Tesi sul tema "Nanostructural characterisation"
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1
Buha, Joka School of Materials Science & engineering UNSW. "Interrupted ageing of Al-Mg-Si-Cu alloys". Awarded by:University of New South Wales. School of Materials Science and engineering, 2005. http://handle.unsw.edu.au/1959.4/20794.
Testo completoAbstract (sommario):
This thesis systematically investigates the effects of a recently developed modified ageing procedure of aluminium alloys, termed the T6I6 temper, on the microstructural development and mechanical properties of the Al ??? Mg ??? Si - Cu alloy 6061. For the T6I6 temper, a conventional single stage T6 temper is interrupted by an ageing period at a reduced temperature (65??C) to facilitate secondary precipitation, before resuming the final ageing at the temperature of the initial T6 treatment. The T6I6 temper was found to cause simultaneous increases in tensile properties, hardness, and toughness as compared with 6061 T6. Al ??? Mg ??? Si ??? Cu alloys are medium strength alloys widely used in the automotive industry and their further improvement is underpinned by stringent demands for weight reduction placed on the transportation industry in recent years. The potential for further improvement of the mechanical properties was found in the control of secondary precipitation that may take place even in some fully aged alloys when exposed to reduced temperatures. The overall improvement in the mechanical properties of 6061 T6I6 was attributed to the formation of finer and more densely dispersed precipitates in the final microstructure. The refinement of precipitates was facilitated by control of the precipitation processes and gradual evolution of the microstructure throughout each stage of the T6I6 treatment. The results indicated that the concentration and the chemical environment of the vacancies controlled the precipitation processes in this alloy. Findings also show that the proportion of the different precipitate phases present in the final microstructure, as well as the amount of the solute in these precipitates, can be controlled and modified utilizing secondary precipitation. A number of analytical techniques were used in this study. The evolution of the microstructure was studied using Transmission Electron Microscopy (TEM), High Resolution TEM (HRTEM) and Three Dimensional Atom Probe (3DAP). Vacancy-solute interactions were studied using Positron Annihilation Lifetime Spectroscopy (PALS) and 3DAP. The distribution of the solute was studied using 3DAP and Nuclear Magnetic Resonance (NMR). Differential Scanning Calorimetry (DSC) was used to identify precipitation reactions and to determine the stability of vacancy-associated aggregates.
2
Krishnan, Jagadamma Lethy. "Characterisation of nanostructured light emitters". Thesis, University of Strathclyde, 2012. http://oleg.lib.strath.ac.uk:80/R/?func=dbin-jump-full&object_id=17192.
Testo completoAbstract (sommario):
Group III-nitride semiconductors are the dominant inorganic solid state light emitting materials, spanning the UV to infra-red spectral range. InGaN/GaN based LEDs and lasers are commercially available and intense research is being pursued to improve their efficiency. One practical approach is the development of functionalised and/or improved materials patterned on a nanometre length scale. This thesis presents the optical, morphological and compositional characterisation of III-nitride based nanostructured light emitters. The III-nitride nanostructures studied are GaN coalesced above arrays of either nanopyramids or nanocolumns, semipolar and nonpolar InGaN QWs on the facets of GaN nanopyramids, and thin epilayers of AlInN and AlInGaN. Spatially resolved optical characterisation of nano-ELOG GaN layers revealed a shift in the exciton-related band edge emission across the coalesced layer. This is related to Si doping and to strain effects. Study of the semipolar {1011} InGaN QWs grown on the facets of GaN nanopyramids identified a blue shift in QW emission energy as the sampled region is moved up the pyramid facets. This shift is found to follow the release of the tensile strain towards the top of nanopyramid. Luminescence properties of nearly lattice matched AlInN epilayers investigated using CL, PL and PLE spectroscopic techniques revealed that the emission and bandgap energy of the AlInN layers are at higher energy than that of GaN. Results obtained from polarisation resolved PL measurements of AlInN epilayers point to two possible implications: the observed higher energy AlInN emission is either related to defects or this emission is due to carrier recombination occurring in InN clusters similar to those of InGaN epilayers. Optical properties of thin AlInGaN epilayers investigated using PL and PLE spectroscopy revealed a redshift in bandgap energy with increase in InN fraction. The observed spatial intensity fluctuations are discussed in terms of the InN compositional fluctuations and inhomogeneous strain effects.
3
Smith, Nathan. "Characterisation of zinc oxide nanostructures". Thesis, Swansea University, 2015. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.678399.
Testo completo
4
Collins, Christopher J. A. "Electrical characterisation of organic nanostructures". Thesis, Bangor University, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.519555.
Testo completo
5
Borowiak-Palen, Ewa. "Synthesis and characterisation of molecular nanostructures". Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2004. http://nbn-resolving.de/urn:nbn:de:swb:14-1092389977453-71487.
Testo completoAbstract (sommario):
In this thesis, bulk and local scale spectroscopic and microscopic tools have been applied to investigate the purified raw material of SWCNT and synthesized MWBNNT, BN-nanocapsules, B-doped SWCNT and SiC nanostructures. Using bulk scale sensitive techniques, including optical absorption spectroscopy, Raman spectroscopy, high-resolution electron energy-loss spectroscopy, the average response of the whole sample is obtained. On the other hand, on a local scale transmission and scanning electron microscopy as well as TEM-electron energy-loss spectroscopy provide information on single tubes or other nanostructures. First, diverse chemical and oxidation methods for the purification of as-produced SWCNT were presented. Purified samples were investigated using TEM and OAS. The analysis of the optical absorption spectra in the UV-Vis energy range revealed that some of the chemical treatments are harmful to nanotubes. In contrast to the chemical treatments an oxygen burning procedure was used on the raw material in high vacuum and a temperature range 450?650oC. The purification processes of SWCNT by HNO3 and oxygen burning procedures resulted in SWCNT comprised of selected diameters and a reduced diameter distribution. Both HNO3 and oxygen burning treatments can be used to selectively remove SWCNT with smaller diameters from the samples. In addition, an adapted substitution reaction was used for the synthesis of multiwall boron nitride nanotubes. It was shown that the IR-response of MWBNNT can be used as a fingerprint to analyse MWBNNT. As in h-BN for the analysis one has to be aware of the sample texture and the LO-TO splitting of the IR-active modes. TEM images and B1s and N 1s excitation edges of the grown material reveal the presence of multiwall BN nanotubes with an inner diameter of 3.1 nm and with a larger interplanar distance than in h-BN. The electronic properties of the multiwall BN nanotubes as derived from the q-dependent dielectric function e(w,q) are dominated by the band structure of the hexagonal-like BN sheets, as revealed by the large degree of momentum dispersion observed for the p and s+p plasmons, in agreement with that previously reported for different graphitic allotropic forms. Moreover, a fast and highly efficient synthesis route to produce BN nanocapsules with a narrow size distribution was developed. This was achieved by an adapted substitution process using SWCNT as templates followed by a rapid cooling treatment. The IR responses reveal the strong dipole active fingerprint lines of h-BN with distinct differences, which are due to texturing effects and which highlight the BN nanocapsules potential application as a reference source when deriving the sp2 to sp3 ratio in BN species due to their random orientation Furthermore, the idea of substitution was used for the systematic studies of B-doped SWCNT. The experiments carried out have resulted in 1, 5, 10, and 15 % boron incorporated into the single wall carbon nanotubes. Core level excitation spectroscopy of the B1s and C1s edges revealed that the boron atoms substitute carbon atoms in the tube lattice keeping an sp2-like bond with their nearest C neighbour atoms. Our results show that a simple rigid band model as has been applied previously to intercalated SWCNT is not sufficient to explain the changes in the electronic properties of highly doped B-SWCNT and a new type of a highly defective BC3 SWNT with new electronic properties is obtained. Finally, different silicon carbide nanostructures were produced. The spectroscopic and microscopic data led to a good understanding of the formation process. NH3 acts as a source of hydrogen that plays a key role in the formation of the structures through its ability to decompose SiC at high temperature such that along with the stacking faults that arise from the many polytypes of SiC the produced SiC nanorods become porous then hollow and eventually are completely decomposed.
6
Oliver, Rachel. "Growth and characterisation of nitride nanostructures". Thesis, University of Oxford, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.400219.
Testo completo
7
Chivall, J. M. "Growth and characterisation of uranium nanostructures". Thesis, University College London (University of London), 2012. http://discovery.ucl.ac.uk/1348539/.
Testo completoAbstract (sommario):
Uranium is the only element in the periodic table to exhibit a charge-density wave and superconductivity at ambient pressure. The competition between these effects in technologically important high-temperature superconducting systems has come under increasing scrutiny, and uranium offers a model system in which to study the CDW. However, the element is difficult to grow in single-crystals in the bulk. We describe the growth by magnetron sputtering and characterisation of single-crystal epitaxial thin- films of alpha-uranium in the (110) orientation on the Nb(110)/A-plane sapphire buffer layer/substrate system. We use X-ray scattering methods to determine the influence of the thickness of the component layers of the samples on the microstructure, and find that there is a non-trivial dependence of the microstructural state of the uranium layers on both the thickness of the uranium layers themselves and the thickness of the niobium buffer layers upon which they are grown. In particular, the widths of the uranium peaks decrease when the uranium layer thickness is increased, but increase when the buffer layer thickness is increased. An extensive review of the methods for characterisation of thin- film microstructures using X-ray diffraction is given, and several widely used models and interpretations are critically discussed, in particular those given in the many instances in which two-component line shapes are seen in transverse scans from thin- films. We also use X-ray diffraction from high-intensity synchrotron sources to characterise the charge-density wave state in these samples, and discuss the effect of uranium-layer thickness on its characteristics as a function of temperature. Important differences between the CDW seen in bulk uranium and in thin- films are seen and discussed in terms of the microstructure of the films. In particular, no incommensurate{commensurate transition is seen in the films, and a large intensity asymmetry is seen between the 2+2+1± and 2+2-1± CDW satellites. Furthermore, the correlation-length of the CDW is limited in the plane of the film, and dependent on the thickness of the uranium layers.
8
Ho, G. W. "Synthesis, characterisation and properties of nanostructured materials". Thesis, University of Cambridge, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.604107.
