Showing papers on "Grain boundary published in 2013"
TL;DR: In this paper, single-crystal islands and polycrystals containing tilt and mirror twin grain boundaries are characterized, and the influence of the grain boundaries on the material properties of molybdenum disulphide is assessed.
Abstract: Despite recent progress in the synthesis and characterization of molybdenum disulphide, little is yet known about its microstructure. Using refined chemical vapour deposition synthesis, high-quality crystals of monolayer molybdenum disulphide have now been grown. Single-crystal islands and polycrystals containing tilt and mirror twin grain boundaries are characterized, and the influence of the grain boundaries on the material properties of molybdenum disulphide is assessed.
1,911 citations
TL;DR: The atomic scale study of structural defects presented here brings new opportunities to tailor the properties of MoS2 via controlled synthesis and defect engineering.
Abstract: Monolayer molybdenum disulfide (MoS2) is a two-dimensional direct band gap semiconductor with unique mechanical, electronic, optical, and chemical properties that can be utilized for novel nanoelectronics and optoelectronics devices. The performance of these devices strongly depends on the quality and defect morphology of the MoS2 layers. Here we provide a systematic study of intrinsic structural defects in chemical vapor phase grown monolayer MoS2, including point defects, dislocations, grain boundaries, and edges, via direct atomic resolution imaging, and explore their energy landscape and electronic properties using first-principles calculations. A rich variety of point defects and dislocation cores, distinct from those present in graphene, were observed in MoS2. We discover that one-dimensional metallic wires can be created via two different types of 60° grain boundaries consisting of distinct 4-fold ring chains. A new type of edge reconstruction, representing a transition state during growth, was als...
1,722 citations
TL;DR: The controlled vapour phase synthesis of molybdenum disulphide atomic layers is reported and a fundamental mechanism for the nucleation, growth, and grain boundary formation in its crystalline monolayers is elucidated.
Abstract: Single-layered molybdenum disulphide with a direct bandgap is a promising two-dimensional material that goes beyond graphene for the next generation of nanoelectronics. Here, we report the controlled vapour phase synthesis of molybdenum disulphide atomic layers and elucidate a fundamental mechanism for the nucleation, growth, and grain boundary formation in its crystalline monolayers. Furthermore, a nucleation-controlled strategy is established to systematically promote the formation of large-area, single- and few-layered films. Using high-resolution electron microscopy imaging, the atomic structure and morphology of the grains and their boundaries in the polycrystalline molybdenum disulphide atomic layers are examined, and the primary mechanisms for grain boundary formation are evaluated. Grain boundaries consisting of 5- and 7- member rings are directly observed with atomic resolution, and their energy landscape is investigated via first-principles calculations. The uniformity in thickness, large grain sizes, and excellent electrical performance signify the high quality and scalable synthesis of the molybdenum disulphide atomic layers.
1,645 citations
TL;DR: In this article, the microstructure of monolayer molybdenum disulfide has been studied and the authors show that triangular islands are single crystals and merge to form faceted tilt and mirror boundaries that are stitched together by lines of 8- and 4- membered rings.
Abstract: Recent progress in large-area synthesis of monolayer molybdenum disulfide, a new two-dimensional direct-bandgap semiconductor, is paving the way for applications in atomically thin electronics. Little is known, however, about the microstructure of this material. Here we have refined chemical vapor deposition synthesis to grow highly crystalline islands of monolayer molybdenum disulfide up to 120 um in size with optical and electrical properties comparable or superior to exfoliated samples. Using transmission electron microscopy, we correlate lattice orientation, edge morphology, and crystallinity with island shape to demonstrate that triangular islands are single crystals. The crystals merge to form faceted tilt and mirror boundaries that are stitched together by lines of 8- and 4- membered rings. Density functional theory reveals localized mid-gap states arising from these 8-4 defects. We find that mirror boundaries cause strong photoluminescence quenching while tilt boundaries cause strong enhancement. In contrast, the boundaries only slightly increase the measured in-plane electrical conductivity.
1,430 citations
TL;DR: In this article, a controlled vapor phase synthesis of molybdenum disulfide atomic layers and elucidate a fundamental mechanism for the nucleation, growth, and grain boundary formation in its crystalline monolayers are reported.
