Graphene Honeycomb Research Articles

Enhanced Photocurrent Generation of Graphene/Au@ZnO Honeycomb Film

A bio‐inspired graphene/Au@ZnO photoelectrode has been prepared via breath figure method, in which Au@ZnO nanospheres were uniformly distributed in the whole honeycomb film. The size of the honeycomb holes effects the light using efficiency. The honeycomb film with smaller holes in more ordered array shows better antireflective property. All the formed graphene/Au@ZnO honeycomb photoelectrodes show a fast, stable, and reversible response of photocurrent accompanied by each switch‐on and switch‐off event. Au@ZnO‐modified graphene honeycomb film can combine the advantages of increased light harvesting provided by honeycomb structure, efficient charge separation from Au nanoparticles (NPs), and efficient electron transfer provided by graphene. Au@ZnO‐ modified graphene honeycomb film shows a two‐fold increase of photocurrent generation than ZnO‐modified graphene honeycomb film and a three‐fold increase of photocurrent generation than Au@ZnO‐modified graphene smooth film, respectively. The rational design and engineering of multi components with different functions in a hybrid bio‐inspired structure hold great promise for further efficient solar energy conversion devices.

Constructing Three-Dimensional Honeycombed Graphene/Silicon Skeletons for High-Performance Li-Ion Batteries.

Silicon has been considered to be an attractive high-capacity anode material for next-generation lithium-ion batteries (LIBs). Currently, the commercial application of Si-based anodes is still restricted by its limited cycle life and rate capacity, which could be ascribed to the colossal volumetric change during the cycling process and poor electronic conductivity. We report the design of a unique Si-based nanocomposite of three-dimensional (3D) honeycombed graphene aerogel and the reduced graphene oxide sheets preprotected silicon secondary particles (SiNPs@rGO1). Through simple electrostatic self-assembly and hydrothermal processes, SiNPs are able to be wrapped with rGO1 to form reunited SiNPs@rGO1, and embedded into the backbone of 3D graphene honeycomb (rGO2). Such an intriguing design (namely, SiNPs@rGO1/rGO2) not only provides a conductive skeleton to improve the electrical conductivity, but also possesses abundant void spaces to accommodate the dramatic volume changes of SiNPs. Meanwhile, the outer rGO1 coats protect the inner SiNPs away from the electrolyte and prevent the destruction of the solid electrolyte interphase (SEI) film. As a result, the 3D honeycombed architecture achieves a high cyclability and excellent rate capability.

Spin-valley skyrmions in graphene at filling factor ν=−1

We model quantum Hall skyrmions in graphene monolayer at quarter filling by a theory of CP3 fields and study the energy minimizing skyrmions in presence of valley pseudospin anisotropy and Zeeman coupling. We present a diagram of all types of skyrmions in a wide range of the anisotropy parameters. For each type of skyrmion, we visualize it on three Bloch spheres, and present the profiles of its texture on the graphene honeycomb lattice, thus providing references for the STM/STS imaging of spin-pseudospin textures in graphene monolayer in quantum Hall regime. Besides the spin and pseudospin skyrmions for the corresponding degrees of freedom of an electron in the N=0 Landau level, we discuss two unusual types -- the "entanglement skyrmion" whose texture lies in the space of the entanglement of spin and pseudospin, as well as the "deflated pseudospin skyrmion" with partial entanglement. For all skyrmion types, we study the dependence of the energy and the size of a skyrmion on the anisotropy parameters and perpendicular magnetic field. We also propose three ways to modify the anisotropy energy, namely the sample tilting, the substrate anisotropy and the valley pseudospin analogue of Zeeman coupling.

Flyweight, Superelastic, Electrically Conductive, and Flame-Retardant 3D Multi-Nanolayer Graphene/Ceramic Metamaterial.

A ceramic/graphene metamaterial (GCM) with microstructure-derived superelasticity and structural robustness is achieved by designing hierarchical honeycomb microstructures, which are composited with two brittle constituents (graphene and ceramic) assembled in multi-nanolayer cellular walls. Attributed to the designed microstructure, well-interconnected scaffolds, chemically bonded interface, and coupled strengthening effect between the graphene framework and the nanolayers of the Al2 O3 ceramic (NAC), the GCM demonstrates a sequence of multifunctional properties simultaneously that have not been reported for ceramics and ceramics-matrix-composite structures, such as flyweight density, 80% reversible compressibility, high fatigue resistance, high electrical conductivity, and excellent thermal-insulation/flame-retardant performance simultaneously. The 3D well-ordered graphene aerogel templates are strongly coupled with the NAC by the chemically bonded interface, exhibiting mutual strengthening, compatible deformability, and a linearly dependent relationship between the density and Young's modulus. Considerable size effects of the ceramic nanolayers on the mechanical properties are revealed in these ceramic-based metamaterials. The designed hierarchical honeycomb graphene with a fourth dimensional control of the ceramic nanolayers on new ways to scalable fabrication of advanced multifunctional ceramic composites with controllable design suggest a great potential in applications of flexible conductors, shock/vibration absorbers, thermal shock barriers, thermal insulation/flame-retardant skins, and porous microwave-absorbing coatings.

