This thesis, submitted to the University of Manchester, covers a range of topics related to current research in two-dimensional materials under the title: 'Investigations into the interfacial interaction of graphene with hexagonal boron nitride.'In the last decade, two-dimensional materials have become a rich source of original research and potential applications. The main advantage lies in the ability to produce novel composite structures, so-called 'layered heterostructures', which are only a few atomic layers thick. One can utilise the unique properties of several species of crystal separately, or how they interact to realise a diverse range of uses. Two such crystals are graphene and hexagonal boron nitride. Hexagonal boron nitride has, so far, been used primarily as a substrate for graphene, allowing researchers to get the most out of graphene's impressive individual properties. However, in this thesis, the non-trivial van der Waals interaction between graphene and hexagonal boron nitride is examined. The interface potential reveals itself as a relatively large-scale, orientation-dependant superlattice, which is described in chapters 1 and 2.I In Chapter 4, the effect of this superlattice is examined by measurement of its effect upon the electrons in graphene, where its modulation leads to the creation of second and third generation Dirac points, revealing Hofstadter's Butterfly. As well as an excellent example of the physics possible with graphene, it also presents a new tool with which to create novel devices possessing tailored electronic properties. II In chapter 5, the consequential effect of the superlattice potential on the structure of graphene is studied. Results are discussed within the framework of the Frenkel-Kontorova model for a chain of atoms on a static background potential. Results are consistent with relaxation of the graphene structure leading to the formation of a commensurate ground state. This has exciting consequences for the production of heterostructures by demonstrating that alignment angle can have large effects upon the physical properties of the crystals. III In chapter 6, the van der Waals potential is shown to be responsible for the self-alignment of the two crystals. This effect is important for the fabrication of perfectly aligned devices and may lead to new applications based on nanoscale motion.

Made available in DSpace on 2014-07-29T15:25:21Z (GMT). No. of bitstreams: 1 tesedefendidarodrigo.pdf: 1424160 bytes, checksum: ba481cb9361d272aa1bf192493e601ce (MD5) Previous issue date: 2012-06-12 / To evaluate corneal endothelium by means of specular microscopy exam in patients with keratoconus, before and after Cornealring® corneal intrastromal ring surgery. A hundred and two eyes of sixty-seven patients, aged between 12 and 45, with the average age of 27,31 ± 8,15 years, 30 of whom females and 37 males, were selected to be submitted to the implant of Cornealring® corneal ring segments and evaluation of the corneal endothelium in the pre and post surgery (six months after the procedure) in the External Diseases and Cornea Ward of the Instituto Panamericano da Visão. Data have been stored for later analysis by software SPSS (Statistical Package of Social Science) for Windows 15.0. Only the eyes receiving two rings with 250 μ thickness showed significant statistical difference (p = 0,008). The remaining eyes showed no statistical difference (initial x final). In relation to the thickness and to the number of implanted rings, a decrease in the final coefficient was observed in most implanted rings. However, only the eyes that received two rings 250 μ thick showed significant difference (p=0,003), with a decrease of 10,1% in the coefficients of variation average. In relation to the thickness and number of implanted rings, a decrease in the percentage of hexagonal cells was observed (before and after surgery). Only the eyes that received two rings 250 μ thick showed a highly significant decrease (p=0,001) in relation to the hexagonal percentage of 9,7%. Although the parameters in the specular microscopy showed alterations mainly in the thicker rings, all the patients showed transparent corneas without clinical signs of endothelial suffering along the six months of follow-up. A longer period of study must be considered for better understanding and interpretation of the results obtained, in order to establish any relationship between eventual complications and the thickness of the implanted ring. / A tese tem por objetivo avaliar o endotélio corneal por meio de microscopia especular, de pacientes portadores de ceratocone, antes e após a cirurgia corneal pela técnica de anel intraestromal, utilizando o implante Cornealring®. Cento e dois olhos de 67 pacientes, com idade entre 12 e 45 anos (com média 27,31 ± 8,15 anos), sendo 30 do sexo feminino e 37 do sexo masculino, foram selecionados para serem submetidos