Magnetism in disordered graphene and irradiated graphite

The magnetic properties of disordered graphene and irradiated graphite are systematically studied using a combination of mean-field Hubbard model and first-principles calculations. By considering large-scale disordered models of graphene, I conclude that only single-atom defects can induce ferromagnetism in graphene-based materials. The preserved stacking order of graphene layers is shown to be another necessary condition for achieving a finite net magnetic moment of irradiated graphite. Ab initio calculations of hydrogen binding and diffusion and of interstitial-vacancy recombination further confirm the crucial role of stacking order in pi-electron ferromagnetism of proton-bombarded graphite.     

B、N、S掺杂石墨烯表面质子吸附的第一性原理研究

表面吸附是石墨烯中质子(H~+)输运的基础步骤.本文基于第一性原理计算研究了B、N、S掺杂对石墨烯表面H~+吸附行为的影响.结果表明,B、N、S掺杂对石墨烯的晶体结构、内聚能及电子性质有重要影响,且影响程度与掺杂元素的性质密切相关,进而导致对石墨烯表面H~+吸附行为产生不同程度的影响.从吸附能的观点看,N、B、S掺杂对石墨烯表面H~+的吸附有利.研究结果对石墨烯在氢贮存、氢同位素分离、燃料电池等领域的应用具有重要的指导意义.     

Low-energy theory of disordered graphene

At low values of external doping, graphene displays a wealth of unconventional transport properties. Perhaps most strikingly, it supports a robust "metallic" regime, with universal conductance of the order of the conductance quantum. We here apply a combination of mean-field and bosonization methods to explore the large scale transport properties of the system. We find that, irrespective of the doping level, disordered graphene is subject to the common mechanisms of Anderson localization. However, at low doping a number of renormalization mechanisms conspire to protect the conductivity of the system, to an extend that strong localization may not be seen even at temperatures much smaller than those underlying present experimental work.     

锯齿形石墨烯反点网络加工与输运性质研究

具有特定边界的石墨烯纳米结构在纳电子学、自旋电子学等研究领域表现出良好的应用前景.然而石墨烯加工成纳米结构时,无序的边界不可避免地会降低其载流子迁移率.氢等离子体各向异性刻蚀技术是加工具备完美边界石墨烯微纳结构的一项关键技术,刻蚀后的石墨烯呈现出规则的近原子级平整的锯齿形边界.本文研究了氮化硼上锯齿形边界石墨烯反点网络的磁输运性质,低磁场下可以观测到载流子围绕着一个空位缺陷运动时的公度振荡磁阻峰.随着磁场的增大,朗道能级简并度逐渐增大,载流子的磁输运行为从Shubnikov-de Haas振荡逐渐向量子霍尔效应转变.在零磁场附近可以观测到反点网络周期性空位缺陷的边界散射所导致的弱局域效应.研究结果表明,在氮化硼衬底上利用氢等离子体刻蚀技术加工锯齿形边界石墨烯反点网络,其样品质量会明显提高,这种简单易行的方法为后续高质量石墨烯反点网络的输运研究提供了新思路.     

Effect of substitutional impurities on the electronic transport properties of graphene

Density-functional theory in combination with the nonequilibrium Green's function formalism is used to study the effect of substitutional doping on the electronic transport properties of hydrogen passivated zig-zag graphene nanoribbon devices. B, N and Si atoms are used to substitute carbon atoms located at the center or at the edge of the sample. We found that Si-doping results in better electronic transport as compared to the other substitutions. The transmission spectrum also depends on the location of the substitutional dopants: for single atom doping the largest transmission is obtained for edge substitutions, whereas substitutions in the middle of the sample give larger transmission for double carbon substitutions. The obtained results are explained in terms of electron localization in the system due to the presence of impurities.     

