Complex structure of triangular graphene: electronic, magnetic and electromechanical properties

We have investigated electronic and magnetic properties of graphene nanodisks (nanosize triangular graphene) as well as electromechanical properties of graphene nanojunctions. Nanodisks are nanomagnets made of graphene, which are robust against perturbation such as impurities and lattice defects, where the ferromagnetic order is assured by Lieb's theorem. We can generate a spin current by spin filter, and manipulate it by a spin valve, a spin switch and other spintronic devices made of graphene nanodisks. We have analyzed nanodisk arrays, which have multi-degenerate perfect flat bands and are ferromagnet. By connecting two triangular graphene corners, we propose a nanomechanical switch and rotator, which can detect a tiny angle rotation by measuring currents between the two corners. By making use of the strain induced Peierls transition of zigzag nanoribbons, we also propose a nanomechanical stretch sensor, in which the conductance can be switched off by a nanometer scale stretching.     

Vapor trapping growth of single-crystalline graphene flowers: synthesis, morphology, and electronic properties

We report a vapor trapping method for the growth of large-grain, single-crystalline graphene flowers with grain size up to 100 μm. Controlled growth of graphene flowers with four lobes and six lobes has been achieved by varying the growth pressure and the methane to hydrogen ratio. Surprisingly, electron backscatter diffraction study revealed that the graphene morphology had little correlation with the crystalline orientation of underlying copper substrate. Field effect transistors were fabricated based on graphene flowers and the fitted device mobility could achieve ～4200 cm(2) V(-1) s(-1) on Si/SiO(2) and ～20?000 cm(2) V(-1 )s(-1) on hexagonal boron nitride (h-BN). Our vapor trapping method provides a viable way for large-grain single-crystalline graphene synthesis for potential high-performance graphene-based electronics.     

Nitrogen-doped graphene microtubes with opened inner voids: Highly efficient metal-free electrocatalysts for alkaline hydrogen evolution reaction

A facile method was developed to fabricate nitrogen-doped graphene microtubes (N-GMT) with ultra-thin walls of 1–4 nm and large inner voids of 1–2 μm. The successful introduction of nitrogen dopants afforded N-GMT more active sites for significantly enhanced hydrogen evolution reaction (HER) activity, achieving a current density of 10 mA·cm^–2 at overpotentials of 0.464 and 0.426 V vs. RHE in 0.1 and 6 M KOH solution, respectively. This HER performance surpassed that of the best metal-free catalyst reported in basic solution, further illustrating the great potential of N-GMT as an efficient HER catalyst for real applications in water splitting and chlor-alkali processes.

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The electronic properties of graphene and its bilayer

We present a discussion of some of the physical properties of graphene and its bilayer. In particular, we focus our attention on the calculation of the transparency of graphene and on the dependence of the energy gap of the biased graphene bilayer on the electronic density. We show that the transparency of graphene is controlled by the value of the fine structure constant over a frequency range from the infra-red to the ultra-violet. We derive the dependence of the energy gap of the graphene bilayer on the external applied electric field.     

Direct imaging of graphene edges: atomic structure and electronic scattering

We report an atomically resolved scanning tunneling microscopy investigation of the edges of graphene grains synthesized on Cu foils by chemical vapor deposition. Most of the edges are macroscopically parallel to the zigzag directions of graphene lattice. These edges have microscopic roughness that is found to also follow zigzag directions at atomic scale, displaying many ～120° turns. A prominent standing wave pattern with periodicity ～3a/4 (a being the graphene lattice constant) is observed near a rare-occurring armchair-oriented edge. Observed features of this wave pattern are consistent with the electronic intervalley backscattering predicted to occur at armchair edges but not at zigzag edges.     

Modelling the role of size, edge structure and terminations on the electronic properties of trigonal graphene nanoflakes

Graphene nanoflakes provide a range of opportunities for engineering graphene for future applications, due to the large number of configurational degrees of freedom associated with the addition of different types of corners and edge states in the structure. Since these materials can, in principle, span the molecular to macroscale dimensions, the electronic properties may also be discrete or continuous, depending on the application in mind. However, since the widespread use of graphene nanoflakes will require them to be predictable, stable and robust against variations associated with some degree of structural polydispersivity, the development of a complete understanding of the relationship between structure, properties and property dispersion is essential. In this paper we used electronic structure computer simulations to model the thermodynamic, mechanical and electronic properties of trigonal graphene nanoflakes with acute (highly reactive) corners. We find that these acute corners introduce new features that are different to the obtuse corners characteristic of hexagonal graphene nanoflakes, as well as different electronic states in the vicinity of the Fermi level. The structure and properties are sensitive to size and functionalization, and may provide new insights into the engineering of graphene nanoflake components.     

