http://www.dspace.cam.ac.uk:80/handle/1810/34586 The Departement of Materials Science and Metallurgy Thu, 20 Jun 2013 00:53:19 GMT 2013-06-20T00:53:19Z http://www.dspace.cam.ac.uk:80/handle/1810/244605 Title: Preparation, characterization and in vitro evaluation of nanostructured chitosan/apatite and chitosan/Si-doped apatite composites Authors: Solis, Yaimara; Davidenko, Natalia; Carrodeguas, Raul G; Cruz, Jeny; Hernandez, Andy; Tomas, Miriela; Cameron, Ruth; Peniche, Carlos Abstract: Chitosan/apatite composites are attracting great attention as biomaterials for bone repair and regeneration procedures. The reason is their unique set of properties: bioactivity and osteoconductivity provided by apatite and resorbability supplied by chitosan among others. Thus, in this work chitosan/apatite and chitosan/Si-doped apatite composites were prepared and characterized. Particle size, surface area, in vitro physiological stability, enzymatic biodegradation and bioactivity were evaluated. Unimodal particle size distribution was obtained for composites with high chitosan/apatite ratios while bimodal distribution was present in composites with low chitosan/apatite ratio. Physiological stability decreased with Si-doping and with the chitosan content. Acetylation degree and molecular weight of chitosan did not affect in vitro stability. Rate of enzymatic degradation increased with the chitosan content in composites. Si-doped apatite composites also showed increased degradation with respect to non-doped ones. The bioactivity of the composites was evidenced by the deposition on their surface of a calcium phosphate layer with apatite morphology after immersion in simulated body fluid. Both, biodegradation and bioactivity were dependent on the molecular weight of the polymeric chitosan matrix. These results suggest that the chitosan/apatite composites obtained are promising materials for bone regeneration applications Tue, 01 Jan 2013 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244605 2013-01-01T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244515 Title: Collagen fibre implant for tendon and ligament biological augmentation. In vivo study in an ovine model Authors: Enea, D; Gwynne, J; Kew, S; Arumugam, M; Shepherd, J; Brooks, R; Ghose, S; Best, S; Cameron, R; Rushton, N Abstract: Purpose Although most in vitro studies indicate that collagen is a suitable biomaterial for tendon and ligament tissue engineering, in vivo studies of implanted collagen for regeneration of these tissues are still lacking. The objectives of this study were the following: (1) to investigate the regeneration of the central third of the ovine patellar tendon using implants made of an open array of collagen fibres (reconstituted, extruded bovine collagen); and (2) to compare two collagen crosslinking chemistries: carbodiimide and carbodiimide associated with ethyleneglycoldiglycidylether. Methods Forty-eight Welsh Mountain sheep were operated on their right hind leg. The central third of patellar tendon was removed and substituted with carbodiimide (n = 16) and carbodiimide–ethyleneglycoldiglycidylether-crosslinked implants (n = 16). In the control group the defect was left empty (n = 16). The central third of contralateral unoperated tendons was used as positive controls. Half of the sheep in each group were killed at 3- and 6-month time points. After proper dissection, tendon sub-units (medial, central and lateral) were tested to failure (n = 6 for each group), whilst 2 non-dissected samples were used for histology. Results Both the implants had significantly lower stress to failure and modulus with respect to native tendon at both 3- and at 6-month time points. The implants did not statistically differ in stress to failure, whilst carbodiimide-crosslinked implants had significantly higher modulus than carbodiimide–ethyleneglycoldiglycidylether-crosslinked implants both at 3 and at 6 months. Histology showed carbodiimide-crosslinked implants to have a better integration with the native tendon than carbodiimide–ethyleneglycoldiglycidylether-crosslinked implants. Carbodiimide-crosslinked implants appeared partially resorbed and showed increased tissue ingrowth with respect to carbodiimide–ethyleneglycoldiglycidylether-crosslinked implants. Conclusions To deliver collagen implants as an open array of fibres allows optimal tendon–implant integration and good ingrowth of regenerated tissue. In the present study the resorption rate of both the examined implants was too low due to the high level of crosslinking. This led to only minor substitution of the implant with regenerated tissue, which in turn produced a low-strength implanted region. Further studies are needed to find the right balance between strength and resorption rate of collagen fibres. Tue, 19 Jun 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244515 2012-06-19T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244509 Title: Exploring the application of stem cells in tendon repair and regeneration. Authors: Ahmad, Z; Wardale, J; Brooks, R; Henson, F; Noorani, A; Rushton, N Abstract: PURPOSE: To conduct a systematic review of the current evidence for the effects of stem cells on tendon healing in preclinical