Individual Grain Boundaries Research Articles

Effect of Carrier Localization on Electrical Transport and Noise at Individual Grain Boundaries in Monolayer MoS2.

Despite its importance in the large-scale synthesis of transition metal dichalcogenides (TMDC) molecular layers, the generic quantum effects on electrical transport across individual grain boundaries (GBs) in TMDC monolayers remain unclear. Here we demonstrate that strong carrier localization due to the increased density of defects determines both temperature dependence of electrical transport and low-frequency noise at the GBs of chemical vapor deposition (CVD)-grown MoS2 layers. Using field effect devices designed to explore transport across individual GBs, we show that the localization length of electrons in the GB region is ∼30-70% lower than that within the grain, even though the room temperature conductance across the GB, oriented perpendicular to the overall flow of current, may be lower or higher than the intragrain region. Remarkably, we find that the stronger localization is accompanied by nearly 5 orders of magnitude enhancement in the low-frequency noise at the GB region, which increases exponentially when the temperature is reduced. The microscopic framework of electrical transport and noise developed in this paper may be readily extended to other strongly localized two-dimensional systems, including other members of the TMDC family.

Mass transfer in polycrystalline ytterbium disilicate under oxygen potential gradients at high temperatures

To elucidate the mass transfer mechanism in polycrystalline Yb2Si2O7 at high temperatures, the oxygen permeability of Yb2Si2O7 wafers was evaluated under various oxygen potential gradients (dμO) at temperatures up to 1673 K, using 18O2. The 18O was found to concentrate at grain boundaries (GBs), indicating that oxygen preferentially diffuses along the wafer GBs. It was also determined that oxygen permeation is controlled by the mutual GB diffusion of ytterbium and oxide ions, and that ytterbium and oxide ions interdiffuse along the GBs without experiencing any acceleration or retardation effects. The lattice and GB diffusion coefficients for oxygen were directly determined from secondary ion mass spectrometry (SIMS). The oxygen GB diffusion coefficients determined from SIMS-18O line profiles along cross-sections of individual GBs were almost the same as those obtained from SIMS-18O depth profiles. The oxygen GB diffusion coefficient under a dμO was clearly smaller than that in the absence of a dμO (representing self-diffusion data). This work also determined the electronic transference numbers for Yb2Si2O7 with and without the application of a dμO, on the basis of the oxygen GB diffusion coefficients predicted using oxygen permeation data.

Energetics of Hydrogen Segregation to α-Fe Grain Boundaries for Modeling Stress Corrosion Cracking

The physics of embrittlement is dictated by the various interactions between the impurities/defects and the local structure in polycrystalline material systems. In this study, a physically motivated model that describes the degree of interaction of hydrogen (H) defects on the segregation behavior to α-Fe grain boundaries (GBs) is developed. Molecular statics simulations were performed to quantify the segregation behavior of 1–2 H atoms at various interstitial sites around the , , , and symmetric tilt GBs. The results provide insights into the concentration profile of hydrogen defects along different GBs. Furthermore, the model accurately links the intrinsic GB character by quantifying the segregation length scale for the individual GBs based on the segregation behavior of defects. Finally, the metrics provided in this work are essential to comprehensively understanding the effect of hydrogen on the macroscopic behavior of α-Fe.

Oxygen permeation mechanism in polycrystalline mullite at high temperatures

AbstractThe oxygen permeability of polycrystalline mullite wafers, serving as a model environmental barrier coating layer on SiC fiber‐reinforced SiC matrix composites, was evaluated at temperatures above 1673 h with an oxygen tracer gas (18O2). Oxygen permeation occurred by grain‐boundary (GB) diffusion of oxygen from the high oxygen partial pressure (high‐Po2) surface to the low‐Po2 surface, with simultaneous GB diffusion of aluminum in the opposite direction. This GB interdiffusion of both oxygen and aluminum proceeded without acceleration or retardation, maintaining the Gibbs‐Duhem relationship. Oxygen permeation related to the GB diffusion of silicon was negligibly small compared to that generated by aluminum GB diffusion, resulting in decomposition of the mullite near the low‐Po2 surface. The GB diffusion coefficients for oxygen in the vicinity of the high‐Po2 surface were determined directly from the SIMS‐18O line profiles along individual GBs, as assessed from cross sections of the exposed wafer. The coefficients thus obtained were comparable to those determined in the absence of an oxygen potential gradient and those calculated from an oxygen permeation trial under the assumption of nearly ionic conductivity.

