Nanowhiskers semicondutores de compostos III-V apresentam grande potencial para aplicações tecnológicas. Controlando as condições de crescimento, tais como temperatura e diâmetro, é possível alternar entre as fases cristalinas zincblend e wurtzita, dando origem ao politipismo. Esse efeito tem grande influência nas propriedades eletrônicas e óticas do sistema, gerando novas formas de confinamento para os portadores. Um modelo teórico capaz de descrever com exatidão as propriedades eletrônicas e óticas presentes nessas nanoestruturas politípicas pode ser utilizado para o estudo e desenvolvimento de novos tipos de nanodispositivos. Neste trabalho, apresento a construção do Hamiltoniano k.p no ponto Γ para as estruturas cristalinas zincblend e wurtzita baseada no formalismo da teoria de grupos. Utilizando o grupo de simetria do ponto Γ, é possível obter as representações irredutíveis das bandas de energia, partindo de orbitais atômicos e do número de átomos na célula primitiva unitária. Além disso, as operações de simetria do grupo são utilizadas para calcular os elementos de matriz não nulos e independentes do Hamiltoniano k.p. O estudo da simetria dos estados de base pertencentes às representações irredutíveis das bandas de energia, juntamente com a aproximação da função envelope, permitiu a formulação de um modelo polítipico wurtzita/zincblend para cálculo da estrutura de bandas em nanowhiskers. Embora o interesse seja em super-redes politípicas, o modelo proposto foi aplicado a um poço quântico de InP com o intuito de extrair a física envolvida na interface wurtzita/zincblend. / Semiconductor nanowhiskers made of III-V compounds exhibit great potential for technological applications. Controlling the growth conditions, such as temperature and diameter, it is possible to alternate between zincblend and wurtzite crystalline phases, giving origin to the polytypism. This effect has great influence in the electronic and optical properties of the system, generating new forms of confinement to the carriers. A theoretical model capable to accurately describe electronic and optical properties in these polytypical nanostructures can be used to study and develop new kinds of nanodevices. In this study, I present the development of the k.p Hamiltonian in the Γ point for the zincblend and wurtzite crystal structures based on the formalism of group theory. Using the symmetry group of the Γ point, it is possible to obtain the irreducible representations of the energy bands, considering the atomic orbitals and the number of atoms in the primitive unit cell. Also, the group symmetry operations are used to calculate the non-zero and independent matrix elements of the k.p Hamiltonian. The study of the basis states symmetry of irreducible representations in the energy bands, alongside with the envelope function approximation, allowed the formulation of a wurtzite/zincblend polytypical model to calculate the electronic band structure of nanowhiskers. Although the interest is in polytypical superlattices, the proposed model was applied to a single quantum well of InP to extract the physics of the wurtzite/zincblend interface.

Nanowhiskers semicondutores de compostos III-V apresentam grande potencial para aplicações tecnológicas. Controlando as condições de crescimento, tais como temperatura e diâmetro, é possível alternar entre as fases cristalinas zincblend e wurtzita, dando origem ao politipismo. Esse efeito tem grande influência nas propriedades eletrônicas e óticas do sistema, gerando novas formas de confinamento para os portadores. Um modelo teórico capaz de descrever com exatidão as propriedades eletrônicas e óticas presentes nessas nanoestruturas politípicas pode ser utilizado para o estudo e desenvolvimento de novos tipos de nanodispositivos. Neste trabalho, apresento a construção do Hamiltoniano k.p no ponto Γ para as estruturas cristalinas zincblend e wurtzita baseada no formalismo da teoria de grupos. Utilizando o grupo de simetria do ponto Γ, é possível obter as representações irredutíveis das bandas de energia, partindo de orbitais atômicos e do número de átomos na célula primitiva unitária. Além disso, as operações de simetria do grupo são utilizadas para calcular os elementos de matriz não nulos e independentes do Hamiltoniano k.p. O estudo da simetria dos estados de base pertencentes às representações irredutíveis das bandas de energia, juntamente com a aproximação da função envelope, permitiu a formulação de um modelo polítipico wurtzita/zincblend para cálculo da estrutura de bandas em nanowhiskers. Embora o interesse seja em super-redes politípicas, o modelo proposto foi aplicado a um poço quântico de InP com o intuito de extrair a física envolvida na interface wurtzita/zincblend. / Semiconductor nanowhiskers made of III-V compounds exhibit great potential for technological applications. Controlling the growth conditions, such as temperature and diameter, it is possible to alternate between zincblend and wurtzite crystalline phases, giving origin to the polytypism. This effect has great influence in the electronic and optical properties of the system, generating new forms of confinement to the carriers. A theoretical model capable to accurately describe electronic and optical properties in these polytypical nanostructures can be used to study and develop new kinds of nanodevices. In this study, I present the development of the k.p Hamiltonian in the Γ point for the zincblend and wurtzite crystal structures based on the formalism of group theory. Using the symmetry group of the Γ point, it is possible to obtain the irreducible representations of the energy bands, considering the atomic orbitals and the number of atoms in the primitive unit cell. Also, the group symmetry operations are used to calculate the non-zero and independent matrix elements of the k.p Hamiltonian. The study of the basis states symmetry of irreducible representations in the energy bands, alongside with the envelope function approximation, allowed the formulation of a wurtzite/zincblend polytypical model to calculate the electronic band structure of nanowhiskers. Although the interest is in polytypical superlattices, the proposed model was applied to a single quantum well of InP to extract the physics of the wurtzite/zincblend interface.