studies and human studies. METHODS: A systematic search of the PubMed, CINAHL (Cumulative Index to Nursing and Allied Health Literature), Cochrane, and Embase databases was performed for stem cells and tendons with their associated terminology. Data validity was assessed, and data were collected on the outcomes of trials. RESULTS: A total of 27 preclinical studies and 5 clinical studies met the inclusion criteria. Preclinical studies have shown that stem cells are able to survive and differentiate into tendon cells when placed into a new tendon environment, leading to regeneration and biomechanical benefit to the tendon. Studies have been reported showing that stem cell therapy can be enhanced by molecular signaling adjunct, mechanical stimulation of cells, and the use of augmentation delivery devices. Studies have also shown alternatives to the standard method of bone marrow-derived mesenchymal stem cell therapy. Of the 5 human studies, only 1 was a randomized controlled trial, which showed that skin-derived tendon cells had a greater clinical benefit than autologous plasma. One cohort study showed the benefit of stem cells in rotator cuff tears and another in lateral epicondylitis. Two of the human studies showed how stem cells were successfully extracted from the humerus and, when tagged with insulin, became tendon cells. CONCLUSIONS: The current evidence shows that stem cells can have a positive effect on tendon healing. This is most likely because stem cells have regeneration potential, producing tissue that is similar to the preinjury state, but the results can be variable. The use of adjuncts such as molecular signaling, mechanical stimulation, and augmentation devices can potentially enhance stem cell therapy. Initial clinical trials are promising, with adjuncts for stem cell therapy in development. Fri, 27 Jul 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244509 2012-07-27T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244508 Title: A study of surface morphology and phase separation of polymer/cellulose liquid crystal composite membranes Authors: Tu, Mei; Han, Wanqing; Zeng, Rong; Best, Serena; Cameron, Ruth Abstract: This paper explores the effects of the incorporation of liquid crystalline phases into polyurethane (PU) matrix with the aim of creating composites with biomimetic surfaces. The surface morphology and phase separation structure of polyurethane/butyl hydroxypropyl cellulose ester (PU/BPC) composite membranes with different BPC contents and underwent different post-treatments were investigated by using polarized optical microscopy (POM), scanning electron microscopy (SEM) and small angle X-ray scattering (SAXS). Well-dispersed liquid crystal (LC) domains occurred on the PU/BPC composite membranes surfaces. As the increment of BPC content, the LC domains tended to form enlarged quasispherical aggregates with poorly molecular orientation, and low degree of regular phase separation occurred between the LC domains and PU substrate. Membranes with different LC contents underwent heat treating and cooling at three different conditions exhibited distinct surface morphologies, meanwhile, a sharp peak emerged in the SAXS pattern, which indicated that the ordered arrangement of BPC molecular chains existed in the LC domains and the phase separation structure between substrate and LC domain had changed. Sharper and more intense SAXS peaks were found in the membranes that annealing in oven, indicating more regular arrangement of LC domains and more obvious phase separation presented than those cooling to 20 °C or −20 °C respectively. Results suggested that the surface morphology of polymer/LC membranes could be controlled through adjusting LC contents or post treatment conditions. Sat, 04 Aug 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244508 2012-08-04T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244382 Title: Production, characterisation and properties of carbon nanotube fibres Authors: Sundaram, Rajyashree Meenakshi Abstract: Assembling carbon nanotubes (CNTs) in to ensembles like fibres, yarns or films is aimed at exploiting the exceptional properties of individual CNTs at a macroscopic scale. Amongst the various production methods, continuous and direct spinning of CNT assemblies (specifically, fibres) from the gas-phase chemical vapour deposition (CVD) reactor is attractive due to its scale-up potential. Although, these fibres exhibit remarkable mechanical, electrical and thermal properties, their performance is limited by compositional heterogeneity – stemming from the presence of non-CNT inclusions and nanotubes with a wide distribution of diameters, lengths and chiralities. The work presented here attempts to explore strategies to control the composition of the CNT fibres. Reduction of impurity levels can stem to either post-spinning removal of extraneous materials or to direct synthesis of pure fibres. In this work, each of these routes has been pursued. A post-spinning purification methodology, involving sonication of the continuously spun fibres in a solvent has been attempted and the structure, composition and performance of the purified fibres have been assessed. In addition, having evaluated