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A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy. Johannesburg, 2014. / The synthesis of good crystalline nanomaterials by green methods is one of the means to preventing global warming. Application of microwave thermal methods and the use of green solvents to synthesize nanomaterials contribute to this goal. Further, the low cost synthesis of nanomaterials contributes to their ease of availability in the market at affordable costs. In this study, different NiSx phases and GaN nanomaterials were obtained by microwave-assisted solution phase synthesis. NiS2, GaN, -NiS, ( & ) NiS and Ni3S2 phases were obtained by using different reagents and applying different reaction parameters. These materials were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and photoluminescence, to evaluate their crystalline phases, morphologies, particle size distribution and optical properties respectively. Hierarchical structures of cubic phase NiS2 and spherical HDA capped nanostructures were synthesized by a MW-assisted hydrothermal technique. The product phase purity was optimized and the effect of precursor concentration and capping agents were discussed. Further, optical properties of bare and HDA capped NiS2 materials are reported. Detailed analysis of the PL properties shown by these materials in the UV-vis range has been given by considering their calculated DOE energy band diagrams. Single phase -NiS nanostructures with uniformly distributed hierarchical networks were synthesized for the first time in this study. The materials were evaluated for thermal stability under an oxidative environment and at temperatures between 150 oC and 600 oC. NiS materials showed stability at 300 oC and NiO formation was observed from 350 oC to 600 oC. The annealing effect on the crystalline size and IR absorption of the annealed samples is reported by XRD and FTIR studied. The EPR properties of the annealed materials were studied and compared to the oxidized materials. The transition temperature of the -NiS was further confirmed by performing electrical measurements on the as-synthesized material. Further, hydrostatic pressure sensing properties, ethanol and tomato VOCs sensing properties of the -NiS/PVA composite based devices were carried out and the results are reported. The ethanol gas sensing properties of the devices prepared showed the highest response when compared to hydrostatic pressure sensing and tomato VOCs gas sensing. UV-blue emitting GaN nanostructures were obtained for the first time by a onestep MW-assisted solvothermal technique. Sensor devices based on the hexagonal wurtzite structures obtained and their PVA composites (GaN/PVA) were prepared with different GaN NPs concentrations. A very high response to hydrostatic pressure was achieved for the devices prepared. The sensitivity of a GaN/PVA composite based device was analyzed for tomato VOCs detection and the results are presented. Binary phase ( & ) synthesis of NiS materials is commonly reported for the synthesis of Ni:S ratio of 1:1 stoichiometry. This is due to the formation of both phases at temperatures lower than 200 oC. Here, the effect of NaOH and the S source was investigated as reaction parameters. It was found that the concentration of OH- ions in solution plays a huge role in the formation of binary phase NiS as well as its morphology distribution in the nanostructures. Hexagonal nanoplatelets, nanorods and nanorodbased flower-like structures were obtained when different reaction parameters were varied in the presence of NaOH. Further, the solubility of different S precursors in the solvent used was studied and found to affect both the morphology and crystalline phase distribution of the products. Preliminary work on the synthesis of Ni3S2 and Se and Te doped Ni3S2 is presented in the last chapter. The crystallite sizes of the materials were determined by use of the Scherrer equation and the elemental composition was confirmed by EDS analysis. The relative humidity gas sensing of the samples materials was determined and sensitivity response of the material to humidity was obtained for the first time.