Anisotropy of self-diffusion in forsterite grain boundaries derived from molecular dynamics simulations

Diffusion rates and associated deformation behaviour in olivine have been subjected to many studies, due to the major abundance of this mineral group in the Earth’s upper mantle. However, grain boundary (GB) transport studies yield controversial results. The relation between transport rate, energy, and geometry of individual GBs is the key to understand transport in aggregates with lattice preferred orientation that favours the presence and/or alignment of specific GBs over random ones in an undeformed rock. In this contribution, we perform classical molecular dynamics simulations of a series of symmetric and one asymmetric tilt GBs of $$\hbox {Mg}_2\hbox {SiO}_4$$ forsterite, ranging from 9.58° to 90° in misorientation and varying surface termination. Our emphasis lies on unravelling structural characteristics of high- and low-angle grain boundaries and how the atomic structure influences grain boundary excess volume and self-diffusion processes. To obtain diffusion rates for different GB geometries, we equilibrate the respective systems at ambient pressure and temperatures from 1900 to 2200 K and trace their evolution for run durations of at least 1000 ps. We then calculate the mean square displacement of the different atomic species within the GB interface to estimate self-diffusion coefficients in the individual systems. Grain boundary diffusion coefficients for Mg, Si and O range from $$10^{-18}$$ to $$10^{-21}\,\hbox {m}^3$$ /s, falling in line with extrapolations from lower temperature experimental data. Our data indicate that higher GB excess volumes enable faster diffusion within the GB. Finally, we discuss two types of transport mechanisms that may be distinguished in low- and high-angle GBs.

Phase transformations at interfaces: Observations from atomistic modeling

We review the recent progress in theoretical understanding and atomistic computer simulations of phase transformations in materials interfaces, focusing on grain boundaries (GBs) in metallic systems. Recently developed simulation approaches enable the search and structural characterization of GB phases in single-component metals and binary alloys, calculation of thermodynamic properties of individual GB phases, and modeling of the effect of the GB phase transformations on GB kinetics. Atomistic simulations demonstrate that the GB transformations can be induced by varying the temperature, loading the GB with point defects, or varying the amount of solute segregation. The atomic-level understanding obtained from such simulations can provide input for further development of thermodynamics theories and continuous models of interface phase transformations while simultaneously serving as a testing ground for validation of theories and models. They can also help interpret and guide experimental work in this field.

Atomic structures and oxygen dynamics of CeO2 grain boundaries.

Material performance is significantly governed by grain boundaries (GBs), a typical crystal defects inside, which often exhibit unique properties due to the structural and chemical inhomogeneity. Here, it is reported direct atomic scale evidence that oxygen vacancies formed in the GBs can modify the local surface oxygen dynamics in CeO2, a key material for fuel cells. The atomic structures and oxygen vacancy concentrations in individual GBs are obtained by electron microscopy and theoretical calculations at atomic scale. Meanwhile, local GB oxygen reduction reactivity is measured by electrochemical strain microscopy. By combining these techniques, it is demonstrated that the GB electrochemical activities are affected by the oxygen vacancy concentrations, which is, on the other hand, determined by the local structural distortions at the GB core region. These results provide critical understanding of GB properties down to atomic scale, and new perspectives on the development strategies of high performance electrochemical devices for solid oxide fuel cells.

Improved dielectric properties of (Y + Mg) co-doped CaCu3Ti4O12 ceramics by controlling geometric and intrinsic properties of grain boundaries

Significantly improved dielectric properties of CaCu3Ti4O12 ceramics were accomplished by co-doping with Y3+ and Mg2+ to simultaneously control the geometric and intrinsic properties of grain boundaries (GB), respectively. Substitution of these dopants strongly suppressed grain growth while enhancing the resistivity of an individual GB, respectively. The resulting small grained microstructure with very high GB density and greatly increased resistivity resulted in a strong increase in the total resistance of GBs. This caused a large decrease in the loss tangent (tanδ~0.013). Furthermore, nonlinear current–voltage properties were also improved.