Formation and electronic properties of hydrogenated few layer graphene

Motivated by the controversial experimental conclusions on the affinity of few layer graphenes (FLGs) towards hydrogen plasma, we systematically investigate the hydrogenation of FLGs within the framework of density functional theory. The approaching hydrogen atoms from both sides of an FLG induce a structural transition from a layered structure into a hydrogen passivated thin diamond film (HP-TDF). The very low transition barrier of FLG hydrogenation indicates the feasibility of FLG hydrogenation through the proposed mechanism. The increasing formation energy with the thickness of FLGs implies that hydrogenation of single layer graphene is easier than that of FLG, which is in agreement with most experimental observations. Moreover, the electronic properties of HP-TDFs and the hydrogenated bilayer graphene ribbons are also studied.     

Connecting dopant bond type with electronic structure in N-doped graphene

Robust methods to tune the unique electronic properties of graphene by chemical modification are in great demand due to the potential of the two dimensional material to impact a range of device applications. Here we show that carbon and nitrogen core-level resonant X-ray spectroscopy is a sensitive probe of chemical bonding and electronic structure of chemical dopants introduced in single-sheet graphene films. In conjunction with density functional theory based calculations, we are able to obtain a detailed picture of bond types and electronic structure in graphene doped with nitrogen at the sub-percent level. We show that different N-bond types, including graphitic, pyridinic, and nitrilic, can exist in a single, dilutely N-doped graphene sheet. We show that these various bond types have profoundly different effects on the carrier concentration, indicating that control over the dopant bond type is a crucial requirement in advancing graphene electronics.     

An efficient preconditioning scheme for plane-wave-based electronic structure calculations

We propose a preconditioning scheme for diagonalizing large Hamiltonian matrices arising in first-principles plane-wave pseudopotential calculations. The new scheme is based on the Neumann expansion for the inverse, shifted Hamiltonian from which only a nonlocal part is omitted. The preconditioner is applied to a gradient vector using the fast Fourier transformation technique. In the framework of the Davidson-type diagonalization algorithm, we have found the present preconditioning scheme to be more efficient than widely accepted diagonal scaling methods.     

Atomic and electronic structure of Si dangling bonds in quasi-free-standing monolayer graphene

Nano Research - Si dangling bonds at the interface of quasi-free-standing monolayer graphene (QFMLG) are known to act as scattering centers that can severely affect carrier mobility. Herein, we...     

Review of functionalization,structure and properties of graphene/polymer composite fibers

Fibrous materials usually have good mechanical, heat-resistant, acid-resistant, alkali-resistant and moisture regained properties which originate from its composition, condensed structure and crosslinking styles. However, these materials often lack of good electrical conductivity, flame retardance, anti-static and anti-radiation properties which are desired for varied specific applications. Graphene, as a new emerging nanocarbon material, has some unique properties including superb thermal and electrical conductivity, strong mechanical and anti-corrosive property, extremely high surface area etc. Therefore, graphene has attracted extensive interests in recent years. Upon modification with graphene, fibers exhibit a number of enhanced or new properties such as adsorption performance, anti-bacteria, hydrophobicity and conductivity which are beneficial for broader applications. In this review, the strategies to modify the fibers with graphene and the corresponding effects on the fibers as well as the relevant applications in varied areas were discussed.     

Stable structure of platinum carbides: A first principles investigation on the structure,elastic, electronic and phonon properties

A comprehensive first principles study of structural, elastic, electronic, phonon and thermodynamical properties of novel metal carbide, platinum carbide (PtC) is reported within the density functional theory scheme. The ground state properties such as lattice constant, elastic constants, bulk modulus, shear modulus and finally the enthalpy of PtC in zinc blende (ZB) and rock-salt (RS) structures are determined. The energy band structure and electron density of states for the two phases of PtC are also presented. Of these phases zinc blende phase of PtC is found stable and phase transition from ZB to RS structure occurs at the pressure of about 37.58 GPa. The phonon dispersion curves and phonon DOS are also presented. All positive phonon modes in phonon dispersion curves of ZB-PtC phase indicate a stable phase for this structure. Within the GGA and harmonic approximation, thermodynamical properties are also investigated. All results reveal that the synthesized PtC would favor ZB phase. The compound is stiffer and ductile in nature.     

