Unconventional Crystallization Pathways for Functional Metal Oxide Film Formation via Deep-Ultraviolet (DUV) Photoactivation

Abstract

Researches and developments of various metal oxide materials on Earth with special and diverse physical/chemical/electrical properties have been continued to date in various shapes and structures ranging from nanostructure synthesis to thin film formation. Also, to exhibit various physicochemical functions, it is highly preferred that metal oxides have a crystalline phase with a stable lattice and structure of constituent atoms rather than a disordered amorphous phase. So as to have a crystalline phase of the metal oxide, material synthesis has been performed through universal thermal processing, but there are many disadvantages such as low energy efficiency, high thermal budget, and large processing time. To achieve this, a technology capable of low-temperature crystallization of a metal oxide system using light (photo) energy has attracted much attention over the past decades as an alternative to conventional thermal processing. There are several reports of techniques for raising the surface temperature with strongly condensed light energy up to the high thermal energy required for oxide crystallization and are commercialized and used for specific purposes in the industry. However, the enormous cost and complex techniques for dealing with powerful light energies comparable to the required high thermal budgets greatly limit the universality and accessibility in the synthesis of crystallized materials Also, there are still many questions about the comprehensive interaction between this light energy and the metal oxide system. The technique of continuous light energy irradiation in the wavelength of ultraviolet (UV) from the various UV lamp to the metal oxides has also been researched and developed in parallel with the intensified laser light technique. Above all, the technology using UV lamps with excellent accessibility provides an optimal platform to directly induce photo-electronic excitation transition reactions with various metal oxide systems. However, most of the previous reports have demonstrated a metal oxide film formation of the high-quality amorphous phase rather than the crystalline phase. Besides, the direct photoactivation and its mechanism of metal oxide crystallization from UV light energy have not been properly reported. We generated and developed the direct photochemical activation & mechanism using UV lamps in metal oxide films fabricated by various chemical vapor/solution deposition. The vigorous photo-electronic excitation transition reactions inside the metal oxides through photochemical activation proved to be a kind of medium to promote low-temperature crystallization, various microstructural evolution, and phase transformation. We demonstrated a very different metal oxide crystallization method and its pathway using DUV photochemical activation. Also, it not only provides guidance to control the microstructural evolution and behavior of metal oxide films but also presents great potential to suggest new alternatives to the existing phase diagram relationship. In Chapter 1, we briefly reviewed the progress research status of the low temperature crystallization process of metal oxides using various UV light sources with the information on the photo-electronic excitation and transition reactions caused by UV photon energy. In Chapter 2, we proposed a new crystallization approach in accordance with various investigations of two different crystallization pathways of amorphous titanium dioxide (TiO2) thin films at relatively low temperatures under deep-UV (DUV) photoactivation conditions. This DUV photoactivation not only lowered the onset crystallization temperature, but also induced phase transformation into the brookite phase as an intermediate crystalline phase, and generated the microstructural evolutions and texture behaviors of crystal grains. In Chapter 3 and 4, we developed a DUV-photocombustion protocol by taking advantage of the combination of the principles of DUV photochemical activation and solution combustion synthesis (SCS). The synergistic effects of DUV-photocombustion significantly lowered the crystallization temperatures of VO2 (binary oxide) (600°C → 250°C) and BiVO4 (ternary oxide) (500°C → 350°C). All various oxide film characterizations exhibit that ammonium nitrate (AN), as a UV photoactivable & carbon-free oxidizing agent, is a key additive that promotes not only the near-stoichiometric combustion synthesis but also the DUV photoactivation reactions, leading to the low-temperature crystallization with various microstructural and morphological evolutions and behaviors. We believe that the low-temperature metal oxide crystallization via DUV photochemical activated combustion can provide a progressive strategy for the synthesis and formation of new functional crystalline metal oxide films via various solution chemistry.