High-angle Twist Grain Research Articles

Carrier Separation Enhanced by High Angle Twist Grain Boundaries in Cesium Lead Bromide Perovskites.

Grain boundaries (GBs) have a profound impact on mechanical, chemical, and physical properties of polycrystalline materials. Comprehension of atomic and electronic structures of different GBs in materials can help to understand their impact on materials' properties. Here, with aberration-corrected scanning transmission electron microscopy (STEM), the atomic structure of a 90° twist GB s in CsPbBr3 is determined, and its impact on electron-hole pair separation is predicted. The 90° twist GB has a coherent interface and the same chemical composition as the bulk except for the lattice twist. Density functional theory (DFT) calculation results indicate that the twist GB has an electronic structure similar to that of the bulk CsPbBr3. An electronic potential at the GBs enhances the separation of photogenerated carriers and promotes the motion of electrons across the GBs. These results extend the understanding of atomic and electronic structure of GBs in halide perovskites and propose a potential strategy to eliminate the influence of GBs by GB engineering.

Nucleation and growth of helium bubble at (110) twist grain boundaries in tungsten studied by molecular dynamics**Project supported by the National Natural Science Foundation of China (Grant Nos. 11305136, 11675230, 11375242, and U1832112).

Migration of He atoms and growth of He bubbles in high angle twist grain boundaries (HAGBs) in tungsten (W) are investigated by atomic simulation method. The energy and free volume (FV) of grain boundary (GB) are affected by the density and structure of dislocation patterns in GB. The migration energy of the He atom between the neighboring trapping sites depends on free volume along the migration path at grain boundary. The region of grain boundary around the He bubble forms an ordered crystal structure when He bubble grows at certain grain boundaries. The He atoms aggregate on the grain boundary plane to form a plate-shape configuration. Furthermore, high grain boundary energy (GBE) results in a large volume of He bubble. Thus, the nucleation and growth of He bubbles in twist grain boundaries depend on the energy of grain boundary, the dislocation patterns and the free volume related migration path on the grain boundary plane.

Molecular dynamical investigation on plastic behavior of Cu(100) twist-grain boundary under shear load

By using molecular dynamics with EAM potential, yield strength of copper column with (100) twist grain boundary and twist grain boundary effect are investigated under shear load. The results reveal that dislocations nucleate at the misfit dislocation network of low-angle twist grain boundary and dislocations stacking influence the yield strength of copper column. For high-angle twist grain boundary, twist grain boundary sliding decreases the yield stress of copper column. Meanwhile it is found that, with increasing twist angle, copper column yielding strength increases firstly. When twist angle reaches the critical value, the yielding strength decreases with increasing twist angle. It is concluded that different mechanisms mediate the yielding of copper column. Dislocation nucleation controls the yield stress of copper column for low-angle twist grain boundary and yielding of copper column is dominated by grain boundary sliding for high-angle twist grain boundary.

Cooperative atomic displacements at sheared twist (001) grain boundaries

Molecular dynamics simulations show that the shear deformation of Cu bi-crystalscontaining a symmetric high-angle twist (001) grain boundary induces the formation offorce chains in which a few atoms are pushed against each other. Short-range andrelatively long-range atomic displacements take place in the neighborhood ofa force chain when its size respectively increases and decreases, promoting themigration of atoms across the grain boundary. The thickness of the layer affectedby atomic diffusion processes increases approximately with the square root oftime.

Order and structural units in simulations of twist grain boundaries in silicon at absolute zero

In contrast to previous results of simulations of the ground-state structures of high-angle twist grain boundaries (GB's) in silicon at absolute zero, in which amorphous intergranular films were found, recent experimental results imply structural order in some high-angle twist boundaries. With a novel protocol for simulating twist GB's, which allows the number of atoms at the boundary to vary, we have found new, lower energy, ordered structures. We have also found structural units common to many of these boundaries, which is further evidence that these boundary structures are not arbitrary or disordered. We give a detailed exposition of the results for five boundaries and conclude that there is structural order present in all of them.

Are the Structures of Twist Grain Boundaries in Silicon Ordered at 0 K?

Contrary to previous simulation results on the existence of amorphous intergranular films at high-angle twist grain boundaries (GBs) in elemental solids such as silicon, recent experimental results imply structural order in some high-angle boundaries. With a novel protocol for simulating twist GBs, which allows the number of atoms at the boundary to vary, we have found new low-energy ordered structures. We give a detailed exposition of the results for the simplest boundary. The validity of our results is confirmed by first-principles calculations.

