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'Physics' in all fields;
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Displaying Results 201 - 225 of 3911 on page 9 of 157
Marked
Mark
Young brown dwarfs and their environments : testing star and planet formation
(2014)
Dawson, Paul
Young brown dwarfs and their environments : testing star and planet formation
(2014)
Dawson, Paul
Abstract:
THESIS 10767
This thesis presents a new contribution to our understanding of the formation of brown dwarfs. From a description of their discovery, to an analysis of their disks and photospheres, it sheds new light on the conditions under which stars, brown dwarfs, and planets, form and evolve. It explores new challenges to further studies of young brown dwarfs, and offers a means of advance.
http://hdl.handle.net/2262/79556
Marked
Mark
Modification and analysis of layered materials with charged particle beams
(2014)
Fox, Daniel
Modification and analysis of layered materials with charged particle beams
(2014)
Fox, Daniel
Abstract:
THESIS 10385
In this work a range of highly focused charged particle beams were used to modify the structure and geometry of materials. Recently discovered two-dimensional (2D) materials were the focus of the work. Fundamental studies on the beam-sample interactions were conducted. This knowledge was then used to achieve nanoscale control of the modification.
http://hdl.handle.net/2262/80025
Marked
Mark
A theoretical study of the strain-modified RKKY interaction in magnetically doped carbon materials
(2015)
Gorman, Paul D.
A theoretical study of the strain-modified RKKY interaction in magnetically doped carbon materials
(2015)
Gorman, Paul D.
Abstract:
THESIS 10636
Graphene-related materials continue to be in the scientific limelight due to near-continuous discoveries regarding their properties and production. The applications envisaged for these super materials are numerous and varied across many disciplines. In addition to this, the simplicity with which the physical properties of these materials can be described makes them particularly popular in theoretical examinations, where the simple dispersion relation for their electrons is of key utility. In this thesis we will examine a number of topics relating to graphene systems, focusing on those doped with magnetic impurities. We will utilize a combination of analytical and numerical methods to explore these topics in graphene and carbon nanotubes.
http://hdl.handle.net/2262/80052
Marked
Mark
Electrochemical energy storage properties of solution processed nanomaterial networks
(2015)
Higgins, Thomas M.
Electrochemical energy storage properties of solution processed nanomaterial networks
(2015)
Higgins, Thomas M.
Abstract:
THESIS 10897
Recently, exciting progress has been made towards the creation of electronic devices via unconventional fabrication routes. For example, liquid processing techniques such as printing and spraying are enabling the production of nanomaterial-based electronic devices that are assembled directly on everyday substrates, such as plastic films. This approach is interesting as it presents an opportunity to develop electronic devices that are mechanically flexible and even transparent. Clearly, these possibilities will lead to a host of applications that are impossible using traditional manufacturing techniques.
http://hdl.handle.net/2262/80070
Marked
Mark
Physics in the extreme: X-ray and optical studies of magnetic white dwarfs and related objects
(2017)
Kennedy, Mark R.
Physics in the extreme: X-ray and optical studies of magnetic white dwarfs and related objects
(2017)
Kennedy, Mark R.
Abstract:
In this thesis, we present results from optical and X-ray studies of compact binary systems. These observations are used to constrain orbital parameters - mainly periods and inclinations for several cataclysmic variables - and examine the accretion structures within the systems. In Chapters 3 and 4, optical and X-ray studies of two interesting cataclysmic variables systems, Lanning 386 and MASTER OTJ192328.22+612413.5, are presented. The X-ray and optical data of these objects suggest that both systems have a high inclination and harbour weakly magnetic white dwarfs with spin periods 17-22 min. There is also a strong focus on studying the long term behaviour in intermediate polars in this thesis. Chapter 5 presents results from Kepler observations of FO Aquarii taken over 69 consecutive days. The shape of the light curve is consistent with the outer edge of the accretion disc being eclipsed and the detected period is longer than the last reported period of 1254.284(16) s, suggesting...
http://hdl.handle.net/10468/4038
Marked
Mark
Contact interface studies of metal and carbon electrodes to germanium nanowires, carbon nanotubes and thin carbon films in associated devices
(2013)
Lutz, Tarek
Contact interface studies of metal and carbon electrodes to germanium nanowires, carbon nanotubes and thin carbon films in associated devices
(2013)
Lutz, Tarek
Abstract:
THESIS 10451
In current state-of-the art electronic devices the electrical contact between source- and drain-electrode plays a crucial role in the performance of the nano size devices. Especially for carbon based materials such as carbon nanotubes and thin graphite layers, the electrical contacting issues become more and more critical to develop carbon based electronic devices. In the present thesis the correlation between charge- injection, of in terms of contact-resistance and specific contact resistivity into a nano-object or nanostructured material and the associated morphology of the contact- interface region, by transmission electron means, is investigated.
