• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 177
  • 46
  • 42
  • 39
  • 1
  • Tagged with
  • 4862
  • 2174
  • 1865
  • 235
  • 129
  • 128
  • 109
  • 104
  • 85
  • 85
  • 80
  • 80
  • 80
  • 73
  • 72
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Synthesis of BODIPY dyads to study electronic energy transfer

Bai, Dan January 2013 (has links)
Nature’s efficiency of converting sunlight into chemically stored energy have inspired many scientists research into the field of “artificial photosynthesis.” In order to produce renewable sustainable energy many chemists are looking for viable modus operandi to make carefully designed artificial analogues which could duplicate the essential features of natural photosynthesis. The mechanism of electronic excitation energy transfer between weakly coupled chromophores is well known as Förster resonance energy transfer (FRET). This thesis covers the work of the qualitatively and quantitative study the electro-energy transfer process within tailor-made boron dipyrromethene (Bodipy) systems.

High speed imaging detectors with diamond dynode materials

Taillandier, Virgil January 2014 (has links)
The primary focus of this thesis has been to experimentally study the secondary electron emission of diamond films; to identify materials and process parameters producing a high performance dynode for novel fast imaging photomultiplier designs. Material investigations and performance measurements were made to measure the effect of film thickness, substrate, surface termination, dopant concentration and crystallinity; in order to identify the optimum parameters for dynode manufacture and performance. In addition, a novel gain structure for two dimensional imaging using a diamond-coated image charge dynode in combination with a transparent mesh, was investigated by simulation and experimentally. Experimental work involved 65 CVD (Chemical Vapour Deposition) diamond samples on silicon, molybdenum, niobium substrates or free standing, micro- nano- crystalline and hydrogen or cesium terminated. The secondary electron (SE) yield was found to be far lower than the results found in the literature. Several hypotheses regarding the sample’s low SE yield measured have been considered, for example charge mobility could be reduced due to grain boundaries, high amounts of boron impurities, surface contamination and/or low electron affinity. The main cause identified, after bad quality films was the negative electron affinity of the surface produced when hydrogen terminated. Measurements have indicated that the majority of samples were not fully hydrogen terminated and thus could not produce the potentially high SE yield. The hydrogen termination has also shown to desorb under beam irradiation causing a problem of repeatability of the measurements. However, despite the poor results an interesting phenomenon has been measured: three time domains were observed on the SE yields during beam irradiation. These three processes correspond to different phases of trapping holes, electrons and hydrogen desorption. They have helped to better understand what happens during the process of secondary electron emission of diamond and could be used to determine the initial yield independent of the termination lost when modelling.

The use of holographic optical tweezing in the spectroscopic analysis of micron-sized compartments

Mistry, Nileshkumar S. January 2014 (has links)
Optical tweezers is an exciting and unique scientific instrument, which uses a highly focused laser to isolate and manipulate micron-sized dielectric particles in three-dimensions. The work undertaken in this thesis is divided into two themes. Firstly, it describes the design, construction and adaptation of a holographic optical tweezer (HOT) capable of multi-spectroscopy experiments. Secondly, the holographic optical tweezer is used to isolate and spectroscopically investigate microparticles of different colloidal systems. Chapter 1 provides an introduction to the field of optical manipulation from the first observation by Ashkin to the present day where novel beam shaping technology is being used to simultaneously manipulate multiple particles. It also covers an introduction to the three colloidal systems which have been investigated using the HOT. Chapter 2 outlines the components of a generic optical tweezer and describes in detail the one used in the current study. Chapter 3 covers the application of the HOT on aqueous droplets suspended in air. The binary coalescence and bistable axial trapping positions of aqueous aerosols were investigated using elastically-scattered laser light from the trapped droplet. This is a new technique which was developed by the Optical Tweezers group at the University of Leicester. The work described in chapter 4 the HOT adapted to perform resonance Raman measurements and used to investigate the catalytic cycle of membrane-bound human cytochrome P450 incorporated into a vesicle. Chapter 5 lays down the preliminary work in generating a stable reverse emulsion of water-in-fluorocarbon. The emulsion is being developed as a micro-reactor vessel to monitor biological processes in the near future.

A new route for the fabrication of nanoparticles in superfluid helium

Spence, Daniel James January 2015 (has links)
This project intends to establish a new route for the fabrication of nanoparticles using superfluid helium droplets as the growth medium, which offers a high degree of control over the growth of nanoparticles, allowing tailoring to specific applications. Starting from the characterization of the UHV superfluid helium droplet source, we progressively add more and more dopants to the droplets, leading to the formation of small clusters composed of a few molecules and/or metal atoms, and large nanoparticles composed of >104 atoms. The helium droplet source and small clusters are characterized using mass spectrometry while nanoparticles are investigated by transmission electron microscope imaging. By sequential doping we have obtained binary clusters, unlocking the possibility of producing heterogeneous clusters of any two or more materials. In addition, we have fabricated core-shell nanoparticles, and have provided microscopy images that clearly show core-shell structure for nanoparticles fabricated by this route for the first time. Finally, in this project we have obtained the first direct evidence for the presence of quantized vortices in nanoscale superfluid helium, which are then utilized as templates for the growth of nanowires. This opens up new a landscape of helium nanodroplet technology and new possibilities for nanoscience and nanotechnology.

