Electronic Structure Of Grain Boundaries Research Articles

Atomic and electronic structure of grain boundaries in a-Al2O3: A combination of machine learning, first-principles calculation and electron microscopy

To accurately determine the atomic and electronic structures of symmetric tilt grain boundaries (GBs) in α-Al2O3, this work employed an artificial-neural-network (ANN) interatomic potential, density-functional-theory (DFT) calculation and scanning transmission electron microscopy (STEM) observation. An ANN-based simulated annealing method was demonstrated to efficiently screen candidate low-energy structures with reasonably high accuracy. For Σ7 and Σ31GBs with the [0001] tilt axis, which were absent in the training datasets for the ANN potential, their lowest-energy structures predicted from ANN and DFT calculations were in quantitative agreement with STEM images in terms of both Al- and O-column positions. The exact GB structures have enabled us to analyze quantitatively the relationship between their atomic and electronic structure. This work will be an important model case where a combination of machine-learning, theoretical calculation and experiment has successfully solved the problem of determining complicated GB structures and their electronic structures in α-Al2O3.

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.

Engineering of atomic-scale flexoelectricity at grain boundaries

Flexoelectricity is a type of ubiquitous and prominent electromechanical coupling, pertaining to the electrical polarization response to mechanical strain gradients that is not restricted by the symmetry of materials. However, large elastic deformation is usually difficult to achieve in most solids, and the strain gradient at minuscule is challenging to control. Here, we exploit the exotic structural inhomogeneity of grain boundary to achieve a huge strain gradient (~1.2 nm−1) within 3–4-unit cells, and thus obtain atomic-scale flexoelectric polarization of up to ~38 μC cm−2 at a 24° LaAlO3 grain boundary. Accompanied by the generation of the nanoscale flexoelectricity, the electronic structures of grain boundaries also become different. Hence, the flexoelectric effect at grain boundaries is essential to understand the electrical activities of oxide ceramics. We further demonstrate that for different materials, altering the misorientation angles of grain boundaries enables tunable strain gradients at the atomic scale. The engineering of grain boundaries thus provides a general and feasible pathway to achieve tunable flexoelectricity.

Unveiling the Electronic Structure of Grain Boundaries in Anatase with Electron Microscopy and First-Principles Modeling.

Polycrystalline anatase titanium dioxide has drawn great interest, because of its potential applications in high-efficiency photovoltaics and photocatalysts. There has been speculation on the electronic properties of grain boundaries but little direct evidence, because grain boundaries in anatase are challenging to probe experimentally and to model. We present a combined experimental and theoretical study of anatase grain boundaries that have been fabricated by epitaxial growth on a bicrystalline substrate, allowing accurate atomic-scale models to be determined. The electronic structure in the vicinity of stoichiometric grain boundaries is relatively benign to device performance but segregation of oxygen vacancies introduces barriers to electron transport, because of the development of a space charge region. An intrinsically oxygen-deficient boundary exhibits charge trapping consistent with electron energy loss spectroscopy measurements. We discuss strategies for the synthesis of polycrystalline anatase in order to minimize the formation of such deleterious grain boundaries.

The Atomic and Electronic Structure of 0° and 60° Grain Boundaries in MoS2

We have investigated atomic and electronic structure of grain boundaries in monolayer MoS2, where relative angles between two different grains are 0 and 60 degree. The grain boundaries with specific relative angle have been formed with chemical vapor deposition growth on graphite and hexagonal boron nitride flakes; van der Waals interlayer interaction between MoS2 and the flakes restricts the relative angle. Through scanning tunneling microscopy and spectroscopy measurements, we have found that the perfectly stitched structure between two different grains of MoS2 was realized in the case of the 0 degree grain boundary. We also found that even with the perfectly stitched structure, valence band maximum and conduction band minimum shows significant blue shift, which probably arise from lattice strain at the boundary.

