Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2007. / Includes bibliographical references (leaves 146-161). Also available in electronic version.

<p> Silicon nanocrystal&rsquo;s (SiNCs) size dependent optical properties and nontoxic nature portend potential applications across a broad range of industries. With any of these applications, a thorough understanding of SiNC photophysics is desirable to tune their optical properties while optimizing quantum yield. However, a detailed understanding of the photoluminescence (PL) from SiNCs is convoluted by the complexity of the decay mechanisms, including a stretched-exponential relaxation and the presence of both nanosecond and microsecond decays.</p><p> In this dissertation, a brief history of semiconductor nanocrystals is given, leading up to the first discovery of room temperature PL in SiNCs. This is then followed by an introduction to the various nanocrystal synthetic schemes and a discussion of quantum dot photophysics in general. Three different studies on the PL from SiNCs are then presented. In the first study, the stretched nature of the time dependent PL is analyzed via chromatically-resolved and full-spectrum PL decay measurements. The second study analyzes the size dependence of the bimodal PL decay, where the amplitude of the nanosecond and microsecond decay are related to nanocrystal size, while the third project analyzes the temperature and microstructure dependencies of the PL from SiNC solids.</p><p> After an indepth look at the PL from SiNCs, this report examines preliminary results of SiNC and silver nanocrystal self-assembly. When compared to metal and metal chalcogenide nanoparticles, there is a dearth of literature on the self-assembly of SiNCs. To understand these phenomena, we analyze the size dependent ability of SiNCs to form a &lsquo;superlattice&rsquo; and compare this with silver nanocrystals. Although the results on self-assembly are still somewhat preliminary, it appears that factors such as SiNC concentration and size dispersity play a key role in SiNC self-assembly, while suggesting intrinsic differences between the self-assembly of SiNCs and silver nanocrystals.</p><p> Finally, at the end of this dissertation, a corollary project is presented on the computational analysis of fluorescent silver nanoclusters (AgNCs). Due to their small size and non-toxic nature, AgNCs are an ideal fluorophore for biological systems, yet there is a limited understanding of their photophysics, which is the focus of this part of the dissertation.</p><p>

abstract: Deoxyribonucleic acid (DNA) has been treated as excellent building material for nanoscale construction because of its unique structural features. Its ability to self-assemble into predictable and addressable nanostructures distinguishes it from other materials. A large variety of DNA nanostructures have been constructed, providing scaffolds with nanometer precision to organize functional molecules. This dissertation focuses on developing biologically replicating DNA nanostructures to explore their biocompatibility for potential functions in cells, as well as studying the molecular behaviors of DNA origami tiles in higher-order self-assembly for constructing DNA nanostructures with large size and complexity. Presented here are a series of studies towards this goal. First, a single-stranded DNA tetrahedron was constructed and replicated in vivo with high efficiency and fidelity. This study indicated the compatibility between DNA nanostructures and biological systems, and suggested a feasible low-coast method to scale up the preparation of synthetic DNA. Next, the higher-order self-assembly of DNA origami tiles was systematically studied. It was demonstrated that the dimensional aspect ratio of origami tiles as well as the intertile connection design were essential in determining the assembled superstructures. Finally, the effects of DNA hairpin loops on the conformations of origami tiles as well as the higher-order assembled structures were demonstrated. The results would benefit the design and construction of large complex nanostructures. / Dissertation/Thesis / Ph.D. Biochemistry 2012

abstract: As engineered nanomaterials (NMs) become used in industry and commerce their loading to sewage will increase. However, the fate of widely used NMs in wastewater treatment plants (WWTPs) remains poorly understood. In this research, sequencing batch reactors (SBRs) were operated with hydraulic (HRT) and sludge (SRT) retention times representative of full-scale biological WWTPs for several weeks. NM loadings at the higher range of expected environmental concentrations were selected. To achieve the pseudo-equilibrium state concentration of NMs in biomass, SBR experiments needed to operate for more than three times the SRT value, approximately 18 days. Under the conditions tested, NMs had negligible effects on ability of the wastewater bacteria to biodegrade organic material, as measured by chemical oxygen demand (COD). NM mass balance closure was achieved by measuring NMs in liquid effluent and waste biosolids. All NMs were well removed at the typical biomass concentration (1~2 gSS/L). However, carboxy-terminated polymer coated silver nanoparticles (fn-Ag) were removed less effectively (88% removal) than hydroxylated fullerenes (fullerols; >90% removal), nano TiO2 (>95% removal) or aqueous fullerenes (nC60; >95% removal). Although most NMs did not settle out of the feed solution without bacteria present, approximately 65% of the titanium dioxide was removed even in the absence of biomass simply due to self-aggregation and settling. Experiments conducted over 4 months with daily loadings of nC60 showed that nC60 removal from solution depends on the biomass concentration. Under conditions representative of most suspended growth biological WWTPs (e.g., activated sludge), most of the NMs will accumulate in biosolids rather than in liquid effluent discharged to surface waters. Significant fractions of fn-Ag were associated with colloidal material which suggests that efficient particle separation processes (sedimentation or filtration) could further improve removal of NM from effluent. As most NMs appear to accumulate in biosolids, future research should examine the fate of NMs during disposal of WWTP biosolids, which may occur through composting or anaerobic digestion and/or land application, incineration, or landfill disposal. / Dissertation/Thesis / M.S. Civil and Environmental Engineering 2012