Analysis of metal ions in water using SAM-modified EQCM electrodes

Hewas, Agab Mohamed January 2014 (has links)
The toxic nature of some metal ions makes it necessary to monitor their concentrations in the environment. In this thesis there are two fundamental objectives. The first is to develop a portable electrochemical sensor, which is capable for detecting and monitoring Pb[superscript 2+], Ni[superscript 2+] and Co[superscript 2+] rapidly, selectively and accurately. The second is to develop an extraction procedure for these metal ions by using some ionic liquids (ILs), with the QCM and colorimetric indicators being used to investigate this process. Mercaptosuccinic acid (MSA), 2, 2'-Thiodisuccinic acid (2.2'-TDS), 4-Mercaptobenzoic acid (4-MBA) and 4-Mercaptophenylacetic acid (4-MPAA) thiols were used to prepare self-assembled monolayers on the gold electrodes of 10MHz AT-cut piezoelectric quartz crystal resonators. The extent of immobilisation of each ligand was determined by measuring the resonant frequency change of the crystal and the quality of the SAM was investigated voltammetrically, with [Fe(CN)[subscript 6]][superscript 3-/4-] as a probe redox couple. The focus was on Au-MSA as the most effective ligand for Pb[superscript 2+], Ni[superscript 2+] and Co[superscript 2+]. Adsorption of these metal ions, individually and competitively, by Au-MSA from aqueous solution was measured as a function of solution concentration and the data fitted to a range of isotherms. Structural and compositional information on the SAMs was acquired using Raman spectroscopy and XPS.

Coordination and reactivity of post d 10 cations

Forfar, Laura C. January 2014 (has links)
This thesis investigates the formation and subsequent coordination and reactivity of post d 10 metal cations, specifically cations of the group 11 metals and group 15 and 16 compounds. Cation formation is facilitated by abstraction of a chloride from the starting compound by Lewis acidic gallium trichloride or by exchange of the chloride by reaction with silver hexafluoroantimonate. Simple metal salts, MX (M = Cu, Au, X = Cl, M = Ag, X = OTt) were reacted with GaCl), generating cations in situ, and addition of P 4 yielded three new M -P 4 complexes. Copper and silver formed coordination polymers, whereas gold reacted to form a homoleptic cation, [Au(rrP4)2][GaCI4], in the first example of a white phosphorus complex of gold. The gold complex was particularly sensitive to air and moisture and thus stabilised cations were generated by the abstraction of a halide from ligated copper and gold complexes to further probe the M-P4 interaction. The coordination of white phosphorus yielded complexes of the type [LM(rrp4)][X] and the solid-state structures, characterised by X-ray crystallography, showed P 4 coordinated in 112- fashion in all instances. All the complexes showed the P 4 unit to be fluxional in solution, with the room temperature 3lpeH} NMR spectra showing broad peaks. At low temperatures, the fluxionality within the gold complexes was slowed on the NMR timescale and the peaks resolved into 2nd order splitting patterns. The dynamic behaviour of the P 4 unit coordinated to [LAu t was investigated computationally which revealed a dynamic pathway via a 11 1- transition state. The reactivities of the complexes were tested through a variety of experiments.

Applications of holographic optical tweezers to the manipulation and coalescence of airborne microparticles

Power, Rory M. January 2014 (has links)
Aerosols play a key role in atmospheric science but remain the most poorly quantified factor in predicting anthropogenic radiative forcing. The interaction of aerosol with water vapour regulates the lifetime and reflectivity of clouds, while coagulation governs aerosol removal and a better ' understanding of these processes is crucial. In particular, the ubiquity of organic aerosols, their characteristic phase-behaviour and surface chemistry, provides a fertile area for study and cannot be reproduced in bulk-phases. This thesis reports single-particle studies of aerosol using holographic optical tweezers for the trapping and manipulation of micron-sized aerosol particles. Novel microscopic and spectroscopic techniques were developed and used in tandem for their characterisation. A study of optical trapping forces and their interaction with micro particles was carried out. The influence of a range of experimental parameters and their influence on trapping geometry were explored through particle position tracking, allowing the complex interplay between thermal, optical and inter-particle forces to be studied. Notably, particle size-resolved measurements of Brownian dynamics have experimentally demonstrated resonant behaviour in trapping forces. Studies of aerosol coalescence in optical tweezers have allowed the hydrodynamics of single aerosol particles to be explored. In this regard, the underdamped relaxation of 'inviscid particles was observed from the elastically-scattered light signature. Correspondingly, the frequency of damped surface oscillations was used to probe aerosol surface tension, of key importance in regulating cloud droplet number concentration. This was extended to the study of viscous particles, critical in understanding the kinetics of cloud droplet activation. Overdamped relaxation in morphology was observed, spanning 12 orders of magnitude in time scale and viscosity in amorphous aerosol. Furthermore, the relationship between bulk-diffusivity and viscosity was shown to decouple from the classical theories of Stokes and Einstein, highlighting the microscopic and macroscopic factors governing the phase-behaviour of metastable states approaching a glass-transition.

