Hydrogen-free graphene edges

Spiral-Driven Vertical Conductivity in Nanocrystalline Graphene

The structure of graphene grown in chemical vapor deposition (CVD) is sensitive to the growth condition, particularly the substrate. The conventional growth of high-quality graphene via the Cu-catalyzed cracking of hydrocarbon species has been extensively studied; however, the direct growth on noncatalytic substrates, for practical applications of graphene such as current Si technologies, remains unexplored. In this study, nanocrystalline graphene (nc-G) spirals are produced on noncatalytic substrates by inductively coupled plasma CVD. The enhanced out-of-plane electrical conductivity is achieved by a spiral-driven continuous current pathway from bottom to top layer. Furthermore, some neighboring nc-G spirals exhibit a homogeneous electrical conductance, which is not common for stacked graphene structure. Klein-edge structure developed at the edge of nc-Gs, which can easily form covalent bonding, is thought to be responsible for the uniform conductance of nc-G aggregates. These results have important implications for practical applications of graphene with vertical conductivity realized through spiral structure.

A Generalized Nomenclature Scheme for Graphene Pores, Flakes, and Edges, and an Algorithm for Their Generation and Numbering

In the present study, we generalize our recently proposed nomenclature scheme for porous graphene structures to include graphene flakes and (periodic) edges, i.e., nanographenes and graphene nanoribbons. The proposed nomenclature scheme is a complete scheme that similarly treats all these structures. Beyond this generalization, we study the geometric features of graphene flakes and edges based on ideas from the graph theory, as well as the pore-flake duality. Based on this study, we propose an algorithm for the systematic generation, identification, and numbering of graphene pores, flakes, and edges. The algorithm and the nomenclature scheme can also be used for flakes and edges of similar honeycomb systems.

Theoretical insights into the methane catalytic decomposition on graphene nanoribbons edges

Catalytic methane decomposition (CMD) is receiving much attention as a promising application for hydrogen production. Due to the high energy required for breaking the C-H bonds of methane, the choice of catalyst is crucial to the viability of this process. However, atomistic insights for the CMD mechanism on carbon-based materials are still limited. Here, we investigate the viability of CMD under reaction conditions on the zigzag (12-ZGNR) and armchair (AGRN) edges of graphene nanoribbons employing dispersion-corrected density functional theory (DFT). First, we investigated the desorption of H and H₂ at 1200 K on the passivated 12-ZGNR and 12-AGNR edges. The diffusion of hydrogen atom on the passivated edges is the rate determinant step for the most favourable H₂ desorption pathway, with a activation free energy of 4.17 eV and 3.45 eV on 12-ZGNR and 12-AGNR, respectively. The most favourable H₂ desorption occurs on the 12-AGNR edges with a free energy barrier of 1.56 eV, reflecting the availability of bare carbon active sites on the catalytic application. The direct dissociative chemisorption of CH₄ is the preferred pathway on the non-passivated 12-ZGNR edges, with an activation free energy of 0.56 eV. We also present the reaction steps for the complete catalytic dehydrogenation of methane on 12-ZGNR and 12-AGNR edges, proposing a mechanism in which the solid carbon formed on the edges act as new active sites. The active sites on the 12-AGNR edges show more propensity to be regenerated due lower free energy barrier of 2.71 eV for the H₂ desorption from the newly grown active site. Comparison is made between the results obtained here and experimental and computational data available in the literature. We provide fundamental insights for the engineering of carbon-based catalysts for the CMD, showing that the bare carbon edges of graphene nanoribbons have performance comparable to commonly used metallic and bi-metallic catalysts for methane decomposition.

Functional groups in graphene oxide

Graphene oxide consists of diverse surface chemistry which allows tethering GO with additional functionalities and tuning its intrinsic properties. This review summarizes recently advanced methods to covalently modify GO for specific applications.

