Graphene Grain Boundaries Research Articles

Unraveling the Sinuous Grain Boundaries in Graphene

Grain boundaries (GBs) in graphene are stable strings of pentagon‐heptagon dislocations. The GBs have been believed to favor an alignment of dislocations, but increasing number of experiments reveal diversely sinuous GB structures whose origins have long been elusive. Based on dislocation theory and first‐principles calculations, an extensive analysis of the graphene GBs is conducted and it is revealed that the sinuous GB structures, albeit being longer than the straight forms, can be energetically optimal once the global GB line cannot bisect the tilt angle. The unusually favorable sinuous GBs can actually decompose into a series of well‐defined bisector segments that effectively relieve the in‐plane stress of edge dislocations, and the established atomic structures closely resemble recent experimental images of typical GBs. In contrast to previously used models, the sinuous GBs show improved mechanical properties and are distinguished by a sizable electronic transport gap, which may open potential applications of polycrystalline graphene in functional devices.

Grain boundaries in graphene on SiC(0001̅) substrate.

Grain boundaries in epitaxial graphene on the SiC(0001̅) substrate are studied using scanning tunneling microscopy and spectroscopy. All investigated small-angle grain boundaries show pronounced out-of-plane buckling induced by the strain fields of constituent dislocations. The ensemble of observations determines the critical misorientation angle of buckling transition θc = 19 ± 2°. Periodic structures are found among the flat large-angle grain boundaries. In particular, the observed θ = 33 ± 2° highly ordered grain boundary is assigned to the previously proposed lowest formation energy structural motif composed of a continuous chain of edge-sharing alternating pentagons and heptagons. This periodic grain boundary defect is predicted to exhibit strong valley filtering of charge carriers thus promising the practical realization of all-electric valleytronic devices.

Non-equilibrium grain boundaries with excess energy in graphene

Exploring an approach to grain boundary (GB) mesoscopic structure in 3D polycrystals, we develop a model description for equilibrium and non-equilibrium states of GBs in graphene. Non-equilibrium states of GBs in graphene are analyzed for the first time. On the base of this model, dependence of total energy for symmetric GBs on the misorientation angle, E(θ), in graphene was calculated. It was found that for the same average misorientation angle the energy in a non-equilibrium state of a GB in graphene can significantly exceed the energy in the equilibrium state. In graphene, the GB energy depends to a greater extent on the deviation from the equilibrium state than on the misorientation angle. It is proposed to account for the presence of non-equilibrium GBs in the explanations of unusual physical properties of graphene.

Formation of Graphene Grain Boundaries on Cu(100) Surface and a Route Towards Their Elimination in Chemical Vapor Deposition Growth

Grain boundaries (GBs) in graphene prepared by chemical vapor deposition (CVD) greatly degrade the electrical and mechanical properties of graphene and thus hinder the applications of graphene in electronic devices. The seamless stitching of graphene flakes can avoid GBs, wherein the identical orientation of graphene domain is required. In this letter, the graphene orientation on one of the most used catalyst surface — Cu(100) surface, is explored by density functional theory (DFT) calculations. Our calculation demonstrates that a zigzag edged hexagonal graphene domain on a Cu(100) surface has two equivalent energetically preferred orientations, which are 30 degree away from each other. Therefore, the fusion of graphene domains on Cu(100) surface during CVD growth will inevitably lead to densely distributed GBs in the synthesized graphene. Aiming to solve this problem, a simple route, that applies external strain to break the symmetry of the Cu(100) surface, was proposed and proved efficient.

Chemical sensing with switchable transport channels in graphene grain boundaries.

Grain boundaries can markedly affect the electronic, thermal, mechanical and optical properties of a polycrystalline graphene. While in many applications the presence of grain boundaries in graphene is undesired, here we show that they have an ideal structure for the detection of chemical analytes. We observe that an isolated graphene grain boundary has ~300 times higher sensitivity to the adsorbed gas molecules than a single-crystalline graphene grain. Our electronic structure and transport modelling reveal that the ultra-sensitivity in grain boundaries is caused by a synergetic combination of gas molecules accumulation at the grain boundary, together with the existence of a sharp onset energy in the transmission spectrum of its conduction channels. The discovered sensing platform opens up new pathways for the design of nanometre-scale highly sensitive chemical detectors.

Raman spectroscopic investigation of polycrystalline structures of CVD-grown graphene by isotope labeling.