京都大学 / 新制・課程博士 / 博士(理学) / 甲第23459号 / 理博第4753号 / 新制||理||1681(附属図書館) / 京都大学大学院理学研究科化学専攻 / (主査)教授 倉田 博基, 教授 島川 祐一, 教授 寺西 利治 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Cellulose nanowhiskers reinforced poly(vinyl alcohol)(PVA) nanofiber web is successfully fabricated using electrospinning technique and the mechanical properties of the single electrospun fiber are measured using nanoindentation method. The morphology and mechanical properties of highly aligned electrospun fiber webs are investigated. It is found that the modulus and tensile strength of aligned webs are higher than those of isotropic electrospun fiber webs. Experimental results are compared with a longitudinal Halpin-Tsai model. Ice-templated(IT) cellulose microfibril porous foams are successfully fabricated via unidirectional freezing methods. The morphology and growth mechanism of IT surfaces are investigated successfully using cellulose microfibrils and hydrophillic substrates. By controlling the temperature gradient between cellulose microfibril suspensions and secondary freezing mediums, various surface structures including honey-comb like structures, ellipse-shape channel strcutures, fully developed multichannel structures are obtained. For the honey-comb like patterned surface, high contact angles are observed. On the other hand, for the layered patterned surface, anisotropic wetting properties are observed.

Cellulose nanowhisker (CNW) prepared by acid hydrolysis of softwood Kraft pulp was incorporated as nanofiller in rigid polyurethane (PU) foam synthesis. The density, morphology, chemical structure, mechanical properties and thermal behavior of the products were characterized. The nanocomposites exhibited better performance especially at high CNW¡¯s content which was probably due to the high specific strength and aspect ratio of CNW, the hydrogen bonding and crosslinking between CNW and polymer matrix, a higher crosslinking density compared to the control, and the function of CNW as an insulator and mass transfer insulator. Lignin polyol was synthesized through oxypropylation and used for rigid PU foam preparation. The density, morphology, chemical structure, compressive property and thermal behavior of the product were characterized. Lingin-based rigid PU foam showed improved compressive property compared to its commercial counterpart. Ethanol organosolv lignin-based PU showed a slightly stronger compressive property than Kraft lignin-based PU. The enhancement was primarily attributed to the rigid phenolic structure and the high hydroxyl functionality of lignin. Lignin-based PU generated more char than common PUs which was possibly related to the better flame retardant property. This study provided an alternative way to valorize the two most abundant biopolymers and resulted in relatively environmentally benign rigid PU nanocomposite foam.

Cellulosic fibers sized from the macro-scale to the nano-scale were prepared hierarchically from kenaf bast fibers using chemicals. The process began with a hermetical alkaline retting followed by a bleaching treatment. The bleached fibers were hydrolyzed using inorganic acid, from which microfibers and cellulose nanowhiskers (CNWs) were fabricated. Inorganic nanoparticle impregnation (INI) was used to treat the retted fibers for the improvement of the interfacial compatibility between the fiber and polypropylene (PP) matrix. The retted fibers and INI-treated fibers were used as reinforcement for the PP polymer composites. Film casting process was used to make CNW/PVA composites. The hermetical retting process used in this study produced fibers with high cellulose contents (81-92%) by removing the lignin and hemicelluloses. Higher retting temperature resulted in higher fiber surface hardness and elastic moduli. The tensile strengths and tensile moduli of the fibers decreased as the temperature increased. The SEM images showed the micropores in the cell wall structure for the fibers retted at over 130°C, providing the possibility to anchor nanoparticles into the cell wall. Surface morphology of the INI-treated fibers was examined with SEM, and showed that the CaCO3 nanoparticle crystals grew onto the fiber surface. Energy-dispersive X-ray spectroscopy (EDS) was used to verify the CaCO3 particle deposits on the fiber surface. As the size scale of the fibers decreased, the fiber crystallinity increased from 49.9% (retted fibers) to 83.9% (CNWs). About 23% á-cellulose in the raw kenaf bast fibers had been converted into CNWs. The retted fibers without INI treatment had poor compatibility with the polypropylene matrix. The INI treatment improved the compatibility between the fibers and the PP matrix, resulting in an improvement in kenaf fiber/PP composite tensile moduli and tensile strengths. The CNWs prepared from kenaf bast fiber gave excellent reinforcement for PVA composites. A nine percent increase of CNWs in the CNW/PVA composites yielded significant improvements in tensile strength and modulus of about 46% and 152%, respectively, compared with pure PVA.

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