Zigzag-edged Graphene Research Articles

Phosphorus-Doped Graphene Aerogel as Self-Supported Electrocatalyst for CO2 -to-Ethanol Conversion.

Electrochemical reduction of carbon dioxide (CO2) to ethanol is a promising strategy for global warming mitigation and resource utilization. However, due to the intricacy of C─C coupling and multiple proton–electron transfers, CO2‐to‐ethanol conversion remains a great challenge with low activity and selectivity. Herein, it is reported a P‐doped graphene aerogel as a self‐supporting electrocatalyst for CO2 reduction to ethanol. High ethanol Faradaic efficiency (FE) of 48.7% and long stability of 70 h are achieved at −0.8 VRHE. Meanwhile, an outstanding ethanol yield of 14.62 µmol h−1 cm−2 can be obtained, outperforming most reported electrocatalysts. In situ Raman spectra indicate the important role of adsorbed *CO intermediates in CO2‐to‐ethanol conversion. Furthermore, the possible active sites and optimal pathway for ethanol formation are revealed by density functional theory calculations. The graphene zigzag edges with P doping enhance the adsorption of *CO intermediate and increase the coverage of *CO on the catalyst surface, which facilitates the *CO dimerization and boosts the EtOH formation. In addition, the hierarchical pore structure of P‐doped graphene aerogels exposes abundant active sites and facilitates mass/charge transfer. This work provides inventive insight into designing metal‐free catalysts for liquid products from CO2 electroreduction.

Multifunctional molecular spintronic device based on zigzag-edged trigonal graphene: A first-principles study

A zigzag-edged trigonal graphene (ZTG)-based molecular spintronic device is proposed and its spin-polarized transport properties are investigated using a first-principles method based on density functional theory in conjunction with nonequilibrium Green's function technique. Multiple high-efficiency physical effects, including spin-filtering, giant magnetoresistance, spin-rectifying and low-bias voltage negative differential resistance effects, are observed in the proposed system. The underlying mechanisms are analyzed by the bias voltage-dependent spin-resolved transmission spectra, projected density of states and local density of states. The results indicate that the proposed system has great potential in developing high-performance multifunctional molecular spintronic devices.

Multiple hydrogen bonds enhancing formaldehyde adsorption on functionalized zigzag graphene nanoribbons

In this work, based on density functional theory, the edge and the center of both armchair-edged graphene nanoribbons (AGNRs) and zigzag-edged graphene nanoribbons (ZGNRs) have been modified through five oxygen-containing groups (epoxy, methoxy, formyl, hydroxyl and carboxyl) to form functionalized AGNRs and ZGNRs, in which distorted surface of AGNRs can be observed. Computational results indicated that ZGNR is appealing choice to act as a candidate for designing formaldehyde (HCHO) sensor. Simulation results demonstrate that the interactions between HCHO and oxygen-containing ZGNRs were mainly determined by the multiple hydrogen bonds. The novel design of carboxyl modified ZGNRs on the center of surface with the largest adsorption energy of − 0.510 eV and the relatively long recovery time of 4.15 × 10-4 s could act as a physical sensor to detect HCHO molecules. Dynamics simulation results indicated that HCHO shows very strong hydrogen bonding interactions during competitive adsorption with N2 and O2 at 298.15 K and 308.15 K. Consequently, simulation results offer a molecular-level insight into the interactions between HCHO and functionalized ZGNR surfaces, and might provide useful information for the design of HCHO sensors.

Hydrogen Atoms on Zigzag Graphene Nanoribbons: Chemistry and Magnetism Meet at the Edge.

