Temporal Evolution of Microscopic Structure and Functionality during Crystallization of Amorphous Indium-Based Oxide Films.

Understanding the crystallization mechanism of amorphous metal-oxide thin films remains of importance to avoid the deterioration of multifunctional flexible electronics. We derived the crystallization mechanism of indium-based functional amorphous oxide films by using in situ X-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. Crystallization begins with surface nucleation, especially at low annealing temperatures, and proceeds simultaneous nucleation and growth in the bulk. Three-dimensional crystal growth in the film was observed when the crystallite size was sufficiently smaller than the film thickness. When the growing crystallites reached the film surface, the crystallization was dominated by two- or lower-dimensional growth. Such crystallization can be explained within the framework of the modified Avrami theory and can be varied for tailoring the electrical properties of the amorphous In2O3 film. After tailoring the film crystallinity and crystallite size, the carrier mobility was improved to >100 cm2/V·s in 30 min. Our results show that a carrier mobility of >90 cm2/V·s can be implemented for the In2O3 film with a crystallinity of >40% and a crystallite size of >70 nm by an optimized annealing process. The incorporation of Ga element into amorphous In2O3 films obviously increases the activation energy of nucleation and migration. In contrast, Sn dopants can promote the crystal growth. This is attributed to two kinds of migration mechanisms during the annealing in air, one of which is the dominant migration mechanism of oxygen interstitials in crystallized indium-tin oxide (ITO) films and the other dominated by oxygen vacancies in In2O3 and IGO films. Combining the modified Avrami theory with TEM observations, we predicted the structural evolution kinetics for indium-based amorphous oxide films and gained new insights for understanding the temporal structure-functionality relationship during crystallization.

Intermediate-Spin Iron(III) Complexes Having a Redox-Noninnocent Macrocyclic Tetraamido Ligand.

An iron(III) complex having a dibenzotetraethyltetraamido macrocyclic ligand (DTTM4-), (NEt4)2[FeIII(DTTM)Cl] (1), was synthesized and characterized by crystallographic, spectroscopic, and electrochemical methods. Complex 1 has a square-pyramidal structure in the S = 3/2 spin state. The complex exhibited two reversible redox waves at +0.36 and +0.68 V (vs SCE) in the cyclic voltammogram measured in CH2Cl2 at room temperature. The stepwise oxidation of 1 using chemical oxidants allowed us to observe clear and distinct spectral changes with distinct isosbestic points for each step, in which oxidation occurred at the phenylenediamido moiety rather than the iron center. One-electron oxidation of 1 by 1 equiv of [RuIII(bpy)3](ClO4)3 (bpy = 2,2'-bipyridine) in CH2Cl2 afforded square-pyramidal (NEt4)[Fe(DTTM)Cl] (2), which was in the S = 1 spin state involving a ligand radical and showed a slightly distorted square-pyramidal structure. Complex 2 showed an intervalence charge-transfer band at 900 nm, which was assigned on the basis of time-dependent density functional theory calculations, to indicate that the complex is in a class IIA mixed-valence ligand-radical regime with Hab = 884 cm-1. Two-electron oxidation of 1 by 2 equiv of [(4-Br-Ph)3N•+](SbCl6) in CH2Cl2 afforded two-electron-oxidized species of 1, [Fe(DTTM)Cl] (3), which was in the S = 1/2 spin state; complex 3 exhibited a distorted square-pyramidal structure. X-ray absorption near-edge structure spectra of 1-3 were measured in both CH3CN solutions and BN pellets to observe comparable rising-edge energies for the three complexes, and Mössbauer spectra of 1-3 showed almost identical isomer shifts and quadruple splitting parameters, indicating that the iron centers of the three complexes are intact to be in the intermediate-spin iron(III) state. Thus, in complexes 2 and 3, it is evident that antiferromagnetic coupling is operating between the unpaired electron(s) of the ligand radical(s) and those of the iron(III) center.

A Ruthenium(III)-Oxyl Complex Bearing Strong Radical Character.

