Layers In Unit Cell Research Articles

Charge control of antiferromagnetism at PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3 interface

The interfacial spin state of the multiferroic heterostructure PbZr0.52Ti0.48O3/La0.67Sr0.33MnO3 and its dependence on ferroelectric polarization is investigated with magnetic second-harmonic generation at 78 K. The spin alignment of Mn ions in the first unit cell layer at the heterointerface can be tuned from ferromagnetic to antiferromagnetic exchange coupled, while the bulk magnetization remains unchanged. Multiple domains of both phases coexist as the ferroelectric polarization is switched. The results will help promote the development of new interface-based functionalities and device concepts.

Triclinic deformation of InGaN layers grown on vicinal surface of GaN (00.1) substrates

We report on a triclinic unit cell deformation of fully strained InGaN layers grown on vicinal GaN (00.1) substrates. The samples were examined using the high-resolution X-ray diffraction (HR XRD) using a set of asymmetrical reflections and one symmetrical reflection of 00.2. The substrate miscut induced triclinic deformation of the layer unit cells, breaking the hexagonal symmetry. The experimental results are compared with predictions of the theory of elasticity. We formulate equations for unit cell parameters of layers grown on substrates cut in any direction, based on the equations given by Romanov et al. [J. Appl. Phys. 100, 023522 (2006)]. Additionally, the paper provides a recipe of the XRD measurements necessary to establish unit cell parameters (useful for composition determination of ternary compounds) of the hexagonal mismatched layers grown on off-axis substrates.

Layer-by-Layer Evolution of a Two-Dimensional Electron Gas Near an Oxide Interface

We report the momentum-resolved measurement of a two-dimensional electron gas at the LaTiO(3)/SrTiO(3) interface by angle-resolved photoemission spectroscopy (ARPES). Thanks to an advanced sample preparation technique, the orbital character of the conduction electrons and the electronic correlations can be accessed quantitatively as each unit cell layer is added. We find that all of these quantities change dramatically with distance from the interface. These findings open the way to analogous studies on other heterostructures, which are traditionally a forbidden field for ARPES.

Modulation of electron carrier density at the n-type LaAlO3/SrTiO3 interface by water adsorption

We investigate the energetic stability and dissociation dynamics of water adsorption at the LaAlO3 surface of the n-type LaAlO3/SrTiO3 (LAO/STO) interface and its effect on the electronic properties of the interface by carrying out first-principles electronic structure calculations. In an ambient atmosphere at room temperature the configuration of 1 monolayer (ML) of water molecules including 3/4 ML of dissociated water molecules adsorbed at the surface is found to be most stable, whereas the configuration of 1 ML of dissociated water molecules is metastable. Water molecule dissociation induces an up-shift of the valence band maximum (VBM) of the LAO surface, reducing the gap between the VBM of the LAO surface and the conduction band minimum of the STO. For the LAO/STO interface with three LAO unit-cell layers, once the coverage of dissociated water molecules reaches 1/2 ML the gap is closed, the interface becomes metallic and the carrier density at the LAO/STO interface increases with increasing coverage of dissociated water molecules. Our findings suggest two ways to control the conductivity at the LAO/STO interface: (I) an insulator–metal transition by adsorbing an amount of water at the bare surface; (II) a carrier density change by the transition between the most stable and the metastable adsorption configurations for 1 ML coverage in an ambient atmosphere at room temperature.

Direct observations of the influence of solution composition on magnesite dissolution

