Metal nanoparticles can possess intriguing properties due to their nanoscale dimensions, and are intensively applied in research. With the development of synthetic systems, classic crystal growth theories become limited and cannot explain current conditions very well. The aim of this project is to find out the factors that influence crystal growth at the nanoscale and develop general methods to prepare shape-controlled nanomaterials. The growth process of CuPt nanorods is studied and a ligand mediated mechanism is proposed. It reveals that surface ligands are crucial in guiding the one dimensional growth through their mutual interactions. Solvent effect is discovered to be able to control the nanoparticles morphology, by indirectly tuning the interactions between ligands and the surface of a particle. Based on this mechanism, titanate nanosheets with a monolayer thickness are prepared with the assistance of surface ligands. An effective, one-step method is developed to prepare CuPd nanowire networks, which demonstrates its versatility in the preparation of other alloyed networks. The growth process of CuPt nanoparticles are investigated, and show that the growth pathway can be a reversed, surface-to-core crystallization route. The effect of dealloying, including acid etching and galvanic replacement, is studied and used to fabricate nanoparticles with various morphologies. The findings in this project highlight the influence of surface ligands in the synthesis of nanocrystals, provide new perspectives of crystal growth mechanisms and offer practical knowledge for nanostructuring materials.

No description available.

by Yang Hua. / "August 1999." / Thesis (Ph.D.)--Chinese University of Hong Kong, 1999. / Includes bibliographical references. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese. / by Yang Hua.

Synthetic Laponite-clay particles with a platelet-like shape display strong gelation when dispersed in aqueous solutions because of their positively charged rims and negatively charged flat surfaces. In this thesis, my aim was to modify the surfaces of these clay particles such that we can both access their liquid crystalline (LC) discotic phase and further build transparent and mechanically resilient coatings with a 3D "brick-and-mortar" structure that is similar to that observed in natural mother of pearl (nacre). I first introduce a simple strategy that successfully suppresses Laponite's ageing phenomenon and enables the system's isotropic-to-LC phase transition. By grafting Laponite particle surfaces with comb-like polymers, poly (L-lysine)-g-poly (ethylene glycol) (PLL-PEG), I was able to screen negative surface charges and ensure steric stabilisation. Besides using long-chain polymers, I also coated the positively charged Laponite rims with small, barrel-shaped molecules cucurbit[7]uril (CB[7]). By carefully tuning the ratio between CB[7] and Laponite, the system experienced a macroscopic phase separation into a Laponite-poor suspension and a birefringent LC gel. Inspired by the hierarchical structure of nacre, here I also demonstrate a simple approach to fabricate polymer-clay hybrid films via a water-evaporation process. In this third method, Laponite platelets were bridged by natural abundant polymers (carboxymethyl cellulose) through hydrogen bonding. This hybrid material possesses high transparency, flexibility and an outstanding fire-retardant property. After Ca2+ ion-coordination of these cellulose-Laponite composite films, the interface between the polymers and clays was further strengthened, leading to enhanced mechanical properties along with improved thermal- and water-resistance. I also present that using Dextran as a depletant, sterically stabilised Laponite can access its liquid crystal phase under low clay concentration. Finally, I show that Laponite can be coated with various polymers (PEO, chitosan, sodium alginate) for the purpose of obtaining LC gels and hybrid films. I believe that our findings on surface-modification of clay particles can open new routes to large-scale and inexpensive production of bio-inspired functional materials.

by Leung Kwok Kuen. / Thesis (Ph.D.)--Chinese University of Hong Kong, 1997. / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / by Leung Kwok Kuen.

No description available.

No description available.

