Ho Nyung Lee

Strong polarization enhancement in asymmetric three-component ferroelectric superlattices

Theoretical predictions—motivated by recent advances in epitaxial engineering—indicate a wealth of complex behaviour arising in superlattices of perovskite-type metal oxides. These include the enhancement of polarization by strain and the possibility of asymmetric properties in three-component superlattices. Here we fabricate superlattices consisting of barium titanate (BaTiO3), strontium titanate (SrTiO3) and calcium titanate (CaTiO3) with atomic-scale control by high-pressure pulsed laser deposition on conducting, atomically flat strontium ruthenate (SrRuO3) layers. The strain in BaTiO3 layers is fully maintained as long as the BaTiO3 thickness does not exceed the combined thicknesses of the CaTiO3 and SrTiO3 layers. By preserving full strain and combining heterointerfacial couplings, we find an overall 50% enhancement of the superlattice global polarization with respect to similarly grown pure BaTiO3, despite the fact that half the layers in the superlattice are nominally non-ferroelectric. We further show that even superlattices containing only single-unit-cell layers of BaTiO3 in a paraelectric matrix remain ferroelectric. Our data reveal that the specific interface structure and local asymmetries play an unexpected role in the polarization enhancement.

Quantitative mapping of switching behavior in piezoresponse force microscopy

The application of ferroelectric materials for nonvolatile memory and ferroelectric data storage necessitates quantitative studies of local switching characteristics and their relationship to material microstructure and defects. Switching spectroscopy piezoresponse force microscopy (SS-PFM) is developed as a quantitative tool for real-space imaging of imprint, coercive bias, remanent and saturation responses, and domain nucleation voltage on the nanoscale. Examples of SS-PFM implementation, data analysis, and data visualization are presented for epitaxial lead zirconate titanate (PZT) thin films and polycrystalline PZT ceramics. Several common artifacts related to the measurement method, environmental factors, and instrument settings are analyzed.

Reversible redox reactions in an epitaxially stabilized SrCoO x oxygen sponge

Fast, reversible redox reactions in solids at low temperatures without thermomechanical degradation are a promising strategy for enhancing the overall performance and lifetime of many energy materials and devices. However, the robust nature of the cations oxidation state and the high thermodynamic barrier have hindered the realization of fast catalysis and bulk diffusion at low temperatures. Here, we report a significant lowering of the redox temperature by epitaxial stabilization of strontium cobaltites (SrCoO(x)) grown directly as one of two distinct crystalline phases, either the perovskite SrCoO(3-δ) or the brownmillerite SrCoO(2.5). Importantly, these two phases can be reversibly switched at a remarkably reduced temperature (200-300 °C) in a considerably short time (< 1 min) without destroying the parent framework. The fast, low-temperature redox activity in SrCoO(3-δ) is attributed to a small Gibbs free-energy difference between two topotatic phases. Our findings thus provide useful information for developing highly sensitive electrochemical sensors and low-temperature cathode materials.

Anisotropic ferroelectric properties of epitaxially twinned Bi3.25La0.75Ti3O12 thin films grown with three different orientations

Epitaxially twinned (001)- , (118)- , and (104)-oriented La-substituted Bi4Ti3O12 (BLT) ferroelectric films have been grown by pulsed laser deposition on (001)- , (011)- , and (111)-oriented SrTiO3 single-crystal substrates, respectively, covered with SrRuO3. Well-defined (001)-oriented BLT films were grown at a substrate temperature as low as 600 °C. By x-ray diffraction characterization it has been found that the low-index three-dimensional epitaxial orientation relationship BLT(001)‖SrRuO3(001)‖SrTiO3(001); BLT[110]‖SrRuO3[100]‖SrTiO3[100] is valid for all epitaxially twinned BLT thin films grown on SrRuO3-covered SrTiO3 substrates in spite of their different orientations. The (104)-oriented BLT films showed an about 1.5 times higher remanent polarization (2Pr=31.9 μC/cm2) than the (118)-oriented BLT films (2Pr=20.7 μC/cm2), while (001)-oriented BLT films revealed only a small polarization component (2Pr=1.1 μC/cm2), thus demonstrating the ferroelectric anisotropy.

