Cell-by-cell mapping of carrier concentrations in high-temperature superconductors

The correlation between atomic and electronic structure in the vicinity of grain boundaries in YBa₂Cu₃O_7- is investigated on the scale of the coherence length, using a combination of Z-contrast imaging and electron energy loss spectroscopy in the scanning transmission electron microscope. The detector arrangement of the microscope enables both techniques to be performed simultaneously, allowing the exact crystallographic location of the probe to be determined from the image. As the pre-edge feature of the oxygen K-absorption edge is very sensitive to the partial density of unoccupied states, it can be calibrated to give an accurate measure of local hole concentrations (±5%). Spatial resolution is limited only by the 2.2 Å probe size, thus allowing the electronic properties of the material to be inferred on a scale less than the coherence length. Results are discussed from the study of YBa₂Cu₃O_7- films prepared by laser ablation on yttria-stabilized-zirconia (YSZ) substrates, where a symmetric (near 5) grain boundary shows no significant hole depletion in the boundary region, while an asymmetric (near 17) boundary shows depletion extending over a region far greater than the atomic disorder shown in the image. Application of this technique to the study of interfaces in other superconducting systems is discussed.     

Atomic Structure and Properties of the (310) Symmetrical Tilt Grain Boundary (STGB) in SrTiO3 Part II: Comparison with Experimental Studies

The atomistic simulation results presented in Part I for SrTiO₃ (310) symmetrical tilt grain boundary (STGB, the so-called = 5 GB with 36.8° symmetrical misorientation about [001]) are analyzed in the context of available experimental studies. In particular, atomic imaging studies of SrTiO₃ GBs via high resolution TEM and incoherent Z-contrast STEM imaging; and determination of oxygen positions by combining electron energy loss spectroscopy (EELS) and bond-valence-sum rules, are compared with simulation results. The atomistic simulation data on the GB energies are compared with relative experimental estimates obtained via a novel approach of faceting of focused ion beam (FIB) induced microvoids.While there are considerable differences in details of simulation and experimental results, some basic trends seem to emerge about the core structural framework of GBs in SrTiO₃. The paper highlights the limitations of both, experimental and simulation techniques, and argues in favor of synergistic use of diverse experimental and simulation approach to determine the atomic structure and properties of GBs.     

The Electronic Structure of Pristine and Doped (100) Tilt Grain Boundaries in SrTiO3

To understand the electronic properties of doped grain boundaries, we reviewed the atomic scale techniques currently available to study the electronic structure at pristine SrTiO₃ grain boundaries. The knowledge gained from the pristine boundaries is used to interpret experimental and theoretical results from a Mn doped 5 SrTiO₃ grain boundaries. Mn atoms are shown to preferentially substitute at specific Ti sites at the grain boundary core. Furthermore, the formal oxidation state of the Mn atoms at the grain boundary core was found to be reduced compared to the Mn atoms substituting for Ti in the bulk. This change of valence did not, however, significantly affect the atomic structure of the grain boundary, as determined by Z-contrast imaging and electron energy-loss spectroscopy, which revealed similar fine-structure features at both the doped and pristine grain boundary. We conclude, therefore, that composition and atomic structure have different effects on the local electronic structure and should be treated separately in any segregation and electrical conductivity models for grain boundaries.     

[001] Twist boundaries in Cu3Au for T ∈ 0 K

Four different [001] twist boundaries in Cu₃Au spanning a wide range of the rotation angle are investigated for temperatures below the bulk order-disorder transition temperature. All the boundaries show almost identical behavior for the plane averaged stoichiometry and the order parameter at a given temperature. Within the accuracy of the calculations no segregation of either Cu or Au is observed to the boundary. At the boundary and below the transition temperature there is a layer which is more disordered than the rest of the crystal, and its width increases as the bulk order-disorder transition temperature is approached. This indicates wetting phenomena, which are in agreement with the experimental and theoretical results on other boundaries.     