the origins of the compositional heterogeneity associated with the CVD-based synthesis employed for fibre spinning, CNT fibres with minimal impurities could be directly produced by coordinating the pyrolysis chemistry of the CVD-reactants. As a step further, morphology control of the constituent nanotubes, specifically with respect to limiting their diameter distribution and regulating the number of walls, was also attempted. As a result, CNT fibres composed of exclusively multiwalled/doublewalled/singlewalled nanotubes with minimal impurities have been produced. Having developed protocols to exercise a definite degree of control on the contents of the CNT fibres, effects of the composition on the fibre-performance have also been examined. Tue, 13 Nov 2012 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244382 2012-11-13T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244252 Title: Effect of fiber crosslinking on collagen-fiber reinforced collagen-chondroitin-6-sulfate materials for regenerating load-bearing soft tissues Authors: Shepherd, JH; Ghose, S; Kew, SJ; Moavenian, A; Best, SM; Cameron, RE Tue, 01 Jan 2013 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244252 2013-01-01T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244229 Title: Synthetic Collagen Fascicles for the Regeneration of Tendon Tissue Authors: Kew, SJ; Gwynne, JH; Enea, D; Brooks, R; Rushton, N; Best, SM; Cameron, RE Abstract: The structure of an ideal scaffold for tendon regeneration must be designed to provide a mechanical, structural and chemotactic microenvironment for native cellular activity to synthesise functional (i.e. load bearing) tissue. Collagen fibre scaffolds for this application have shown some promise to date, although the microstructural control required to mimic the native tendon environment has yet to be achieved allowing for minimal control of critical in vivo properties such as degradation rate and mass transport. In this report we describe the fabrication of a novel multi-fibre collagen fascicle structure, based on type-I collagen with failure stress of 25 – 49 MPa, approximating the strength and structure of native tendon tissue. We demonstrate a microscopic fabrication process based on the automated assembly of type-I collagen fibres with the ability to produce a controllable fascicle-like, structural motif allowing variable numbers of fibres per fascicle. We have confirmed that the resulting post-fabrication type-I collagen structure retains the essential phase behaviour, alignment and spectral characteristics of aligned native type-I collagen. We have also shown that both ovine tendon fibroblasts and human white blood cells in whole blood readily infiltrate the matrix on a macroscopic scale and that these cells adhere to the fibre surface after seven days in culture. The study has indicated that the synthetic collagen fascicle (SCF) system may be a suitable biomaterial scaffold to provide a rationally-designed implantable matrix material to mediate tendon repair and regeneration. Sun, 01 Jan 2012 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244229 2012-01-01T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/244221 Title: Stress relief cracking in creep resisting low alloy ferritic steels. Authors: Tait, Robert Andrew Abstract: Early investigations of the phenomenon of Stress Relief Cracking in low alloy steels suggested that the observed failure along prior austenite grain boundaries was in large part promoted by the relative strengthening of the grains with respect to the grain boundaries, during the stress relieving heat treatment. In this respect, it was felt that certain Cr-Mo-V steels containing vanadium carbide should be particularly susceptible to this mode of failure, since V4 C3 is well known to confer very good creep strengthening in these low alloy steels. As a result of this work, it has been established that the above view is both inaccurate and misleading. Most of the experiments were performed on two samples of commercially produced 1/2Cr 1/2Mo 1/2V steel. One of these samples had proved to be susceptible to stress relief cracking during fabrication. Detailed observations of the austenitic grain growth and secondary hardening response of the two samples are described. The results facilitated the design of a series of high temperature hardness and tensile tests in which the separate effects of grain strengthening and grain size in promoting high temperature intergranular failure could be identified. In particular, it is observed that although both steels exhibit very similar strength characteristics, the effect of grain size in promoting low ductility in association with intergranular failure is more marked in one steel than in the other. The steel in which a marked tendency towards stress relief cracking had been observed is seen to show low ductilities at high temperatures even in fine grain size specimens ; suggesting that the state of the boundaries, independent of the strengthening effect, is an important factor in promoting high temperature intergranular failure. By using an anisothermal stress relaxation testing procedure, it was possible to identify precisely the conditions of stress,temperature