Effect of graphene grain boundaries on MoS_2/graphene heterostructures

The grain boundaries of graphene are disordered topological defects, which would strongly affect the physical and chemical properties of graphene. In this paper, the spectral characteristics and photoresponse of MoS_2/graphene heterostructures are studied. It is found that the blueshift of the G and 2 D peaks of graphene in Raman spectrum is due to doping. The lattice mismatch at the graphene boundaries results in a blueshift of MoS_2 features in the photoluminescence spectra, comparing to the MoS_2 grown on SiO_2. In addition, the photocurrent signal in MoS_2/hexagonal single-crystal graphene heterostructures is successfully captured without bias, but not in MoS_2/polycrystalline graphene heterostructures.The electron scattering at graphene grain boundaries affects the optical response of MoS_2/graphene heterostructures. The photoresponse of the device is attributed to the optical absorption and response of MoS_2 and the high carrier mobility of graphene. These findings offer a new approach to develop optoelectronic devices based on two-dimensional material heterostructures.     

Electronic properties of grains and grain boundaries in graphene grown by chemical vapor deposition

We synthesize hexagonal shaped single-crystal graphene, with edges parallel to the zig-zag orientations, by ambient pressure CVD on polycrystalline Cu foils. We measure the electronic properties of such grains as well as of individual graphene grain boundaries, formed when two grains merged during the growth. The grain boundaries are visualized using Raman mapping of the D band intensity, and we show that individual boundaries between coalesced grains impede electrical transport in graphene and induce prominent weak localization, indicative of intervalley scattering in graphene.     

Grain Growth During Superplastic Deformation

Significant grain growth occurring during superplastic deformation is related to the micro-mechanism of superplastic flow. Observations performed on the deformed surface of superplastically deformed tensile and shear Pb-62%Sn samples and bi-axially formed AA7475 samples directly indicate that cooperative grain boundary sliding, i.e. sliding of grain groups, is accompanied by cooperative grain boundary migration that can result in an enhanced grain growth. Such a long range correlation in migration of sliding grain boundaries is related to movement of grain boundary dislocations having a step associated with its core. Observed correlation between grain size and strain measured in different regions of a superplastically formed Ti-alloy part and alignment of grain boundaries along shear surfaces support coupling of grain boundary sliding and migration. A model of grain growth considering climb of cellular dislocations, topological defects in a grain array, has been expanded to incorporate gliding and mixed cellular dislocations.     

A cosmological model for corrugated graphene sheets

Defects play a key role in the electronic structure of graphene layers flat or curved. Topological defects in which an hexagon is replaced by an n-sided polygon generate long range interactions that make them different from vacancies or other potential defects. In this work we review previous models for topological defects in graphene. A formalism is proposed to study the electronic and transport properties of graphene sheets with corrugations as the one recently synthesized. The formalism is based on coupling the Dirac equation that models the low energy electronic excitations of clean flat graphene samples to a curved space. A cosmic string analogy allows to treat an arbitrary number of topological defects located at arbitrary positions on the graphene plane. The usual defects that will always be present in any graphene sample as pentagon–heptagon pairs and Stone-Wales defects are studied as an example. The local density of states around the defects acquires characteristic modulations that could be observed in scanning tunnel and transmission electron microscopy.     

Study of the effects of grain size on the mechanical properties of nanocrystalline copper using molecular dynamics simulation with initial realistic samples

Abstract

Molecular dynamics simulations have been performed to study the mechanical properties of a columnar nanocrystalline copper with a mean grain size between 9.0 and 24 nm. A melting–cooling method has been used to generate the initial samples: this method produces realistic samples that contain defects inside the grains such as dislocations and vacancies. The results of uniaxial tensile tests applied to these samples reveal the presence of a critical mean grain size between 16 and 20 nm, for which there is an inversion of the conventional Hall–Petch relation. The principal mechanisms of deformation present in the samples correspond to a combination of dislocations and grain boundary sliding. In addition, this analysis shows the presence of sliding planes generated by the motion of perfect edge dislocations that are absorbed by grain boundaries. It is the initial defects present inside the grains that lead to this mechanism of deformation. An analysis of the atomic configurations further shows that nucleation and propagation of cracks are localised on the grain boundaries especially on the triple grains junctions.     

Effects of topological defects and local curvature on the electronic properties of planar graphene

A formalism is proposed to study the electronic and transport properties of graphene sheets with corrugations as the one recently synthesized. The formalism is based on coupling the Dirac equation that models the low energy electronic excitations of clean flat graphene samples to a curved space. A cosmic string analogy allows to treat an arbitrary number of topological defects located at arbitrary positions on the graphene plane. The usual defects that will always be present in any graphene sample as pentagon–heptagon pairs and Stone–Wales defects are studied as an example. The local density of states around the defects acquires characteristic modulations that could be observed in scanning tunnel and transmission electron microscopy.     