Properties of electronic potential barriers at grain boundaries in Cu(In,Ga)Se2 thin films

The lack of an efficiency increase with increasing Ga content in Cu(In,Ga)Se2 solar cells has attracted much scientific interest. It has been claimed that the physical properties of grain boundaries are responsible for this curious effect. Here, we present an in-depth analysis of electronic potential barriers at grain boundaries (GBs) in a series of Cu(In,Ga)Se2 (CIGSe) thin films using Kelvin probe force microscopy (KPFM) measurements, extending our previous study [Baier et al., Sol. Energy Mater. Sol. Cells 103 (2012) 86–92]. Here, (i) we show, by comparison with data of the crystal lattice orientations, that localization of GBs purely from KPFM topography data allows reliable localization of GBs. (ii) We consider the averaging effect of KPFM due to long-range electrostatic forces for the analysis of the electronic GB properties to determine the real potential barrier height for each individual GB; we determine potential variations ranging from −400 to +400mV. (iii) We consider the different physical origin of positive and negative potential barriers and present a quantitative analysis of the results to determine charge carrier concentration and defect densities at GBs. From our data and analysis we do not observe any systematic variation of these quantities with the Ga content.

Grain boundary diffusion: non-random topology and effective medium approximation

Driven by the success of grain boundary (GB) engineering, it is desirable to establish a relationship between the properties of polycrystalline materials and their GB network structures. Here we particularly explore GB diffusion using a two-dimensional GB network model to establish a connection between the effective diffusivity of GB networks and the network topology for random and two crystallographically consistent networks. For all the networks, the effective diffusivity against GB characters exhibits two distinct behaviours, that is, composite and percolation, based on the diffusivity contrast of individual GB properties. Generalised effective medium (GEM) equation, which combines the effective medium theory and the percolation theory, can be applied to predict the behaviour of crystallographically consistent networks when incorporating the shift of the effective diffusivity percolation threshold. The shift of the percolation threshold resulting from local constraints can be connected to the population of triple junctions. Notably, the population of J2 plays a key role in determining the shift by changing the connectivity of high-diffusivity boundaries.

Energy of slip transmission and nucleation at grain boundaries

Grain boundaries (GBs) provide a strengthening mechanism in engineering materials by impeding dislocation motion. In a polycrystalline material, there is a wide distribution of GB types with characteristic slip transmission and nucleation behaviors. Slip–GB reactions are not easy to establish analytically or from experiments; furthermore, there is a strong need to quantify the energy barriers of the individual GBs. We introduce a methodology to calculate the energy barriers during slip–GB interaction, in concurrence with the generalized stacking fault energy curve for slip in a perfect face-centered cubic material. By doing so, the energy barriers are obtained at various classifications of GBs for dislocation transmission through the GB and dislocation nucleation from the GB. The character and structure of the GB plays an important role in impeding slip within the material. The coherent twin (Σ3) boundary provides the highest barrier to slip transmission. From this analysis, we show that there is a strong correlation between the energy barrier and interfacial boundary energy. GBs with lower static interfacial energy offer a stronger barrier against slip transmission and nucleation at the GB. The results of our simulations are in agreement with experimental observations as demonstrated for Σ3, Σ13 and Σ19 boundaries. The Σ3 GB represents a higher-energy barrier compared to the Σ13 and Σ19 GBs, where, for the latter case, complex stacking fault structures are present in experiments and simulations.

Large Discrete Resistance Jump at Grain Boundary in Copper Nanowire

Copper is the current interconnect metal of choice in integrated circuits. As interconnect dimensions decrease, the resistivity of copper increases dramatically because of electron scattering from surfaces, impurities, and grain boundaries (GBs) and threatens to stymie continued device scaling. Lacking direct measurements of individual scattering sources, understanding of the relative importance of these scattering mechanisms has largely relied on semiempirical modeling. Here we present the first ever attempt to measure and calculate individual GB resistances in copper nanowires with a one-to-one correspondence to the GB structure. Large resistance jumps are directly measured at the random GBs with a value far greater than at coincidence GBs and first-principles calculations. The high resistivity of the random GB appears to be intrinsic, arising from the scaling of electron mean free path with the size of the lattice relaxation region. The striking impact of random GB scattering adds vital information for understanding nanoscale conductors.

New method to characterise grain boundary plane orientations based on EBSD orientation microscopy for serial sectioned surfaces

A new 'elementary facet' method has been developed to measure grain boundary (GB) plane orientations using three-dimensional electron backscattered diffraction on serial sectioned surfaces in a sample. The method defines different types of elementary facets in an orthogonal coordinate system in the sample. These elementary facets are used to pave GBs. For a flat GB, its projected areas on the orthogonal coordinate planes in the sample are obtained from the numbers of different facets paved in the GB. From the projected areas of the GB plane in the three coordinate planes, the GB plane normal is derived. For uneven GBs, a convolution method has been developed to obtain the GB plane orientations at different GB locations. In essence, this new method measures the GB orientations at every elementary facet that is used to pave the GBs in the sample. Together with the measured crystal orientations of different grains, the GB five parameters at all facets can be obtained readily. The distribution of GB five parameters for the whole GB population in a sample can be derived from the collection of properties of individual GB facets in the sample.