Testo completoAbstract (sommario):
Synthesis of controlled quality and quantity of nanostructured materials form the basis of future nanotechnology building blocks. The understanding of fundamental properties, creation of complex hierarchical nanostructured materials and development of nanotechnologies are research areas which follow closely after the synthesis of nanomaterials. First and foremost, the key growth parameters of vapour-phase synthesis were identified so as to control the growth of nanomaterials with desired physical dimensions and chemical compositions. The synthesized nanomaterials were characterised using various chemical and structural analysis techniques in a complementary fashion. In addition, various self-assembly growth techniques were used to engineer the growth of complex nanostructures. The use of lithography; photolithography and ion beam lithography to generate micro to nano dimension catalyst patterns proved to be a valuable guide for selective growth of nanowires, whilst the use of zinc oxide polyhedron crystals and grain-boundary textured Cu templates have successfully produced a variety of interesting hierarchical nanostructures. Essentially, the templates act as a structure directing medium to intercede the growth in a confined manner. Success in the growth of one-dimensional single-crystal nanowires is the focus of interest since they offer the potential to answer fundamental questions about the effect of dimensionality on physical properties and are expected to play a central role in applications ranging from molecular electronics to scanning microscopy probes. Finally, with the availability of nanostructures in desirable crystal structures and chemical compositions, studies on their physical properties and phenomena were performed. Interesting properties such as the wettability and electrical properties were investigated. In particular, silicon carbide nanowire flowers show remarkable surface hydrophobicity and elasticity attributed to the unconventional multi-directional assembly of nanowires. The zinc oxide nanowires, on the other hand, exhibit superior electrical conductivity due to their clean surfaces and perfect crystallinity nature.
9
Houchin, Rachael May. "Spectroscopic and microscopic characterisation of carbon nanostructures". Thesis, Durham University, 2011. http://etheses.dur.ac.uk/3414/.
Testo completoAbstract (sommario):
Currently, carbon nanotubes (CNTs) are produced using a variety of techniques which yield CNT materials with wide ranging levels of chemical purity and structural perfection. Consequently, characterising CNT materials accurately is of utmost importance if potential applications of CNTs are to be realised on a large scale. In this work four commercially available CNT samples are characterised using a number of techniques, namely: scanning electron microscopy (SEM); high resolution transmission microscopy (HRTEM); energy dispersive x-ray analysis (EDX); Auger electron spectroscopy (AES); low-loss electron energy loss spectroscopy (low-loss EELS); ultra-violet photoemission spectroscopy (UPS); x-ray photoemission spectroscopy (XPS) and Raman spectroscopy. The information provided by these techniques is assessed in their ability to characterise different CNT materials. The definition of CNT ‘quality’ is also discussed and the ability of these techniques to determine such a property is considered. A significant part of ascertaining the ‘quality’ of a CNT sample lies in understanding the nature and number of defects in the walls of these materials. In this work, defects are introduced into the lattice of different CNT species using 1.5 keV Ar+ ions and the effects are monitored using XPS. In particular, the resultant reactivity of irradiated CNTs to ambient atmospheric oxygen is investigated, which is found to be markedly enhanced for CNTs with one wall when compared to those with multiple walls. It is also demonstrated that the type of incident ion and irradiation dose can be used to selectively control the level and nature of the surface composition of oxygen functionalised SWCNTs. Many applications of fullerenes require detailed understanding of how these molecules interact with surfaces and the perturbations this induces. In this work the interaction of C60 with highly ordered pyrolytic graphite (HOPG) and Ni(110) is studied using scanning tunnelling microscopy (STM), low energy electron diffraction (LEED), XPS and UPS. Investigation focuses on a novel two-dimensional solid-vapour phase in the C60-HOPG system and the C60-induced reconstruction of the Ni(110) surface.
10
White, Richard. "Synthesis and characterisation of three-dimensional nanostructures". Thesis, University of Cambridge, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.612473.
Testo completo
11
Schönherr, Piet. "Growth and characterisation of quantum materials nanostructures". Thesis, University of Oxford, 2016. https://ora.ox.ac.uk/objects/uuid:7dca792e-4236-4d19-aa59-7c9c3cb5d0e4.
Testo completoAbstract (sommario):
The three key areas of this thesis are crystal synthesis strategies, growth mechanisms, and new types of quantum materials nanowires. The highlights are introduction of a new catalyst (TiO2) for nanowire growth and application to Bi2Se3, Bi2Te3, SnO2, and Ge nanowires; demonstration of step-flow growth, a new growth mechanism, for Bi2Te3 sub-micron belts; and the characterisation of the first quasi-one dimensional topological insulator (orthorhombic Sb-doped Bi2Se3) and topological Dirac semimetal nanowires (Cd3As2). Research into new materials has been one of the driving forces that have contributed to the progress of civilisation from the Bronze Age four thousand years ago to the age of the semiconductor in the 20th century. At the turn to the 21st century novel materials, so-called quantum materials, started to emerge. The fundamental theories for the description of their properties were established at the beginning of the 20th century but expanded significantly during the last three decades based, for example, on a new interpretation of electronic states by topological invariants. Hence, topological insulator (TI) materials such as mercury-telluride are one manifestation of a quantum material. In theory, TIs are characterised by an insulating interior and a surface with spin-momentum locked conduction. In real crystals, however, the bulk can be conducting due to crystal imperfections. Nanowires suppress this bulk contribution inherently by their high surface-to-volume ratio. Additionally, trace impurity elements can be inserted into the crystal to decrease the conductance further. These optimised TI nanowires could provide building blocks for future electronic nanodevices such as transistors and sensors. Initial synthesis efforts using vapour transport techniques and electronic transport studies showed that TI nanowires hold the promise of reduced bulk contribution. This thesis expands the current knowledge on synthesis strategies, crystal growth mechanisms, and new types of quantum materials nanowires. Traditionally, gold catalyst nanoparticles were used to grow TI nanowires. We demonstrate that they are suitable to produce large amounts of nanowires but have undesired side-effects. If a metaloxide catalyst nanoparticle is used instead, quality and even quantity are significantly improved. This synthesis strategy was used to produce a new TI which is built from chains of atoms and not from atomic layers as in case of previously known TIs. The growth of large nanowires with a layered crystal structure leads to step-flowgrowth, an intriguing phenomenon in the growth mechanism: New layers grow on top of previous layers with a single growth frontmoving fromthe root to the tip. These wires are ideal for further electronic characterisation that requires large samples. The nanowire growth of tin-oxide will also be discussed, a side project that arose from my growth studies, which is useful for sensor applications. Under certain conditions it forms tree-like structures in a single synthesis step. All of the aforementioned growth studies are carried out at atmospheric pressure. A separate growth study is carried out in ultra-high vacuum to assess the transferability of the growth process towards the cleanliness requirements of the semiconductor industry. If two quantum materials are joined together, exotic physics may emerge at the interface. One of the goals of TI research is the experimental observation of Majorana fermions, exotic particles which are their ownantiparticles with potential applications in quantum computing that may appear in superconductor/TI hybrid structures. We have synthesised such structures and initial characterisation suggests that the resistivity increases when they are cooled below the critical temperature of the superconductor. Beyond TIs, a new type of quantum material, called a topological Dirac semimetal, opens new realms of exotic physics to be discovered. Nanowires are grownfroma material which has recently been discovered to be a topological Dirac semimetal. Their growth mechanism is characterised and an extremely high electron mobility at room temperature is measured. The contribution of this thesis to the field is summarised in Fig. 1. Its core is the study of the growth mechanism of quantum materials which will be vital for future development of applications and fundamental research.
12
Ureña, Begara Fernando. "Local characterisation of strain in silicon nanostructures". Thesis, University of Newcastle upon Tyne, 2014. http://hdl.handle.net/10443/2378.
Testo completoAbstract (sommario):
Strain engineering is used in the microelectronics industry for fabricating micro- and nano-electromechanical systems (MEMS and NEMS) and state-of-the-art metal-oxide-semiconductor field-effect transistors (MOSFETs). In these devices suspended silicon beams, films and nanowires are widely used. However, the mechanical, thermal and electrical properties of silicon change significantly at the nanoscale. Therefore, an accurate knowledge of the size effect on these properties, the role of the surface and an accurate characterisation of the stress and strain distribution in these devices is needed for a complete understanding of the device operation. Likewise, state-of-the-art MOSFETs incorporate strain into the channel to improve performance due to a carrier mobility enhancement compared with unstrained silicon channel transistors. However, the mobility enhancement especially at high vertical electric fields (where commercial MOSFETs operate), is still not well understood. The SiO2/Si interface roughness exhibits, at the nanoscale, scaling behaviour with the scale of observation. However, to date, there is no experimental study of the SiO2/Si interface roughness scaling behaviour with strain. This study is needed to better understand the surface roughness scattering-limited mobility of electrons and holes in strained devices. Raman spectroscopy is a widely used technique to characterise strain. However, the conversion of Raman peak shifts to strain values requires a strain-shift coefficient. Traditionally, the reported strain-shift coefficients have been determined from experiments performed in bulk material. The applied stress has also been limited within the range 0 – 2 GPa. This range is reasonable for bulk silicon characterisation but is too narrow for silicon nanostructures and devices where higher stress values are often favourable for improving performance. Consequently, there is an outstanding need to find appropriate strain-shift coefficients for silicon nanowires and thin films under large values of stress. In this thesis strain in silicon nanostructures is experimentally and theoretically investigated for strain values ranging from 0 to 3.6%. Strain has been characterised using scanning electron microscopy (SEM), Raman spectroscopy, and theoretically with analytical calculations and finite element simulations. The combination of these techniques and the large number of samples (up to 85) has allowed the accurate determination of the ii strain-shift coefficient for the technologically important (100) silicon surface and for stress values up to 4.5 GPa. The work also enables a better understanding of the changes in silicon properties with strain when device dimensions are reduced to the nanoscale. The size dependency of the Young‟s modulus, fracture strain, thermal conductivity and the role of the surface in the size dependent physics are also investigated. It is found that some properties such as the fracture strain change with the dimensions of the sample whereas others such as the Young‟s modulus and thermal conductivity do not change. Finally, the impact of uniaxial and biaxial strain on the surface roughness of silicon nanostructures and thin films has been analysed by atomic force microscopy (AFM). It is found that the silicon surface roughness changes in different manner with uniaxial and biaxial strain. The results show that the silicon surface roughness is self-affine with strain and that this behaviour has to be considered within the models used to describe the carrier mobility in MOSFETs at high vertical electric fields.
13
Etok, S. E. "Structural characterisation and in vitro behaviour of apatite coatings and powders". Thesis, Faculty of Medicine and Biosciences, 2009. http://hdl.handle.net/1826/3973.
Testo completoAbstract (sommario):
Hydroxyapatite (HAP) coatings are used in orthopaedic surgery for bone regeneration.
Current methods of phase quantification of HAP coatings suffer from drawbacks. A
novel methodology of quantitative phase analysis of HAP coatings has been devised and
validated. This method, based on whole pattern fitting with a fundamental parameters
approach, incorporates amorphous calcium phosphate (ACP) and apatite phases into
structural refinements.
A comparison of the structural and chemical properties of plasma sprayed (PS) and novel
electrodeposited (ED) HAP coatings has been conducted. ED coatings contained less
ACP and more preferred orientation than the PS coatings, although the stoichiometry was
similar.
In vitro investigations of PS and ED coatings in simulated body fluid and foetal calf
serum revealed that both are bioactive. A carbonated apatite layer produced on the ED
coatings was -0.7μm thick with a stoichiometry and chemical constituents similar to that
of natural bone apatite.