Magnetoresistance effect in vertical NiFe/graphene/NiFe junctions

For convenient and efficient verification of the magnetoresistance effect in graphene spintronic devices, vertical magnetic junctions with monolayer graphene sandwiched between two NiFe electrodes are fabricated by a relatively simple way of transferring CVD graphene onto the bottom ferromagnetic stripes. The anisotropic magnetoresistance contribution is excluded by the experimental result of magnetoresistance (MR) ratio dependence on the magnetic field direction. The spin-dependent transport measurement reveals two distinct resistance states switching under an in-plane sweeping magnetic field. A magnetoresistance ratio of about 0.17 % is obtained at room temperature and it shows a typical monotonic downward trend with the bias current increasing. This bias dependence of MR further verifies that the spin transport signal in our device is not from the anisotropic magnetoresistance. Meanwhile, the I—V curve is found to manifest a linear behavior, which demonstrates the Ohmic contacts at the interface and the metallic transport characteristic of vertical graphene junction.     

Scattering theory of electron transport in single layer graphene with a time-periodic potential

We applied the scattering approach to studying the transport properties of charge carriers through single layer graphene in the presence of a time-periodic potential. Using the method, expressions for the second-quantized current operator, conductivity and shot noise are obtained. The results obtained in this study demonstrate that the applied external field provides sidebands for charge carriers to tunnel through the graphene, and these sidebands changed the transport properties of the system. The results obtained in this study might be of interest to basic understanding of photon-assisted tunneling (PAT) and designers of electron devices based on graphene.     

Transport properties in monolayer–bilayer–monolayer graphene planar junctions

The transport study of graphene based junctions has become one of the focuses in graphene research. There are two stacking configurations for monolayer–bilayer–monolayer graphene planar junctions. One is the two monolayer graphene contacting the same side of the bilayer graphene, and the other is the two-monolayer graphene contacting the different layers of the bilayer graphene. In this paper, according to the Landauer–Büttiker formula, we study the transport properties of these two configurations. The influences of the local gate potential in each part, the bias potential in bilayer graphene,the disorder and external magnetic field on conductance are obtained. We find the conductances of the two configurations can be manipulated by all of these effects. Especially, one can distinguish the two stacking configurations by introducing the bias potential into the bilayer graphene. The strong disorder and the external magnetic field will make the two stacking configurations indistinguishable in the transport experiment.     

Quasi-particle spectrum and density of electronic states in AA- and AB-stacked bilayer graphene

The present work deals with the analysis of the quasi-particle spectrum and the density of states of monolayer and bilayer (AB- and AA-stacked) graphene. The tight binding Hamiltonian containing nearest-neighbor and next-nearest neighbor hopping and onsite Coulomb interaction within two triangular sub-lattice approach for monolayer graphene, along-with the interlayer coupling parameter for bilayer graphene has been employed. The expressions of quasi-particle energies and the density of states (DOS) are obtained within mean-field Green’s function equations of motion approach. It is found that next-nearest-neighbour intralayer hopping introduce asymmetry in the electronic states above and below the zero point energy in monolayer and bilayer (AA- and AB-stacked) graphene. The behavior of electronic states in monolayer and bilayer graphene is different and highly influenced by interlayer coupling and Coulomb interaction. It has been pointed out that the interlayer coupling splits the quasi-particle peak in density of states while the Coulomb interaction suppresses the bilayer splitting and generates a gap at Fermi level in both AA- and AB-stacked bilayer graphene. The theoretically obtained quasi-particle energies and density of states in monolayer and bilayer (AA- and AB-stacked) graphene has been viewed in terms of recent ARPES and STM data on these systems.     

Two-dimensional phonon transport in graphene

Properties of phonons-quanta of the crystal lattice vibrations-in graphene have recently attracted significant attention from the physics and engineering communities. Acoustic phonons are the main heat carriers in graphene near room temperature, while optical phonons are used for counting the number of atomic planes in Raman experiments with few-layer graphene. It was shown both theoretically and experimentally that transport properties of phonons, i.e. energy dispersion and scattering rates, are substantially different in a quasi-two-dimensional system such as graphene compared to the basal planes in graphite or three-dimensional bulk crystals. The unique nature of two-dimensional phonon transport translates into unusual heat conduction in graphene and related materials. In this review, we outline different theoretical approaches developed for phonon transport in graphene, discuss contributions of the in-plane and cross-plane phonon modes, and provide comparison with available experimental thermal conductivity data. Particular attention is given to analysis of recent results for the phonon thermal conductivity of single-layer graphene and few-layer graphene, and the effects of the strain, defects, and isotopes on phonon transport in these systems.     