Biosensors based on Bloch surface waves in one-dimensional photonic crystal with graphene nanolayers.

In biosensors research, much effort has been made to achieve high sensitivity to detect lower concentrations of analyte in a solution by testing different kinds of materials. In this paper, we present a biosensor based on Bloch surface waves made of photonic crystal (PhC) including graphene nanolayers under the Kretschmann configuration. The band structures, surface modes, reflectivity, and sensitivity of the PhC biosensor are calculated by the transfer matrix method and results are compared with those of the structure without graphene layers. Our investigations show that the angular sensitivity of the biosensor considerably increases in the presence of the graphene layers. Moreover, we study the effect of the number of the graphene layers placed on the surface of the biosensor on the performance of our proposed biosensor. The results reveal that the sensitivity of the biosensor is enhanced by increasing the number of graphene layers on the surface due to the π-stacking interactions between graphene's honeycomb cells and the carbon rings in biomolecules. Furthermore, our results show that the phase sensitivity is higher than the angular sensitivity, which can promote the accuracy of the calculations.

On topological indices of honeycomb networks and Graphene networks

In QSAR/QSPR study, topological indices such as the Shultz index, generalized Randic index,Zagreb index, general sum-connectivity index, atom-bond connectivity ($ABC$) index and geometric-arithmetic ($GA$) index exploited to estimate the bioactivity of chemical compounds. In this paper, we compute first general Zagreb index, general Randic connectivity index, general sum-connectivity index, $ABC$, $GA$, $ABC_4$ and $GA_5$ indices of the line graphs of subdivision graphs of honeycomb networks and graphene.

Strain-Induced Landau Levels in Arbitrary Dimensions with an Exact Spectrum.

Certain nonuniform strain applied to graphene flakes has been shown to induce pseudo-Landau levels in the single-particle spectrum, which can be rationalized in terms of a pseudomagnetic field for electrons near the Dirac points. However, this Landau level structure is, in general, approximate and restricted to low energies. Here, we introduce a family of strained bipartite tight-binding models in arbitrary spatial dimension d and analytically prove that their entire spectrum consists of perfectly degenerate pseudo-Landau levels. This construction generalizes the case of triaxial strain on graphene's honeycomb lattice to arbitrary d; in d=3, our model corresponds to tetraxial strain on the diamond lattice. We discuss general aspects of pseudo-Landau levels in arbitrary d.

Electrical and thermal conductivities of the graphene, boron nitride and silicon boron honeycomb monolayers

Density of states, electrical and thermal conductivities of electrons in graphene, boron nitride and silicon boron single sheets are studied within the tight-binding Hamiltonian model and Green's function formalism, based on the linear response theory. The results show that while boron nitride keeps significantly the lowest amounts overall with an interval of zero value in low temperatures, due to its insulating nature, graphene exhibits the most electrical and thermal conductivities, slightly higher than silicon boron except for low temperature region where the latter surpasses, owing to its metallic character. This work might make ideas for creating new electronic devices based on honeycomb nanostructures.

Versatile two-dimensional stanene-based membrane for hydrogen purification

The two-dimensional (2D) honeycomb graphene lattices were demonstrated to be promising hydrogen separation membranes. To effectively operate as hydrogen separation membranes, porous defects and enlargement of pores in these materials must be created. The defect-free 2D stanene (Sn), which is realized in experiment, possesses densely packed intrinsic pores as large as 4.66 Å. This work analysed honeycomb 2D Sn-based membranes for hydrogen purification using density functional theory calculations, and found that a fluorinated 2D Sn membrane exhibits excellent hydrogen purification. This purification performance was then tuned for better balanced permeability and selectivity by applying a moderate strain to the membrane. The defect-free fluorinated 2D Sn membrane was identified as suitable for hydrogen purification.