Nonorthogonal tight-binding model for germanium

We present a pair of nonorthogonal tight-binding (TB) models for germanium within the NRL-TB approach. One uses an ${\mathrm{sp}}^{3}$ basis, and is optimized for total-energy calculations by fitting to the total energy and band structures of several high-symmetry lattice structures. The other uses an ${\mathrm{sp}}^{3}{d}^{5}$ basis to accurately reproduce the diamond lattice band structure, including three conduction bands. We present tests of the ${\mathrm{sp}}^{3}$ TB model on bulk properties, including high-symmetry lattice structure energies and volumes and the diamond lattice elastic constants, phonons, and band structure. We also present results for point defect formation and relaxation energies and low index surface energies and stresses, many of which have not been calculated using the density-functional theory (DFT), as well as some medium size clusters. Taking advantage of the computational efficiency of the TB approach, we go beyond the capabilities of standard density-functional theory, combining it with molecular dynamics to simulate finite temperature properties of Ge. We get good agreement with experiment for the atomic mean-squared displacement and the melting point approximated using the Lindemann criterion, as well as the linear thermal-expansion coefficient. In another demonstration of the efficiency of the TB approach, we present results for the structure and electronic properties of a high angle twist grain boundary (GB). In agreement with DFT simulation we see a range of structures with comparable energies, all with electronic states deep in the band gap. In contrast to previous work we find some different geometries with perfect fourfold coordination of all atoms in the GB. Despite the perfect coordination, these structures also have deep electronic states in the gap, indicating that the GB will be electrically active.

Tight-binding molecular-dynamics simulation of impurities in ultrananocrystalline diamond grain boundaries

Ultrananocrystalline diamond (UNCD) films grown from hydrogen-poor plasmas have grain sizes of 3-10 nm, resulting in a large number of grain boundaries. We repon on density-functional-based tight-binding molecular-dynamics calculations of high-energy high-angle twist (100) grain boundaries in diamond as a model for the UNCD grain boundaries. About one-half of the carbons in the grain boundary are threefold coordinated and are responsible for states introduced into the band gap. Simulations were also performed for N, Si, and H impurities in (100) twist grain boundaries where substitution energies, optimized geometries, and electronic structures were calculated. Substitution energies were found to be substantially lower for the grain boundaries compared to the bulk diamond crystal. Nitrogen increases the number of threefold-coordinated carbons while hydrogen saturates dangling bonds. The electronic structure of UNCD is characterized by a large number of states in the band gap attributed to the bonding disorder and impurities in the grain boundaries.

Huge differences between low- and high-angle twist grain boundaries: The case of ultrathin (001) Si films bonded to (001) Si wafers

Ultrathin (001) Si films bonded onto (001) Si wafers, inducing grain boundaries with twist angles varying from 0.5° to 12°, were studied by transmission electron microscopy. A great structural difference between low (ψ<5°) and high (ψ>6°) twist angles was observed. In low twist angle grain boundaries, “twist interfacial dislocations” are dissociated and produce rough interfaces with no oxide precipitates. It is the opposite in high-angle grain boundaries: there is no dissociation, the interfaces are smoother but contain oxide precipitates. These differences are not attributed to the thin thickness of one grain, but to the large atomic differences between high- and low-angle twist grain boundaries, which is not the case for tilt grain boundaries.

Self-diffusion in high-angle fcc metal grain boundaries by molecular dynamics simulation

Abstract Recent molecular dynamics simulations of high-energy high-angle twist grain boundaries (GBs) in Si revealed a universal liquid-like high-temperature structure which, at lower temperatures, undergoes a reversible structural and dynamical transition from a confined liquid to a solid; low-energy boundaries, by contrast, were found to remain solid all the way up to the melting point. Here we demonstrate for the case of palladium that fcc metal GBs behave in much the same manner. Remarkably, at high temperatures the few representative high-energy high-angle (tilt or twist) boundaries examined here exhibit the same, rather low self-diffusion activation energy and an isotropic liquid-like diffusion mechanism that is independent of the boundary misorientation. These observations are in qualitative agreement with recent GB self- and impurity-diffusion experiments by Budke et al. on Cu. Our simulations demonstrate that the decrease in the activation energy at elevated temperatures is caused by a structural transition, from a solid boundary structure at low temperatures to a liquid-like structure at high temperatures. Consistent with the experiments, the transition temperature decreases with increasing GB energy, that is with increasing degree of short-range GB structural disorder. By contrast, the degree of long-range structural disorder in the zero-temperature GB appears to play no role in whether or not the GB undergoes such a transition at elevated temperatures.

