No description available.

Thesis made openly available per email from author, 8-2-2015.

在本論文中，我們利用簡單的濕化學氧化鋅(ZnO)納米線陣列模板法成功地製備了一系列具有不同化學成份、晶體結構和形貌的準一維鋅-鐵-氧納米結構陣列。 / 垂直排列的ZnO納米線陣列首先生長在不同的襯底上，然后进一步被用作其他納米結構陣列的生長模板。ZnO納米線不僅僅起到骨架定型的作用，最終還可以为后續納米結構提供原料组分。通過控制ZnO和氯化鐵溶液的反應時間，在煅燒后，我們可以製備ZnO/鐵酸鋅(ZnFe₂O₄)納米線纜陣列，以及化學/非化學計量的ZnFe₂O₄、ZnFe₂O₄/α-三氧化二鐵(α-Fe₂O₃)和α-Fe₂O₃納米管陣列。ZnFe₂O₄和α-Fe₂O₃納米管陣列都表現出了對可見光的吸收，它們的帶隙經估算分別是2.3 eV和1.7 eV。 / 通過電子能量損失譜(EELS)，可以得到ZnFe₂O₄納米管陣列的一些細節的結構信息。我們分別研究了兩個不同系列(溫度和化學計量)的ZnFe₂O₄納米管。研究發現，樣品的磁性和它們的晶體結構有著非常緊密的關係。首先，對於溫度系列的樣品，當樣品的燒結溫度從600 °C降到400 °C時，更多的三價鐵離子(Fe³⁺)佔據了尖晶石結構中的A位置(四面體位置)而並非它們本應佔據的平衡B位置(八面體位置)。這種偏離了正常尖晶石結構的情況使得A和B位置上的Fe³⁺的超交換作用增加，進而增加了樣品的阻隔溫度(TB)，磁各向異性常數(K)，3K和300 K下的飽和磁化強度(MS)和3K下的矯頑力(HC)。同時使3K和300K下的MS的比值變小。其次，對於化學計量系列的樣品，通過比較在同一燒結溫度下製備的化學計量和非化學計量的ZnFe₂O₄納米管，我們發現在鐵鋅比大於2的納米管中，Fe³⁺佔據A和B位置的比例和化學計量的樣品是类似的。這些多出的Fe³⁺也會增加超交換作用，從而導致較大的TB, K, MS(3K和300 K)，HC(3K)和較小的MS(3 K)/MS(300 K)比值。最後，作為非化學計量的極端情況，α-Fe₂O₃納米管在小的外加磁場下表現出了典型的Morin相變，在大的外加磁場下出現了場致spin-flop轉變。 / 另一方面，我們發現，當使用羅丹明B(RhB)作為指示劑時，ZnO/ZnFe₂O₄納米線纜陣列表現出了優於纯ZnO和纯ZnFe₂O₄納米管陣列的可見光降解活性，但是它們的降解路徑各不相同。ZnO由於染料敏化機制而具有可見光降解能力，但是其降解活性最差。ZnO/ZnFe₂O₄納米線纜陣列和ZnFe₂O₄納米管陣列的基本降解原理是相同的，那就是，利用有可見光活性的ZnFe₂O₄中的光生電子和空穴所生成的活性自由基降解RhB。但是，ZnO/ZnFe₂O₄納米線纜陣列的降解能力明顯優於ZnFe₂O₄納米管陣列，這是由於ZnO與ZnFe₂O₄之間的II型能帶匹配顯著地促進了光生電子和空穴的分離。 / In the present thesis, several kinds of pseudo-one-dimensional Zn-Fe-O nanostructure arrays with tunable chemical compositions, crystal structures and morphologies are successfully synthesized via a simple wet-chemical ZnO-nanowire-array templating method. / Vertically-aligned ZnO nanowire arrays are firstly fabricated on several different substrates and then serve as templates for other nanostructured arrays growth. The ZnO nanowires not only act as morphology-defining skeleton but also contribute chemically to the final composition of the nanostructures. By controlling the reaction time between ZnO and FeCl₃ solution, ZnO/ZnFe₂O₄ nanocable arrays, stoichiometric ZnFe₂O₄ nanotube arrays, nonstoichiometric ZnFe₂O₄ nanotube arrays, ZnFe₂O₄/α-Fe₂O₃ nanotube arrays and α-Fe₂O₃ nanotube arrays can be synthesized in a controlled manner after calcination. Both ZnFe₂O₄ and α-Fe₂O₃ nanotube arrays exhibit visible light absorption and their bandgap are estimated to be ~2.3 eV and ~1.7 eV, respectively. / The detailed structural information of the ZnFe₂O₄ nanotube arrays are obtained by electron energy loss spectroscopy (EELS). In particular, EELS are carried out for two different series (i.e., temperature and stoichiometric series). The magnetic properties of these samples are found to closely correlate to their structural characteristics. Firstly, with