and microstructure under which stress relief cracking occurs. In particular, it was observed that the susceptible sample failed by nucleation controlled intergranular cavitation in association with a particular stress/strain-rate/temperature regime where the deformation process is rate controlled by the diffusion of carbon. Further experiments described in the latter part of the thesis examine the effect of purity in promoting intergranular failure both at low and high temperatures. The results suggest that the presence of impurities like phosphorus, which are known to segregate to prior austenite grain boundaries, may act to allow easier nucleation and growth of cavities during failure by inter granular cavitation. The conclusions of the thesis outline the mechanism of stress relief cracking and make discussion in terms of current theories of intergranular failure at high temperatures. Recommendations are made concerning both the assessment of susceptibility of steels to this mode of failure, as well as the adoption of safer stress relief procedures. Tue, 26 Oct 1976 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/244221 1976-10-26T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243639 Title: Defect-induced magnetism and transport phenomena in epitaxial oxides Authors: Schoofs, Frank Abstract: This work focuses on the impact of defects, intrinsic or artificially introduced, on the functional properties of thin, epitaxial oxide films. In the first part, the origin of the ferromagnetic properties of Mn-doped and undoped zinc oxide is studied. The deposition conditions are found to have a significant impact on the structural, transport and magnetic properties of the thin films. Combining x-ray magnetic circular dichroism and magnetometry experiments, it is established that the transition metal dopants (i.e. Mn) have no influence on the ferromagnetic nature of the zinc oxide, but that localised magnetic moments on intrinsic defects are in fact responsible for the ferromagnetic behaviour. A relation between strain (related to defect concentration) and magnetisation is established. In the second part of this dissertation, artificially introduced defects are employed in order to discover the fundamental conduction mechanism behind the two-dimensionally conductive LaAlO3/SrTiO3 interface. All experiments, from varying deposition temperature, to oxygen pressure, to laser fluence or to the insertion of (doped) perovskite layers, point towards a structurally governed conduction mechanism, although the exact details are still unclear. Distinct transitions in the resistance versus temperature curves are observed at different values than the bulk phase transformation temperature. These transitions form the boundaries of different conduction modes, with tendencies towards non-Fermi-liquid behaviour observed in certain two-dimensionally conducting samples in limited temperature regimes. By optimising the (defect) structure at the interface, i.e. by introducing a single unit cell of (La0.5,Sr0.5)TiO3 or SnTiO3, it is shown that the sheet carrier density can be dramatically enhanced, up to an order of magnitude higher than unmodified LaAlO3/SrTiO3 interfaces with a value of 1e14 cm−2 at 200 K. Finally, attempts at functionalising the conductive heterointerface by doping and inserting (anti)ferromagnetic layers are made. Mon, 02 Jul 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243639 2012-07-02T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243440 Title: The interplay between physical and chemical properties of protein films affects their bioactivity Authors: Grover, CN; Farndale, RW; Best, SM; Cameron, RE Abstract: Although mechanical properties, roughness, and receptor molecule expression have all been shown to influence the cellular reactivity of collagen-based biomaterials, their relative contribution, in a given system remains unclear. Here, we study films containing combinations of collagen, gelatin, and soluble and insoluble elastin, crosslinking of which results in altered film stiffness and roughness. Collagen and gelatin have similar amino acid sequences but altered cell-binding sites. We studied cell response with both C2C12 myoblast cells (which possess RGD-recognizing integrins α(V) β(3) and α(5) β(1) ) and C2C12-α2+ cells (which, in addition, express the collagen-binding integrin α(2) β(1) ) to establish the effect of altering the available binding sites on cell adhesion and spreading on films. Systematically altering the composition, crosslinking and cell type, allows us to deconvolute the effects of physical parameters and available binding sites on the cell reactivity of films in this system. Collagen-based films were rougher and stiffer and supported lower cell surface coverage than gelatin-based films. Additionally, C2C12-α2+ cells showed preferential attachment to collagen-based films compared with C2C12 cells, but no significant difference was seen using gelatin-based films. The cell count and surface coverage were found to decrease significantly on all films after crosslinking (Coll XL coverage = 2-6%, Gel XL coverage = 20-32%), but cell area and aspect ratio on collagen films were affected to a greater extent than