Novel cyclohexane-based receptors for anion binding and transport

Cooper, James Arthur January 2016 (has links)
This thesis describes a series of novel anionophores and details their synthesis, anion binding and anion transport capabilities. These novel anionophores are based on a cyclohexane scaffold CA, Figure 1), and are simpler to synthesise than the cholapod and decal ins CB and C in Figure 1 respectively) previously reported by the Davis group. Association constants for chloride were evaluated for the new cyclohexane-based receptors in DMSO-d6/0.S% H20 and, where solubility permitted, in water-saturated chloroform. For the novel cyclohexane systems A, higher association constants were obtained where R = Me as opposed to R = H as well as for those receptors possessing electron-deficient aromatics. However the association constants of these novel systems were found to be somewhat lower than those seen previously with cholapods and decalins. Anion transport by these new cyclohexanes was evaluated by the lucigenin method, with the majority of receptors displaying measurable anion transport activity. As with anion binding, higher levels of transport were obtained where R = Me as opposed to R = H and where aromatics were electron-deficient. Certain cyclohexane-based receptors were found to have similar or improved rates of transport when compared to previously reported receptors from the Davis group, including one which is the most active anion transporter reported by the Davis group. Some progress has also been made towards altering the identity of the hydrogen bond donors on the scaffold, further modifying appended alkyl units and a novel squaramide macrocycle.

A real-space study of phase behaviour and slow dynamics in colloid-polymer mixtures

Zhang, Isla January 2014 (has links)
Simple interactions in colloidal systems can often lead to complex phase behaviour and dynamics. In the work presented throughout this thesis, we study several related problems in colloid-polymer mixtures through real-space imaging, mainly through the use of confocal microscopy. We studied the phase separation dynamics in colloid-polymer mixtures crossing over from the regime of fluid-fluid phase separation into gelation. We related dynamics at the mesoscopic structural level to particle-level dynamics. Slow particle dynamics limited the rate of phase separation, and could lead to the arrest of phase separation and thus to gelation. The long-time stability and eventual collapse of colloidal gels were studied. We used a combination of microscopic and macroscopic imaging to observe both changes in the microstructure prior to collapse, and the sedimentation kinetics during collapse. Collapse timescales and collapse behaviour were linked to the phase behaviour and microstructure. The phase behaviour of a colloid-polymer mixture with bidisperse colloidal particles was studied and compared to the equivalent monodisperse systems. Multiple interaction ranges and strengths in the bidisperse system led to novel phase behaviour, including a near-critical region of the phase diagram where larger particles undergo phase separation but smaller particles remain in a one-phase state. Finally, self-assembly of clusters using particles with complementary shapes to promote site-specific bonding was attempted. Electrostatic charge was used to direct bonding in order to encourage the formation of linear clusters. While the formation of well-controlled structures was limited by kinetic effects, site-specific bonding appeared to be successful.

Catalysis in carbon nanoreactors

Solomonsz, William Arran January 2014 (has links)
The work presented in this thesis describes the preparation and application of hollow carbon nanostructures as containers of preparative chemical reactions. The effects of nanoscale confinement in carbon nanoreactors have been shown to dramatically affect the selectivity, activity and stability of catalytic chemical transformations. The optimum structural properties of the nanoreactor have been established by comparing the regioselectivity of molecular catalysts of the hydrosilylation reaction confined in a range of carbon nanostructures. In wide, internally corrugated hollow graphitised nanofibres, the effects of confinement were more prevalent than inside narrower, atomically smooth carbon nanotubes. The specific nature of the interactions induced at the unique reaction environment provided by nanofibres via confinement of nanopartic1e catalytic centres and reactant molecules was elucidated by exploration of the properties of the hydrosilylation reaction using a range of aromatic and aliphatic substrates. The synergy between increased local concentrations of aromatic reactants and the stabilisation of specific reaction intermediate species were critical in determining the regioselectivity of the reaction pathway in nanoreactors. The magnitude of local concentration effects have been quantified for the first time using a competitive hydrosilylation reaction methodology. A greater than three-fold increase in the addition of aromatic hydrosilanes relative to aliphatic analogues has been attributed to maximal1t-1t interactions between the graphitic internal surfaces of nanoreactors and the aromatic reactant molecules. The effects of local concentrations of aromatic molecules were harnessed in the silane oxidation reaction, yielding vital information regarding the critical dimensions of molecules and oligomeric structures that are subject to confinement at the graphitic step-edges of carbon nanoreactors.

Page generated in 0.0381 seconds