Graphene Grain Research Articles

The effects of graphene intrinsic defects on the formation of extrinsic defects by plasma treatment

Two-dimensional materials, like graphene, are expected to make highly efficient permeation membranes for separation and sensing applications; however, defects of a specific size and location must be added to the film to make them selectively permeable. Though techniques like TEM drilling give precise information on size and location, this method is not scalable for eventual commercial use. Plasma treatments are a scalable alternative, but there is less information on where the defect formation begins and how intrinsic defects affect pore formation. This work seeks to understand these effects by using Raman spectroscopy at the same location before and after plasma treatments to map how the intrinsic defects affect the formation of extrinsic defects to create nanopores. Defect density increases with increasing RF power, independent of the gas used during the plasma treatment, suggesting that the mechanism is due to ion bombardment. Coalesced and arrested graphene syntheses are used to study the relative trends of defect formation and map existing graphene grain boundary locations. The results show that regions starting with high densities of intrinsic defects, potentially due to strain, were found to degrade into nanocrystalline graphene first on the amorphization trajectory, even before grain boundaries.

First principles study of electronic structure and transport in graphene grain boundaries

Grain boundaries play a major role for electron transport in graphene sheets grown by chemical vapor deposition. Here we investigate the electronic structure and transport properties of idealized graphene grain boundaries (GBs) in bi-crystals using first principles density functional theory (DFT) and non-equilibrium Greens functions. We generated 150 different grain boundaries using an automated workflow where their geometry is relaxed with DFT. We find that the GBs generally show a quasi-1D bandstructure along the GB. We group the GBs in four classes based on their conductive properties: transparent, opaque, insulating, and spin-polarizing and show how this is related to angular mismatch, quantum mechanical interference, and out-of-plane buckling. Especially, we find that spin-polarization in the GB correlates with out-of-plane buckling. We further investigate the characteristics of these classes in simulated scanning tunnelling spectroscopy and diffusive transport along the GB which demonstrate how current can be guided along the GB.

Mapping nanoscale carrier confinement in polycrystalline graphene by terahertz spectroscopy

Terahertz time-domain spectroscopy (THz-TDS) can be used to map spatial variations in electrical properties such as sheet conductivity, carrier density, and carrier mobility in graphene. Here, we consider wafer-scale graphene grown on germanium by chemical vapor deposition with non-uniformities and small domains due to reconstructions of the substrate during growth. The THz conductivity spectrum matches the predictions of the phenomenological Drude–Smith model for conductors with non-isotropic scattering caused by backscattering from boundaries and line defects. We compare the charge carrier mean free path determined by THz-TDS with the average defect distance assessed by Raman spectroscopy, and the grain boundary dimensions as determined by transmission electron microscopy. The results indicate that even small angle orientation variations below 5° within graphene grains influence the scattering behavior, consistent with significant backscattering contributions from grain boundaries.

New local pseudopotential for multilayer carbon materials and its application in wave packet dynamics

Formerly we created a one-electron local pseudopotential which proved to be successful in studying transport phenomena in various sp2 carbon nanosystems, e.g. graphene grain boundaries and nanotube networks. In this work, we present an extended version of the local pseudopotential which correctly describes the electronic structure of van der Waals stacks of carbon sheets, e.g. AA, AB bi-layer graphene, ABC (rhombohedral) tri-layer graphene, as well as AA, AB, and ABC graphite, etc., even in case of external electric fields. We utilize this potential to study the hopping dynamics of wave packets between the graphene sheets in multilayer systems. The frequency of the hopping is proportional to the band splitting, caused by the interlayer coupling. For the case of AB and ABC graphene there is a backscattering near the Fermi level, causing a Zitterbewegung-like phenomenon, an interference between +kBloch and −kBloch states. For the rhombohedral tri-layer graphene the time dependence of the wave packet probability density is an aperiodic function for the first- and third layer (the two outer layers), but a quasi-periodic function for the second (inner) layer.

Thermodynamic Window for Size-Controlled Pore Formation in Graphene for Large-Scale Molecular Sieves.

Nanopores in graphene monolayers are a promising option for molecular separation applications, such as desalination and carbon capture. Graphene's atomic thickness allows for an optimal balance between molecular selectivity and permeability, while its chemical stability and robust mechanical properties make it appealing for a wide range of commercial applications. However, scaling to large areas with controlled pore size distribution is an open challenge in ultrathin membranes. Here, using first-principles calculations, we identify a suitable thermodynamic window in a chemical vapor deposition system for directly growing graphene monolayers with a controlled pore size distribution. As an example, our calculations show that a postgrowth annealing step with a supersaturation range of 19.7-25 kJ/mol at 1000 K results in the creation of a controllable pore density at graphene grain boundaries, with pore sizes falling within the range of 5-8 Å. Such pores isolate hydrated Cl ions from water molecules, effectively desalinating seawater. Thus, it allows the design of targeted synthesis of large-scale 2D layers for membrane applications.