Although the unconventional π-magnetism at the zigzag edges of graphene holds promise for a wide array of applications, whether and to what degree it plays a role in their chemistry remains poorly understood. Here, we investigate the addition of a hydrogen atom─the simplest yet the most experimentally relevant adsorbate─to zigzag graphene nanoribbons (ZGNRs). We show that the π-magnetism governs the chemistry of ZGNRs, giving rise to a site-dependent reactivity of the carbon atoms and driving the hydrogenation process to the nanoribbon edges. Conversely, the chemisorbed hydrogen atom governs the π-magnetism of ZGNRs, acting as a spin-1/2 paramagnetic center in the otherwise antiferromagnetic ground state and spin-polarizing the charge carriers at the band extrema. Our findings establish a comprehensive picture of the peculiar interplay between chemistry and magnetism that emerges at the zigzag edges of graphene.

Geometry dependent current–voltage characteristics of T-stub zigzag-edged graphene nanoribbon quantum waveguides

Geometry dependent current–voltage characteristics of T-stub zigzag-edged graphene nanoribbon quantum waveguides

All-carbon multifunctional molecular spintronic device: A first-principles study

Based on the first-principles density functional theory combined with the nonequilibrium Green’s function methodology, we have designed and investigated the spin-polarized transport properties of an all-carbon molecular device, in which a C60 fullerene is sandwiched between two ferromagnetic zigzag-edge graphene nanoribbon electrodes. The results show that the spin-polarized properties are strongly dependent on the external magnetic field modulation and spin-filtering, giant magnetoresistance and spin-rectifying effects are realized in this device. The mechanisms are proposed for these interesting phenomena. These results demonstrate that the designed all-carbon device holds great potentials in the development of multifunctional molecular spintronic device.

Tunable giant magnetoresistance ratio in bilayer CuPc molecular devices.

We investigate the influence of the distance between the buffer layer and the central molecule on the electrical transport, spin-filter transport, magnetoresistance effects and thermoelectric properties of a bilayer CuPc molecular device with V-shaped zigzag-edged graphene nanoribbon (VZGNR) electrodes by combining density functional theory and the non-equilibrium Green's function. The results show that the spin-dependent total conductance and spin filter efficiency of the bilayer CuPc molecular device reach a maximum with a parallel spin configuration (PC) when the carbon atom at the edge of the electrode is in the center of the carbon atom at the edge of the bilayer molecules due to the stronger coupling interaction between the double-layer molecules and the leads. Moreover, the spin polarization of the bilayer CuPc molecular device is reversed at certain distances; there is a minimum spin filter efficiency (SFE) of −99.93448% and a maximum SFE of 97.91% observed in the anti-parallel spin configuration (APC) of the device and there is a minimum SFE of −26.03175% and a maximum SFE of 99.99996% observed with the PC at zero bias. The SFE oscillates with increasing considered bias voltage in the PC and APC when the distances are d = 0 Å and d = −1.06 Å, and a negative differential resistance (NDR) effect was observed. For the PC and APC, there is a giant magnetoresistance (MR) effect and the MR ratio exceeds 5.21 × 107% (99.9996%), and the MR ratio oscillates with increasing considered bias voltage when d = 0 Å. The MR ratio could be reserved by applying a certain bias voltage. These transport behaviors can be well understood by analyzing the transmission spectra, projected density of states and scattering states. There are pure spin Seebeck coefficients and pure charge Seebeck coefficients at certain temperatures when the distances are certain values, which means that the corresponding temperature differences could produce pure spin current and pure charge current, respectively. Our results provide new ideas for designing ultrahigh-performance spintronic molecular devices.

Spin thermoelectric effects of new-style one-dimensional carbon-based nanomaterials

Based on first-principles methods, the authors of this paper investigate spin thermoelectric effects of one-dimensional spin-based devices consisting of zigzag-edged graphene nanoribbons (ZGNRs), carbon chains and graphene nanoflake. It is found that the spin-down transmission function is suppressed to zero, while the spin-up transmission function is about 0.25. Therefore, an ideal half-metallic property is achieved. In addition, the phonon thermal conductance is obviously smaller than the electronic thermal conductance. Meantime, the spin Seebeck effects are obviously enhanced at the low-temperature regime (about 80K), resulting in the fact that spin thermoelectric figure of merit can reach about 40. Moreover, the spin thermoelectric figure of merit is always larger than the corresponding charge thermoelectric figure of merit. Therefore, the study shows that they can be used to prepare the ideal thermospin devices.