Crystal engineering is a rapidly developing area of research with goals aimed at designing functional molecular solids using reliable intermolecular interactions. By designing these intermolecular interactions using principles of supramolecular chemistry, favorable molecular arrangements can be achieved, which is manifested in desirable properties. We have applied crystal engineering strategies to the synthesis of unique materials for advanced applications including a metal-organic semiconductor, photochromic co-crystals, and a co-crystalline thin film for photolithography. We designed a metal-organic complex based on Ag(I) that exhibits π-π stacking interactions in the organic ligands, which is favorable for electrical conductivity in organic-based semiconductors. The nanocrystalline complex exhibits remarkable electrical conductivity and is also designed to undergo a [2+2] cycloaddition reaction, resulting in over a 70% increase in electrical conductivity. The increase in conductivity is supported by an increased contribution of Ag(I) ions to the top edge of the valence band, as well as new Ag···C(phenyl) interactions that can provide a charge transport pathway. Co-crystallization strategies were used to switch a non-photochromic compound photochromic upon incorporation into a series of co-crystals. Previously in our group, a co-crystalline thin film was applied for photolithography, and here, the crystal structure of the co-crystalline film is elucidated. We have also applied principles of crystal engineering to the synthesis of materials that are candidates for electrical property characterization measurements. First, we utilize Ag(I) to synthesize 0D and 1D metal-organic complexes. These complexes are also designed to undergo [2+2] photocycloaddition reactions and upon reaction, an increase in dimensionality by at least one order (i.e. 0D to 1D) is achieved. In one complex, photodimerization resulted in a 3D metal-organic framework (MOF), and we successfully applied a ‘green’ synthetic method to the synthesis of the 3D MOF via vortex grinding. We also report the X-ray crystal structure and solid-state packing of an organic molecule involving tetrathiafulvalene, a classic organic semiconductor. The molecule is susceptible to solvent uptake/loss and exhibits π-π stacking arrangements that are not ideal for favorable electrical properties. Through co-crystallization strategies, we achieve a unique ‘lock-arm’ motif that results in infinite stacking in the tetrathiafulvalene core, an ideal property for semiconductivity. This thesis will also focus on the solid-state [2+2] photodimerization reactions of styrylthiophenes, molecules that rarely undergo the reaction in either the solution or solid state. There have been very few efforts to attain regiocontrol of the products and high yielding photodimerizations of thiophenes are rare. We utilized a ditopic resorcinol template to afford the head-to-head photodimerization product, and by using a dicarboxylic acid-based co-crystal former, we were able to synthesize the head-to-tail photodimer. Both products were achieved in quantitative yield. We have also expanded our approach by employing silver templates, which have previously been successfully applied to photodimerizations of olefins substituted with six-membered rings (i.e. phenyl). We examined photoreactivity in Ag(I) coordination complexes with both α- and β-substituted thiophenes. Both head-to-head and head-to-tail products can be achieved and, in some complexes, both products are produced. In our studies examining thiophene photoreactivity with dicarboxylic acid templates, we discovered a unique co-crystal wherein two strong supramolecular synthons contribute equally to the solid-state packing. Due to this rare observation, we performed a survey of the Cambridge Crystallographic Database for co-crystals dominated equally by the same two strong supramolecular synthons. We found that co-crystals including both of these synthons are quite rare, and our co-crystal was the first to include a monopyridine. We discuss differences in pKas between the hydrogen-bond donor and acceptor to understand situations where these interactions do not form. We also highlight optimization of crystal symmetry and favorable secondary interactions, such as weak hydrogen bonding and π-π stacking, which may lead to and support the unique synthon formation. Lastly, we utilize co-crystallization strategies to modify the degree of dynamic molecular motion in the azo functional group, a group that is known to exhibit pedal motion in the solid state. The molecular motion is related to the thermal expansion behavior of the crystals and only upon co-crystallization with a ditopic receptor is the molecular motion capability of the azo group unlocked and ‘colossal’ thermal expansion properties achieved. By systematically modifying the non-azo component, we achieve thermal expansion ranging from ‘colossal’ to nearly zero, as well as rare negative thermal expansion.