Abstract: Single layered molybdenum disulfide with a direct bandgap is a promising two-dimensional material that goes beyond graphene for next generation nanoelectronics. Here, we report the controlled vapor phase synthesis of molybdenum disulfide atomic layers and elucidate a fundamental mechanism for the nucleation, growth, and grain boundary formation in its crystalline monolayers. Furthermore, a nucleation-controlled strategy is established to systematically promote the formation of large-area single- and few-layered films. The atomic structure and morphology of the grains and their boundaries in the polycrystalline molybdenum disulfide atomic layers are examined and first-principles calculations are applied to investigate their energy landscape. The electrical properties of the atomic layers are examined and the role of grain boundaries is evaluated. The uniformity in thickness, large grain sizes, and excellent electrical performance of these materials signify the high quality and scalable synthesis of the molybdenum disulfide atomic layers.
1,245 citations
TL;DR: It is shown that the elastic stiffness of CVD-graphene is identical to that of pristine graphene if postprocessing steps avoid damage or rippling, and its strength is only slightly reduced despite the existence of grain boundaries.
Abstract: Pristine graphene is the strongest material ever measured. However, large-area graphene films produced by means of chemical vapor deposition (CVD) are polycrystalline and thus contain grain boundaries that can potentially weaken the material. We combined structural characterization by means of transmission electron microscopy with nanoindentation in order to study the mechanical properties of CVD-graphene films with different grain sizes. We show that the elastic stiffness of CVD-graphene is identical to that of pristine graphene if postprocessing steps avoid damage or rippling. Its strength is only slightly reduced despite the existence of grain boundaries. Indentation tests directly on grain boundaries confirm that they are almost as strong as pristine. Graphene films consisting entirely of well-stitched grain boundaries can retain ultrahigh strength, which is critical for a large variety of applications, such as flexible electronics and strengthening components.
763 citations
TL;DR: It is demonstrated that high-crystalline mono- and few-layer WS2 flakes and even complete layers can be synthesized on sapphire with the domain size exceeding 50 × 50 μm(2) and it is interesting to see that, only through a mild sample oxidation process, the grain boundaries are easily recognizable by scanning electron microscopy due to its altered contrasts.
Abstract: Atomically thin tungsten disulfide (WS2), a structural analogue to MoS2, has attracted great interest due to its indirect-to-direct band-gap tunability, giant spin splitting, and valley-related physics. However, the batch production of layered WS2 is underdeveloped (as compared with that of MoS2) for exploring these fundamental issues and developing its applications. Here, using a low-pressure chemical vapor deposition method, we demonstrate that high-crystalline mono- and few-layer WS2 flakes and even complete layers can be synthesized on sapphire with the domain size exceeding 50 × 50 μm2. Intriguingly, we show that, with adding minor H2 carrier gas, the shape of monolayer WS2 flakes can be tailored from jagged to straight edge triangles and still single crystalline. Meanwhile, some intersecting triangle shape flakes are concomitantly evolved from more than one nucleus to show a polycrystalline nature. It is interesting to see that, only through a mild sample oxidation process, the grain boundaries are ...
710 citations
TL;DR: In this paper, a high-entropy FeCoNiCrMn alloy with a single face-centered cubic phase was synthesized and subsequently annealed at different temperatures to systematically investigate the grain growth behavior.
598 citations
TL;DR: Very-high-rate shear deformation with high strain gradients was applied in the top surface layer of bulk nickel, where a 2D nanometer-scale laminated structure was induced, producing a stronger, more thermally robust nickel microstructure.
Abstract: Heavy plastic deformation may refine grains of metals and make them very strong. But the strain-induced refinement saturates at large strains, forming three-dimensional ultrafine-grained (3D UFG) structures with random orientations. Further refinement of this microstructure is limited because of the enhanced mobility of grain boundaries. Very-high-rate shear deformation with high strain gradients was applied in the top surface layer of bulk nickel, where a 2D nanometer-scale laminated structure was induced. The strongly textured nanolaminated structure (average lamellar thickness of 20 nanometers) with low-angle boundaries among the lamellae is ultrahard and ultrastable: It exhibits a hardness of 6.4 gigapascal--which is higher than any reported hardness of the UFG nickel--and a coarsening temperature of 40 kelvin above that in UFG nickel.