On the micromechanical modelling of the effective diffusion coefficient of a polycrystalline material

This study focuses on calculation of the effective diffusion coefficient of a polycrystalline material accounting for the grain size and shapes. Polycrystal is modelled as a composite consisting of a matrix with high diffusivity (grain boundaries and triple junctions) and inhomogeneities with low diffusivity (bulk grains including crystal defects like dislocations). The segregation at the grain boundaries is accounted for. Typical micromechanical models are re-written for diffusivity assuming that the grains have the shape of ellipsoids of revolution (spheroids). The results are compared with (1) numerical results for hydrogen diffusion in an imaginary polycrystalline material and (2) experimental results for diffusion of hydrogen in nickel polycrystal available in the literature. The approach can be used for extraction of information on diffusivity along the grain boundaries.     

The effect of Au nanoparticles on the strain-dependent electrical properties of CVD graphene

We conducted an experimental study of the effect of Au nanoparticles (NPs) on the strain-dependent electrical properties in chemical vapor deposition grown graphene. We used 5-nm thick Au NPs as an effective cover (and doping) layer for graphene, and found that Au NPs decrease electrical resistance by two orders of magnitude. In addition, the Au NPs suppress the effect of strain on resistance because the intrinsic topological cracks and grain boundaries in graphene are filled with Au nanoparticles. This method has a big potential to advance industrial production of large-area, high-quality electronic devices and graphene-based transparent electrodes.     

Structural defects in germanium single crystals

The origin and distribution of structural defects (dislocations, small angle grain boundaries, slip lines, polycrystalline inclusions) is studied in large single crystals of germanium applied in manufacturing optical elements.     

Evolution of dislocation patterns in a tricrystal model subjected to cyclic loading

When a polycrystalline aggregate is deformed beyond the elastic range, dislocations pile up at grain boundaries and make some patterned structures within the grains. If the external load is reversed, most dislocations at grain boundaries and in the patterned structure are supposed to disappear or change their arrangements, but the details are not yet well known. In this study, we examined such changes in dislocation structures by a crystal plasticity analysis. Models for the polycrystalline aggregates consist of three grains and the generation and degeneration of the geometrically necessary dislocations during cyclic loading are examined in detail. The results show that there are some groups of dislocations that do not dissipate but evolve upon reverse loading.     

Special interaction between quasiperiodic grain boundaries and lattice dislocations in crystalline solids

A theoretical model is suggested which describes phason imperfections (specific excitations) in a quasiperiodic grain boundary in a polycrystalline solid as dilatation flexes. In the framework of the model, an elastic stress field of the quasiperiodic grain boundary is calculated as a stress field created by an ensemble of dilatation flexes (phason imperfections) located in the boundary. It is shown that there is a special elastic interaction between crystal lattice defects and quasiperiodic grain boundaries comprising phason imperfections. The strengthening effect in plastically deformed polycrystalline solids is quantitatively described which is related to the special elastic interaction between lattice dislocations and quasiperiodic grain boundaries. Received: 29 October 1996 / Revised: 22 August 1997 / Accepted: 13 November 1997     

The influence of tilt grain boundaries on the mechanical properties of bicrystalline graphene nanoribbons

The mechanical properties of bicrystalline graphene nanoribbons with various tilt grain boundaries (GBs) which typically consist of repeating pentagon–heptagon ring defects are investigated based on the method of molecular structural mechanics. The GB models are constructed via the theory of disclinations in crystals, and the elastic properties and ultimate strength of bicrystalline graphene nanoribbons are calculated under uniaxial tensile loads in perpendicular and parallel directions to grain boundaries. The dependence of mechanical properties is analyzed on the chirality and misorientation angles of graphene nanoribbons, and the experimental phenomena that Young's modulus and ultimate strength of bicrystalline graphene nanoribbons can either increase or decrease with the grain boundary angles are further verified and discussed. In addition, the influence of GB on the size effects of graphene Young's modulus is also analyzed.     

Extended topological defects as sources and outlets of dislocations in spherical hexagonal crystals

Extended topological defects (ETDs) arising in spherical hexagonal crystals due to their curvature are considered. These prevalent defects carry a unit total topological charge and are surrounded by scalene pentagonal boundaries. Topological peculiarities of reactions between ETDs and dislocations are considered. Similarly to boundaries of the usual planar crystalline order the ETDs emit and absorb the dislocations without preservation of their dislocational charge. Dislocations located inside the ETD area lose it and the enforced ETD decay can proceed in different ways without conservation of the total Burgers vector of the dislocations emitted.     

Microscopic evidence for the modification of the electronic structure at grain boundaries of Cu(In(1-x),Ga(x))Se2 films

We investigated the electronic properties around grain boundaries of polycrystalline Cu(In(1-x),Ga(x)Se(2)) films as a function of Ga content, using scanning tunneling spectroscopy. Spectra acquired on samples with low Ga content (x=0 and 0.33) reveal downward band bending with respect to adjacent p-type grains, suggesting type inversion at the surface of grain boundaries. Such a behavior was not observed for samples with high Ga contents. These results are consistent with our atomic force microscopy data and may shed light on the origin of the x-dependent efficiency for polycrystalline Cu(In(1-x),Ga(x)Se(2))-based solar cells.     