A Singular Molecule-to-Molecule Transformation on Video: The Bottom-Up Synthesis of Fullerene C60 from Truxene Derivative C60H30

Singular reaction events of small molecules and their dynamics remain a hardly understood territory in chemical sciences since spectroscopy relies on ensemble-averaged data, and microscopic scanning probe techniques show snapshots of frozen scenes. Herein, we report on continuous high-resolution transmission electron microscopic video imaging of the electron-beam-induced bottom-up synthesis of fullerene C₆₀ through cyclodehydrogenation of tailor-made truxene derivative 1 (C₆₀H₃₀), which was deposited on graphene as substrate. During the reaction, C₆₀H₃₀ transformed in a multistep process to fullerene C₆₀. Hereby, the precursor, metastable intermediates, and the product were identified by correlations with electron dose-corrected molecular simulations and single-molecule statistical analysis, which were substantiated with extensive density functional theory calculations. Our observations revealed that the initial cyclodehydrogenation pathway leads to thermodynamically favored intermediates through seemingly classical organic reaction mechanisms. However, dynamic interactions of the intermediates with the substrate render graphene as a non-innocent participant in the dehydrogenation process, which leads to a deviation from the classical reaction pathway. Our precise visual comprehension of the dynamic transformation implies that the outcome of electron-beam-initiated reactions can be controlled with careful molecular precursor design, which is important for the development and design of materials by electron beam lithography.

Stability of hydrogen-terminated graphene edges

Hydrogen passivation is an important method used to stabilize a specific graphene edge. Although several hydrogen-terminated graphene edges have been proposed in theory, a comprehensive exploration of highly stable hydrogen-terminated graphene edges is still absent. According to the bare graphene-edge databases, a series of hydrogen-terminated graphene edges have been proposed. The energy stability of hydrogen-terminated zigzag and armchair graphene edges is fully investigated. The six most stable hydrogen-terminated zigzag edges and six armchair edges of graphene are determined. Hydrogen passivation makes hydrogen-terminated graphene edges energetically more stable than bare graphene edges. The additional hydrogen atoms balance the dangling bonds of carbon atoms at edges by forming hydrogen-carbon covalent bonds. Hydrogen-terminated graphene edges with six-membered carbon rings have better global stability than those composed of non-hexagonal structural units. The effects of the experimental temperatures and hydrogen partial pressures on the stability of hydrogen-terminated graphene edges are fully investigated. Furthermore, hydrogen passivation can open the band gap of graphene effectively. These results provide a deep understanding of hydrogen-terminated graphene nanostructures.

Electrostatic forces above graphene nanoribbons and edges interpreted as partly hydrogen-free

Graphene nanoribbons' electronic transport properties strongly depend on the type of edge, armchair, zigzag or other, and on edge functionalization that can be used for band-gap engineering. For only partly hydrogenated edges interesting magnetic properties are predicted. Electric charge accumulates at edges and corners. Scanning force microscopy has so far shown the centre of graphene nanoribbons with atomic resolution using a quartz crystal tuning fork sensor of high stiffness. Weak long-range electrostatic forces related to the charge accumulation on the edges of graphene nanoribbons could not be imaged so far. Here, we show the electrostatic forces at the corners and edges of graphene nanoribbons are amenable to measurement. We use soft cantilevers and a bimodal imaging technique to combine enhanced sensitivity to weak long-range electrostatic forces with the high resolution of the second-frequency shift. Additionally, in our work the edges of the nanoribbons are mainly hydrogen-free, opening to the route to investigations of partly hydrogenated magnetic nanoribbons.

Direct observation and catalytic role of mediator atom in 2D materials

The structural transformations of graphene defects have been extensively researched through aberration-corrected transmission electron microscopy (AC-TEM) and theoretical calculations. For a long time, a core concept in understanding the structural evolution of graphene defects has been the Stone-Thrower-Wales (STW)-type bond rotation. In this study, we show that undercoordinated atoms induce bond formation and breaking, with much lower energy barriers than the STW-type bond rotation. We refer to them as mediator atoms due to their mediating role in the breaking and forming of bonds. Here, we report the direct observation of mediator atoms in graphene defect structures using AC-TEM and annular dark-field scanning TEM (ADF-STEM) and explain their catalytic role by tight-binding molecular dynamics (TBMD) simulations and image simulations based on density functional theory (DFT) calculations. The study of mediator atoms will pave a new way for understanding not only defect transformation but also the growth mechanisms in two-dimensional materials.

Local Carbon Concentration Determines the Graphene Edge Structure

Although the structures and properties of various graphene edges have attracted enormous attention, the underlying mechanism that determines the appearance of various edges is still unknown. Here, a global search of graphene edge structures is performed by using the particle swarm optimization algorithm. In addition to locating the most stable edges of graphene, two databases of graphene armchair and zigzag edge structures are built. Graphene edge self-passivation plays an important role in the stability of the edges of graphene, and self-passivated edge structures that contain both octagons and triangles are found for the first time. The obvious "apical dominance" feature of armchair edges is found. The appearance of the experimentally observed ac(56), ac(677), and Klein edges can be explained by the local carbon concentration. Additionally, the graphene edge database is also significant for the study of the open end of nanotubes or fullerenes.

