Real-space pseudopotential calculations for graphene dots embedded in hexagonal boron nitride

A major challenge for graphene-based applications is the creation of a tunable electronic band gap as would be present for traditional semiconductor alloys. Since hexagonal boron nitride has a very similar lattice structure to graphene, it is a natural candidate for modifying the electronic structure of graphene by forming a hybrid phase sheet containing domains of graphene and hexagonal boron nitride, as has been done experimentally. Here we investigate the properties of such hybrid sheets using pseudopotential-density functional theory implemented in real space. We find for a graphene dot comparable in size to those observed in experiment, the band gap of the sheet is not significantly modified.     

Magical Allotropes of Carbon: Prospects and Applications

The invention of carbon and its allotropes have transformed the electronic and optoelectronic industry due to their encouraging properties in a large spectrum of applications. The interesting characteristic of carbon is its ability to form many allotropes due to its valency. In recent decades, various allotropes and forms of carbon have been invented, including fullerenes, carbon nanotubes (CNTs), and graphene (GR). Since the inception of nanotechnology, carbon allotropes-based nanocomposites have become a leading sector of research and advancement due to their unique bonding properties. Fullerenes and CNTs-based polymer nanocomposites have attracted significant research interest due to their vast applications in every sphere of science and technology. Current research impetus reveals that carbon and its allotropes have revolutionized the industry and academia due to their fascinated properties. Recent advances in various aspects of graphene, CNTs, graphene nanoribbons, fullerenes, carbon encapsulates, and their nanocomposites with polymeric materials and their different applications are reported in this review article. Also, current status and future prospects of graphene-based polymer nanocomposites are presented in common along with proper citations extracted from the scientific literature. Moreover, this article is a unique collection of vital information about GR, CNTs, fullerenes, and graphene-based polymer nanocomposites in a single platform.     

二维硼碳基纳米结构上的吸附及其性质

综述金属原子与非金属原子和分子在石墨烯、BC₃平面等二维硼碳基纳米结构上的吸附所表现出的各种物理性质及可能的应用.纯净的石墨烯为零带隙的半金属、无磁且自旋轨道耦合效应非常弱,BC₃平面为间接带隙半导体,但金属原子与非金属原子和分子的吸附可能使石墨烯体系在Dirac点处打开带隙、具有强自旋轨道耦合效应,可能使石墨烯体系与二维BC₃体系具有磁有序、超导电性及应用在氢存储上.另外石墨烯表现出非常好的分子探测性能.     

Scanning probe microscopy of graphene

A material with novel fundamental properties that challenge our current understanding is always exciting for research. If the novel properties extend to the realm of device engineering and promise a revolution in applications, then the scope of its research knows no bounds. The story of graphene, the two dimensional form of carbon, has followed this path. Graphene has been the subject of numerous experimental and theoretical investigations since 2004 when an elegant and a simple technique to make monolayer graphene set the stage for extensive research. Many other techniques to make graphene were developed in parallel to this technique. As graphene is replete with unique structural and electronic properties scanning probe microscopy has proved to be an exciting and a rewarding venture. In this review we discuss the findings of scanning probe microscopy and how it has served as an indispensable tool to understand the properties of graphene and further graphene research.     

Electronic properties and quantum transport in Graphene-based nanostructures

Carbon nanotubes (CNTs) and graphene nanoribbons (GNRs) represent a novel class of low-dimensional materials. All these graphene-based nanostructures are expected to display the extraordinary electronic, thermal and mechanical properties of graphene and are thus promising candidates for a wide range of nanoscience and nanotechnology applications. In this paper, the electronic and quantum transport properties of these carbon nanomaterials are reviewed. Although these systems share the similar graphene electronic structure, confinement effects are playing a crucial role. Indeed, the lateral confinement of charge carriers could create an energy gap near the charge neutrality point, depending on the width of the ribbon, the nanotube diameter, the stacking of the carbon layers regarding the different crystallographic orientations involved. After reviewing the transport properties of defect-free systems, doping and topological defects (including edge disorder) are also proposed as tools to taylor the quantum conductance in these materials. Their unusual electronic and transport properties promote these carbon nanomaterials as promising candidates for new building blocks in a future carbon-based nanoelectronics, thus opening alternatives to present silicon-based electronics devices.     