PS coatings produced a nanocrystalline carbonated apatite layer (-4μm). For the first
time it has been possible to model crystalline HAP and nanocrystalline apatite as
independent phases and obtain accurate lattice parameters for each.
A positive linear correlation has been made between microstrain and the solubility of
HAP and carbonated apatites. Dissolution studies have shown that the behaviour of HAP
and carbonated apatite is dominated by crystallite size at low undersaturation and by
crystallite size and microstrain at high undersaturation for crystallites between -30OA-
1000A. Metastable equilibrium occurred for crystallites <_400A at low undersaturation.
Carbonate content did not affect the solubility or dissolution behaviour.
A novel technology for coating polymeric tape with HAP for potential use in anterior
cruciate ligament reconstruction has been devised. Mechanical tests have demonstrated
that no adverse properties are induced by the coating technology. Cell culture studies
have shown that the HAP layer is capable of enhanced attachment, proliferation and
differentiation of osteoblast cells compared to uncoated tape.
14
Grant, Victoria Anne. "Growth and characterisation of III-V semiconductor nanostructures". Thesis, University of Nottingham, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.490983.
Testo completoAbstract (sommario):
This thesis describes the growth and characterisation of III-V semiconductor materials and nanostructures. The material was grown by molecular beam epitaxy (MBE) and characterised using a range of techniques including atomic force microscopy (AFM), cross-sectional scanning tunnelling microscopy (XSTM) and x-ray diffraction (XRD).
15
Nicholls, David Peter. "The growth and characterisation of zinc oxide nanostructures". Thesis, University of Bristol, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.441689.
Testo completo
16
Baynes, Nicholas de Brissac. "Ultrafast characterisation of gallium arsenide devices and nanostructures". Thesis, University of Cambridge, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.388822.
Testo completo
17
Mofor, Augustine Che. "Fabrication and characterisation of device quality ZnO nanostructures /". Göttingen : Cuvillier, 2007. http://www.gbv.de/dms/bs/toc/538227826.pdf.
Testo completo
18
Hudelist, Florian. "Design and characterisation of nanostructured gradient index lenses". Thesis, Heriot-Watt University, 2010. http://hdl.handle.net/10399/2355.
Testo completoAbstract (sommario):
The design and characterisation of nanostructured gradient index lenses is investigated in this thesis. Nanostructured gradient index materials achieve their refractive index pro le by creating a pattern with feature sizes of =5 and smaller from two glasses with di erent refractive indices. These structures are fabricated by the stack-anddraw technology generally used for photonic crystal bres. The rigorous theoretical analysis is performed with the Fourier modal method or the nite di erence time domain algorithm. A comprehensive introduction of the Fourier modal method for one and two dimensional gratings is given. Due to the inherit periodicity of the Fourier modal method, an algorithm to calculate the transmitted eld of isolated non-periodic lamellar gratings is developed and tested experimentally with a multi layer lens grating in the microwave regime. Furthermore, the eld stitching method for the analysis of large two dimensional gratings with very small feature sizes is developed. The numerical performance is tested with a di ractive element consisting of 32 32 pixels and shown to reduce the required memory as well as the computation time by more than an order of magnitude in certain con gurations. Considerations of symmetries in the grating structure are also included in the derivation of the eld stitching method. The e ective medium theory for nanostructured gradient index materials is introduced which allows to describe nanostructured materials with the equations for standard gradient index lenses. The stack-and-draw fabrication process is described including the choice of glass types, assembly and drawing of the preforms. For the design of the required binary pattern, the simulated annealing algorithm is used in conjunction with the e ective medium theory. In order to provide experimental evidence of the simulations, two lenses were assembled from PTFE rods with a diameter of 6mm and characterised in the microwave regime at = 3 cm. It is shown that with this wavelength to feature size ratio, the nanostructured gradient index lenses can have properties nearly identical to conventional gradient index lenses. Finally, a spherical and an elliptical nanostructured microlens are characterised in the optical regime. On the elliptical microlens, phase and intensity measurements are performed and compared to simulations obtained with the Fourier modal method.
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Mkwizu, Samwel Peter. "Electrochemical Synthesis and Characterisation of Multimetallic Nanostructured Electrocatalysts". Thesis, University of Pretoria, 2015. http://hdl.handle.net/2263/53528.
Testo completoAbstract (sommario):
This thesis concerns investigations on novel multistage electrochemical deposition
of nanostructured systems composed of noble metals platinum, ruthenium, and
gold. Various electrochemical synthetic pathways were systematically explored
producing multilayered nanoscale electrode systems composed of Pt, Ru, or Au on
glassy carbon or crystalline gold used as substrates. Electrochemical pathways
involved sequential surface-limited redox-replacement (SLRR) reactions of
underpotentially-deposited or overpotentially-deposited copper, potentiostatic
dealloying, direct spontaneous deposition of noble metals (without intermediary
steps involving redox-replacement templating reactions) as well as sequential
codeposition of noble metals (with or without SLRR templating reactions).
Fundamental studies were conducted using thermodynamic and kinetic models, in
situ electrochemical techniques and ex situ microscopic, spectroscopic, or
spectrophotometric techniques employed for probing factors controlling electrode
dynamics, electrocatalysis, morphology, bulk and surface compositional properties
of the noble metal-based electrode systems.
Unique multilayered multimetallic nanoclusters synthesized (with binary active
sites of Pt with Ru or Au) exhibited superior electrocatalytic activity towards
methanol or formic acid oxidation reactions when benchmarked to equivalent monometallic multilayered Pt. Hydrodynamic electrokinetic studies of the oxygen
reduction reaction (ORR) on the multilayered monometallic Pt and bimetallic Rucontaining
nanoclusters revealed that the monometallic nanoclusters exhibited
direct four-electron ORR whereas electrocatalysis on the bimetallic ones could be
tuned to proceed via a two-electron reaction pathway. Electrocatalytic bifunctional
reaction mechanisms were especially enhanced by the nanostructured systems
investigated.
Characterisation of multilayered nanoclusters surface and near-surface metal
contents revealed interactions between metal centers, carbon and oxygen containing
surface functional groups on the glassy carbon, which appeared to have played a
significant role in the overall stabilization and catalytic activity of the
electrochemically immobilized nanoclusters.
Physico chemical models and characteristics of intermediary Cu adlayers in the
electrosynthetic pathways revealed role of Cu surface coverage (within the
framework of electrochemical isotherms with kinetic and thermodynamic
parameters) and heterogeneity effects, adatom substrate interactions as well as
adatom adatom lateral interactions within individual adlayers during multilayer Pt
growth on crystalline Au. New relationships for computing trends in the apparent
Gibbs free energy of the SLRR reaction provided insights on reaction energetics of
the interfacial Pt layered growth.Thesis (PhD)--University of Pretoria, 2015.
Chemistry
PhD
Unrestricted
20
Qin, Lang. "The growth and characterisation of zinc-based nanostructures". Thesis, University of Salford, 2009. http://usir.salford.ac.uk/26868/.
Testo completoAbstract (sommario):
Functional nanomaterials have been studied intensively ever since the selforganisation of nanostructures was first reported in the mid-1990s. The templateassisted growth of nanostructures is considered to be one of the most important branches in nanoscience. This thesis aims to develop new fabrication routes to ordered semiconducting nanoarrays and other metallic structures. Using a combination of ac electrodeposition of precursor Zn nanowires and subsequent heat treatment at 400 °C, the growth of an ordered array of semiconducting ZnO nanowires embedded in an anodic aluminium oxide (AAO) template is studied. Investigations on the transformation process have been carried out. The ZnO nanowire arrays are also examined in terms of their structural, optical and magnetic properties. The effect of heat treatment is also examined at a higher temperature of 800 °C. It is demonstrated that the AAO can be used as a reactive template in the fabrication of ordered arrays of spinel ZnAl2O4 nanotube/net arrays. This involves the growth of monocrystalline Zn nanowires into an AAO template using a pulse dc electrodeposition technique followed by a heat treatment in air. The formation of the nanotube structure is attributed to the Kirkendall effect. The results suggest that this route may be extended to fabricate other spinel structures such as MgAl2O4 or BaAl2O4. ZnO whiskers obtained in these samples by different pre-treatment of the AAO templates have also been investigated. Attempts to produce Ni-doped ZnO nanowires using pulse dc electrodeposition resulted in the formation of Ni5Zn2i dendrites. A detailed morphological and purity study of these structures has been made.
21
Dunnill, Charles. "Synthesis, characterisation and properties of tantalum based inorganic nanofibres". Connect to e-thesis, 2007. http://theses.gla.ac.uk/173/.
Testo completoAbstract (sommario):
Thesis (Ph.D.) - University of Glasgow, 2007.Ph.D. thesis submitted to the Department of Chemistry, Faculty of Physical Sciences, University of Glasgow, 2007. Includes bibliographical references. Print version also available.
22
Sala, Simone. "Wavefront and nanostructure characterisation with X-ray ptychography". Thesis, University College London (University of London), 2018. http://discovery.ucl.ac.uk/10057099/.
Testo completoAbstract (sommario):
X-ray ptychography is a scanning diffraction microscopy technique suited for the phase-sensitive investigation of wavefronts and specimens. It returns complex-valued wave functions and transmission functions producing high-resolution (nanoscale) phase- contrast images. This work focuses on the implementation and application of X-ray ptychography in the context of synchrotron radiation facilities. It presents an experimental protocol developed for multiscale X-ray imaging and tested at the I13-1 Coherence Branchline at Diamond Light Source. This protocol combines both near-field and far-field ptychography with other imaging methods, providing a flexible way of conducting experiments on hierarchical structures at any high-brilliance X-ray facility. This work also reports ptychography experiments performed at free-electron lasers, aimed at characterising their pulsed beam. Both the average and individual wavefronts are retrieved through a novel application of a reconstruction algorithm based on singular- value decomposition, giving direct insight on pulse-to-pulse fluctuations and confirming ptychography as a powerful beam diagnostics technique. Additional ptychography experiments are also discussed, which were carried out at storage rings on flat, weakly-scattering biogenic samples to characterise their 3D nanos- tructures. Their data analysis pipeline is presented in detail, from data acquisition to rendered volumes. Furthermore, one of these last experiments constitutes the first successful 3D ptychography experiment run on real-life samples at the I13-1 Coherence Branchline at Diamond Light Source.
23
Etok, Susan Essien. "Structural characterisation and in vitro behaviour of apatite coatings and powders". Thesis, Cranfield University, 2005. http://dspace.lib.cranfield.ac.uk/handle/1826/3973.
