E. Vasco

Bio: E. Vasco is an academic researcher from Spanish National Research Council. The author has contributed to research in topics: Surface diffusion & Stress relaxation. The author has an hindex of 9, co-authored 29 publications receiving 277 citations.

Growth dynamics and strain relaxation mechanisms in BaTiO 3 pulsed laser deposited on SrRuO 3 ∕ SrTiO 3

Abstract: J. Q. He,1,3,* E. Vasco,2 R. Dittmann,1 and R. H. Wang3 1Institut für Festkörperforschung, Forschungzentrum Jülich GmbH, D-52425 Jülich, Germany 2Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, 28049 Cantoblanco, Madrid, Spain 3Physics Department and Center for Electron Microscopy, Wuhan University, Wuhan 430072, China Received 11 November 2005; published 20 March 2006

Postcoalescence evolution of growth stress in polycrystalline films.

Abstract: The growth stress generated once grains coalesce in Volmer-Weber-type thin films is investigated by time-multiscale simulations comprising complementary modules of (i) finite-element modeling to address the interactions between grains happening at atomic vibration time scales ($\ensuremath{\sim}0.1\text{ }\text{ }\mathrm{ps}$), (ii) dynamic scaling to account for the surface stress relaxation via morphology changes at surface diffusion time scales ($\ensuremath{\sim}\ensuremath{\mu}\mathrm{s}$--ms), and (iii) the mesoscopic rate equation approach to simulate the bulk stress relaxation at deposition time scales ($\ensuremath{\sim}\mathrm{sec} $--h). On the basis of addressing the main experimental evidence reported so far on the topic dealt with, the simulation results provide key findings concerning the interplay between anisotropic grain interactions at complementary space scales, deposition conditions (such as flux and mobility), and mechanisms of stress accommodation-relaxation, which underlies the origin, nature and spatial distribution, and the flux dependence of the postcoalescence growth stress.

Diffusion and nucleation of yttrium atoms on Si(111)7×7: A growth model

Abstract: The atomic diffusion of Y atoms on the Si(111)7×7 surface at room temperature is studied by using a combination of scanning tunneling microscopy and kinetic Monte Carlo simulations. The experiment provides the occupancy statistics of faulted and unfaulted half-cells by Y atoms. A model taking into account the attractive interactions among adsorbates which provides the best quantitative agreement with the experimental data by kinetic Monte Carlo simulations is introduced. For low Y coverages, single Y adatoms as well as clusters can be identified inside the 7×7 reconstruction half-cells in the scanning tunneling microscopy images. Single Y adatoms are highly mobile inside the halves, resulting in a characteristic fuzzy appearance of the half-cell. The Y adatoms as well as the clusters present strong electronic effects, which gives us information about their interaction with the substrate Si atoms.

Morphology evolution of thermally annealed polycrystalline thin films

Abstract: Investigation of the morphology evolution of annealed polycrystalline Au(111) films by atomic force microscopy and x-ray diffraction leads to a continuous model that correlates such an evolution to local interactions between grains triggering different mechanisms of stress accommodation (grain zipping and shear strain) and relaxation (gap filling and grain rotation). The model takes into consideration findings concerning the in-plane reorientation of the grains during the coalescence to provide a comprehensive picture of the grain-size dependence of the interactions (underlying the origin of the growth stress in polycrystalline systems); and in particular it sheds light on the postcoalescence compressive stress as a consequence of the kinetic limitations for the reorientation of larger surface structures.

Intrinsic Compressive Stress in Polycrystalline Films is Localized at Edges of the Grain Boundaries.

Abstract: The intrinsic compression that arises in polycrystalline thin films under high atomic mobility conditions has been attributed to the insertion or trapping of adatoms inside grain boundaries. This compression is a consequence of the stress field resulting from imperfections in the solid and causes the thermomechanical fatigue that is estimated to be responsible for 90% of mechanical failures in current devices. We directly measure the local distribution of residual intrinsic stress in polycrystalline thin films on nanometer scales, using a pioneering method based on atomic force microscopy. Our results demonstrate that, at odds with expectations, compression is not generated inside grain boundaries but at the edges of gaps where the boundaries intercept the surface. We describe a model wherein this compressive stress is caused by Mullins-type surface diffusion towards the boundaries, generating a kinetic surface profile different from the mechanical equilibrium profile by the Laplace-Young equation. Where the curvatures of both profiles differ, an intrinsic stress is generated in the form of Laplace pressure. The Srolovitz-type surface diffusion that results from the stress counters the Mullins-type diffusion and stabilizes the kinetic surface profile, giving rise to a steady compression regime. The proposed mechanism of competition between surface diffusions would explain the flux and time dependency of compressive stress in polycrystalline thin films.

Recent advances in pulsed-laser deposition of complex oxides

Abstract: Pulsed-laser deposition (PLD) is one of the most promising techniques for the formation of complex-oxide heterostructures, superlattices, and well controlled interfaces. The first part of this paper presents a review of several useful modifications of the process, including methods inspired by combinatorial approaches. We then discuss detailed growth kinetics results, which illustrate that 'true' layer-by-layer (LBL) growth can only be approached, not fully met, even though many characterization techniques reveal interfaces with unexpected sharpness. Time-resolved surface x-ray diffraction measurements show that crystallization and the majority of interlayer mass transport occur on timescales that are comparable to those of the plume/substrate interaction, providing direct experimental evidence that a growth regime exists in which non-thermal processes dominate PLD. This understanding shows how kinetic growth manipulation can bring PLD closer to ideal LBL than any other growth method available today.

On the origin of increased phonon scattering in nanostructured PbTe based thermoelectric materials.

Abstract: We have investigated the possible mechanisms of phonon scattering by nanostructures and defects in PbTe-X (X = 2% Sb, Bi, or Pb) thermoelectric materials systems. We find that among these three compositions, PbTe-2% Sb has the lowest lattice thermal conductivity and exhibits a larger strain and notably more misfit dislocations at the precipitate/PbTe interfaces than the other two compositions. In the PbTe-Bi 2% sample, we infer some weaker phonon scattering BiTe precipitates, in addition to the abundant Bi nanostructures. In the PbTe-Pb 2% sample, we also find that pure Pb nanoparticles exhibit stronger phonon scattering than nanostructures with Te vacancies. Within the accepted error range, the theoretical calculations of the lattice thermal conductivity in the three systems are in close agreement with the experimental measurements, highlighting the important role of misfit dislocations, nanoscale particles, and associated interfacial elastic strain play in phonon scattering. We further propose that such particle-induced local elastic perturbations interfere with the phonon propagation pathway, thereby contributing to further reduction in lattice thermal conductivity, and consequently can enhance the overall thermoelectric figure of merit.

Epitaxial CdSe-Au Nanocrystal Heterostructures by Thermal Annealing

Abstract: The thermal evolution of a collection of heterogeneous CdSe−Au nanosystems (Au-decorated CdSe nanorods, networks, vertical assemblies) prepared by wet-chemical approaches was monitored in situ in the transmission electron microscope. In contrast to interfaces that are formed during kinetically controlled wet chemical synthesis, heating under vacuum conditions results in distinct and well-defined CdSe/Au interfaces, located at the CdSe polar surfaces. The high quality of these interfaces should make the heterostructures more suitable for use in nanoscale electronic devices.