the decrease of the calcination temperature from 600 °C to 400 °C, more Fe³⁺ions occupy A sites (tetrahedral sites in spinel structure) rather than their equilibrium B sites (octahedral sites in spinel structure). The deviation from the normal spinel structure leads to the enhancement of superexchange interactions between Fe³⁺ions in A and B sites, and thus results in an increase in blocking temperature (TB), magnetic anisotropic constant (K), saturation magnetization (MS, at 3 K and 300 K), coercivity (HC, at 3 K) and a decrease in MS(3 K)/MS(300 K) ratios. Secondly, by comparing stoichiometric and nonstoichiometric ZnFe₂O₄ nanotubes calcinated at the same temperature, we found that the nonstoichiometric nanotubes (Fe:Zn > 2) shows similar ratios of Fe³⁺in A and B sites to that of the stoichiometric one. The extra Fe³⁺in the crystal also enhances the superexchange interactions of Fe³⁺, which results in larger TB, K, MS(at 3 K and 300 K) and HC(at 3 K), and smaller MS(3 K)/MS(300 K) ratio. Lastly, α-Fe₂O₃ nanotubes, as an extreme case of the nonstoichiometric sample, show typical Morin-transition characterization under small external field, and field-induced spin-flop transition at large external field. / On the other hand, we found that the visible-light-driven photodegradation activities of ZnO/ZnFe₂O₄ nanocable arrays are superior to those of the ZnO nanowire arrays and ZnFe₂O₄ nanotube arrays using RhB as the probe molecules. All the three nanostructures show degradation of RhB molecules under visible light irradiation, but they take different degradation pathways. The degradation of RhB in the presence of ZnO nanowire arrays is attributed to the dye-sensitized mechanism, and the photodegradation activity is the worst. ZnO/ZnFe₂O₄ nanocable arrays and ZnFe₂O₄ nanotube arrays have the same degradation mechanism, that is, reactive radicals produced by photogenerated electron-hole pairs in the visible-light-active ZnFe₂O₄ are responsible for the photodegradation of RhB. However, the nanocable arrays show much higher degradation capability. This is owing to the type II band alignment between ZnO and ZnFe₂O₄, which greatly promotes the separation of photogenerated electronsand holes in ZnFe₂O₄. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Guo, Xuan = 準一維鋅-鐵-氧納米結構陣列 : 控制製備, 磁學性質以及光催化方面的應用 / 郭璇. / Thesis (Ph.D.) Chinese University of Hong Kong, 2013. / Includes bibliographical references (leaves 107-117). / Abstracts also in Chinese. / Guo, Xuan = Zhun yi wei xin-tie-yang na mi jie gou zhen lie : kong zhi zhi bei, ci xue xing zhi yi ji guang cui hua fang mian de ying yong / Guo Xuan.