In situ observations during atomic force microscopy experiments and ex situ observations after static and flow-through experiments were used to explore the effect of three different electrolytes on magnesite (MgCO3) dissolution at pH 2. The experiments showed that the magnesite dissolution rate varied in the order NO3->Cl->SO42- when these anions were present in solution. Under the experimental conditions magnesite dissolution occurred via the removal of successive single surface layers, where changes in magnesite reactivity in the presence of different electrolytes could be observed as variations in the cycle length for the removal of one unit cell layer. The cycles began with the formation of sporadically distributed etch pits followed by the nucleation of homogeneously distributed etch pits. Coalescence of the etch pits formed isolated sections of the remnant surface, which then dissolved away. The timing of sporadic and homogeneous etch pit nucleation was constant despite the presence of different anions. However, the cycles in surface roughness and etch pit spreading rates indicate that the different anions affect step retreat rates and hence dissolution rates. Differences in magnesite reactivity can be attributed to the direct interaction of sulphate with the magnesite surface and the indirect effects of chloride and nitrate on the magnesite surface hydration and hydration of the Mg2+ ion in solution. In all experiments during the dissolution process evidence for the precipitation of a new phase was observed, either directly as precipitates forming on the magnesite surface in the AFM and after the experiments, seen in SEM analysis, or as changes in the Mg outlet concentration during flow-through experiments. EDX and Raman spectroscopy were used to analyse the composition of the precipitate and although it could not be definitively identified, considering previous observations the precipitate is most likely a hydrated Mg-carbonate phase with a MgCO3·xH2O composition. Thus, the formation of a precipitate can facilitate further magnesite dissolution by increasing the undersaturation of the interfacial solution.

Investigation of initial epitaxial growth of (Ba, Sr)TiO3 thin film on (0 0 1) SrTiO3 substrate using first-principles calculations

Initial epitaxial growth of (Ba, Sr)TiO3 (BST) thin film on (001) SrTiO3 (STO) substrate is investigated by first-principles calculations. Binding energy, density of state and bond populations are calculated for (AO) (A=Ba or Sr), (TiO) and (ATiO3) unit-cell layers deposition on (001) STO substrate based on layer-by-layer growth mode including SrO-terminated and TiO2-terminated surfaces. Three possible interfaces including Normal stacking interface, Ruddlesden–Popper-type interface and Magnéli-type interface are considered. It is found that BST thin film can epitaxially grow on (001) STO substrate with a two-layers growth mode. This work is crucial in theoretical understanding the epitaxial growth mechanism and preparing epitaxial thin films for perovskite ferroelectrics.

Structural investigation of interface and defects in epitaxial Bi3.25La0.75Ti3O12 film on SrRuO3/SrTiO3 (111) and (100)

The structure of La-doped bismuth titanate (BLT), Bi3.25La0.75Ti3O12, is investigated with atomic resolution high-angle annular dark field (HAADF) scanning transmission electron microscopy. The images reveal evidence of the tilting of TiO6 octahedra within the perovskite-like layers of the BLT unit cell. The tendency of La ions to substitute Bi ions and occupy the top part of the (Bi2O2)2+ layer, previously observed from electron energy loss spectroscopy (EELS) mapping experiments, is explained based on the tolerance factors and stress relief mechanism. The atomic resolution HAADF images also reveal the presence of the out-of-phase boundaries (OPBs). The role of OPBs in BLT is discussed in terms of its fatigue resistance as the OPBs provide extra nucleation sites for ferroelectric domains during polarization reversals. Further, we show evidence that the first deposited atomic layer at the interface also governs the subsequent film growth, resulting in the modulation of the “defect-free” and the “defected” regions throughout the BLT film in the lateral direction, parallel to the film-substrate interface. As demonstrated from atomic-resolved elemental mapping with EELS, the generation of the defect-free and regions with defect in the BLT film are likely to be induced by the presence of surface steps and dislocations at the film-substrate interface. These effects are discussed in terms of the large remanent polarization and enhanced fatigue resistance in BLT.