Graphene in high magnetic fields

Carbon-based nano-materials, such as graphene and carbon nanotubes, represent a fascinating research area aiming at exploring their remarkable physical and electronic properties. These materials not only constitute a playground for physicists, they are also very promising for practical applications and are envisioned as elementary bricks of the future of the nano-electronics. As for graphene, its potential already lies in the domain of opto-electronics where its unique electronic and optical properties can be fully exploited. Indeed, recent technological advances have demonstrated its effectiveness in the fabrication of solar cells and ultra-fast lasers, as well as touch-screens and sensitive photo-detectors. Although the photo-voltaic technology is now dominated by silicon-based devices, the use of graphene could very well provide higher efficiency. However, before the applied research to take place, one must first demonstrates the operativeness of carbon-based nano-materials, and this is where the fundamental research comes into play. In this context, the use of magnetic field has been proven extremely useful for addressing their fundamental properties as it provides an external and adjustable parameter which drastically modifies their electronic band structure. In order to induce some significant changes, very high magnetic fields are required and can be provided using both DC and pulsed technology, depending of the experimental constraints. In this article, we review some of the challenging experiments on single nano-objects performed in high magnetic and low temperature. We shall mainly focus on the high-field magneto-optical and magneto-transport experiments which provided comprehensive understanding of the peculiar Landau level quantization of the Dirac-type charge carriers in graphene and thin graphite.     

Density functional theory study on the adsorption of alkali metal ions with pristine and defected graphene sheet

The graphene-based materials along with the adsorption of alkali metal ions are suitable for energy conversion and storage applications. Hence in the present work, we have investigated the structural and electronic properties of pristine and defected graphene sheet upon the adsorption of alkali metal ions (Li⁺, Na⁺, and K⁺) using density functional theory (DFT) calculations. The presence of vacancies or vacancy defects enhances the adsorption of alkali ions than the pristine sheet. From the obtained results, it is found that the adsorption energy of Li⁺ on the vacancies defected graphene sheet is higher (3.05?eV) than the pristine (2.41?eV) and Stone–Wales (2.50?eV) defected sheets. Moreover, the pore radius of the pristine and defected graphene sheets are less affected by metal ions adsorption. The increase in energy gap upon the adsorption of metal ions is found to be high in the vacancy defected graphene than that of other sheets. The metal ions adsorption in the defective vacancy sheets has high charge transfer from metal ions to the graphene sheet. The bonding characteristic between the metal ions and graphene sheet are analysed using QTAIM analysis. The influence of alkali ions on the electronic properties of the graphene sheet is examined from the Total Density of States (TDOS) and Partial Density of States (PDOS).     

Investigation of the transport mechanism in graphene-induced metal-oxide-semiconductor capacitors (MOSCAP)

Graphene is the most promising contender for the future generation of electronic and photonic devices, based on its extraordinary properties. The effect of the metal interface with graphene, however, which completely alters its properties, is of great importance. The effects of the substrate supporting the graphene matrix, the graphene/metal contact resistance and the overall metal oxide semiconductor capacitors (MOSCAP) for possible CMOS circuitry have been thoroughly investigated in this research work. We have fabricated a structure with pertinent deposition techniques and performed a detailed electrical analysis to obtain the transport characteristics. Nickel (Ni) is chosen as the transition metal which makes the chemisorption bonding with graphene while qualifying as an interface. We present an analysis of the metal contacts, a study of the metal resistivity at various planes, a study of the graphene (carbon) atom's resistance at the atomistic scale, the graphene based MOSCAP leakages, the necessary charge accumulation at the metal–graphene interface and the charge inversion just beneath the oxide layer.     

Phonon dispersions in graphene sheet and single-walled carbon nanotubes

In the present research paper, phonons in graphene sheet have been calculated by constructing a dynamical matrix using the force constants derived from the second-generation reactive empirical bond order potential by Brenner and co-workers. Our results are comparable to inelastic X-ray scattering as well as first principle calculations. At Γ point, for graphene, the optical modes (degenerate) lie near 1685 cm^???1. The frequency regimes are easily distinguishable. The low-frequency (ω→ 0) modes are derived from acoustic branches of the sheet. The radial modes can be identified with ω→ 584 cm^???1. High-frequency regime is above 1200 cm^???1 (i.e. ZO mode) and consists of TO and LO modes. The phonons in a nanotube can be derived from zone folding method using phonons of a single layer of the hexagonal sheet. The present work aims to explore the agreement between theory and experiment. A better knowledge of the phonon dispersion of graphene is highly desirable to model and understand the properties of carbon nanotubes. The development and production of carbon nanotubes (CNTs) for possible applications need reliable and quick analytical characterization. Our results may serve as an accurate tool for the spectroscopic determination of the tube radii and chiralities.     

Computer simulation and study of radiation defects in graphene

Computer simulation has been used to study the properties of radiation defects in a graphene sheet. Several possible stable configurations of 3D radiation defects in graphene have been simulated. Several types of absorbed species were calculated involving a single carbon atom and a dumbbell absorbed at graphene sheet surface and a dumbbell configuration at a vacancy created by radiation knocking out a carbon atom. The calculations also included the effect of structural relaxation. The defect caused by the atom being displaced from the structure converted the sheet from metallic to semiconducting.     

