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Register a new userStep bunching-induced vertical lattice mismatch and crystallographic tilt in vicinal BiFeO3(001) films
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Epitaxial (001) BiFeO3 thin films grown on vicinal SrTiO3 substrates are under large anisotropic stress from the substrates. The variations of the crystallographic tilt angle and the c lattice constant, caused by the lattice mismatch, along the film thickness were analyzed quantitatively using the X-ray diffraction technique. By generalizing the Nagai model, we estimated how step bunching resulted in the vertical lattice mismatch between adjacent BiFeO3 layers, which induced the strain relaxation and crystallographic tilt. The step bunching was confirmed by the increased terrace width on the BiFeO3 surface.
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The structural and ferroelectric domain variants of highly-strained BiFeO3 films grown on vicinal LaSrAlO4 substrates were studied by piezoelectric force microscopy and high-resolution X-ray reciprocal space mapping. Through symmetry breaking of the substrate surface, ferroelastic domain variants in the highly-strained MC phase BiFeO3 can be greatly reduced. Single-domain film can be obtained on substrates with large miscut angle, which is accompanied by the reduction of structural variants in the mixed-phase nanodomains. These findings lead to better understanding of the phase evolution and polarization rotation process in the strain-driven morphotropic phase system.
For more than three decades, measurement of terrace width distributions (TWDs) of vicinal crystal surfaces have been recognized as arguably the best way to determine the dimensionless strength $tilde{A}$ of the elastic repulsion between steps. For sufficiently strong repulsions, the TWD is expected to be Gaussian, with $tilde{A}$ varying inversely with the squared variance. However, there has been a controversy over the proportionality constant. From another perspective the TWD can be described as a continuous generalized Wigner distribution (CGWD) essentially no more complicated than a Gaussian but a much better approximation at the few calibration points where exact solutions exist. This paper combines concisely the experimentally most useful results from several earlier papers on this subject and describes some advancements that are in progress regarding numerical tests and in using Schrodinger-equation formalism to give greater understanding of the origin of the CGWD and to give hope of extensions to more general interaction potentials between steps. There are many implications for future experiments.
Increasing demand and high prices of advanced catalysts motivate a constant search for novel active materials with reduced content of noble metals. The development of thin films and core-shell catalysts seem to be a promising strategy along this path. Using Density Functional Theory we have analyzed a number of surface properties of supported bimetallic thin films with composition A3B (where A = Pt, Pd, B = Cu, Ag, Au). We focus on surface segregation, dissolution stability and surface electronic structure. We also address the chemisorption properties of Pd3Au thin films supported by different substrates, by probing the surface reactivity with CO. We find a strong influence of the support in the case of mono- and bilayers, while the surface strain seems to be the predominant factor in determining the surface properties of supported trilayers and thicker films. In particular, we show that the studied properties of the supported trilayers can be predicted from the lattice mismatch between the overlayer and the support. Namely, if the strain dependence of the corresponding quantities for pure strained surfaces is known, the properties of strained supported trilayers can be reliably estimated. The obtained results can be used in the design of novel catalysts and predictions of the surface properties of supported ultrathin catalyst layers.
We study current-induced step bunching and wandering instabilities with subsequent pattern formations on vicinal surfaces. A novel two-region diffusion model is developed, where we assume that there are different diffusion rates on terraces and in a small region around a step, generally arising from local differences in surface reconstruction. We determine the steady state solutions for a uniform train of straight steps, from which step bunching and in-phase wandering instabilities are deduced. The physically suggestive parameters of the two-region model are then mapped to the effective parameters in the usual sharp step models. Interestingly, a negative kinetic coefficient results when the diffusion in the step region is faster than on terraces. A consistent physical picture of current-induced instabilities on Si(111) is suggested based on the results of linear stability analysis. In this picture the step wandering instability is driven by step edge diffusion and is not of the Mullins-Sekerka type. Step bunching and wandering patterns at longer times are determined numerically by solving a set of coupled equations relating the velocity of a step to local properties of the step and its neighbors. We use a geometric representation of the step to derive a nonlinear evolution equation describing step wandering, which can explain experimental results where the peaks of the wandering steps align with the direction of the driving field.
Ferroelectric BaTiO3 films with large polarization have been integrated with Si(001) by pulsed laser deposition. High quality c-oriented epitaxial films are obtained in a substrate temperature range of about 300 deg C wide. The deposition temperature critically affects the growth kinetics and thermodynamics balance, resulting on a high impact in the strain of the BaTiO3 polar axis, which can exceed 2% in films thicker than 100 nm. The ferroelectric polarization scales with the strain and therefore deposition temperature can be used as an efficient tool to tailor ferroelectric polarization. The developed strategy overcomes the main limitations of the conventional strain engineering methodologies based on substrate selection: it can be applied to films on specific substrates including Si(001) and perovskites, and it is not restricted to ultrathin films.
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