The effect of MFI zeolite lamellar and related mesostructures on toluene disproportionation and alkylation

The effect of MFI zeolite nanosheet morphology and related mesostructures on the accessibility of acid sites evaluated from FTIR spectra of different adsorbed probe molecules and conversions and selectivities in toluene disproportionation and alkylation with isopropyl alcohol was investigated. Two-dimensional MFI lamellar zeolite (1.5 nm thickness of layers) and related mesoporous sponge morphologies, most probably with 10-ring channel system not yet structurally fully determined, were synthesized and characterized by XRD, SEM, TEM, and adsorption measurements. FTIR spectroscopy using pyridine, deuterated acetonitrile and 2,6-di-tert-butyl pyridine evidenced high concentrations of accessible Lewis acid sites at the expense of Bronsted acid sites. Ordered mesoporous zeolites exhibited slightly higher toluene conversion in its alkylation with isopropyl alcohol than lamellar MFI or conventional MFI due to easy access to active sites. The lamellar zeolite consisting of less than a unit cell layer was slightly more active than MFI but still exhibited high selectivity to p-cymene. It evidences that alkylation of toluene with isopropyl alcohol proceeds mainly in the 10-ring channel system of MFI zeolite, although the effect of large external surface cannot be neglected. Increased para-selectivity compared with thermodynamic value is achieved even in the case of extremely thin zeolite nanosheets.

Design and Performance of a Double-Layer Miniaturized-Element Frequency Selective Surface

In this letter, a novel bandpass frequency selective surface (FSS) is proposed. Unlike the traditional FSSs, our building block makes use of a periodic cell with much smaller size than the resonant wavelength. The proposed FSS is composed by two layers of metallic unit cells printed on the substrate. The coupling between parallel patches in different metallic layers can produce the significant distributed capacitance of the FSS; as a result, the size of the FSS unit cell can be decreased to λ/16. Our simulated result shows that the transmission characteristic of the miniaturized FSS is very stable since the relative change rate of resonant frequency is less than 2% with respect to different incident angles from 0 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> to 45 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> . The measured results show good agreements with the simulated ones. In addition, our result also implies that the structure has wide application space, based on analysis of three other variations on the periodic structure.

Structures of the 2:1 Layers of Pyrophyllite and Talc

AbstractTo determine the relationships between the symmetry of the overall pyrophyllite and talc structure and the symmetry of individual layers, the geometry and symmetry of each 2:1 layer of pyrophyllite and talc were analyzed. For each, the previously published, refined unit cell may be rotated clockwise by ~60° for comparison to a layer unit cell. In pyrophyllite, the layer unit cell is ideal and shown to be orthogonal with C2/m symmetry. The agreement between the refined atomic coordinates and those calculated for the layer with C2/m symmetry confirms that the symmetry of the pyrophyllite layer is C2/m. The obliquity of the pyrophyllite refined cell results from the layer stacking and the choice of unit cell, but the interlayer stacking sequence does not disturb the layer symmetry. In contrast, talc has an oblique layer cell, without a mirror plane. For the most part, the distortion of the talc 2:1 layer is probably caused by an elongation of unshared O-O lateral edges around M1 that creates a slight corrugation of the octahedral sheet surface. Perhaps of lesser importance, the distortion of the talc layer cell may result from Coulombic interactions between cations of adjacent layers, and these cation-to-cation distances are sufficiently large (~6–7.5 Å) that the weak van der Waals forces that stabilize the stacking are not overcome. Because pyrophyllite has a vacant octahedral site, similar interactions are not present, and this results in a more idealized layer symmetry.Phyllosilicates consisting of layers with an orthogonal cell and mirror plane (pyrophyllite, kaolinite, sudoite) were shown to have similar stacking faults. In these structures, the 2:1 or 1:1 layers have uniform orientation, and stacking faults occur owing to interstratifications of two alternative interlayer displacements in the same crystal that are related by a mirror plane in the projection on the (001) plane. In talc, stacking faults are associated with layer rotations by ±120°, whereas the lateral displacement between the adjacent tetrahedral sheets across the interlayer region is relatively ordered.

Prediction of Weak Topological Insulators in Layered Semiconductors

We report the discovery of weak topological insulators by abinitio calculations in a honeycomb lattice. We propose a structure with an odd number of layers in the primitive unit cell as a prerequisite for forming weak topological insulators. Here, the single-layered KHgSb is the most suitable candidate for its large bulk energy gap of 0.24eV. Its side surface hosts metallic surface states, forming two anisotropic Dirac cones. Although the stacking of even-layered structures leads to trivial insulators, the structures can host a quantum spin Hall layer with a large bulk gap, if an additional single layer exists as a stacking fault in the crystal. The reported honeycomb compounds can serve as prototypes to aid in the finding of new weak topological insulators in layered small-gap semiconductors.

