published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Thesis (Ph. D.)--University of Hong Kong, 1995. / Includes bibliographical references.

The electronic structure of a variety of metal-heteroatom multiply bonded complexes, including some active alkyne metathesis catalysts, have been investigated using He(I) and He(II) ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and Fenske-Hall molecular orbital calculations. Utilizing this electronic structure information, confirmation of the proposed mechanism for the alkyne metathesis reaction which involves formation of a metallacyclobutdienyl intermediate was ascertained. Also, the important relationships between metallatetrahedral and metallacyclobutadienyl complexes, both of which have been mentioned as possible intermediates in the alkyne metathesis reaction and for which examples have been prepared and isolated, are discussed in significant detail. In the final chapters the electronic structure of some corresponding metal-nitrogen triply bonded complexes are discussed as well as the results probing the charge distribution in metal-heteroatom multiply bonded systems as determined by the XPS experiment.

Thesis (Ph. D.)--University of Texas at Austin, 1999. / Vita. Includes bibliographical references (leaves 129-137). Available also in a digital version from Dissertation Abstracts.

Alternative plasmonic materials of Transparent Nitrides and Conductive Oxides (TNCOs) including Indium Tin Oxide (ITO), Al-doped ZnO (AZO) and Titanium Nitride (TiN), have been proposed as novel material platforms for Si-compatible plasmonics and metamaterials, showing enhanced light-matter interaction over a broad spectral range. It has been recently shown that these materials feature reduced optical losses compared with conventional noble metals such as Au and Ag in the visible and near-infrared spectral range. However, it is still an open challenge to tailor the structural and optical properties of these materials, and to further reduce their optical losses, in order to effectively utilize them in photonic devices. In this thesis work, I demonstrate wide tunability of the optical and structural properties of ITO, AZO and TiN thin films, by using post-deposition annealing treatments, enabling significant reduction of their optical losses. By measuring the optical bandgaps of the investigated materials, I show that the tunability of the optical properties originates from the modulation of the free carrier concentration induced by the annealing treatment. Moreover, I perform XRD characterization of the fabricated films, indicating that the annealing also effectively tunes the grain size, which is consistent with the change of the optical properties. Eventually, I investigate the role of the annealing gases for ITO and AZO, demonstrating that free-carrier modulation in ITO and AZO is due to the change in the density of oxygen vacancies after post-deposition annealing. In particular, TNCOs possess epsilon-near-zero (ENZ) condition in near-infrared range with optical loss ε^"<1, thus providing enhanced internal fields in the medium at the ENZ condition. In collaboration with Prof. Nader Engheta and the previous post-doc in our group Dr. Antonio Capretti, we demonstrate enhanced second-harmonic generation (SHG) and third-harmonic generation (THG) from ITO thin films driven by ENZ condition. It results that the SHG generation efficiency is comparable with that of a crystalline quartz plate of thickness 0.5 mm, and that the THG generation efficiency is ∼600 times larger than crystalline silicon. As an application for the fabricated TiN material, I investigate PL intensity and lifetime in Hyperbolic Metamaterials (HMMs) coupled with emitting Si Quantum Dots (QDs). In collaboration with Hiroshi Sugimoto in Prof. Minoru Fujii’s group and the previous post-doc in our group Dr. Sandeep Inampudi, we demonstrate up to 1.6-times enhanced decay rate of QDs emission. Photonic devices based on TNCO plasmonic materials offer an effective approach for the engineering of novel Si-based photonic devices with enhanced light-matter coupling over a broad spectral range. As an application for the fabricated ITO, in collaboration with Hongwei Zhao in Prof. Jonathan Klamkin’s group, electro-absorption modulators are numerically investigated to show high extinction ration of greater than 6dB, while insertion loss is less than 1.3dB for wavelength range from 1.25 µm to 1.42 µm. Additionally, we demonstrate tunable optical properties of ITO thin films in mid-infrared spectrum by thermal annealing of ITO in oxygen environment. In collaboration with Sajan Shrestha and Adam Overvig in Prof. NanFang Yu’s group, we fabricate 2D periodic arrays of ITO and show wide tuning of plasmonic resonances of ITO nanostructure from 4 µm to 10 µm. Combining with the tunability of ITO thin films in near-infrared, the ITO material platform provides a promising method for the control and engineering of Si-based tunable plasmonic and metamaterial devices in the infrared spectrum. Finally, in collaboration with my colleague Ren Wang, we experimentally demonstrate silicon nanodisk arrays with tunable anapole mode excitation in the visible spectrum. The proposed high index nanostructures can be used to enhance absorption rate for applications in semiconductor photodetector.

No description available.

