This dissertation describes experimental studies of the superconducting proximity effect in single-crystal Pb, Sn, and Zn nanowires of lengths up to 60 um, with both ends of the nanowires in contact with macroscopic electrodes that are either superconducting (Sn or Pb) or non-superconducting (Au). The Pb, Sn, and Zn nanowires are fabricated using a template-based electrochemical deposition method. Electric contacts to the nanowires are formed in situ during electrochemical growth. This method produces high transparency contacts between a pair of macroscopic electrodes and a single nanowire, circumventing the formation of oxide or other poorly conducting interface layers. Extensive analyses of the structure and the composition of the nanowire samples are presented to demonstrate that (1) the nanowires are single crystalline and (2) the nanowires are clean without any observable mixing of the materials from the electrodes. The nanowires being investigated are significantly longer than the nanowires with which electrode-induced superconductivity was previously investigated by other groups. We have observed that in relatively short (~6 um) Sn and Zn nanowires, robust superconductivity is induced at the superconducting transition temperatures of the electrodes. When Sn and Pb nanowires are in contact with a pair of Au electrodes, superconductivity is suppressed completely. For nanowires of 60 um in length, although the suppression of superconductivity by Au electrodes is only partial, the induced superconductivity at the higher transition temperatures of the electrodes remains full and robust. Therefore, an anomalous superconducting proximity effect has been observed on a length scale which far exceeds the expected length based on the existing theories of the proximity effect. The measured current-voltage characteristic of the nanowires reveals more details such as hysteresis, multiple Andreev reflection, and phase-slip centers. An interesting relation between the proximity effect and the residual-resistance-ratio of the nanowires has also been observed. Possible mechanisms for this proximity effect are discussed based on these experimental observations.

Two types of Mo₂C have been investigated: polycrystalline β-Mo₂C and single crystal α-Mo₂C. The β-Mo₂C material was synthesized via a temperature-programmed method, and then characterized using x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), CO chemisorption, and N₂ physisorption. The catalytic activity of the β-Mo₂C was tested for cumene hydrogenation under high pressure conditions, and the effect of sulfur and oxygen poisons on cumene hydrogenation was also investigated. As a complement to the work done on polycrystalline β-Mo₂C, UHV studies of single crystal α-Mo₂C were undertaken to provide fundamental information about a well-characterized Mo₂C surface. The (0001) surface of α-Mo₂C was investigated using XPS and low energy electron diffraction (LEED). It was found that an ion-bombarded surface could be prepared as either Mo-terminated or C-terminated by choosing either low annealing temperatures (~1000 K) or high annealing temperatures (~1500 K), respectively. CO and O₂ adsorption was also studied on α-Mo₂C (0001) using thermal desorption spectroscopy (TDS), XPS, Auger electron spectroscopy (AES), and LEED. Finally, thiophene adsorption was investigated on α-Mo₂C (0001). / Ph. D.

<p> A number of silicate compounds BaTSiO4 (T = Co, Mg, Zn), Na0.5K0.5GaSiO4 and Sr1-xCaxBeSiO4 (x = 0.0 ~ 0.4) have been characterized by a combination of diffraction techniques. These compounds crystallize with the same (√3XA, C) superstructure of the hexagonal kalsilite (KAlSiO4) structure and belong to the large structural family of stuffed tridymite-derivatives. Their crystal structures have been refined by using powder neutron data (BaTSiO4, T = Mg, Zn), powder X-ray data (Na0.5K0.5 GaSiO4) and single crystal X-ray data (BaCoSiO4 and Sr1-xCaxBeSiO4, x = 0.0 and 0.27). This study shows that these kalsilite-like structures can accommodate cavity and tetrahedral atoms of variable sizes by relatively minor framework distortions and atomic displacements. The formation of the (√3XA, C) superstructure can be correlated with the relative sizes of the tetrahedral and cavity atoms.</p> / Thesis / Master of Science (MSc)

No description available.

No description available.

No description available.

