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.

A visible-light active TiO2 photocatalyst multilayered with WO3.

Sputter-deposited TiO2 films with high visible-light photocatalytic activity were successfully realized by a hybrid TiO2/Pt/WO3 film structure with Pt nanoparticles uniformly distributed at the interface of the TiO2 and WO3 films. The TiO2/Pt/WO3 hybrid films enable the complete decomposition of CH3CHO under visible-light irradiation. The water contact angle of the TiO2/Pt/WO3 hybrid films reaches below 5° under visible-light irradiation. Pt nanoparticles are considered to act as a cocatalyst to improve the electron-hole separation efficiency. We demonstrate that the photogenerated holes in WO3 are transferred to the surface of the TiO2 film with less hole-trapping and induce high visible-light photocatalytic activity and hydrophilic behavior, and the photogenerated electrons are accumulated in the Pt nanoparticles. The highly hydrophilic thin films with high visible-light photocatalytic activity can be applied to various indoor products possessing self-cleaning and antifogging properties.

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.

Visible-light active photocatalytic WO3 films loaded with Pt nanoparticles deposited by sputtering.

Visible-Light active photocatalytic tungsten trioxide (WO3) films were deposited at a substrate temperature of 800 degrees C by dc reactive magnetron sputtering using a W metal target. In addition, Platinum (Pt) was deposited on the WO3 film surfaces at room temperature, also by sputtering. In the early stages of Pt growth, formation of Pt nanoparticles could be expected because of the island structure observed in Volmer-Weber-type growth mode. The surface coverage of Pt on the WO3 films was estimated quantitatively by X-ray photoelectron spectroscopy and was found to be approximately 60% after 7 s deposition. High resolution electron microscopy (HREM) demonstrated that Pt nanoparticles with a diameter of about 2.5 nm were generated and dispersed uniformly on the entire surface area of the columnar polycrystalline WO3 films. These Pt-loaded films exhibited high photocatalytic activity in the decomposition of acetaldehyde (CH3CHO) under visible light irradiation.

Novel emission properties of melem caused by the heavy metal effect of lanthanides(III) in a LB film.

A novel emissive molecular system is constructed by the intercalation of the fluorophore melem (triamino-tri-s-triazine) within a Langmuir-Blodgett (LB) film of stearic acid with the periodic arrangement of lanthanides (Ln(III)), mainly Pr(III) with supporting of Eu(III). From emission spectra, decay curves, quantum yields and XPS measurements, it is clarified that the external heavy metal effect of Pr(III) on melem is much stronger in the film than in the bulk solid state, resulting in producing an unusual triplet state of melem. The triplet state of melem in the LB film donates the excitation energy to Pr(III) in the LB film, which is completely different from the energy transfer pathway of Pr-melem complex in the solid state through the singlet state of melem.

Molecular distortion effect on ff-emission in a Pr(III) complex with 4,7-diphenyl-1,10-phenanthroline.

The electronic and structural behaviour of a Pr(III) complex with 4,7-diphenyl-1,10-phenanthroline, [Pr(bathophen)(2)(NO(3))(3)], is investigated with respect to the effect of configuration changes on the Pr(III) centre. [Pr(bathophen)(2)(NO(3))(3)] luminesces from the excited states of the ligand and the metal ion. The fluorescence, ff-emission ((1)D(2)-->(3)H(4)), and phosphorescence bands appear at 394, 608.2 and 482 nm, respectively, in the solid state. In acetonitrile, the complex also shows multiple emissions. From the time-resolved emission and the lifetime measurements, the excitation energy-transfer in [Pr(bathophen)(2)(NO(3))(3)] is clarified, that is, the upper excited triplet level of the ligand acts as an energy donor, while the (1)D(2) levels of Pr(III) is the acceptor. Additionally, the emission phenomena of the complex can be modified by molecular distortion, particularly by rotation of the phenyl groups in the ligand.