School of Chemistry - Theses
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ItemWater soluble fullerene and SPIO based nanoparticles by solvent exchange and polymer self-assembly methods( 2017)The synthesis of nanomaterials with tailored components and structures has drawn significant attention for industrial and research applications. Buckminsterfullerene (C60), metallofullerene and related materials have potential uses in varied fields such as MRI contrast agents, solar cells, bio-medicines and drug delivery systems. In particular, gadofullerene (Gd@C82) has drawn research attention as a new type of MRI contrast agent taking advantage of the Gd atom inside the fullerene cage. However, one of the main challenges of C60 or metallofullerene Gd@C82 for application in the biological sciences is their negligible solubility in aqueous solution due to their hydrophobic surfaces. In this research, different strategies have been investigated to explore water-soluble fullerene or related inorganic nanoparticles for potential MRI applications. A number of different strategies were investigated to design water-soluble fullerenes. In the second chapter, the solvent exchange technique was explored, which is a method for transferring C60 from an organic solvent into water under stirring or sonication. The use of tetrahydrofuran (THF) as a solvent to produce water dispersable fullerenes has been widely reported and extensively utilised. In this research, we have developed a related method of solvent exchange based on the use of N,N-dimethylformamide (DMF). The DMF approach results in the formation of fullerene nanoparticle agglomerates that are highly stable in phosphate buffered saline solution and also in water, while the THF agglomerates are only stable in water as previously reported. Moreover, we found that both of these approaches result in the significant degradation of the fullerene cage as shown by various techniques such as matrix assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry, X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), fourier transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance (1H NMR). This discovery was not previously reported in the literature. Our results have shown that the solvent exchange technique using THF results in partial oxidation and degradation of C60. In contrast, the DMF evaporative method results in greater oxidation and degradation of C60 but significantly enhanced colloidal stability in buffer solution. Another approach to modify fullerenes is through the use of polymers. RAFT polymerization is one of the widely used controlled living polymerization processes as it allows the synthesis of tailored polymer structures with controlled molecular weight and narrow molecular weight distribution. In our research, the parameters of RAFT polymerization of poly(2-(diethylamino)ethyl methacrylate) (PDEAEMA) and poly(ethylene glycol) methyl ether methacrylate (PEGMA) were explored by the high throughput instrument, the Chemspeed platform, to explore the optimal polymerization conditions. The Chemspeed is a parallel synthesis platform with high-through output and automated robot synthesis instrument, which have been used to systematically investigate the parameters of different polymerizations and to discover new materials with accelerated efficiency. The polymers synthesised were pH responsive, meaning they changed structure with variation in pH. In particular, PDEAEMA polymers are attractive for its pH responsiveness with pKa ~ 7 which is between the pH values of healthy and cancerous tissues. The kinetics of RAFT polymerization of both PDEAEMA and PEGMA were investigated and homopolymers with different chain length were synthesized. These pre-synthesized PEGMA-b-PDEAEMA amphiphilic block copolymers have a great potential as a smart platform for both MRI contrast agent and drug delivery. In the forth chapter, the polymer/fullerene complex nanoparticles (PEGMA-b-PDEAEMA/C60 and PEGMA-b-PDEAEMA/Gd@C82) were successfully synthesized via a self-assembly method and then were characterized by dynamic light scattering (DLS), ultraviolet-visible (UV-vis) spectrophotometry, cryo transmission electron microscopy (cryo-TEM) and matrix assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry. The disassembly of the complex PEGMA-b-PDEAEMA/C60 nanoparticles could be triggered by lowering the pH in the solution which makes the nanoparticle as a smart platform with pH responsiveness. In addition, the PEGMA-b-PDEAEMA/Gd@C82 nanoparticles were also explored utilizing the self-assembly strategy developed for PEGMA-b-PDEAEMA/C60 nanoparticles. The particles demonstrated stable incorporation of Gd@C82. The MRI contrast behavior of these materials showed variation with the different length of block copolymers used. Our results indicate these materials are interesting as potential MRI contrast agents. In chapter five, this self-assembly synthetic strategy was applied to synthesize PEGMA-b-PDEAEMA/iron oxide nanoparticles as superparamagnetic iron oxide (SPIO) nanoparticles which have been widely used as MRI T2 contrast agents to improve the MRI imaging quality. The pH responsive PEGMA-b-PDEAEMA/Fe3O4 nanoparticles fabricated by the self-assembly method were characterized in detail by dynamic light scattering (DLS) and cryo transmission electron microscopy (cryo-TEM). Moreover, the pH responsive behavior of the complex PEGMA-b-PDEAEMA/Fe3O4 nanoparticles was also investigated via DLS and cryo-TEM. The complex PEGMA-b-PDEAEMA/Fe3O4 nanoparticles as a new MRI T2 contrast agent was also explored. The results indicated that the MRI response was tunable based on the polymer building block and also on the concentration of polymer used in the formulation. The simple and modular synthesis of water soluble C60, Gd@C82 and superparamagnetic iron oxide nanoparticles using pH responsive polymers provides a potential smart system for advanced MRI imaging, bio-medicines and drug delivery in the future.