石墨烯与金属的欧姆接触理论研究

石墨烯材料应用于多种电子器件时不可避免地要与金属电极接触,它们之间的接触电阻直接影响了器件的性能.为了揭示影响金属电极与石墨烯间接触电阻的因素,提出有效地抑制这些影响的措施,本文建立了一种求解接触电阻的物理模型,将载流子的输运分为金属与正下方石墨烯之间、正下方石墨烯与邻近石墨烯之间的两个过程,分别研究各个过程的输运概率;结合金属电极与石墨烯接触对载流子分布的影响分析接触电阻,据此分别探讨了金属电极材料、栅极电压、掺杂浓度、金属与石墨烯原子距离等对接触电阻的影响.为验证理论分析结果的正确性,制作了金与石墨烯接触的实验样品,实验测得的接触电阻与理论分析结果符合.理论分析结果表明,可通过选择与石墨烯功函数接近的金属材料,降低二氧化硅层厚度,增加载流子平均自由程,改进金属材料的表面形态使其更光滑,减小金属与石墨烯耦合长度等方法降低石墨烯与金属电极的接触电阻.     

Magnetothermoelectric transport in modulated and unmodulated graphene

We draw motivation from recent experimental studies and present a comprehensive study of magnetothermoelectric transport in a graphene monolayer within the linear response regime. We employ the modified Kubo formalism developed for thermal transport in a magnetic field. Thermopower as well as thermal conductivity as a function of the gate voltage of a graphene monolayer in the presence of a magnetic field perpendicular to the graphene plane is determined for low magnetic fields (～1 T) as well as high fields (～8 T). We include the effects of screened charged impurities on thermal transport. We find good qualitative and quantitative agreement with recent experimental work on the subject. In addition, in order to analyze the effects of modulation, which can be induced by various means, on the thermal transport in graphene, we evaluate the thermal transport coefficients for a graphene monolayer subjected to a periodic electric modulation in a magnetic field. The results are presented as a function of the magnetic field and the gate voltage.     

Electronic and transport properties of boron-doped graphene nanoribbons

We report a spin polarized density functional theory study of the electronic and transport properties of graphene nanoribbons doped with boron atoms. We considered hydrogen terminated graphene (nano)ribbons with width up to 3.2 nm. The substitutional boron atoms at the nanoribbon edges (sites of lower energy) suppress the metallic bands near the Fermi level, giving rise to a semiconducting system. These substitutional boron atoms act as scattering centers for the electronic transport along the nanoribbons. We find that the electronic scattering process is spin-anisotropic; namely, the spin-down (up) transmittance channels are weakly (strongly) reduced by the presence of boron atoms. Such anisotropic character can be controlled by the width of the nanoribbon; thus, the spin-up and spin-down transmittance can be tuned along the boron-doped nanoribbons.     

Thermal transport in graphene

The recent advances in graphene isolation and synthesis methods have enabled potential applications of graphene in nanoelectronics and thermal management, and have offered a unique opportunity for investigation of phonon transport in two-dimensional materials. In this review, current understanding of phonon transport in graphene is discussed along with associated experimental and theoretical investigation techniques. Several theories and experiments have suggested that the absence of interlayer phonon scattering in suspended monolayer graphene can result in higher intrinsic basal plane thermal conductivity than that for graphite. However, accurate experimental thermal conductivity data of clean suspended graphene at different temperatures are still lacking. It is now known that contact of graphene with an amorphous solid or organic matrix can suppress phonon transport in graphene, although further efforts are needed to better quantify the relative roles of interface roughness scattering and phonon leakage across the interface and to examine the effects of other support materials. Moreover, opportunities remain to verify competing theories regarding mode specific scattering mechanisms and contributions to the total thermal conductivity of suspended and supported graphene, especially regarding the contribution from the flexural phonons. Several measurements have yielded consistent interface thermal conductance values between graphene and different dielectrics and metals. A challenge has remained in establishing a comprehensive theoretical model of coupled phonon and electron transport across the highly anisotropic and dissimilar interface.     