Ultrahigh Spin Filter Efficiency, Giant Magnetoresistance and Large Spin Seebeck Coefficient in Monolayer and Bilayer Co-/Fe-/Cu-Phthalocyanine Molecular Devices.

The spin related electrical and thermoelectric properties of monolayer and bilayer MPc (M = Co, Fe, Cu) molecular devices in a parallel spin configuration (PC) and an anti-parallel spin configuration (APC) between the V-shaped zigzag-edged graphene nanoribbon electrodes and the center bilayer MPc molecules are investigated by combining the density functional theory and non-equilibrium Green’s function approaches. The results show that there is an ultrahigh spin filter efficiency exceeding 99.99995% and an ultra-large total conductance of 0.49996G0 for FePc-CoPc molecular devices in the PC and a nearly pure charge current at high temperature in the APC and a giant MR ratio exceeding 9.87 × 106% at a zero bias. In addition, there are pure spin currents for CuPc and FePc molecular devices in the PC, and an almost pure spin current for FePc molecular devices in the APC at some temperature. Meanwhile, there is a high SFE of about 99.99585% in the PC and a reserved SFE of about −19.533% in the APC and a maximum MR ratio of about 3.69 × 108% for the FePc molecular device. Our results predict that the monolayer and bilayer MPc (M = Co, Fe, Cu) molecular devices possess large advantages in designing high-performance electrical and spintronic molecular devices.

Designing multifunctional single-molecule devices by mononuclear or binuclear manganese phthalocyanines

Molecular-scale magnetic devices have attracted great attention of research in recent years. In this work, by using the non-equilibrium Green’s function (NEGF) method in combination with density functional theory (DFT), we investigated the spin transport properties of manganese phthalocyanine based spintronic devices constructed by a mononuclear manganese phthalocyanine (MnPc) or binuclear manganese phthalocyanine (Mn2Pc2) sandwiched between two zigzag-edge graphene nanoribbon (zGNR) electrodes. The calculation results show that both MnPc and Mn2Pc2 devices are good spin filters manifesting excellent spin filtering effect, which is originated from the distinct difference in the energy alignments of the spin-up and spin-down molecular electronic states with respect to the Fermi energy (EF) of zGNR electrodes. More interestingly, the spin polarization of the tunneling electrons through the devices is intimately opposite to that of Mn atom(s). Specifically, for a spin-up polarization of Mn in the devices, almost only the spin-down electrons are allowed to go through the device, and vice versa. In addition, spin filtering efficiency (SFE) of the devices is evidently enhanced in Mn2Pc2 devices in comparison with that of the MnPc devices, of which the value of SFE can reach almost 100%. This enhancement is attributed to the fact that the spin electronic states of Mn2Pc2 devices are closer to EF and there is a weaker localization in their spatial distributions induced by the external bias voltage. Furthermore, both MnPc and Mn2Pc2 devices can act as NOT logic gates by taking the spin polarization characteristics of Mn atom(s) and current of devices as input and output signals, respectively. This work demonstrates that mononuclear MnPc and binuclear Mn2Pc2 molecules have potential applications in designing multifunctional spintronic single-molecule devices.

Two-Electron Electrochemical Reduction of CO2 on B-Doped Ni–N–C Catalysts: A First-Principles Study

