Graphene Honeycomb Structure Research Articles

Topological regulations of Stone-Wales graphene

Designed synthesis of graphene-based materials containing Stone-Wales defects represents a promising strategy for graphene modification, because of their highly diverse structures and tunable properties. In graphene-based materials, Stone-Wales defects are caused by the in-plane atomic rearrangement and re-linkage of graphene's honeycomb structure. The resultant Stone-Wales graphene's are topoisomers of graphene. Topology is naturally a major factor in regulating the structures and properties of Stone-Wales graphene, but the regulation rules are mostly unclear. In this study, 478 low-energy Stone-Wales graphene allotropes with diverse topology are predicted using topology-based global optimization, for which the thermodynamic, electronic, and elastic properties are evaluated using the first-principles calculations. The dependencies of the predicated physical properties on topological quantities and patterns are explicitly determined. The current study may lay the groundwork for design, synthesis, and property tuning of graphene-based materials.

Density functional study of twisted graphene L1-FePd heterogeneous interface

Graphene on L10-FePd(001), which has been experimentally studied in recent years, is a heterogeneous interface with a significant lattice symmetry mismatch between the honeycomb structure of graphene and tetragonal alloy surface. In this work, we report on the density functional study of its atomic-scale configurations, electronic and magnetic properties, and adsorption mechanism, which have not been well understood in previous experimental studies. We propose various atomic-scale models, including simple nontwisted and low-strain twisted interfaces, and analyze their energetical stability by performing structural optimizations using the van der Waals interactions of both DFT-D2 and optB86b-vdW functionals. The binding energy of the most stable structure reached EB=−0.22 eV/atom for DFT-D2 (EB=−0.19 eV/atom for optB86b-vdW). The calculated FePd-graphene spacing distance was approximately 2 Å, which successfully reproduced the experimental value. We also find out characteristic behaviors: the modulation of π-bands, the suppression of the site-dependence of adsorption energy, and the rise of moiré-like corrugated buckling. In addition, our atomic structure is expected to help build low-cost computational models for investigating the physical properties of L10 alloys/two-dimensional interfaces.

A mathematical basis for the graphene

We present a new basis of representation for the graphene honeycomb structure that facilitates the solution of the eigenvalue problem by reducing it to one dimension. We define spaces in these geometrical basis that allow us to solve the Hamiltonian in the edges of the lattice. We conclude that it is enough to analyze a one-dimensional problem in a set of coupled ordinary second-order differential equations to obtain the behavior of the solutions in the whole graphene structure.

Point and Space Groups of Graphene

Graphene — the first discovered ideal two-dimensional material combines a wealth of exhibited novel physical phenomena with an overwhelming potential for practical applications. It is a one atom thick material where the carbon atoms sit on a honeycomb lattice. The honeycomb lattice is the cause for the many fascinating properties that graphene turns up with. Conventionally a material scientist would first like to know the symmetry that any newly discovered material possesses, as this can help determine the physical properties of the material. In this article, we discuss the space and point symmetry groups that the hallmark honeycomb structure of graphene possesses.

Monolayer graphene electrodes as alignment layer for ferroelectric liquid crystal devices

Transparent electrodes and alignment layers are two essential parts of the liquid crystal (LC) cell fabrication. Conventionally, indium tin oxide (ITO) acts as transparent electrode and a polyimide film is used as alignment layer. In present study, both of these are replaced by using a monolayer graphene film deposited on glass substrate. The graphene film used to fabricate LC cell show excellent optical transmittance (~95%) over 450–800 nm range and electrical resistance of 328.77 Ω/square. Moreover, the alignment of FLC mesogens over graphene monolayer is achieved owing to the π-π electron stacking between benzene rings of FLC and honeycomb structure of graphene. Polarizing optical microscopy (POM) shows homogeneous planar alignment of filled FLC over graphene under crossed polarizers. The cell is switched between bright and dark states under the application of electric field to demonstrate the working of LC cell. Further, dielectric relaxation spectroscopy is used to measure the dielectric constant and absorption of FLC. This application of graphene would lead to thin and defect-free devices based on LC.