稀磁半導體是通過摻雜磁性過渡金屬進入半導體基體而形成的。他們是一類部分磁性離子占居半導體本身離子態的新型半導體，擁有很多優異的特性，因而在工業和商業等方面具有廣泛的應用和巨大的潛能。磁性離子的出現使得他們擁有了有別于普通半導體材料的獨特磁性特性。室溫鐵磁性作爲特性之一已經在很多稀磁半導體中觀察到。然而，稀磁半導體中鐵磁序的來源一直備受爭議，因爲磁性摻雜劑本身擁有磁性性能。因此很難確定得到的産物確實是半導體與磁性過渡金屬摻雜劑的固溶體，還是半導體基包裹磁性過渡金屬團簇、沈析物或者二相，這些包裹的成分呈現了磁性特性。另外，絶大部分磁性過渡金屬在半導體基中溶解率都比較低。因此，人們越來越感興趣通過摻雜非過渡金屬摻雜劑來開發磁性系統。在這類系統中，可以排除鐵磁信號來源于摻雜劑，並且由于沒有鐵磁相互吸引力，可以實現較高的摻雜率。近來這樣的磁性系統已經被發展，例如碳摻雜的氧化鋅，碳摻雜的氧化钛等。 / 受這些想法的啓發，我們選擇非磁性元素鋁、鎢作爲摻雜劑，並將他們摻雜進入B₄C和SiC半導體基體中。利用高溫固相燒結法，以碳，硼和矽，鋁和鎢粉末作爲起始材料在2200ºC合成了鋁摻雜的B₄C，鋁摻雜的SiC和鎢摻雜的B₄C化合物。在鋁摻雜的B₄C體系中，鋁的摻雜沒有改變晶體結構。所有的摻雜樣品均保持與未摻雜樣品相同的晶體結構，即B₄C（ICDD-PDF：75-0424，空間群R-3m）。並且所有的結果，包括X射線衍射圖，拉曼譜，X射線光電子譜，都表明C-B-C鏈上的部分碳原子被摻雜的鋁原子所取代。替代導致這個系統中隨著鋁含量的增加而表現出鐵磁特性。更重要的是，在這個系統中觀察到了典型的自旋玻璃行爲。而在鋁摻雜的SiC系統中，所有的樣品，包括未摻雜樣品和摻雜樣品，都是包含了6H，4H和15R多型的混合物。相比之下，在這個系統中鐵磁序的變化與鋁摻雜的B₄C系統中的也有所不同。這表明在這兩種半導體中由于摻雜效應誘導的鐵磁序機制是不一樣的。爲了進一步的調查，嘗試以錳作爲摻雜劑。錳摻雜的SiC結果證實在SiC系統中鐵磁特征確實不依賴于摻雜劑。這兩個SiC系統證明了在摻雜的SiC系統中，鐵磁行爲與混合物中的一種SiC多型(15R-SiC)而非摻雜劑有關。鎢摻雜的B₄C是一個有趣的體系。 / 輕摻雜樣品呈現了鐵磁特性，但隨著鎢含量的增加鐵磁序減弱直至消失。在高摻雜的樣品中産生了B₂W₅二相，實驗證明它是沒有鐵磁性能的。作為比較，在相同燒結條件下合成了鎢和鋁共摻雜的B₄C。發現鋁和鎢各自發揮摻雜效應并促成共同的結果。一個虛弱的自旋玻璃特性在這個共摻雜的體系中被觀察到。 / 總之，四個不同的摻雜體系被合成。他們都表現出磁性特性的變化。其中鋁摻雜的B₄C體系還表現出自旋玻璃特性。這項工作證明非磁性摻雜劑確實能影響半導體的磁性特性，並促使摻雜的半導體表現出鐵磁性能。這使得這些摻雜的半導體表現的就像稀磁半導體材料一樣。這項工作提供了理想的系統來研究和闡明非過渡金屬摻雜的寬禁帶半導體中自旋序的來源問題。 / Diluted magnetic semiconductors (DMSs) are formed by doping magnetic transition metals (TMs) into semiconductor matrices. They are a type of new semiconductors with fraction of magnetic ions on the sites of semiconductor constituent ions. DMSs have some excellent properties which enhance their potentials for using in a wide range of industrial and commercial applications. The presence of substituted magnetic ions results in unique magnetic properties which distinguish them from the ordinary semiconductor materials. Room temperature ferromagnetic (FM) ordering，as one of the unique properties, has been observed in many DMSs. However, the origin of FM ordering in the DMSs is still controversial due to the presence of intrinsic magnetic properties possessed by the magnetic TM dopants. It is difficult to determinate whether the obtained products is indeed a solid solution of the semiconductor matrix and the magnetic TM dopant or it remains as semiconductor matrix engulfing a magnetic TM clusters, precipitates, or second phases that are responsible for the observed magnetic properties. Moreover, most of the magnetic TMs have extremely low solubility in the semiconductor matrix. Therefore, an increasing interest is provoked on developing magnetic systems by doping non-TM dopants. In this new type of system, the origin of the FM signal from the dopant can be eliminated, and higher doping ratio in the system can be expected due to no magnetic attraction. Recently, such magnetic systems have been obtained, for examples C-doped ZnO and C-doped TiO₂. / Inspired by these ideas, we had selected the non-magnetic element Al and W as the dopants, and introducing them to the semiconductor matrices, B₄C and SiC. The Al-doped B₄C, Al-doped SiC and W-doped B₄C compounds were synthesized by high temperature solid-state sintering of carbon, boron/silicon, and aluminum/tungsten powders at 2200ºC. In the Al-doped B₄C system, doping of Al had not altered the crystal structure. All of the doped samples retained the same structure as the un-doped sample, which corresponded to the B₄C (ICDD-PDF:75-0424, space group R-3m). All results, including results of XRD, Raman and XPS, indicated that the parts of the carbon atoms on the C-B-C chains were replaced by some of the doped Al atoms. The result of this replacement was that the system showed FM feature with increasing of the Al content. More important, a typical spin-glass behavior was observed in this system. While in the Al-doped SiC system, all of the samples, including un-doped and doped samples, were