ao implante dos segmentos do anel Cornealring® e avaliação do endotélio corneal no pré e pós-operatório, seis meses após o procedimento, no serviço de Doenças Externas e Córnea do Instituto Panamericano da Visão. Os dados foram armazenados, para posterior análise no software Statistical Package of Social Science (SPSS) para Windows, versão 15.0. Apenas os olhos que receberam dois anéis de espessura de 250 μ apresentaram diferença estatisticamente significativa da densidade endotelial (p = 0,008). Nos demais olhos, não houve diferença estatística (antes e após a cirurgia corneal). Notou-se que, na maioria das espessuras de anéis utilizados, houve uma diminuição do coeficiente de variação após a cirurgia corneal. Porém, só os olhos que receberam dois anéis de espessuras de 250 μ mostraram diferença significativa (p=0,003), com uma diminuição de 10,1% na média do coeficiente de variação. Observou-se diminuição no percentual de células hexagonais em relação, no tocante à espessura e à quantidade de anéis implantados. Somente os olhos que receberam os dois anéis de espessura de 250 μ mostraram diminuição altamente significativa (p=0,001), em relação ao percentual hexagonal de 9,7%. Apesar de os parâmetros na microscopia especular mostrarem-se alterados, sobretudo nos anéis com espessura maior, todos os pacientes apresentaram córneas transparentes sem sinais clínicos de sofrimento endotelial ao longo dos seis meses de seguimento. Maior tempo de estudo deve ser considerado para melhor compreensão e interpretação dos resultados encontrados, a fim de ser estabelecida alguma relação entre possíveis complicações e a espessura do anel implantado.

The deformation behaviour and twinning mechanisms of commercially pure titanium alloys were investigated using complementary diffraction techniques and crystal plasticity modelling. The main motivation for conducting this investigation was to improve understanding of the deformation of titanium to help achieve the long term aim of reducing manufacturing and design costs. The deformation behaviour was characterised with tension, compression and channel die compression tests for three important variables: orientation; temperature from 25 C to 600 C; and composition for two contrasting alloys, CP-G1 and CP-G4. The experimental data used to characterise the behaviour and determine the mechanisms causing it were: textures determined by X-ray diffraction; twin area fractions for individual modes determined using electron back-scatter diffraction; and lattice strains measured by neutron diffraction. A strong effect of the orientation–stress state conditions on the flow curves (flow stress anisotropy) was found. The propensity for prism hai slip was the dominant cause of the behaviour – samples that were more favourably oriented for prism hai slip had lower flow stresses. Twinning was the most significant secondary deformation mode in the CP-G1 alloy but only had a minor effect on flow stress anisotropy in most cases. In the CP-G4 alloy twinning generally did not play a significant role indicating that hc + ai slip modes were significant in this alloy. Differences in the flow stress anisotropy between the two alloys were found to occur largely in the elasto-plastic transition and initial period of hardening. Modelling results indicated that larger relative resolved shear stress values for secondary deformation modes in the higher purity alloy increased the initial anisotropy. Decreasing flow stresses with increasing temperature were largely caused by a decrease in the critical resolved shear stress (CRSS) values for slip, but also by a decrease in the Hall-Petch parameter for slip. The propagation of twinning was found to be orientation dependent through a Schmid law in a similar way to slip – it was activated at a CRSS and hardened so that an increasing resolved shear stress was required for it to continue operating. The CRSS values determined for the individual twin modes were – 65MPa, 180MPa, 83MPa for {1012}, {1122} and {1011} twinning, respectively. Further, twinning was found to be temperature insensitive except when the ability to nucleate twins posed a significant barrier (for {1011} twinning). Also, the CRSS for {1012} twinning was clearly shown to increase with decreasing alloy purity. A thorough method for determining crystal plasticity modelling parameters based on experimental data was formulated. Additionally, twinning was modelled in a physically realistic manner influenced by the present findings using the visco-plastic self-consistent (VPSC) model. In particular: the activity of twinning decreased in a natural way due to greater difficulty in its operation rather than through an enforced saturation; and hardening or softening due to changes in orientation and dynamic Hall-Petch hardening were important. The rigorous modelling procedure gave great confidence in the key experimental findings.