A novel green approach for reduction of free standing graphene oxide: electrical and electronic structural investigations

Ion beam irradiation technique has been proposed, for efficient, fast and eco-friendly reduction of graphene oxide (GO), as an alternative to the conventional methods. 5 MeV, Au+ ion beam has been used to reduce the free standing GO flake. Both electronic and nuclear energy loss mechanisms of the irradiation process play a major role in removal of oxygen moieties and recovery of graphene network. Atomic resolution scanning tunnelling microscopy analysis of the irradiated GO flake shows the characteristic honeycomb structure of graphene. X-ray absorption near edge structure analysis at C K-edge reveals that the features of the irradiated GO flake resemble the few layer graphene. Resonant Rutherford backscattering spectrometry analysis evidenced an enhanced C/O ratio of ∼23 in the irradiated GO. In situ sheet resistance measurements exhibit a sharp decrease of resistance (few 100 s of Ω) at a fluence of 6.5 × 1014 ions cm−2. Photoluminescence spectroscopic analysis of irradiated GO shows a sharp blue emission, while pristine GO exhibits a broad emission in the visible—near IR region. Region selective reduction, tunable electrical and optical properties by controlling C/O ratio makes ion irradiation as a versatile tool for the green reduction of GO for diverse applications.

Fabrication of the graphene honeycomb structure as a scaffold for the study of cell growth

The good biocompatibility of the graphene honeycomb structure was evaluated by the proliferation of HeLa and MCF-7 cell lines.

Dynamical Strain‐Induced Charge Pumping in Monolayer Graphene

Emergence of high pseudo‐magnetic field as a result of mechanical deformations is one of the intriguing characteristics of the graphene honeycomb structure. For a time‐dependent nonuniform strain and in the context of Berry curvature approach, the topological charge pumping is studied theoretically. Calculations have been performed in time–momentum parametric space. Within the Berry curvature approach, it has been revealed that time‐dependent deformations result in nonzero valley‐dependent charge pumping in the gapped graphene when there is a population imbalance between the valleys. This indicates that the valley polarization can be measured by the amount of topological charge pumping in the system. Results also show that the strain can remove the valley degeneracy at nonzero Fermi energies. Therefore, population imbalance or valley polarization can be realized as a result of the external strain. Strain has been considered as gauge field that couples oppositely with two valleys of the Brillouin zone.

Nanostructured graphene for spintronics

Zigzag edges of the honeycomb structure of graphene exhibit magnetic polarization making them attractive as building blocks for spintronic devices. Here, we show that devices with zigzag edged triangular antidots perform essential spintronic functionalities, such as spatial spin-splitting or spin filtering of unpolarized incoming currents. Near-perfect performance can be obtained with optimized structures. The device performance is robust against substantial disorder. The gate-voltage dependence of transverse resistance is qualitatively different for spin-polarized and spin-unpolarized devices, and can be used as a diagnostic tool. Importantly, the suggested devices are feasible within current technologies.

Topologically Protected States in One-Dimensional Systems

We study a class of periodic Schrodinger operators, which we prove have Dirac points. We then show that the introduction of an via adiabatic modulation of a periodic potential by a domain wall results in the bifurcation of spatially localized \edge states, associated with the topologically protected zero- energy mode of an asymptotic one-dimensional Dirac operator. The bound states we construct can be realized as highly robust TM- electromagnetic modes for a class of photonic waveguides with a phase-defect. Our model also captures many aspects of the phenomenon of topologically protected edge states for two-dimensional bulk structures such as the honeycomb structure of graphene.

Graphene and liquid crystal mediated interactions

ABSTRACTThe two-dimensional graphene-honeycomb structure can interact with the liquid crystal’s (LC) benzene rings through π–π electron stacking. This LC–graphene interaction gives rise to a number of interesting physical and optical phenomena in the LC. In this paper, we present a combination of a review and original research of the exploration of novel themes of LC ordering at the nanoscale graphene surface and its macroscopic effects on the LC’s nematic and smectic phases. We show that monolayer graphene films impose planar alignment on the LC, creating pseudo-nematic domains (PNDs) at the surface of graphene. In a graphene-nematic suspension, these PNDs enhance the orientational order parameter, exhibiting a giant enhancement in the dielectric anisotropy of the LC. These anisotropic domains interact with the external electric field, resulting in a non-zero dielectric anisotropy in the isotropic phase as well. We also show that graphene flakes in an LC reduce the free ion concentration in the nematic media by an ion-trapping process. The reduction of mobile ions in the LC is found to have subsequent impacts on the LC’s rotational viscosity, allowing the nematic director to respond quicker on switching the electric field on and off. In a ferroelectric LC (smectic-C* phase), suspended graphene flakes enhance the spontaneous polarisation by improving the tilted smectic-C* ordering resulting from the π–π electron stacking. This effect accelerates the ferroelectric-switching phenomenon. Graphene can possess strain chirality due to a soft shear mode. This surface chirality of graphene can be transmitted into LC molecules exhibiting two types of chiral signatures in the LCs: an electroclinic effect (a polar tilt of the LC director perpendicular to, and linear in, an applied electric field) in the smectic-A phase, and a macroscopic helical twist of the LC director in the nematic phase. Finally, we show that a graphene-based LC cell can be fabricated without using any aligning layers and ITO electrodes. Graphene itself can be used as the electrodes as well as the aligning layers, obtaining an electro-optic effect of the LC inside the cell.