Proton-coupled electron-transfer oxidation of a RuII -OH2 complex, having an N-heterocyclic carbene ligand, gives a RuIII -O. species, which has an electronically equivalent structure of the RuIV =O species, in an acidic aqueous solution. The RuIII -O. complex was characterized by spectroscopic methods and DFT calculations. The oxidation state of the Ru center was shown to be close to +3; the Ru-O bond showed a lower-energy Raman scattering at 732 cm-1 and the Ru-O bond length was estimated to be 1.77(1) Å. The RuIII -O. complex exhibits high reactivity in substrate oxidation under catalytic conditions; particularly, benzaldehyde and the derivatives are oxidized to the corresponding benzoic acid through C-H abstraction from the formyl group by the RuIII -O. complex bearing a strong radical character as the active species.

On the Crystal Structural Control of Sputtered TiO2 Thin Films.

In this study, we focused on the origin on the selective deposition of rutile and anatase TiO2 thin films during the sputtering process. The observation on microstructural evolution of the TiO2 films by transmission electron microscopy revealed the coexistence of rutile and anatase TiO2 phases in the initial stage under the preferential growth conditions for the anatase TiO2; the observations further revealed that the anatase phase gradually dominated the crystal structure with increasing film thickness. These results suggest that the bombardment during the sputtering deposition did not obviously affect the TiO2 crystal structure, and this was also confirmed by off-axis magnetron sputtering experiments. We also investigated the mechanism of the effect of Sn impurity doping on the crystal structure using first-principles calculations. It is found that the formation energy of Sn-doped rutile TiO2 is lower than that of Sn-doped anatase TiO2; this suggests that the Sn-doped TiO2 favours the rutile phase. These results offer a guideline for the utilization of selective deposition of rutile and anatase TiO2 thin films in various industrial applications.

Stabilization of metastable ferroelectric Ba1-xCaxTi2O5 by breaking Ca-site selectivity via crystallization from glass.

The thermal stability and dielectric and structural properties of ferroelectric Ba1-xCaxTi2O5 (0 ≤ x ≤ 0.30) prepared by crystallization from glass are investigated. The Ba1-xCaxTi2O5 compounds with x < 0.10 are thermally stable phases, while those with x ≥ 0.10 are metastable phases. The ferroelectric transition temperature drastically decreases from 470 to 220°C with increasing x. Crystal structure analyses reveal that one of two possible Ba sites is occupied by Ca in the stable phase region, while Ca-site selectivity is broken in the metastable phase region. The Ca-site selectivity introduces local distortion and makes the crystal lattice unstable. However, the local distortion is suppressed by the occupancy of Ca into both Ba sites. Accordingly, the metastable ferroelectric phase can be obtained beyond the substitution limit of Ca by crystallization from the glassy state. The stabilization mechanism provides possible wide control of the functionality of materials by expanding the composition range.

Tetravalent dysprosium in a perovskite-type oxide.

The existence of tetravalent dysprosium in perovskite-type oxide barium zirconate is confirmed in this work. This discovery will stimulate many researchers in diverse fields such as catalysts, solid state ionics, sensors, and fluorescent materials.

Local environment analysis of dopants in ceramics by x-ray absorption near-edge structure with the aid of first-principles calculations.

Three types of functional ceramic materials, (1) dilute magnetic semiconductor, (2) phosphor and (3) electrolyte of a solid fuel cell, are fabricated by the conventional solid state reaction method. Local environments of dopants in these ceramic materials here synthesized are systematically investigated by using the x-ray absorption near-edge structure (XANES) with the aid of first-principles calculations. Our present analytical method by combined use of XANES and first principles calculations has successfully explained the local environment of dopants in the above ceramic materials.

Identification of ultradilute dopants in ceramics.

The properties of ceramic materials are strongly influenced by the presence of ultradilute impurities (dopants). Near-edge X-ray absorption fine structure (NEXAFS) measurements using third-generation synchotron sources can be used to identify ultradilute dopants, provided that a good theoretical tool is available to interpret the spectra. Here, we use NEXAFS analysis and first-principles calculations to study the local environments of Ga dopants at levels of 10 p.p.m in otherwise high-purity MgO. This analysis suggests that the extra charge associated with substitutional Ga on a Mg site is compensated by the formation of a Mg vacancy. This defect model is then confirmed by positron lifetime measurements and plane-wave pseudopotential calculations. This powerful combination of techniques should provide a general method of identifying the defect states of ultradilute dopants in ceramics.