Charging effect at grain boundaries of MoS2

Grain boundaries (GBs) are inherent extended defects in chemical vapor deposited (CVD) transition metal dichalcogenide (TMD) films. Characterization of the atomic structure and electronic properties of these GBs is crucial for understanding and controlling the properties of TMDs via defect engineering. Here, we report the atomic and electronic structure of GBs in CVD grown MoS2 on epitaxial graphene/SiC(0001). Using scanning tunneling microscopy/spectroscopy, we find that GBs mostly consist of arrays of dislocation cores, where the presence of mid-gap states shifts both conduction and valence band edges by up to 1 eV. Our findings demonstrate the first charging effect near GBs in CVD grown MoS2, providing insights into the significant impact GBs can have on materials properties.

Al grain boundary segregations in doped intermetallic NiAl and their effect on brittleness at room temperature

The nature of intermetallic NiAl brittleness is investigated by Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and electron energy-loss fine structure (EELFS) means. It is found that the reason for this phenomenon is the ordering of intermetallic NiAl, accompanied by the formation of aluminum structural segregations at grain boundaries. Doping with 0.1 wt % La transforms the atomic and electronic structures of grain boundaries, eliminating NiAl intergranular fracturing at room temperature. Doping is accompanied by Fermi level shifts and an increase in conduction electron density neff. These factors are responsible for the lower covalence of interatomic bonds in pure intermetallic NiAl. The effect A1 segregations have on the critical deformation of the generating grain boundary fracture in NiAl is discussed.

Stability and electronic structure of the low-Σ grain boundaries in CdTe: a density functional study

Using first-principles density functional calculations, we investigate the relative stability and electronic structure of the grain boundaries (GBs) in zinc-blende CdTe. Among the low-Σ-value symmetric tilt Σ3 (111), Σ3 (112), Σ5 (120), and Σ5 (130) GBs, we show that the Σ3 (111) GB is always the most stable due to the absence of dangling bonds and wrong bonds. The Σ5 (120) GBs, however, are shown to be more stable than the Σ3 (112) GBs, even though the former has a higher Σ value, and the latter is often used as a model system to study GB effects in zinc-blende semiconductors. Moreover, we find that although containing wrong bonds, the Σ5 (120) GBs are electrically benign due to the short wrong bond lengths, and thus are not as harmful as the Σ3 (112) GBs also having wrong bonds but with longer bond lengths.

Atomic and electronic structure of grain boundaries in crystalline organic semiconductors

Grain boundaries in organic crystalline semiconductors play an important role in charge carrier transport. We have developed a model for atomic and electronic structure calculations of grain boundaries in naphthalene polycrystals. Atomic structure was obtained by the Monte Carlo algorithm, while electronic structure was obtained by the charge patching method. The results for two naphthalene polycrystals are presented and discussed. They show the existence of trap states produced by grain boundaries.

Comparative study of the luminescence and intrinsic point defects in the kesterite Cu2ZnSnS4and chalcopyrite Cu(In,Ga)Se2thin films used in photovoltaic applications

The kesterite Cu${}_{2}$ZnSnS${}_{4}$ (CZTS) is attracting considerable interest because first-principles calculations predict that its electronic properties must be similar to their associated chalcopyrite Cu(In,Ga)Se${}_{2}$ (CIGS) compounds [Chen et al., Phys. Rev. B 81, 245204 (2010)]. Here, the authors report on first experimental evidence of the close resemblance in the luminescence of Cu-poor kesterites and Cu-poor chalcopyrites used in photovoltaic applications. Microluminescence measurements suggest that even the very distinct electronic structure of grain boundaries in CIGS is present, to some extent, in CZTS. The similarities between CIGS and CZTS become more pronounced as the efficiency of the CZTS solar cells gradually increases. The implications of these results for the future development of CZTS solar cells are discussed.