Testo completoAbstract (sommario):
Hydroxyapatite (HAP) coatings are used in orthopaedic surgery for bone regeneration. Current methods of phase quantification of HAP coatings suffer from drawbacks. A novel methodology of quantitative phase analysis of HAP coatings has been devised and validated. This method, based on whole pattern fitting with a fundamental parameters approach, incorporates amorphous calcium phosphate (ACP) and apatite phases into structural refinements. A comparison of the structural and chemical properties of plasma sprayed (PS) and novel electrodeposited (ED) HAP coatings has been conducted. ED coatings contained less ACP and more preferred orientation than the PS coatings, although the stoichiometry was similar. In vitro investigations of PS and ED coatings in simulated body fluid and foetal calf serum revealed that both are bioactive. A carbonated apatite layer produced on the ED coatings was -0.7μm thick with a stoichiometry and chemical constituents similar to that of natural bone apatite. PS coatings produced a nanocrystalline carbonated apatite layer (-4μm). For the first time it has been possible to model crystalline HAP and nanocrystalline apatite as independent phases and obtain accurate lattice parameters for each. A positive linear correlation has been made between microstrain and the solubility of HAP and carbonated apatites. Dissolution studies have shown that the behaviour of HAP and carbonated apatite is dominated by crystallite size at low undersaturation and by crystallite size and microstrain at high undersaturation for crystallites between -30OA- 1000A. Metastable equilibrium occurred for crystallites <_400A at low undersaturation. Carbonate content did not affect the solubility or dissolution behaviour. A novel technology for coating polymeric tape with HAP for potential use in anterior cruciate ligament reconstruction has been devised. Mechanical tests have demonstrated that no adverse properties are induced by the coating technology. Cell culture studies have shown that the HAP layer is capable of enhanced attachment, proliferation and differentiation of osteoblast cells compared to uncoated tape.
24
Bocharova, Vera. "Electrically Conductive Low Dimensional Nanostructures: Synthesis, Characterisation and Application". Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2009. http://nbn-resolving.de/urn:nbn:de:bsz:14-ds-1231161926227-23379.
Testo completoAbstract (sommario):
Miniaturization has become a driving force in different areas of technology including microelectronics, sensoric- and bio-technologies and in fundamental science. Because of the well-known limitations of conventional lithographic methods, newly emerging bottom-up approach, utilizing self-assembly of various nanoobjects including single polymer molecules and carbon nanotubes constitutes a very promising alternative for fabrication of ultimately small devices. Carbon nanotubes are attractive materials for nanotechnology and hold much promise to revolutionize fundamental science in a investigation of phenomena, associated with the nanometer–sized objects.It was found in this work that grafted chains of poly(2-vinylpyridine) form a shell covering the carbon nanotubes that makes them dispersible in organic solvents and in acidic water (CNTs-g-P2VP).The positively charged poly(2-vinylpyridine) shell is responsible for the selective deposition of carbon nanotubes onto oppositely charged surfaces. It was established that the deposition CNTs-g-P2VP from aqueous dispersions at low pH is an effective method to prepare ultra-thin films with a tunable density of carbon nanotubes.It was shown that poly(2-vinylpyridine) grafted to carbon nanotubes is a universal support for the immobilization of various nanoclusters at the carbon nanotube's surface. Prussian Blue nanoparticles were selectively attached to the surface of CNTs-g-P2VP.Conducting polymer nanowires are another very promising kind of nanomaterials that could be also suitable for applications in nanodevices and nanosensors. In this work was developed a simple method to control the conformation and orientation of single adsorbed polyelectrolyte molecules by co-deposition with octylamine. A simple chemical route to conductive polypyrrole nanowires by the grafting of polypyrrole from molecules of polystyrensulfonic acid was developed. The dc conductivity of individual polypyrrole nanowires approaches the conductivity of polypyrole in bulk.The conductivity can be described using variable-range hopping model.
25
Gao, Nan. "Fabrication and characterisation of nanostructured surfaces with extreme wettabilities". Thesis, University of Nottingham, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.581967.
Testo completoAbstract (sommario):
The wetting phenomena related to superhydrophobic and superhydrophilic surfaces have been investigated over the past decade with attention on the micro and nano-scaled structures. Particularly, nanoparticles have been applied to create the structures and change surface wettability. In the present PhD work, it is aimed to produce extremely wettable and non- wettable surfaces out of Si02 nanoparticles and study the characteristics with respect to wetting and surface structures. The nanoparticles are presented and prepared in various solutions, which will be then deposited for the creation of micro and nano-scaled structures. Since the sizes of the individual nanoparticles are extremely small and different, the formed structures and therefore the wettability will be influenced accordingly. Various concentrations of nanoparticles in the solutions are set. The experiments are performed to characterise the wettability of the formed surfaces. The static contact angles are measured, while it is also needed to measure the advancing and receding contact angles for some cases, especially on the superhydrophobic surfaces. The structures of the surfaces are observed using Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM). It is noticed that the wettability of the formed surfaces is related to the concentration of nanoparticles, and the size can also make a difference in surface structures. The investigation further explores a variety of experimental conditions where the prepared solutions are promptly evaporated. This significantly changes the surface wettability and leads to very distinctive surface structures. The experimental work demonstrates that the surface structures and roughness play an important role in prompting the wettability of a solid surface. The distribution and assembling of Si02 nanoparticles on surfaces are connected to the processing conditions. Under these circumstances, the uniformity and consistency of the formed surface structures in micro and nano-scale are closely related to the size of nanoparticles. Thus, the use of Si02 nanoparticles for the creation of surface structures and the exhibited characteristics are reflected through the investigations and discussions in this PhD thesis. It sets up a platform to compare and examine the suitability of the theories for wetting phenomena on the formed surfaces, and provides good references for future work on the features of functional surfaces based on various nanoparticles.
26
Maude, Steven. "Design and characterisation of functionalised self-assembled peptide nanostructures". Thesis, University of Leeds, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.555840.
Testo completoAbstract (sommario):
This work develops the Pl1 peptide family as new materials, utilising a strategy of mod- ifying existing peptides to tune them to specific end uses. Several peptides, based on the self-assembling, ,B-sheet forming Pl1-2 sequence, were screened as potential tissue engineering materials. This was accomplished by studying the effect of net peptide charge on aggregation and secondary structure in physiological conditions, applying both proton NMR and IR spectroscopies. Single increments of net peptide charge produce large increases in critical aggregation concentration and decreases in peptide aggregate fraction at fixed concentration. No self-assembly was detected for peptides with net charges > 3, even at high mM concentration. At least a small net charge is required for solubility. Based on these studies, a peptide with a -2 charge, Pn-4, was selected for further study. The remainder of the studies explore the use of P n-4 as a scaffold for small (peptide) and large (protein) groups. Using TEM and AFM, the self-assembly of Pn-4 has been compared to Pn-4 functionalised with bioactive RGD peptides. Two modified Pn-4 peptides (LR66 and LR46) self-assembled to some extent. Two others (LR56 and LR44) did not appear to display Pn-4-type self-assembly, though, on their inclusion in mixtures with Pn-4, the resulting structures appeared more like those of unmodified Pn-4. Finally, an amphiphilic fusion protein, KSI-(Pn-4h-His6, comprised of ketosteroid isomerase, a trimeric Pn-4 and a hexahistidine tag was investigated. KSI-(Pn-4h-His6 was sparingly soluble in H20 near neutral pH, but more soluble at low and high pH. AFM and TEM studies indicated KSI-(Pn-4h-His6 in H20 is in the form ofround particles. No Pn-4-like aggregates were observed, implying the attached protein disrupts self-assembly. Tensiometer studies at the air-water interface demonstrate that KSI-(Pn-4h-His6 is a promising protein surfactant, with surface tension reductions - at concentrations up to 1 mg ml-1 - bettering or closely matching that of the model ,B-Iactoglobulin.
27
Bennington-Gray, Suzanne. "Magnetic characterisation of nanostructured magnetic systems using SQUID magnetometry". Thesis, Queen's University Belfast, 2019. https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.766289.
Testo completo
28
Bocharova, Vera. "Electrically Conductive Low Dimensional Nanostructures: Synthesis, Characterisation and Application". Doctoral thesis, Technische Universität Dresden, 2008. https://tud.qucosa.de/id/qucosa%3A23607.
Testo completoAbstract (sommario):
Miniaturization has become a driving force in different areas of technology including microelectronics, sensoric- and bio-technologies and in fundamental science. Because of the well-known limitations of conventional lithographic methods, newly emerging bottom-up approach, utilizing self-assembly of various nanoobjects including single polymer molecules and carbon nanotubes constitutes a very promising alternative for fabrication of ultimately small devices. Carbon nanotubes are attractive materials for nanotechnology and hold much promise to revolutionize fundamental science in a investigation of phenomena, associated with the nanometer–sized objects.It was found in this work that grafted chains of poly(2-vinylpyridine) form a shell covering the carbon nanotubes that makes them dispersible in organic solvents and in acidic water (CNTs-g-P2VP).The positively charged poly(2-vinylpyridine) shell is responsible for the selective deposition of carbon nanotubes onto oppositely charged surfaces. It was established that the deposition CNTs-g-P2VP from aqueous dispersions at low pH is an effective method to prepare ultra-thin films with a tunable density of carbon nanotubes.It was shown that poly(2-vinylpyridine) grafted to carbon nanotubes is a universal support for the immobilization of various nanoclusters at the carbon nanotube's surface. Prussian Blue nanoparticles were selectively attached to the surface of CNTs-g-P2VP.Conducting polymer nanowires are another very promising kind of nanomaterials that could be also suitable for applications in nanodevices and nanosensors. In this work was developed a simple method to control the conformation and orientation of single adsorbed polyelectrolyte molecules by co-deposition with octylamine. A simple chemical route to conductive polypyrrole nanowires by the grafting of polypyrrole from molecules of polystyrensulfonic acid was developed. The dc conductivity of individual polypyrrole nanowires approaches the conductivity of polypyrole in bulk.The conductivity can be described using variable-range hopping model.
29
Alshadokhi, Mohammed. "Design and characterisation of nanostructured microelectrodes for biomedical applications". Thesis, University of Southampton, 2014. https://eprints.soton.ac.uk/375029/.
Testo completoAbstract (sommario):
The aim of the project is to create a nanostructured microelectrode pH sensor to measure the pH of brain fluids. This work will describe the fabrication, characterization and development of the nanostructured Pd electrodes and their assessment for use as pH sensors in the brain fluid. The palladium hydride α+β transition is located between the α and β phases where H/Pd atomic ratios range between 0.02 and 0.6. This region was selected to fabricate the pH sensor because its potential (Epd-H) is stable and independent of the hydrogen– palladium composition. In addition, Epd-H follows a linear relationship with pH. A nanostructured Pd film was chosen to fabricate the pH sensor in order to obtain a large electroactive area because Epd-H is not stable with microelectrode; a large area is needed to reach the equilibrium between palladium and hydrogen. Also, neurobiological pH measurements require a small sensor as a result of the limited biological sample quantities available. The nanostructured pH sensor H1-e Pd was made by liquid crystal templating method (LCT). Scanning electron microscopy (SEM) and electrochemical characterisation were used to estimate the radius a and electroactive area of the nanostructured Pd film after the deposition process. The α+β transition was prepared by loading hydrogen electrochemically before the experiments were carried out. The nanostructured Pd hydride electrode was then used to estimate the pH in different solutions including artificial cerebral spinal fluid (aCSF) and real brain fluid. The results obtained demonstrate the applicability of such electrodes to function as pH sensors in brain fluid. The biomedical applications requires high efficiency that can be affected by the biological samples contaminations onto the electrode. Thus, attempts were made to develop the H1-e Pd pH sensors performance by covering their surface with coats to stop the biological impurities.