This research provides the original work on the geometry factors selection for single crystal sapphire optical fiber (SCSF) to improve the optical property in sensing applications. Single crystal sapphire fibers were fabricated with a Laser Heated Pedestal Growth (LHPG) system, which was constructed in-house at Virginia Tech. The cost effective, high efficiency and fully operational Laser-heated Pedestal Growth (LHPG) system as well as the fiber fabrication process were also demonstrated in this research. The results indicated the windmill single crystal sapphire optical fiber (SCSF) will readily improve the performance of current fiber optic sensors in the harsh environment and potentially enable those that are limited by the optical property of unclad single crystal sapphire optical fiber (SCSF). / Ph. D.

A temperature-programmed desorption (TPD) study of CO and NH₃ adsorption on MnO(100) with complimentary density functional theory (DFT) simulations was conducted. TPD reveals a primary CO desorption signal at 130 K from MnO(100) in the low coverage limit giving an adsorption energy of -35.6 ±2.1 kJ/mol on terrace sites. PBE+U gives a more reasonable structural result than PBE, and the adsorption energy obtained by PBE+U and DFT-D3 Becke-Johnson gives excellent agreement with the experimentally obtained ΔE_ads for adsorption at Mn²⁺ terrace sites. The analysis of NH₃-TPD traces revealed that adsorption energy on MnO(100) is coverage-dependent. At the low-coverage limit, the adsorption energy on terraces is -58.7±1.0 kJ/mol. A doser results in the formation of a transient NH₃ multilayers that appears in TPD at around 110K. For a terrace site, PBE+U predicts a more realistic surface adsorbate geometry than PBE does, with PBE+U with Tkatchenko-Scheffler method with iterative Hirshfeld partitioning (TSHP) provides the best prediction. DFT simulations of the dehydrogenation elementary step of the ethyl and methyl fragments on α-Cr2O₃(101̅2) were also conducted to complement previous TPD studies of these subjects. On the nearly-stoichiometric surface of α-Cr₂O₃(101̅2), CD₃₋ undergoes dehydrogenation to produce CD₂=CD₂ and CD₄. Previous TPD traces suggest that the α-hydrogen (α-H) elimination of methyl groups on α-Cr₂O₃(101̅2) is the rate-limiting step, and has an activation barrier of 135±2 kJ/mol. DFT simulations showed that PBE gives reasonable prediction of the adsorption sites for CH3- fragments in accordance with XPS spectra, while PBE+U did not. Both PBE and PBE+U failed to predict the correct adsorption sites for CH₂=. When the simulation is set in accordance with the experimentally observed adsorption sites for the carbon species, PBE gives very accurate prediction on the reaction barrier when an adjacent I adatom is present, while PBE+U failed spectacularly. When the simulation is set in accordance with the DFT-predicted adsorption sites, PBE is still able to accurately predict the reaction barrier (<1% to 8.7% error) while PBE+U is less accurate. DFT is also used to complement the previous study of the β-H elimination an ethyl group on the α-Cr₂O₃(101̅2) surface. The DFT simulation shows that absent surface Cl adatoms, PBE predicts an activation barrier of 92.6 kJ/mol, underpredicting the experimental activation barrier by 28.7%, while PBE+U predicts a barrier of 27.0 kJ/mol, under-predicting the experimental barrier by 79.2%. The addition of chlorine on the adjacent cation improved the prediction on barrier by PBE+U marginally, while worsened the prediction by PBE marginally. Grant information: Financial support provided by the U.S. Department of Energy through grant DE-FG02 97ER14751. / Doctor of Philosophy / Nowadays, density functional theory (DFT), a computational approach to chemistry has become increasingly more popular due to it being less computationally expensive than other traditional computational approaches. One major shortcoming of DFT is its inability to explain the electronic interactions within transition metal oxides, where the electronic configuration within one cation is intimately linked to those on adjacent cations. To address this, DFT+U, a variant of DFT, has been developed to better account for these special electronic interactions. However, not enough experimental comparisons have been established to verify the accuracy of DFT and DFT+U. Our lab focuses on providing high quality experimental benchmarks that can be readily compared to by the DFT community. To establish the experimental benchmarks, we use a technique called temperature-programmed desorption (TPD), which focuses on measuring the rate at which gas molecules leave a sample surface populated with a pre-determined amount of gas molecules as the temperature of the surface is raised at constant but slow temperature ramp rate. Through analysis of the results, the adsorption energy can be obtained for a desorption process, or an activation barrier if the desorption is the result of a surface reaction. Some simple calculations involving PBE, a popular functional used in the DFT community, and its variant PBE+U were conducted for comparison purposes. The transition metal oxide surfaces chosen in this study is MnO(100) and of α-Cr₂O3(101̅2), because they both possess the special electronic interactions between their own cations. For adsorption studies, we determined adsorption energies of carbon monoxide (CO), and ammonia (NH₃) on MnO(100) single crystal surface. For CO, TPD study revealed that CO undergoes weak adsorption on the surface, with no dissociation of CO detected. PBE predicts an unreasonable surface adsorption geometry while PBE+U predicts a reasonable one. When coupled with a particular dispersion correction method named DFT-D3 Becke-Johnson, PBE+U predicts a very accurate adsorption energy of CO on MnO(100). TPD shows that NH₃ undergoes a stronger adsorption on MnO(100) with no dissociation of NH₃. Similarly, PBE+U predicted a more reasonable adsorption geometry while PBE did not. Coupled with a dispersion correction named Tkatchenko-Scheffler method with iterative Hirshfeld partitioning (TSHP), PBE+U provides an accurate prediction of adsorption energy. In comparison to previous experimental works based on TPD results, the simple decomposition reactions of an ethyl group and a methyl group were also studied on α-Cr₂O₃(101̅2) surface using DFT. Overall, PBE gave better prediction on the activation barrier than PBE+U did in comparison to experimentally observed barriers.

