Graphene Grain Boundaries Research Articles

Grain Boundaries and Gas Barrier Property of Graphene Revealed by Dark-Field Optical Microscopy

We demonstrate that dark-field (DF) optical microscopy is a powerful tool to visualize grain boundaries (GBs) and grain structure of graphene grown by chemical vapor deposition (CVD). Copper oxide nanoparticles sparsely formed along the graphene GBs by postgrowth mild oxidation allow one to determine the position and structure of the GBs by the DF microscope. As DF imaging offers a much higher sensitivity than bright-field (BF) microscopy, some GBs were observed even without the postgrowth oxidation. We found that periodic Cu steps formed below graphene can be also used to visualize the grain structure of the as-grown graphene by DF microscopy. Moreover, DF imaging is applicable to study of the gas barrier property of CVD graphene. Interestingly, the dissolved oxygen inside Cu foil enhanced oxidation of the Cu surface below graphene in spite of the fact that the graphene protects the underlying Cu from the exterior gas. Our work highlights the wide availability of DF optical microscopy in characterizing g...

Mapping electronic states of triple anti-parallel and symmetric zigzag grain boundaries of graphene on highly oriented pyrolytic graphite

We studied the structures and electronic spectra of two periodic GBs in graphene on highly oriented pyrolytic graphite (HOPG) surface by using scanning tunneling microscopy and spectroscopy (STM/S). It was found that a GB consisting of triple anti-parallel dislocation arrays of pentagonal-heptagonal (5–7) carbon rings induced an enhanced localized state above the Dirac point, which was attributed to van Hove singularities (VHSs). The localized state was site dependent with the variation between 0.47 and 0.36 eV. Another periodic GB with a symmetric zigzag structure presented VHSs at −0.038 and 0.12 eV near the Fermi level. Moreover, intervalley scattering was observed on GBs. This finding further suggested that defect engineering might tune the electronic structures of graphene and even the performance of graphene-based electronic devices by properly mediating and designing ordered GBs in a graphene layer.

Impact of Mismatch Angle on Electronic Transport Across Grain Boundaries and Interfaces in 2D Materials

We study the impact of grain boundaries (GB) and misorientation angles between grains on electronic transport in 2-dimensional materials. Here we have developed a numerical model based on the first-principles electronic bandstructure calculations in conjunction with a method which computes electron transmission coefficients from simultaneous conservation of energy and momentum at the interface to essentially evaluate GB/interface resistance in a Landauer formalism. We find that the resistance across graphene GBs vary over a wide range depending on misorientation angles and type of GBs, starting from 53 Ω μm for low-mismatch angles in twin (symmetric) GBs to about 1020 Ω μm for 21° mismatch in tilt (asymmetric) GBs. On the other hand, misorientation angles have weak influence on the resistance across MoS2 GBs, ranging from about 130 Ω μm for low mismatch angles to about 6000 Ω μm for 21°. The interface resistance across graphene-MoS2 heterojunctions also exhibits a strong dependence on misorientation angles with resistance values ranging from about 100 Ω μm for low-mismatch angles in Class-I (symmetric) interfaces to 1015 Ω μm for 14° mismatch in Class-II (asymmetric) interfaces. Overall, symmetric homo/heterojunctions exhibit a weak dependence on misorientation angles, while in MoS2 both symmetric and asymmetric GBs show a gradual dependence on mismatch angles.

Kapitza thermal resistance across individual grain boundaries in graphene

We study heat transport across individual grain boundaries in suspended monolayer graphene using extensive classical molecular dynamics (MD) simulations. We construct bicrystalline graphene samples containing grain boundaries with symmetric tilt angles using the two-dimensional phase field crystal method and then relax the samples with MD. The corresponding Kapitza resistances are then computed using nonequilibrium MD simulations. We find that the Kapitza resistance depends strongly on the tilt angle and shows a clear correlation with the average density of defects in a given grain boundary, but is not strongly correlated with the grain boundary line tension. We also show that quantum effects are significant in quantitative determination of the Kapitza resistance by applying the mode-by-mode quantum correction to the classical MD data. The corrected data are in good agreement with quantum mechanical Landauer-Bütticker calculations.

Bimodal Grain-Size Scaling of Thermal Transport in Polycrystalline Graphene from Large-Scale Molecular Dynamics Simulations.