30
Crisan, Alina Daniela. "Synthesis and characterisation of nanostructured magnetic FePt-based alloys". Le Mans, 2009. http://cyberdoc.univ-lemans.fr/theses/2009/2009LEMA1002.pdf.
Testo completoAbstract (sommario):
Ce travail de thèse est une contribution à l'étude et au développement d'aimants permanents de type spring magnet. Ces matériaux sont constitués de grains magnétiques durs dispersés dans une matrice magnétique douce. Le couplage des grains magnétiques durs à travers la matrice douce permet d'augmenter significativement la coercivité et l'aimantation rémanente et par conséquent la valeur du produit (BH)max, facteur de qualité ou de mérite des aimants permanents. L'objectif de ce travail est de réaliser ce type d'aimant permanent à partir d'alliages nano cristallisés à base de FePt. Pour cela, la méthode de trempe ultra-rapide sur roue a été utilisée pour produire un précurseur métastable à base de FePtNbB, suivi d'un traitement de dévitrification partielle permettant d'obtenir des grains nano cristallisées de phase dure, de symétrie quadratique à face centrée (phase L10) avec une forte anisotropie uniaxiale (7MJ/m3), disperses dans une matrice résiduelle magnétiquement douce de composition Fe(PtNb). Les mesures par calorimétrie différentielle et diffraction in situ du rayonnement synchrotron en température et sous pression ont permis d'optimiser les paramètres de cristallisation. Les propriétés structurales des échantillons ont été étudiées par diffraction de rayons X, microscopie électronique à transmission, analyse dispersive en énergie X et spectrométrie Mössbauer du 57 Fe alors que les caractéristiques magnétiques ont été mesurées avec des magnétomètres SQUID et VSM. Les performances obtenues à 300K montrent que ces échantillons sont de bons candidats comme aimants permanents: coercivité de 850kA/m, champs rémanent de 0. 7T et facteur de mérite (BH)max de 69kJ/m3The present work is a contribution to the study and development of a new class of permanent magnets based on the exchange spring effect. These materials are made of hard magnetic grains dispersed in a soft magnetic matrix. The coupling of hard magnetic grains through the soft matrix allows a significant improvement of the coercivity and remanence, and by consequence, an improved maximum energy product may be obtained in these alloys. The aim of this work is to synthesize this type of magnetic material starting with nanocrystalline FePt-based alloys. The rapid solidification by melt spinning method has been used to cast metastable precursor alloys with composition FePtNbB. After annealing, a microstructure made of hard magnetic nanograins of face-centred-tetragonal symmetry (L10 phase) with high uniaxial magnetic anisotropy (7 MJ/m3), dispersed in a residual soft magnetic phase with composition FePt(NbB) has been obtained. The differential scanning calorimetry studies as well as in situ X-ray diffraction of synchrotron radiation in temperature and applied pressure have allowed the optimization of parameters of crystallization of the metastable precursor. The structural parameters of the samples were investigated using X-ray diffraction, transmission electron microscopy, energy dispersive spectroscopy and 57Fe Mössbauer spectrometry while the magnetic parameters were determined using VSM and SQUID magnetometry. The performances obtained at 300K, coercivity 850kA/m, remanence 0. 7 T and (BH)max 69 kJ/m3 show that these alloys are good candidates as permanent magnets
31
Severs, John. "Microstructural characterisation of novel nitride nanostructures using electron microscopy". Thesis, University of Oxford, 2014. http://ora.ox.ac.uk/objects/uuid:6229b51e-70e7-4431-985e-6bcb63bd99d1.
Testo completoAbstract (sommario):
Novel semiconductor nanostructures possess a range of notable properties that have the potential to be harnessed in the next generation of optical devices. Electron microscopy is uniquely suited to characterising the complex microstructure, the results of which may be related to the growth conditions and optical properties. This thesis investigates three such novel materials: (1) GaN/InGaN core/shell nanowires, (2) n-GaN/InGaN/p-GaN core/multi-shell microrods and (3) Zn3N2 nanoparticles, all of which were grown at Sharp Laboratories of Europe. GaN nanowires were grown by a Ni-catalysed VLS process and were characterised by various techniques before and after InGaN shells were deposited by MOCVD. The majority of the core wires were found to have the expected wurtzite structure and completely defect free – reflected in the strong strain-free photoluminescence peak –with a- and m- axis orientations identified with shadow imaging. A small component, <5%, were found to have the cubic zinc-blende phase and a high density of planar faults running the length of the wires. The deposited shells were highly polycrystalline, partially attributed to a layer of silicon at the core shell interface identified through FIB lift-out of cross section samples, and accordingly the PL was very broad likely due to recombination at defects and grain boundaries. A high throughput method of identifying the core size indirectly via the catalyst particle EDX signal is described which may be used to link the shell microstructure to core size in further studies. An n-GaN/InGaN/p-GaN shell structure was deposited by MOCVD on the side walls of microrods etched from c-axis GaN film on sapphire, which offers the possibility of achieving non-polar junctions without the issues due to non-uniformity found in nanowires. Threading dislocations within the core related to the initial growth on sapphire were shown to be confined to this region, therefore avoiding any harmful effect on the junction microstructure. The shell defect density showed a surprising relationship to core size with the smaller diameter rods having a high density of unusual 'flag' defects in the junction region whereas the larger diameter sample shells appeared largely defect free, suggesting the geometry of the etched core has an impact on the strain in the shell layers. The structure of unusual 'flag' defects in the m-plane junctions was characterised via diffraction contrast TEM, weak beam and atomic resolution ADF STEM and were shown to consist of a basal plane stacking faults meeting a perfect or partial dislocation loop on a pyramidal plane, the latter likely gliding in to resolve residual strain due to the fault formed during growth. Zn3N2 has the required bandgap energy to be utilised as a phosphor with the additional advantage over conventional materials of its constituent elements not being toxic or scarce. The first successful synthesis of Zn3N2 nanoparticles appropriate to this application was confirmed via SAD, EDX and HRTEM, with software developed to fit experimental polycrystalline diffraction patterns to simulated components suggesting a maximum Zn3N2 composition of ~30%. There was an apparent decrease in crystallinity with decreasing particle size evidenced in radial distribution function studies with the smallest particles appearing completely amorphous in 80kV HRTEM images. A rapid change in the particles under the electron beam was observed, characterised by growth of large grains of Zn3N2 and ZnO which increased with increasing acceleration voltage suggesting knock-on effects driving the change. PL data was consistent with the bandgap of Zn3N2 blue shifted from 1.1eV to around 1.8eV, confirming the potential of the material for application as a phosphor.
32
McAuley, Matthew Bryan. "Optical hydrogen sensors : fabrication and characterisation of palladium nanostructures". Thesis, Queen's University Belfast, 2016. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.707830.
Testo completoAbstract (sommario):
Reliable, fast and sensitive hydrogen detection is central to safety in the emerging hydrogen economy. Extensive research has highlighted nanostructured materials as an approach to meet these demands. In this work, arrays of palladium nanotubes and nanorods have been characterised as optical hydrogen sensors. The fabrication of nanorod and nanotube arrays using a bottom-up template method is presented. Electrodeposition into alumina templates is shown to produce ordered arrays of structures across glass substrates, confirmed through microscopy techniques. Utilising the purpose built characterisation facility, the sensitivity and response of palladium nanotubes have been demonstrated to improve upon nanorods, which in turn offer performance enhancements over traditional thin film sensing elements.
33
Gorodyska, Ganna. "Generation and Characterisation of Nanostructures from Single Adsorbed Polyelectrolyte Molecules". Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2005. http://nbn-resolving.de/urn:nbn:de:swb:14-1129637907155-76458.
Testo completoAbstract (sommario):
Visualization and study of reconformation of polyelectrolytes (PEs) of different architecture is of great fundamental and practical interest. Verification of theoretical predictions with experiment is of essential importance. On the other hand, a wide range of bottom-up techniques based on patterning of matter on the length scale of a few nanometers have been recently developed. Particularly interesting is the possibility of using self-assembled single molecule structures as templates for the deposition of inorganic matter, in particular metals. Synthetic "normal-sized" polymers of various architecture, like poly-2-vinylpyridine (P2VP) or polystyrene-poly(2-vynil pyridine) P2VP7-PS7 star-like block copolymer, adsorbed on solid substrates have been visualized for the first time with molecular resolution by AFM in different conformation. This finding allowed us to study largely discussed problem, a coil-to-globule transition of PEs. It was found that PE molecules undergo conformational transitions from stretched worm-like coil to compact globule via set of necklace-like globules, as the fraction of charged monomers decreases with an increase of pH and ionic strength. These results are in good agreement with recently developed DRO theory for weakly charged flexible PEs in poor solvent. The size of the deposited single molecules correlates very well with molecular dimensions in solution obtained in light scattering experiments. PE single molecules of various architectures was mineralized in different conformations that constitutes the route to nanoparticles with desired shape (including wire-shape and star-shaped), size, and composition (including metallic, magnetic and semiconductive nanoparticles). It was shown that molecular details of the adsorbed linear flexible PE molecules determine the dimensions of the nanostructures after metallization and that observed sizes are consistent with the decoration of single molecules with nanoclusters. Thus those metallized nanoparticles (cluster assembles) reflect the conformation of original adsorbed PE molecules. The dimensions of the obtained nanowires are significantly smaller than those previously reported. All of these features are of the potential benefit in applications for nanodevices. Metallization of the PS7-P2VP7 improves AFM resolution due to the selective deposition of Pd clusters along the P2VP chains. For the first time, the number of the P2VP second generation arms of the heteroarm block-copolymer was directly counted in the single molecule AFM experiment. Simple contrasting procedure was developed to improve AFM visualization of positively charged polymer chains deposited on the substrates of relatively high roughness. This method allows increasing the thickness of the resulting structures up to 10 nm, and, consequently, provide visualization of polymer chains on rough surfaces. This innovation is important for the development of single molecule experiments with polymer chains. The reaction of HCF-anion could be used for recognition of polycation molecules, when polycations, polyanions and neutral molecules coexist on the surface. Recently, the study was strongly restricted to atomically smooth surfaces. The contrasting procedure extends the range of substrates (Si-wafers, chemically modified or patterned Si-wafers, polished glasses, polymer films, etc) appropriate for the experiments. Thus, polymer single molecules can be considered not only as representative of the ensemble molecules, but also as individual nanoscale objects which can be used for future nanotechnology for the fabrication of single molecule electronic devices. Also these findings are important from fundamental point of view, since developed approach can be successfully applied for investigation of various "classical" problems in polymer science, such as polymer reconformation, interpolyelectrolyte complex formation, polymer diffusion, adsorption, etc.
