Graphene Edges Research Articles

Termination of graphene edges created by hydrogen and deuterium plasmas

Edge engineering is important for both fundamental research and applications as the device size decreases to nanometer scale. This is especially the case for graphene because a graphene edge shows totally different electronic properties depending on the atomic structure and the termination. It has recently been shown that an atomically precise zigzag edge can be obtained by etching graphene and graphite using hydrogen (H) plasma. However, edge termination had not been studied directly. In this study, termination of edges created by H-plasma is studied by high-resolution electron energy loss spectroscopy to show that the edge is sp2 bonded and the edge carbon atom is terminated by only one H atom. This suggests that an ideal zigzag edge, which is not only atomically precise but also sp2 bonding, can be obtained by H-plasma etching. Etching of the graphite surface with plasma of a different isotope, deuterium (D), is also studied by scanning tunneling microscopy to show that D-plasma anisotropically etches graphite less efficiently, although it can make defects more efficiently, than H-plasma.

Defect-rich graphene skin modified carbon felt as a highly enhanced electrode for vanadium redox flow batteries

Vanadium redox flow batteries (VRFBs) with high energy density, long cycle life, flexible design and rapid response have attracted great attention in large-scale energy storage applications. However, the low activity of traditional carbon felt electrodes severely limits its practical implementation. Herein, we report the fabrication of modified carbon felt (CF) by coating defect-rich graphene through chemical vapor deposition (CVD) and subsequent Ar plasma treatment as a highly improved electrode (denoted as Ar-GCF) for VRFB. The modification of defect-rich graphene skin can not only expose a large number of graphene edges as highly active sites, but also offers copious oxygen-containing functional groups to improve the electronic conductivity of Ar-GCF and accelerate the VO2+/VO2+ redox, which are demonstrated by both electrochemical measurements and spin-polarized density functional theory (DFT) calculations. As a result, the VRFB with Ar-GCF electrode exhibited an improved energy efficiency (EE) by 7.10% compared with that of VRFB using an unmodified CF electrode. Moreover, the Ar-GCF electrode showed excellent stability, with the charge transfer resistance (Rct) increased by merely 15.79% after 800 cycles, while the Rct of CF electrode increased by 102.40% after 600 cycles.

Reconstruction of Zigzag Graphene Edges: Energetics, Kinetics, and Residual Defects.

Ab initio calculations are performed to study consecutive reconstruction of a zigzag graphene edge. According to the obtained energy profile along the reaction pathway, the first reconstruction step, formation of the first pentagon-heptagon pair, is the slowest one, while the growth of an already nucleated reconstructed edge domain should occur steadily at a much higher rate. Domains merge into one only in 1/4 of cases when they get in contact, while in the rest of the cases, residual defects are left. Structure, energy, and magnetic properties of these defects are studied. It is found that spontaneous formation of pairs of residual defects (i.e., spontaneous domain nucleation) in the fully reconstructed edge is unlikely at temperatures below 1000 K. Using a kinetic model, we show that the average domain length is several micrometers at room temperature and it decreases exponentially upon increasing the temperature at which the reconstruction takes place.

Enhanced electrical conductivity of pitch-derived carbon via graphene template effects for high electrically conductive composites

Large-scale preparation of cheap and high-performance carbonaceous materials is in urgent need due to the huge demand of carbonaceous materials-organic binder composites for Joule heating. Here, carbon-based electrothermal composites with high electrical conductivity were fabricated by adjusting the morphology and structure of pitch-based carbonaceous materials (PC) through the use of graphene as structure-directing agent to tune the orientation and carbonization of coal pitch. It is demonstrated that the addition of graphene can effectively promote the formation of graphitized carbon, increase the content of sp2C, reduce defective carbon and increase the graphite interlayer spacing. 1% graphene-added PC-PVDF composite exhibits 290% increase in the carrier concentration, 190% enhancement in mobility, and 67% reduction in the volume resistivity compared to PC-PVDF composite. Molecular simulations elucidate that the graphene edges favor pitch carbonization and improve the orientation factor and energy gap of carbon materials. This study provides clues for design of low-cost pitch-derived carbon materials-binder composites.