Large Neutral Barrier at Grain Boundaries in Chalcopyrite Thin Films

The electronic structure of grain boundaries in polycrystalline Cu(In,Ga)Se2 thin films and their role on solar cell device efficiency is currently under intense investigation. A neutral barrier of about 0.5 eV has been suggested as the reason for the benign behavior of grain boundaries in chalcopyrites. Previous experimental investigations have in fact shown a neutral barrier but only a few 10 meV high, which cannot be expected to have a significant influence on the solar cell efficiency. Here we show that a full investigation of the electrical behavior of charged and neutral grain boundaries shows the existence of an additional narrow neutral barrier, several 100 meV high, which is tunneled through by the majority carriers but is sufficiently high to explain the benign behavior of the grain boundaries.

Electronic structure of grain boundaries in the Fe86-Mn12.7-C1.3alloy

The importance of studying Fe-Mn-C alloys is related to their wide use as constructional materials in mechanical engineering. In this work an effort has been made to elucidate the physical origin of the magnetization in austenitic steels by calculating the electronic structure of grain boundaries. To theoretically investigate the magnetic properties of a crystal of ferromagnetic bcc iron, the wave functions of the iron atom calculated in view of the spin polarization of a core by the Hartree-Fock method with local exchange-correlation potentials have been used as base functions. This has made it possible to optimize the choice of a zero approximation for the description of the electronic states of ferromagnetic iron and to attain good agreement with the experimental values of the magnetic moment (μ theor = 2.23μ B , μ exp = 2.218μ B ), of the exchange splitting of the crystal term P 4 (Δ theor = Δ exp = 0.112 Ry), and of the cross-sections of the Fermi surface. A similar approach has been used to investigate the magnetic states of clusters (nanoclusters) based on the method of scattered waves. The approach developed for clusters of the alloy under investigation makes it possible to calculate the alloy magnetic properties in relation to the cluster size for a varied lattice parameter.

A Neutral Barrier at CGS Grain Boundaries - Compositional and Structural Dependencies

AbstractUnlike other photoactive layers (e.g. Si) polycrystalline chalcopyrite layers achieve higher efficiency than monocrystalline ones. Although grain boundaries may play an important role for this phenomenon, there is currently no commonly accepted model for the electronic structure of grain boundaries. First experimental results on CuGaSe2 absorbers indicated a small neutral barrier for majority carriers (20-40meV) at sigma 3 grain boundaries, which is the predominant grain boundary in polycrystalline chalcopyrite absorbers [1]. These results are in discrepancy with theory, which predicts a 10 times higher barrier; we suspect that the copper excess might reduce the barrier height.Here we present a study using Hall-effect and Kelvin Probe Force Microscopy (KPFM) measurements to investigate the composition dependence of the barrier height of epitaxially grown CuGaSe2 layers containing a sigma 3 grain boundary as a function of the Cu/Ga ratio. First results show that the barrier height is independent of the copper content. In addition, we present initial results on sigma 9 grain boundaries which are observed to occur in high efficiency absorbers.

The electronic structure of grain boundaries in metals and alloys

We study the electronic structure of Ni with Σ5 (2 1 0) and TiNi with Σ5 (3 1 0) tilt grain boundaries using the full-potential linearized augmented plane wave (FLAPW) method. The calculated electron energy loss spectra, as well as X-ray absorption and emission spectra in nickel and its alloy are analyzed in dependence on composition and structure.

Theoretical Study of the Atomic and Electronic Structure of Grain Boundaries in SiC

Theoretical Study of the Atomic and Electronic Structure of Grain Boundaries in SiC

Atomic and electronic structures of doped grain boundaries in SrTiO3

The atomic and electronic structures of pristine, Mn- and Nb-doped grain boundaries in SrTiO3 are investigated by atomistic simulations and cluster calculations. The atomic structures of (310) symmetric tilt grain boundaries in SrTiO3 are determined by atomistic simulation using empirical potentials. The defect energies of Mn(Nb)-doped models are calculated and discussed in relation to the concentration profiles of Mn(Nb) in SrTiO3 grain boundaries. The local electronic structures near Mn(Nb)-doped grain boundaries in SrTiO3 are determined using embedded cluster calculations based on the density functional theory. The charge density of each system is calculated to elucidate the electronic structure of the grain boundary. The calculation results agree well with previous experimental observations of the atomic structures and grain boundary charges near the Mn(Nb)-doped grain boundary in SrTiO3.