We evaluate the electrocatalytic activity of graphene-supported NiN4 active center in facilitating two-electron CO2 electrochemical reduction into CO and HCOOH, as well as the competing hydrogen evolution reaction. NiN4 center is found to be more stable in zigzag-edge and armchair-edge proximity of graphene, confirming experimental evidence. In an attempt to reduce the CO2 reduction overpotential, we construct a neighboring-site environment of NiN4 center and BN substitutional defect. B-doped structures are found to be capable of reducing the CO2 reduction energy barrier through direct (HCOOH pathway) or indirect (CO pathway) participation in facilitating CO2 reduction-related key intermediates. In most Ni sites, the presence of adjacent BN site is also discovered to change the product selectivity from CO to HCOOH. Our result predicts that B-doped NiN4 sites on the interior side (NiN4BN-G) and on the armchair-edge side of tilted orientation (t-NiN4BN-AGNR) are HCOOH-selective. Although the rest of zigzag-edge and armchair-edge sites have a tendency to selectively produce CO and HCOOH, respectively, the undesirable hydrogen evolution reaction is found to be more dominant and therefore obscuring the potential. Our result demonstrates that the hydrogen evolution reaction is most likely to occur on top of a neighboring C atom instead of on the Ni center, in contrast to the commonly understood mechanism. To ultimately suppress the activity of the hydrogen evolution reaction, we predict that an effective electrocatalyst optimization cannot be realized by only selecting the best metal center and dopant pair but also by altering the neighbor’s electronic structure.

Effect and Mechanism of Hydrogen-Assisted Sulfur Intercalation for Decoupling Graphene from Cu(1 1 1) Substrate: A First-Principles Study

Intercalation has been experimentally proven to be an efficient approach for decoupling graphene from metal substrates such as Cu, which enables the easy transfer of graphene from a substrate. Thereby we propose a simple and effective method for injecting the H2S gas under an annealing environment to form sulfur compound intercalations, to help decouple graphene from the Cu substrate. Using density functional theory calculations, two different S-intercalating mechanisms, direct S intercalation and Hydrogen (H)-assisted S intercalation, have been found to occur at the zigzag and armchair edges of graphene, respectively. By forming various two-dimensional (2D) sulfide (Cu2S, CuS, and CuS2) intercalations depending on the temperature and S concentration, the electron interaction between the Cu substrate and graphene as well as the exfoliation energy of graphene from the Cu substrate can be effectively reduced. This allows graphene to decouple from the Cu substrate and become almost free-standing with a neutral charge under controlled experimental conditions. The stability of the sulfide intercalations has been proven by thermodynamic and kinetic simulations. H-assisted S intercalation may promote the subsequent transfer of graphene, enabling the practical application of large-sized graphene sheets grown by chemical vapor deposition on metal substrates.

A Nano‐Imaging Study of Graphene Edge Plasmons with Chirality‐Dependent Dispersions (Advanced Optical Materials 10/2021)

The graphene plasmons at the edges of a monolayer graphene have been demonstrated to exhibit superior electromagnetic ?eld con?nements. In article number 2100207, Huanjun Chen and co-workers show that, with a nano-imaging technique, the edge plasmon modes at zigzag and armchair edges exhibit chirality-dependent dispersions. In addition, due to the stronger molecule absorption ability of the zigzag edge, the associated plasmons exhibit a broader tuning range upon chemical doping via HNO3 exposure. This outcome highlights the potential of graphene zigzag edge in applications of nanoscale molecular sensing.

The Mechanism of Graphene Vapor-Solid Growth on Insulating Substrates.

Wafer-scale single-crystal graphene film directly grown on insulating substrates via the chemical vapor deposition (CVD) method is desired for building high-performance graphene-based devices. In comparison with the well-studied mechanism of graphene growth on transition metal substrates, the lack of understanding on the mechanism of graphene growth on insulating surfaces greatly hinders the progress. Here, by using first-principles calculation, we systematically explored the absorption of various carbon species CHx (x = 0, 1, 2, 3, 4) on three typical insulating substrates [h-BN, sapphire, and quartz] and reveal that graphene growth on an insulating surface is dominated by the reaction of active carbon species with the hydrogen-passivated graphene edges and thus is less sensitive to the type of the substrate. The dominating gas phase precursor, CH3, plays two key roles in graphene CVD growth on an insulating substrate: (i) to feed the graphene growth and (ii) to remove excessive hydrogen atoms from the edge of graphene. The threshold reaction barriers for the growth of graphene armchair (AC) and zigzag (ZZ) edges were calculated as 3.00 and 1.94 eV, respectively; thus the ZZ edge grows faster than the AC one. Our theory successfully explained why the circumference of a graphene island grown on insulating substrates is generally dominated by AC edges, which is a long-standing puzzle of graphene growth. In addition, the very slow graphene growth rate on an insulating substrate is calculated and agrees well with existing experimental observations. The comprehensive insights on the graphene growth on insulating surfaces at the atomic scale provide guidance on the experimental design for high-quality graphene growth on insulating substrates.

