Hexagonal Graphene Lattice Research Articles

Mechanistic Insight into Dual-Metal-Site Catalysts for the Oxygen Reduction Reaction

Incorporating a second transition metal to iron–nitrogen–carbon single-atom catalysts (Fe–N–C SACs) to design dual-metal-site catalysts (DMSCs) was demonstrated to offer a promising opportunity to enhance the oxygen reduction reaction (ORR). However, due to the many possible structural configurations and the dynamic structure evolution of metal centers under reaction conditions, it is challenging to clearly elucidate the structure–property relationship at the atomic level. Here, we develop a computational workflow integrating configuration generations, phase diagram constructions, and reaction free energy calculations to provide an insightful understanding of the active site structures and catalytic mechanisms of ORR on DMSCs. Using Fe–Cu as an example, we generate 31 configurations by tiling the hexagonal lattice of graphene and investigate their atomic structures under reaction conditions. We find that for a wide range of electrode potentials, the Fe site is covered by an *OH intermediate, while the Cu site is not covered by any intermediate. With the OH-ligated structures, we identify the configurations which possess higher catalytic activity than Fe–N–C and Pt(111). We demonstrate that ORR on Fe–Cu DMSCs proceeds via the associative pathway, and the desorption of *OH is the rate-determining step. Further analysis reveals a linear correlation between the limiting potential and the magnetic moment on Fe and suggests a closer distance between the two metal sites benefits the catalytic activity. These mechanistic insights pave the way for the rational design of efficient platinum group metal-free DMSCs for ORR.

Using polycyclic aromatic hydrocarbons for graphene growth on Cu(111) under ultra-high vacuum

Ultra-high vacuum deposition of the polycyclic aromatic hydrocarbons azupyrene and pyrene onto a Cu(111) surface held at a temperature of 1000 K is herein shown to result in the formation of graphene. The presence of graphene was proven using scanning tunneling microscopy, x-ray photoelectron spectroscopy, angle-resolved photoemission spectroscopy, Raman spectroscopy, and low energy electron diffraction. The precursors, azupyrene and pyrene, are comparatively large aromatic molecules in contrast to more commonly employed precursors like methane or ethylene. While the formation of the hexagonal graphene lattice could naively be expected when pyrene is used as a precursor, the situation is more complex for azupyrene. In this case, the non-alternant topology of azupyrene with only 5- and 7-membered rings must be altered to form the observed hexagonal graphene lattice. Such a rearrangement, converting a non-alternant topology into an alternant one, is in line with previous reports describing similar topological alterations, including the isomerization of molecular azupyrene to pyrene. The thermal synthesis route to graphene, presented here, is achievable at comparatively low temperatures and under ultra-high vacuum conditions, which may enable further investigations of the growth process in a strictly controlled and clean environment that is not accessible with traditional precursors.

Enhanced photoluminescence of layered terbium hydroxides by graphene quantum dots in-situ synthesized in the interlayer

Graphene quantum dots (GQDs) intercalated layered terbium hydroxides (LTbH) were prepared through in-situ hydrothermal carbonization of the intercalated citrate anions (CA) and showed an enhanced photoluminescence (PL) due to the sensitization of GQDs for LTbHs. Structural and compositional analysis indicated the GQDs were consisted of well-identified hexagonal lattice of graphene, together with some defects caused by oxygen-containing functional groups. The basal spacings decreased from 8.6 Å of NO3ˉ-LTbHs to 7.3 Å of GQD-LTbHs resulted by the strong interaction between the intercalated GQDs and LTbHs.

A Wavy-Structured Highly Stretchable Thermoelectric Generator with Stable Energy Output and Self-Rescuing Capability

A Wavy-Structured Highly Stretchable Thermoelectric Generator with Stable Energy Output and Self-Rescuing Capability

Graphene as an alignment agent, an electrode, and a source of surface chirality in a smectic-A liquid crystal.

