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Modelling the structure and properties of supported ultra thin films.Many material properties are governed by the presence of interfaces. For example, epitaxial growth of a material onto a (lattice mismatched) substrate surface will result in structural modifications such as dislocations, grain-boundaries, cracks and fractures, which may influence, for example, the mechanical, catalytic and electronic properties of the material. Moreover, whereas certain structural modifications may enhance the desired material properties, others, such as dislocations within supported superconductors, may prove deleterious. The structural characterisation of interfaces and the relationship of structure to properties are central to materials science and engineering.
Supported positive misfit oxide nano clusters.A simulated amorphisation and recrystallisation strategy has been used to generate atomistic models of CaO and SrO nanoparticles, approximately 13x13x3 nm, supported on an MgO(001) substrate. The (single crystal) CaO nanoparticle exhibits a cubic ‘slab’ morphology in contrast to the SrO nanoparticle, which has a convex-lens or ‘droplet’ morphological appearance with a height of 3.5nm and a diameter of 14-15nm. In addition, the SrO nanoparticle comprises about ten interconnecting misoriented crystallites. The epitaxial relationships that exist between the nanoparticle and substrate are identified and correlated with the lattice misfit together with the interfacial structures and complex dislocation networks that evolve within the nanoparticles. The atomistic structure of various screw-edge dislocation cores (in CaO), grain-boundaries and grain-junctions (in SrO) that form within the nanoparticles are presented graphically. In addition, the structures of the nanoparticles are compared with previous simulations performed on the analogous CaO and SrO thin-films supported on MgO(001).related references:
Supported oxide nano clusters.The atomistic structures of SrO (15×15×2.2nm), CaO (14×14×2nm) and MgO(12×12×2nm), nanoparticles, supported on BaO(001) and synthesised using a simulated amorphisation and recrystallisation strategy, are presented. The SrO and CaO exhibit cubic ‘slab’ morphologies in contrast to the MgO nanoparticle, which comprises various misaligned interconnecting crystallites. The lattice misfit was found to have a profound influence on the structure of the nanoparticles. The SrO nanoparticle (-7% misfit) was found to lie coherent with respect to the substrate across the entire area covered by the SrO. Conversely, only small regions of the CaO were found to be coherent with the BaO substrate (-15% misfit), with screw-edge dislocations located at regions where the ions became misaligned. The MgO nanoparticle (-31% misfit) exhibited no regions of coherence with respect to the underlying BaO substrate. Defects (vacancies and substitutionals) and defect clusters including voids were also identified for each system and act to help reduce locally the lattice misfit thereby enhancing the stability. Specifically, the results indicate that as the lattice misfit associated with the system increases, so the interfacial layer of the substrate becomes more defective. Arguments, based on the results of the study, are presented, which suggest that the area of the nanoparticle in contact with the substrate is linked with the critical thickness to dislocation evolution for a particular system. That the limitations of periodic boundary conditions can be eliminated when simulating nanoparticles compared with thin films, which cover completely the substrate material, is discussed.related references:
MgO/SrTiO3(001) and BaO/SrTiO3(001) thin film interfaces. Large-scale atomistic simulations of oxide interfaces have been performed where the thin film is forced to undergo a controlled amorphous transition prior to recrystallising. The MO/SrTiO3 (001) (M=Ba or Mg) systems explored using this simulation technique demonstrate markedly different structural characteristics owing to the difference in lattice parameter associated with each system. In particular regions of SrTiO3 (001) (TiO2 terminated) supported MgO undergo a transformation into a pseudo-hexagonal type structure, reducing the lattice misfit from +7.4% to ca. -0.5 to -4%, whereas the +34% lattice misfit associated with the BaO/SrTiO3 system is reduced to -0.3% via a 45o rotation of the BaO with respect to the underlying SrTiO3. Moreover, the plane exposed by the SrTiO3 (001) support (either SrO or TiO2) exacts a critical influence on the structure of the thin film. In particular, BaO, when supported on a TiO2 terminated SrTiO3 , comprises a high concentration of dislocations. Conversely, for the analogous SrO terminated system no dislocations are present within the thin film.related references:
Ceria / Zirconia interfaces.