Computing Edge States without Hard Truncation

We present a numerical method which accurately computes the discrete spectrum and associated bound states of Hamiltonians which model electronic "edge" states localized at boundaries of one and two-dimensional crystalline materials. The problem is non-trivial since arbitrarily large finite "hard" truncations of the Hamiltonian in the infinite bulk direction tend to produce spurious bound states partially supported at the truncation. Our method, which overcomes this difficulty, is to compute the Green's function of the Hamiltonian by imposing an appropriate boundary condition in the bulk direction; then, the spectral data is recovered via Riesz projection. We demonstrate our method's effectiveness by studies of edge states at a graphene zig-zag edge in the presence of defects modeled both by a discrete tight-binding model and a continuum PDE model under finite difference discretization. Our method may also be used to study states localized at domain wall-type edges in one and two-dimensional materials where the edge Hamiltonian is infinite in both directions; we demonstrate this for the case of a tight-binding model of distinct honeycomb structures joined along a zig-zag edge.

Tunable conductance and spin filtering in twisted bilayer copper phthalocyanine molecular devices.

We investigate theoretically the quantum transport properties of a twisted bilayer copper phthalocyanine (CuPc) molecular device, in which the bottom-layer CuPc molecule is connected to V-shaped zigzag-edged graphene nanoribbon electrodes. Based on a non-equilibrium Green's function approach in combination with density-functional theory, we find that the twist angle effectively modulates the electron interaction between the bilayer CuPc molecules. HOMO (highest occupied molecular orbital)–LUMO (lowest unoccupied molecular orbital) gap, spin filtering efficiency (SFE) and spin-dependent conductance of the bilayer CuPc molecular device could be modulated by changing the twist angle. The conductance reaches its maximum when the twist angle θ is 0° while the largest SFE is achieved when θ = 60°. The twist angle-induced exotic transport phenomena can be well explained by analyzing the transmission spectra, molecular energy level spectra and scattering states of the twisted bilayer CuPc molecular device. The tunable conductance, HOMO–LUMO gap and spin filtering versus twist angle are helpful for predicting how a two-molecule system may behave with twist angle.

Generation of pure spin current in graphene nanoribbons with continous antidots

Spin caloritronics, which combines the characteristics of thermoelectronics with the characteristics of spintronics, has a wide range of promising applications in high-speed and low-dissipation devices. In this paper, according to the density functional theory combined with nonequilibrium Green’s function method, we propose a scheme for generating pure spin current with spin dependent Seebeck effect in the zigzag-edged graphene nanoribbons by introducing continuous antidots (hexagonal defects). Specifically, by creating an antidot at one edge of the nanoribbon, an X-shape transmission spectrum around the Fermi level is formed, which results from the disrupted edge of the nanoribbon. The mechanism is well explained by the cooperation between the varying localization features of the eigenstates around the Fermi level for the unit cell and the scattering states at the Fermi level for the device. Therefore, the electrons of the two spin channels flow in the opposite directions under a temperature gradient, generating the spin current and charge current. By slightly tuning the chemical potential of the device, the charge current can be zero, while the spin current is not equal to zero. With the increase of adjacent antidot number along the width of the nanoribbons, the structures are more disrupted, thus promoting the pure spin current due to the increase of the spin Seebeck coefficient. However, for zigzag graphene nanoribbons with <i>W</i> zigzag carbon chains, the pure spin current decreases when the number of the antidots are more than (<i>W</i>/2–1), which results from the decrease of the spin conductance. So, the maximum pure spin current can be obtained when the number of the continuous antidots introduced along the width of nanoribbons reaches (<i>W</i>/2–1). These findings indicate a novel strategy for thermally generating the spin current by introducing continuous antidots along the nanoribbon bandwidth in zigzag graphene nanoribbons and will be greatly instructive in designing the graphene spintronic devices.

