In scientifically intriguing and technologically important multifunctional ABO
perovskite oxides, oxygen vacancies are most common defects. They cause lattice expansion and can alter the key ...functional properties. Here, it is demonstrated that contrary to weak isotropic expansion in bulk samples, oxygen vacancies produce strong anisotropic strain in epitaxial thin films. This anisotropic chemical strain is explained by preferential orientation of elastic dipoles of the vacancies. Elastic interaction of the dipoles with substrate-imposed misfit strain is suggested to define the dipolar orientation. Such elastic behavior of oxygen vacancies is anticipated to be general for perovskite films and have critical impacts on the film synthesis and response functions.
Single-crystal epitaxial films of technologically important and scientifically intriguing multifunctional ABO
3
perovskite-type metal oxides are essential for advanced applications and understanding ...of these materials. In such films, a film-substrate misfit strain enables unprecedented crystal phases and unique properties that are not available in their bulk counterparts. However, the prerequisite growth of strained epitaxial films is fundamentally restricted by misfit relaxation. Here we demonstrate that introduction of a small oxygen deficiency concurrently stabilizes epitaxy and increases lattice strain in thin films of archetypal perovskite oxide SrTiO
3
. By combining experimental and theoretical methods, we found that lattice distortions around oxygen vacancies lead to anisotropic local stresses, which interact with the misfit strain in epitaxial films. Consequently, specific crystallographic alignments of the stresses are energetically favorable and can facilitate epitaxial growth of strained films. Because anisotropic oxygen-vacancy stresses are inherent to perovskite-type and many other oxides, we anticipate that the disclosed phenomenon of epitaxial stabilization by oxygen vacancies is relevant for a very broad range of functional oxides.
Anisotropic elastic dipoles of oxygen vacancies interact with substrate-induced misfit strain in epitaxial oxide films. This interaction leads to specific spatial alignment of the dipoles that facilitates coherent growth.
Control of lattice strain in epitaxial films of ABO
3
perovskite oxides is crucial for modern understanding and applications of these scientifically and technologically important materials. Here, we ...show that oxygen vacancies have unique impacts on lattice strain in such films. We suggest that in the presence of substrate-imposed misfit in epitaxial or highly oriented films, the crystallographic alignment of anisotropic elastic dipole tensors of oxygen vacancies is energetically favorable. The dipolar alignment leads to an enhanced above-misfit magnitude of maximal lattice strain and to increased inhomogeneous strain or strain gradients. The vacancy-induced remarkably strong elastic effects are experimentally validated by varying the misfit strain and oxygen content in thin films of perovskite niobate (ANbO
3
) and titanate (ATiO
3
) ferroelectrics. It is anticipated that elastic effects of oxygen vacancies are relevant for controlling strain in epitaxial films of a broad range of functional oxides.
Misfit-induced crystallographic alignment of elastic dipoles of oxygen vacancies enhances anisotropic lattice strain beyond the misfit magnitude and raises inhomogeneous strain in epitaxial perovskite oxide films.
Misfit strains arising from a film–substrate mismatch can induce novel phases and properties in the epitaxial films of perovskite oxides. Here we employ yet another effect, namely, strain-assisted ...formation of oxygen vacancies. We demonstrate the misfit-promoted presence of oxygen vacancies and related substitutional incorporation of anion dopants in the epitaxial films of archetypal perovskite oxide SrTiO 3 . Both the oxygen vacancies and hydrogen or nitrogen dopants are introduced in situ during the pulsed-laser deposition of the films using compressive substrates. The films exhibit peculiar chemical expansion and optical properties, which are consistent with substitutional anion doping.