Includes bibliographical references (p. 58-63).

Includes bibliographical references (p. 74-79).

No description available.

For nearly a decade, quasi-one-dimensional conductors have been the subject of intensive study. Theoretically, much attention has been devoted to the development of one-dimensional Fermi gas models, some which may be solved exactly, and to the calculation of their response functions. After a review of this theory, a different approach is adopted in the investigation of two models. The dielectric response theory of the three-dimensional Coulomb gas has been applied to an anisotropic system in which the particles interact with an effective one-dimensional long-range potential. Within the framework of the approximation of Singwi, Tosi, Land, and Sjolander, the dielectric properties of the model are examined in order to determine the conditions under which it is unstable with respect to formation of a charge density wave state. It is found that the positive neutralizing background must be polarizable in order for such an instability to occur. The same approximation method, when applied to a one-dimensional fermion gas with a ʃ-function interaction may be compared with the exact solution of Yang. This solution, which exists in the form of coupled integral equations, has been calculated numerically, and, as predicted by the Lieb-Mattis theorem, the ground state is found to be non-magnetic. The approximation of Singwi et al. proves to give better correlation energies than other inexact methods, particularly at higher densities. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Molibdatos têm atraído grande atenção devido à existência de compostos com caráter elétrico unidimensional como conseqüência da presença de cadeias contendo ligações de Mo-O ou Mo-Mo em suas estruturas cristalinas. Com o objetivo de estudar molibdatos com esta característica, amostras policristalinas do sistema K-Mo-O foram preparadas pelo método de reação de difusão no estado sólido e caracterizadas por difratometria de raios x, propriedades elétricas e magnéticas. Estes resultados demonstram a existência de uma nova fase neste sistema com estequiometria KxMoO2-&#948;. Medidas da resistência elétrica em função da temperatura deste material mostram comportamento metálico anômalo que está relacionado a um ordenamento antiferromagnético. Foi observado ainda que a anomalia na resistência elétrica em baixas temperaturas (T < TM) comporta-se segundo uma lei de potência com expoente próximo de 0,5, o que sugere que o comportamento elétrico do KxMoO2-&#948; pode ser descrito por um mecanismo de condutividade unidimensional. / Molybdates have attracted great attention due to the existence of compounds which show one-dimensional electrical behavior as consequence of the channel containing Mo-O or Mo-Mo bonds in their crystalline structure. In order to study molybdates exhibiting onedimensional conductivity, polycrystalline samples of the K-Mo-O system were prepared using the solid state diffusion reaction method and characterized by X-ray powder diffractometry, electrical and magnetic properties. These results demonstrate the existence of a new phase in this system with KxMoO2-&#948; stoichiometry. Electrical resistance as a function of temperature measurements for this compound have shown anomalous metallic behavior which is related to an antiferromagnetic ordering. It has been also observed that the anomaly in the electrical resistance at low temperatures (T < TM) is fitted by power law temperature dependence with an exponent near 0.5 which suggests that the electrical behavior of the KxMoO2-&#948; can be well described by the one-dimensional conducting mechanism.

Molibdatos têm atraído grande atenção devido à existência de compostos com caráter elétrico unidimensional como conseqüência da presença de cadeias contendo ligações de Mo-O ou Mo-Mo em suas estruturas cristalinas. Com o objetivo de estudar molibdatos com esta característica, amostras policristalinas do sistema K-Mo-O foram preparadas pelo método de reação de difusão no estado sólido e caracterizadas por difratometria de raios x, propriedades elétricas e magnéticas. Estes resultados demonstram a existência de uma nova fase neste sistema com estequiometria KxMoO2-&#948;. Medidas da resistência elétrica em função da temperatura deste material mostram comportamento metálico anômalo que está relacionado a um ordenamento antiferromagnético. Foi observado ainda que a anomalia na resistência elétrica em baixas temperaturas (T < TM) comporta-se segundo uma lei de potência com expoente próximo de 0,5, o que sugere que o comportamento elétrico do KxMoO2-&#948; pode ser descrito por um mecanismo de condutividade unidimensional. / Molybdates have attracted great attention due to the existence of compounds which show one-dimensional electrical behavior as consequence of the channel containing Mo-O or Mo-Mo bonds in their crystalline structure. In order to study molybdates exhibiting onedimensional conductivity, polycrystalline samples of the K-Mo-O system were prepared using the solid state diffusion reaction method and characterized by X-ray powder diffractometry, electrical and magnetic properties. These results demonstrate the existence of a new phase in this system with KxMoO2-&#948; stoichiometry. Electrical resistance as a function of temperature measurements for this compound have shown anomalous metallic behavior which is related to an antiferromagnetic ordering. It has been also observed that the anomaly in the electrical resistance at low temperatures (T < TM) is fitted by power law temperature dependence with an exponent near 0.5 which suggests that the electrical behavior of the KxMoO2-&#948; can be well described by the one-dimensional conducting mechanism.

Cette thèse étudie le passage liquide de Luttinger liquide de Fermi au moyen de l'application d'une pression hydrostatique dans le conducteur organique quasi unidimensionnel TMTSF2PF6. La dépendance en température de la résistivité le long the l'axe de moindre conductivité fait apparaître un régime haute température qui s'interpréter par des chaînes de Luttinger alors qu'au dessous de 100K le système évolue vers un régime de chaînes couplés avec une physique de type liquide de Fermi.Ce modèle est aussi confirmé par l'étude de l'effet Hall dans le même composé.