Supramolecular chemistry is a branch of chemistry that focuses on chemical systems that are made up of a discrete number of assembled molecular components held together by various non-covalent interactions. Supramolecular systems are rarely designed from first principles. Rather, chemists have a range of well-studied structural and functional building blocks that they are able to use to build up larger functional architectures. We have chosen tetraphenylethylene (TPE) as the supramolecular building block in designing various functional materials because of interesting optical and electronic properties of TPE derivatives. We have utilized several intermolecular interactions like hydrogen bonding, coordinate bonding and halogen bonding to obtain materials with remarkable optical and electronic properties in the solid state as well as solution phase that can have potential applications in fields like crystal engineering, material science and organic electronics. TPE functionalized with four carboxylic acid groups was synthesized and crystallized with various bis(pyridines) to yield organic semiconducting materials. These crystals have been characterized by single crystal X-ray diffraction and conducting properties have been studied using conducting probe-atomic force microscopy. Semiconducting properties of these materials can be tuned based on bis(pyridine) component. Two different tetrapyridyl substituted TPEs have been synthesized and their photoluminescent properties have been studied in solution. Fluorescence emission was found to be switchable as a function of solvent mixture as well as pH. Both compounds have been structurally characterized in their free base form as well as in their protonated form as tetraperchlorate salts via X-ray diffraction. These three compounds have been utilized as supramolecular building blocks in metal organic frameworks (MOFs) as well as organic co-crystals mediated by hydrogen bonding as well as halogen bonding. A fluorescent complex of the tetraacid with zinc has been obtained which crystallized in a non-centrosymmetric space group due to solvent and water ligands on the zinc center. A coordination polymer has been obtained via solvothermal synthesis using tetrapyridyl TPE and zinc chloride. Lewis basicity of tetrapyridyl TPE's has also been utilized in organic co-crystalline assemblies mediated by halogen bonding interactions with iodoperfluoroarenes. Finally, Halobenzoyl esters of TPE based compounds have been synthesized and halogen bonding properties of these compounds have been exploited to achieve solid state porous networks.

Nonlinear optical phenomena associated with high-order soliton breakup in photonic crystal fibres and squeezed state generation in three dimensional photonic crystal microcavities are investigated. In both cases, the properties of periodically patterned, high-index contrast dielectric structures are engineered to control the dispersion and local field enhancements of the electromagnetic field. Ultra-short pulse propagation in a polarization-maintaining microstructured fibre (with 1 um core diameter and 1.1 m length) is investigated experimentally and theoretically. For an 80 MHz train of 130 fs pulses with average propagating powers in the fibre up to 13.8 mW, the output spectra consist of multiple discrete solitons that shift continuously to lower energies as they propagate in the lowest transverse mode of the fibre. The number of solitons and the amount that they shift both increase with the launched power. All of the data is quantitatively consistent with solutions of the nonlinear Schrodinger equation, but only when the Raman nonlinearity is treated without approximation, and self-steepening is included. The feasibility of using a parametric down-conversion process to generate squeezed electromagnetic states in 3D photonic crystal microcavity structures is investigated for the first time. The spectrum of the squeezed light is theoretically calculated by using an open cavity quantum mechanical formalism. The cavity communicates with two main channels, which model vertical radiation losses and coupling into a single-mode waveguide respectively. The amount of squeezing is determined by the correlation functions relating the field quadratures of light coupled into the waveguide. All of the relevant model parameters are realistically estimated using 3D finite-difference time-domain (FDTD) simulations. Squeezing up to ~20% below the shot noise level is predicted for reasonable optical excitation levels. To preserve the squeezed nature of the light generated in the microcavity, a unidirectional coupling geometry from the microcavity to a ridge waveguide in a slab photonic crystal structure is studied. The structure was successfully fabricated in a silicon membrane, and experimental measurements of the efficiency for the signal coupled out of the structure are in good agreement with the result of FDTD simulations. The coupling efficiency of the cavity mode to the output channel is ~60%.