京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19311号 / 工博第4108号 / 新制||工||1633(附属図書館) / 32313 / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 川上 養一, 教授 髙岡 義寛, 教授 藤田 静雄 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Master of Science / Department of Chemical Engineering / James H. Edgar / Pure boron exists in two main polymorphs, the common β-rhombohedral boron and the relatively rare α-rhombohedral boron. α-rhombohedral boron (α-B) possesses several extraordinary properties: self-healing from radiation damage and a high hole mobility. In addition, the [superscript]10B isotope has a large thermal neutron capture cross section. Such properties make it an excellent candidate for novel electronic device, such as direct energy conversion devices (alphacells and betacells) and neutron detectors. However, research on the properties and applications of α-B has been limited due to the difficulty to produce high quality α-B crystals of significant size. The preparation of α-rhombohedral boron is challenging for several reasons: first, α-rhombohedral boron has a low thermodynamic stability; it is only stable below 1100°C, at higher temperature β-rhombohedral boron is the stable polymorph. In addition, at elevated temperatures, boron is highly reactive, which make it is difficult to produce pure boron crystals. The primary goal of this research was to produce high quality α-B crystals of significant size. The main focus of this study was to explore the feasibility of producing α-B from a copper flux. Copper is a promising solvent for α-B crystal growth: the eutectic temperature of copper-boron is low, 996°C, and the phase diagram of copper-boron is relatively simple, and there are not many intermediate boride-copper compounds. In addition, copper is easily removed from crystals by etching with concentrated nitric acid. Last but not least, copper is less expensive than other metal solvents such as platinum. Boron crystal growth from a platinum solvent and vapor-liquid-solid growth by chemical vapor deposition were also performed for comparison. A series of crystals were grown over a range of initial boron concentrations (9.9 to 27.7 mole %) and cooling rates. Small irregular-shaped black crystals (>100μm) and well-faceted red crystals in various shapes, as large as 500 microns were produced. The crystals were characterized by optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, x-ray diffraction analysis, and Raman spectroscopy. The correlation between experiment results and experimental parameters (source materials, the purity of growth atmosphere, and crucible materials, etc.) are reported. Suggestions about further investigation for α-B crystal growth are proposed.

The main objective of nanotechnology is to build self-powered nanosystems that are ultrasmall in size, exhibit super sensitivity, extraordinary multi functionality, and extremely low power consumption. As we all know that 21st century has brought two most important challenges for us. One is energy shortage and the other is global warming. Now to overcome these challenges, it is highly desirable to develop nanotechnology that harvests energy from the environment to fabricate self-power and low-carbon nanodevices. Therefore a self-power nanosystem that harvests its operating energy from the environment is an attractive proposition. This is also feasible for nanodevices owing to their extremely low power consumption. One advantageous approach towards harvesting energy from the environment is the utilization of semiconducting piezoelectric materials, which facilitate the conversion of mechanical energy into electrical energy. Among many piezoelectric materials ZnO has the rare attribute of possessing both piezoelectric and semiconducting properties. But most applications of ZnO utilize either the semiconducting or piezoelectric property, and now it’s time to fully employ the coupled semiconducting-piezoelectric properties to form  the basis for electromechanically coupled nanodevices. Since wurtzite zinc oxide (ZnO) is structurally noncentral symmetric and has the highest piezoelectric tensor among tetrahedrally bonded semiconductors, therefore it becomes a promising candidate for energy harvesting applications. ZnO is relatively biosafe and biocompatible as well, so it can be used at large scale without any harm to the living environment. The synthesis of another transition metal oxide known as Co3O4 is also important due to its potential usage in the material science, physics and chemistry fields. Co3O4 has been studied extensively due to low cost, low toxicity, the most naturally abundant, high surface area, good redox, easily tunable surface and structural properties. These significant properties enable Co3O4 fruitful for developing variety of nanodevices. Co3O4 nanostructures have been focused considerably in the past decade due to their high electro-chemical performance, which is essential for developing highly sensitive sensor devices. I started my work with the synthesis of ZnO nanostructures with a focus to improve the amount of harvested energy by utilizing oxygen plasma treatment. Then I grow ZnO nanorods on different flexible substrates, in order to observe the effect of substrate on the amount of harvested energy. After that I worked on understanding the mechanism and causes of variation in the resulting output potential generated from ZnO nanorods. My next target belongs to an innovative approach in which AFM tip decorated with ZnO nanorods was utilized to improve the output energy. Then I investigated Co3O4 nanostructures though the effect of anions and utilized one of the nanostructure to develop a fast and reliable pH sensor. Finally to take the advantage of higher degree of redox chemistry of NiCo0O4 compared to the single phase of nickel oxide and cobalt oxide, a sensitive glucose sensor is developed by immobilizing glucose oxidase. However, there were problems with the mechanical robustness, lifetime, output stability and environmental adaptability of such devices, therefore more work is going on to find out new ways and means in order to improve the performance of fabricated nanogenerators and sensors.