Plasma-stimulated penetrability of hydrogen as a tool for studying phase transitions at grain boundaries

It is well known that the diffusion of hydrogen atoms through the intrinsic defects of a crystal lattice has characteristics different from those of bulk diffusion and, at certain parameters for some polycrystalline metals, ensures the determining contribution to the transfer of hydrogen atoms through the material. Grain boundaries (and dislocations) are the most important and shortest paths, the diffusion through which is much faster than bulk diffusion through a crystal lattice. It is particularly important to take into account this diffusion in materials with grains having sizes of about several nanometers. The possibility of using the method of the plasma-stimulated penetrability of hydrogen to analyze phase transitions at the grain boundaries is demonstrated on the example of polycrystalline niobium foils. In contrast to the existing methods, this method proposed for studying grain-boundary diffusion and phase transitions is simple and ensures control over the surface. The temperature characteristics of the diffusion of hydrogen atoms through niobium grain boundaries have been measured.     

Grain boundaries in Cu(In,?Ga)(Se,?S)2 thin-film solar cells

The paper reviews the current status of the research on grain boundaries in polycrystalline Cu(In, Ga)(S, Se)₂ alloys used as absorber materials for thin-film solar cells. We discuss the different concepts that are available to explain the relatively low electronic activity of grain boundaries in these materials. Numerical simulations that have been undergone so far to model the polycrystalline solar cells are briefly summarized. In addition, we give an overview on the experiments that have been conducted so far to elucidate the structural, defect-chemical, and electronic properties of grain boundaries in Cu(In, Ga)(S, Se)₂ thin-films.     

Effect of copper surface morphology on grain size uniformity of graphene grown by chemical vapor deposition

The graphene grain boundaries (GGBs) of polycrystalline graphene grown by chemical vapor deposition (CVD) typically constitute a major reason of deterioration of the electrical properties of graphene-based devices. To reduce the density of GGB by increasing the grain size, CVD growth conditions with a reduced CH₄ flow rate have been widely applied and, recently, electropolishing of copper (Cu) foil substrates to flatten the surface has been undertaken prior to graphene growth. In this study, we show that polycrystalline graphene layer grown on typical Cu foil features two heterogeneous regions with different average grain sizes: small-grain regions (SGRs) and large-grain regions (LGRs). Statistical analysis of the grains of the graphene layers grown under different process conditions showed that SGRs (which form on Cu striations) limit the average grain size, the ability to control the grain size through adjustment of growth conditions, and global grain-size uniformity. Analysis showed that the surface-flattening process significantly improves grain-size uniformity, and monolayer coverage, as well as the average grain size. These results suggest that a process for flattening the surfaces of Cu substrates is critical to controlling the quality and uniformity of CVD-grown graphene layers for practical device applications.     

Conductivity tensor of striped quantum hall phases

We study the transport properties of pinned striped quantum Hall phases. We show that, under quite general assumptions, the macroscopic conductivity tensor satisfies a semicircle law. In particular, this result is valid for both smectic and nematic stripe phases, independent of the presence of topological and orientational defects such as dislocations and grain boundaries. As a special case, our results explain the experimental validity of a product rule for the dissipative part of the resistivity tensor, which was previously derived by MacDonald and Fisher (cond-mat/9907278) for a perfect stripe structure.     

纳米结构中磁斯格明子的原位电子全息研究

斯格明子(skyrmion)磁序结构与晶体微观结构的关联是新型功能磁材料和器件研发的重要问题.本文利用微纳加工技术制备了形状、尺寸均可控的磁纳米结构,通过电子全息术观察定量地分析了斯格明子磁序结构,确定了材料晶格缺陷和空间受限效应对斯格明子磁结构形成和稳定机制的影响,系统地分析了斯格明子基元的磁功能与材料微结构的关联.文中主要探讨了两个问题:1)斯格明子在磁纳米结构中的空间受限效应.重点研究斯格明子磁序随外磁场和温度变化的演变规律,探索其演变过程的拓扑属性和稳定性;2)晶格缺陷对斯格明子磁结构的影响,重点考察晶界原子结构手性反转对斯格明子磁序的影响.这些研究结果可为研发以磁斯格明子为基元的磁信息存储器及自旋电子学器件提供重要实验基础.     

Fracture of polycrystalline graphene membranes by in situ nanoindentation in a scanning electron microscope

Failure of polycrystalline graphene grown by chemical vapor deposition was investigated by nanoindentation in a scanning electron microscope. Circular graphene membranes were subject to central point loads using a nanomanipulator combined with an atomic force microscope cantilever as a force sensor. The grain boundaries of the polycrystalline graphene were visualized by Raman spectroscopy coupled with a carbon isotope labeling technique. Graphene membranes without any grain boundary had a failure strength of 45.4 ± 10.4 GPa, compared to 16.4 ± 5.1 GPa for those with grain boundaries when a Young's modulus was assumed to be 1 TPa. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)