Conduction band population in graphene in ultrashort strong laser field: Case of massive Dirac particles

The Dirac-like quasiparticles in honeycomb graphene lattice are taken to possess a non-zero effective mass. The charge carriers involve to interact with a femtosecond strong laser pulse. Due to the scattering time of electrons in graphene ([Formula: see text]10–100 fs), the one femtosecond optical pulse is used to establish the coherence effect and, consequently, it can be realized to use the time-dependent Schrödinger equation for electron coupled with strong electromagnetic field. Generalized wave vector of relativistic electrons interacting with electric field of laser pulse causes to obtain a time-dependent electric dipole matrix element. Using the coupled differential equations of a two-state system of graphene, the density of probability of population transition between valence (VB) and conduction bands (CB) of gapped graphene is calculated. In particular, the effect of bandgap energy on dipole matrix elements at the Dirac points and resulting CB population (CBP) is investigated. The irreversible electron dynamics is achieved when the optical pulse end. Increasing the energy gap of graphene results in decreasing the maximum CBP.

Mini-Dirac cones in the band structure of a copper intercalated epitaxial graphene superlattice

The electronic band structure of an epitaxial graphene superlattice, generated by intercalating a monolayer of Cu atoms, is directly imaged by angle-resolved photoelectron spectroscopy. The 3.2 nm lateral period of the superlattice is induced by a varying registry between the graphene honeycomb and the Cu atoms as imposed by the heteroepitaxial interface Cu/SiC. The carbon atoms experience a lateral potential across the supercell of an estimated value of about 65 meV. The potential leads to strong energy renormalization in the band structure of the graphene layer and the emergence of mini-Dirac cones. The mini-cones’ band velocity is reduced to about half of graphene's Fermi velocity. Notably, the ordering of the interfacial Cu atoms can be reversibly blocked by mild annealing. The superlattice indeed disappears at ∼220 °C.

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).

Dirac cones beyond the honeycomb lattice: A symmetry-based approach

Recently, several new materials exhibiting massless Dirac fermions have been proposed. However, many of these do not have the typical graphene honeycomb lattice, which is often associated with Dirac cones. Here, we present a classification of these different two-dimensional Dirac systems based on the space groups, and discuss our findings within the context of a minimal two-band model. In particular, we show that the emergence of massless Dirac fermions can be attributed to the mirror symmetries of the materials. Moreover, we uncover several novel Dirac systems that have up to twelve inequivalent Dirac cones, and show that these can be realized in (twisted) bilayers. Hereby, we obtain systems with an emergent SU(2N) valley symmetry with N=1,2,4,6,8,12. Our results pave the way to engineer different Dirac systems, besides providing a simple and unified description of materials ranging from square- and $\beta$-graphynes, to Pmmn-Boron, TiB$_2$, phosphorene, and anisotropic graphene.

Macrocycles inserted in graphene: from coordination chemistry on graphene to graphitic carbon oxide.

Tuning electronic structures and properties through chemical modifications has become the focus of recent research on graphene. The adsorption of metal atoms on graphene showed strong potential but is limited due to weak binding. On the other hand, macrocyclic molecules are well known for their strong and selective binding with metal atoms in solutions through coordination bonds. The alliance of the two substances will largely benefit the two parallel fields: it will provide a scaffold for coordination chemistry as well as a controllable method for tuning the electronic structure of graphene through strong binding with metals. Here, using crown ether as an example, we demonstrate that the embedment of macrocyclic molecules into the graphene honeycomb lattice can be very thermochemically favored. The combination also leads to a family of new materials that has potential in many areas including photolysis and two-dimensional superconductivity.

Fabrication of artificial graphene in a GaAs quantum heterostructure

The unusual electronic properties of graphene, which are a direct consequence of its two-dimensional honeycomb lattice, have attracted a great deal of attention in recent years. Creation of artificial lattices that re-create graphene's honeycomb topology, known as artificial graphene, can facilitate the investigation of graphenelike phenomena, such as the existence of massless Dirac fermions, in a tunable system. In this work, the authors present the fabrication of artificial graphene in an ultrahigh quality GaAs/AlGaAs quantum well, with lattice period as small as 50 nm, the smallest reported so far for this type of system. Electron-beam lithography is used to define an etch mask with honeycomb geometry on the surface of the sample, and different methodologies are compared and discussed. An optimized anisotropic reactive ion etching process is developed to transfer the pattern into the AlGaAs layer and create the artificial graphene. The achievement of such high-resolution artificial graphene should allow the observation for the first time of massless Dirac fermions in an engineered semiconductor.