http://hdl.handle.net/2262/80300
Marked
Mark
Molecular electronics: first principles and model approaches
(2015)
Melo Souza, Amaury de
Molecular electronics: first principles and model approaches
(2015)
Melo Souza, Amaury de
Abstract:
THESIS 10665
This thesis deals with the application and the development of computational tools for the study of ground state and electronic transport properties of molecular junctions. The calculations can be divided in two main categories, depending on whether they are based on first-principles or model Hamiltonians, respectively. From the point of view of first- principles calculations, the ground state properties are studied within density functional theory (DFT) and the electronic transport within the non-equilibrium Green?s functions (NEGF) formalism. From the point of view of model Hamiltonians, the electronic transport is calculated by means of the density matrix formalism by solving master equations (ME), where all the parameters can be obtained from first-principles calculations. These approaches allow us to study the electronic transport in different regimes: for strong electrode-molecule coupling, where tunneling is the doming mechanism, as well as for weak electronic...
http://hdl.handle.net/2262/80348
Marked
Mark
Design and characterization of widely tunable semiconductor lasers based on etched slots
(2015)
Nawrocka, Marta
Design and characterization of widely tunable semiconductor lasers based on etched slots
(2015)
Nawrocka, Marta
Abstract:
THESIS 10558
Tunable semiconductor lasers play an important role in numerous technologies for optical communication and sensing systems. They are widely used in dense wavelength division multiplexed (DWDM) systems and in other important markets such as trace gas detection for environmental emission motoring. Tunable lasers are also becoming a key component in optical packet and burst switching systems to reduce the latency and increase the capacity of current optical transmission networks.
http://hdl.handle.net/2262/80420
Marked
Mark
Femtosecond laser ablation of metals
(2015)
O'Connell, Gearoid
Femtosecond laser ablation of metals
(2015)
O'Connell, Gearoid
Abstract:
THESIS 10560
Femtosecond laser ablation of metals and dielectrics is currently very topical due to its new and distinctive ablation process and the growing number of applications. The ablation process is rather complex and results in the formation of two distinct plumes, a fast, atomic component, and a slow component comprised mainly of nanoparticles. This thesis describes the results of an investigation into the femtosecond laser ablation of some metals. The experiments were performed for gold, silver, aluminium and tin to compare the differences in their ablation behaviour.
http://hdl.handle.net/2262/80440
Marked
Mark
The structure and flow of disordered wet foams
(2014)
O'Donovan, Cathal B.
The structure and flow of disordered wet foams
(2014)
O'Donovan, Cathal B.
Abstract:
THESIS 10834
This thesis concerns the structure and flow properties of wet foams. We primarily use computer simulations of soft particles to model the behaviour of disordered wet foams. We investigate the properties of 2D and 3D polydis- perse packings of soft particles and the flow of 2D soft discs as model systems for wet foams.
http://hdl.handle.net/2262/80446
Marked
Mark
Liquid phase exfoliation and dispersion of two dimensional materials
(2014)
Smith, R?n?n
Liquid phase exfoliation and dispersion of two dimensional materials
(2014)
Smith, R?n?n
Abstract:
THESIS 10804
Over the last few years, it has become clear that due to their high surface area and novel properties, two dimensional materials will be extremely useful both for fundamental studies and for a range of applications. Previous work has shown that graphene can be produced in large quantities by exfoliation of graphite in certain solvents or surfactants. Of these methods, surfactant dispersion is perhaps the most promising because the use of toxic solvents is avoided. Although previous work has shown that such methods give reasonably large quantities of good-quality graphene, there is still much scope for improvement as well as further investigation.
http://hdl.handle.net/2262/80512
Marked
Mark
Tuning and characterization of self-assembled plasmonic nanoparticle arrays
(2015)
Ualibek, Oral
Tuning and characterization of self-assembled plasmonic nanoparticle arrays
(2015)
Ualibek, Oral
Abstract:
THESIS 10517
This thesis details the development and use of a novel nano-entity fabrication technique. The technique is based on a glancing angle deposition (GLAD) on transparent stepped templates. It is demonstrated that the technique is functionally independent of the deposited material as evidenced by the growth of ordered nanoparticle (NP) arrays of Ag, Au and Cu. Furthermore, the morphology of the systems grown can be changed through choice of the deposition parameters. This allows a complete tuning of the plasmonic resonances measured mainly with reflectance anisotropy spectroscopy. Finally, the self-assembled noble metal NP arrays have been used as building blocks for Surface Enhanced Raman Spectroscopy (SERS) experiments, demonstrating a strong enhancement in the Raman signal measured.