Wide bandgap tunability in complex transition metal oxides by site-specific substitution

Fabricating complex transition metal oxides with a tunable bandgap without compromising their intriguing physical properties is a longstanding challenge. Here we examine the layered ferroelectric bismuth titanate and demonstrate that, by site-specific substitution with the Mott insulator lanthanum cobaltite, its bandgap can be narrowed by as much as 1 eV, while remaining strongly ferroelectric. We find that when a specific site in the host material is preferentially substituted, a split-off state responsible for the bandgap reduction is created just below the conduction band of bismuth titanate. This provides a route for controlling the bandgap in complex oxides for use in emerging oxide optoelectronic and energy applications.

Effect of epitaxial strain on ferroelectric polarization in multiferroic BiFeO3 films

Multiferroic BiFeO3 epitaxial films with thicknesses ranging from 40to960nm were grown by pulsed laser deposition on SrTiO3 (001) substrates with SrRuO3 bottom electrodes. X-ray characterization shows that the structure evolves from angularly distorted tetragonal with c∕a≈1.04 to more bulklike distorted rhombohedral (c∕a≈1.01) as the strain relaxes with increasing thickness. Despite this significant structural evolution, the ferroelectric polarization along the body diagonal of the distorted pseudocubic unit cells, as calculated from measurements along the normal direction, barely changes.

Suppressed Dependence of Polarization on Epitaxial Strain in Highly Polar Ferroelectrics

A combined experimental and computational investigation of coupling between polarization and epitaxial strain in highly polar ferroelectric PbZr(0.2)Ti(0.8)O3 (PZT) thin films is reported. A comparison of the properties of relaxed (tetragonality c/a approximately 1.05) and highly strained (c/a approximately 1.09) epitaxial films shows that polarization, while being amongst the highest reported for PZT or PbTiO3 in either film or bulk forms P(r) approximately 82 microC/cm(2)), is almost independent of the epitaxial strain. We attribute this behavior to a suppressed sensitivity of the A-site cations to epitaxial strain in these Pb-based perovskites, where the ferroelectric displacements are already large, contrary to the case of less polar perovskites, such as BaTiO3. In the latter case, the A-site cation (Ba) and equatorial oxygen displacements can lead to substantial polarization increases.

Nonlinear Dynamics of Domain-Wall Propagation in Epitaxial Ferroelectric Thin Films

We investigated the ferroelectric domain-wall propagation in epitaxial Pb(Zr,Ti)O3 thin film over a wide temperature range (3-300 K). We measured the domain-wall velocity under various electric fields and found that the velocity data is strongly nonlinear with electric fields, especially at low temperature. We found that, as one of surface growth issues, our domain-wall velocity data from ferroelectric epitaxial film could be classified into the creep, depinning, and flow regimes due to competition between disorder and elasticity. The measured values of velocity and dynamical exponents indicate that the ferroelectric domain walls in the epitaxial films are fractal and pinned by a disorder-induced local field.

Room-Temperature Multiferroic Hexagonal LuFeO3 Films

The crystal and magnetic structures of single-crystalline hexagonal LuFeO(3) films have been studied using x-ray, electron, and neutron diffraction methods. The polar structure of these films are found to persist up to 1050 K; and the switchability of the polar behavior is observed at room temperature, indicating ferroelectricity. An antiferromagnetic order was shown to occur below 440 K, followed by a spin reorientation resulting in a weak ferromagnetic order below 130 K. This observation of coexisting multiple ferroic orders demonstrates that hexagonal LuFeO(3) films are room-temperature multiferroics.

Multiple conducting carriers generated in LaAlO3/SrTiO3 heterostructures

We have found that there is more than one type of conducting carriers generated in LaAlO3/SrTiO3 heterostructures by comparing the sheet carrier density and mobility from optical transmission spectroscopy with those from dc-transport measurements. When multiple types of carriers exist, optical characterization dominantly reflects the contribution from the high-density carriers whereas dc-transport measurements may exaggerate the contribution of the high-mobility carriers even though they are present at low density. Since the low-temperature mobility determined by dc-transport in the LaAlO3/SrTiO3 heterostructures is much higher than that extracted by optical method, we attribute the origin of high-mobility transport to the low-density conducting carriers.