on gelatin films. The results show that, in this system, the composition, and more significantly, crosslinking, of films affects the cell reactivity to a greater extent than their stiffness or roughness. Sun, 08 Jul 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243440 2012-07-08T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243439 Title: Investigating the morphological, mechanical and degradation properties of scaffolds comprising collagen, gelatin and elastin for use in soft tissue engineering Authors: Grover, CN; Best, SM; Cameron, RE Abstract: Collagen-based scaffolds can be used to mimic the extracellular matrix (ECM) of soft tissues and provide support during tissue regeneration. To better match the native ECM composition and mechanical properties as well as tailor the degradation resistance and available cell binding motifs, other proteins or different collagen types may be added. The present study has explored the use of components such as gelatin or elastin and investigated their effect on the bulk physical properties of the resulting scaffolds compared to those made from pure collagen type I. The effect of altering the composition and crosslinking was evaluated in terms of the scaffold structure, mechanical properties, swelling, degradation and cell attachment. Results demonstrate that scaffolds based on gelatin had reduced tensile stiffness and degradation time compared with collagen. The addition of elastin reduced the overall strength and stiffness of the scaffolds, with electron microscopy results suggesting that insoluble elastin interacts best with collagen and soluble elastin interacts best with gelatin. Carbodiimide crosslinking was essential for structural stability, strength and degradation resistance for scaffolds of all compositions. In addition, preliminary cell adhesion studies showed these highly porous structures (pore size 130-160 μm) to be able to support HT1080 cell infiltration and growth. Therefore, this study suggests that the use of gelatin in place of collagen, with additions of elastin, can tailor the physical properties of scaffolds and could be a design strategy for reducing the overall material costs. Sun, 08 Jul 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243439 2012-07-08T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243438 Title: Crosslinking and composition influence the surface properties, mechanical stiffness and cell reactivity of collagen-based films. Authors: Grover, CN; Gwynne, JH; Pugh, N; Hamaia, S; Farndale, RW; Best, SM; Cameron, RE Abstract: This study focuses on determining the effect of varying the composition and crosslinking of collagen-based films on their physical properties and interaction with myoblasts. Films composed of collagen or gelatin and crosslinked with a carbodiimide were assessed for their surface roughness and stiffness. These samples are significant because they allow variation of physical properties as well as offering different recognition motifs for cell binding. Cell reactivity was determined by the ability of myoblastic C2C12 and C2C12-α2+ cell lines (with different integrin expression) to adhere to and spread on the films. Significantly, crosslinking reduced the cell reactivity of all films, irrespective of their initial composition, stiffness or roughness. Crosslinking resulted in a dramatic increase in the stiffness of the collagen film and also tended to reduce the roughness of the films (R(q)=0.417±0.035μm, E=31±4.4MPa). Gelatin films were generally smoother and more compliant than comparable collagen films (R(q)=7.9±1.5nm, E=15±3.1MPa). The adhesion of α2-positive cells was enhanced relative to the parental C2C12 cells on collagen compared with gelatin films. These results indicate that the detrimental effect of crosslinking on cell response may be due to the altered physical properties of the films as well as a reduction in the number of available cell binding sites. Hence, although crosslinking can be used to enhance the mechanical stiffness and reduce the roughness of films, it reduces their capacity to support cell activity and could potentially limit the effectiveness of the collagen-based films and scaffolds. Sun, 08 Jul 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243438 2012-07-08T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/243436 Title: Osteoinduction by combining bone morphogenetic protein (BMP)-2 with a bioactive novel nanocomposite Authors: Sharma, A; Meyer, F; Best, SM; Cameron, RE; Rushton, N Sun, 08 Jul 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/243436 2012-07-08T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/242431 Title: The consequences of high injected carrier densities on carrier localisation and efficiency droop in InGaN/GaN quantum well structures Authors: Hammersley, S; Watson-Parris, D; Dawson, P; Godfrey, M; Badcock, T; Kappers, M; McAleese, C; Oliver, R; Humphreys, C Abstract: There is a great deal of interest in the underlying causes of efficiency droop in InGaN/GaN quantum well light emitting diodes, with several physical mechanisms being put forward to explain the phenomenon. In this paper we report on the observation of a reduction in the localisation induced S-shape