On the accurate analysis of linear elastic meta-material properties for use in design optimization problems

In the field of meta-materials engineering, asymptotic homogenization has proven to be a valuable tool to estimate material properties for many different design needs. Homogenization is used in the design of an object to tie the global material properties to the local material microstructure of the meta-material. The governing equations of the theory are derived from a small-parameter expansion that assumes the scaling length of the unit cell, the smallest repeatable structure found in the material, is much smaller than the entire material length scale. However, for those design problems that do not meet this scaling requirement, the meta-material properties given by homogenization may be inaccurate, leading to error in the material design process.To demonstrate some of these limitations, the more general set of macroscopic analyses derived from the average stress and average strain theorems are applied to several designs in which the assumptions of homogenization theory are no longer valid, those chiefly being materials made of a single or a few layers of unit cells. Then, by applying both materials analyses to simple design problems, the necessity of using a meta-material design method to link global and local properties in a manner consistent with global design methods is demonstrated. In particular, the use of homogenization in designing single layer materials is shown to incur a very large error in the design process, whereas the averaging method is much more accurate.

TO CONTROL THE PROPAGATION CHARACTERISTIC OF ONE-DIMENSIONAL PLASMA PHOTONIC CRYSTAL USING EXPONENTIALLY GRADED DIELECTRIC MATERIAL

The efiect of exponentially graded material on the modal dispersion characteristics, group velocity and efiective group index as well as phase index of refraction of a binary One-Dimensional Plasma Photonic Crystals (1D-PPCs) has been studied. The dispersion relation is derived by solving Maxwell's equations and using the transfer matrix method. The anomalous dispersion characteristics are observed for difierent values of selection parameters. The introduction of graded dielectric layers in 1D-PPCs provides additional parameters for controlling the propagation characteristics of 1D-PPCs. Also, the band gap is shown to become larger with the increase of plasma frequency and plasma width. Hence the structure having plasma and exponentially graded dielectric layer in unit cell is more useful for controlling and tuning of the plasma functioning devices than the structure having plasma and homogeneous dielectric layer in one unit cell.

Electrical and structural properties of ABO3/SrTiO3 interfaces

ABSTRACTElectrical transport and microstructure of interfaces between nm-thick films of various perovskite oxides grown by pulsed laser deposition (PLD) on TiO2- terminated SrTiO3 (STO) substrates are compared. LaAlO3/STO and KTaO3/STO interfaces become quasi-2DEG after a critical film thickness of 4 unit cell layers. The conductivity survives long anneals in oxygen atmosphere. LaMnO3/STO interfaces remain insulating for all film thicknesses and NdGaO3/STO interfaces are conducting but the conductivity is eliminated after oxygen annealing. Medium-energy ion spectroscopy and scanning transmission electron microscopy detect cationic intermixing within several atomic layers from the interface in all studied interfaces. Our results indicate that the electrical reconstruction in the polar oxide interfaces is a complex combination of different mechanisms, and oxygen vacancies play an important role.

Epitaxial oxide growth on polar (111) surfaces

Obtaining atomically smooth surfaces and interfaces of perovskite oxide materials on polar (111) surfaces presents a particular challenge as these surfaces and interfaces will reconstruct. Here, the effect of the use of screening buffer layers on the epitaxial growth on such polar surfaces is investigated. Both transmission electron microscopy and in situ reflective high energy electron diffraction data imply that the buffer layers, SrRuO3 or LaAlO3, restore a near bulk-like termination at growth temperature, allowing for coherent growth of BiFeO3 and CaTiO3 for all deposited unit cell layers of the film material.