426 citations
TL;DR: In this paper, three-dimensional atom-probe tomography studies demonstrate that the distribution of Al is highly inhomogeneous in the sintered bulk samples, and Al-containing precipitates including Al(Cu,Zn)−O−N, Al-O-N and Al−N are distributed in the matrix.
420 citations
TL;DR: In this article, the authors present an approach for processing bulk nanocomposites containing interfaces that are stable under irradiation, which is the key factor in reducing the damage and imparting stability in certain nanomaterials under conditions where bulk materials exhibit void swelling and/or embrittlement.
TL;DR: A novel mechanism of radiation damage healing in metals is proposed, which may guide further improvements in radiation resistance of metallic materials through grain boundary engineering.
Abstract: Structural transformations at interfaces are of profound fundamental interest as complex examples of phase transitions in low-dimensional systems. Despite decades of extensive research, no compelling evidence exists for structural transformations in high-angle grain boundaries in elemental systems. Here we show that the critical impediment to observations of such phase transformations in atomistic modelling has been rooted in inadequate simulation methodology. The proposed new methodology allows variations in atomic density inside the grain boundary and reveals multiple grain boundary phases with different atomic structures. Reversible first-order transformations between such phases are observed by varying temperature or injecting point defects into the boundary region. Owing to the presence of multiple metastable phases, grain boundaries can absorb significant amounts of point defects created inside the material by processes such as irradiation. We propose a novel mechanism of radiation damage healing in metals, which may guide further improvements in radiation resistance of metallic materials through grain boundary engineering.
TL;DR: The analysis shows how the versatile GB in MS(2) (if carefully engineered) should enable new developments for electronic and opto-electronic applications.
Abstract: Guided by the principles of dislocation theory, we use the first-principles calculations to determine the structure and properties of dislocations and grain boundaries (GB) in single-layer transition metal disulfides MS2 (M = Mo or W). In sharp contrast to other two-dimensional materials (truly planar graphene and h-BN), here the edge dislocations extend in third dimension, forming concave dreidel-shaped polyhedra. They include different number of homoelemental bonds and, by reacting with vacancies, interstitials, and atom substitutions, yield families of the derivative cores for each Burgers vector. The overall structures of GB are controlled by both local-chemical and far-field mechanical energies and display different combinations of dislocation cores. Further, we find two distinct electronic behaviors of GB. Typically, their localized deep-level states act as sinks for carriers but at large 60°-tilt the GB become metallic. The analysis shows how the versatile GB in MS2 (if carefully engineered) should...
TL;DR: In this paper, the grain boundary mismatch in the carbon-carbon bonds at the boundary of the two-dimensional structure of graphene has been found to increase the strength of synthesized graphene.
Abstract: The two-dimensional structure of graphene is known to impart high strength, but can be hard to synthesize without grain boundaries. Here, the authors find that strength increases with grain boundary mismatch, which results from low atomic-scale strain in the carbon–carbon bonds at the boundary.
25 Nov 2013
TL;DR: It is demonstrated how this can be achieved using an approach that combines the accuracy of structural characterization in transmission electron microscopy with the 3D chemical sensitivity of atom probe tomography, which indicates that ω is the most influential crystallographic parameter in this regime.
Abstract: Grain boundary segregation leads to nanoscale chemical variations that can alter a material's performance by orders of magnitude (e.g., embrittlement). To understand this phenomenon, a large number of grain boundaries must be characterized in terms of both their five crystallographic interface parameters and their atomic-scale chemical composition. We demonstrate how this can be achieved using an approach that combines the accuracy of structural characterization in transmission electron microscopy with the 3D chemical sensitivity of atom probe tomography. We find a linear trend between carbon segregation and the misorientation angle ω for low-angle grain boundaries in ferrite, which indicates that ω is the most influential crystallographic parameter in this regime. However, there are significant deviations from this linear trend indicating an additional strong influence of other crystallographic parameters (grain boundary plane, rotation axis). For high-angle grain boundaries, no general trend between carbon excess and ω is observed; i.e., the grain boundary plane and rotation axis have an even higher influence on the segregation behavior in this regime. Slight deviations from special grain boundary configurations are shown to lead to unexpectedly high levels of segregation.
TL;DR: In this article, a phase field model was proposed to predict the kinetics of phase transformation at segregation decorated grain boundaries, showing that strong interface segregation of austenite stabilizing elements (here Mn) and the release of elastic stresses from the host martensite can generally promote phase transformation.