Structural, electronic and magnetic properties of manganese doping in the upper layer of bilayer graphene

First-principles spin-polarized calculations have been conducted to investigate the structural, electronic and magnetic properties of 3d transition metal Mn doping into two typical sites in the upper layer of bilayer graphene with the AB Bernal structure. One of the doping sites is above the center of a carbon hexagon of the lower graphene layer (called the H site) and the other is directly on top of a carbon atom of the lower graphene layer (called the T site). We found that Mn doping enlarges the interlayer distance in bilayer graphene. Charge density distribution indicates that the region between the upper and lower graphene layer has apparent covalent-bonding characters due to the Mn doping. In the spin-polarized band structure of H?site doping, the π and π(*) bands separate from each other at the Dirac point both in majority spin and minority spin. In the band structure of T site doping, the Fermi level is located above the Dirac point and moves to the conduction bands in majority spin and minority spin, making the bilayer graphene n doped. A high spin polarization of 95% is achieved due to the H site doping. The local moment of Mn for H and T site doping is reduced to 1.76?μ(B) and 1.88?μ(B), respectively, which are smaller than the value (5?μ(B)) in the free state.     

Atomic structure,electronic properties,and thermal stability of diamond-like nanowires and nanotubes

Density functional tight-binding calculations are used to investigate the structure, electronic properties, energy stability, and thermal behavior (0–1500 K) of extended monolithic (nanowires) and hollow (nanotubes) diamond-like carbon nanostructures. The results indicate that diamond-like nanowires and nanotubes may be both metallic and semiconducting, depending on their morphology and size. A new type of hybrid (sp ³ + sp ²) nanostructure is identified, which has the form of a monolithic diamond-like (sp ³) wire inside a graphite-like (sp ²) shell. Diamond-like nanowires are shown to be more stable than nanotubes of comparable size.     

Nitrogen-doped tungsten oxide nanowires: low-temperature synthesis on Si, and electrical, optical, and field-emission properties

Very dense and uniformly distributed nitrogen-doped tungsten oxide (WO(3)) nanowires were synthesized successfully on a 4-inch Si(100) wafer at low temperature. The nanowires were of lengths extending up to 5 mum and diameters ranging from 25 to 35 nm. The highest aspect ratio was estimated to be about 200. An emission peak at 470 nm was found by photoluminescence measurement at room temperature. The suggested growth mechanism of the nanowires is vapor-solid growth, in which gaseous ammonia plays a significant role to reduce the formation temperature. The approach has proved to be a reliable way to produce nitrogen-doped WO(3) nanowires on Si in large quantities. The direct fabrication of WO(3)-based nanodevices on Si has been demonstrated.     

Multiscale simulations of carbon nanotube nucleation and growth: electronic structure calculations

Several first-principles surface and bulk electronic structure calculations relating to the nucleation and growth of single-wall carbon nanotubes are described. Density-functional theory in various forms is used throughout. In the surface-related calculations, a 38-atom Ni cluster and several low-index Ni surfaces are investigated using pseudopotentials and plane-wave expansions. The energetic ordering of the sites for C atom adsorption is found to be the same, with the Ni(100) facet favored. The bulk diffusion coefficient of C in Ni as a function of cluster size and temperature is calculated from various molecular dynamics approaches. In another group of bulk-related calculations, Gaussian orbital basis sets are used to study a cluster or "flake" containing 14 C atoms. The flake is a segment of three hexagons from an "unrolled" carbon nanotube, with an armchair termination. The binding energies of C, Ni, Co, Fe, Cu, and Au atoms to it were calculated in an effort to gain insight into the mechanism for the high catalytic activity of Ni, Co, and Fe and the lack of it in Cu and Au. The binding energies of Cu and Au are about 1 eV less than those of the three catalytic elements. Similar methods are used to study the initial stages of nanotube growth within the context of classical nucleation theory. Finally, issues relating to the establishment of a fundamental catalytic mechanism are addressed.