Diamond-like carbon (DLC) is a biocompatible material that has many potential biomedical applications, including in orthopaedics. DLC layers doped with Cr at atomic percent (at.%) of 0, 0.9, 1.8, ...7.3, and 7.7 at.% were evaluated with reference to their osteoinductivity with human bone marrow mesenchymal stromal cells (hMSCs), immune activation potential with RAW 264.7 macrophage-like cells, and their effect on apoptosis in Saos-2 human osteoblast-like cells and neonatal human dermal fibroblasts (NHDFs). At mRNA level, hMSCs on DLC doped with 0.9 and 7.7 at.% of Cr reached higher maximum values of both RUNX2 and alkaline phosphatase. An earlier onset of mRNA production of type I collagen and osteocalcin was also observed on these samples; they also supported the production of both type I collagen and osteocalcin. RAW 264.7 macrophages were screened using a RayBio™ Human Cytokine Array for cytokine production. 10 cytokines were at a concentration more than 2 × as high as the concentration of a positive control, but the values for the DLC samples were only moderately higher than the values on glass. NHDF cells, but not Saos-2 cells, had a higher expression of pro-apoptotic markers Bax and Bim and a lower expression of anti-apoptotic factor BCL-XL in proportion to the Cr content. Increased apoptosis was also proven by annexin V staining. These results show that a Cr-doped DLC layer with a lower Cr content can act as an osteoinductive material with relatively low immunogenicity, but that a higher Cr content can induce cell apoptosis.
•UNCD and DLC films were modified by UV/O3 treatments, O2 or NH3-containing plasmas.•Surface composition, wettability and surface energy change upon modifications.•Higher efficiency of UNCD ...modifications was observed.•Cell attachment and growth were influenced by the surface termination and roughness.
Diamond and diamond-like carbon (DLC) films possess a set of excellent physical and chemical properties which together with a high biocompatibility make them attractive candidates for a number of medical and biotechnological applications. In the current work thin ultrananocrystalline diamond (UNCD) and DLC films were comparatively investigated with respect to cell attachment and proliferation after different surface modifications. The UNCD films were prepared by microwave plasma enhanced chemical vapor deposition, the DLC films by pulsed laser deposition (PLD). The films were comprehensively characterized with respect to their basic properties, e.g. crystallinity, morphology, chemical bonding nature, etc. Afterwards the UNCD and DLC films were modified applying O2 or NH3/N2 plasmas and UV/O3 treatments to alter their surface termination. The surface composition of as-grown and modified samples was studied by X-ray photoelectron spectroscopy (XPS). Furthermore the films were characterized by contact angle measurements with water, formamide, 1-decanol and diiodomethane; from the results obtained the surface energy with its dispersive and polar components was calculated. The adhesion and proliferation of MG63 osteosarcoma cells on the different UNCD and DLC samples were assessed by measurement of the cell attachment efficiency and MTT assays. The determined cell densities were compared and correlated with the surface properties of as-deposited and modified UNCD and DLC films.
The complex index of refraction in the spectral range of 0.74 to 4.5 eV is studied by variable-angle spectroscopic ellipsometry in ferroelectric K0.5Na0.5NbO3 films. The 20-nm-thick cube-on-cube-type ...epitaxial films are grown on SrTiO3(001) and DyScO3(011) single-crystal substrates. The films are transparent and exhibit a significant difference between refractive indices Δn = 0.5 at photon energies below 3 eV. The energies of optical transitions are in the range of 3.15-4.30 eV and differ by 0.2-0.3 eV in these films. The observed behavior is discussed in terms of lattice strain and strain-induced ferroelectric polarization in epitaxial perovskite oxide films.
In perovskite oxide ferroelectrics, gradients of lattice strain are known to induce nanoscale topological structures, leading to novel or enhanced functionality. Here, we experimentally detect and ...theoretically analyze thickness distribution of structural properties in epitaxial Pb_{0.5}Sr_{0.5}TiO_{3} films grown on (001) SrTiO_{3} substrates. We show that the relaxation of substrate-imposed stress produces a strain gradient, which leads to the formation of distinct ferroelectric phases as a function of distance from the film-substrate interface within the same film. Charge carriers trapped at phase boundaries stabilize the induced phases and manifest themselves under electric field. Crosstalk between the phases, where polarization may rotate in one phase and invert in the other one, opens perspectives for advanced ferroelectric thin film devices.
Very large lattice strain and strain-induced polarization are achieved in KNbO3 using epitaxial growth of a thin KNbO3 film onto a (001)-oriented SrTiO3 single-crystal substrate. We demonstrate ...experimentally that epitaxy produces dramatic changes of interband transitions in the film compared to those of a reference KNbO3 crystal: the energies of transitions change, some transitions are substantially suppressed and new ones appear in the film. A comparison of the experimental observations with theoretical calculations points to yet unexplored phenomena. Our results indicate that optical refraction and electro-optical coefficients of ferroelectric films can be controlled by epitaxial growth, which is of importance for emerging photonic and optoelectronic applications.
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