Grain boundaries in graphene are inherent in wafer-scale samples prepared by chemical vapor deposition. They can strongly influence the mechanical properties and electronic and heat transport in graphene. In this work, we employ extensive molecular dynamics simulations to study thermal transport in large suspended polycrystalline graphene samples. Samples of different controlled grain sizes are prepared by a recently developed efficient multiscale approach based on the phase field crystal model. In contrast to previous works, our results show that the scaling of the thermal conductivity with the grain size implies bimodal behavior with two effective Kapitza lengths. The scaling is dominated by the out-of-plane (flexural) phonons with a Kapitza length that is an order of magnitude larger than that of the in-plane phonons. We also show that, to get quantitative agreement with the most recent experiments, quantum corrections need to be applied to both the Kapitza conductance of grain boundaries and the thermal conductivity of pristine graphene, and the corresponding Kapitza lengths must be renormalized accordingly.

Thermal boundary resistance between the polycrystalline graphene and the amorphous SiO2 substrate

The interface between graphene and substrate plays a very important role in graphene-based advanced devices. We examine the thermal boundary resistance R of the graphene/silicon dioxide (Gr/SiO2) interface by using molecular dynamics simulations. R decreases monotonically with the increase of temperature and exhibits a strong dependence on the substrate coupling strength. Due to the polycrystalline nature of graphene, we show that the presence of periodic 5–7, 5–8–5 and 5–7–5–7 grain boundaries in graphene enhances phonon transmission across the Gr/SiO2 interface, which are attributed to both the increased overlap in the phonon spectra and more inelastic scattering at the interface.

A visualization method for probing grain boundaries of single layer graphene via molecular beam epitaxy

Graphene, a member of layered two-dimensional (2D) materials, possesses high carrier mobility, mechanical flexibility, and optical transparency, as well as enjoying a wide range of promising applications in electronics. Adopting the chemical vaporization deposition method, the majority of investigators have ubiquitously grown single layer graphene (SLG), which inevitably involves polycrystalline properties. Here we demonstrate a simple method for the direct visualization of arbitrarily large-size SLG domains by synthesizing one-hundred-nm-scale MoS2 single crystals via a high-vacuum molecular beam epitaxy process. The present study based on epitaxial growth provides a guide for probing the grain boundaries of various 2D materials and implements higher potentials for the next-generation electronic devices.

Charge and spin transport on graphene grain boundaries in a quantizing magnetic field

We study charge and spin transport along grain boundaries in single layer graphene in the presence of a quantizing magnetic field. Transport states in a grain boundary are produced by hybridization of Landau zero modes with interfacial states. In selected energy regimes quantum Hall edge states can be deflected either fully or partially into grain boundary states. The degree of edge state deflection is studied in the nonlocal conductance and in the shot noise. We also consider the possibility of grain boundaries as gate-switchable spin filters, a functionality enabled by counterpropagating transport channels laterally confined in the grain boundary.

Electron Mobility across Grain Boundaries in Graphene Synthesized using Chemical Vapor Deposition Process

Chemical vapor deposition (CVD) is currently the only method for large-scale synthesis of graphene. However, the CVD process introduces grain boundaries (GBs) when individual grains coalesce with various mismatch angles. These GBs contain atomic dislocations and defects, which are believed to alter graphene's mechanical, electrical, and thermal properties. Specifically, the GBs can act as “potential barriers” when charges move from one grain to neighboring grains. This barrier effect will not only change the electrical conductivity but also the thermal conductivity of graphene. Besides high-resolution, 3-dimensional topography images, Atomic force microscopy (AFM) can also obtain the electrical properties at the nanoscale. In this report, the potential barrier effect of graphene GBs is studied with AFM. During the experiment, the probe is brought into contact with the graphene while positively (or negatively) biased. This process injects net charges into the graphene. The electrostatic potential across the GBs can be measured by AFM as an indication of the potential barrier effect. GBs with lower potential difference correspond to lower potential barrier, and vice versa. The dependency of the barrier effect on the mismatch angles was also measured. Considering the 6 folds’ symmetry of graphene atomic lattice, the mismatch angle is in the range of 0° ∼ 30°, with 30° the maximum mismatch angle. Our results can be well fitted with a sinusoidal function with π/3 period, which supports our hypothesis that higher mismatch angle contains higher density of dislocations and defects that increase the potential barrier of GBs.

Novel transparent conductor with enhanced conductivity: hybrid of silver nanowires and dual-doped graphene

We present hybrid transparent conducting films based on silver nanowires (Ag NWs) and doped graphene through novel dual co-doping method by applying various dopants (HNO3 or Au for p-doping and N2H4 for n-doping) on top and bottom sides of graphene. We systematically investigated the effect of dual-doping on their surface as well as electrical and optical properties of graphene and Ag NW/graphene hybrid films through the combination study with various dopant types (p/p, p/n, n/p, and n/n). We found that the p/p-type dual-doped (p-type dopant: HNO3) graphene and its hybrid formation with Ag NWs appeared to be the most effective in enhancing the electrical properties of conductor (doped graphene with ΔR/R0=84% and Ag NW/doped graphene hybrid with ΔR/R0=62%), demonstrating doped monolayer graphene with high optical transmittance (TT=97.4%), and sheet resistance (Rs=188Ω/sq.). We also note that dual-doping improved such electrical properties without any significant debilitation of optical transparency of conductors (doped graphene with ΔTT=0.1% and Ag NW/doped graphene hybrid with ΔTT=0.4%). In addition, the enhanced conductivity of p-type dual-doped graphene allows a hybrid system to form co-percolating network in which Ag NWs can form a secondary conductive path at grain boundaries of polycrystalline graphene.