http://hdl.handle.net/2262/80530
Marked
Mark
Development of a cantilever based device for the investigation of malaria vaccine cross-reactivity
(2015)
Walther, Michael
Development of a cantilever based device for the investigation of malaria vaccine cross-reactivity
(2015)
Walther, Michael
Abstract:
THESIS 10520
The invention of the atomic force microscope and subsequent implementation of cantilevers as transducers for sensing applications laid the foundation for the current use of cantilevers as biosensors. This application involves functionalisation of the surface with a bio-recognition layer, immersion of the sensors in a physiological environment and operation in either the static or the dynamic mode to transduce the biological interaction into a measurable signal. When operated in dynamic mode, the cantilever is actuated at its resonance frequency. Mass uptake on the sensor results in a resonance frequency shift, thereby transforming the cantilever into a sensitive microbalance.
http://hdl.handle.net/2262/80542
Marked
Mark
Localised surface plasmon mediated energy transfer in quantum dot systems
(2015)
Zhang, Xia
Localised surface plasmon mediated energy transfer in quantum dot systems
(2015)
Zhang, Xia
Abstract:
THESIS 10599
Nonradiative energy transfer to metal nanoparticles (NPs) is a technique used for optical- based distance measurements which is often implemented in sensing. Both Forster resonant energy transfer (FRET) and nanometal surface energy transfer (NSET) mechanisms have been proposed for emission quenching in proximity to metal NPs. Here quenching of emission of colloidal quantum dots (QD) in proximity to a monolayer of gold NPs is investigated. Five differently sized CdTe QDs are used to probe the wavelength dependence of the quenching mechanism as their emission peak moves from on resonance to off resonance with respect to the localized surface plasmon (LSP) peak of the Au NP layer.
http://hdl.handle.net/2262/80552
Marked
Mark
Silicon-based resonant-cavity-enchanced photodiode with a buried SiO2 reflector
(1999)
Sinnis, Vasileios S.; Seto, M.; 't Hooft, G. W.; Watabe, Y.; Morrison, Alan P.; Ho...
Silicon-based resonant-cavity-enchanced photodiode with a buried SiO2 reflector
(1999)
Sinnis, Vasileios S.; Seto, M.; 't Hooft, G. W.; Watabe, Y.; Morrison, Alan P.; Hoekstra, W.; de Boer, W. B.
Abstract:
We report on a silicon-based resonant cavity photodiode with a buried silicon dioxide layer as the bottom reflector. The buried oxide is created by using a separation by implantation of oxygen technique. The device shows large Fabry-Perot oscillations. Resonant peaks and antiresonant troughs are observed as a function of the wavelength, with a peak responsivity of about 50 mA/W at 650 and 709 nm. The leakage current density is 85 pA/mm(2) at -5 V, and the average zero-bias capacitance is 12 pF/mm(2). We also demonstrate that the buried oxide prevents carriers generated deep within the substrate from reaching the top contacts, thus removing any slow carrier diffusion tail from the impulse response. (C) 1999 American Institute of Physics. (DOI: 10.1063/1.123499).
http://hdl.handle.net/10468/4410
Marked
Mark
Photons in restricted topologies
(2016)
Ballantine, Kyle
Photons in restricted topologies
(2016)
Ballantine, Kyle
Abstract:
THESIS 10965
In this work we discuss three different aspects of the topological classification of propagating beams of light. This topological classification relies on global properties of the light beam, which are insensitive to local disorder or perturbations. However, topological invariants calculated for scalar fields may not fully describe a beam of light, which consists of a vector field which points in a particular direction at a given time. We consider the extension of the topological classification of light beams to cases where the polarisation is allowed to vary. When the phase of a scalar electric field varies around a point, that field carries angular momentum, proportional to the magnitude of the change of phase. When the polarisation also varies, we show that there is a new angular momentum which is carried by such a beam. This generalised angular momentum accounts for the possible winding in both the direction of polarisation and the phase around a point. We show ...