Establishing the excitation field in tip-enhanced Raman spectroscopy to study nanostructures within two-dimensional systems

The optical field generated by a nanoplasmonic probe is revealed in tip-enhanced Raman spectroscopy (TERS) experiments. The TERS intensity profile of nano-objects smaller than the probe’s apex has a donut-like shape which resembles the magnitude of the field generated by a point-dipole source, being well described by the Dyadic Green’s function. Having prior knowledge on the excitation field generated by the TERS probe, we measured the width of shear solitons caused by lattice reconstruction in low-angle twisted bilayer graphene, a prominent platform for twistronics, and the extend of defect-induced light emission from graphene edges.

Direct Observation of Dynamically Localized Quantum Optical States.

Quantum-correlated biphoton states play an important role in quantum communication and processing, especially considering the recent advances in integrated photonics. However, it remains a challenge to flexibly transport quantum states on a chip, when dealing with large-scale sophisticated photonic designs. The equivalence between certain aspects of quantum optics and solid-state physics makes it possible to utilize a range of powerful approaches in photonics, including topologically protected boundary states, graphene edge states, and dynamic localization. Optical dynamic localization allows efficient protection of classical signals in photonic systems by implementing an analogue of an external alternating electric field. Here, we report on the observation of dynamic localization for quantum-correlated biphotons, including both the generation and the propagation aspects. As a platform, we use sinusoidal waveguide arrays with cubic nonlinearity. We record biphoton coincidence count rates as evidence of robust generation of biphotons and demonstrate the dynamic localization features in both spatial and temporal space by analyzing the quantum correlation of biphotons at the output of the waveguide array. Experimental results demonstrate that various dynamic modulation parameters are effective in protecting quantum states without introducing complex topologies. Our Letter opens new avenues for studying complex physical processes using photonic chips and provides an alternative mechanism of protecting communication channels and nonclassical quantum sources in large-scale integrated quantum optics.

Enhancing oxygen reduction reaction activity of pyrolyzed Fe–N–C catalyst by the inclusion of BN dopant at the graphitic edges

We study the detailed mechanism of oxygen reduction reaction (ORR) at the graphitic edges of BN-doped pyrolyzed Fe–N–C catalyst (FeN4-BN-edge) by utilizing the combination of DFT and microkinetic calculations. We find that the ORR mechanism on FeN4-BN-edge active sites is more selective toward the associative reduction pathway for all relevant potential ranges instead of the dissociative reduction pathway. This situation arises because the O2 dissociation energies on the FeN4-BN-edge active sites are too high for the dissociative reduction pathway to take place with sufficient ORR rates. The inclusion of BN-dopant at the graphitic edge of pyrolyzed Fe–N–C is found to improve the ORR activity of the catalyst. One factor contributed to this improvement originates from the easy formation of active site configurations at the graphitic edges, which is induced by the interaction between neighboring FeN4 and BN configurations. Another factor is the formation of the FeN4-BN configuration at the zigzag edge of graphene. These active sites could improve the onset potential for the associative reduction pathway up to 0.12 V larger than that at the standard FeN4 active site in the basal plane of graphene.