Defects Mediated Corrosion in Graphene Coating Layer.

Mixed results were reported on the anticorrosion of graphene-coated metal surfaces-while graphene serves as an effective short-term barrier against corrosion and oxidation due to its low permeability to gases, the galvanic cell between graphene and the metal substrate facilitates extensive corrosion in the long run. Defects in the graphene layer provide pathways for the permeation of oxidizing species. We study the role of defects in graphene in the anticorrosion using first-principles theoretical modeling. Experiments in the highly reactive environment indicate that the oxidized products primarily distribute along the grain boundaries of graphene. We analyze the thermodynamics of the absorption of S and O on the grain boundaries of graphene on the basis of density functional theory. The insertion of S and O at the vacancy sites is energetically favorable. The interstitial impurities facilitate structural transformation of graphene and significantly decrease the mechanical strength of the graphene layer. Furthermore, the presence of the interstitial S and O reduces the chemical stability of graphene by enhancing the formation of vacancies and promoting dispersive growth of corrosive reactants along the grain boundaries.

Nanolines of transition metals ruled by grain boundaries in graphene: An ab initio study

We have performed an ab initio investigation of the energetic stability, and the electronic properties of transition metals (TMs = Mn, Fe, Co, and Ru) adsorbed on graphene, upon the presence of grain boundaries (GBs); where we found an energetic preference for the TMs lying on the GB sites (TM/GB). Further energy barrier calculations, of the transition metals in TM/GB, reveal that the GBs promote the formation of energetically favorable diffusion channels on graphene. By increasing the concentration of the TM adatoms, the energetic stability of the TM/GB systems has been strengthened; giving rise to TM nanolines (TM-NLs). The electronic properties of those TM-NLs were characterized through extensive electronic band structure calculations, where the energy bands of the TM/GB systems indicate the appearance of an anisotropic spin-polarized electronic current along the TM-NLs on graphene.

Precise Identification of Graphene's Crystal Structures by Removable Nanowire Epitaxy.

Monitoring crystallographic orientations of graphene is important for the reliable generation of graphene-based nanostructures such as van der Waals heterostructures and graphene nanoribbons because their physical properties are dependent on crystal structures. However, facile and precise identification of graphene's crystallographic orientations is still challenging because the majority of current tools rely on complex atomic-scale imaging. Here, we present an identification method for the crystal orientations and grain boundaries of graphene using the directional alignment between epitaxially grown AuCN nanowires and the underlying graphene. Because the nanowires are visible in scanning electron microscopy, crystal orientations of graphene can be inspected with simple procedures. Kernel density estimation that we used in analyzing the nanowire directions enables precise measurement of graphene's crystal orientations. We also confirm that the imaged nanowires can be simply removed without degrading graphene's quality, thus showing that the present method can be practically used for measuring graphene's crystal structures.

Adsorption and diffusion of sodium on graphene with grain boundaries

Effects of grain boundaries (GBs) in graphene on adsorption and diffusion of sodium were investigated using first principle calculations. Results showed that the presence of GBs in graphene enhanced the adsorption of sodium, with their adsorption energies in the range of −1.32∼-0.79 eV, which were lower than the value of −0.67 eV for sodium adsorbed on pristine graphene. The diffusion energy barriers were in the range of 0.09–0.35 eV when sodium was diffused along GBs of graphene, whereas they were decreased when sodium was gradually diffused into the GBs. Results showed that graphene with GBs had a larger energy storage capacity for sodium than the pristine one, indicating that it can be used as a good anode material for sodium ion batteries.