mixtures containing 6H-SiC, 4H-SiC and 15R-SiC polytype. By comparison, the changes in the FM ordering in this system were also different from those in the Al-B₄C system. This indicated that the induced mechanisms of FM ordering due to the doping effects on the two semiconductors were not the same. To further investigation, attempts were made by using Mn as dopants. The result of Mn-doped SiC confirmed that the FM feature in SiC systems indeed did not depend on with the dopants. The two doped SiC systems demonstrated that the FM behavior might be related to one type of SiC polytype (15R-SiC) in the mixture rather than dopants. / The W-doped B₄C system was an interesting one. The lightly doped sample showed FM feature, while the FM ordering became weak and disappeared with the increase of W content. A second phase B₂W₅ was produced in the highly doped samples. It was demonstrated that B₂W₅ had no FM feature. To compare, (W, Al) co-doped B₄C were also synthesized under the same heating condition. It was found that the doping effects of Al and of W played its own individual role, and facilitated a common outcome. A weak spin-glass feature was observed in the co-doped system but not in the W-doped system. / In summary, four systems were fabricated. All of them showed changes in magnetic behaviors. The Al-doped B₄C system showed spin-glass features for the first time. The results of this work demonstrated that the non-magnetic dopants indeed could affect the magnetic properties of semiconductors, and urged the doped-semiconductors to exhibit FM features. This made the doped-semiconductors behaved as DMS materials even there was no 3d electron in the whole of systems. This work provides an ideal system to illustrate the origin of spin order in non-TM doped wide-gap semiconductors. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Zhang, Caihong = 摻雜半導體中的誘導磁性特性 / 張彩虹. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2013. / Includes bibliographical references. / Abstracts also in Chinese. / Zhang, Caihong = Shan za ban dao ti zhong de you dao ci xing te xing / Zhang Caihong. / Abstract --- p.i / 摘要 --- p.iv / Acknowledgments --- p.vi / Table of contents --- p.vii / List of figure captions --- p.x / List of Table captions --- p.xiii / Chapter Chapter 1 --- Introduction --- p.1 / Chapter 1.1 --- Diluted magnetic semiconductors --- p.1 / Chapter 1.1.1 --- Origin --- p.1 / Chapter 1.1.2 --- Background --- p.2 / Chapter 1.1.3 --- Current development --- p.3 / Chapter 1.1.4 --- Present challenges --- p.5 / Chapter 1.2 --- Objectives of this work --- p.7 / References --- p.9 / Chapter Chapter 2 --- Magnetism and Semiconductors --- p.12 / Chapter 2.1 --- Magnetism --- p.12 / Chapter 2.1.1 --- Types of magnetism --- p.12 / Chapter 2.1.2 --- Spin glass --- p.15 / Chapter 2.1 --- B₄C --- p.18 / Chapter 2.2.1 --- Introduction --- p.18 / Chapter 2.2.2 --- Properties and applications --- p.19 / Chapter 2.2.3 --- Synthesis and phase diagram --- p.20 / Chapter 2.2.4 --- Structure and characterization --- p.21 / Chapter 2.2 --- SiC --- p.25 / Chapter 2.3.1 --- Properties and applications --- p.25 / Chapter 2.3.2 --- SiC polytypes --- p.26 / Chapter 2.3.3 --- SiC-base DMSs --- p.28 / References --- p.30 / Chapter Chapter 3 --- Methodology and Instrumentation --- p.34 / Chapter 3.1 --- Sample preparation --- p.34 / Chapter 3.2 --- Sample characterization --- p.36 / Chapter 3.3 --- Instrumentation --- p.37 / Chapter 3.3.1 --- High temperature furnace --- p.37 / Chapter 3.3.2 --- X-ray diffraction --- p.38 / Chapter 3.3.3 --- Raman spectroscopy --- p.39 / Chapter 3.3.4 --- X-ray photoelectron spectroscopy --- p.40 / Chapter 3.3.5 --- Scanning electron microscopy and Energy-dispersive X-Ray spectroscopy --- p.41 / Chapter 3.3.6 --- Transmission electron microscopy --- p.42 / Chapter 3.3.7 --- Physical properties measuring system --- p.43 / Chapter 3.3.7.1 --- Vibrating sample magnetometer --- p.43 / Chapter 3.3.7.2 --- AC measurement system --- p.44 / References --- p.46 / Chapter Chapter 4 --- Al-doped B₄C System --- p.47 / Chapter 4.1 --- Introduction --- p.47 / Chapter 4.2 --- Experiments --- p.48 / Chapter 4.3 --- Results and discussion --- p.49 / Chapter 4.3.1 --- Confirmation of the un-doped B₄C sample --- p.49 / Chapter 4.3.2 --- Confirmation of Al into the B₄C matrix --- p.51 / Chapter 4.3.3 --- Evaluation of magnetic properties --- p.59 / Chapter 4.3.4 --- Magnetism in the Al-B₄C --- p.62 / Chapter 4.4 --- Conclusions --- p.65 / References --- p.66 / Chapter Chapter 5 --- Al-Doped SiC System --- p.68 / Chapter 5.1 --- Introduction --- p.68 / Chapter 5.2 --- Experiment I --- p.69 / Chapter 5.2.1 --- Experimental condition 1 --- p.69 / Chapter 5.2.2 --- Results and discussion --- p.70 / Chapter 5.2.2.1 --- X-ray diffraction patterns --- p.70 / Chapter 5.2.2.2 --- Raman spectra --- p.72 / Chapter 5.2.2.3 --- Transmission electron microscopy images --- p.77 / Chapter 5.2.2.4 --- DC magnetization --- p.79 / Chapter 5.2.2.5 --- AC susceptibilities --- p.80 / Chapter 5.3 --- Experiment II --- p.81 / Chapter 5.3.1 --- Experimental condition 2 --- p.81 / Chapter 5.3.2 --- Results and discussion --- p.82 / Chapter 5.4 --- Experiment III --- p.88 / Chapter 5.4.1 --- Experimental condition 3 --- p.88 / Chapter 5.4.2 --- Results and discussion --- p.89 / Chapter 5.5 --- Mn-doped SiC --- p.91 / Chapter 5.5.1 --- Experimental condition --- p.91 / Chapter 5.5.2 --- Results and discussion --- p.92 / Chapter 5.6 --- Summary --- p.93 / Chapter 5.7 --- Conclusions --- p.94 / References --- p.95 / Chapter Chapter 6 --- W-doped and (W, Al)-codoped B₄C System --- p.98 / Chapter 6.1 --- Introduction --- p.98 / Chapter 6.2 --- Experiments --- p.99 / Chapter 6.3 --- Results and discussion --- p.101 / Chapter 6.3.1 --- X-ray diffraction patterns --- p.101 / Chapter 6.3.2 --- Raman spectra --- p.104 / Chapter 6.3.3 --- Scanning electron microscopy images --- p.106 / Chapter 6.3.4 --- DC magnetization --- p.108 / Chapter 6.3.5 --- AC susceptibilities --- p.111 / Chapter 6.4 --- Conclusions --- p.113 / References --- p.114 / Chapter Chapter 7 --- Summary and Suggestions for Future Work --- p.115 / Chapter 7.1 --- Summary --- p.115 / Chapter 7.2 --- Suggestions --- p.117 / References --- p.120

Liang, Zhixiong. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2011. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

Tang, Qin. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2011. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

by Yu Chung Keung. / Chinese title: / Bibliography: leaves 158-164 / Thesis (M.Phil.) -- Chinese University of Hong Kong, 1983

by Yip Kam-wa. / Parallel title in Chinese characters. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1991. / Bibliography: leaf 64. / List of Figures --- p.iii / Acknowledgement --- p.vi / Abstract --- p.vii / Chapter 1. --- Introduction --- p.1 / Chapter 2. --- Effective-mass Approximation for Lattice-matched Heterostructures --- p.4 / Chapter 2.1 --- Introduction --- p.4 / Chapter 2.2 --- Formulation --- p.5 / Chapter 2.3 --- Model --- p.10 / Chapter 2.4 --- Exact Solution and Effective-mass Approximation for the Reflection Coefficient of 1-D Heterojunction --- p.15 / Chapter 2.5 --- Comparison --- p.20 / Chapter 2.6 --- Kinetic Energy Operator Ordering --- p.26 / Chapter 2.7 --- Conclusion --- p.30 / Chapter 3. --- Effective Hamiltonian for Semiconductor Heterostructures in a Uniform Magnetic Field --- p.33 / Chapter 3.1 --- Introduction --- p.33 / Chapter 3.2 --- Bloch Electron in a Uniform Magnetic Field --- p.34 / Chapter 3.3 --- Effective Hamiltonian for Heterostructures in a Uniform Magnetic Field --- p.40 / Chapter 3.4 --- Conclusion --- p.42 / Chapter 4. --- Effective-mass Approximation for a 1-D Strained Heterostructure --- p.43 / Chapter 4.1 --- Introduction --- p.43 / Chapter 4.2 --- Formulation --- p.44 / Chapter 4.3 --- Model --- p.50 / Chapter 4.4 --- Comparison --- p.54 / Chapter 4.5 --- Conclusion --- p.62 / Chapter 5. --- Conclusion --- p.63 / References --- p.64 / Appendix A --- p.65 / Appendix B --- p.67 / Appendix C --- p.68