Imaging Beam-Sensitive Materials by Electron Microscopy

Electron microscopy allows the extraction of multidimensional spatiotemporally correlated structural information of diverse materials down to atomic resolution, which is essential for figuring out their structure-property relationships. Unfortunately, the high-energy electrons that carry this important information can cause damage by modulating the structures of the materials. This has become a significant problem concerning the recent boost in materials science applications of a wide range of beam-sensitive materials, including metal-organic frameworks, covalent-organic frameworks, organic-inorganic hybrid materials, 2D materials, and zeolites. To this end, developing electron microscopy techniques that minimize the electron beam damage for the extraction of intrinsic structural information turns out to be a compelling but challenging need. This article provides a comprehensive review on the revolutionary strategies toward the electron microscopic imaging of beam-sensitive materials and associated materials science discoveries, based on the principles of electron-matter interaction and mechanisms of electron beam damage. Finally, perspectives and future trends in this field are put forward.

Atomic Scale Imaging of Reversible Ring Cyclization in Graphene Nanoconstrictions

We present an atomic level study of reversible cyclization processes in suspended nanoconstricted regions of graphene that form linear carbon chains (LCCs). Before the nanoconstricted region reaches a single linear carbon chain (SLCC), we observe that a double linear carbon chain (DLCC) structure often reverts back to a ribbon of sp² hybridized oligoacene rings, in a process akin to the Bergman rearrangement. When the length of the DLCC system only consists of ∼5 atoms in each LCC, full recyclization occurs for all atoms present, but for longer DLCCs we find that only single sections of the chain are modified in their bonding hybridization and no full ring closure occurs along the entire DLCCs. This process is observed in real time using aberration-corrected transmission electron microscopy and simulated using density functional theory and tight binding molecular dynamics calculations. These results show that DLCCs are highly sensitive to the adsorption of local gas molecules or surface diffusion impurities and undergo structural modifications.

Black Phosphorus: Degradation Favors Lubrication

Due to its innate instability, the degradation of black phosphorus (BP) with oxygen and moisture was considered the obstacle for its application in ambient conditions. Here, a friction force reduced by about 50% at the degraded area of the BP nanosheets was expressly observed using atomic force microscopy due to the produced phosphorus oxides during degradation. Energy-dispersive spectrometer mapping analyses corroborated the localized concentration of oxygen on the degraded BP flake surface where friction reduction was observed. Water absorption was discovered to be essential for the degraded characteristic as well as the friction reduction behavior of BP sheets. The combination of water molecules as well as the resulting chemical groups (P-OH bonds) that are formed on the oxidized surface may account for the friction reduction of degraded BP flakes. It is indicated that, besides its layered structure, the ambient degradation of BP significantly favors its lubrication behavior.

Ultrathin Metal Crystals: Growth on Supported Graphene Surfaces and Applications

Controlled nucleation and growth of metal clusters in metal deposition processes is a long-standing issue for thin-film-based electronic devices. When metal atoms are deposited on solid surfaces, unintended defects sites always lead to a heterogeneous nucleation, resulting in a spatially nonuniform nucleation with irregular growth rates for individual nuclei, resulting in a rough film that requires a thicker film to be deposited to reach the percolation threshold. In the present study, it is shown that substrate-supported graphene promotes the lateral 2D growth of metal atoms on the graphene. Transmission electron microscopy reveals that 2D metallic single crystals are grown epitaxially on supported graphene surfaces while a pristine graphene layer hardly yields any metal nucleation. A surface energy barrier calculation based on density functional theory predicts a suppression of diffusion of metal atoms on electronically perturbed graphene (supported graphene). 2D single Au crystals grown on supported graphene surfaces exhibit unusual near-infrared plasmonic resonance, and the unique 2D growth of metal crystals and self-healing nature of graphene lead to the formation of ultrathin, semitransparent, and biodegradable metallic thin films that could be utilized in various biomedical applications.