A ‘simulated amorphisation and recrystallisation’
technique is employed to explore the structural modifications
and epitaxial relationships, which evolve within a CeO2
thin-film when supported on zirconia and yttrium stabilised zirconia
(YSZ). The epitaxial relationship identified within the CeO2/YSZ(111)
system facilitates a reduction in the lattice misfit from ca. 6.1%
(bulk misfit) to ca. 1% thereby, in part, stabilising the system. Moreover,
edge dislocation networks, defects, including vacancies, substitutions
and interstitials evolve in both the CeO2 and underlying YSZ to reduce
further this residual misfit. Graphical techniques are employed to
characterise and present, the epitaxial relationships and atomistic structure
of the dislocation networks and defects that evolve within the system.
Since experimentation is, at present, unable to explore the structural
features that evolve within supported thin films with three-dimensional
atomistic resolution, simulation provides an invaluable complement
with which to study supported catalysts.
related references:
Rock salt /rock salt oxide thin film interface.
The SrO/MgO(001) interface was constructed
by placing four layers of SrO onto an MgO(001) substrate
using a simulation cell size of 28 × 28 (3455 Å2
). The SrO was constrained to occupy `on-top' positions thereby accommodating
the entire +20% lattice misfit associated with this system. Dynamical
simulation was then applied to the system for 150 ps at 2100 K
followed by 90 ps at 1500 K, after which the system was energy minimised
at 0 K and the resulting structure analysed.
The strain within the SrO associated with this initial configuration is considerable. Consequently, during the dynamical simulation, the Sr and O species move to alleviate the strain, generating an amorphous SrO thin film. Upon prolonged dynamical simulation, regions of the SrO start to recrystallise. The picture depicts the structure of the system after 25 ps and shows a SrO region, within the amorphous SrO, that is starting to recrystallise into a cubic structure. During prolonged dynamical simulation, the SrO thin film was observed to recrystallise completely, revealing a rocksalt structure. For many simulations the starting configuration is likely to influence the final structure. Moreover, there is a danger, particularly within incommensurate interface systems, of generating structures, which may be artefacts of the initial configuration. Our approach of forcing the thin film through an amorphous phase eliminates any possible influence or ` memory' of the (strained) starting configuration. Inspection of the final energy minimised structure reveals the presence of four dislocations within the SrO thin film (see figure).. The Burgers vector for this dislocation lies at ca. 45° to the line of the dislocation indicating that the dislocation has both screw and edge character. The combined screw-edge character arises in order to maintain charge continuity within this material of both positive and negative species: For a `pure' screw dislocation, the core would have to spiral over 2 layers to maintain charge connectivity and for a `pure' edge dislocation, two MgO planes would have to be inserted. Accordingly, the screw component allows for the insertion of a single edge dislocation. A stable interface is associated with unstrained thin film and support lattices in conjunction with optimised interfacial interactions. Clearly, for incommensurate systems such a configuration is not possible and a compromise must be sought. The average cation–anion bond distances, for both the MgO and SrO, are equivalent to their `bulk' values suggesting that there is little strain within the lattice. Inspection of the atom positions of the SrO with respect to the MgO support at the interfacial plane reveals that the Sr and O no longer accommodate positions directly on top of the underlying MgO, rather the SrO exists within particular `domains', which lie either almost commensurate with the underlying MgO support or rotated at low angles. Dislocations are present at the domain boundaries within the SrO thin film facilitating the rotations. The rotated domains are effectively quenching lattice strain and enhancing the interactions across the interfacial plane. related references:
Email: watsong AT tcd.ie Last updated: Apr 14 2012 Back to Top |