Doctor of Philosophy / Department of Chemical Engineering / James H. Edgar / The sublimation recondensation crystal growth of aluminum nitride, titanium nitride, and yttrium nitride were explored experimentally and theoretically. Single crystals of these nitrides are potentially suitable as substrates for AlGaInN epitaxial layers, which are employed in ultraviolet optoelectronics including UV light-emitting diodes and laser diodes, and high power high frequency electronic device applications. A thermodynamic analysis was applied to the sublimation crystal growth of aluminum nitride to predict impurities transport (oxygen, carbon, and hydrogen) and to study the aspects of impurities incorporation for different growth conditions. A source purification procedure was established to minimize the impurity concentration and avoid degradation of the crystal’s properties. More than 98% of the oxygen, 99.9% of hydrogen and 90% of carbon originally in the source was removed. The AlN crystal growth process was explored in two ways: self-seeded growth with spontaneous nucleation directly on the crucible lid or foil, and seeded growth on SiC and AlN. The oxygen concentration was 2 ~ 4 x 1018cm-3, as measured by secondary ion mass spectroscopy in the crystals produced by self-seeded growth. Crystals grown from AlN seeds have visible grain size expansion. The initial AlN growth on SiC at a low temperature range (1400°C ~1600°C) was examined to understand the factors controlling nucleation. Crystals were obtained from c-plane on-axis and off-axis, Si-face and C-face, as well as m-plane SiC seeds. In all cases, crystal growth was fastest perpendicular to the c-axis. The growth rate dependence on temperature and pressure was determined for TiN and YN crystals, and their activation energies were 775.8±29.8kJ/mol and 467.1±21.7kJ/mol respectively. The orientation relationship of TiN (001) || W (001) with TiN [100] || W [110], a 45o angle between TiN [100] and W [100], was seen for TiN crystals deposited on both (001) textured tungsten and randomly orientated tungsten. Xray diffraction confirmed that the YN crystals were rock-salt structure, with a lattice constant of 4.88Å. Cubic yttria was detected in YN sample from the oxidation upon its exposed to air for limited time by XRD, while non-cubic yttria was detected in YN sample for exposures more than one hour by Raman spectra.

Thesis (Ph.D)--Electrical and Computer Engineering, Georgia Institute of Technology, 2010. / Committee Chair: Yoder, Douglas; Committee Member: Graham, Samuel; Committee Member: Allen, Janet; Committee Member: Klein, Benjamin; Committee Member: Voss, Paul. Part of the SMARTech Electronic Thesis and Dissertation Collection.

One of the keys to successful commercialization of photonic devices is compatibility with complementary metal-oxide-semiconductor technology (CMOS), the major platform of the microelectronics industry. Silicon photonics, with plasmonic materials are promising candidates for next generation chip-scale technology. The majority of plasmonics research has focused on noble metals, which are not CMOS compatible. Transition metal nitrides are an emerging class of alternative plasmonic materials that are complementary metal-oxide-semiconductor compatible and have shown promising results when compared to devices utilizing noble metals. This dissertation highlights, a CMOS compatible method to produce such alternative plasmonic materials using atomic layer deposition (ALD), specifically ultrathin plasmonic titanium nitride, aluminum metal and zirconium nitride. A post-deposition hydrogen plasma treatment is also introduced to improve the metallic properties of the ultrathin films. Additionally, this dissertation proposes a core-multishell (CMS) nanowire (NW) device structure that utilizes these materials to enable the creation of photonic devices, specifically detailing designs for cloaking and photoelectrochemical (PEC) water splitting applications. It is shown theoretically that zirconium nitride cloaks a silicon nanowire without substantially compromising the absorption of light, resulting in a less-intrusive, better performing silicon nanowire photosensor, and outperforms a gold cloak in the wavelength region of 400-500 nm. It is demonstrated theoretically that emerging plasmonic materials TiN and ZrN are promising candidates to improve the ideal photocurrent density hematite photoanodes in core-multishell nanowire devices, allowing hematite to remain electrically thin enough to effectively transport charge carriers while absorbing light similar to thick hematite features.

<p dir="ltr">Hybrid metamaterials have garnered significant attention in recent years owing to their unique properties not found in natural materials. These materials are engineered by integrating two or more distinct materials at the nanoscale, forming various microstructures such as particle-in-matrix, pillar-in-matrix, and multilayers. The recent development of vertically aligned nanocomposites (VANs) offers a platform in forming pillar-in-matrix metamaterials in a self-assembled fashion. Transition metal nitrides, such as titanium nitride (TiN), are interesting materials for VAN designs due to their outstanding plasmonic properties, chemical stability, and compatibility with various functional materials. However, the current range of material selection and morphological demonstrations in two-phase nitride-based nanocomposites is limited. There is a growing need for a deeper understanding of the self-assembly growth mechanism and greater freedom in structural and property tunability of nitride-based VANs to develop the next generation of integrated photonic and electronic devices.</p><p dir="ltr">This dissertation investigates the design, growth mechanisms, and tunability of nitride-based VANs for advanced metamaterial applications. The first chapter focuses on integrating ferromagnetic CoFe₂ into a plasmonic TiN matrix to achieve anisotropic optical and magnetic properties, as well as coupling effects between the two phases. In the second chapter, a third phase, gold (Au), is introduced into TiN-CoFe₂ VANs in a core-shell configuration, demonstrating enhanced tunability in microstructure and resultant properties, such as distinct hyperbolic behavior and switchable magnetic easy axis. The third chapter extends the exploration into three-dimensional (3D) nanostructured films by combining different VAN films (e.g., TiN-CoFe₂, TaN-CoFe₂) in multilayer configurations, demonstrating highly tunable optical properties along with ferromagnetic response. This 3D nanocomposite approach highlights the potential for advanced tunability in metamaterials beyond traditional two-phase VAN designs. The fourth chapter explores the control of stoichiometry and phase composition in TiN-CuO systems. By systematically adjusting oxygen partial pressure during deposition, a gradual transition from metallic to dielectric behavior in these nanocomposite films has been observed. This investigation provides valuable insights into the comprehensive understanding of the interaction processes within hybrid nanocomposites during self-assembly. Overall, this thesis presents diverse methodologies for tuning microstructures and functionalities within nitride-based VAN systems, showing potentials for advanced applications in optics, magnetics, and beyond in metamaterial research.</p>