Effects on the Microstructure Evolution and Properties of Graphene/Copper Composite during Rolling Process.

Rolling treatments have been identified as a promising fabrication and deformation processing technique for graphene/metal composites with high performance. However, it is still a challenge to choose appropriate rolling parameters to achieve high strength, ductility and electrical conductivity of the composite simultaneously. In this study, graphene/Cu composites were prepared with an in situ growth method and rolling treatment. The effects of rolling deformation and temperature on the microstructural evolution of graphene and Cu grains, interface bonding between graphene and the matrix, mechanical and electrical properties were systemically investigated. The cold-rolled composite with 85% deformation displayed a maximum ultimate strength of 548 MPa, a high elongation of 8.8% and a good electrical conductivity of 86.2% IACS. This is attributed to oriented graphene arrangement and matrix grain refinement. Our research provides a comprehensive understanding for the rolling behavior of graphene/Cu composites, and can promote the development of graphene-based composites with high performance.

Nucleation and growth of graphene at different temperatures by plasma enhanced chemical vapor deposition

Temperature plays an important role in the synthesis of graphene by chemical vapor deposition. In the low-temperature plasma-enhanced chemical vapor deposition (PECVD) process, although the cleavage of hydrocarbon precursor no longer depends on growth temperature, which can affect the nucleation and growth of graphene by the diffusion and adsorption of active species, the catalytic ability of substrate and other factors, and ultimately lead to various growth of graphene. In this paper, the nucleation and growth of graphene at different temperatures by PECVD were studied. We noted that the nucleation and growth of graphene varied greatly in different temperature zones. Graphene could not be grown at low temperatures (< 600 ℃). At the medium and low temperature (650 ∼ 800 ℃), high nucleation density of graphene with nanometer size were produced. In the middle and high temperature region (850 ∼ 900 ℃), larger grains of graphene grown, and in the high temperature region (> 950 ℃), the coexistence of large and small grains was finally formed. In addition, the models of the nucleation and growth of graphene in different temperature zones were proposed.

Small-data-based machine learning interatomic potentials for graphene grain boundaries enabled by structural unit model

Small-data-based machine learning interatomic potentials for graphene grain boundaries enabled by structural unit model

Mdapy: A flexible and efficient analysis software for molecular dynamics simulations

The mdapy is a library for pre- and postprocessing molecular dynamics simulation data. Benefitting from the just-in-time compile technology of TaiChi mdapy can be written in pure Python while possessing similar speed to those written in C++. mdapy is designed with highly paralleled and makes full advantages of modern computer resources on both multicore CPU and GPU architecture. The package implements a fast module to find the neighbors of particles in both free and periodic boundaries, based on which it offers a wide variety of methods to analyze atomic environments, such as standard centrosymmetry parameters, radial distribution function and newer methods, such as atomic entropy fingerprint. In addition, mdapy can be used to create the geometric structure of polycrystals with metallic or graphene grain boundaries by Voronoi diagram. mdapy can directly read the DUMP and DATA format defined in LAMMPS code, and, in practice, it accepts any other format by converting it into NumPy ndarray format. This design philosophy enables seamless integration with abundant scientific ecosystems in the Python community and easy cooperation with other analysis codes like OVITO or freud. Program summaryProgram Title: mdapyCPC Library link to program files:https://doi.org/10.17632/dtdkxvcsc9.1Developer's repository link:https://github.com/mushroomfire/mdapyCode Ocean capsule:https://codeocean.com/capsule/5271472Licensing provisions: BSD 3-clauseProgramming language: Python, C++Nature of problem: Atomic environment analysis and generation of the initial structure are important in molecular dynamics simulations. Many analysis methods relying on particle neighbors, such as radial distribution functions and atomic entropy, are computationally intensive and need to be carefully implemented to scale to large systems. Traditional code is often written in C++ or Fortran to guarantee performance while causing difficulty in installation and secondary development due to the complexity of the programming language.Solution method: mdapy is written in parallel to quickly perform neighbor finding, provides a set of analysis methods and creates an atomic geometry structure on multicore CPU and GPU. Over 95 percent of mdapy is written in Python with a uniform API to call. All data are stored in NumPy ndarray, making users easy to install, use and secondary develop.Additional comments including restrictions and unusual features:1. mdapy provides fast parallel implementations of neighbor finding in periodic and free boundary system.2. mdapy can generate a polycrystalline model with graphene grain boundaries.3. mdapy is helpful for analyzing the melting process by the mean squared displacement and Lindemann index at the atomic level.4. mdapy is very easy to install via pip install mdapy without any compile steps and can be run on Windows, Linux and Mac OS with Python 3.7-3.10.5. mdapy has detailed documentation to make it easier to use.