temperature dependence of the peak photoluminescence energy with increasing excitation power density. This S-shape dependence is a key fingerprint of carrier localisation. Over the range of excitation power density where the depth of the S shape is reduced we also observe a reduction in the integrated photoluminescence intensity per unit excitation power, i.e. efficiency droop. Hence the onset of efficiency droop occurs at the same carrier density as the onset of carrier delocalisation. We correlate these experimental results with the predictions of a theoretical model of the effects of carrier localisation due to local variations in the concentration of the randomly distributed In atoms on the optical properties of InGaN/GaN quantum wells. On the basis of this comparison of theory with experiment we attribute the reduction in the Sshape temperature dependence to the saturation of the available localised states. We propose that this saturation of the localised states is a contributory factor to efficiency droop whereby non localised carriers recombine non-radiatively. Tue, 17 Apr 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/242431 2012-04-17T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/242430 Title: Atom probe tomography characterisation of a laser diode structure grown by molecular beam epitaxy Authors: Bennett, Samantha; Smeeton, Tim; Saxey, David; Smith, George; Hooper, Stewart; Heffernan, Jonathan; Humphreys, Colin; Oliver, Rachel Abstract: Atom probe tomography (APT) has been used to achieve three-dimensional characterization of a III-nitride laser diode (LD) structure grown by molecular beam epitaxy (MBE). Four APT data sets have been obtained, with fields of view up to 400 nm in depth and 120 nm in diameter. These data sets contain material from the InGaN quantum well (QW) active region, as well as the surrounding p- and n-doped waveguide and cladding layers, enabling comprehensive study of the structure and composition of the LD structure. Two regions of the same sample, with different average indium contents (18% and 16%) in the QW region, were studied. The APT data are shown to provide easy access to the p-type dopant levels, and the composition of a thin AlGaN barrier layer. Next, the distribution of indium within the InGaN QW was analyzed, to assess any possible inhomogeneity of the distribution of indium (“indium clustering”). No evidence for a statistically significant deviation from a random distribution was found, indicating that these MBE-grown InGaN QWs do not require indium clusters for carrier localization. However, the APT data show steps in the QW interfaces, leading to well-width fluctuations, which may act to localize carriers. Additionally, the unexpected presence of a small amount (x = 0.005) of indium in a layer grown intentionally as GaN was revealed. Finally, the same statistical method applied to the QW was used to show that the indium distribution within a thick InGaN waveguide layer in the n-doped region did not show any deviation from randomness. Tue, 06 Mar 2012 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/242430 2012-03-06T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/242429 Title: Growth and optical characterization of multilayers of InGaN quantum dots Authors: Zhu, Tontong; El-Ella, Haitham; Reid, Benjamin; Holmes, Mark; Taylor, Robert; Kappers, Menno; Oliver, Rachel Abstract: We report on the growth (using metal-organic vapour phase epitaxy) and optical characterization of single and multiple layers of InGaN quantum dots (QDs), which were formed by annealing InGaN epilayers at the growth temperature in nitrogen. The size and density of the nanostructures have been found to be fairly similar for uncapped single and three layer QD samples if the GaN barriers between the dot layers are grown at the same temperature as the InGaN epilayer. The distribution of nanostructure heights of the final QD layer of three is wider and is centred around a larger size if the GaN barriers are grown at two temperatures (first a thin layer at the dot growth temperature, then a thicker layer at a higher temperature). Micro-photoluminescence studies at 4.2 K of capped samples have confirmed the QD nature of the capped nanostructures by the observation of sharp emission peaks with full width at half maximum limited by the resolution of the spectrometer. We have also observed much more QD emission per unit area in a sample with three QD layers, than in a sample with a single QD layer, as expected. Sun, 01 Jan 2012 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/242429 2012-01-01T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/242371 Title: Characterisation of polar (0001) and non-polar (11-20) ultraviolet nitride semiconductors Authors: Chang, Tse Yang Abstract: UV and deep-UV emitters based on AlGaN/AlN heterostructures are very inefficient due to the high lattice mismatch of these films with sapphire substrates, leading to high dislocation densities. This thesis describes the characterisation of the nanostructures of a range of UV structures, including c-plane (polar) AlGaN epilayers grown on AlN template, and nonpolar GaN/AlGaN MQWs grown on a-plane GaN template. The results are