TL;DR: In this article, the grain boundary diffusion process using an Nd70Cu30 eutectic alloy has been applied to hot-deformed anisotropic Nd-Fe-B magnets, resulting in a substantial enhancement of coercivity, at the expense of remanence.
TL;DR: In this paper, the electrical characteristics of field effect transistors (FETs) with single-crystal molybdenum disulfide (MoS2) channels synthesized by chemical vapor deposition (CVD) were reported.
Abstract: We report the electrical characteristics of field-effect transistors (FETs) with single-crystal molybdenum disulfide (MoS2) channels synthesized by chemical vapor deposition (CVD). For a bilayer MoS2 FET, the field-effect mobility is ∼17 cm2 V−1 s−1 and the on/off current ratio is ∼108, which are much higher than those of FETs based on CVD polycrystalline MoS2 films. By avoiding the detrimental effects of the grain boundaries and the contamination introduced by the transfer process, the quality of the CVD MoS2 atomic layers deposited directly on SiO2 is comparable to or better than the exfoliated MoS2 flakes. The result shows that CVD is a viable method to synthesize high quality MoS2 atomic layers.
TL;DR: A microscopic mechanism that is responsible for the low electron mobility observed in CVD-grown graphene is uncovered, and the possibility of using electronic barriers to realize tunable plasmon reflectors and phase retarders in future graphene-based plAsmonic circuits is suggested.
Abstract: Individual grain boundaries are imaged using a scanning plasmon interferometry technique, revealing mechanistic insights on electronic transport and plasmon propagation in graphene.
TL;DR: Grain boundaries are observed and characterized in chemical vapor deposition-grown sheets of hexagonal boron nitride (h-BN) via ultra-high-resolution transmission electron microscopy at elevated temperature, consistent with recent theoretical model predictions.
Abstract: Grain boundaries are observed and characterized in chemical vapor deposition-grown sheets of hexagonal boron nitride (h-BN) via ultra-high-resolution transmission electron microscopy at elevated temperature. Five- and seven-fold defects are readily observed along the grain boundary. Dynamics of strained regions and grain boundary defects are resolved. The defect structures and the resulting out-of-plane warping are consistent with recent theoretical model predictions for grain boundaries in h-BN.
TL;DR: In this article, the mechanism of the coercivity enhancement by the grain boundary diffusion process (GBDP) using Dy vapor based on detailed microstructural characterizations was discussed, and it was shown that both the (Nd,Dy) 2 Fe 14 B shell and the Nd-rich GB phase layer are required micro-structural features for the GBDP.
TL;DR: In this paper, the evolution of dislocation storage in deformed copper was studied with cross-correlation-based high-resolution electron backscatter diffraction, and the average dislocation density increases with imposed macroscopic strain in accord with Ashby's theory of work hardening.
TL;DR: In this article, a regular nanocrystalline solution model was developed to identify the conditions under which binary nanocrystine alloy systems with positive heats of mixing are stable with respect to both grain growth and phase separation.
TL;DR: A crystal growth method that employs only a cyclic heat treatment to obtain a single crystal of more than several centimeters in a copper-based shape-memory alloy is reported, providing a method of fabricating a single-crystal or large-grain structure important for shape- memory properties, magnetic properties, and creep properties, among others.
Abstract: In polycrystalline materials, grain growth occurs at elevated temperatures to reduce the total area of grain boundaries with high energy. The grain growth rate usually slows down with annealing time, making it hard to obtain grains larger than a millimeter in size. We report a crystal growth method that employs only a cyclic heat treatment to obtain a single crystal of more than several centimeters in a copper-based shape-memory alloy. This abnormal grain growth phenomenon results from the formation of a subgrain structure introduced through phase transformation. These findings provide a method of fabricating a single-crystal or large-grain structure important for shape-memory properties, magnetic properties, and creep properties, among others.
TL;DR: In this paper, the authors demonstrate that the highly defective structure of reduced graphene oxide sheets assembled into free-standing, paper-like films can be fully repaired by means of high temperature annealing (graphitization).