Simulating structural transitions with kinetic Monte Carlo: The case of epitaxial graphene on SiC.

We have developed a kinetic Monte Carlo numerical scheme, specifically suited to simulate structural transitions in crystalline materials, and implemented it for the case of epitaxial graphene on SiC. In this process, surface Si atoms selectively sublimate, while the residual C atoms rearrange from a position occupied in the SiC hexagonal lattice to the graphene honeycomb structure, modifying their hybridization (from sp(3) to sp(2)) and bond partners (from Si-C to C-C). The model is based on the assumption that the Monte Carlo particles follow the evolution of their reference crystal until they experience a thermally activated reversible transition to another crystal structure. We demonstrate that, in a formulation based on three parallel lattices, the method is able to recover the complex evolution steps of epitaxial graphene on SiC. Moreover, the simulation results are in noteworthy agreement with the overall experimental scenario, both when varying the structural properties of the material (e.g., the initial surface configuration or polarity) as well as the process conditions (e.g., the temperature and pressure).

Nano-electromechanical rotation of graphene and giant enhancement in dielectric anisotropy in a liquid crystal

A nematic liquid crystal (LC) is doped with dilute concentrations of pristine monolayer graphene (GP) flakes, and the LC + GP hybrids are found to exhibit a dramatic increase in the dielectric anisotropy. Electric field-dependent conductance studies reveal that the graphene flakes follow the nematic director that mechanically rotates on increasing an applied electric field. Further studies show that the π–π electron stacking, between the graphene's honeycomb structure and the LC's benzene rings, stabilizes pseudo-nematic domains that collectively amplify the dielectric anisotropy by improving the orientational order parameter in the nematic phase. These anisotropic domains interact with the external electric field, resulting in a nonzero dielectric anisotropy in the isotropic phase as well. The enhancement in dielectric anisotropy, due to the LC–graphene coupling, is found to have subsequent positive impacts on the LC's orientational threshold field and elasticity that allows the nematic director to respond quicker on switching the electric field off.

A Comparison of Electrochemical ORR Activity of Boron in Graphene Oxide; Incorporated As a Charge-Adsorbate and/or Substitutional p-Type Dopant

The present contribution aims to examine electrocatalytic oxygen reduction (ORR) performance of boron (B) in two diverse classes of B-doped graphitic materials; (i) 3-D graphene organic framework (GOF) as a descriptive of adsorbate-induced doped materials and (ii) BGs as illustration of the substitutionally boron-integrated graphene. Pillared GOF is easily synthesised under mild solvothermal condition (reaction temperature of 80 °C for 8 h) by reacting graphene oxide (GO) suspension with 1,4 phenyl diboronic acid as stitches to graphene layers. Trivalent boron is integrated into sp2 graphene honeycomb structure using thermal annealing method which is believed to produce fewer layer graphene with less agglomeration, higher surface area and electrical conductivity. Here, for tuneable boron surrogation, mixtures of GO and boric acid (H3BO3, as precursor) are annealed at elevated temperatures of 600 and 1200 °C to form substitutionally boron-doped BG1 and BG2. X-ray photoelectron spectroscopy (XPS) analysis verified the total embedded boron contents of 1.90 and 4.19 at% corresponding to BCO2 and BCO2/ BC3 structures in BG1 and BG2, respectively. The electron transfer efficiency of the state of the art pillared boronate lattices with CBO2 active chemistries is examined alongside the commonly literature cited boron doped materials (BGs) via cyclic voltammetry (CV) and rotating ring disk (RDE) tests. Generally, collected CV data confirms noticeable catalytic modulation of pristine GO by synergistic electron accepting-donating effect of amalgamated boron in GOF and BGs. In fact, the difference in electronegativity of boron and its neighbouring (C/O) atoms’ provides positively charged site for O2 chemisorption. Subsequently, B enables to donate a portion of accumulated C=C π conjugation to the adsorbed oxygen from its 2pz orbital and finally reduces the adsorbed oxygen. The materials are also subjected to RDE test to obtain Koutechy-Levich plots for estimating the total number of exchanged electrons during O2 reduction. According to cathodic responses, BG1 completes the ORR in a way similar to that of reduced graphene oxide (rGO) through a 2e- pathway with H2O2 production. This in turn highlights the insignificant functionality of boron during O2 reduction when it is in CBO2 form. However, ahead of incorporation of three-coordinated BC3 in BG2 sample, the reaction is managed by a 4e- reduction mechanism to produce H2O. This enlightens the frontier-controlling role of boron as an effective donating pole when it is in BC3 configuration. Surprisingly, GOF shows an exclusive performance comparable to that of BG2 (with highly active BC3 configuration) which is synthesised under much more rigorous thermal condition. To conclude, GOF has potential to be considered as a rival to previously introduced graphene doped materials for replacing the expensive Pt/C cathodic catalyst in fuel cells. The advantage of GOF compared to conventional BG materials through literature is considered to be due to the merits of moderate hydrothermal synthesis conditions and hence scale-up capability. Figure 1