A liquid crystal (LC) cell was fabricated by putting together a monolayer graphene-coated glass substrate on one side, and a rubbed planar-aligning polyimide layer on an indium tin oxide (ITO) coated glass substrate on the other side. The monolayer graphene film served as the planar-alignment agent as well as the transparent electrode on one side of the cell. The cell was filled with an achiral LC alkoxyphenylbenzoate (9OO4). The presence of the graphene film on one substrate resulted in an induced chiral signature in the otherwise achiral LC 9OO4. The induced chirality was probed utilizing the electroclinic effect (a polar tilt of the LC director perpendicular to, and linear in, an applied electric field) in the smectic-A phase. The electroclinic effect showed significant pretransitional behavior on approaching the smectic-A to smectic-C transition temperature from above. The electroclinic effect revealed a low-frequency relaxation process indicating that the chirality was induced on the LC molecules at the graphene interface and did not propagate into the bulk. A soft shear mode can break the symmetry of the hexagonal lattice of graphene on a substrate and, consequently, graphene possesses strain chirality. The noncovalent π-π interaction between the LC and the strained graphene induces molecular conformational deracemization in the LC at the graphene interface, and the LC exhibits surface-induced chirality.

On-Surface Synthesis of Unsaturated Carbon Nanostructures with Regularly Fused Pentagon-Heptagon Pairs.

Multiple fused pentagon–heptagon pairs are frequently found as defects at the grain boundaries of the hexagonal graphene lattice and are suggested to have a fundamental influence on graphene-related materials. However, the construction of sp2-carbon skeletons with multiple regularly fused pentagon–heptagon pairs is challenging. In this work, we found that the pentagon–heptagon skeleton of azulene was rearranged during the thermal reaction of an azulene-incorporated organometallic polymer on Au(111). The resulting sp2-carbon frameworks were characterized by high-resolution scanning probe microscopy techniques and feature novel polycyclic architectures composed of multiple regularly fused pentagon–heptagon pairs. Moreover, the calculated analysis of its aromaticity revealed a peculiar polar electronic structure.

Stripping voltammetric analysis of mercury ions at nitrogen-doped reduced graphene oxide modified electrode

Selective and sensitive monitoring of toxic heavy metals is prerequisite to the curb of water contamination. Herein, we present a differential pulse anodic stripping voltammetric (DPASV) method for the determination of mercury in water samples with a glassy carbon electrode (GCE) modified with nitrogen-doped reduced graphene (NRGO). Through thermal treatment of graphene oxide (GO) in NH3 atmosphere, nitrogen atoms can be facilely doped into the hexagonal lattice of graphene to produce NRGO. The NRGO modified GCE (abbreviated as NRGO/GCE) shows an impressively favorable electron transfer activity and a high stripping voltammetric response for mercury compared to that of reduced graphene oxide (RGO) modified electrode. Not only does the NRGO/GCE show an extraordinary low detection limit of 0.58 nM, it also demonstrates a wide linear concentration range from 1 nM to 800 nM and a high sensitivity of 2.69 μA/μM. Due to the excellent selectivity of the NRGO/GCE to mercury, the interference from other heavy metals such as Fe(III), Co(II), Ni(II), Cu(II), Mn(II), Cd(II), Al(III), Ca(II), and Pb(II) were almost negligible. Those outstanding features guarantee the proposed anodic stripping voltammetric method highly applicable to the assay of mercury ions in real water samples, which is validated by the conventional atomic fluorescence spectrometry.

Utilizing Cyclic Voltammetry to Understand the Energy Storage Mechanisms for Copper Oxide and its Graphene Oxide Hybrids as Lithium-Ion Battery Anodes.

Invited for this month's cover is the group of Robert A. W. Dryfe at the University of Manchester in collaboration with William Blythe Ltd. (Lancashire). The image shows bees building a graphene-containing battery that powers an external circuit, depicted by a purple curve. This curve represents a cyclic voltammogram, specifically of copper oxide during a charge/discharge cycle in a Li-ion half-cell, as presented in the article. The use of bees has both scientific and geographic significance. For the former, bees are assisting in building the honeycomb-like hexagonal graphene lattice, just as they do in nature. Geographically, bees are symbolic to the locations of both collaborators on this project, the University of Manchester and William Blythe (Lancashire). The Full Paper itself is available at 10.1002/cssc.201902784.

On mesoscopic description of interfaces in graphene

The article discusses a mesoscopic approach to the description of interfaces (IFs) in graphene. The approach is based on the representation of defective carbon rings with broken six-fold symmetry in hexagonal lattice of graphene as singular defects, i.e. wedge disclinations, in an elastic continuum. The angle of the sector inserted into or removed from the hexagonal lattice that results in the formation of localized quadrate, pentagon, heptagon, and octagon carbon rings, defines the strength (charge) of disclinations. The mesoscopic approach views IF in graphene as an ensemble of disclinations distributed along a line. Elementary building blocks for IFs with periodic motives of carbon atoms are structural units (SUs) containing disclination sets with zero total disclination charge. The junction of SUs of different type is considered as a virtual disclination. The approach works well in graphene when analyzing elastic fields and stored energies of IFs of two types: grain boundaries (GBs) that induce misorientation of neighboring domains of graphene lattice, and zero misorientation interfaces (ZMIs) that do not possess such a property.