Decorated graphyne and its boron nitride analogue as versatile nanomaterials for CO detection

The sensitivity of a new two-dimensional (2D) carbon allotrope built from sp- and sp²-hybridised carbon atoms, graphyne (GY), as well as its boron nitride analogue (BN-yne) towards CO molecule has been theoretically investigated. Indeed, a theoretical understanding of the interaction between gas molecules and extended carbon-based network structures is crucial for developing new materials that could have a wide range of applications. Here, we report our first-principles calculations to explore the impact of metal decoration on the GY and BN-yne upon the CO adsorption. We predict that Ca and Li decorations significantly enhance the CO-sensing ability of the GY and BN-yne compared to that of their pristine sheets. Owing to strong interactions between CO and the decorated GY and BN-yne, dramatic changes in the electronic properties of the sheets together with large band gap variations were observed. The present study sheds a deep insight into the sensing properties of the novel carbon-based 2D structures beyond the graphene sheet.     

金属催化制备石墨烯的研究进展

石墨烯作为一种新兴的碳素材料,从一出现就引起了众多学者的关注.石墨烯具有许多新奇的特性,使得石墨烯在光电领域及微电子工业等有极大的应用潜力.但是目前难以实现大尺寸、高质量、宏量石墨烯的可控制备,限制了石墨烯的广泛应用.本文分析了各种石墨烯制备方法的利弊,重点从层数控制及大面积制备等方面对金属催化法进行了阐述,固态碳源金属催化法可以实现宏量制备大尺寸、高质量、薄且均匀的石墨烯.综述了金属催化制备石墨烯的相关机理研究,指出了目前研究的局限,并对石墨烯相变机理的下一步研究方向进行了展望.     

Carbon nanotubes: opportunities and challenges

Carbon nanotubes are graphene sheets rolled-up into cylinders with diameters as small as one nanometer. Extensive work carried out worldwide in recent years has revealed the intriguing electrical and mechanical properties of these novel molecular scale wires. It is now well established that carbon nanotubes are ideal model systems for studying the physics in one-dimensional solids and have significant potential as building blocks for various practical nanoscale devices. Nanotubes have been shown to be useful for miniaturized electronic, mechanical, electromechanical, chemical and scanning probe devices and materials for macroscopic composites. Progress in nanotube growth has facilitated the fundamental study and applications of nanotubes. Gaining control over challenging nanotube growth issues is critical to the future advancement of nanotube science and technology, and is being actively pursued by researchers.     

Introduction to graphene electronics – a new era of digital transistors and devices

The speed of silicon-based transistors has reached an impasse in the recent decade, primarily due to scaling techniques and the short-channel effect. Conversely, graphene (a revolutionary new material possessing an atomic thickness) has been shown to exhibit a promising value for electrical conductivity. Graphene would thus appear to alleviate some of the drawbacks associated with silicon-based transistors. It is for this reason why such a material is considered one of the most prominent candidates to replace silicon within nano-scale transistors. The major crux here, is that graphene is intrinsically gapless, and yet, transistors require a band-gap pertaining to a well-defined ON/OFF logical state. Therefore, exactly as to how one would create this band-gap in graphene allotropes is an intensive area of growing research. Existing methods include nanoribbons, bilayer and multi-layer structures, carbon nanotubes, as well as the usage of the graphene substrates. Graphene transistors can generally be classified according to two working principles. The first is that a single graphene layer, nanoribbon or carbon nanotube can act as a transistor channel, with current being transported along the horizontal axis. The second mechanism is regarded as tunnelling, whether this be band-to-band on a single graphene layer, or vertically between adjacent graphene layers. The high-frequency graphene amplifier is another talking point in recent research, since it does not require a clear ON/OFF state, as with logical electronics. This paper reviews both the physical properties and manufacturing methodologies of graphene, as well as graphene-based electronic devices, transistors, and high-frequency amplifiers from past to present studies. Finally, we provide possible perspectives with regards to future developments.     

Theoretical investigation of manganese adsorption on graphene and graphane: A first-principles comparative study

Within the framework of spin-polarized generalized gradient approximation (σGGA) of the density functional theory (DFT) and pseudopotential method, the structural, magnetic, and electronic properties of graphene and graphane upon the adsorption of manganese atoms have been theoretically investigated. In contrast to the recent results (New J. Phys. 12, 063020 (2010)), Mn atom has been found to be adsorbed on a hollow-site configuration and no appreciable indication to substitute one of the C atoms of the graphene sheet. Unlike the recent results on Mn-doped graphane (Carbon 48, 3901 (2010)), the Mn adatom prefers to adsorb on the top of a carbon atom, forming a bridge with the uppermost hydrogen atoms. The magnetic moment of the Mn-doped graphene is found to be larger than that of the Mn-doped graphane. The structural parameters and electronic properties of both Mn-doped graphene and Mn-doped graphane are determined and compared with the available data.     