Solute-atom segregation at symmetric twist and tilt boundaries in binary metallic alloys on an atomic-scale

Monte Carlo and overlapping distributions Monte Carlo (ODMC) techniques are employed to simulate grain boundary (GB) segregation in a number of single-phase binary metallic alloys—the Au-Pt, Cu-Ni, Ni-Pd, and Ni-Pt systems. For a series of symmetric [001] twist and [001] tilt boundaries, with coincident site lattice (CSL) structures, we demonstrate that the Gibbsian interfacial excess of solute is a systematic function of the misorientation angle. We also explore in detail whether the GB solid solution behavior is ideal or nonideal by comparing the results of Monte Carlo and ODMC simulations. The range of binding free energies of specific atomic sites at GBs for solute atoms is also studied. The simulational results obtained demonstrate that the thermodynamic and statistical thermodynamic models commonly used to explain GB segregation are too simple to account for the microscopic segregation patterns observed, and that it is extremely difficult. If not impossible, to extract the observed microscopic information employing macroscopic models.     

Atomic Structure of [0001] Tilt Boundaries in GaN

Crystals of GaN grown by molecular-beam epitaxi on (0001) sapphire show [0001] tilt boundaries of low and high angles (V. Potin et al., Physical Review B, 61, 5587 (2000)). The structure of these boundaries plays a role in the optoelectrical properties shown by this semiconductor. In this work we have studied the atomic structure of 7, 13 and 19 boundaries (in the coincidence side lattice notation) by means of atomic computer simulation using a modified version of the empirical Stillinger-Weber interatomic potential. For each boundary we studied the interface of lowest period and found the atomic structure of minimum energy by applying the method of quasidynamic relaxation. Special attention has been paid to the 13 where we have studied two different interfaces comparing our results with the experimental results obtained in a study of bicrystals of ZnO (A.N. Kiselev et al., Phil. Mag. A, 76, 633 (1997)). In these boundaries the atomic structure can be understood in terms of edge dislocations located along the boundary. The a edge dislocation in GaN has been studied with the same potential and the stable structures are presented here for comparison.     

Static and Dynamic Electron Holography of Electrically Active Grain Boundaries in SrTiO3

The dynamics of electrostatic potential barriers at grain boundaries (GBs) in Nb-doped SrTiO₃ bicrystals is investigated using a unique combination of bulk and in-situ TEM electrical measurements across isolated GBs, coupled with electron holography under in-situ applied bias. The Nb bulk-doped bicrystals exhibit a positive GB potential that suppresses reversibly under applied bias greater than the nonlinearity threshold in the current-voltage curve. This suppression is interpreted as break-down of the potential barrier to current transport.The results on Nb bulk-doped bicrystals have been compared to those in which Mn has been added as a grain boundary specific dopant. This acceptor doping of the grain boundary causes an appreciable increase in the grain boundary resistance and extension of the nonlinear regime. A preliminary account of static electron holography shows a relatively flat potential profile across the GB, indicating probable compensation of donor states at the GB core with Mn-acceptors. Interestingly, the phase profile under applied bias in this case exhibits a reversible dip at the GB which is interpreted as an activation of GB trap states due to Mn-acceptor dopants trapping extra electrons (the majority charge carriers) at the GB core, inducing a negative GB potential, and diminishing current transport until the threshold bias is exceeded.The synergistic combination of nanoscale TEM measurements coupled with traditional macroscopic electrical measurements is emphasized.     

Structure,energy and solute segregation behaviour of [110] symmetric tilt grain boundaries in yttria-stabilized cubic zirconia

Yttria-stabilized cubic zirconia bicrystals with [110] symmetric tilt grain boundaries are systematically fabricated by the diffusion bonding method. It is revealed that the grain-boundary atomistic structures, excess energies and solute segregation behaviours are strongly dependent on the macroscopic geometries of the boundaries. High-resolution transmission electron microscopy combined with lattice statics calculations suggests that the grain-boundary structures are characterized by the accumulation of coordination-deficient cation sites at their cores, whose densities have a clear correlation with excess energies and amounts of solute segregation. The orientation dependence of grain-boundary properties in cubic zirconia can thus be linked and understood via local grain-boundary atomistic structures with the characteristic miscoordinated cation sites.     