Grain-Boundary-Induced Ultrasensitive Molecular Detection of Graphene Film.

Graphene has been considered a promising platform for molecular detection due to the graphene-enhanced Raman scattering (GERS) effect. However, the GERS performance of pristine graphene is limited by a low chemically active surface and insufficient density of states (DOS). Although diverse defects have been introduced, it remains a great challenge to improve the enhancement performance. Here, we show that graphene grain boundaries (GBs) possess stronger adsorption capacity and more abundant DOS. Thus, GERS performance increases with the atomic percentage of GBs, which makes nanocrystalline graphene (NG) film a superior GERS substrate. For R6G as a probe molecule, a low detection limit of 3 × 10-10 M was achieved. Utilizing the high chemical activity of GBs, we also fabricated NG film decorated with Au particles using a one-step quenching strategy, and this hybrid film exhibits an extremely low limit of detection down to 5 × 10-11 M, outperforming all the reported graphene-based systems.

Anisotropic bending of graphene with grain boundaries: Insights from tight-binding simulations

Graphene grain boundaries (GBs) are associated with distinct mechanical and physical properties. To utilize these properties requires comprehensive investigation of the GB-controlled nanomechanics. Currently, there is short of fundamental understanding of the out-of-plane bending of graphene in the presence of GBs and the mechanical properties of related curved structures. Here with density functional theory-based tight-binding objective molecular simulations, we revealed anisotropic nonlinear bending of graphene with tilt GBs. The anisotropy and nonlinearity are caused by the dislocation cores which display out-of-plane protrusions. In addition, we investigated the GB-controlled elastic responses of curved graphene configurations, and established the coupling between the elastic modulus and local curvature near the GBs. Our findings serve as useful references for 3D graphene structure design utilizing GBs and can be generalized to other 2D materials.

Shear-coupling of graphene grain boundaries: Elementary mechanisms, effects of topology, and role of buckling

Engineering grain boundary structure is a promising method for rational control of the microstructure and mechanical properties of two-dimensional materials. In bulk materials, shear stresses can drive grain boundary migration through the dislocations in grain boundaries. However, while shear coupling of grain boundaries has been studied in bulk materials like nanocrystalline copper, its translation to two-dimensional materials where out-of-plane deformation can relieve in-plane shear is not yet established. We investigate how the low flexural rigidity of graphene effects shear coupled grain boundary motion using atomic scale simulations of flat and buckled grain boundaries. We define the coupling shear strain as the strain at which a grain boundary has advanced by one Burgers vector and is at equilibrium and the critical shear strain as the strain at which migration of the first dislocation in the grain boundary becomes thermodynamically favorable. We show that the out-of-plane deformation does not influence the coupling shear strain and is governed only by the grain boundary topology. While the critical shear strain is altered somewhat by the low flexural rigidity due to buckling induced softening, it is also still dominated by the grain boundary topology. Our atomic scale results are synthesized into two models that predict the coupling and critical shears.

Strength criterion of graphene GBs combining discrete bond strength and varied bond stretch

The strength criterion of graphene grain boundaries (GBs) under complex stress states plays an important role in guiding the design of high-performance graphene-related materials and devices under severe mechanical environments like ultrastrong structural materials or flexible electronics. However, finding a unified strength criterion that can adapt to different GB angles and loading states is still a challenge. In this work, we proposed a general method to construct the strength criterion of graphene GBs that includes the atomic details of defect. Firstly, systematic molecular dynamics (MD) simulations were carried out to explore the fracture behaviors of graphene GBs with different GB angles and loading states. Then, bond strength of heptagon ring σ t h R was obtained by projecting the applied stress field to the fractured bond direction, which gives the same value for the same type of C-C bond of same GB angle under different loading states. Besides, to further unify the bond strength of different GB angles, the residual stress caused by other pentagon–heptagon defects was considered by using the disclination dipole theory, in which the intrinsic bond strength σ t h 0 for different GB angles collapses to the same value. With this scenario, the strength criterion of graphene GBs is presented as the bond stretch stress generated by external load exceeds any bond strength of the hexagon–heptagon defect. Furthermore, a unified bond strength is found, linearly decreasing with the equilibrium bond length, which can further be applied to the strength criterion of other defective structures like two types of SW defects (without analytical solution of residual stress) and even pristine graphene. The present results provide a comprehensive understanding of the failure of graphene by considering the balance of bond strength of defective structure and bond stretch stress generated by external load, which may pave the way to construct the atomic structure based strength criterion of other two-dimensional materials.