Graphene grown on transition metal substrates: Versatile templates for organic molecules with new properties and structures

The interest in graphene (a carbon monolayer) adsorbed on metal surfaces goes back to the 60's, long before isolated graphene was produced in the laboratory. Owing to the carbon-metal interaction and the lattice mismatch between the carbon monolayer and the metal surface, graphene usually adopts a rippled structure, known as moiré, that confers it interesting electronic properties not present in isolated graphene. These moiré structures can be used as versatile templates where to adsorb, isolate and assemble organic-molecule structures with some desired geometric and electronic properties. In this review, we first describe the main experimental techniques and the theoretical methods currently available to produce and characterize these complex systems. Then, we review the diversity of moiré structures that have been reported in the literature and the consequences for the electronic properties of graphene, attending to the magnitude of the lattice mismatch and the type of interaction, chemical or physical, between graphene and the metal surface. Subsequently, we address the problem of the adsorption of single organic molecules and then of several ones, from dimers to complete monolayers, describing both the different arrangements that these molecules can adopt as well as their physical and chemical properties. We pay a special attention to graphene/Ru(0001) due to its exceptional electronic properties, which have been used to induce long-range magnetic order in tetracyanoquinodimethane (TCNQ) monolayers, to catalyze the (reversible) reaction between acetonitrile and TCNQ molecules and to efficiently photogenerate large acenes.     

Comparative Study of Monolayer and Bilayer Epitaxial Graphene Field-Effect Transistors on SiC Substrates

Monolayer and bilayer graphenes have generated tremendous excitement as the potentially useful electronic materials due to their unique features.We report on monolayer and bilayer epitaxial graphene field-effect transistors(GFETs)fabricated on SiC substrates.Compared with monolayer GFETs,the bilayer GFETs exhibit a significant improvement in dc characteristics,including increasing current density Ids,improved transconductance g_m,reduced sheet resistance R_(on),and current saturation.The improved electrical properties and tunable bandgap in the bilayer graphene lead to the excellent dc performance of the bilayer GFETs.Furthermore,the improved dc characteristics enhance a better rf performance for bilayer graphene devices,demonstrating that the quasifree-standing bilayer graphene on SiC substrates has a great application potential for the future graphene-based electronics.     

Weyl-gauge symmetry of graphene

The conformal invariance of the low energy limit theory governing the electronic properties of graphene is explored. In particular, it is noted that the massless Dirac theory in point enjoys local Weyl symmetry, a very large symmetry. Exploiting this symmetry in the two spatial dimensions and in the associated three dimensional spacetime, we find the geometric constraints that correspond to specific shapes of the graphene sheet for which the electronic density of states is the same as that for planar graphene, provided the measurements are made in accordance to the inner reference frame of the electronic system. These results rely on the (surprising) general relativistic-like behavior of the graphene system arising from the combination of its well known special relativistic-like behavior with the less explored Weyl symmetry. Mathematical structures, such as the Virasoro algebra and the Liouville equation, naturally arise in this three-dimensional context and can be related to specific profiles of the graphene sheet. Speculations on possible applications of three-dimensional gravity are also proposed.     

Silicene: compelling experimental evidence for graphenelike two-dimensional silicon

Because of its unique physical properties, graphene, a 2D honeycomb arrangement of carbon atoms, has attracted tremendous attention. Silicene, the graphene equivalent for silicon, could follow this trend, opening new perspectives for applications, especially due to its compatibility with Si-based electronics. Silicene has been theoretically predicted as a buckled honeycomb arrangement of Si atoms and having an electronic dispersion resembling that of relativistic Dirac fermions. Here we provide compelling evidence, from both structural and electronic properties, for the synthesis of epitaxial silicene sheets on a silver (111) substrate, through the combination of scanning tunneling microscopy and angular-resolved photoemission spectroscopy in conjunction with calculations based on density functional theory.     

石墨烯纳米带的制备与电学特性调控

石墨烯由于其独特的晶体结构展现出了特殊的电学特性,其导带与价带相交于第一布里渊区的六个顶点处,形成带隙为零的半金属材料,具有优异的电子传输特性的同时也限制了其在电子学器件中的使用.因而科研人员尝试各种方法来打开其带隙并调控其能带特性,主要有利用缺陷、应力、掺杂、表面吸附、结构调控等手段.其中石墨烯纳米带由于量子边界效应和限制效应,存在带隙.本综述主要介绍了制备各类石墨烯纳米带的方法,并通过精确调控其细微结构,从而对其进行精确的能带调控,改变其电学特性,为其在电子学器件中的应用提供一些可行的方向.