We explore the evolution of epitaxial TiO2 films on a wide range of (001)-oriented perovskite substrates. We observe epitaxial stabilization of anatase to >150 °C above the bulk anatase-to-rutile transition temperature for films grown on substrates with -2.0% to 6.0% lattice mismatch. Continuum elastic models are used to calculate the strain energy density and to construct a model for the preferred epitaxial orientation of anatase. This model is consistent with experimental observations that the strain energy density dominates and leads to the stabilization of 00l- and h00-oriented anatase below and above lattice mismatch values of 5%, respectively. Additionally, TiO 2 nanocrystallite size is found to decrease with lattice mismatch and is discussed in terms of energy competition and possible changes in the nucleation and growth process. To further probe the competition between bulk free, surface, interface, and strain energies in metastable (kinetically limited) as-grown TiO2 films, ex post facto annealing was completed to assess the equilibrium state of the films. These studies confirm the continuum elastic model and highlight the relative importance of the different energies. We then implement our understanding of energy competition to deterministically increase surface area and enhance light absorption via in situ growth processes and ex post facto annealing. © 2014 American Chemical Society.

Correlated electron oxides prove a diverse landscape of exotic materials' phenomena and properties. One example of such a correlated oxide material is strontium ruthenate (SrRuO3) which is known to be a metallic itinerant ferromagnet and for its widespread utility as a conducting electrode in oxide heterostructures. We observe that the complex electronic structure of SrRuO3 is also responsible for unexpected optical properties including high absorption across the visible spectrum (commensurate with a low band gap semiconductor) and remarkably low reflection compared to traditional metals. By coupling this material to a wide band gap semiconductor (TiO 2) we demonstrate dramatically enhanced visible light absorption and large photocatalytic activities. The devices function by photo-excited hot-carrier injection from the SrRuO3 to the TiO2 and the effect is enhanced in thin films due to electronic structure changes. This observation provides an exciting new approach to the challenge of designing visible-light photosensitive materials. The correlated electron "metal" SrRuO3 exhibits strong visible light absorption. Overlaid on the AM1.5G solar spectrum, it can be seen that SrRuO3 absorbs more than 75 times more light than TiO2. The structural, chemical, and electronic compatibility of TiO2 and SrRuO3 further enables the fabrication of heterojunctions with exciting photovoltaic and photocatalytic response driven by hot-carrier injection. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

n-n Schottky, n-n ohmic, and p-n Schottky heterojunctions based on TiO2 /correlated "metallic" oxide couples exhibit strong solar-light absorption driven by the unique electronic structure of the "metallic" oxides. Photovoltaic and photocatalytic responses are driven by hot electron injection from the "metallic" oxide into the TiO2 , enabling new modalities of operation for energy systems.

We report measurements of the temperature dependence of the optical reflectivity, dR/dT of fifteen metallic elements at a wavelength of λ = 1.03 μm by time-domain thermoreflectance (TDTR); and the thermoreflectance of thin-films of Pt, Ta, Al, Au, SrRuO(3), and LaNiO(3) over the wavelength range 0.4 < λ < 1.6 μm using variable angle spectroscopic ellipsometry. At λ = 1.03 μm, Al, Ta, Re, Ru, have high values of thermoreflectance, dR/dT > 6∙10(-5) K(-1), and are good choices as optical transducers for TDTR experiments using a Yb:fiber laser oscillator. If low optical reflectivity and the associated high degree of steady-state heating are not a concern, LaNiO(3) provides an exceptionally sensitive thermometer in the infrared; (1/R)(dR/dT) > 2.5∙10(-4) K(-1) in the wavelength range 0.85 < λ < 1.3 μm. This compilation of data will assist in the design and interpretation of optical pump-probe studies of thermal properties.

We report the results of time-domain thermoreflectance (TDTR) measurements of two strongly bonded metal-oxide systems with unusually large thermal conductances. We find that TDTR data for the epitaxial SrRuO3/SrTiO3 interface is consistent with an interface conductance G>0.8GWm-2K-1. For an Al/MgO interface at a pressure of 60 GPa, we find G≈1.1GWm-2K-1. Both are within 40% of the maximum possible conductance for these systems, as predicted by simple theory.