On the electrocatalytically active sites in graphene-based vanadium redox flow batteries

There is too much controversy and too little interdisciplinary analysis of the role of carbon electrodes in a wide range of electrochemical and electrocatalytic processes. Here we focus on vanadium redox flow batteries (VRFB), which play a central role in the transition to renewable energy sources in the electricity sector of the global economy. We used density functional theory (DFT) to determine the relationship between chemically and electrocatalytically active sites on the positive electrode and the effects of chemical surface modification that introduces a variety of oxygen functional groups. Carefully selected aromatic (Ar) model clusters were analyzed to assess the extent of the electron density accumulation on and around the free carbon sites at graphene edges. The results reveal trends that help to resolve the controversies surrounding the role of phenolic or carboxyl groups in the redox mechanism. We conclude, in agreement with other chemical and electrocatalytic reactions that involve oxygen transfer (e.g., CO2 gasification or oxygen reduction reaction), that carbene-type edge carbon atoms are responsible for VO2+ adsorption and reduction of V5+ to V4+ in VO2+. The presence of Ar-OH and Ar-COOH groups can actually inhibit the redox process as a consequence of hydrogen and/or oxygen migration to the adjacent active sites. The presence of ArO groups, while affecting the electron density at the active sites, is confirmed to have a positive effect of stabilizing the free zigzag carbon sites in a triplet ground state.

Development of near-infrared light responsive cup-stacked carbon nanofiber/ITO electrodes modified with poly(N-isopropylacrylamide)

• Energy conversion among light, heat, and mechanical motion. • Photothermal conversion-based switchable mechano-electrochemical properties. • Control of electrochemical responses upon ON/OFF switching of the NIR irradiation. Reversibly activation and inhibition of electrochemical reactions at phase transition polymer-modified electrodes is of great interest from the viewpoints of application to sensing and energy conversion and storage. We synthesized the thermo-responsive poly( N -isopropylacrylamide) (PNIPA) via electrochemically induced free-radical polymerization at the surface of a cup-stacked carbon nanofiber-modified indium-tin-oxide (CSCNF/ITO) electrode. Here, CSCNFs not only allow fast electron transfer reactions with redox species because of highly ordered electroactive graphene edges at their surface but also absorb the near-infrared (NIR) light and convert to heat. As the PNIPA layer swelled at 25 ˚C, a [Fe(CN) 6 ] 3–/4– ion easily diffused to the CSCNF/ITO electrode surface due to the hydrophobic hydration of PNIPA, leading to the larger current responses. As the photothermal conversion of the CSCNF/ITO electrode occurred upon the NIR light irradiation (>940 nm), the contracted structure of PNIPA was formed due to its phase transition, resulting in the suppression of the electron transfer reaction for [Fe(CN) 6 ] 3–/4– even in the electrolyte solution at temperature below the lower critical solution temperature of PNIPA (ca. 32 ˚C). After turning off the NIR irradiation, the current response immediately turned back to the original values thanks to cooling of the electrode surface by the electrolyte solution (25 ˚C). The electron transfer reaction to dissolved redox species is now reversibly and repeatedly tunable by the phase transition of the grafted PNIPA based on not only temperature switching of the electrolyte solution but also the photothermal conversion of the CSCNF/ITO electrode upon the NIR light irradiation.

Novel nanostructures suspended in graphene vacancies, edges and holes

Under electron beam irradiation, graphene is inclined to form defects (such as vacancies and holes) that can trap foreign atoms to form new structures. The interactions between these structures and graphene have garnered considerable research interest as they can yield exciting properties. This review focuses on the fabrication and characterization of free-standing nanostructures suspended in graphene using transmission electron microscopy, which enables the observations with atomic resolution and investigations of the dynamic behavior of atoms/structures in such materials. Additionally, the review discusses the influence of novel metal/nonmetal dopants in graphene vacancies with varying bond configurations and the catalytic activities of single atoms/clusters located at the graphene edges. Moreover, the dynamic forming process of freestanding single-atom-thick two-dimensional (2D) clusters/metal/metallenes and 2D clusters/metal/metallenes oxides is discussed. Understanding the behavior, stabilities, and macroscopic effects of these nanostructures is vital for the practical deployment of novel atom/molecule scale nanotechnology. Overall, accumulative evidence confirms the growing number of these novel nanostructures, implying a bright future for further exciting discoveries.