http://hdl.handle.net/2262/82893
Marked
Mark
Morphology and time evolution of thermal jets associated with low-mass young stars
(2016)
Ainsworth, Rachael Evelyn
Morphology and time evolution of thermal jets associated with low-mass young stars
(2016)
Ainsworth, Rachael Evelyn
Abstract:
THESIS 11249
Jets are one of the most ubiquitous processes in the Universe, observed from astrophysical objects spanning an extreme range of masses. Despite the difference in scale, the similar morphologies and other characteristics between astrophysical jets suggest that the launching mechanism may be universal, although this suggestion is still controversial. It is widely accepted that many astrophysical jets, including those from young stars, are driven by accretion and launched through magnetohydrodynamic (MHD) processes, in particular, the centrifugal acceleration of material along magnetic field lines anchored to a rotating disk. However, many open questions remain: Where is the precise origin of jet launch? What is the role of the magnetic field? How does the jet evolve with time? These questions are best answered within the domain of low-mass star formation, as the large numbers of (relatively) nearby young stellar objects (YSOs) uniquely allows the region of jet launch ...
http://hdl.handle.net/2262/82887
Marked
Mark
Mechanically alloyed Sm-Co hard magnetic materials
(2002)
Gallagher, Kathleen
Mechanically alloyed Sm-Co hard magnetic materials
(2002)
Gallagher, Kathleen
Abstract:
THESIS 7062
In the recent search for new high-temperature permanent magnet materials, research has concentrated on bulk Sm2Co17-type magnets. The usual approach to development of coercivity in bulk Sm-Co based materials is to prepare a nanophase cellular microstructure with cell walls that act as pinning sites (precipitation hardening). An alternative approach is to mechanically alloy the materials, with the goals of reducing particle size, forming metastable phases, and introducing planar crystalline defects that act as pinning sites. The objectives of this work were to produce SmZr(Co,Fe) alloys by mechanical alloying, to study the effect of composition on magnetic properties, and to assess whether these materials would make good high temperature magnets. In this work, SmzZry(Co1-xFex)100-z-y nanocrystalline powders, where 13 ? z ? 15, 0 ? y ? 5, and 0 ? x ? 0.5 (compositions intermediate to SmCo5 and Sm2Co17), were prepared by mechanical alloying followed by an optimized anne...
http://hdl.handle.net/2262/83128
Marked
Mark
The surface energetics of low dimensional nanomaterials
(2017)
Ferguson, Auren
The surface energetics of low dimensional nanomaterials
(2017)
Ferguson, Auren
Abstract:
THESIS 11405
Over the past two decades, there has been great interest in the discovery and research of low dimensional nanomaterials. These materials, such as carbon nanotubes and graphene, are amongst the strongest and most conductive materials ever discovered and show great promise for applications such as, electronics, renewable energy and polymer composites. Great strides in their understanding have been achieved, however, one material property that isn't well understood is their surface energy. Surface energy is a measure of the energy required to produce a unit area of surface and it controls how a material interacts with its environment. Therefore, understanding the surface energetics of low dimensional materials is very important, particularly in the case of the exfoliation of layered 2D materials in liquid, e.g. the exfoliation of graphite to produce graphene. This thesis develops a framework for the measurement and analysis of the surface energy of layered materia...
http://hdl.handle.net/2262/83125
Marked
Mark
The growth, formation and magnetic behaviour of noble metal capped atomic wires
(2016)
Duignan, Martin James
The growth, formation and magnetic behaviour of noble metal capped atomic wires
(2016)
Duignan, Martin James
Abstract:
THESIS 11313
Transition metal nanostructures have been observed to have several interesting magnetic properties. Nanowire of atomic widths on a vicinal surfaces show these interesting properties clearly, which also vary with the width of the wires. If these nanostructures are to be of future use in devices they will have to be able to survive outside of a ultra high vacuum environment. The most obvious means to prevent the nanowires from oxidising is to cap noble metal or inert overlayer. It is a system of this type that is the subject of the study, specifically cobalt atomic wires on a vicinal platinum surface, which have been capped with four to eight monolayers of gold. During this work the growth of the wires, and the capping layer was investigated using low energy electron diffraction, scanning tunnelling microscopy and Auger electron spectroscopy. Along with their lifetime before oxidisation outside of a ultra high vacuum environments by x-ray absorption and photoemission ...
http://hdl.handle.net/2262/83124
Marked
Mark
Interactions between quantum emitters in the presence of plasmonic nanostructures
(2016)
Karanikolas, Vasilios D.
Interactions between quantum emitters in the presence of plasmonic nanostructures
(2016)
Karanikolas, Vasilios D.