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ItemNanoparticle assembly using an atomic force microscope( 2017)Friction is caused by the energy dissipation between two interfaces in relative motion. Almost 100 million terajoules of energy is lost to overcome friction every year. It is almost one fifth of the energy produced globally. Therefore, understanding friction behaviour is one of the most fundamental topics of scientific research. At the macro-scopic size scale, three empirical laws of friction were discovered almost 400 years ago. Although these three laws of friction work well in the macroscale world, the understanding of friction on the microscale and nanoscale is still very challenging and remains controversial. Atomic force microscope (AFM) is a powerful tool to quantify nanoscale forces. In our thesis, we demonstrate the normal and lateral force calibration. Beyond these, as the optical lever sensitivity calibration of the AFM is becoming a crucial parameter in force measurements, we calculate the ratio of the dynamic and static sensitivities for several common AFM cantilevers, whose shapes vary considerably, and experimentally verify these results. The dynamic-to-static optical lever sensitivity ratio is found to range from 1.09 to 1.41 for the cantilevers studied – in stark contrast to the constant value of 1.09 used widely in current calibration studies. This analysis shows that accuracy of the thermal noise method for the static spring constant is strongly dependent on cantilever geometry – neglecting these dynamic-to-static factors can introduce errors exceeding 100%. We also discuss a simple experimental approach to non-invasively and simultaneously determine the dynamic and static spring constants and optical lever sensitivities of cantilevers of arbitrary shape, which is applicable to all AFM platforms that have the thermal noise method for spring constant calibration. With good force calibration in both the normal and lateral direction, this Ph.D project focuses on the investigation of friction of nanospheres of different sizes. Chemically synthesised gold nanospheres with various diameters were manipulated on Si/SiO2 and gold substrates. Using atomic force microscopy in contact mode and dynamic mode, the static friction and sliding friction of gold nanospheres on the Si/SiO2 substrates were measured and analysed. The friction experiment data in contact mode manipulation indicated that the static friction of gold nanospheres did not reveal size dependence. However, the static friction is higher related to the surface chemistry between the particle ligand and the substrate. Also, we have observed that the relative humidity also plays an important role in the quantify of static friction coefficient of gold nanospheres on a substrate. In contact mode AFM manipulation, the sliding friction of nanospheres revealed size dependence. However, with plotting the logarithm of sliding friction as a function of the logarithm of nanoparticles’ diameter, the slope of linear fitting did not correspond to any contact area models. We believe that this discrepancy is caused by the large deviation of sliding friction in contact mode manipulation. The dynamic mode nanoparticle manipulation was also performed in this Ph.D project. Different sizes of gold nanospheres were manipulation on a gold substrate. Our experiment data indicates the sliding friction is clearly dependent on the size of nanospheres. From plotting the logarithm of sliding friction as a function of the logarithm of nanoparticles’ diameter, the slope of linear fitting suggests that the experimental data is in accord with the DMT contact area model. A new fabrication method, which combines electron beam lithography and atomic force microscopy manipulation, is demonstrated to build a series of symmetric or asymmetric 3D gold nanostructures with nanoscale interparticle separation. The topography of these structures is provided by scanning electron microscopy, and the plasmon modes of these structures were elucidated by the polarised dark field microscopy. The spectra of the symmetric structures exhibit little polarisation dependence, which are in good accord with the COMSOL modelling. More interestingly, the symmetric 3D pentamer exhibits a Fano-like resonance and can provide a drastically enhanced localized electric field in the interstices of 3D structures. For the asymmetric 3D pentamer structures, where one disk is situated 5 nm away from the center of the bottom tetramer, a Fano-like dip is only revealed at a particular polarisation angle. When the top particle is manipulated onto a single disk of the bottom tetramer, there is no Fano-like resonance.