Sticking of atomic hydrogen on graphene

Recent years have witnessed an ever growing interest in the interactions between hydrogen atoms and a graphene sheet. Largely motivated by the possibility of modulating the electric, optical and magnetic properties of graphene, a huge number of studies have appeared recently that added to and enlarged earlier investigations on graphite and other carbon materials. In this review we give a glimpse of the many facets of this adsorption process, as they emerged from these studies. The focus is on those issues that have been addressed in detail, under carefully controlled conditions, with an emphasis on the interplay between the adatom structures, their formation dynamics and the electric, magnetic and chemical properties of the carbon sheet.

Nonlinear fracture toughness measurement and crack propagation resistance of functionalized graphene multilayers

Despite promising applications of two-dimensional (2D) materials, one major concern is their propensity to fail in a brittle manner, which results in a low fracture toughness causing reliability issues in practical applications. We show that this limitation can be overcome by using functionalized graphene multilayers with fracture toughness (J integral) as high as ~39 J/m², measured via a microelectromechanical systems-based in situ transmission electron microscopy technique coupled with nonlinear finite element fracture analysis. The measured fracture toughness of functionalized graphene multilayers is more than two times higher than graphene (~16 J/m²). A linear fracture analysis, similar to that previously applied to other 2D materials, was also conducted and found to be inaccurate due to the nonlinear nature of the stress-strain response of functionalized graphene multilayers. A crack arresting mechanism of functionalized graphene multilayers was experimentally observed and identified as the main contributing factor for the higher fracture toughness as compared to graphene. Molecular dynamics simulations revealed that the interactions among functionalized atoms in constituent layers and distinct fracture pathways in individual layers, due to a random distribution of functionalized carbon atoms in multilayers, restrict the growth of a preexisting crack. The results inspire potential strategies for overcoming the relatively low fracture toughness of 2D materials through chemical functionalization.

Approaches for Achieving Superlubricity in Two-Dimensional Materials

Controlling friction and reducing wear of moving mechanical systems is important in many applications, from nanoscale electromechanical systems to large-scale car engines and wind turbines. Accordingly, multiple efforts are dedicated to design materials and surfaces for efficient friction and wear manipulation. Recent advances in two-dimensional (2D) materials, such as graphene, hexagonal boron nitride, molybdenum disulfide, and other 2D materials opened an era for conformal, atomically thin solid lubricants. However, the process of effectively incorporating 2D films requires a fundamental understanding of the atomistic origins of friction. In this review, we outline basic mechanisms for frictional energy dissipation during sliding of two surfaces against each other, and the procedures for manipulating friction and wear by introducing 2D materials at the tribological interface. Finally, we highlight recent progress in implementing 2D materials for friction reduction to near-zero values-superlubricity-across scales from nano- up to macroscale contacts.

Atomic structure of defects and dopants in 2D layered transition metal dichalcogenides

Transmission electron microscopy can directly image the detailed atomic structure of layered transition metal dichalcogenides, revealing defects and dopants.

Hydrogen Adsorption on Nearly Zigzag-Edged Nanoribbons: A Density Functional Theory Study

The realistic shapes of N doped graphene nanoribbons (GNRs) can be realized by considering nearly zigzag-edged (NZE) imperfections and pyridine defects (3NV). The paper focuses on NZE-GNRs with 3NV that is populated by Scandium abbreviated as Sc/NZE-3NVGNRs. Systematic calculations have clarified roles of the nano-shapes of NZE-3NVGNRs in its formation, energetics, stability and electron states functionalized with Sc using density functional theory (DFT) formalisms. According to DFT calculations, the magnitude of the spin that is attributed to the rise of magnetic order is closely linked to the altered shape of the ribbon edges. Also, calculations show that the stability of Sc functionalization at the 3NV and NZE site is thermodynamically stable and is dictated by a strong binding energy (BE). The magnitude of the BE is enhanced when the zigzag edge is short or the ribbon width is narrow, suggesting a reduced clustering of Sc atoms over the Sc-doped NZE-3NVGNRs. Results also show that as the length of the zigzag edge in Sc/NZE-3NVGNRs increases it creates considerable distortion on the appearance of the structure. Finally, the Sc/NZE-3NVGNRs as a potential candidate for hydrogen storage was evaluated and it was found that it could adsorb multiple hydrogen molecules.