용융탄산염형 연료전지 스택의 균질 물성치 추정에 관한 연구

용융탄산염형 연료전지 시스템의 성능 향상을 위해 핵심 부분인 스택을 수치적으로 모사할 필요성이 대두되고 있다. 스택은 다양한 재질, 즉 금속, 세라믹, 폴리머 등으로 이루어져 있고 또한 단위전지 수백 장이 적층되어 제작되기 때문에 이를 전부 세세히 모델링하여 고려하는 것은 거의 불가능하다. 이에 따라 스택 전체를 하나의 균질한 물질로 대체할 수 있는 균질 물성치 도출이 요구되고 있다. 본 논문에서는 단위전지 압축률을 도입하고 모든 압축 변형이 분리판과 매트릭스에서만 일어난다는 등의 몇 가지 기본 가정 하에서 스택의 두 가지 영역, 즉 활성 영역과 매니폴드 영역에서 각각 균질 물성치를 추정하였다. 추정된 물성치를 실제 문제에 적용하여 성공적인 결과를 얻을 수 있었다. The performance and efficiency of a Molten Carbonate Fuel Cell system will improve with the aids of numerical simulations such as finite element analysis. For best simulation results, the virtual model must accurately reflect the actual model including the material properties. It is very difficult, however, to make a detailed numerical model of the stack that consists of hundreds of layers of unit cells composed of various materials like metal, ceramics, polymer, etc. Instead, a practical approach is to find a homogenized material property of the stack as a whole as an approximate replacement. In this paper, the compression ratio of a unit cell is introduced, and a new method is proposed to estimate the homogeneous material properties for both the active and the manifold regions of the stack under the assumption that the compressive deformation occurs only at the separators and matrices in the unit cells. The estimated properties are applied successfully to simulating an actual stack.

Short-range order and Li + ion diffusion mechanisms in Li5La9□ 2( TiO3) 16 (LLTO)

The Li-rich phases of superionic LLTO lithium conductor were modelled by a 2 a p × 2 a p × 4 a p perovskite supercell hosting one Li 5La 9⎕ 2(TiO 3) 16 formula unit. The disordered La/Li/⎕ distribution in the A-type perovskite cages was resolved into several ordered configurations, characterized by a short-range order parameter σ related to the number of nearest-neighbour La-vacant A sites. Quantum-mechanical ab initio calculations of the total energy were performed by the periodic code CRYSTAL09, on the basis of a B3LYP Hamiltonian and Gaussian-type localized basis sets. The structural configurations were least-energy optimized within the Pm monoclinic symmetry, finding a progressive stabilization for larger σ values, i.e. with clustering of La-vacancies and then with increasing connectivity of LaO 12 coordination polyhedra. By the frozen ion technique, one- and two-dimensional mechanisms of lithium diffusion within (001) layers of thickness a p were investigated. The lowest energy barrier of 0.31 eV, in good agreement with the experimental activation energy of 0.35 eV, was determined for the 1D pathways. The result holds both for one and for two mobile Li atoms per 2 a p × 2 a p layer unit-cell, according to a peculiar correlation between the motions of neighbouring ions.

Dirichlet-to-Neumann Map Method with Boundary Cells for Photonic Crystals Devices

AbstractIn a two-dimensional (2D) photonic crystal (PhC) composed of circular cylinders (dielectric rods or air holes) on a square or triangular lattice, various PhC devices can be created by removing or modifying some cylinders. Most existing numerical methods for PhC devices give rise to large sparse or smaller but dense linear systems, all of which are expensive to solve if the device is large. In a previous work [Z. Hu et al., Optics Express, 16 (2008), 17383-17399], an efficient Dirichlet-to-Neumann (DtN) map method was developed for general 2D PhC devices with an infinite background PhC to take full advantage of the underlying lattice structure. The DtN map of a unit cell is an operator that maps the wave field to its normal derivative on the cell boundary and it allows one to avoid computing the wave field in the interior of the unit cell. In this paper, we extend the DtN map method to PhC devices with a finite background PhC. Since there is no bandgap effect to confine the light in a finite PhC, a different technique for truncating the domain is needed. We enclose the finite structure with a layer of empty boundary and corner unit cells, and approximate the DtN maps of these cells based on expanding the scattered wave in outgoing plane waves. Our method gives rise to a relatively small and sparse linear systems that are particularly easy to solve.