Abstract:
THESIS 11217
In this thesis the spontaneous emission, of a single quantum emitter, and the energy transfer, between a pair of quantum emitters, rates are investigated in the presence of conducting nanostructures. The competition between these two rates leads to the introduction of the energy transfer efficiency. Metallic and graphene nanostructures are investigated. The excitation of surface plasmon modes, hybrid modes of the conduction band electrons and the electromagnetic field, is crucial for enhancing the spontaneous emission and energy transfer rates, compared with their free space values. The near field of the quantum emitters can directly excite the surface plasmon modes. The surface plasmon wavelengths depend on the shape and material properties of the nanostructures. The spectral and distance dependences of the spontaneous emission and energy transfer processes are studied. It is of particular interest, from the point of view of applications, to be able to manipulate t...
http://hdl.handle.net/2262/83132
Marked
Mark
Electronic properties of doped carbon-based nanostructures
(2016)
Lawlor, James
Electronic properties of doped carbon-based nanostructures
(2016)
Lawlor, James
Abstract:
THESIS 11188
Carbon-based nanostructures have been at the forefront of scientific research for the last couple of decades due to their unique properties, giving rise to many potential applications especially for electronics. Their simple structure and the ease with which many of their electronic properties can be studied makes them particularly interesting from a theoretical perspective. These electronic properties can be modified by altering the atomic structure, for example through the introduction of dopants to the system. In this thesis a thorough investigation of several topics related to doping in bulk graphene, narrow strips of graphene (nanoribbons) and carbon nanotubes is be undertaken. A tight-binding Green function formalism is developed and then employed in the study of impurity modelling, Friedel oscillations, impurity scattering and sublattice segregation of impurities. The distinct advantage of using such a mathematically transparent approach is that it better rev...
http://hdl.handle.net/2262/83133
Marked
Mark
LOFAR tied-array imaging and spectroscopy of solar radio bursts
(2016)
Morosan, Diana E.
LOFAR tied-array imaging and spectroscopy of solar radio bursts
(2016)
Morosan, Diana E.
Abstract:
THESIS 11418
The Sun is the source of the most powerful explosions in the solar system such as solar ares and coronal mass ejections (CMEs). These explosions can occur up to several times per day and they release up to 1032 ergs (1025 J) of energy. Radio emission at frequencies ranging from a few kHz to a few hundred GHz is often associated with these energetic phenomena. At low radio frequencies (<200 MHz), the Sun has only been studied at selective frequency bands due to instrumental and imaging limitations of previous radio telescopes. With the recent commissioning of the Low Frequency Array (LOFAR), the low frequency Sun can now be studied with unprecedented spectral, spatial and temporal resolution. The low frequency radio spectrum and the possibility of imaging of radio bursts at low frequencies was explored with LOFAR, which has only been done before on limited occasions with older radio telescopes. A new observational method was applied to solar observations, tied-arr...
http://hdl.handle.net/2262/83465
Marked
Mark
Simulation and analysis of foam structure
(2017)
Whyte, David
Simulation and analysis of foam structure
(2017)
Whyte, David
Abstract:
THESIS 11397
We use the method of bond-orientational order parameter analysis on X-ray tomographic data to investigate the interal structure of an experimental sample of ~25 000 microbubbles. By employing appropriate cutoff distances in the parameter space, we see that over the 7 days of the experiment the amount of ordering in the sample increases signficantly. In line with previous experiments and simulations, we see a preference for face-centred cubic (fcc) ordering over hexagonal close-packed (hcp). We present a simple geometrical argument concerning the ideal shapes of bubbles at an arbitrary liquid fraction between the dry and wet limits. By applying the appropriate transformation to an fcc bubble at a given liquid fraction, we obtain a ?trial? hcp bubble of the same surface area. This surface can be relaxed, proving that the hcp structure has lower energy than the fcc. We perform Surface Evolver simulations of fcc and hcp bubbles over the full range of liquid fractions. T...
http://hdl.handle.net/2262/83457
Marked
Mark
The structures and extents of the chromospheres of late-type stars
(2015)
? Riain, Neal
The structures and extents of the chromospheres of late-type stars
(2015)
? Riain, Neal
Abstract:
THESIS 10990
The chromosphere is the region of a star, above what is traditionally defined as the stellar surface, from which photons freely escape. As the definition implies, this region is characterised by complexity, non-equilibrium, and specifying its structure is a vastly non-linear, nonlocal problem. In this work we are concerned with the chromospheres of late-type stars, objects of spectral type K to M, the thermodynamic structure, extent, and heating mechanisms of whose chromospheres are not well understood. We use a number of observational and computational methods in order to gain a detailed quantitative understanding of these chromospheres. We construct a model to compute the mm, thermal bremsstrahlung flux from the chromospheres of late-type objects, based on a number of simplifying assumptions concerning their thermodynamic structure. We compare this model with archival and recent observations, and find that the model is capable of reproducing the observed flux from...
http://hdl.handle.net/2262/85162
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