High angle [001] twist boundaries in platelet colonies of BiSrCaCuO (2223) superconductor.

The microstructure of BiPbSrCaCuO (2223) ceramic was studied by polarised light microscopy and the etch-pits technique. The high-angle twist grain boundaries in the platelet colonies were confirmed to be [001] coincidence type twisted boundaries or rotation twin boundaries with the (001) twinning plane.

Molecular Dynamics Simulation of Impurities in Nanocrystalline Diamond Grain Boundaries

ABSTRACTNanocrystalline diamond films grown on Si substrates at 800°C from hydrogen-poorplasmas have a number of highly desirable mechanical and electronic properties. Impurities were found by SIMS measurements to be uniformly distributed throughout the thickness of the films at a level of 1017–1018 cm−3. It is likely that the impurities are located at the grain boundaries, which play a crucial role in controlling important characteristics of the films, such as electrical conductivity and electron emission. Density-functional based tight-binding (DFTB) molecular dynamics simulations were performed for diamond high-energy high-angle (100) twist grain boundaries with impurities such as N, Si and H

Mechanical properties of twist grain boundaries in Cu

In a previous paper we studied the Young's and shear modulus of a high-angle twist grain boundary (Σ5) in Cu, using the EAM, and related it to the uniaxial strain derivatives of single crystals. In this paper, we discuss elastic properties of ten additional twist grain boundaries, from 8.8–43.6°. The monolayer Young's modulus at each boundary was calculated and found to be 20–50% higher than the bulk value for all eleven boundaries for both csl and type1 structures. The monolayer shear modulus at each boundary was calculated and found to be 93–98% lower than the bulk value for six grain boundaries with csl structure and found to decrease with increasing twist angle. The critical shear stress was also calculated for eleven boundaries with csl structure and found to roughly decrease with increasing twist angle.

Reconstruction of grain boundaries in copper and gold by simulation

The reconstructions of high-angle twist grain boundaries on the four densest atomic planes in fcc copper, as described by a Lennard-Jones potential, and gold, as described by an embedded-atom-method potential, are investigated using the recently developed method of grand-canonical simulated quenching. It is found that the grain boundaries on the widely spaced (111) and (100) planes do not reconstruct, while those on the less widely spaced (110) and (113) planes do reconstruct. The effect that reconstruction can have on the physical properties of an interfacial system is illustrated by comparing the elastic properties and ideal cleavage energies of reconstructed grain boundaries with those of corresponding unreconstructed grain boundaries.

Self-diffusion along twist grain boundaries in Cu

In a previous paper we studied vacancy diffusion in two high-angle twist grain boundaries in Cu, using the EAM. In this paper, we discuss vacancy diffusion along four additional twist grain boundaries, from 8.8–43.6°. Vacancy formation energies in all the possible sites were calculated (0.14–1.42 eV) and found to be directly related to the degree of coincidence with the neighboring crystal planes. The optimal migration paths were found to coincide with the screw dislocations which comprise the boundary. Vacancy migration energies were found to be low (0.02–0.52 eV). The activation energies for self-diffusion at the boundaries were found to be less than half of the bulk value, in general agreement with experiment. Calculated diffusion rates, δD, for medium-high angle twist grain boundaries were in reasonable agreement with experimental data for polycrystalline material. Diffusion rates were found to decrease with increasing twist angle, in contrast with two sets of conflicting experimental data.

Simulated quenching to the zero-temperature limit of the grand-canonical ensemble

A formalism for obtaining the zero-temperature structure of monocomponent solids in the grand-canonical ensemble is developed. The new methodology, grand-canonical simulated quenching, is validated by simulating systems initially containing a vacancy and an interstitial. As a first application, the reconstruction of a high-angle twist grain boundary in fcc Cu is investigated.

Thermoelastic behavior of structurally disordered interface materials: Homogeneous versus inhomogeneous effects.

The effects of homogeneous (temperature-induced) and inhomogeneous (interface-induced) structural disorder on the thermoelastic properties of interface materials are explored by molecular-dynamics simulation. The interface systems considered are superlattices composed of high-angle twist grain boundaries on the (100) plane of a fcc metal at various modulation wavelengths and temperatures. For comparison, the thermoelastic behavior of a perfect fcc crystal is also investigated at the same temperatures. By allowing thermal expansion in some cases, and not in other cases, the explicit effects of the thermal expansion on the elastic properties are explored. We find competing thermal effects of the atomic-level disordering on one hand and the consequent volume expansion on the other. We conclude that atomic-level structural disorder, be it homogeneous or inhomogeneous, can lead to elastic stiffening, provided that the related volume expansions do not dominate the elastic behavior and result in a softening.

