A comprehensive study of the growth of TiO2 via pulsed-laser deposition: Annealing effects and substrate-film interface physics

Date

2019-05

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Abstract

Titanium dioxide (titania; TiO2) is considered to be a well-studied material; the first study of titania was published by Vegard in July, 1916 (Vegard, 1916). Despite its long publication record, we still do not know everything there is to know about this material. Titania is a widely-used material in industry and a large part of research on it is driven by real-life applications, including coatings for reflective optics, self-cleaning coatings, and gas-sensing devices. Depending on the application, there are prerequisites for the substrate. Additionally, for industrial applications, the substrate usually needs to be inexpensive; otherwise, mass production may become cost prohibitive. Of the many considerations, lattice mismatch can be a large limiting factor when selecting substrates. That is why some substrates, such as glass, quartz, and silicon are used for growing various crystallographic phases of titania. Other substrates, like strontium titanate (SrTiO3) and lanthanum aluminate (LaAlO3), are only used for growing a specific crystallographic phase. Note, however, that using different substrates to produce different phases of titania severely limits the utility, if not the veracity, of the phase map of the system. Given that over the last 100 years, titania phase behavior on sapphire and anatase-sapphire interface physics have never been studied, and given the recent surge in interest for using TiO2 in a wide variety of applications that span a number of different research areas, the goal of my dissertation is to address this gap in the body of knowledge on titanium dioxide. My dissertation will address two major topics:

  1. The growth of TiO2 thin films. Sapphire is commonly used as a single-crystal substrate (in the (0001), or c-cut orientation) to grow rutile titania, but has never been used for growth of the anatase crystalline phase, presumably due to the large lattice mismatch ( 21%). In fact, because the anatase, rutile, and brookite crystallographic phases of titania have never been grown using a single technique on one single-crystal substrate, a phase map of titania on sapphire does not exist. My dissertation will show that anatase TiO2 can be grown on c-cut sapphire using pulsed-laser deposition (PLD), and I will produce the first complete phase map of this material grown on sapphire substrates. In addition, because of the large lattice mismatch between anatase and sapphire, we will shed new light on the physics of titania growth — specifically, at the substrate-film interface. This will provide valuable information in the general area of the physics of film formation in case of large lattice mismatch with the substrate and will develop an optimized PLD growth protocol for the two principal tetragonal phases of titania.
  2. The effects of post-growth thermal treatment of TiO2 thin films. My dissertation will also provide information on how the phase of titania grown on sapphire substrates changes after being subjected to different annealing conditions. The annealing study will be the first such study that uses a wide range of temperatures and gas pressures, and will help to further understand the physics of titania phase formation and provide additional information on phase transformation between rutile and anatase, specifically when grown on high lattice mismatch substrates. By addressing these topics I hope to expand the body of knowledge on this material specifically, and, more generally, the physics of thin film growth on high lattice-mismatch conditions. I will offer a better understanding of how titania phase changes with post-treatment conditions. Since titania is a popular material for many applications and research groups, my dissertation promises to contribute significantly to many areas of science and industry.

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Keywords

Titania, Sapphire, Pulsed laser deposition, Phase

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