Orientation dependent interlayer stacking structure in bilayer MoS2 domains

We have studied the atomic structure of small secondary domains that nucleate on monolayer MoS₂ grown by chemical vapour deposition (CVD), which form the basis of bilayer MoS₂. The small secondary bilayer domains have a faceted geometry with three-fold symmetry and adopt two distinct orientations with 60° rotation relative to an underlying monolayer MoS₂ single crystal sheet. The two distinct orientations are associated with the 2H and 3R stacking configuration for bilayer MoS₂. Atomic resolution images have been recorded using annular dark field scanning transmission electron microscopy (ADF-STEM) that show the edge termination, lattice orientation and stacking sequence of the bilayer domains relative to the underlying monolayer MoS₂. These results provide important insights that bilayer MoS₂ growth from 60° rotated small nuclei on the surface of monolayer MoS₂ could lead to defective boundaries when merged to form larger continuous bilayer regions and that pure AA' or AB bilayer stacking may be challenging unless from a single seed.

Direct observation of multiple rotational stacking faults coexisting in freestanding bilayer MoS2

Electronic properties of two-dimensional (2D) MoS₂ semiconductors can be modulated by introducing specific defects. One important type of defect in 2D layered materials is known as rotational stacking fault (RSF), but the coexistence of multiple RSFs with different rotational angles was not directly observed in freestanding 2D MoS₂ before. In this report, we demonstrate the coexistence of three RSFs with three different rotational angles in a freestanding bilayer MoS₂ sheet as directly observed using an aberration-corrected transmission electron microscope (TEM). Our analyses show that these RSFs originate from cracks and dislocations within the bilayer MoS₂. First-principles calculations indicate that RSFs with different rotational angles change the electronic structures of bilayer MoS₂ and produce two new symmetries in their bandgaps and offset crystal momentums. Therefore, employing RSFs and their coexistence is a promising route in defect engineering of MoS₂ to fabricate suitable devices for electronics, optoelectronics, and energy conversion.

Microfluidic Electrochemical Impedance Spectroscopy of Carbon Composite Nanofluids

Understanding the internal structure of composite nanofluids is critical for controlling their properties and engineering advanced composite nanofluid systems for various applications. This goal can be made possible by precise analysis with the help of a systematic robust platform. Here, we demonstrate a microfluidic device that can control the orientation of carbon nanomaterials in a suspension by applying external fields and subsequently examine the electrochemical properties of the fluids at microscale. Composite nanofluids were prepared using carbon nanomaterials, and their rheological, thermal, electrical, and morphological characteristics were examined. The analysis revealed that microfluidic electrochemical impedance spectroscopy (EIS) in the device offered more reliable in-depth information regarding the change in the microstructure of carbon composite nanofluids than typical bulk measurements. Equivalent circuit modelling was performed based on the EIS results. Furthermore, the hydrodynamics and electrostatics of the microfluidic platform were numerically investigated. We anticipate that this microfluidic approach can serve as a new strategy for designing and analyzing composite nanofluids more efficiently.

Highly dispersible edge-selectively oxidized graphene with improved electrical performance

We prepared liquid phase exfoliated edge-selectively oxidized graphene (LPEOG) with a high concentration in water (∼14.7 mg ml^-1) and a high ratio of a single layer (70%). The edge of graphite was selectively oxidized by step II oxidation of the modified Hummers method, and we subsequently exfoliated the edge-selectively oxidized graphite (EOG) into LPEOG. The edge selective oxidation of the LPEOG was confirmed by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), zeta-potentiometry, Raman spectroscopy, Fourier-transform infrared spectroscopy (FT-IR), atomic force microscopy (AFM), and transmission electron microscopy (TEM). The highly concentrated LPEOG ink can be used in solution processing such as simple drawing or spin casting. Reduced LPEOG showed a higher conductivity (120 000 S m^-1) than that of reduced graphene oxide (68 800 S m^-1) despite the small lateral size. A transparent conducting film prepared from the LPEOG ink showed a lower surface resistance (∼2.97 kΩ sq^-1) at a higher transmittance (>83.0 %T) compared to those of the graphene oxide based film. These results indicate that preservation of π-conjugation of the basal plane of graphene is critical for electrical performance of graphene. Our method facilitates solution processing of graphene for a wide range of applications.

Current and future directions in electron transfer chemistry of graphene

The participation of graphene in electron transfer chemistry, where an electron is transferred between graphene and other species, encompasses many important processes that have shown versatility and potential for use in important applications.