The O-doped layer under electron irradiation induced high sensitivity simultaneous electrochemical determination of colorants in the graphene intercalated carbon film

In this work, we studied the high electrochemical activity induced by the O-doped layer on the graphene intercalated carbon (GIC) film under low energy electron irradiation. The GIC film was prepared by using the electron cyclotron resonance (ECR) plasma sputtering system and the top layer was deposited with 5 % oxygen doped under electron irradiation with different energy. Based on the nanostructure characterization, the electron irradiation played the key role to activate the bonding with oxygen and carbon atoms. In our experiments, doping 5 % oxygen under electron irradiation with energy of 50 V promoted more graphene edges and functional groups formed. The GIC film with O-doped top layer under electron irradiation of 50 V exhibits high electrochemical activity in Fe(CN)64−/3− and Fe3+/2+ redox systems, and realized the simultaneous detecting of sunset yellow (SY) and tartrazine (TT). The mechanism could be that the graphene edge and functional group formed by the activated oxygen atoms provide more active sites to accelerate the electron transfer.

Predicting magnetic edge behavior in graphene using neural networks

Magnetic moments near zigzag edges in graphene allow complex nanostructures with customized spin properties to be realized. However, computational costs restrict theoretical investigations to small or perfectly periodic structures. Here, we demonstrate that a machine-learning approach, using only geometric input, can accurately estimate magnetic moment profiles, allowing arbitrarily large and disordered systems to be quickly simulated. Excellent agreement is found with mean-field Hubbard calculations, and important electronic, magnetic, and transport properties are reproduced using the estimated profiles. This approach allows the magnetic moments of experimental-scale systems to be quickly and accurately predicted, and will speed up the identification of promising geometries for spintronic applications. While machine-learning approaches to many-body interactions have largely been limited to exact solutions of complex models at very small scales, this Letter establishes that they can be successfully applied at very large scales at mean-field levels of accuracy.

Molecular Dynamics Simulations of Interactions between Human Telomeric i-Motif Deoxyribonucleic Acid and Functionalized Graphene.

The work deals with molecular dynamics (MD) simulations of protonated, human telomeric i-motif deoxyribonucleic acid (DNA) with functionalized graphene. We studied three different graphene sheets: unmodified graphene with hydrogen atoms attached to their edges and two functionalized ones. The functionalization of graphene edge consists in attaching partially protonated or dissociated amine and carboxyl groups. We found that in all cases the protonated i-motif adsorbs strongly on the graphene surface. The biased MD simulations showed that the work necessary to drag the i-motif out from amine-doped graphene is about twice larger than that in other cases. In general, the system i-motif/amine-doped graphene stands out from the rest, e.g., in this case, the i-motif adsorbs its side with 3′ and 5′ ends oriented in the opposite to surface direction. In other cases, the DNA fragment is adsorbed to graphene by 3′ and 5′ ends. In all cases, the adsorption on graphene influences the i-motif internal structure by changing the distances between i-motif strands as well as stretching or shortening the DNA chain, but only in the case of amine-doped graphene the adsorption affects internal H-bonds formed between nucleotides inside the i-motif structure.

Pentagon‐Containing Doublet Graphene Fragments with Edge‐Dependent Spin/Charge Distribution

Comprehensive SummaryThe synthesis of stable open‐shell hydrocarbon system comprising of pentagon defects and zigzag edges remains unexplored, which can be considered as model compounds for unraveling the spin‐localized states in graphene edges. With concise synthetic approaches, two π‐extended fluorenyl radicals were synthesized and isolated in their crystalline state. X‐ray crystallographic analysis and studies on the magnetic, optical and electrochemical properties of neutral and charged species have revealed an interesting edge‐dependence of the spin/charge distribution, that is, the spin and charge distribution shifted from the pentagon defects to the zigzag edges with the elongation of conjugation. Such phenomenon was unprecedented for hydrocarbon radicals and can be rationalized by the recovery of Clar Sextets in the dominant resonance structures. Remarkably, one of the radicals exhibited exceptional stability in air‐saturated solutions with the half‐life time up to 260 d, thus providing opportunities for the applications as electronic materials.