Abstract: The complete restoration of a perfect carbon lattice has been a central issue in the research on graphene derived from graphite oxide since this preparation route was first proposed several years ago, but such a goal has so far remained elusive. Here, we demonstrate that the highly defective structure of reduced graphene oxide sheets assembled into free-standing, paper-like films can be fully repaired by means of high temperature annealing (graphitization). Characterization of the films by X-ray photoelectron and Raman spectroscopy, X-ray diffraction and scanning tunneling microscopy indicated that the main stages in the transformation of the films were (i) complete removal of oxygen functional groups and generation of atomic vacancies (up to 1,500 °C), and (ii) vacancy annihilation and coalescence of adjacent overlapping sheets to yield continuous polycrystalline layers (1,800–2,700 °C) similar to those of highly oriented graphites. The prevailing type of defect in the polycrystalline layers were the grain boundaries separating neighboring domains, which were typically a few hundred nanometers in lateral size, exhibited long-range graphitic order and were virtually free of even atomic-sized defects. The electrical conductivity of the annealed films was as high as 577,000 S·m−1, which is by far the largest value reported to date for any material derived from graphene oxide, and strategies for further improvement without the need to resort to higher annealing temperatures are suggested. Overall, this work opens the prospect of truly achieving a complete restoration of the carbon lattice in graphene oxide materials.
TL;DR: It is shown that dislocations and GBs in two-dimensional (2D) metal dichalcogenides MX2 can actually improve the material by giving it a qualitatively new physical property: magnetism.
Abstract: Grain boundaries (GBs) are structural imperfections that typically degrade the performance of materials. Here we show that dislocations and GBs in two-dimensional (2D) metal dichalcogenides MX2 (M = Mo, W; X = S, Se) can actually improve the material by giving it a qualitatively new physical property: magnetism. The dislocations studied all display a substantial magnetic moment of ∼1 Bohr magneton. In contrast, dislocations in other well-studied 2D materials are typically nonmagnetic. GBs composed of pentagon–heptagon pairs interact ferromagnetically and transition from semiconductor to half-metal or metal as a function of tilt angle and/or doping level. When the tilt angle exceeds 47°, the structural energetics favor square–octagon pairs and the GB becomes an antiferromagnetic semiconductor. These exceptional magnetic properties arise from interplay of dislocation-induced localized states, doping, and locally unbalanced stoichiometry. Purposeful engineering of topological GBs may be able to convert MX2 i...
TL;DR: The formation of deformation, annealing and growth twins in face-centered cubic materials is discussed in this paper, where the influence of metallurgical variables on twinning can only be rationalized in terms of the model.
TL;DR: In this paper, a bulk Al 7075 alloy with an ultrafine grain (UFG) structure was fabricated via cryomilling, hot isostatic pressing and extrusion, followed by solution treatment and artificial aging.
TL;DR: In this paper, the authors identify an unconventional pure-shuffle mechanism for the nucleation of (1¯012) twins, which then grow through the conventional glide shuffle mechanism entailing the glide of twinning disconnections.
Abstract: The propagation of deformation twins in hexagonal-close-packed metals is commonly described by a conventional glide-shuffle mechanism. The widely accepted convention is that this process is also responsible for twin nucleation, but lacks direct confirmation. Using atomistic simulations, we identify an unconventional pure-shuffle mechanism for the nucleation of (1¯012) twins, which then grow through the conventional glide-shuffle mechanism entailing the glide of twinning disconnections. The pure-shuffle nucleation of twins at grain boundaries can be ascribed to a high-stress concentration and pre-existing grain boundary dislocations.
TL;DR: The fracture of polycrystalline graphene is explored by performing molecular dynamics simulations with realistic finite-grain-size models, emphasizing the role of grain boundary ends and junctions, with a surprising systematic decrease of tensile strength and failure strain, while the elastic modulus rises.
Abstract: The fracture of polycrystalline graphene is explored by performing molecular dynamics simulations with realistic finite-grain-size models, emphasizing the role of grain boundary ends and junctions. The simulations reveal a ∼50% or more strength reduction due to the presence of the network of boundaries between polygonal grains, with cracks preferentially starting at the junctions. With a larger grain size, a surprising systematic decrease of tensile strength and failure strain is observed, while the elastic modulus rises. The observed crack localization and strength behavior are well-explained by a dislocation-pileup model, reminiscent of the Hall–Petch effect but coming from different underlying physics.