Topological defects, such as point defects, dislocations and grain boundaries, have a dramatic influence on the chemical and physical properties of large-scale graphene grown by chemical vapor deposition (CVD) method. Here we demonstrate the Raman visualization of polycrystalline structures in an isotopically modified CVD graphene. By means of the reversible reaction of methane on a copper catalyst, the etching of (12)C-lattice and surface deposition of (13)C-atoms occur in CVD graphene by sequentially introducing hydrogen and isotopic methane after standard growth of graphene with full monolayer coverage. Spatial Raman spectroscopic mapping on labeled graphene reveals pronounced network-like (13)C-rich regions, which are further identified to exist along the grain boundaries of graphene by low-energy electron microscopy. The structural defects inside the graphene grains are also targeted in the isotope labeling process. Our work opens a new way to investigate multiple grain structures in CVD graphene with a simple spectroscopic technique.

Shear strain induced modulation to the transport properties of graphene

Applying shear strain has been considered as a hopeful method to open a band gap of graphene. To study the transport properties of graphene under shear strain, a device was fabricated to apply shear strain, up to 16.7%, to graphene grown by chemical vapor deposition method. A top gate with ionic liquid as the dielectric material was used to tune the carrier density. The conductance of the Dirac point and carrier mobility is found to increase with a comparatively small increasing strain but then decrease with a larger one. Such a behavior might be related to several factors: the wrinkles, the transverse conducting channels, and the grain boundaries of graphene. Our study is helpful to further understand the strain engineering in graphene.

Periodic grain boundaries formed by thermal reconstruction of polycrystalline graphene film.

Grain boundaries consisting of dislocation cores arranged in a periodic manner have well-defined structures and peculiar properties and can be potentially applied as conducting circuits, plasmon reflectors and phase retarders. Pentagon-heptagon (5-7) pairs or pentagon-octagon-pentagon (5-8-5) carbon rings are known to exist in graphene grain boundaries. However, there are few systematic experimental studies on the formation, structure and distribution of periodic grain boundaries in graphene. Herein, scanning tunneling microscopy (STM) was applied to study periodic grain boundaries in monolayer graphene grown on a weakly interacting Cu(111) crystal. The periodic grain boundaries are formed after the thermal reconstruction of aperiodic boundaries, their structures agree well with the prediction of the coincident-site-lattice (CSL) theory. Periodic grain boundaries in quasi-freestanding graphene give sharp local density of states (LDOS) peaks in the tunneling spectra as opposed to the broad peaks of the aperiodic boundaries. This suggests that grain boundaries with high structural quality can introduce well-defined electronic states in graphene and modify its electronic properties.

Scattering of flexural acoustic phonons at grain boundaries in graphene

We investigate the scattering of long-wavelength flexural phonons against grain boundaries in graphene using molecular dynamics simulations. Three symmetric tilt grain boundaires are considered: one with a misorientation angle of $17.9^\circ$ displaying an out-of-plane buckling 1.5 nm high and 5 nm wide, one with a misorientation angle of $9.4^\circ$ and an out-of-plane buckling 0.6 nm high and 1.7 nm wide, and one with a misorientation angle of $32.2^\circ$ and no out-of-plane buckling. At the flat grain boundary, the phonon transmission exceeds 95 % for wavelengths above 1 nm. The buckled boundaries have a substantially lower transmission in this wavelength range, with a minimum transmission of 20 % for the $17.9^\circ$ boundary and 40 % for the $9.4^\circ$ boundary. At the buckled boundaries, coupling between flexural and longitudinal phonon modes is also observed. The results indicate that scattering of long-wavelength flexural phonons at grain boundaries in graphene is mainly due to out-of-plane buckling. A continuum mechanical model of the scattering process has been developed, providing a deeper understanding of the scattering process as well as a way to calculate the effect of a grain boundary on long-wavelength flexural phonons based on the buckling size.

Predicting interface structures: FromSrTiO3to graphene

We present here a fully first-principles method for predicting the atomic structure of interfaces. Our method is based on the {\it ab initio} random structure searching (AIRSS) approach, applied here to treat two dimensional defects. The method relies on repeatedly generating random structures in the vicinity of the interface and relaxing them within the framework of density functional theory (DFT). The method is simple, requiring only a small set of parameters that can be easily connected to the chemistry of the system of interest, and efficient, ideally adapted to high-throughput first-principles calculations on modern parallel architectures. Being first-principles, our method is transferable, an important requirement for a generic computational method for the determination of the structure of interfaces. Results for two structurally and chemically very different interfaces are presented here, grain boundaries in graphene and grain boundaries in strontium titanate (SrTiO$_3$). We successfully find a previously unknown low energy grain boundary structure for the graphene system, as well as recover the previously known higher energy structures. For the SrTiO$_3$ system we study both stoichiometric and non-stoichiometric compositions near the grain boundary and find previously unknown low energy structures for all stoichiometries. We predict that these low energy structures have long-range distortions to the ground state crystal structure emanating into the bulk from the interface.