Atomic and Electronic Structure Analysis of Σ=3, 9 and 27 Boundary, and Multiple Junction in β-SiC

AbstractThe atomic structures of σ=3, 9 and 27 boundaries, and multiple junctions in β-SiC were studied by high-resolution electron microscopy (HREM). Especially, the existence of the variety of structures of σ=3 incoherent twin boundaries and σ=27 boundary was shown by HREM. The structures of σ=3, 9 and 27 boundary were explained by structural unit models. Electron energy-loss spectroscopy (EELS) was used to investigate the electronic structure of grain boundaries. The spectra recorded from bulk, {111}σ=3 coherent twin boundary (CTB) and {1211}σ=3 incoherent twin boundary (ITB) did not show significant differences. Especially, the energy-loss corresponding to carbon 1s-to Φ* transition was not found. It indicates that C atoms exist at grain boundary on the similar condition of bulk

Effect of boron and sulphur on the electronic structure of grain boundaries in Ni

The first-principles discrete variational method is employed to study the effect of boron and sulphur on the electronic structure of the Ni Σ11(1 1 3 ̄ )/[1 1 0] grain boundary (GB). The calculated results show that boron does not strongly influence (only slightly decreases) the bonding between the atoms of the metal. In addition, B forms the strong bonding state with its neighbouring metal atoms. Our study also indicates that S strongly decreases the bonding between the atoms of the metal, and that the bonding tendency between S and the atoms of the metal across the GB plane is very weak. The calculations of environment-sensitive embedding energies show B has the strong site-competition ability and can successfully drive out S from the GB region. We conclude that the influence of impurities segregating on the GBs is closely associated with their effects on: (i) the decrease of the bonding between the atoms of the metal due to the presence of impurities; (ii) the bonding between the impurity atom and the atoms of the metal; and (iii) the site-competition ability of impurity atoms.

Theoretical studies of grain boundaries in covalent materials

Atomic and electronic structures of grain boundaries in covalent materials such as semiconductors or covalent ceramics have been investigated using various theoretical methods. Significant advances have been made in the understanding of grain boundaries in elemental semiconductors. Currently, ab initio calculations for grain boundaries are possible by virtue of the development of the first-principles molecular dynamics method. A first ab initio approach to grain boundaries in SiC by using the models of the {122} Σ = 9 boundary is presented. It has been shown that interfacial C-C and Si-Si wrong bonds have significant effects on grain boundaries in SiC. The C-C and Si-Si bonds have bond lengths and bond charges similar to those in bulk diamond and Si, and the C-C bonds generate peculiar localized states at the valence-band edges.

Effect of impurities on the electronic structure of grain boundaries and intergranular cohesion in iron and tungsten

The semi-empirical and first-principles calculations of the energetics and the electronic structures of impurities on a ∑3 (111) grain boundary (GB) in Fe and W have revealed important features of the impurity effects. In both Fe and W, impurities such as N, O, P, S and Si, weaken the intergranular cohesion resulting in ‘loosening’ of the GB. The presence of B and C on the contrary, enhances the interatomic interaction across the GB. Boron plays a dual role in both Fe and W; not only does its presence at GBs enhance the intergranular cohesion, but it also accomplishes ‘site competition cleansing’ by displacing the other impurity atoms off the GB. Micro alloying with 10–50 ppm B may be an effective way of improving the ductility of both Fe-base alloys and W. Another important result is understanding the decohesion effect of H in Fe. Contrary to general belief, H does not contribute its electron to the Fe d-bands at all. Instead, the electron stays very strongly localized close to its proton. There is also virtually no hybridization with the Fe electrons and virtually no effect on Fe atoms magnetization. As a result the interatomic bonding along the Fe-atom chains via H atoms is very inefficient, thus resulting in significant weakening of the intergranular cohesion. These results are important for understanding the fundamental physics of intergranular embrittlement.