Transport in graphene nanostructures

Graphene nanostructures are promising candidates for future nanoelectronics and solid-state quantum information technology. In this review we provide an overview of a number of electron transport experiments on etched graphene nanostructures. We briefly revisit the electronic properties and the transport characteristics of bulk, i.e., two-dimensional graphene. The fabrication techniques for making graphene nanostructures such as nanoribbons, single electron transistors and quantum dots, mainly based on a dry etching ??paper-cutting?? technique are discussed in detail. The limitations of the current fabrication technology are discussed when we outline the quantum transport properties of the nanostructured devices. In particular we focus here on transport through graphene nanoribbons and constrictions, single electron transistors as well as on graphene quantum dots including double quantum dots. These quasi-one-dimensional (nanoribbons) and quasi-zero-dimensional (quantum dots) graphene nanostructures show a clear route of how to overcome the gapless nature of graphene allowing the confinement of individual carriers and their control by lateral graphene gates and charge detectors. In particular, we emphasize that graphene quantum dots and double quantum dots are very promising systems for spin-based solid state quantum computation, since they are believed to have exceptionally long spin coherence times due to weak spin-orbit coupling and weak hyperfine interaction in graphene.     

Quantum Monte Carlo investigations of adsorption energetics on graphene

We have performed calculations of adsorption energetics on the graphene surface using the state-of-the-art diffusion quantum Monte Carlo method. Two types of configurations are considered in this work: the adsorption of a single O, F, or H atom on the graphene surface and the H-saturated graphene system (graphane). The adsorption energies are compared with those obtained from density functional theory with various exchange-correlation functionals. The results indicate that the approximate exchange-correlation functionals significantly overestimate the binding of O and F atoms on graphene, although the preferred adsorption sites are consistent. The energy errors are much less for atomic hydrogen adsorbed on the surface. We also find that a single O or H atom on graphene has a higher energy than in the molecular state, while the adsorption of a single F atom is preferred over the gas phase. In addition, the energetics of graphane is reported. The calculated equilibrium lattice constant turns out to be larger than that of graphene, at variance with a recent experimental suggestion.     

Formation of graphene quantum dots by “Planting” hydrogen atoms at a graphene nanoribbon

Different technological approaches for creating graphene quantum dots by the adsorption of hydrogen atoms are considered. The adsorption can occur both at convex portions of a distorted graphene nanoribbon and in the structure formed by two distorted graphene nanoribbon rows superimposed on each other at the places free from the ribbon crossings. It is shown that settlement of hydrogen atoms at convex portions of the nanoribbons is energetically favorable. This gives rise to the creation of insulating graphane (CH) nanodomains separating the conducting regions. As a result, a graphene quantum dot appears. The variation of the electron spectra of graphene quantum dots with the length of these graphane regions is discussed.     

Influence of Mechanical Stretching on Adsorption Properties of Nitrogen-Doped Graphene

This paper presents the results of quantum chemical modeling of chemisorption of atomic hydrogen and epoxy, carboxyl, and hydroxyl functional groups on nitrogen-doped graphene. It is shown that the substitutional nitrogen atom does not bind to adsorbing groups directly, but significantly increases the adsorption activity of neighboring carbon atoms. Mechanical stretching of doped graphene reduces the adsorption energy of all the aforementioned radicals. This reduction is significantly greater for the epoxy group than for the other functional groups. The results obtained confirm that, upon a sufficient stretching of a nitrogen-doped graphene sheet, the dissociation of molecular hydrogen and oxygen with subsequent precipitation of the resulting radicals onto graphene can be energetically favorable.