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ItemUnderstanding cadmium telluride nanocrystal solar cells( 2016)Solution-processed solar cells use cheaper manufacturing methods that could potentially bring down the cost of solar energy to be cost-competitive with fossil fuels. However, solution processed solar cells still have many issues that keep them from being commercially viable. While several solution processed techniques have resulted in efficiencies comparable to solar cells available on the market, there are many issues with long-term performance and stability. The work presented here is focused on one of the major candidates for commercial development of solution-processed solar cells. Nanocrystal (NC) cadmium telluride (CdTe) solar cells have reached power conversion efficiencies of over 12 % which is comparable to commercially available CdTe solar cells that use high cost vacuum depositions. However, these efficiencies are reached with a biasing treatment and degrade over time. We refer to the improvement in PCE through biasing treatments as “the biasing effect”. The work presented in this thesis will give a comprehensive analysis of NC CdTe solar cells by understanding the role of processing steps on chemical and electronic properties of CdTe films. Following this, we will show the major cause of degradation of NC CdTe films and devices. We will give comprehensive understanding and show the cause of the biasing effect. Finally we will show some possibilities for stabilizing and improving the current nanocrystal CdTe device which is an integral part of commercializing this technology. This thesis will answer the following questions: 1. What is the purpose of the cadmium chloride treatment and temperature annealing in NC CdTe films? 2. What is the cause of degradation and the biasing effect? 3. Can we stabilize and improve the device?
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ItemSurface engineering for mechanically robust superhydrophobic films( 2016)The inherent surface roughness of superhydrophobic surfaces renders them mechanically fragile and limits their use in many applications from self-cleaning to anti-fouling. With the view of improving the mechanical durability of these films several steps have been taken to both identify and understand the underlying principles for the apparent dichotomy between superhydrophobicity and mechanical durability. Rough surface coatings with variable surface roughness have been developed and examined using atomic force and electron microscopy, contact angle goniometry nanoindentation as well as industry based mechanical testing. Prepared predominately by bottom up strategies such as sol-gel processing, a diverse variety of superhydrophobic surfaces were prepared exhibiting contact angles greater than 150° and sliding angles less than 10°. Subsequently, several synthetic protocols have been developed to overcome these difficulties. Within conventional sol-gel derived coatings, by normalizing against the surface topology, the enhancement in abrasion resistance can be correlated to crosslinked polymer material property ratios H/E and H3/E2, providing a rationale for polymer choice to wear improve wear behavior in future coatings. Understanding of geometric limitations led to the development of polymer spheres prepared through emulsion synthesis which were utilized as sacrificial templates within a siloxane matrix to yield films with crater-like surface roughness. Surface roughness was controlled through the template geometry and concentration. The intrinsic hydrophobicity of the MTMS matrix provides enhanced longevity towards wear. This was subsequently improved through the development of polyhedral silsequioxane chemistry. Further design of the crater-like surface was inspired by mimicking the fascinating assembly of particles in natural materials. Hierarchical assembly of anisotropic particles to achieve mutually exclusive properties inspired work toward the preparation of biomimetic, superhydrophobic coatings predominantly from the incorporation of silica and polyaniline fibers and rods into craterlike surfaces.
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ItemSurface plasmon spectroscopy of redox processes on single gold nanocrystals( 2016)Gold nanocrystals (Au NCs) are important materials for catalysis, sensing and photonics. Au NCs exhibit strong scattering signals in the visible and near infrared range due to localised surface plasmon resonances. Due to statistical averaging, when measurements are made on ensembles of particles, the precise determination of the effect of gold nanocrystal size, shape and local environment on specific application performance is not feasible. This ensemble problem is overcome by applying a combination of dark field imaging with surface plasmon spectroscopy, enabling the scattering spectra of individual nanocrystals to be measured. This approach allows changes in the electron concentration of a single Au NC to be observed via localised surface plasmon resonance shifts. In this thesis, the dark field microscopy technique has been expanded to study single gold nanocrystal electrodeposition, gas-phase adsorption, photoreduction, and solid-state charging. The first key objective was to understand how nanocrystal morphology and surface properties influence underpotential deposition. The second key objective was to understand how metal oxide supports influence charge transport during hydrogen adsorption and photoexcitation. These studies clearly demonstrate that the above parameters are crucial to the electrochemical and catalytic properties of Au NCs. By performing the measurements on single nanocrystals, the chemical kinetics and charging rates could be uncovered with detail never before achieved at this scale. In addition to using standard dark field microscopy techniques in this work, an upgraded laser illuminated dark field system was developed to optically reveal single particle charging rates by measuring electron transfer in real-time. The added sensitivity of this new approach has enabled the optical detection of fewer than 150 electrons as they are transferred to a single gold nanorod. In order to characterise the performance of the laser system, a reliable and reproducible method to rapidly charge single gold nanocrystals was developed. Au NCs were integrated in an ion gel capacitor, enabling them to be charged in a solid, transparent and highly capacitive device, ideal for transmission microscopy.
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