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.

No description available.

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.

In this dissertation, the design and fabrication of deep-ultraviolet photodetectors were investigated based on gallium oxide and its alloys, through the heterogeneous integration with metallic and other inorganic materials. The crystallographic properties of oxide films grown directly and indirectly on silicon, magnesium oxide, and sapphire are examined, and the challenges that hinder the realization of efficient and reliable deep-ultraviolet photodetectors are described. In recent years, single-crystalline heterojunction photodiodes employing beta-polymorph gallium oxide thin films as the main absorption layers have been studied. However, reports in the literature generally lack a thorough examination of epitaxial growth processes of high-quality single-crystalline beta-polymorph gallium oxide thin films on metals, such as transition metal nitrides. My research was initiated by demonstrating an ultraviolet-C photodetector based on an amorphous aluminum gallium oxide photoconductive layer grown directly on (100)-oriented silicon. The solar-blind photodetector exhibited a peak spectral responsivity of 1.17 A/W. This is the first reported gallium oxide-based photodetector to have been grown and fabricated directly on silicon. The growth of high-quality monoclinic crystals on cubic silicon is a challenging process, which is largely due to the large lattice mismatch that compromises the crystal quality of the oxide layer, and leads to the degradation of device performance. This issue was addressed by growing the material on substrates with metal nitride templates, which resulted in improvements to the oxide crystal quality. Consequently, high optical gain ultraviolet-C photodetectors were fabricated based on a beta-polymorph gallium oxide photoconductive layer grown on magnesium oxide and silicon substrates with titanium nitride templates. The enhanced solar-blind photodetectors exhibited peak spectral responsivity levels as high as 276 A/W. Moreover, thin polymorphic gallium oxide films were grown on c-plane sapphire using pulsed laser deposition for the first time. The stacked thin films, namely epsilon- and beta-polymorph gallium oxide, were sequentially grown under the same conditions. X-ray diffraction measurements and transmission electron microscopy micrographs confirmed a heteroepitaxially grown beta-polymorph gallium oxide on a heterogeneously nucleated epsilon-polymorph gallium oxide polymorphic heterostructure on c-plane sapphire, which had rocking-curve widths of 1.4° (β-Ga2O3 (−603)) and 0.6° (ε-Ga2O3 (006)).

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.

Nanostructured materials are materials consisting of nanoparticulate building blocks on the scale of nanometers (i.e. 10-9 m). Composition, crystallinity and morphology can enhance or even induce new properties of the materials, which are desirable for todays and future technological applications. In this work, we have shown new strategies to synthesise metal oxide and metal nitride nanomaterials. The first part of the work deals with the study of nonaqueous synthesis of metal oxide nanoparticles. We succeeded in the synthesis of In2O3 nanopartcles where we could clearly influence the morphology by varying the type of the precursors and the solvents; of ZnO mesocrystals by using acetonitrile as a solvent; of transition metal oxides (Nb2O5, Ta2O5 and HfO2) that are particularly hard to obtain on the nanoscale and other technologically important materials. Solvothermal synthesis however is not restricted to formation of oxide materials only. In the second part we show examples of nonaqueous, solvothermal reactions of metal nitrides, but the main focus lies on the investigation of the influence of different morphologies of metal oxide precursors on the formation of the metal nitride nanoparticles. In spite of various reports, the number and variety of nanocrystalline metal nitrides is marginally small by comparison to metal oxides; hence preformed metal oxides as precursors for the preparation of metal nitrides are a logical choice. By reacting oxide nanoparticles with cyanamide, urea or melamine, at temperatures of 800 to 900 °C under nitrogen flow metal nitrides could be obtained. We studied in detail the influence of the starting material and realized that size, crystallinity, type of nitrogen source and temperature play the most important role. We have managed to propose and verify a dissolution-recrystallisation model as the formation mechanism. Furthermore we could show that the initial morphology of the oxides could be retained when ammonia flow was used instead. / Nanostrukturierte Materialien sind Materialien, die aus nanopartikulären Baueinheiten in der Größenordnung von Nanonmetern (d.h. 10-9 m) bestehen. Zusammensetzung, Kristallinität und Morphologie können die natürlichen Eigenschaften dieser Materialien verbessern oder zusätzliche Eigenschaften erzeugen, die für heutige und zukünftige Anwendungen und Verfahren wünschenswert sind. In dieser Arbeit präsentieren wir neue Strategien zur Synthese von Nanopartikeln der Metaloxide und Metalnitride. Im einführenden Teil wird die nichtwässrige Synthese von Metaloxidnanopartikeln beschrieben. Uns gelang die Darstellung von In2O3 Nanopartikeln, deren Größe und Form wir durch die Wahl des Prekursors und des Lösemittels deutlich beeinflussen konnten; von ZnO Mesokristallen durch den Einsatz von Acetonitril als Lösemittel; von Übergangsmetalloxiden (Nb2O5, Ta2O5 and HfO2), die besonders schwer im Nanomaßstab zu erhalten sind und von anderen, technisch relevanten Materialien. Die Möglichkeiten der solvothermalen Synthese sind nicht mit der Darstellung von Oxidmaterialen erschöpft. Im zweiten Teil zeigen wir einige Beispiele nichtwässriger, solvothermaler Synthese von Metalnitriden auf; das Hauptaugenmerk liegt aber auf einer Betrachtung der Einflüsse der Morphologie von Metaloxidnanopartikelprekursoren auf die Bildung der Metalnitridnanopartikel. Die Anzahl und Vielfalt bekannter nanokristalliner Metalnitride ist verschwindend klein im Vergleich zu den Metaloxiden, die in der Fachliteratur etabliert sind und demzufolge einen reichen Baukasten an Prekursoren zur Darstellung von Metalnitriden liefern. Durch die Reaktion von Metaloxidnanopartikeln mit Cyanamid, Urea oder Melamine bei Temperaturen von 800 bis 900 °C unter Stickstofffluss konnten Metalnitride erhalten werden. Eine detaillierte Studie der Reaktionsbedingungen und des Reaktionsablaufs zeigte auf, dass Größe und Kristallinität der Metaloxide, die Art der Stickstoffquelle und die Temperatur die entscheidenden Faktoren sind und legte eine Auflösungs-Rekristallisation als Modelmechanismus dieser Art Reaktion nahe. Darüber hinaus konnte gezeigt worden, dass die anfängliche Morphologie des Oxids unter einem Ammoniafluss beibehalten werden konnte.