Spatial Control of Lithium Deposition by Controlling the Lithiophilicity with Copper(I) Oxide Boundaries

Spatial control of lithium deposition is the most important issue in lithium‐metal batteries because of the considerable control of lithium dendrite suppression via the uniform distribution of Li+ flux. Although seed materials are crucial for the behavior of lithium deposition, in‐depth studies on their physical and chemical control have not been conducted. Here, we describe a new design of seed structure comprising a wrinkled Cu/graphene substrate surrounded by copper(I) oxide (Cu2O) on a graphene grain boundary over a large area, which is fabricated by the oxidation of the Cu surface via graphene boundary defects by using chemical vapor deposition (CVD). Scanning and transmission electron microscopy results reveal that Cu2O on the graphene boundary can render a preferential reaction with lithium during the first deposition and assist in the uniform deposition of lithium by preventing the agglomeration of lithium clusters during the second deposition. This two‐step process is attributed to the degree of selectivity due to the difference in lithium affinity, which allows long‐term electrochemical stability and a high rate capability via boundary effects. This study highlights the significance of the boundary effect, which can open new avenues for the formation of a large family of seed structures in lithium‐metal batteries.

Improved Crystallinity of Graphene Grown on Cu/Ni (111) through Sequential Mobile Hot-Wire Heat Treatment

Over the past few years, many efforts have been devoted to growing single-crystal graphene due to its great potential in future applications. However, a number of issues remain for single-crystal graphene growth, such as control of nanoscale defects and the substrate-dependent nonuniformity of graphene quality. In this work, we demonstrate a possible route toward single-crystal graphene by combining aligned nucleation of graphene nanograins on Cu/Ni (111) and sequential heat treatment over pregrown graphene grains. By use of a mobile hot-wire CVD system, prealigned grains were stitched into one continuous film with up to ∼97% single-crystal domains, compared to graphene grown on polycrystalline Cu, which was predominantly high-angle tilt boundary (HATB) domains. The single-crystal-like graphene showed remarkably high thermal conductivity and carrier mobility of ∼1349 W/mK at 350 K and ∼33 600 (38 400) cm2 V-1 s-1 for electrons (holes), respectively, which indicates that the crystallinity is high due to suppression of HATB domains.

Engineering Graphene Grain Boundaries for Plasmonic Multi-Excitation and Hotspots.

Surface plasmons, merging photonics and electronics in nanoscale dimensions, have been the cornerstones in integrated informatics, precision detection, high-resolution imaging, and energy conversion. Arising from the exceptional Fermi-Dirac tunability, ultrafast carrier mobility, and high-field confinement, graphene offers excellent advantages for plasmon technologies and enables a variety of state-of-the-art optoelectronic applications ranging from tight-field-enhanced light sources, modulators, and photodetectors to biochemical sensors. However, it is challenging to co-excite multiple graphene plasmons on one single graphene sheet with high density, a key step toward plasmonic wavelength-division multiplexing and next-generation dynamical optoelectronics. Here, we report the heteroepitaxial growth of a polycrystalline graphene monolayer with patterned gradient grain boundary density, which is synthesized by creating diverse nanosized local growth environments on a centimeter-scale substrate with a polycrystalline graphene ring seed in chemical vapor deposition. Such geometry enables plasmonic co-excitation with varied wavelength diversification in the nanoscale. Via using high-resolution scanning near-field optical microscopy, we demonstrate rich plasmon standing waves, even bright plasmonic hotspots with a size up to 3 μm. Moreover, by changing the grain boundary density and annealing, we find the local plasmonic wavelengths are widely tunable, from 70 to 300 nm. Theoretical modeling supports that such plasmonic versatility is due to the grain boundary-induced plasmon-phonon interactions through random phase approximation. The seed-induced heteroepitaxial growth provides a promising way for the grain boundary engineering of two-dimensional materials, and the controllable grain boundary-based plasmon co-generation and manipulation in one single graphene monolayer will facilitate the applications of graphene for plasmonics and nanophotonics.