Enhanced current flow through meandering grain boundaries in YBa2Cu3O7−δ films

In YBa2Cu3O7−δ (YBCO) coated conductors grown by metal organic deposition, in-plane meandering of grain boundaries (GBs) has been linked to higher critical current density. The authors investigate this link in individual GBs using transport measurements and scanning Hall probe microscopy with current reconstruction. They observe current-induced flux entry into a coated conductor, then model its behavior by imaging YBCO films with single, straight GBs tilted at various angles to the applied current. They find a strong dependence of critical current on angle, sufficient to explain the enhancement observed for meandering GBs.

Correlation Between Grain‐Boundary Segregation and Grain‐Boundary Plane Orientation in Nb‐Doped TiO2

Grain‐boundary (GB) segregation has been measured in Nb‐doped TiO2 (rutile) by Auger electron spectroscopy analyses of the two surfaces exposed by inter‐granular fracture. In addition, the orientations of the exposed GB planes on both sides of the fracture surface have been determined. A strong anisotropy of GB composition is found. Measurements on 166 individual GB fracture surfaces show that boundary planes with low and high levels of segregation tend to be clustered along the (110)–(010) and (001)–(011) regions of the stereographic triangle, respectively. The matching halves of GB planes with either high or low segregation also tend to be terminated by planes in the same respective regions of orientation space. High and low levels of segregation occur in regions of GB plane orientation space estimated to correspond to high and low GB energy, respectively. This trend is consistent with qualitative arguments based on Gibbsian adsorption.

Mathematical modelling of impurity diffusion in grain boundary neighborhood in a dislocation wall force field (modified fisher model)

Abstract In this work a computer simulation of impurity diffusion in the vicinity of individual bicrystalline grain boundary has been carried out with the use of a modified Fisher model in which the grain's boundary influence (in the form of internal mechanical stresses) onto impurity atoms is taken into account. In computations the monotone difference schemes of the second order of accuracy on irregular grids were used. For such schemes under intrinsic restrictions on the grid steps the stability in the uniform norm has been proved.

Determination of the Grain Boundary Misorientation for the Analysis of the Discontinuous Ordering Reaction in Fe-50 at. % Co

The discontinuous ordering (DO) reaction represents the mechanism of atomic ordering in alloys at such low temperatures, at which the mobility of atoms in the bulk of the alloy can be neglected and the ordering process occurs only in the regions swept by migrating grain boundaries (GBs). In order to study the relationship between the grain boundary crystallography and the kinetics of the DO reaction in the Fe-50 at.% Co alloy, the orientations of all the grains in a 8x4 mm 2 sample have been determined in the scanning electron microscope by an automated analysis of the backscattered Kikuchi patterns, and the crystallographic orientation image of the sample has been constructed. With the help of this image, completed by a light microscopy map of the sample, all GBs have been numbered sequentially and the maximal width of the ordered phase has been determined in the light microscope (LM) for each individual GB. Afterwards, the misorientation parameters for all GBs investigated in the LM have been determined (totally 226 GBs). Such a procedure allowed us to establish a relationship between the rate of the DO reaction at individual GBs and their misorientation parameters. The data are presented as the dependencies of the GB migration rate on the misorientation angles around the axis close to the low-index , and axes. It is shown that the GBs close to the special Σ3 , Σ19 , Σ9 , Σ11 , Σ5 , Σ13 and Σ17 misorientations (here Σ is the inverse density ofthe coincidence sites) exhibit a low DO rate. This is interpreted in terms of a decreased diffusivity in the special GBs. At the same time, GBs with a misorientation angle slightly below the value of 46.6° for the special Σ19 boundary exhibit the highest DO rate. A model is suggested which establishes a relationship between DO rate, diffusivity and mobility of the GB.

Measurement of Grain Boundary Parameters by Laser-Spot Photoconductivity

ABSTRACTAn experimental technique has been developed to study the electrical properties of semiconductor grain boundaries (GBs) by a focused laser beam. The laser beam is trained on a GB while the photoconductivity of the sample is measured. Both the steady-state and transient signals are recorded as functions of temperature. From these data, we obtain well-defined GB parameters, including the barrier height, interface charge density, trap energy and thermal capture cross-section. This technique allows us to examine localized regions of individual GBs in a semiconductor with multiple grains.