34
Bellini, Eleonora. "Characterisation of magnetic nanostructures for spintronic applications by electron microscopy". Thesis, University of Glasgow, 2011. http://theses.gla.ac.uk/3011/.
Testo completoAbstract (sommario):
The work presented in this PhD thesis concerns the characterisation of the physical structure, composition and domain structure of advanced magnetic materials by electron microscopy within the FP6 European Research Training Network "Spinswitch". In particular the investigations concerned MgO/CoFeB/MgO multilayers to be employed in magnetic sensors (this work was done in collaboration with INESC-MN Lisbon-Portugal); Ni80Fe20/Cu electrodeposited nanowires to be employed as spin transfer torque devices (this work was done in collaboration with NIRDTP Iasi-Romania and University of Salamanca); multilayers with perpendicular anisotropy which represent potential candidates to be employed in the next generation of MRAMs (this work was done in collaboration with Spintec-CEA-Grenoble). Chapter 1 will provide an overview of the physics behind the topics treated during this work and a description of the general motivations of the research carried out. Chapter 2 will provide an overview of all the experimental techniques employed for the fabrication and characterisation of the samples investigated for this research. Chapter 3 aims to present an investigation using conventional transmission electron microscopy (CTEM) and Lorentz microscopy (LTEM) to characterise respectively the physical microstructure and the domain structure of the CoFeB free layer, embedded in a multilayer composed by SiN/MgO(50)/CoFeB(t)/MgO(15), with t from 30 Å down to 14 Å. We carried out first the investigation of the physical structure performed by selected area diffraction and bright field imaging of planar samples and physically the plan view sections show the structure of the films appears similar. The magnetization reversal behaviour observed during Lorentz TEM experiments are found to vary considerably with the CoFeB thickness, with both domain wall formation and magnetisation rotation seen. In the thicker film the behaviour was characteristic of a typical soft magnetic material with uniaxial anisotropy. However the magnetic reversal of the thinner film was more complex. A particular characteristic of the 14 Å CoFeB layer is the variation of domain wall angle seen when varying the orientation of the applied field This wall asymmetry suggests the presence of a unidirectional anisotropic energy term. To assist in the interpretation of these experimental results a modified Stoner–Wohlfarth model has been constructed and calculations have been carried out by using a MATLAB code. The purpose of the project presented in Chapter 4 was the advanced characterisation of multilayered electrodeposited NiFe/Cu nanowires grown in alumina and polycarbonate templates. In particular the objective was the characterisation of the structure and local chemistry of the nanowires by TEM and the classification of nanowire switching deduced by Lorentz microscopy experiments, which are challenging for this specific material system. In order to perform TEM studies on single nanowires, they should be extracted from their template. The chemical etching used to remove the nanowires from the template in addition to issues related to the deposition of Cu, led to nanowires with edge and compositional irregularities, detrimental for their magnetic properties. Indeed, we were not able to classify the nanowire switching and investigate domain walls forming during the reversal process, but we could only observe a change in the magnetising state. A lot of the work described in this chapter deals with the difficulties associated with imaging these challenging nanowires. Issues were discovered that may have resulted from deposition and/or etching for TEM preparation, therefore we do rely heavily on simulations and calculations. The research presented in Chapter 5 will describe the investigation of the reorientation process of the easy axis for two different multilayer systems magnetised out of plane, and the evolution of their domain structure for increasing temperature, and trying to understand the role of the insertion of a Co/Pt/Ni/Pt multilayer from a microscopic point of view. The two multilayers represent the free layer of a perpendicular MTJ (pMTJ) and this study represents a state of development of materials for pMTJs. Experiments were performed by MOKE magnetometry in polar configuration and Lorentz Microscopy in Fresnel mode. Materials were prepared in Spintec-CEA, Grenoble (France) where the MOKE experiments were also carried out, and Lorentz Microscopy experiments were performed in Glasgow. For the first multilayer (with Co/Pt/Ni/Pt) we found that for lower temperatures (25°C - 220°C) the specimen appears to have a strong perpendicular anisotropy. We observed a small scale random domain structure that we can ascribe to perpendicularly magnetised domains. For higher temperatures (220°C - 300°C) we found a behaviour typical of a soft magnetic material magnetised in plane with low anisotropy and high susceptibility. For the second multilayer (without Co/Pt/Ni/Pt), for instrumental reasons, we were not able to investigation of the magnetic behaviour of the specimen for temperatures above 110°C. The magnetisation is out of plane for all the temperatures investigated. The sample develops a different domain structure when the sample is heated below 100°C or above 100°C. In the first case isotropic serpentine domain structure is visible, with a large periodicity, whereas in the second case, an anisotropic stripe domain structure is visible with a small periodicity.
35
Turnbull, Susan B. "Characterisation of focused ion beam nanostructures by transmission electron microscopy". Thesis, University of Glasgow, 2009. http://theses.gla.ac.uk/572/.
Testo completoAbstract (sommario):
Ion irradiation is an effective tool for the modifcation and control of the properties of magnetic thin films. Basic magnetic properties such as coercivity and local anisotropy direction can be altered in NiFe (Permalloy) films, whilst for Co/Pd multilayers, ion irradiation results in a transition from perpendicular to in-plane magnetisation. This ability to tailor magnetic properties in a controlled manner can be used as a tool for nanoscale patterning. Results are presented from investigations into the effect of Ga+ ion dose on the magnetic and structural properties of permalloy thin film systems. Systems consisting of a permalloy layer of either 10nm or 20nm, and one or more non-magnetic layers of Al or Au were deposited by thermal evaporation and irradiated in a focused ion beam (FIB) with a 30kV Ga ion source. The presence of the non-magnetic layers allows irradiation induced mixing with the magnetic layer, effectively creating alloyed regions with different properties to the rest of the film. At low ion doses, no signifcant effect on either the magnetic or structural properties were observed. Bright field TEM images of the irradiated regions revealed that increasing the dose to 1x10^15 ions/cm^2 and above caused an increase in mean grain size from ~5nm to ~30nm. The Fresnel mode of Lorentz microscopy revealed that a reduction in the mean moment was also observed at these doses but no clear changes in coercivity or magnetisation reversal behaviour were observed until the systems were rendered non-magnetic. This occurred at 1x10^16 and 3x10^16 ions/cm^2 for systems with 10nm NiFe and 20nm NiFe respectively. Milling of the samples was evident at these high doses, meaning that it was not possible to magnetically pattern these systems without occasioning a change of 2nm and 6nm respectively in the thickness of the samples. Based on the above, structures were created to control the location of magnetic domain walls (DW). Lines were written by FIB in simple elements with dimensions <1micron, the aim being to create a higher density of DW than could be realised in equivalent homogeneous elements. Structures containing high DW densities are attractive for measuring domain wall magnetoresistive effects and have potential application in DW-based storage or logic devices. One geometry of interest is an element with `zigzag' edges. Results are be presented in chapter 4 showing how these can support either quasi-uniform magnetisation or multi-domain structures separated by DW with spacing <100nm. In chapter 5 irradiation of magnetic structures was again carried out, but this time in magnetic wires to create defect or pinning sites. Domain wall traps fabricated by ion irradiation were characterised, and irradiation line defects introduced along the wire. The lines were patterned at 90± and 45± to the length of the wire, and successfully pinned the domain walls at predefned locations. A 90 degree line irradiated at a dose of 1x10^15 ions/cm^2 was not able to provide a strong enough pinning site for a domain wall. However, when the angle of the line was changed to ±45 degrees it was possible to reproducibly pin domain walls at these sites. A relationship between the orientation of the irradiated line and the chirality of the domain wall that pinned at the site was observed. The effcts of irradiation on Co/Pd multilayers with perpendicular magnetic anisotropy was investigated in chapter 6. Irradiation causes magnetic systems with perpendicular magnetisation to undergo a transition from out-of-plane magnetisation to in-plane. A grid pattern was devised so that magnetic states with both in-plane and out-of-plane magnetisation could be observed. A combination of differential phase contrast microscopy and simulations of integrated magnetic induction were used to determine the orientation of magnetisation within the lines.
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Vempati, Sesha Pavan Kumar. "Thin film and nanostructures zinc oxide : characterisation and device applications". Thesis, Queen's University Belfast, 2012. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.580108.
Testo completoAbstract (sommario):
This thesis reports the preparation and characterization of ZnO films and nanostructures and their incorporation in simple devices. The characterization includes imaging techniques - atomic force-, scanning electron- and transmission electron- microscopies (and accompanying analysis) - as well as X-ray diffraction (confirming wurzite structure in all the form of ZnO), photoluminescence (elucidating exciton and defect bands), Raman spectroscopy (dopent incorporation, including defects) and, importantly, optical absorption since it is crucial to confirm the various forms of ZnO as transparent conducting oxide. Also, electron scanning tunnelling microscopy reveals interesting bias- and polarity-dependent changes in 'topography' images originating with different density-of-states contributions from the conduction band, valence band and defect (surface) states. A new fabrication methodology, based on metal-salt decomposition, is introduced to prepare un doped and Co-doped thin films and nanowires on quartz where the doped ZnO exhibited Co2+ substitution of Zn2+ while retaining good optical transmission. Extending the wet-chemical approach synthesis a simple change in reaction temperature led to two quite different forms of ZnO-nanostructure:- nanocrystals and nanosheets. The potential of ZnO-nanosheets as a phosphor coating for producing (bluish-) white light from UV-LEDs is demonstrated. The nanocrystals were used in poly(3, 4- ethylenedioxythiophene)-poly(styrenesulfonate)(pEDOT:PSS) host matrix to form a nanocomposite exhibiting the fascinating property of negative photoconduction, explained in terms of decreased conductivity of the ZnO stemming from a charge transfer interaction with the PEDOT:PSS. The well-known difficulty in forming stable p-type 2nO films was confirmed by fabricating Li-doped 2nO/n-type 2nO homojunctions which failed to show rectifying behaviour, where the oxygen vacancies may indicate extinction of p-type behavior. However, a number of successful n-2nO (Ga- and In-doped 2nO films, honeycomb structured intrinsic .n-2nOIPEDOT:PSS and Ag-doped 2nO nanorods/p-Si heterojunctions were fabricated and their photoresponse examined in detail - these data, in particular polarity-dependent wavelength selectivity, are discussed and analysed in terms of basic band structure and carrier transport properties.