Nitrogen cluster doping for high-mobility/conductivity graphene films with millimeter-sized domains.

Directly incorporating heteroatoms into the hexagonal lattice of graphene during growth has been widely used to tune its electrical properties with superior doping stability, uniformity, and scalability. However the introduction of scattering centers limits this technique because of reduced carrier mobilities and conductivities of the resulting material. Here, we demonstrate a rapid growth of graphitic nitrogen cluster-doped monolayer graphene single crystals on Cu foil with remarkable carrier mobility of 13,000 cm2 V-1 s-1 and a greatly reduced sheet resistance of only 130 ohms square-1. The exceedingly large carrier mobility with high n-doping level was realized by (i) incorporation of nitrogen-terminated carbon clusters to suppress the carrier scattering and (ii) elimination of all defective pyridinic nitrogen centers by oxygen etching. Our study opens up an avenue for the growth of high-mobility/conductivity doped graphene with tunable work functions for scalable graphene-based electronic and device applications.

An analytical theory for propagation in graphene

A new approach to modeling scattering within the hexagonal lattice of graphene produces a theory that could be useful for predicting graphene’s properties, and perhaps for topological insulators as well.

P-Type Doping of Graphene with Cationic Nitrogen

Tailoring electrical properties of graphene by nitrogen doping is currently of great significance in a broad area of advanced applications. Bonding configuration of nitrogen atoms in graphene plays a vital role in controlling its electrical, chemical, and optical properties. Here, we report for the first time a simple bottom-up synthesis of a novel cationic nitrogen-doped graphene (CNG) by a solution plasma (SP). A mixture of ionic liquid and organic solvent was used as starting precursor. CNG exhibited an orthorhombic structure possibly due to the presence cationic nitrogen in hexagonal graphene lattice. Nitrogen doping content was found to be as high as 13.4 atom %. Electrical characterization demonstrated that the CNG exhibited a p-type semiconducting behavior with superior electrical conductivity and carrier concentration. Such unique electrical characteristics of CNG are mainly attributed to the presence of cationic nitrogen with preserved planar structure.

Schur complement solver for Quantum Monte-Carlo simulations of strongly interacting fermions

We present a non-iterative solver based on the Schur complement method for sparse linear systems of special form which appear in Quantum Monte-Carlo (QMC) simulations of strongly interacting fermions on the lattice. While the number of floating-point operations for this solver scales as the cube of the number of lattice sites, for practically relevant lattice sizes it is still significantly faster than iterative solvers such as the Conjugate Gradient method in the regime of strong inter-fermion interactions, for example, in the vicinity of quantum phase transitions. The speed-up is even more dramatic for the solution of multiple linear systems with different right-hand sides. We present benchmark results for QMC simulations of the tight-binding models on the hexagonal graphene lattice with on-site (Hubbard) and non-local (Coulomb) interactions, and demonstrate the potential for further speed-up using GPU.

Topological insulator-metal transition and molecular electronics device based on zigzag phagraphene nanoribbon

In this work, we investigate the electronic transport properties of a graphene allotrope composed of 5–6-7 carbon aromatic rings called phagraphene and compare with the results of the transition-voltage spectroscopy (TVS) and propose the behavior at low voltage characteristic of a topological insulator. Phagraphene properties were compared to those of graphene in a zigzag nanoribbon configuration, zigzag graphene vs zigzag phagraphene nanoribbon (zzGNR and zzPGNR). The molecular geometry and the electronic properties were calculated by density functional theory (DFT) without spin, and the electronic transport and TVS were obtained by means of DFT combined with non-equilibrium Green´s function when we couple the optimized geometry of zzGNR and zzPGNR to the leads (left and right), forming the molecular junction that will be subjected to the action of an external bias voltage (Ve) to generate the molecular device. The results exhibit (i) a metal-insulator transition when Ve is increased until Ve = 1.4 V which corresponds to the nonlinear region (resonance), showing the field effect transistor behaviour for zzGNR junctions; and (ii) two nonlinear regions (two negative differential resistances), showing a resonant tunnel diode behaviour with two operation windows (Ve = 0.5 V and Ve = 1.7 V) for the zzPGNR junction. In addition, the zzPGNR junction exhibits topological insulator characteristics upon introducing topological defects such as pentagons and heptagons in the hexagonal lattice of graphene, and when Ve = 1.7 V, there occurs a topological insulator-metal transition that can be seen in the behaviour of the density of states, transmittance, and frontier molecular orbitals with Ve.