A proposed nomenclature for graphene pores: a systematic study of their geometrical features and an algorithm for their generation and enumeration

In the present study we propose a nomenclature scheme for graphene pores based on the arrangement of 2-fold coordinated atoms in the pore boundary. An expansion of that scheme is also proposed, to include pores with functionalized and/or reconstructed edges. The proposed nomenclature provides a unique, unambiguous, uniform and consistent way to name and identify graphene pores. Moreover, using graph theory we systematically study the geometrical features of graphene pores and we present an algorithm for their generation, identification and enumeration. The present nomenclature scheme could be also used for naming graphene flakes and edges and can be expanded to include similar systems.

Unveiling a geometric angstrom era of transistors: Physical gate length scaled down to one atom thickness

The world shortest gate length transistor was born using MoS2 as channel and the edge of graphene as gate, exhibiting on/off ratios up to 1.02 × 105 and subthreshold swing values down to 117mVdec−1. This work unveils that the geometric Angstrom era of transistors has come into reality. However, there is still a long way to go for the sub-1 nm- Lg transistors form the lab to the fab.

Multilayer graphene sunk growth on Cu(111) surface

The controllable growth of multilayer graphene is a challenging research topic. Prior results show graphene adlayers can grow beneath pre-existing graphene layers on a Cu(111). The conventional inverted-wedding-cake (IWC) model used to describe this incurs an energy disadvantage due to deformation in the overlying graphene. We propose an alternative theoretical model, the sunk growth mode, for understanding multilayer graphene growth on Cu substrates. Extensive density functional theory (DFT) calculations show that multilayer graphene grown via this sunk mode is energetically favourable compared to the on-terrace growth mode for Cu(111). These results reveal that graphene underlayers tend to grow in a sunk growth mode, minimizing deformation in the overlayers, reducing deformation energy. Further density functional tight binding-molecular dynamic (DFTB-MD) simulations on Cu(111) substrates yield sunken structures consistent with our sunk growth mode. Moreover, AFM investigations of experimentally grown multilayer graphene on polycrystaline Cu show that while friction data indicates multiple graphene edges in the sample, the topological height measurement indicates flat graphitic sheets, further confirming our sunk growth mode. This discovery provides a novel and more reasonable model for the “underlayer” growth of multilayer graphene and can be extended to a general theory for the multilayer graphene growth on various substrates.

Tailoring magnetism in silicon-doped zigzag graphene edges

Recently, the edges of single-layer graphene have been experimentally doped with silicon atoms by means of scanning transmission electron microscopy. In this work, density functional theory is applied to model and characterize a wide range of experimentally inspired silicon doped zigzag-type graphene edges. The thermodynamic stability is assessed and the electronic and magnetic properties of the most relevant edge configurations are unveiled. Importantly, we show that silicon doping of graphene edges can induce a reversion of the spin orientation on the adjacent carbon atoms, leading to novel magnetic properties with possible applications in the field of spintronics.

Switching performance of optically generated spin current at the graphene edge

We investigate the response of the optically generated spin current as a function of time at the graphene edge for a possible application of optospintronic devices. The spin current is generated by optically excited edge plasmon in which the induced electric field is rotating on the graphene plane. According to the inverse Faraday effect, the in-plane rotation of electric field generates magnetization in the direction perpendicular to the graphene plane. The diffusive spin current flows by decaying distribution of the magnetization from the edge. By solving the time-dependent, diffusion equation in the presence/absence of the spin-source term, we evaluate the on/off response of spin current. The switching speed becomes fast when the spin-diffusion length is sufficiently large compared with the decaying length of the magnetization.

Density Functional Theory Studies on Boron-Modified Graphene Edges for Electroreduction of Nitrogen

Density Functional Theory Studies on Boron-Modified Graphene Edges for Electroreduction of Nitrogen