Atomic Structure and Properties of the (310) Symmetrical Tilt Grain Boundary (STGB) in SrTiO3. Part I: Atomistic Simulations

The relaxed structure and energy of the (310) symmetrical tilt grain boundary (STGB) in SrTiO₃ have been calculated using static lattice energy minimization methods. In principle, the (310) GB plane can either be a cation-rich, positively charged SrTiO plane or a negatively charged oxygen plane, and both scenarios have been considered in this report. The effect of point-defect reconstruction at the GB core region, manifested either as completely missing columns or as half-filled columns of ions as suggested by experiments, has been analyzed. The results indicate that while Schottky defects are very strongly preferred energetically at the GB core, there is not significant gain in energy by having half-filled columns, as opposed to fully-dense and fully-empty columns, at the GB core. The simulation results have been analyzed in the context of Pauling's rules of crystal chemistry and bicrystallography. The results form the basis for an objective comparison with experimental studies in Part II of the paper.     

A new type of periodie boundary condition useful for high-temperature atomistic simulations of grain boundaries: Applications in semiconductors

A new type of boundary condition, named Möbius or antiperiodic boundary conditions, is proposed and tested, both analytically and within the context of numerical simulations. It is shown that these boundary conditions are very useful for twist grain boundary atomistic simulations. By contrast to the use of the ordinary Born von Kármán periodic boundary conditions, they allow only one grain boundary per box instead of two. The risk of migration and overinteraction of two grain boundaries at high temperature is thus avoided while more complex grain boundaries can also be tackled at the same computer price. Such examples are presented and discussed.     

纯铜[0 1 1]倾侧型非共格∑3晶界结构稳定性分子动力学模拟研究

有研究表明, 非共格∑₃晶界的行为在中低层错能面心立方金属晶界 特征分布演化中发挥着重要作用. 为了掌握不同界面匹配的非共格∑₃晶界的特性, 本文利用分子动力学(MD)模拟方法首先研究了纯铜的[0 1 1]倾侧型 非共格∑₃晶界在700–1100 K温度范围内和常压下的结构稳定性. MD模拟采用原子间相互作用长程经验多体势, 步长为5×10^-15 s. 模拟结果表明: 所研究的五个非共格∑₃晶界, 其结构稳定性存在很大差异, 其一般规律是, 与(1 1 1)/(1 1 1)共格孪晶界之间的夹角(Φ角)越小, 晶界匹配值越大, 则非共格∑₃晶界越稳定; 反之亦然. Φ角最小的 (2 5 5)/(2 1 1)非共格∑₃晶界较稳定, 在退火过程中几乎不发生变化. 随着Φ角的增大, 非共格∑₃晶界不再稳定, 这类晶界会通过Miller指数较高一侧晶体每三层原子面合并为一层原子面 (或Miller指数较低一侧晶体每一层原子面分解为三层原子面)的机理 转变为亚稳的“台阶”状晶界, 台阶面部分地处于精确的能量极低 的{111}/{111}共格孪晶界上; 当提高温度退火时, 这种“台阶”状晶界最终会全部转变成稳定平直的{111}/{111}共格孪晶界. 关键词： 纯铜 ∑₃晶界')" href="#">非共格∑₃晶界 分子动力学模拟     

Microstructure and transport properties of various 90° grain boundaries in YBa2Cu3O7 thin films

The microstructure and transport properties of various 90° grain boundaries in (103) oriented YBa₂Cu₃O₇(YBCO) thin films grown epitaxially in situ by 90° off-axis sputtering are compared. The (103) films grown on (101) LaAlO₃ and (101) SrTiO₃ substrates have specific sets of 90° grain boundaries in both principal in-plane directions: 90° [010] twist boundaries along the [101] direction, and 90° [010] symmetrical tilt boundaries and 90° [010] basal-plane-faced tilt boundaries along the (301) direction. No weak-link behavior is observed across some of these boundaries by transport critical current density and normalized magnetic field dependence of J _c measurements along both those in-plane directions. High-resolution transmission electron microscopy reveals variations in the structure and microfaceting of the 90° boundaries, which may contribute to the absence of weak-link behavior. These results have important implications for understanding the behavior of step-edge Josephson junctions.     