Insertion of Line Defect in Nanoribbons of Graphene, Boron Nitride, and Hybrid of Them: An AIMD Study

Using constant-temperature ab initio molecular dynamics simulation we demonstrated a way to insert extended line defects (ELDs) at the grain boundary in hybrid graphene and boron nitride nanoribbons (BNCNRs) as well as in pure graphene nanoribbons (GNRs) and pure boron nitride nanoribbons (BNNRs). Our systematic studies have shown that 5-8-5 and 8-8-8 extended line defects can be installed and stabilized by depositing different adatoms such as carbon, boron, and nitrogen at the grain boundaries of graphene–graphene, boron nitride–boron nitride, and graphene–boron nitride junctions. The electronic and magnetic structures of these nanoribbons are highly modulated in the presence of these ELDs.

Evidence of van Hove singularities in ordered grain boundaries of graphene.

It has long been under debate whether the electron transport performance of graphene could be enhanced by the possible occurrence of van Hove singularities in grain boundaries. Here, we provide direct experimental evidence to confirm the existence of van Hove singularity states close to the Fermi energy in certain ordered grain boundaries using scanning tunneling microscopy. The intrinsic atomic and electronic structures of two ordered grain boundaries, one with alternative pentagon and heptagon rings and the other with alternative pentagon pair and octagon rings, are determined. It is firmly verified that the carrier concentration and, thus, the conductance around ordered grain boundaries can be significantly enhanced by the van Hove singularity states. This finding strongly suggests that a graphene nanoribbon with a properly embedded ordered grain boundary can be a promising structure to improve the performance of graphene-based electronic devices.

Tuning structures and electronic spectra of graphene layers with tilt grain boundaries

Despite the structures and properties of tilt grain boundaries of graphite surface and graphene have been extensively studied, their effect on the structures and electronic spectra of graphene layers has not been fully addressed. Here we study effects of one-dimensional tilt grain boundaries on structures and electronic spectra of graphene multilayers by scanning tunneling microscopy and spectroscopy. A tilt grain boundary of a top graphene sheet in graphene multilayers leads to a twist between consecutive layers and generates superstructures (Moir\'e patterns) on one side of the boundary. Our results demonstrate that the twisting changes the electronic spectra of Bernal graphene bilayer and graphene trilayers dramatically. We also study quantum-confined twisted graphene bilayer generated between two adjacent tilt grain boundaries and find that the band structure of such a system is still valid even when the number of superstructures is reduced to two in one direction. It implies that the electronic structure of this system is driven by the physics of a single Moir\'e spot.

First Principles Study of the Role of Defects for Graphene Lithiation

Since the discovery of freestanding graphene, this 2D structure is considered as a potential alternative for graphite, because of its outstanding electronic and mechanical properties. It has attracted much attention as a promising electrode material for electrochemical energy storage devices such as Li-ion batteries due to high surface area and superior electronic conductivity. For Li-ion battery applications, graphene is used as a buffer layer to suppress the large volume expansion observed in high capacity anode materials and as a coating material to enhance conductivity. It was shown that graphene sheets prepared by reduction of graphite oxide (GO) deliver a reversible capacity and good cycling stability. Large variation in the capacity is reported by several studies for graphene, which is prepared with the exact similar techniques. Such discrepancy is suggested to result from the disordered and/or ordered structure of graphene sheets affecting lithium storage capacity, suggesting that the structural characteristics influence the resulting electrochemical performance.Note that earlier experimental studies have identified the existence of defects such as Stone-Wales and double vacancy defects, as well as grain boundaries in graphene. Thus, for complete understanding of the electrochemical performance of graphene, the role of different types of defects on the electrochemical performance should be identified as well as the defect density influence should be evaluated. Such comparative study along with pristine graphene can provide vast amount of information not only for the electrochemical performance, but also, for the structural and electronic structure differences with the types of defects, defect densities, and Li concentration.In this study, we use density functional theory to build atomistic level understanding of topological defects in graphene and their role on the electrochemical performance. Li configurations are generated combining an algorithm whose results are tested using a genetic algorithm method. We explore the effect of each defect type on Li adsorption and obtain an atomic level understanding of the interaction between defected graphene and Li. The electrochemical performance of these defective sheets under varying Li concentration is studied, and compared with those obtained for pristine graphene. Furthermore, the effect of defect density on the electrochemical performance is evaluated. Lithiation voltages as function of each defect type will be reported and compared with those of the pristine graphene. The results are expected to shed light on the role of each defect type on the electrochemical performance of graphene as an alternative anode material for Li ion batteries.