Activated Hopping Transport in Nematic Conducting Aerogels

The transport properties of nematic aerogels, which consist of highly oriented Al$_2$O$_3\cdot$SiO$_2$ nanofibers coated with a graphene shell with a large number of defects, are studied. The temperature dependences of the electrical resistivity in the range of 9-40K strictly follow the formula derived to describe the variable range hopping (VRH) conductivity, in which exponent $\alpha$ changes from 0.4 to 0.9 when the number of layers in the graphene shell decreases from 4-6 to 1-2. The dependence of $\alpha$ on the shell thickness can be explained by a simultaneous change in the dimensionality of hopping transport and the character of the energy dependence of the density of localized states near the Fermi level. The fact that $\alpha$ approaches unity at the minimum graphene shell thickness indicates a gradual transition from VRH transport to nearest neighbor hopping (NNH) transport. The magnetoresistance measured at T = 4.2 K is negative, increases significantly with decreasing graphene shell thickness, and is approximated by a formula for the case of weak localization with a good accuracy. The phase coherence lengths are in a reasonable relation with the graphene grain sizes. The conducting aerogels under study complement the well-known set of materials that exhibit hopping electron transport at low temperatures, which is characteristic of media with strong carrier localization, and also a negative magnetoresistance, which usually manifests itself under weak localization conditions.

From single-layer graphene to HOPG: Universal functionalization strategy with perfluoropolyether for the graphene family materials

Graphene functionalization offers the opportunity to modify the chemical-physical properties of graphene, therefore broadening the variety of its possible applications. In this work, single- and few-layers graphene as well as highly oriented pyrolytic graphite (HOPG) were functionalized with perfluoropolyether (PFPE) chains via a peroxide decomposition and radical reaction. Samples were prepared using different amounts of PFPE peroxide and the effects of the treatment were studied through Raman spectroscopy, grazing angle Fourier-transform Infrared spectroscopy (GA-FTIR), X-ray Photoelectron Spectroscopy (XPS) and water contact angle measurements. A coherent trend of FC bonds with increasing amount of peroxidic precursor was detected, attacking the sp2 reactive sites of the grains of graphene itself and generating new sp3 hybridizations. The perfluorinated chains were found to give hydrophobic properties to graphenic layers, demonstrating the functionalization route as an easy and universal strategy for the preparation of hydrophobic graphenic substrates.

Ameliorating strength-ductility efficiency of graphene nanoplatelet-reinforced aluminum composites via deformation-driven metallurgy

1.5 wt% graphene nanoplatelet-reinforced aluminum matrix composites were prepared by deformation-driven metallurgy to ameliorate strength-ductility efficiency. Severe plastic deformation with its frictional/deformation heat introduced by deformation-driven metallurgy was studied by their strengthening-toughing behaviors related to graphene nanoplatelet dispersion, interfacial bonding, and grain refinement. The synergy strengthening behaviors were studied via modeling analysis. Uniform dispersion of graphene nanoplatelets and ultra-fine microstructures (267.0 nm) were obtained via severe plastic deformation and dynamic recrystallization. High-efficiency interfacial bonding was realized via graphene nanoplatelets-(amorphous Al2O3)-Al semi-direct interface without the formation of Al4C3. The automatic flow of Al2O3 nanodots to compensate for the spatial discontinuity caused by the interlayer slip of graphene was observed to achieve self-compensating spatial continuity. The ultimate tensile strength and elongation reached 468 ± 7 MPa and 19.9 ± 0.6%, respectively, showing an enhancement of strength by 293.3% with almost no loss in ductility.

Quantum Hall Effect across Graphene Grain Boundary.

Charge carrier scattering at grain boundaries (GBs) in a chemical vapor deposition (CVD) graphene reduces the carrier mobility and degrades the performance of the graphene device, which is expected to affect the quantum Hall effect (QHE). This study investigated the influence of individual GBs on the QH state at different stitching angles of the GB in a monolayer CVD graphene. The measured voltage probes of the equipotential line in the QH state showed that the longitudinal resistance (Rxx) was affected by the scattering of the GB only in the low carrier concentration region, and the standard QHE of a monolayer graphene was observed regardless of the stitching angle of the GB. In addition, a controlled device with an added metal bar placed in the middle of the Hall bar configuration was introduced. Despite the fact that the equipotential lines in the controlled device were broken by the additional metal bar, only the Rxx was affected by nonzero resistance, whereas the Hall resistance (Rxy) revealed the well-quantized plateaus in the QH state. Thus, our study clarifies the effect of individual GBs on the QH states of graphenes.