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Gorodyska, Ganna. "Generation and Characterisation of Nanostructures from Single Adsorbed Polyelectrolyte Molecules". Doctoral thesis, Technische Universität Dresden, 2004. https://tud.qucosa.de/id/qucosa%3A24584.
Testo completoAbstract (sommario):
Visualization and study of reconformation of polyelectrolytes (PEs) of different architecture is of great fundamental and practical interest. Verification of theoretical predictions with experiment is of essential importance. On the other hand, a wide range of bottom-up techniques based on patterning of matter on the length scale of a few nanometers have been recently developed. Particularly interesting is the possibility of using self-assembled single molecule structures as templates for the deposition of inorganic matter, in particular metals. Synthetic "normal-sized" polymers of various architecture, like poly-2-vinylpyridine (P2VP) or polystyrene-poly(2-vynil pyridine) P2VP7-PS7 star-like block copolymer, adsorbed on solid substrates have been visualized for the first time with molecular resolution by AFM in different conformation. This finding allowed us to study largely discussed problem, a coil-to-globule transition of PEs. It was found that PE molecules undergo conformational transitions from stretched worm-like coil to compact globule via set of necklace-like globules, as the fraction of charged monomers decreases with an increase of pH and ionic strength. These results are in good agreement with recently developed DRO theory for weakly charged flexible PEs in poor solvent. The size of the deposited single molecules correlates very well with molecular dimensions in solution obtained in light scattering experiments. PE single molecules of various architectures was mineralized in different conformations that constitutes the route to nanoparticles with desired shape (including wire-shape and star-shaped), size, and composition (including metallic, magnetic and semiconductive nanoparticles). It was shown that molecular details of the adsorbed linear flexible PE molecules determine the dimensions of the nanostructures after metallization and that observed sizes are consistent with the decoration of single molecules with nanoclusters. Thus those metallized nanoparticles (cluster assembles) reflect the conformation of original adsorbed PE molecules. The dimensions of the obtained nanowires are significantly smaller than those previously reported. All of these features are of the potential benefit in applications for nanodevices. Metallization of the PS7-P2VP7 improves AFM resolution due to the selective deposition of Pd clusters along the P2VP chains. For the first time, the number of the P2VP second generation arms of the heteroarm block-copolymer was directly counted in the single molecule AFM experiment. Simple contrasting procedure was developed to improve AFM visualization of positively charged polymer chains deposited on the substrates of relatively high roughness. This method allows increasing the thickness of the resulting structures up to 10 nm, and, consequently, provide visualization of polymer chains on rough surfaces. This innovation is important for the development of single molecule experiments with polymer chains. The reaction of HCF-anion could be used for recognition of polycation molecules, when polycations, polyanions and neutral molecules coexist on the surface. Recently, the study was strongly restricted to atomically smooth surfaces. The contrasting procedure extends the range of substrates (Si-wafers, chemically modified or patterned Si-wafers, polished glasses, polymer films, etc) appropriate for the experiments. Thus, polymer single molecules can be considered not only as representative of the ensemble molecules, but also as individual nanoscale objects which can be used for future nanotechnology for the fabrication of single molecule electronic devices. Also these findings are important from fundamental point of view, since developed approach can be successfully applied for investigation of various "classical" problems in polymer science, such as polymer reconformation, interpolyelectrolyte complex formation, polymer diffusion, adsorption, etc.
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Yang, Weina. "Syntheses and characterisations of nanostructural magnesium-based hydrides for hydrogen storage applications". Thesis, Queen Mary, University of London, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.509666.
Testo completo
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Cirera, Hernández Albert. "New technologies and their characterisation for nanostructured SnO2 Gas sensor devices". Doctoral thesis, Universitat de Barcelona, 2000. http://hdl.handle.net/10803/665756.
Testo completoAbstract (sommario):
La presente tesis se ha centrado en el desarrollo de nuevas tecnologías para el desarrollo de sensores de gas semiconductores. Por tal motivo se han desarrollado dos técnicas para el desarrollo de nanopartículas de SnO2.
Paralelamente se ha diseñado y realizado diversos tipos de substratos basados en alúmina y silicio micromecanizados.
Con el objetivo de implementar las nanopartículas en estos substratos se han desarrollado dos nuevas técnicas para dicha implementación, llamadas micropinting y recubrimientos por pulverización.
Finalmente, con los dispositivos obtenidos se ha concebido un algortimo para la cuantificación de CO y CH4 en mezclas binarias.
Desde un punto de vista científico, se ha realizado una caracterización de SnO2 atendiendo a la temperatura de estabilización del material y hallando la evolución nanostructural de dicho óxido de estaño.
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Salazar, Gormez Jorge Ivan. "The production, purification and characterisation of carbon nanostructures for hydrogen storage". Thesis, University of Strathclyde, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.488546.
Testo completo
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Meuleman, Wouter R. A. "Electrodeposition and characterisation of Ni/Cu nanostructured multilayers from citrate solutions". Thesis, University of Newcastle Upon Tyne, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.248298.
Testo completo
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Hu, Jin. "Fabrication, characterisation, and optical applications of electrochemically deposited nanostructured IrOx films". Thesis, University of Southampton, 2008. https://eprints.soton.ac.uk/71855/.
Testo completoAbstract (sommario):
In this work, nanostructured iridium oxide films were fabricated by electrochemical deposition within a template made with polystyrene spheres on gold substrates. SEM pictures show that the nanostructured IrOx films have a very ordered hexagonal structure. To our knowledge, this is the first report of templated deposition of nanostructured IrOx films and only the 2nd of fabrication of templated deposition of nanostructured electrochromic metal oxides. The deposition solution was carried out under voltammetric control with a solution made from iridium tetrachloride, hydrogen peroxide, oxalic acid and potassium carbonate based on a recipe reported by Yamanaka (Anodically Electrodeposited Iridium Oxide-Films (AEIROF) from Alkaline-Solutions for Electrochromic Display Devices, Jpn. J. Appl. Phys. Part 1 - Regul. Pap. Short Notes Rev. Pap. 1989, 28, 632). Both non structured and nanostructured IrOx films were grown with the deposition solution. During the voltammetry, the growth of the film was followed by monitoring the cathodic peaks and anodic peaks which reflect redox reactions Ir(Ⅲ) / Ir(IV) and Ir(IV) / Ir(V) within the IrOx oxide film. During the deposition, the peak currents increase almost linearly with the number of voltammetric cycles thus allowing a fine control of the deposition process. Several attempts were made to estimate the film thickness and establish a relationship between film thickness and deposition cycles. The thickness of the nanostructured films is particularly difficult to measure accurately. Following deposition, the electrochemistry of the nanostructured IrOx films was characterised with cyclic voltammetry in both acid and base solutions. Optical properties of the nanostructured IrOx films were investigated. The transmittance of a nanostructured IrOx films was measured with transparent ITO substrates. The novel idea of coating a thin layer of IrOx film on a nanostructured gold surface was investigated to test whether the electrochromism of the nanostructured IrOx film could be used to alter the surface plasmons of the nanostructured substrate. The nanostructured IrOx film worked as a surface plasmon modulator when its colours shifted between dark and transparent under potential control. The reflectance of thickness-graded nanostructured IrOx films was measured over a range of incident angles for different potentials. Following very recent publications on the theoretical modelling of light interaction with nanostructured gold films, an attempt was made to analyse the results obtained with the nanostructured IrOx films in terms of the interplay between the localised and delocalised surface plasmons.
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Sherif, Mohamed. "Characterisation and development of nanostructured, ultrahigh strength, and ductile bainitic steels". Thesis, University of Cambridge, 2006. https://www.repository.cam.ac.uk/handle/1810/218388.
Testo completoAbstract (sommario):
The purpose of the present work was to characterise and further develop a novel nanostructured type of bainitic steel. Three chemical compositions were considered with different concentrations of Al and Co. The addition of Al and Co is believed to be necessary to produce the desired nanostructure at very low temperatures within a reasonable transformation time. An overview of the mechanical performance of fully bainitic steels vs other steel systems is presented in Chapter 1. An introduction to metallurgical concepts regarding the design and performance of bainite steels is presented in Chapters 1 and 2.Chapter 2 focuses on the design concepts by which the steel chemical composition was optimised, primarily on the basis of cost and the avoidance of carbide precipitation. Chapter 3 deals with the evolution of the microstructure during uniaxial tension, studied using X-ray diffraction. The effect of tempering deformed and undeformed structures, and heating to high temperatures, have also been investigated. In this context, data on bainite-containing steels in the literature are found to be rather limited. Chapter 4 is a comprehensive assessment of the mechanical behaviour of the steels subjected to a variety of processing routes. It is demonstrated that it is possible to outperform current commercially available steels. The microstructural behaviour of strain-aged and as-transformed steels during uniaxial tension studied using in situ neutron diffraction is described in Chapter 5. The evolution of texture with plastic deformation was confirmed as previously observed using conventional X-ray analysis. Evidence regarding the presence of two populations of carbon-depleted and carbon-rich austenite and their response to strain, grain rotation, anisotropy, stress partitioning between phases and the lack of work-hardening to overcome the onset of necking are presented.
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Lau, Desmond, e desmond lau@rmit edu au. "Characterisation of Novel Carbonaceous Materials Synthesised Using Plasmas". RMIT University. Applied Sciences, 2009. http://adt.lib.rmit.edu.au/adt/public/adt-VIT20091119.102551.
Testo completoAbstract (sommario):
Novel carbon materials such as carbon onions, nanotubes and amorphous carbon (a-C) are technologically important due to their useful properties. Normally synthesised using plasmas, their growth mechanisms are not yet fully understood. For example, the growth mechanism of the high density phase of a-C, tetrahedral amorphous carbon (ta-C), has been a subject of debate ever since its discovery. The growth mechanism of carbon nanostructures such as carbon onions and nanotubes is also not well known. The aim of this thesis is two-fold. Firstly, to provide insight into the growth of carbon films, in particular, the driving force behind the formation of diamond-like bonding in a-C which leads to ta-C. Secondly, to investigate the growth of carbon onions and other sp2 bonded carbon nanostructures such as nanotubes. To achieve the first aim, carbon thin films were deposited using cathodic arc deposition at a range of ion energies, substrate temperatures and Ar background gas pressures. These films were characterised using electron microscopy techniques to examine their microstructure, density and sp3 content. It was found that the formation of the ta-C is due to a stress-induced transition whereby a critical stress of 6.5±1.5 GPa is needed to change the phase of the film from highly sp2 to highly sp3. Within this region, a preferentially oriented phase with graphitic sheets aligned perpendicular to the substrate surface was found. By investigating the role of elevated temperatures, the ion energy-temperature
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Chong, K. K. R. "The characterisation of nanostructured magnetic materials using image spectroscopy and electron tomography". Thesis, University of Cambridge, 2007. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.597636.