based primarily on transmission electron microscopy (TEM), cathodoluminescence in the scanning electron microscope (SEM-CL), high-resolution X-ray diffraction (HRXRD) and atomic force microscopy (AFM) measurements. The structural and optical properties of various types of defect were examined in the c-plane AlGaN epilayers. Strain analysis based on in-situ wafer curvature measurements was employed to describe the strain relief mechanisms for different AlGaN compositions and to correlate the strain to each type of defect observed in the epilayers. This is followed by the investigation of AlN template growth optimisation, based on the TMA pre-dose on sapphire method to enhance the quality and the surface morphology of the template further. The initial growth conditions were shown to be critical for the final AlN film morphology. A higher TMA pre-dose has been shown to enable a better Al coverage leading to a fully coalesced AlN film at 1 μm thickness. An atomically smooth surface of the template was achieved over a large 10 x 10 μm AFM scale. Finally, the investigation of UV emitters based on nonpolar crystal orientations is presented. The SiNx interlayer was able to reduce the threading dislocation density but was also found to generate voids with longer SiNx growth time. The relationship between voids, threading dislocations, inversion domain boundaries and their associated V-defects and the variation in MQW growth rate has been discussed in detail. Mon, 09 Apr 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/242371 2012-04-09T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/242232 Title: Refining structures against reflection rank: An alternative metric for electron crystallography. Authors: Eggeman, Alexander; Migley, Paul Abstract: A new metric is proposed to improve the fidelity of structures refined against precession electron diffraction data. The inherent dynamical nature of electron diffraction ensures that direct refinement of recorded intensities against structure factor amplitudes can be prone to systematic errors. Here we show that the relative intensity of precessed reflections, their rank, can be used as an alternative metric for refinement. Experi- mental data from erbium pyrogermanate shows that applying precession reduces the dynamical transfer of intensity between reflections and hence stabilises their rank, enabling accurate and reliable structural refinements. This approach is then applied successfully to an unknown structure of an oxygen-deficient bismuth manganite result- ing in a refined structural model that is similar to a calcium analogue. Mon, 30 Apr 2012 23:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/242232 2012-04-30T23:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/242231 Title: Advances in Imaging and Electron Physics - Volume 170 Authors: Eggeman, Alexander; Midgley, Paul Sun, 01 Jan 2012 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/242231 2012-01-01T00:00:00Z http://www.dspace.cam.ac.uk:80/handle/1810/242179 Title: Indirect measurement of the electrocaloric effect Authors: Young, James Scott Abstract: In August 2011 at the International Symposium on Integrated Functionalities in Cambridge, a whole session was devoted to the electrocaloric effect, which is undergoing a modest renaissance. Surprisingly, the various reports showed that the indirect method of measuring cooling (described in the following sections) did not usually agree with the direct method (actually measuring temperature change with a thermometer). However, there was no obvious systematic error: sometimes the indirect temperature change was larger and sometimes smaller. The discrepancies were beyond the experimental errors. The majority of the present thesis is dedicated to careful reexamination of some of the assumptions made in the indirect method, both during measurement and in the subsequent data analysis that leads to inferred temperature changes. Experimentally, I conclude that the most serious systematic error is likely to be the unwarranted assumption that polarization and field measurements, recorded in hysteresis loops that are traced within a millisecond or less, are all taken at the same temperature. In reality, the experience of the material during such loops is neither isothermal nor adiabatic. Other systematic errors relate to data analyses and are discussed in detail. In some ways, therefore, this thesis has a negative flavour. But it is not designed to criticise prior work. Rather, it is intended to discriminate between reliable experimental procedures and those less convincing. This is a line of research with important technology transfer possibilities, and hence the numerical values of electric cooling must be unusually reliable if we are to avoid unwise capital investment as a country. Description: Theoretical chapter resolving long-term dispute in thermodynamics over electrical work, followed by experimental analyses and exposure of inherent problems of indirect measurement of the electrocaloric effect, including misconceptions in electrocaloric literature and previously unexamined assumptions. Tue, 06 Mar 2012 00:00:00 GMT http://www.dspace.cam.ac.uk:80/handle/1810/242179 2012-03-06T00:00:00Z