Philosophiae Doctor - PhD / This work entails the preparation of various polyanilines with different morphologies and their application in photovoltaic solar cells. Zinc oxide (ZnO) with one-dimensional and flower-like morphology was also prepared by microwave irradiation and used as electron acceptors in photovoltaics devices. The morphological, structural, spectroscopic and electrochemical characteristics of these materials were determined by scanning electron microscopy (SEM), X-Ray diffraction (XRD), Raman, Fourier-transformed infrared spectroscopy (FTIR), ultraviolet and visible spectroscopy (UV-Vis), photoluminescence(PL), thermal gravimetric analysis (TGA) and cyclic voltammetry (CV) experiments. Devices fabricated from these materials were characterized under simulated AM 1.5 at 800 mW. / South Africa

Philosophiae Doctor - PhD / Lithium ion cathode systems based on composites of lithium iron phosphate (LiFePO₄), iron-cobalt-derivatised carbon nanotubes (FeCo-CNT) and polyaniline (PA) nanomaterials were developed. The FeCo-functionalised CNTs were obtained through in-situ reductive precipitation of iron (II) sulfate heptahydrate (FeSO₄.7H₂O) and cobalt (II) chloride hexahydrate (CoCl₂.6H₂O) within a CNT suspension via sodium borohydrate (NaBH₄) reduction protocol. Results from high Resolution Transmission Electron Microscopy (HRTEM) and Scanning Electron Microscopy (SEM) showed the successful attachment FeCo nanoclusters at the ends and walls of the CNTs. The nanoclusters provided viable routes for the facile transfer of electrons during lithium ion deinsertion/insertion in the 3-D nanonetwork formed between the CNTs and adjacent LiFePO₄ particles.

<p> Mechanical signals affect virtually every fundamental single- and multi-cellular process in biology. The local responses of individual molecules to mechanical stimuli at the interface of cell with its adjacent microenvironment (extracellular matrix or material) elicit global responses at the cell and tissue scales. Understanding and manipulating the cell-material interaction can be leveraged to design biomaterials with unique characteristics tailored towards a wide variety of biological applications such as platforms that direct stem cell differentiation for tissue engineering, sensors that can record accurate electrical signals in single cells for neuroscience, and implants that are susceptible to cell adhesion for biomedical applications. In this thesis I present work characterizing the response of cells to mechanical stimuli at the single cell and single molecule scales. At the single cell scale, we provide insights into how mechanical signals such as micro- and nano-topography of metallic and metallic surfaces affect cell adhesion, both in mammalian and bacterial cells. Next we characterize the mechanical response of protein complexes involved in the transmission of mechanical signals across the cytoskeleton to the nucleus. </p><p> The four main contributions of the work presented in this thesis are as follows: 1) We used high resolution scanning electron microscopy to characterize the cell-nanostructure interface and provide insights into the response of individual mammalian cells to nanostructures with complex geometries. 2) We provide a first look at how individual bacterial cells adhere to metallic nanostructures, which could lead to new techniques to thwart infections. 3) We proposed a novel technique to control the growth and arrangements of bacterial cell communities. This method will allow precise small-scale mechanical manipulation of bacterial cells and could be utilized for unraveling the understudied mechanisms of bacterial mechanosensitivity. 4) We performed the first molecular dynamics study on the mechanisms of force transmission to the nucleus of eukaryotic cells through protein complexes known as linkers of the nucleoskeleton and cytoskeleton (LINC complexes). We showed that LINC complexes are highly stable under tensile forces, and that the transmission of force across the complex depends highly on the unique intermolecular covalent bonds formed between the two proteins that construct the complex. Finally, we presented a model for the molecular mechanisms of LINC complex activation and regulation at the nuclear envelope.</p><p>