The mechanics and design of a lightweight three-dimensional graphene assembly

Recent advances in three-dimensional (3D) graphene assembly have shown how we can make solid porous materials that are lighter than air. It is plausible that these solid materials can be mechanically strong enough for applications under extreme conditions, such as being a substitute for helium in filling up an unpowered flight balloon. However, knowledge of the elastic modulus and strength of the porous graphene assembly as functions of its structure has not been available, preventing evaluation of its feasibility. We combine bottom-up computational modeling with experiments based on 3D-printed models to investigate the mechanics of porous 3D graphene materials, resulting in new designs of carbon materials. Our study reveals that although the 3D graphene assembly has an exceptionally high strength at relatively high density (given the fact that it has a density of 4.6% that of mild steel and is 10 times as strong as mild steel), its mechanical properties decrease with density much faster than those of polymer foams. Our results provide critical densities below which the 3D graphene assembly starts to lose its mechanical advantage over most polymeric cellular materials.

The effect of different hydrogen terminations on the structural and electronic properties in the triangular array graphene nanomeshes

Constructing periodic nanoscale holes on graphene to form graphene nanomeshes (GNMs) is an effective way for opening band gaps. The GNMs terminated by di-hydrogenation could open a sizable band gap due to the stronger on-site potential between holes.

Catalytic behavior of noble metal nanoparticles for the hydrogenation and oxidation of multiwalled carbon nanotubes

The catalytic behavior of various noble metal nanoparticles (NPs) supported directly on multiwalled carbon nanotubes (MWCNTs) was observed using environmental transmission electron microscopy (E-TEM). Gasification of the MWCNTs via catalytic hydrogenation or oxidation progressed at ∼450°C in conjunction with certain noble metal NP catalysts at the interface between MWCNTs and the NPs. During such catalytic reactions, the NPs were observed to move rapidly over the MWCNT surfaces. The mobility and wettability of the NPs varied depending on the particular metal NPs employed and the ambient atmosphere. While rhodium NPs exhibited high wettability under both hydrogen and oxygen atmospheres, the wettability of platinum, palladium and iridium NPs on MWCNTs varied with the atmosphere. The metal NPs seemed to have high degrees of crystallinity while their morphologies fluctuated throughout the catalytic reactions.

Characterization of graphene edge functionalization by grating enhanced Raman spectroscopy

We demonstrate a large enhancement of edge related Raman features, associated with armchair and zigzag hydrogen-terminated graphene edges. The graphene edges act as good scatterers to excite LSPP on a noble metal surface.

Atomic Structure of Graphene Subnanometer Pores

The atomic structure of subnanometer pores in graphene, of interest due to graphene's potential as a desalination and gas filtration membrane, is demonstrated by atomic resolution aberration corrected transmission electron microscopy. High temperatures of 500 °C and over are used to prevent self-healing of the pores, permitting the successful imaging of open pore geometries consisting of between -4 to -13 atoms, all exhibiting subnanometer diameters. Picometer resolution bond length measurements are used to confirm reconstruction of five-membered ring projections that often decorate the pore perimeter, knowledge which is used to explore the viability of completely self-passivated subnanometer pore structures; bonding configurations where the pore would not require external passivation by, for example, hydrogen to be chemically inert.

Formation of Klein Edge Doublets from Graphene Monolayers

With increasing possibilities for applications of graphene, it is essential to fully characterize the rich topological variations in graphene edge structures. Using aberration-corrected transmission electron microscopy, dangling carbon doublets at the edge of monolayer graphene crystals have been observed. Unlike the single-atom Klein edge often found at zigzag edges, these carbon dimers were observed in various edge structure environments, but most frequently on the more stable armchair edges. Observation of this Klein edge doublet over time reveals that its existence enhances the stability of armchair edges and is a route to atom abstraction on zigzag edges.

Controlled formation of closed-edge nanopores in graphene

Dangling bonds at the edge of a nanopore in monolayer graphene make it susceptible to back-filling at low temperatures from atmospheric hydrocarbons, leading to potential instability for nanopore applications, such as DNA sequencing. We show that closed edge nanopores in bilayer graphene are robust to back-filling under atmospheric conditions for days. A controlled method for closed edge nanopore formation starting from monolayer graphene is reported using an in situ heating holder and electron beam irradiation within an aberration-corrected transmission electron microscopy. Tailoring of closed-edge nanopore sizes is demonstrated from 1.4-7.4 nm. These results should provide mechanisms for improving the stability of nanopores in graphene for a wide range of applications involving mass transport.