Testo completoAbstract (sommario):
In this dissertation, two advanced TEM techniques, image spectroscopy and electron tomography, are applied to the quantitative structural and chemical characterisation of nanostructured magnetic materials, in both two and three dimensions (3D), at nanometre spatial resolution. In image spectroscopy, chemical information is obtained by acquiring extended energy-selected series of images, which are interpreted using a combination of computer processing and traditional electron energy-loss spectroscopy (EELS) analysis. In tomography, 3D microstructural and chemical information is obtained by acquiring ultra-high tilt series of high-angle-annular dark field (HAADF) or energy-filtered TEM (EFTEM) images, and subsequently applying tomographic reconstruction and visualisation algorithms to the data. The techniques are applied to the characterisation of materials that include closely-spaced FeNi nanoparticles coated in oxide shells, lithographically patterned Ni pillars, platinum nanoparticles, chromium carbides in stainless steel, titanomagnetite and an ALH meteorite sample. Biological samples include magnetotactic crystals and bacteria cells, greigite-containing bacterial cells and amyloid plaque cores. The chemical data from image spectroscopy and morphological data from electron tomography are cross-correlated. Furthermore, experimental tomographic data are compared with tomographic reconstructions of image simulations.
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Morrod, Jessica Katharine. "Growth and characterisation of II-VI wide band-gap heterostructures and nanostructures". Thesis, Heriot-Watt University, 2006. http://hdl.handle.net/10399/132.
Testo completo
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Mohammed, Ali Mohammed Jassim. "Optical characterisation of non polar nanostructures quantum wells ZnO/(Zn,Mg) O". Thesis, Montpellier, 2018. http://www.theses.fr/2018MONTS096/document.
Testo completoAbstract (sommario):
L’oxyde de zinc est un matériau prometteur pour la réalisation de composants optoélectroniques dans la gamme des émetteurs UV. Pour cela il faut développer des hétéro-structures tel que des puits quantiques ZnO/(Zn, Mg)O afin de mieux contrôler les propriétés d’émissions. Ce travail porte sur la caractérisation de telles structures crûes sur le plan A, surface non polaire, de ZnO massif. A partir de mesures de spectroscopie optiques (réflectivité, photoluminescence en excitation continue et photoluminescence résolue en temps) nous avons déterminé les différents phénomènes physiques mis en jeux lors de la recombinaison radiative des porteurs dans ces puits quantiques. Dans un premier temps, nous avons étudié en détail l’émission des photons par les barrières de (Zn, Mg)O. Grace à l’étude en température nous avons montré que l’émission optique de la barrière correspond à la recombinaison de paires électron trou en interactions (excitons) qui sont à basses températures localisées dans des fluctuations de potentiel. Sous l’effet de la température ils se délocalisent et se recombinent comme des exciton libres. L’étude détaillée des déclins temporels de photoluminescence nous à permis de démontrer que nous avions affaire à deux états excitoniques différents qui présentent des dynamiques de recombinaisons différentes. Un modèle est proposé pour rendre compte des différentes observations. La partie principale de ce travail porte sur le comportement des excitons dans le puits quantique. Le résultat majeur de cette étude est la démonstration expérimentale que dans ce cas des complexes excitoniques, ici des trions chargé négativement (exciton en interaction avec un électron libre), se forment à basse température et sont responsable de la luminescence observée. De plus, en variant la densité d’excitation nous avons montré que se former également des bi-excitons (pseudo particule formée de deux exciton en interactions). Le comportement en température de la photoluminescence obtenue dans différente conditions d’excitation à permis de démontrer que sous l’effet de l’énergie thermique les complexes excitoniques se dissociés pour créer des excitons libres. Des mesures en fonction de la polarisation de la lumière émise et de la température ont permis également d’étudier l’état C de l’exciton dans le puits. Les dynamiques de recombinaison des différents complexes excitoniques sont examinées en fonction de la températureThe zinc oxide is a promising material for the realization of optoelectronic devices in the blue-UV range. For this, it is necessary to develop hetero-structures such as ZnO / (Zn, Mg) O quantum wells in order to have better control of the properties of emissions. This work concerns the characterization of such structures grown on the A-plane (non-polar surface) of bulk ZnO. From optical spectroscopies measurements (reflectivity, continuous wave and time-resolved photoluminescence) we determined the various physical phenomena involve during the radiative recombination of the carriers in these quantum wells. At first, we studied in detail the emission of photons by the barriers of (Zn, Mg) O. Thanks to the study in temperature we showed that the optical emission of the barrier corresponds to the recombination of electron hole pairs in interactions (excitons), which are at low temperatures localized in the fluctuations of the potential. Under the influence of the temperature they delocalize and recombine as free exciton. From the detailed study of the temporal decays of photoluminescence we can demonstrate that we deal with two different excitonic states, which present different dynamics of recombination. A model is proposed that explain the various observations. The main part of this work concerns the behavior of the excitons in the quantum well. The major result is the experimental demonstration that excitonics complexes are formed at low temperature, negatively charged trion (exciton in interaction with a free electron), in this system and they are responsible for the observed luminescence. Furthermore, by varying the density of excitation we showed that biexcitons are also form (pseudo-particles formed by two excitons in interactions). The behavior in temperature of the photoluminescence obtained in different conditions of excitation demonstrates that under the influence of the thermal energy the exitonic complexes are broken to create free excitons. Measures according to the polarization of the emitted light and the temperature also allowed studying the C state of the exciton in the quantum well. The dynamics of recombination of the various excitonics complexes are examined according to the temperature
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Marques-Garcia, Lourdes. "Preparation and characterisation of nanostructured bulk Bi2Te3 thermoelectric materials using ultrasound milling". Thesis, Cardiff University, 2016. http://orca.cf.ac.uk/93575/.
Testo completoAbstract (sommario):
Thermoelectric materials have been intensively investigated during the last years for energy harvesting applications. The main drawback of this technology is the low efficiency of the current materials. Significant advances in this respect have been recently achieved by nanostructuring, to mainly reduce the thermal conductivity. In this approach a bulk sample is milled into a nanostructured powder that is then compacted to form a nanobulk sample. The main objective of this thesis is to explore and introduce a new technique, ultrasound milling, based on the crushing of bulk samples by means of ultrasound effects taking place in a liquid medium, for the preparation of nanostructured bulk materials. Bismuth telluride alloys are the industrial standard in thermoelectrics and has been chosen as the material to perform this investigation. The most suitable conditions for the preparation of the nanostructured powders by the ultrasound milling technique have been identified. The optimised powders were used to prepare compacted nanobulk samples. The thermoelectric properties of these samples were finally characterised at room temperature and their performance related to their microstructure. Extraordinarily low thermal conductivity was obtained for both n- and p-type samples prepared (0.5 and 0.35 W/Km respectively), which are within the lowest reported values for any thermoelectric alloy. This reduction was accompanied with a significant decrease in electrical conductivity which led to a non-significant improvement in the figure of merit (Z). However, high ZT values (1 and 1.4 for n- and p-type bismuth telluride respectively) were identified in non-treated samples after a very simple grinding process which was employed as pre-treatment. The figure of merit of these two materials prepared by this simple methodology is close to the best reported values for Bi2Te3. Our results identify ultrasound milling and the simple crushing method as promising tools for the fabrication of nanostructured bulk thermoelectric materials.
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Johansson, Emil. "Synthesis and Characterisation of Potential Hydrogen Storage Materials". Doctoral thesis, Uppsala universitet, Fysik III, 2004. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-4509.
Testo completoAbstract (sommario):
The dissociative and non-dissociative hydrogen uptake in carbon nanostructures and metallic films were investigated by measurements and analysis of solubility isotherms. The total, non-dissociative, uptake for multi-walled nano-barrels and amorphous nanoporous carbon was determined to be 6.2 and 4.2 wt. % respectively at 77 K and the adsorption energies (at lowest coverage) -7.2 and -4.2 kJ/mol. At 298 K the H-uptake was negligible. At low concentrations the H-uptake of Nb-films is strongly affected by the film thickness. For thicknesses less then about 31 nm, the absorption energy was found to be temperature dependent. Such changes have not been observed in Nb films before. The presence of multiple absorption energies was shown to limit the possibility to obtain relevant absorption and interaction energies by traditional Sievert's and van 't Hoff analysis. The Mg1-xNix system (0<0.43) was investigated with respect to the hydrogen uptake. For Mg2Ni the hydrogen uptake, at an external hydrogen pressure of 1 bar, is close to 1.33 H/M (Mg2NiH4). The enthalpy of formation is smaller in the film as compared to bulk material. The changes in the absorption energy are caused by the adhesion to the substrate as well as the nanocrystallinity of the absorbing layers. The optical band gap of Mg2NiH4 was determined to be 2.4 eV. In Mg1-xYx (0<0.17) it was found that the Y-concentration limits the hydrogen uptake at 1 bar. However, the kinetics of the uptake improves substantially with a minimum of 7 at.% of Y. For Mg-Y the optical band gap (3.6 eV) is independent of Y concentration within the concentration range investigated, while the transmittance decreases with increasing Y content.
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Willis, Shawn M. "Advanced optoelectronic characterisation of solar cells". Thesis, University of Oxford, 2011. http://ora.ox.ac.uk/objects/uuid:07683f00-b7ba-4be3-aec0-f389fed34644.
Testo completoAbstract (sommario):
Optoelectronic characterisation techniques are assessed in their application to three solar cell systems. Charge injection barriers are found in PbS/ZnO colloidal quantum dot solar cells through the use of temperature dependent current-voltage and capacitance-voltage measurements. The injection barriers are shown to complicate the Mott-Schottky capacitance analysis which determines built-in bias and doping density. A model that incorporates depletion capacitance and a constant capacitance arising from the injection barriers is given to explain the Mott-Schottky plots. The junction mechanism at the PbS/ZnO interface is found to transition from excitonic to p-n behaviour based on the amount of UV photodoping the cell has received. External quantum efficiency analysis at different photodoping times reveals a growing charge collection region within the material, demonstrating the shift to p-n behaviour. This is further supported by the observance of depletion capacitance behaviour after, but not before, UV photodoping. Defects within GaAs cells containing InAs quantum dots are found to enhance the sub-bandgap performance of the cell using external quantum efficiency analysis. This is verified by illuminated current-voltage analysis using a 1000 nm high pass optical filter to block photons of larger energy than the bandgap. Using capacitance-voltage analysis, high temperature rapid thermal annealing is shown to induce defects in dilute nitride cells, which explains the drop in open circuit voltage compared to lower temperature annealed cells. The doping level of polymer solar cells exposed to air is found to increase with continued exposure using Mott-Schottky capacitance analysis. Current-voltage measurements show the formation of an Al2O3 barrier layer at the polymer/aluminium interface. The usefulness of capacitance-voltage measurements to probe the polymer/fullerene interface is investigated in thermally evaporated thiophene/C60 cells.