SOUZA, Aldilene Saraiva. Electronic Transport in Molecular Systems. 2012. 107 f. Tese (Doutorado em Física) - Programa de Pós-Graduação em Física, Departamento de Física, Centro de Ciências, Universidade Federal do Ceará, Fortaleza, 2012. / Submitted by Edvander Pires (edvanderpires@gmail.com) on 2015-06-08T19:17:13Z No. of bitstreams: 1 2012_tese_assouza.pdf: 11520100 bytes, checksum: 04bcbbf301130ab097bf5c01340034df (MD5) / Approved for entry into archive by Edvander Pires(edvanderpires@gmail.com) on 2015-06-08T19:52:21Z (GMT) No. of bitstreams: 1 2012_tese_assouza.pdf: 11520100 bytes, checksum: 04bcbbf301130ab097bf5c01340034df (MD5) / Made available in DSpace on 2015-06-08T19:52:21Z (GMT). No. of bitstreams: 1 2012_tese_assouza.pdf: 11520100 bytes, checksum: 04bcbbf301130ab097bf5c01340034df (MD5) Previous issue date: 2012 / Nesta tese apresentamos o estudo teórico de transporte eletrônico de dispositivos moleculares em dois problemas distintos. No primeiro, comparamos medidas via microscopia de tunelamento (STM) com cálculos de primeiros princípios onde a tensão aplicada em uma mono camada de moléculas auto-montadas, denominadas: 5-(4-piridina)-1,3,4-oxadiazol-2-tiol (HPYT) e 5-(4-fenil)-1,3,4-oxadiazol-2-tiol (HPOT) mostram a distribuição local de carga. Essas moléculas são depositadas sobre um substrato de ouro tipo (1 1 1). A formação destas camadas moleculares foi confirmada por medidas de STM. Cálculos baseados na teoria do funcional da densidade (DFT) foram realizados para obter a conformação mais estável da interação molécula/substrato. Verificamos uma grande semelhança entre os resultados teóricos e as medidas de imagem de STM. A partir desta comparação, sugerimos que o átomo de enxofre na molécula HPYT e HPOT está ligado à superfície de ouro por uma ligação direta à um único átomo de ouro. Para descrever a corrente de tunelamento ao longo da mono camada molecular sobre a superfície de Au (1 1 1) foi proposto um modelo quântico baseado na técnica de equação mestra. Nós investigamos também, propriedades de transporte de spin em uma cadeia de poliacetileno (como ponte) acoplada à uma nano fita de carbono tipo zigue-zague (ZGNRs) funcionando como eletrodos. Os cálculos de transporte foram efetuados usando técnica de funções de Green fora do equilíbrio (NEGF), combinada com a teoria do funcional da densidade (DFT). Trabalhos anteriores demonstraram que as ZGNRs exibem um ordenamento antiferromagnético (AF) e meia-metalicidade nos estados provenientes da borda, que podem ser destruídos com aplicação de um forte campo elétrico externo. Neste trabalho, nós demonstramos que a ligação entre a ponte molecular e átomos não-equivalentes de carbono (A/B) na sub rede de grafeno ZGNRs pode ocorrer de duas formas produzindo um sistema metálico ou semicondutor fortemente dependente do acoplamento local. Ao considerar o anel de carbono onde a cadeia está ligada, uma ligação se assemelha a uma ligação para no benzeno, enquanto a outra ligação é semelhante a uma ligação meta. Estas geometrias geram transmissão eletrônica distinta, que pode ser controlada sob um campo elétrico transversal.