Direct observation of grain boundaries in chemical vapor deposited graphene

Graphene has received great attention owing to its superior physical properties, making graphene suitable for multiple applications. Numerous graphene growth techniques have been developed in the past decade to provide scalable high quality graphene. Among these techniques, chemical vapor deposition (CVD) on catalytic metal films holds great promises for a large-scale graphene growth. Even though extensive efforts have been devoted to synthesize high quality graphene, formation of defects. In particular, grain boundaries (GBs) have a dominant effect on properties, motivating extensive efforts to tune the CVD growth process to minimize GB. Rapid imaging of GBs will significantly aid in studies of CVD graphene grain structure. Here we report a straightforward technique to optically observe GBs in CVD-grown graphene via optical microscopy, allowing rapid assessment of graphene quality as well as the number of layers. The local oxidation of copper through the damaged GBs induces an optically discernable color change in the underlying copper due to different extend of oxidation between the two copper regions under grains and GBs. Our observation technique for GBs of graphene paves a path for understanding fundamental mechanisms of graphene growth and efficient quality evaluation of large-scale graphene sheet for mass production.

Probing Bilayer Grain Boundaries in Large-Area Graphene with Tip-Enhanced Raman Spectroscopy

Probing Bilayer Grain Boundaries in Large-Area Graphene with Tip-Enhanced Raman Spectroscopy

Mechanical properties of graphene grain boundary and hexagonal boron nitride lateral heterostructure with controlled domain size

In this paper, the mechanical properties of in-plane heterostructure with alternating stripes of graphene grain boundary (GB) and hexagonal boron nitride (h-BN) are investigated using classical Molecular Dynamics method. The graphene GB contains an array of pentagon-heptagon (5–7) defects, and has good interfacial continuity with the lateral BN domain. By studying the dynamic failure process of heterostructure with varying hybridization intervals and GB tilt angles, two different local failure types are noticed. Coupled effects of h-BN hybridization and GB tilt angle on the tensile strength of heterostructure are revealed. For heterostructure with graphene GBs of evenly spaced 5–7 defects, the tensile strength is insensitive to the h-BN hybridization and increases anomalously with the defect density. For heterostructure with graphene GB of unevenly spaced 5–7 defects, lateral h-BN hybridization enhanced the tensile strength of the heterostructure. Such strength enhancement effect is contributed to the bond length mismatch between graphene and h-BN, and deteriorates with the increase of hybridization interval. Our results give helpful insight into the strength characteristics of hybrid two-dimensional nanomaterials based electronic and optical devices.

Special Section Chemical Vapor Deposition Graphene Deposition: Doping Effects and Grain Boundaries in Thermal CVD Graphene on Recrystallized Cu Foil (Adv. Mater. Interfaces 16/2016)

Special Section Chemical Vapor Deposition Graphene Deposition: Doping Effects and Grain Boundaries in Thermal CVD Graphene on Recrystallized Cu Foil (Adv. Mater. Interfaces 16/2016)

Observation of graphene grain boundaries through selective adsorption of rhodamine B using fluorescence microscopy

The graphene grain boundary (GGB), line defect, has a marked influence on an electrical properties and mechanical strength of graphene. Observation of the grain size and defect distribution is important for tuning the properties of graphene. Here we show imaging GGBs with rhodamine B (RB) by fluorescence microscopy (FM), and demonstrate the selective adsorption of RB molecules on the GGBs after a pre-annealing process and RB treatment. The hill structure of oxidized Cu below GGB is formed by a pre-annealing process, and induces RB molecules to selectively physisorb on GGBs due to the interplay between graphene and oxidized Cu, which is directly confirmed by kelvin probe force microscopy (KPFM) and density function theory (DFT) calculation. The selectively adsorbed RB was completely removed by acetone rinsing without disrupting the graphene sheets. This simple observation method provides an efficient and convenient means of evaluating the quality and reliability of graphene as well as other two-dimensional materials, such as WS2, WSe2, MoS2, MoSe2 and BN.

Doping Effects and Grain Boundaries in Thermal CVD Graphene on Recrystallized Cu Foil

Thermal chemical vapor deposition (CVD) of graphene on a copper (Cu) foil is strongly affected by Cu foil recrystallization and grain formation resulting in a heterogeneous graphene layer. Correlated microscopic scanning probe techniques show different chemical and structural properties (Raman microspectroscopy), electrical conductivity (conductive atomic force microscopy), and electronic structure (Kelvin probe force microscopy) across various types of grains. Graphene on clean Cu grains exhibits work function (WF) higher by 300 meV. This is attributed to the contact doping effect. Graphene of higher quality is formed on Cu grains with a surface cuprous oxide (Cu2O) which forms during the thermal CVD process. The graphene WF is higher by 430 meV there. The WF difference is attributed to the contact doping effect and additional strain‐induced doping due to the highly corrugated Cu2O surface. Local electrical conductivity measured by atomic force microscopy and a pair of conductive microprobes corroborates different graphene quality on various Cu grains and reveals that graphene is electrically interrupted at the Cu grain boundaries. Corresponding structural and electronic model of thermal CVD graphene on the Cu foil is presented. The correlation of grain boundaries in Cu foil and graphene is discussed.