Il s'agissait dans ce travail de préparer des composés fluorés de structure de type HTB ou hollandite au sein desquels la présence d'autres anions tels que OH- ou encore O2- viennent se substituer au fluor. Du point de vue structural, les réseaux présentent des canaux selon une seule direction de l'espace. La compétition entre fluoration et hydroxylation a fait l'objet d'une analyse détaillée, notamment par spectroscopie infra-rouge visant à expliquer la stabilité thermique de ces composés à anions mixtes. Enfin, l'acidité de surface, ainsi modifiée par rapport aux oxydes classiques, a été étudiée. Nous avons voulu dans ce travail mieux comprendre par une approche « chimie du solide », les propriétés de morphologie, de stabilité thermique et d'acidité des hydroxyfluorures et oxyhydroxyfluorures. La préparation et la caractérisation de nouveaux oxyhydroxyfluorures de fer et de chrome de structure hollandite a été présentée. Des études par diffraction neutronique couplées à une analyse par spectroscopie infra-rouge et la comparaison avec les hydroxyfluorures de type bronze de tungstène hexagonal nous ont permis de mieux appréhender leurs propriété physico-chimiques. Nos résultats nous ont permis de mettre en avant les rôles joués par les cations, les anions et la structure cristalline sur les différentes propriétés et notamment l'importance d'un nouveau paramètre, /r, caractéristique du cation, qui permet d'expliquer à la force des sites acides et la stabilité thermique de ces composés à structure ouverte. L'eau de constitution ainsi que la taux et la nature des groupements hydroxyles sont des paramètres clefs qui conditionnent la stabilité thermique et les propriétés acido-basiques du matériau final. Ce travail montre également l'influence des cations et du mode de synthèse sur ces caractéristiques.

This thesis presents the synthesis and investigations of physical and chemical properties of multiferroic materials experimentally as well as theoretically. Multiferroics are materials in which at least two of the three ferroic orders, ferroelectricity, ferromagnetism and ferroelasticity occur in the same phase. Multiferroics, have the potential to be used as a four state as well as cross switchable memory devices. The thesis is organized into seven Chapters. Chapter 1 gives a brief overview of the different facets of multiferroics, explaining the origin of Multiferroicity and magnetoelectric coupling, their possible technological applications and the challenges involved. Chapter 2-4 concerns the experimental aspects and chapter 5-7 concerns the theoretical aspects. Chapter 2 deals with experimental investigations on nanoscale charge-ordered rare earth manganites. It shows with decreasing particle size the ferromagnetic interaction increases and the charge-ordering vanishes down to the lowest sizes. Chapter 3 describes magneto-dielectric, magnetic and ferroelectric properties of hexagonal LuMnO3. It also describes the Raman spectroscopy of this compound through the magnetic and ferroelectric transition temperatures. Chapter 4 deals with the anisotropic multiferroic properties in single crystals of hexagonal ErMnO3. In chapter 5 a brief introduction of density functional theory (DFT) is given. Chapter 6 deals with the magneto-structural changes, spin-phonon couplings and crystal field splittings for the different magnetic orderings LuMnO3. Chapter 7 elucidates the role of Lu d0-ness for the ferroelectricity observed of this compound.

