Epitaxial Strain-Controlled Ionic Conductivity in Li-Ion Solid Electrolyte Li_0.33La_0.56TiO₃ Thin Films

Ionic conductive Li_0.33La_0.56TiO₃ (LLT) epitaxial thin films were grown on perovskite SrTiO₃ (100), NdGaO₃ (110), and (LaAlO₃)_0.3-(SrAl_0.5Ta_0.5O₃)_0.7 (100) single crystal substrates by pulsed laser deposition. The use of Li-rich Li_0.84La_0.56TiO_3+δ target together with an optimized laser fluence resulted in the growth of phase pure LLT thin films with high growth rate of 2 nm/min. The <i>a</i>-axis and <i>c</i>-axis oriented films were selectively grown by choosing the substrates. Ionic conductivity at room temperature of LLT epitaxial film on NdGaO₃ (110) substrate was close to that of bulk previously reported, representing the highly crystalline quality. In addition, the unequally strained lattice due to different in-plane lattice constants of orthorhombic NdGaO₃ substrate resulted in laterally anisotropic ionic conductivity with different activation energy perpendicular to NdGaO₃ [11̅0] and [001], 6.7 × 10^–4 S·cm^–1 with 0.34 eV and 4.3 × 10^–4 S·cm^–1 with 0.36 eV, respectively. This result suggests that the lattice engineering can provide a way to control Li ionic conduction

Lateral Solid-Phase Epitaxy of Oxide Thin Films on Glass Substrate Seeded with Oxide Nanosheets

We developed a technique to fabricate oxide thin films with uniaxially controlled crystallographic orientation and lateral size of more than micrometers on amorphous substrates. This technique is lateral solid-phase epitaxy, where epitaxial crystallization of amorphous precursor is seeded with ultrathin oxide nanosheets sparsely (≈10% coverage) deposited on the substrate. Transparent conducting Nb-doped anatase TiO₂ thin films were fabricated on glass substrates by this technique. Perfect (001) orientation and large grains with lateral sizes up to 10 μm were confirmed by X-ray diffraction, atomic force microscopy, and electron beam back­scattering diffraction measurements. As a consequence of these features, the obtained film exhibited excellent electrical transport properties comparable to those of epitaxial thin films on single-crystalline substrates. This technique is a versatile method for fabricating high-quality oxide thin films other than anatase TiO₂ and would increase the possible applications of oxide-based thin film devices

Enhanced Electrical Conduction in Anatase TaON via Soft Chemical Lithium Insertion toward Electronics Application

Metal oxynitride semiconductors with the d⁰ or d¹⁰ electron configuration are promising materials for nontoxic pigments and photocatalysts, but their electrical properties have scarcely been studied. Anatase TaON (δ-TaON) is a metastable polymorph of TaON, and its epitaxial thin films show good semiconducting properties such as a wide tunability of electrical conductivity and a rather high electron mobility comparable to that of anatase TiO₂. However, the density of carrier electrons (<i>n</i>_e) provided by anion vacancies is limited to ∼1 × 10²⁰ cm^–3, so establishing a method for carrier doping of anatase TaON remains a critical issue for its use in electronics applications. In this report, we used soft chemical insertion of Li into interstitial sites of anatase TaON epitaxial thin films by using an <i>n</i>-butyllithium solution, and the resulting material showed a higher <i>n</i>_e (3.5 × 10²⁰ cm^–3) than anion-deficient anatase TaON films. Additionally, the Li-inserted anatase TaON showed an enhanced Hall mobility (μ_H) of over 30 cm²V^–1s^–1 and a lower resistivity of 6.7 × 10^–4 Ωcm at room temperature. In contrast, direct vapor phase deposition of Li-doped TaON caused Li substitution for Ta, where a large difference in charges between Li⁺ and Ta⁵⁺ was compensated by an increase in the O/N ratio. These results indicate that soft chemical insertion of Li after growth of the host crystal is an effective method for carrier doping of anatase TaON

Strain Engineering for Anion Arrangement in Perovskite Oxynitrides

Mixed-anion perovskites such as oxynitrides, oxyfluorides, and oxyhydrides have flexibility in their anion arrangements, which potentially enables functional material design based on coordination chemistry. However, difficulty in the control of the anion arrangement has prevented the realization of this concept. In this study, we demonstrate strain engineering of the anion arrangement in epitaxial thin films of the Ca_1–<i>x</i>Sr_<i>x</i>TaO₂N perovskite oxynitrides. Under compressive epitaxial strain, the axial sites in TaO₄N₂ octahedra tend to be occupied by nitrogen rather than oxygen, which was revealed by N and O K-edge linearly polarized X-ray absorption near-edge structure (LP-XANES) and scanning transmission electron microscopy combined with electron energy loss spectroscopy. Furthermore, detailed analysis of the LP-XANES indicated that the high occupancy of nitrogen at the axial sites is due to the partial formation of a metastable <i>trans-</i>type anion configuration. These results are expected to serve as a guide for the material design of mixed-anion compounds based on their anion arrangements

Reversible Changes in Resistance of Perovskite Nickelate NdNiO₃ Thin Films Induced by Fluorine Substitution

Perovskite nickel oxides are of fundamental as well as technological interest because they show large resistance modulation associated with phase transition as a function of the temperature and chemical composition. Here, the effects of fluorine doping in perovskite nickelate NdNiO₃ epitaxial thin films are investigated through a low-temperature reaction with polyvinylidene fluoride as the fluorine source. The fluorine content in the fluorinated NdNiO_3–<i>x</i>F_<i>x</i> films is controlled with precision by varying the reaction time. The fully fluorinated film (<i>x</i> ≈ 1) is highly insulating and has a bandgap of 2.1 eV, in contrast to NdNiO₃, which exhibits metallic transport properties. Hard X-ray photoelectron and soft X-ray absorption spectroscopies reveal the suppression of the density of states at the Fermi level as well as the reduction of nickel ions (valence state changes from +3 to +2) after fluorination, suggesting that the strong Coulombic repulsion in the Ni 3d orbitals associated with the fluorine substitution drives the metal-to-insulator transition. In addition, the resistivity of the fluorinated films recovers to the original value for NdNiO₃ after annealing in an oxygen atmosphere. By application of the reversible fluorination process to transition-metal oxides, the search for resistance-switching materials could be accelerated