H.Y. Tsoi. / Thesis (M.Phil.)--Chinese University of Hong Kong. / Bibliography: leaf [3]

Since the discovery of titanium dioxide's (TiO2) capability for water-splitting and photocatalytic degradation of organic compounds, semiconductor photocatalysis has received great attention and promises an environmentally clean and sustainable solution by solar hydrogen production and waste water treatment.1,2 In this thesis, the photocatalytic performance of two different photocatalyst systems based on Pd nanoparticle decorated n-type TiO2 nanorods and Ag incorporated ptype BiFeO3 thin films were investigated for solar hydrogen and oxygen production and photodecolourisation of a common textile dye, Rhodamine B. High surface area TiO2 nanorods were grown on glass fibre substrates by a hydrothermal method to produce a mechanically robust photocatalytic filter. Metallic Pd nanoparticles were deposited onto TiO2 nanorods via a photochemical method. It was found that the hybrid Pd/TiO2 catalyst system showed higher photoactivity with a doubled kinetic rate for the photodecolourisation of RhB. Full decolourisation has been achieved in 180 minutes with as-grown TiO2 nanorods whereas this time was reduced to only 90 minutes for the Pd/TiO2 hybrid catalyst. This enhancement was associated with the localised surface plasmon resonance (LSPR) effect due to the interaction of Pd with visible light and the electron scavenging role of Pd for efficient charge separation. The same hybrid Pd/TiO2 photocatalyst system was then developed on FTO coated glass substrates so that the photoelectrochemical experiments can be carried out using a potentiostat. Mott-Schottky curves demonstrated a positive shift in flat band potential and an increased charge carrier density after Pd deposition. The facilitated charge transfer at the interface of Pd and TiO2 was shown by EIS data with a smaller arc size for Pd/TiO2 in Nyquist plots. The photoelectrochemical performance of the bare TiO2 and hybrid Pd/TiO2 samples were compared through the photoelectrocatalysis of Rhodamine B (RhB) and solar hydrogen production in different electrolyte solutions at various applied voltage values. A higher amount of hydrogen by Pd/TiO2 was photogenerated in methanol solution whereas bare TiO2 produced a higher amount of hydrogen in 0.01M Na2SO4 and pure deionised water under the same conditions. The results were discussed by proposing possible reaction mechanisms with an emphasis on the charge trapping role of Pd nanoparticles. P-type BiFeO3 (BFO) thin films were deposited on large scale FTO coated glass substrates by a sol-gel method. A photocurrent density of -0.004mA/cm2 was achieved at 0V vs NHE under AM1.5 G illumination and 1.2μmol of O2 was produced in 2h at an external bias of -0.5V vs Ag/AgCl. These values were significantly increased upon the incorporation of Ag into the BFO matrix. Ag was incorporated into the BiFeO3 matrix at different concentrations as metallic Ag structures and Ag nanowires. The enhancement by Ag modification was attributed to enhanced light absorption due to light scattering effect and efficient charge separation by Ag as they act as electron sinks. These explanations were supported by shifts in flat band and onset potentials after Ag modification in detailed measurements of Mott-Schottky plots and j-v curves.