Temperature dependence of the reconstruction of zigzag edges in graphene

We examine the temperature dependence of graphene edge terminations at the atomic scale using an in situ heating holder within an aberration-corrected transmission electron microscope. The relative ratios of armchair, zigzag, and reconstructed zigzag edges from over 350 frames at each temperature are measured. Below 400 °C, the edges are dominated by zigzag terminations, but above 600 °C, this changes dramatically, with edges dominated by armchair and reconstructed zigzag edges. We show that at low temperature chemical etching effects dominate and cause deviation to the thermodynamics of the system. At high temperatures (600 and 800 °C), adsorbates are evaporated from the surface of graphene and chemical etching effects are significantly reduced, enabling the thermodynamic distribution of edge types to be observed. The growth rate of holes at high temperature is also shown to be slower than at room temperature, indicative of the reduced chemical etching process. These results provide important insights into the role of chemical etching effects in the hole formation, edge sputtering, and edge reconstruction in graphene.

Structural design of graphene for use in electrochemical energy storage devices

This review elucidates the structural design methodologies toward high-performance graphene-based electrode materials for electrochemical energy storage devices.

Extended Klein edges in graphene

Graphene has three experimentally confirmed periodic edge terminations, zigzag, reconstructed 5-7, and arm-chair. Theory predicts a fourth periodic edge of graphene called the extended Klein (EK) edge, which consists of a series of single C atoms protruding from a zigzag edge. Here, we confirm the existence of EK edges in both graphene nanoribbons and on the edge of bulk graphene using atomic resolution imaging by aberration-corrected transmission electron microscopy. The formation of the EK edge stems from sputtering and reconstruction of the zigzag edge. Density functional theory reveals minimal energy for EK edge reconstruction and bond distortion both in and out of plane, supporting our TEM observations. The EK edge can now be included as the fourth member of observed periodic edge structures in graphene.

Direct in situ observations of single Fe atom catalytic processes and anomalous diffusion at graphene edges

Single-atom catalysts are of great interest because of their high efficiency. In the case of chemically deposited sp(2) carbon, the implementation of a single transition metal atom for growth can provide crucial insight into the formation mechanisms of graphene and carbon nanotubes. This knowledge is particularly important if we are to overcome fabrication difficulties in these materials and fully take advantage of their distinct band structures and physical properties. In this work, we present atomically resolved transmission EM in situ investigations of single Fe atoms at graphene edges. Our in situ observations show individual iron atoms diffusing along an edge either removing or adding carbon atoms (viz., catalytic action). The experimental observations of the catalytic behavior of a single Fe atom are in excellent agreement with supporting theoretical studies. In addition, the kinetics of Fe atoms at graphene edges are shown to exhibit anomalous diffusion, which again, is in agreement with our theoretical investigations.

The role of the bridging atom in stabilizing odd numbered graphene vacancies

Vacancy defects in graphene with an odd number of missing atoms, such as the trivacancy, have been imaged at atomic resolution using aberration corrected transmission electron microscopy. These defects are not just stabilized by simple bond reconstructions between under-coordinated carbon atoms, as exhibited by even vacancies such as the divacancy. Instead we have observed reconstructions consisting of under-coordinated bridging carbon atoms spanning the vacancy to saturate edge atoms. We report detailed studies of the effect of this bridging atom on the configuration of the trivacancy and higher order odd number vacancies, as well as its role in defect stabilization in amorphous systems. Theoretical analysis using density functional theory and tight-binding molecular dynamics calculations demonstrate that the bridging atom enables the low energy reconfiguration of these defect structures.

Edge structures for nanoscale graphene islands on Co(0001) surfaces

Low-temperature scanning tunneling microscopy measurements and first-principles calculations are employed to characterize edge structures observed for graphene nanoislands grown on the Co(0001) surface. Images of these nanostructures reveal straight well-ordered edges with zigzag orientation, which are characterized by a distinct peak at low bias in tunneling spectra. Density functional theory based calculations are used to discriminate between candidate edge structures. Several zigzag-oriented edge structures have lower formation energy than armchair-oriented edges. Of these, the lowest formation energy configurations are a zigzag and a Klein edge structure, each with the final carbon atom over the hollow site in the Co(0001) surface. In the absence of hydrogen, the interaction with the Co(0001) substrate plays a key role in stabilizing these edge structures and determines their local conformation and electronic properties. The calculated electronic properties for the low-energy edge structures are consistent with the measured scanning tunneling images.