<p> Recently, there has been growing interest in using transition metals (TM) for catalytic and electromagnetic applications, due to the ability of TMs to form stable compounds in multiple oxidation states. In this research, the focus has been on the synthesis and characterization of carbon-supported TM nanoparticles (NPs), specifically palladium (Pd) and gold (Au) NPs, for catalytic applications, and transition metal oxides (TMO) NPs, specifically Fe₃O₄ NPs for electromagnetic applications. Carbon supports have several advantages, such as enabling even distribution of particles, offering large specific surface area with excellent electron conductivity, and relative chemical inertness. </p><p> In this dissertation, for catalytic applications, emphasis was on removal of trichloroethylene (TCE) from groundwater. For this application, carbon-supported Pd/Au NP catalysts were developed. Pd was chosen because it is more active, stable and selective for desired end-products, and Au has shown to be a good promotor of Pd&rsquo;s catalytic activity. Often, commercially available Pd-based catalysts are made using harsh chemicals, which can be harmful to the environment. Here, an environmentally friendly process with aspects of green chemistry was developed to produce carbon-supported Pd/Au NP catalysts. This process uses a combination of sonochemistry and solvothermal syntheses. The carefully designed carbon-supported Pd/Au NP catalyst material was systematically characterized, tested against TCE, and optimized for increased rate of removal of TCE. Electron microscopy and spectroscopy techniques were used to study the material including structure, configuration and oxidative state. The Pd/Au NPs were found mainly to form clusters with an aggregate-Pd_ShellAu_Core structure. Using state-of-the-art direct detection with electron energy loss spectroscopy, the Pd NPs were found to have an oxidative state of zero (0). The formation of the catalyst material was studied in detail by varying several synthesis parameters including type of solvent, sonication time, synthesis temperature etc. The most optimized catalyst was found remove TCE at double the rate of corresponding commercial Pd-based catalysts in a hydrogen headspace. This material was found to catalyze the removal of TCE via traditional hydrodehalogenation and shows promise for the removal of other contaminants such as trichloropropane (TCP), carbon tetrachloride (CT). </p><p> This green approach to make and optimize TM materials for specific applications was extended to TMOs, specifically magnetite (Fe₃O₄) and further developed for the application of electromagnetism. As catalysts, Fe₃O₄ is used for removal of <i>p</i>-nitrophenol from water. However, since the carbon-supported Pd/Au material system was developed and optimized for catalysis, here, carbon-supported Fe₃O₄ NPs were developed for electromagnetic applications. There has been growing interest in tuning the magnetic properties of materials at room temperature with the use of external electric fields, for long-term applications in data storage and spintronic devices. While a complete reversible change of material properties has not yet been achieved, some success in partial switching has been achieved using multiferroic spinel structures such as Fe₃O₄. These materials experience a change in magnetic moment at room temperature when exposed to the electric fields generated by electrochemical cells such as lithium ion batteries (LIBs) and supercapacitors (SC). In the past, a 1% reversible change was observed in Fe₃O₄ using LIBs. Here, building on the developments from previous material system, Fe₃O₄ NPs were directly hybridized onto the graphene support in order to increase the observable change in magnetic moment. The material was systematically designed and tested for this application, including a study of the material formation. A simple, environmentally friendly synthesis using the solvothermal process was implemented to make the graphene-supported Fe₃O₄ NPs. This new material was found to produce a reversible change of up to 18% in a LIB. In order to overcome some of the difficulties of testing with a LIB, a corresponding hybrid SC was designed, built and calibrated. The graphene-supported Fe₃O₄ NPs were found to produce a net 2% reversibility in the SC, which has not been reported before. The results from both the LIB and SC were analyzed to better understand the mechanism of switching in a spinel ferrite such as Fe₃O₄, which can help optimize the material for future applications. </p><p> The focus of this dissertation was on the development of a methodology for carbon-supported TM and TMO NPs for specific applications. It is envisioned that this approach and strategy will contribute towards the future optimization of similar material systems for a multitude of applications.</p><p>

abstract: Understanding the interplay between the electrical and mechanical properties of single molecules is of fundamental importance for molecular electronics. The sensitivity of charge transport to mechanical fluctuations is a key problem in developing long lasting molecular devices. Furthermore, harnessing this response to mechanical perturbation, molecular devices which can be mechanically gated can be developed. This thesis demonstrates three examples of the unique electromechanical properties of single molecules. First, the electromechanical properties of 1,4-benzenedithiol molecular junctions are investigate. Counterintuitively, the conductance of this molecule is found to increase by more than an order of magnitude when stretched. This conductance increase is found to be reversible when the molecular junction is compressed. The current-voltage, conductance-voltage and inelastic electron tunneling spectroscopy characteristics are used to attribute the conductance increase to a strain-induced shift in the frontier molecular orbital relative to the electrode Fermi level, leading to resonant enhancement in the conductance. Next, the effect of stretching-induced structural changes on charge transport in DNA molecules is studied. The conductance of single DNA molecules with lengths varying from 6 to 26 base pairs is measured and found to follow a hopping transport mechanism. The conductance of DNA molecules is highly sensitive to mechanical stretching, showing an abrupt decrease in conductance at surprisingly short stretching distances, with weak dependence on DNA length. This abrupt conductance decrease is attributed to force-induced breaking of hydrogen bonds in the base pairs at the end of the DNA sequence. Finally, the effect of small mechanical modulation of the base separation on DNA conductance is investigated. The sensitivity of conductance to mechanical modulation is studied for molecules of different sequence and length. Sequences with purine-purine stacking are found to be more responsive to modulation than purine-pyrimidine sequences. This sensitivity is attributed to the perturbation of &pi-&pi stacking interactions and resulting effects on the activation energy and electronic coupling for the end base pairs. / Dissertation/Thesis / Doctoral Dissertation Physics 2014