Comparative study of van der Waals corrections to the bulk properties of graphite

Graphite is a stack of honeycomb (graphene) layers bound together by nonlocal, long-range van der Waals (vdW) forces, which are poorly described by density functional theory (DFT) within local or semilocal exchange-correlation functionals. Several approximations have been proposed to add a vdW correction to the DFT total energies (Stefan Grimme (D2 and D3) with different damping functions (D3-BJ), Tkatchenko–Scheffler (TS) without and with self-consistent screening (TS + SCS) effects). Those corrections have remarkly improved the agreement between our results and experiment for the interlayer distance (from 3.8 to 0.1%) and high-level random-phase approximation (RPA) calculations for interlayer binding energy (from 56.2 to 0.6%). We report a systematic investigation of various structural, energetic and electron properties with the aforementioned vdW corrections followed by comparison with experimental and theoretical RPA data. Comparison between the resulting relative errors shows that the TS + SCS correction provides the best results; the other corrections yield significantly larger errors for at least one of the studied properties. If considerations of computational costs or convergence problems rule out the TS + SCS approach, we recommend the D3-BJ correction. Comparison between the computed -splitting and experimental results shows disagreements of 10% or more with all vdW corrections. Even the computationally more expensive hybrid PBE0 has proved unable to improve the agreement with the measured splitting. Our results indicate that improvements of the exchange-correlation functionals beyond the vdW corrections are necessary to accurately describe the band structure of graphite.

Mott multicriticality of Dirac electrons in graphene

We study the multicritical behavior for the semimetal-insulator transitions on graphene's honeycomb lattice using the Gross-Neveu-Yukawa effective theory with two order parameters: the SO(3) (Heisenberg) order parameter describes the antiferromagnetic transition, and the $\mathbb{Z}_2$ (Ising) order parameter describes the transition to a staggered density state. Their coupling induces multicritical behavior which determines the structure of the phase diagram close to the multicritical point. Depending on the number of fermion flavors $N_f$ and working in the perturbative regime in vicinity of three (spatial) dimensions, we observe first order or continuous phase transitions at the multicritical point. For the graphene case of $N_f=2$ and within our low order approximation, the phase diagram displays a tetracritical structure.

Exhaustive inventory of 2D unit cells commensurate with honeycomb graphene structure

The graphene plane possesses a high hexagonal symmetry. Currently, it is largely studied as 2D crystal, especially due to its very interesting potential electronic properties. It is also frequently studied in the 3D stackings as graphite. When a layer of a chemical species is dropped off on a graphene layer, it is common to observe a right adaptation between both 2D crystal networks. This commensurability is consistently studied in this paper by means of a mathematical approach. Through this method, it has been possible to take an inventory of all 2D unit cells commensurate with hexagonal graphene cell. Hexagonal, rectangular and oblique cells can be perfectly listed for the different values of the number of carbon atoms included in the cell. Numerous experimental examples are given in the field of graphite intercalation compounds. And very especially the graphite-electron donors lamellar compounds have been used in order to illustrate the method and its results.

Mechanically robust honeycomb graphene aerogel multifunctional polymer composites

This work reports the fabrication and characterization of three-dimensional (3D) graphene aerogel (GA)–polydimethylsiloxane (PDMS) composites (GAPC) with outstanding mechanical, electrical and thermal properties. GAPC was fabricated by impregnating 3D GA frameworks with PDMS via ice-bath-assisted infiltration and vacuum curing processes. Because of the well-interconnected 3D GA frameworks, GAPC exhibits extremely large deformability (compressive strain=80% and tensile strain=90%), high electrical and thermal conductivities (1S/cm and 0.68W/(mK), respectively), a stable piezo-resistance effect, rapid electric Joule heating performance ((dT/dt)max>3°C/s under a heating power of 12W/cm3), and high hydrophobicity (contact angle=135°). Furthermore, GAPC exhibits a negative temperature coefficient of expansion with decreased electrical resistivity over a broad temperature range, indicating a typical semiconducting behavior and a dual two-dimensional/3D hopping conduction mechanism.

Quantum walks on graphene nanoribbons using quantum gates as coins

Both discrete and continuous quantum walks on graphs are universal for quantum computation. We define and use discrete quantum walks on the graphene honeycomb lattice to investigate the possibility of using graphene armchair and zigzag nanoribbons to implement quantum gates. The probability distribution of the quantum walker location represents the particle (electron) density distribution on the graphene lattice. We use a universal set of quantum gates as coins that drive the quantum walk and show that different quantum gates result in distinguishable particle distributions on the graphene lattice.