This thesis deals with the influence of the structure andchemical composition of platinum surfaces on the adsorption ofsome molecules. Three main lines can be distinguished : 1) thecharacterisation of clean/modified surfaces, 2) the adsorptionof some simple atoms and molecules on these surfaces andfinally 3) the adsorption of 2-butenal, a bi-functionalmolecule containing a C=C group conjugated with a C=O group.The main tools used in this work are scanning tunnellingmicroscopy, photoelectron spectroscopy and high-resolutionelectron energy loss spectroscopy, in combination with quantumchemical calculations. The platinum (111)(1× 1) and (110)(1× 2) surfacesare chosen as substrates. Pt(111) is a non-reconstructedclose-packed surface, while the Pt(110) surface is open, due toits missing row (MR) reconstruction, which results in thealternation of ridges, {111} microfacets and valleys. Titaniumgrows on Pt(111) in a Volmer-Weber mode. Pronounced reactionsbetween Pt and Ti are detected already at room temperature asthe Ti2p and Pt4f7/2core-level shifts are characteristic of the Pt3Ti alloy. Carbon segregated on the (110) surfaceappears as extended graphitic regions, which smoothen thesubstrate. Sn deposition at room temperature on Pt(110) resultsin the appearance of small islands, randomly spread over thesurface. The presence of mobile Sn ad-atoms and Pt-Sn-Ptalloyed chains in the valley of the MR reconstruction is alsoevidenced. Annealing the surface results in the rearrangementof the tin in the surface layer, together with a globalshortening of the terraces in the [110]direction and anincreased density of (1× n) (n&gt;2) defects. The adsorption of atomic hydrogen and oxygen has beenperformed on the Pt(111)( √ 3x √3)R30º surfacealloy and on Pt(110)(1× 2) respectively. The adsorptionsite of these atoms is changed as compared to the onedetermined on the clean Pt(111) surface (i.e. fcc hollow site): H is adsorbed on-top site on the (√3× √3)surface alloy, and O sits on the ridge of the missing rowreconstruction in bridge site. Carbon monoxide adsorption wasperformed on Pt(111) and Pt(110)(1× 2) surfaces, modifiedor not by tin. On the unmodified (111) surface, CO adsorbsfirst in top site, thenin bridge. Changing the geometry of thesurface to the (110) results in the vanishing of the bridgesite population. Modifying these surfaces by Sn does not changethe CO adsorption site. Finally, 2-butenal (CH3-CH=CH-CH=O) has been adsorbed at 100 K on thePt(111) surface and the Sn/Pt(111) surface alloys. On theunmodified Pt surface, comparison with results obtained forpropene (CH2=CH-CH3) adsorption evidences the involvement of the C=Cgroup of the 2-butenal molecule in the bonding to the Ptsurface. The carbonyl group is also suggested to take part inthe bonding, through a σccη1(O) configuration. This carbonyl group bondingdisappears when Sn is alloyed to the surface, and the formationof a new physisorbed phase is detected. / <p>NR 20140805</p>

