charge/discharge characteristics of graphene materials is similar to those of low-temperature soft carbon materials, such as high capacity, low initial efficiency and large voltage hys- teresis. Although attractive results have been achieved for graphene as anode materials for LIBs, detailed lithium storage mechanisms are still not clear. The effects of the following several structural parameters including disorder degree, surface area, micropores, interlayer spacing, C/O ratio and layer number on the lithium storage properties are discussed. Thermally reduced graphene materials with a highly disordered structure and high surface area has exceptionally high reversible capac- ity. Micropores in graphene materials have a great impact on their electrochemical performance. Although these micropores can provide additional sites for increased reversible lithium storage, it can also results in severe capacity fading and voltage hysteresis. Oxygen functional groups and larger interlayer spacing may provide higher reversible capacity of graphene, but the micropores and defect-based reversible storage may be the main contribution. Effect of layer number on lithium storage mechanisms of graphene and the conclusion are still in debate. Graphene with rich oxygen functional groups is a promising cathode material with high capacity and rate performance for lithium storage. High specific capacity of graphene cathode is mainly ascribed to lithiation reaction of oxygen functional groups, such as epoxide and carbonyl groups. Lithiation of oxygen functional groups still requires further study for a full understanding. Based on the lithium storage characteristics of graphene anode and cathode, lithium ion capacitors with high energy density and graphene composite cathode materials for lithium ion batteries may be designed and developed in the future. Graphene based lithium ion capacitors facilitate the reversible lithium storage, which significantly improves the energy density of lithium ion capacitors compared to those of conventional systems based on activated carbon. LiFePO4 modified with graphene layers has reached 208 mAh/g in specific capacity. The excess capacity is attributed to the reversible reduction-oxidation reaction between the lithium ions of the electrolyte and the exfoli- ated graphene flakes.

Electronic and plasmonic phenomena at graphene grain boundaries

Graphene, a two-dimensional honeycomb lattice of carbon atoms of great interest in (opto)electronics and plasmonics, can be obtained by means of diverse fabrication techniques, among which chemical vapour deposition (CVD) is one of the most promising for technological applications. The electronic and mechanical properties of CVD-grown graphene depend in large part on the characteristics of the grain boundaries. However, the physical properties of these grain boundaries remain challenging to characterize directly and conveniently. Here we show that it is possible to visualize and investigate the grain boundaries in CVD-grown graphene using an infrared nano-imaging technique. We harness surface plasmons that are reflected and scattered by the graphene grain boundaries, thus causing plasmon interference. By recording and analysing the interference patterns, we can map grain boundaries for a large-area CVD graphene film and probe the electronic properties of individual grain boundaries. Quantitative analysis reveals that grain boundaries form electronic barriers that obstruct both electrical transport and plasmon propagation. The effective width of these barriers (∼10-20nm) depends on the electronic screening and is on the order of the Fermi wavelength of graphene. These results uncover a microscopic mechanism that is responsible for the low electron mobility observed in CVD-grown graphene, and suggest the possibility of using electronic barriers to realize tunable plasmon reflectors and phase retarders in future graphene-based plasmonic circuits.

Optical time reversal with graphene

Would you ever guess that a microscopic flake of graphite could reverse the diffraction of light? An experiment that demonstrates just such an effect highlights the exciting optical applications of graphene — an atomic layer of carbon with a two-dimensional honeycomb lattice.

Two-Dimensional Honeycomb Lattice Consisting of a New Organic Radical 2-Cl-6-F-V

We have successfully synthesized new organic radical crystals of 2-Cl-6-F-V [= 3-(2-chloro-6-fluorophenyl)-1,5-diphenylverdazyl]. Two types of dominant intermolecular interactions forming a honeycomb lattice are deduced from the molecular packing in the crystal, and ab initio molecular orbital calculation also indicate such an exchange network. We analyzed the magnetic and thermodynamic properties by using the quantum Monte Carlo method and successfully explained these properties as an \(S = 1/2\) Heisenberg honeycomb lattice with ferromagnetic chain interactions. Our results indicate that 2-Cl-6-F-V is one of the few examples of a purely organic radical compound that forms two-dimensional lattices, and this paper presents the first complete, quantitative description of an actual honeycomb lattice material.