In situ Atomic Force Microscopy study of dissolution of the barite (0 0 1) surface in water at 30 °C

The dissolution behavior of the barite (0 0 1) surface in pure water at 30 °C was investigated using in situ Atomic Force Microscopy (AFM), to better understand the dissolution mechanism and the microtopographical changes that occur during the dissolution, such as steps and etch pits. The dissolution of the barite (0 0 1) surface started with the slow retreat of steps, after which, about 60 min later, the < hk0> steps of one unit cell layer or multi-layers became two-step fronts (fast “f” and slow “s” steps) with a half-unit cell layer showing different retreat rates. The “f” step had a fast retreat rate (≈(14 ± 1) × 10 −2 nm/s) and tended to have a jagged step edge, whereas the “s” step (≈(1.8 ± 0.1) × 10 −2 nm/s) had a relatively straight front. The formation of the “f” steps led to the formation of a new one-layer step, where the front of the “s” step was overtaken by that of the immediate underlying “f” step. The “f” steps also led to the decrease of the < hk0> steps and the increase in the percentage of stable steps parallel to the [0 1 0] direction during the dissolution. Etch pits, which could be observed after about 90 min, were of three types: triangular etch pits with a depth of a half-unit cell, shallow etch pits, and deep etch pits. The triangular etch pits were bounded by the step edges parallel to [0 1 0], [1 2 0], and [ 1 2 ¯ 0 ] and had opposite orientations in the upper half and lower half layers. Shallow etch pits that had a depth of two or more half-unit cell layers had any two consecutive pits pointing in the opposite direction of each other. The triangular etch pit appeared to grow by simultaneously removal of a row of ions parallel to each direction from the three step edges. At first, deep etch pits were elongated in the [0 1 0] direction with a curved outline and then gradually developed to an angular form bounded by the {1 0 0}, {3 1 0}, and (0 0 1) faces. The retreat rate of the (0 0 1) face was much slower than those of the {1 0 0} and {3 1 0} and tended to separate into two rates ((0.13 ± 0.01) × 10 −2 nm/s for the deep etch pits derived from a screw dislocation and (0.07 ± 0.01) × 10 −2 nm/s for those from other line defects). The changes in the dissolution rate of a barite (0 0 1) surface during the dissolution were estimated using the retreat rates and densities of the various steps as well as the growth rates, density, and areas of the lateral faces of the deep etch pits that were obtained from this AFM analysis. Our results showed that the dissolution rate of the barite (0 0 1) surface gradually increased and approached the bulk dissolution rate because of the change in the main factor determining the dissolution rate from the density of the steps to the growth and the density of the deep etch pits on the surface.

Molecular dynamics simulation study on surface structure and surface energy of anatase

Molecular dynamics simulations were performed to investigate the relaxed structures and surface energies of perfect and pit anatase TiO2 surfaces. It is shown that the slab containing more than two unit-cell layers away from the fixed layer expresses the surface characteristics of perfect anatase TiO2 (1 0 1) and (1 0 0) surfaces well, while the slab containing more than one unit-cell layer away from the fixed layer expresses the surface characteristics of the (0 0 1) surface well. Their surface energies follow the sequence (0 0 1) < (1 0 1) < (1 0 0). Simulation results also indicate that the pit edges expose many undercoordinated atoms, and the more highly undercoordinated atoms exhibit the larger displacement vectors. Moreover, the surface energy of the pit surface is higher than that of the perfect surface. The surface energies of pit anatase (1 0 1) surfaces are linearly related to the pit sizes along the [] and [0 1 0] directions, and the changes in their surface energies are less than 0.05 J m−2, while the surface energies increase sharply with the increase in pit depth within 1 nm. Therefore, for anatase (1 0 1) surface, in order to obtain a higher surface energy, one may increase the pit sizes, particularly along the [1 0 1] direction.