A presente dissertação consta de quatro capítulos, sendo que nos dois primeiros apresentamos alguns tópicos dos métodos diretos para determinação de estruturas e nos dois últimos, o desenvolvimento experimental e os resultados finais obtidos na determinação da estrutura cristalina e molecular de um produto natural extraído de E. Nitens, o (2R, 3S) - 2-hidroxi-3-(2&#8217 - hidroxi-isopropil) - 5-hidroximetil-8-metoximetil-1-ceto-1, 2, 3, 4-tetrahidronaftaleno, conhecido pelo nome vulgar Emotina B. A Emotina B, C16H22O5, cristaliza no sistema triclínico, grupo espacial P1. Os parâmetros da cela unitária encontrados foram: a=7,474(2), b=8,922(5), c=12,405(4)&#197, &#945=87,68(4), &#946=78,96(3), &#947=67,40(4)&#176; V=715,042&#1973; dcalc=1,37g/cm3; Z=2 moléculas por cela unitária. Foram coletadas em 1667 reflexões únicas, utilizando o difratômetro automático CAD-4, com radiação monocromatizada de MoK&#945, das quais foram mantidas 1010 reflexões, consideradas observadas segundo o critério I&#62 2&#948(I). A estrutura foi resolvida por métodos diretos (MULTAN-80) e por síntese de Fourier-diferenças sucessivas e refinadas por mínimos quadrados (SHELX-76) até um índice de discordância, R=0,054 para apenas as reflexões observadas e 0,067 para todas as reflexões.As moléculas são aproximadamente planas com variações conformacionais significativas apenas na parte alifática. Relacionam-se através de um pseudo-centro de inversão parcial localizado em (0,0846; 0,0622; 0,4174). Os esquemas de ligação hidrogênio são diferentes nas moléculas A e B e consistem de ligações intra e intermoleculares. / This dissertation consists of four chapters. Chapters 1 and 2 present some theoretical aspects of direct methods for crystal structure determination. Chapters 3 and 4 present the description of the experimental work and the crystal structure determination of a natural product obtained from E. Nitens, the (2R, 3S)-2-hydroxy-3-(2&#8217-hydroxy-isopropyl)-5-hydroximethyl-8-methoxymethyl-1-ceto-1, 2, 3, 4-tetrahydronapghtalen (Emmotin - B). Emmotin - B, C16H22O5, crystallizes in the triclinic system, space group P1. Cell dimensioned are: a=7,474(2), b=8,922(5), c=12,405(4)&#197, &#945=87,68(4), &#946=78,96(3), &#947=67,40(4)&#176; V=715,042&#1973; dcalc=1,37g/cm3; Z=2 molecules/unit cell. The intensities of 1667 unique reflexions were collected using a CAD-4 automatic diffractometer with monochromated MoK&#945 radiations of which only 1010 with I&#62 2&#948(I) were considered observed. The structure was solved by direct methods (MULTAN-80) and successive applications of difference-Fourier calculations. It was refined by least square methods (SHELX-76). The final agreement index was R=0.054, considering only the observed reflexions and 0.067, considering all reflexions. The two independent molecules are almost identical showing discrepancies only in conformation of the aliphatic side chains. They are mutually related by a partial pseudo center of symmetry located at (0.0846, 0.0622, 0.4174). The hydrogen bonding schemes are different in both A and B molecules and consist of intra and intermolecular bonds.