Synthetic optical holography for rapid nanoimaging

Holography has paved the way for phase imaging in a variety of wide-field techniques, including electron, X-ray and optical microscopy. In scanning optical microscopy, however, the serial fashion of image acquisition seems to challenge a direct implementation of traditional holography. Here we introduce synthetic optical holography (SOH) for quantitative phase-resolved imaging in scanning optical microscopy. It uniquely combines fast phase imaging, technical simplicity and simultaneous operation at visible and infrared frequencies with a single reference arm. We demonstrate SOH with a scattering-type scanning near-field optical microscope (s-SNOM) where it enables reliable quantitative phase-resolved near-field imaging with unprecedented speed. We apply these capabilities to nanoscale, non-invasive and rapid screening of grain boundaries in CVD-grown graphene, by recording 65 kilopixel near-field images in 26 s and 2.3 megapixel images in 13 min. Beyond s-SNOM, the SOH concept could boost the implementation of holography in other scanning imaging applications such as confocal microscopy.

Polycrystalline graphene curved by grain boundary for high performance nanoresonators

The grain boundaries (GBs) in polycrystalline graphene (PG) lead to distinctive mechanical and electronic properties relative to their single-crystalline counterparts. In particular, local buckling occurs in the presence of GBs, resulting in arched structures promising for high frequency nanoresonators with raised quality (Q) factor. In this paper such arched PG structures were studied by using software Avogadro and their fundamental vibrations were simulated based on a molecular structure model. The frequency and Q factor of the PG nanoresonators were found to be largely increased due to the GB-induced buckling. The major factors controlling such GB effects were also captured, including the inflection angle of the arched PG, the crystal orientation of its GB and the constraints on its boundaries. The study suggests that the presence of GB may lead to a novel strategy to enhance the structure stiffness of graphene for their potential applications in high performance nanoresonators and nanosensors.

Interfaces between buckling phases in silicene:Ab initiodensity functional theory calculations

The buckled structure of silicene leads to the possibility of new kinds of line defects that separate regions with reversed buckled phases. In the present work we show that these new grain boundaries have very low formation energies, one order of magnitude smaller than grain boundaries in graphene. These defects are stable along different orientations, and they can all be differentiated by STM images. All these defects present local dimerization between the Si atoms, with the formation of $\pi$-bonds. As a result, these defects are preferential adsorption sites when compared to the pristine region. Thus, the combination of low formation energy and higher reactivity of these defects may be cleverly used to design new nano-structures embedded in silicene.

Topological Aspects of Charge-Carrier Transmission across Grain Boundaries in Graphene

Dislocations and grain boundaries are intrinsic topological defects of large-scale polycrystalline samples of graphene. These structural irregularities have been shown to strongly affect electronic transport in this material. Here, we report a systematic investigation of the transmission of charge carriers across the grain-boundary defects in polycrystalline graphene by means of the Landauer-Büttiker formalism within the tight-binding approximation. Calculations reveal a strong suppression of transmission at low energies upon decreasing the density of dislocations with the smallest Burgers vector b = (1,0). The observed transport anomaly is explained from the point of view of resonant backscattering due to localized states of topological origin. These states are related to the gauge field associated with all dislocations characterized by b = (n,m) with n - m ≠ 3q (q ∈ Z). Our work identifies an important source of charge-carrier scattering caused by the topological defects present in large-area graphene samples produced by chemical vapor deposition.

Thermal Stability of Tilt Grain Boundaries in Graphene

We present an assessment of the stability and dynamics of grain boundaries in graphene for different misorientation angles at finite temperature and up to extremely high temperatures, in particular, for a misorientation angle of 6.6, 14.1, 19.66, 27.8, 38.21 and 46.83. We report a high stability against annihilation up to extreme temperatures.

The interplay between strain and size effects on the thermal conductance of grain boundaries in graphene

The effect of strain on the thermal transport across grain boundaries in graphene is investigated using molecular dynamics simulations. The thermal boundary conductance is found to decrease significantly under biaxial tension as expected. In contrast, under biaxial compression, the thermal boundary conductance is strongly affected by the dimensions of the graphene monolayer, increasing with strain for specimen with length-to-width ratio of less than 20 and being insensitive to strain for length-to-width ratio above 20. This rather unexpected size-dependence under biaxial compression is found to be a result of geometric instabilities.