We explore the effect of growth conditions on the cation and anion chemistry, electrical leakage, conduction mechanisms, and ferroelectric and dielectric behavior of BiFeO3. Although it is possible to produce single-phase, coherently strained films in all cases, small variations in the pulsed-laser deposition growth process, specifically the laser repetition rate and target composition, result in films with chemistries ranging from 10% Bi-deficiency to 4% Bi-excess and films possessing Bi gradients as large a 6% across the film thickness. Corresponding variations and gradients in the O chemistry are also observed. As a result of the varying film chemistry, marked differences in surface and domain morphology are observed wherein Bi-deficiency stabilizes atomically smooth surfaces and ordered stripe domains. Subsequent investigation of the current-voltage response reveals large differences in leakage current density arising from changes in both the overall stoichiometry and gradients. In turn, the film stoichiometry drives variations in the dominant conduction mechanism including examples of Schottky, Poole-Frenkel, and modified Poole-Frenkel emission depending on the film chemistry. Finally, slightly Bi-excess films are found to exhibit the best low-frequency ferroelectric and dielectric response while increasing Bi-deficiency worsens the low-frequency ferroelectric performance and reduces the dielectric permittivity.

Self-assembly via nanoscale phase separation offers an elegant route to fabricate nanocomposites with physical properties unattainable in single-component systems. One important class of nanocomposites are optical metamaterials which exhibit exotic properties and lead to opportunities for agile control of light propagation. Such metamaterials are typically fabricated via expensive and hard-to-scale top-down processes requiring precise integration of dissimilar materials. In turn, there is a need for alternative, more efficient routes to fabricate large-scale metamaterials for practical applications with deep-subwavelength resolution. Here, we demonstrate a bottom-up approach to fabricate scalable nanostructured metamaterials via spinodal decomposition. To demonstrate the potential of such an approach, we leverage the innate spinodal decomposition of the VO₂-TiO₂ system, the metal-to-insulator transition in VO₂, and thin-film epitaxy, to produce self-organized nanostructures with coherent interfaces and a structural unit cell down to 15 nm (tunable between horizontally and vertically aligned lamellae) wherein the iso-frequency surface is temperature-tunable from elliptic to hyperbolic dispersion producing metamaterial behavior. These results provide an efficient route for the fabrication of nanostructured metamaterials and other nanocomposites for desired functionalities.

Perovskite titanates such as SrTiO3 (STO) exhibit a wide range of important functional properties, including ferroelectricity and excellent photocatalytic performance. The wide optical band gap of titanates limits their use in these applications; however, making them ill-suited for integration into solar energy harvesting technologies. Our recent work has shown that by doping STO with equal concentrations of La and Cr, we can enhance visible light absorption in epitaxial thin films while avoiding any compensating defects. In this work, we explore the optical properties of photoexcited carriers in these films. Using spectroscopic ellipsometry, we show that the Cr3+ dopants, which produce electronic states immediately above the top of the O 2p valence band in STO reduce the direct band gap of the material from 3.75 eV to 2.4-2.7 eV depending on doping levels. Transient reflectance spectroscopy measurements are in agreement with the observations from ellipsometry and confirm that optically generated carriers are present for longer than 2 ns. Finally, through photoelectrochemical methylene blue degradation measurements, we show that these co-doped films exhibit enhanced visible light photocatalysis when compared to pure STO.

Ferroelectrics, with their spontaneous switchable electric polarization and strong coupling between their electrical, mechanical, thermal, and optical responses, provide functionalities crucial for a diverse range of applications. Over the past decade, there has been significant progress in epitaxial strain engineering of oxide ferroelectric thin films to control and enhance the nature of ferroelectric order, alter ferroelectric susceptibilities, and to create new modes of response which can be harnessed for various applications. This review aims to cover some of the most important discoveries in strain engineering over the past decade and highlight some of the new and emerging approaches for strain control of ferroelectrics. We discuss how these new approaches to strain engineering provide promising routes to control and decouple ferroelectric susceptibilities and create new modes of response not possible in the confines of conventional strain engineering. To conclude, we will provide an overview and prospectus of these new and interesting modalities of strain engineering helping to accelerate their widespread development and implementation in future functional devices.