Reconstruction of Twist Grain Boundaries in Gold

ABSTRACTThe reconstruction of high-angle twist grain boundaries on the four densest atomic planes in gold are investigated using the recently developed method of grand-canonical simulated quenching. It is found that the grain boundaries on the two densest planes, (111) and (100), do not reconstruct, while those on the (110) and (113) planes do.

The effects of segregation on grain boundary cohesive energies in Ni 3−xAl 1+x

The structure and properties of grain boundaries in the intermetallic alloy Ni3A1 have received considerable attention in the past several years due to their interesting mechanical properties. The aim in many of these studies [1-13] was to explain the increase in ductility that occurs when polycrystalline Ni3AI is doped with boron. Experimental observations [1-7] have shown that there is a strong correlation between nickel enrichment at grain boundaries and the ductilization of boron-doped polycrystalline Ni3AI. This grain boundary nickel enrichment has only been observed in Nirich Ni3-xAll+x (i.e., x<0). Some experimental studies have shown that grain boundary nickel enrichment also occurs in Ni-rich, boron-free polycrystalline Ni3-xAll+x [3,7]. Two models have been proposed [14,15] to explain the observed ductilization of polycrstalline Ni3AI. In the first model [e.g. 14], an increase in grain boundary cohesive energy has been attributed to nickel enrichment at the grain boundaries, while in the other model [15], it is suggested that the transmittal of slip across the grain boundary becomes easier in chemically disordered grain boundaries, where the disorder is induced by the nickel enrichment at grain boundaries. While these two purported effects of nickel segregation are not necessarily exclusive, experimental studies have been unable to conclusively validate either. There have been several atomistic simulation studies [8-13] that have been carded out to explore this subject. Most of these simulation studies [8-11] have been done at zero temperature and without direct consideration of the important segregation effects, These studies have shown that the cohesive energy for grain boundaries rich in nickel is lower than that for the same boundaries with aluminum-rich or stoichiometric compositions. One of these same studies [9] has shown that the grain boundary cohesive energy can be reduced even further by placing boron atoms in interstitial sites in the boundary, These results seem to give some credibility to the first model, since these changes in the cohesive energy occur for boundaries that are well ordered, One should note, however, that these simulations are carried out with stoichiometric bulk compositions, not at the nickel rich hulk compositions, where nickel enrichment of the grain boundaries and ductilization are experimentally observed. Due to the nature of the zero-temperature, composition simulation method used in these studies, it was not possible to study the variation in grain boundary cohesive energy that occurs with segregation and equilibrium disordering. Monte Carlo simulation studies [12] at I000 K have shown that grain boundaries become enriched with nickel and aluminum for Ni-rich and Al-rich bulk composition, respectively. In this letter, we report the results of our investigations of segregation effects on grain boundary cohesive energy in ordered, boron-fzee Ni3-xAll+x within ~e framework of ~ atomistic simulation procedure. Segregation to two high angle (001) twist grain boundaries ~ (36.9) and 5.13 (22.6) and to (001) free surfaces are studied using a newly developed, free energy minimization method [16-18]. These simulations are carded out for a range of alloy compositions corresponding to nickel concentrations in the bulk of 73.5, 75.0, and 76.6 atomic percent in the temperature range 300-900 K. This model is also used to investigate the degree of chemical disordering that occurs at the grain boundaries. The two existing models f~ the role of Ni segregation on the ductilization of Ni3AI are discussed in light of the present simulation results. Method The local harmonic CL,I-I) model has been applied with considerable success to both perfect and defected single component solids [16,17]. In this model, the classical vibrational contribution to the free energy for a single component system is given by

A model for ideal cleavage fracture of grain boundaries in b.c.c. metals

Abstract The random grain boundary model is used to determine the work of adhesion of symmetrical high-angle twist grain boundaries (GBs) in b.c.c. metals. In this idealized theoretical model of a high-angle GB, the interactions of atoms across the interface are assumed to be entirely random. Because fully relaxed free surfaces are included naturally, the model is well suited for the determination of GB cleavage-fracture energies. From an investigation of the role of the GB plane, in particular of the interplanar spacing parallel to the GB, it is concluded that, as in the case of free surfaces, ‘special’ and ‘vicinal’ interfaces have fundamentally different physical properties.