Switching characteristics of anthraquinone molecular devices based on graphene electrodes

With the development of microelectronics and the miniaturization of electronic devices, the use of molecular materials to construct various components in electronic circuits has become a most likely development trend. Compared with silicon-based semiconductor components, molecular electronic device has the advantages of small size, high integration, low energy consumption and fast response. In recent years, more and more molecules have been used to design molecular devices such as molecular diodes, molecular switches, molecular field effect transistors and molecular memories. In this paper, sandwich structure devices based on graphene nanoribbon electrodes are constructed. The first-principles calculation method combining density functional theory and non-equilibrium Green’s function is adopted to design the molecular devices with functional characteristics. The effects of redox reactions on the electrical transport properties of molecular devices are systematically discussed. The main research contents of this paper are as follows. The switching characteristics of an anthraquinone molecular device based on graphene electrode are studied. The zigzag-edge nanoribbons and armchair-edge graphene nanoribbons are selected as electrodes. Considering the two isomers of anthraquinone (HQ) and anthraquinone (AQ) molecules in the redox reaction, the double electrode molecular junction is constructed. The effects of redox reaction and electrode structure on the switching characteristics of anthraquinone molecular devices are discussed. It is found that the current in the HQ configuration is significantly greater than that in the AQ configuration, regardless of the zigzag-edge graphene electrode or the armchair-edge graphene electrode. That is, under the redox reaction, the anthraquinone molecules show significant switching characteristics. The switching ratio of zigzag-edge graphene electrode is selected to reach a maximum of 3125, and that of armchair-edge graphene electrode is selected to maximum of 1538. In addition, when the armchair-edge graphene is used as an electrode in the HQ configuration, the negative differential resistance is obviously between 0.7 and 0.9 V.

Spin-resolved transport properties of atomic carbon chain between sawtooth zigzag-edge graphene nanoribbons electrodes

ABSTRACT We investigate the magnetic transport properties for a molecular device model constructed by a carbon atomic chain embedding in two hydrogen-passivated graphene nanoribbons (GNRs) electrodes with sawtooth zigzag edges. The spin-dependent transport simulation is carried out by the first-principles approach based on the density functional theory combing with nonequilibrium Green's function formalism. It is found that the dual spin-filtering effect with the perfect efficiency up to 100, high-performance dual-spin diode effect with the rectification ratio reaches about and obvious negative differential resistance behaviour with a peak-to-valley ratio near are numerically observed in such proposed molecular device. Particularly, a giant magnetoresistance ratio approaching is also observed, which is two orders magnitude higher than the value of a zigzag-edge GNR. These findings suggest that our proposed device has multifunction with highly effective spin-filter, spin-diode, spin-valve performance.

Synthetic Tailoring of Graphene Nanostructures with Zigzag-Edged Topologies: Progress and Perspectives.

Experimental and theoretical investigations have revealed that the chemical and physical properties of graphene are crucially determined by their topological structures. Therefore, the atomically precise synthesis of graphene nanostructures is essential. A particular example is graphene nanostructures with zigzag‐edged structures, which exhibit unique (opto)electronic and magnetic properties owing to their spin‐polarized edge state. Recent progress in the development of synthetic methods and strategies as well as characterization methods has given access to this class of unprecedented graphene nanostructures, which used to be purely molecular objectives in theoretical chemistry. Thus, clear insight into the structure–property relationships has become possible as well as new applications in organic carbon‐based electronic and spintronic devices. In this Minireview, we discuss the recent progress in the controlled synthesis of zigzag‐edged graphene nanostructures with different topologies through a bottom‐up synthetic strategy.