Study of the bilinear biquadratic Heisenberg model on a honeycomb lattice via Schwinger bosons

We analyze the biquadratic bilinear Heisenberg magnet on a honeycomb lattice via Schwinger boson formalism. Due to their vulnerability to quantum fluctuations, non-conventional lattices (kagome, triangular and honeycomb for example) have been cited as candidates to support spin liquid states. Such states without long range order at zero temperature are known in one-dimensional spin models but their existence in higher dimensional systems is still under debate. Biquadratic interaction is responsible for various possibilities and phases as it is well-founded for one-dimensional systems. Here we have used a bosonic representation to study the properties at zero and finite low temperatures of the biquadratic term in the two-dimensional hexagonal honeycomb lattice. The results show an ordered state at zero temperature but much more fragile than that of a square lattice; the behavior at finite low temperatures is in accordance with expectations.

Hexagonally warped Dirac cones and topological phase transition in silicene superstructure

Silicene is a monolayer of silicon atoms forming a two-dimensional honeycomb lattice. We investigate the topological properties of a silicene superstructure generated by an external periodic potential. The superstructure is a quantum spin-Hall (QSH) insulator if it is topologically connected to silicene. It is remarkable that two inequivalent K and K′ points in the silicene Brillouin zone are identified in certain superstructures. In such a case, two Dirac cones coexist at the same Dirac point in the momentum space and they are hexagonally warped by the Coulomb interaction. We carry out a numerical analysis by taking an instance of the (3 × 3) superstructure on the (4 × 4) structure of the Ag substrate. We show that it is a QSH insulator, that there exists no topological phase transition by external electric field, and that the hexagonally warping occurs in the band structure.

Elastic in-plane properties of 2D linearized models of graphene

Graphene is a monolayer of carbon atoms packed into a two-dimensional honeycomb lattice. This allotrope can be considered as mother of all graphitic forms of carbon. The elastic in-plane properties of graphene are studied and various existing linearized models of its elastic deformations are critically re-examined. Problems related to modelling of graphene by nonlinear multi-body potentials of interaction are also discussed. It is shown that experimental results for small deformations can be well described by both the two-parametric molecular mechanics model developed by Gillis in 1984, while some popular models have serious flaws and often the results obtained using these models do not have physical meaning. It is argued that in order to study elastic constants of linearized models of graphene layers, it is very convenient to use the four parameter molecular mechanics model. The advantages of this approach is demonstrated by its application to the Tersoff and Brenner nonlinear interaction potentials, and by its comparison with the Gillis two-parametric model.

Electrodeposited graphene nano-stacks for biosensor applications. Surface groups as redox mediators for laccase

Graphene is a form of carbon material in which the carbon atoms are arranged in a two-dimensional honeycomb lattice. It is a promising candidate material for future electronic applications, including transistors, gas sensors, electromechanical devices and supercapacitors. We explore the applications of graphene in the field of bioelectroanalysis. This communication reports the influence of graphene surface groups on the bioelectrocatalytic activity of a model enzyme, laccase.A graphite oxide (GO) layer cast onto a glassy carbon electrode is subjected to electrochemical reduction. The surface morphology and composition is monitored with scanning electron microscopy, energy-dispersive X-ray microanalysis and infrared reflectance spectroscopy. The results indicate that carbonyl and epoxide groups are fully removed during electroreduction of GO although some OH groups remain after the electrochemical reduction. The OH groups can serve as redox mediators between laccase and the electrode. The surface composition studies reveal that the functionality and the reactivity of graphene can be optimised by electrochemical treatment.Laccase that is physically adsorbed on the electrochemically reduced GO catalyses oxygen reduction without a mediator at potential values up to +0.6V relative to an Ag/AgCl electrode.