Bandgap engineered graphene and hexagonal boron nitride for resonant tunnelling diode

In this article a double-barrier resonant tunnelling diode (DBRTD) has been modelled by taking advantage of single-layer hexagonal lattice of graphene and hexagonal boron nitride (h-BN). The DBRTD performance and operation are explored by means of a self-consistent solution inside the non-equilibrium Green’s function formalism on an effective mass-Hamiltonian. Both p- and n-type DBRTDs exhibit a negative differential resistance effect, which entails the resonant tunnelling through the hole and electron bound states in the graphene quantum well, respectively. The peak-to-valley ratio of approximately 8 (3) for p-type (n-type) DBRTD with quantum well of 5.1 nm (4.3 nm) at a barrier width of 1.3 nm was achieved for zero bandgap graphene at room temperature.

Deformation and fracture in graphene with divacancies of the 555–777 type

The deformation and fracture of graphene sheets containing 555–777 defects have been investigated by molecular dynamics simulations. Each such defect is a divacancy forming a localized configuration of three pentagonal and three septangular cells of carbon atoms in a hexagonal graphene lattice. An emphasis is made on the influence of 555–777 defects in graphene on its mechanical characteristics (stress–strain curve, uniaxial tensile strength, and maximum elastic strain).

Synthesis of large-area monolayer and bilayer graphene using solid coronene by chemical vapor deposition

In this paper, large-area graphene films composed of mono- and bi-layer have been successfully synthesized by atmospheric pressure chemical vapor deposition (CVD) on Cu foils using solid coronene as the carbon precursor. The number of graphene layers was precisely controlled by adjusting the growth temperature range. In addition, the hexagonal lattice of graphene was shown using Raman spectroscopy, transmission electron microscopy (TEM) and selected area electron diffraction (SAED). The continuity and uniformity of graphene films were confirmed by optical microscopy and scanning electron microscopy (SEM). Further, the presence of weak localization was shown by magneto-transport measurements with the carrier mobility of 2120 cm2/Vs at room temperature. This solid-precursor-based atmospheric pressure CVD method provides a simple, inexpensive and safe route for fabrication of graphene films.

Site-Specific Substitutional Boron Doping of Semiconducting Armchair Graphene Nanoribbons.

A fundamental requirement for the development of advanced electronic device architectures based on graphene nanoribbon (GNR) technology is the ability to modulate the band structure and charge carrier concentration by substituting specific carbon atoms in the hexagonal graphene lattice with p- or n-type dopant heteroatoms. Here we report the atomically precise introduction of group III dopant atoms into bottom-up fabricated semiconducting armchair GNRs (AGNRs). Trigonal-planar B atoms along the backbone of the GNR share an empty p-orbital with the extended π-band for dopant functionality. Scanning tunneling microscopy (STM) topography reveals a characteristic modulation of the local density of states along the backbone of the GNR that is superimposable with the expected position and concentration of dopant B atoms. First-principles calculations support the experimental findings and provide additional insight into the band structure of B-doped 7-AGNRs.

Instability of supersonic crack in graphene

The velocity and instability of crack motion in 2D hexagonal lattice of graphene under pure opening loads are investigated by atomistic molecular dynamics simulations. The brittle crack along zigzag direction in a strip can propagate supersonically at even 8.82km/s under uniform normal loading of edge displacements. Crack moving straightly at low speeds produces atomically smooth edges, while kinking occur beyond a critical speed around 8.20km/s equivalent to 65% of Rayleigh-wave speed in graphene, which validates previous theoretical predictions of rapid fracture instability in elastodynamics, and rough and irregular edges with oscillatory overhangs are formed subsequently.

カーボンナノチューブの変形加工

When a topological defect such as 5- or 7-membered ring is introduced in a hexagonal lattice of graphene, it makes curved nature. It is essential to understand how the topological defects in a carbon nanotube (CNT) is introduced, migrated and eliminated, since it may enable us to design many types of CNT-based hybrid structures showing unique properties. In this paper, some experimental studies on deformation process of individual CNTs by transmission electron microscopy is introduced.