Motion estimation and compensation plays an important role in video processing applications. Two-dimensional block-based and mesh-based models are widely used in this area. A 2-D mesh-based model provides a better representation of complex real world motion than a block-based model. Mesh-based motion estimation algorithms are employed in both frame-based and object-based video compression and coding. A hierarchical mesh-based algorithm is applied to improve the motion field generated by a single-layer algorithm. 2-D mesh-based models also enable the manipulation of video objects which is included in the MPEG-4 standard. A video object in a video clip can be replaced by another object by the use of a dynamic mesh structure. In this thesis, a comparative analysis of 2-D block-based and mesh-based motion estimation algorithms in both frame-based and object-based video representations is performed. The experimental results indicate that a mesh-based algorithm produces better motion compensation results than a block-based algorithm. Moreover, a two-layer mesh-based algorithm shows improvement over a one-layer mesh-based algorithm. The application of mesh-based motion estimation and compensation to video object replacement and animation is also performed.

Thesis (Ph.D.); Submitted to the University of California at Berkeley, Berkeley, CA (US); 1 Sep 2003. / Published through the Information Bridge: DOE Scientific and Technical Information. "LBNL--55023" Li, Liang-shi. USDOE Director. Office of Science. Office of Basic Energy Sciences (US) 09/01/2003. Report is also available in paper and microfiche from NTIS.

Comme le quatrième élément le plus abondant dans l’univers, le carbone joue un rôle important dans l’émergence de la vie sur la terre comme nous la connaissons aujourd’hui. L’ère industrielle a vu cet élément au cœur des applications technologiques en raison des différentes façons dont les atomes forment les liaisons chimiques, ce qui donne lieu à une série d’allotropies chacun ayant des propriétés physiques extraordinaires. Par exemple, l’allotrope le plus thermodynamiquement stable du carbone, le cristal de graphite, est connu pour être un très bon conducteur électrique, tandis que le diamant, très apprécié pour sa dureté et sa conductivité thermique, est néanmoins considéré comme un isolant électrique en raison de sa structure cristallographique différente par rapport au graphite. Les progrès de la recherche scientifique ont montré que les considérations cristallographiques ne sont pas le seul facteur déterminant pour une telle variété dans les propriétés physiques des structures à base de carbone. Ces dernières années ont vu l’émergence de nouvelles formes allotropiques de structures de carbone qui sont stables dans les conditions ambiantes, mais avec dimensionnalité réduite, ce qui entraîne des propriétés largement différentes par rapport aux structures en trois dimensions. Parmi ces nouvelles classes d’allotropes il y a le graphene, qui est le premier matériau à deux dimensions. L’isolation réussi de monocouches de graphène a contesté une croyance établie depuis longtemps en physique : le fait que les matériaux purement 2D ne peuvent pas exister dans les conditions ambiantes parce qu'ils sont instables en raison de l’augmentation des fluctuations thermiques lorsqu’ils se prolongent dans les 2D. Afin de minimiser son énergie, un matériau se brisera en îlots coagulées. Le graphène arrive cependant à surmonter cette barrière en formant des ondulations continues sur la surface du substrat et est stable même à température ambiante et pression atmosphérique. Une grande intention dans la communauté scientifique a été donnée au graphène, après les premiers résultats publiés sur les propriétés électroniques de ce matériau. Les propriétés fondamentales et mécaniques du graphène sont fascinants. Grace aux atomes de carbone qui sont emballés dans un mode sp2 hybridé, formant ainsi une structure de réseau hexagonal, le graphène possède le plus grand module de Young et la plus grande capacité d’étirement, en même temps des centaines de fois plus dur que l’acier. Il conduit la chaleur et l’électricité de manière très efficace. L’aspect le plus fascinant à propos du graphène est surement la nature de ses porteurs de charge à basse énergie. En effet, le graphène présente des bandes d’énergie linéaires au point de neutralité de charge, donnant aux porteurs de charge une nature relativiste. De nombreux phénomènes observés dans ce matériau sont des conséquences de la nature relativiste de ses porteurs. Transport balistique, conductivité optique universelle, absence de rétrodiffusion, et une nouvelle classe d’effet Hall quantique sont de bons exemples de phénomènes nouvellement découverts dans ce matériau. Il