Structure and dynamics of TIP3P, TIP4P, and TIP5P water near smooth and atomistic walls of different hydroaffinity.

We perform molecular dynamics simulations to observe the structure and dynamics of water using different water models (TIP3P, TIP4P, TIP5P) at ambient conditions, constrained by planar walls, which are either modeled by smooth potentials or regular atomic lattices, imitating the honeycomb-structure of graphene. We implement walls of different hydroaffinity, different lattice constant, and different types of interaction with the water molecules. We find that in the hydrophobic regime the smooth wall generally represents a good abstraction of the atomically rough walls, while in the hydrophilic regime there are noticeable differences in structure and dynamics between all stages of wall roughness. For a small lattice constant however the smooth and the atomically rough wall still share a number of structural and dynamical similarities. Out of the three water models, TIP5P water shows the largest degree of tetrahedral ordering and is often the one that is least perturbed by the presence of the wall.

Antiferro quadrupolar ordering in Fe intercalated few layers graphene

The π electron cloud above and below the honeycomb structure of graphene causes each carbon atom to carry a permanent electric quadrupole moment which can attach any cation to impart interesting physical properties. We have synthesized Fe intercalated graphene structures to investigate tunable magnetic properties as a result of this chemical modification. An interesting antiferro quadrupolar ordering is observed which arises due to a coupling between magnetic dipole moment of Fe and electric quadrupole moment on graphene surface. In contrast to antiferromagnetic Neel temperature (TN), here the ordering temperature (TQ) increases from 35.5 K to 47.5 K as the magnetic field is raised upto 1 Tesla.

Controllable modulation of the electronic properties of graphene and silicene by interface engineering and pressure

Using first-principles calculations, we show that the band gap and electron effective mass (EEM) of D-X/G/H-D, Si-X/S/H-Si and D-X/S/H-D can be modulated effectively by tuning the pressure (interlayer spacing) and stacking arrangement. The electron effective mass (EEM) is proportional to the band gap. The band gap of confined silicene is more sensitive to pressure than that of confined graphene. Moreover, a heterogeneous interface structure would be beneficial for effectively regulating the band gap and carrier effective masses of confined graphene and silicene. Using a confinement technique and pressure, the integrity of the honeycomb structure of graphene and silicene will be preserved so that the small effective masses and high mobility of graphene and silicene will be retained during compression. The tunable band gap and high carrier mobility of the sandwich structures are promising for building high-performance nanodevices.

Role of pseudospin in quasiparticle interferences in epitaxial graphene probed by high-resolution scanning tunneling microscopy

Pseudospin, an additional degree of freedom related to the honeycomb structure of graphene, is responsible of many of the outstanding electronic properties found in this material. This article provides a clear understanding of how such pseudospin impacts the quasiparticle interferences of monolayer (ML) and bilayer (BL) graphene measured by low temperature scanning tunneling microscopy and spectroscopy. We have used this technique to map, with very high energy and space resolution, the spatial modulations of the local density of states of ML and BL graphene epitaxialy grown on SiC(0001), in presence of native disorder. We perform a Fourier transform analysis of such modulations including wavevectors up to unit-vectors of the reciprocal lattice. Our data demonstrate that the quasiparticle interferences associated to some particular scattering processes are suppressed in ML graphene, but not in BL graphene. Most importantly, interferences with 2qF wavevector associated to intravalley backscattering are not measured in ML graphene, even on the images with highest resolution. In order to clarify the role of the pseudospin on the quasiparticle interferences, we use a simple model which nicely captures the main features observed on our data. The model unambiguously shows that graphene's pseudospin is responsible for such suppression of quasiparticle interferences features in ML graphene, in particular for those with 2qF wavevector. It also confirms scanning tunneling microscopy as a unique technique to probe the pseudospin in graphene samples in real space with nanometer precision. Finally, we show that such observations are robust with energy and obtain with great accuracy the dispersion of the \pi-bands for both ML and BL graphene in the vicinity of the Fermi level, extracting their main tight binding parameters.