Segregation of alloying atoms at a tilt symmetric grain boundary in tungsten and their strengthening and embrittling effects

We investigate the segregation behavior of alloying atoms （Sr, Th, In, Cd, Ag, Sc, Au, Zn, Cu, Mn, Cr, and Ti） near Z3 （ 111 ） [1]-0] tilt symmetric grain boundary （GB） in tungsten and their effects on the intergranular embrittlement by performing first-principles calculations. The calculated segregation energies suggest that Ag, Au, Cd, In, Sc, Sr, Th, and Ti prefer to occupy the site in the mirror plane of the GB, while Cu, Cr, Mn, and Zn intend to locate at the first layer nearby the GB core. The calculated strengthening energies predict Sr, Th, In, Cd, Ag, Sc, Au, Ti, and Zn act as embrittlers while Cu, Cr, and Mn act as cohesion enhancers. The correlation of the alloying atom＇s metal radius with strengthening energy is strong enough to predict the strengthening and embrittling behavior of alloying atoms; that is, the alloying atom with larger metal radius than W acts as an embrittler and the one with smaller metal radius acts as a cohesion enhancer.     

Effect of addition of La2O3 in the grain boundaries of a material based on BaTiO3

Results of an impedance study of capacitors of a material based on BaTiO₃ with variable concentrations of La are presented. It is observed that variation in tanδ is related to variation in covalency of the added La at the grain boundaries. The tanδ is temperature dependent. The discussion brings out that added La controls the electron density at the grain boundaries.     

Identification of grain boundaries as degradation site in n‐channel organic field‐effect transistors determined via conductive atomic force microscopy

One important figure of merit for the commercial usability of organic transistors (OFETs) is their electrical stability. With the aim of identifying the microscopic location of degradation sites within a transistor channel, we have investigated the bias stress stability of OFETs by electrical measurements as well as by conductive atomic force microscopy. Air‐stable n‐channel FETs based on a N,N′‐bis(2‐ethylhexyl)‐1,7(1,6)‐dicyano‐perylene[3,4:9,10]bis (dicarboximide) were fabricated to understand the relation between the thin‐film morphology, the substrate temperature during the vacuum de position with the aim to fabricate transistors with a mobility not dominated by interface traps. The devices showed a maximum carrier mobility of (0.12 ± 0.01) cm²/V s and an on/off ratio up to 10⁷. The electrical performance as well as the bias stress behavior of the semiconductor thin‐films is significantly influenced by grain boundaries. For example, the grain boundary resistance was found to increase upon electrical stress by more than 150% (from 2 ± 0.2 GΩ to 5 ± 1.5 GΩ), while the resistance within the grains remains unchanged. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

3D reconstruction of grains in polycrystalline materials using a tessellation model with curved grain boundaries

A compact and tractable representation of the grain structure of a material is an extremely valuable tool when carrying out an empirical analysis of the material’s microstructure. Tessellations have proven to be very good choices for such representations. Most widely used tessellation models have convex cells with planar boundaries. Recently, however, a new tessellation model — called the generalised balanced power diagram (GBPD) — has been developed that is very flexible and can incorporate features such as curved boundaries and non-convexity of cells. In order to use a GBPD to describe the grain structure observed in empirical image data, the parameters of the model must be chosen appropriately. This typically involves solving a difficult optimisation problem. In this paper, we describe a method for fitting GBPDs to tomographic image data. This method uses simulated annealing to solve a suitably chosen optimisation problem. We then apply this method to both artificial data and experimental 3D electron backscatter diffraction (3D EBSD) data obtained in order to study the properties of fine-grained materials with superplastic behaviour. The 3D EBSD data required new alignment and segmentation procedures, which we also briefly describe. Our numerical experiments demonstrate the effectiveness of the simulated annealing approach (compared to heuristic fitting methods) and show that GBPDs are able to describe the structures of polycrystalline materials very well.