est cependant encore trop tôt pour affirmer que toutes les propriétés physiques du graphene sont bien comprises. Dans cette thèse, nous avons mené des expériences de spectroscopie magnéto-Raman pour répondre à certaines des questions ouvertes dans la physique du graphène, notamment l’effet de couplage de Coulomb sur le spectre d’énergie du graphène, et le changement dans les propriétés physiques du graphène multicouche en fonction de sa cristallographie. Nos echantillions ont été soumis à de forts champs magnétiques, appliqués perpendiculairement aux plans atomiques. Le spectre d’excitation sous champ magnétique montre un couplage entre ces excitations et les modes de vibratoires. Cette approche expérimentale permet de remonter à la structure de bande du graphene en champs nul, ainsi que de nombreuses autres propriétés du matériau. / As the fourth most abundant element in the universe, Carbon plays an important rolein the emerging of life in earth as we know it today. The industrial era has seen this element at the heart of technological applications due to the different ways in which carbon forms chemical bonds, giving rise to a series of allotropes each with extraordinary physical properties. For instance, the most thermodynamically stable allotrope of carbon, graphite crystal, is known to be a very good electrical conductor, while diamond very appreciated for its hardness and thermal conductivity is nevertheless considered as an electrical insulator due to different crystallographic structure compared to graphite. The advances in scientific research have shown that crystallographic considerations are not the only determining factor for such a variety in the physical properties of carbon based structures. Recent years have seen the emergence of new allotropes of carbon structures that are stable at ambient conditions but with reduced dimensionality, resulting in largely different properties compared to the three dimensional structures. Among these new classes of carbon allotropes is the first two-dimensional material: graphene.The successful isolation of monolayers of graphene challenged a long established belief in the scientific community: the fact that purely 2D materials cannot exist at ambient conditions. The Landau-Peierls instability theorem states that purely 2D materials are very unstable due to increasing thermal fluctuations when the material in question extends in both dimensions. To minimize its energy, the material will break into coagulated islands, an effect known as island growth. Graphene happens to overcome such barrier by forming continuous ripples on the surface of its substrate and thus is stable even at room temperature and atmospheric pressure.A great intention from the scientific community has been given to graphene, since 2004. Both fundamental and mechanical properties of graphene are fascinating. Thanks to its carbon atoms that are packed in a sp2 hybridized fashion, thus forming a hexagonal lattice structure, graphene has the largest young modulus and stretching power, yet it is hundreds of times stronger than steel. It conducts heat and electricity very efficiently, achieving an electron mobility as high as 107 cm−2V−1 s−1 when suspended over the substrate. The most fascinating aspect about graphene is the nature of its low energy charge carriers. Indeed, graphene has a linear energy dispersion at the charge neutrality, giving the charge carriers in graphene a relativistic nature. Many phenomena observed in this material are consequences of this relativistic nature of its carriers. Ballistic transport, universal optical conductivity, absence of back-scattering, and a new class of room temperaturequantum Hall effect are good examples of newly discovered phenomena in thismaterial. Graphene has become an active research area in condensed matter physics since 2004. It is however still early to state that all the physical properties of this material are well understood. In this thesis we conducted magneto-Raman spectroscopy experiments to address some of the open questions in the physics of graphene, such as the effect of electron-electron coupling on the energy spectrum of monolayer graphene, and the change in the physical properties of multilayer graphene as a function of the crystallographic stacking order. In all our experiments, the graphene-based systems have been subject to strong continuous magnetic fields, applied normal to the graphene layers. We study the evolution of its energy excitation spectra in the presence of the magnetic field, and also the coupling between these excitations and specific vibrational modes that are already in the system. This experimental approach allows us to deduce the band structure of the studied system at zero field, as well as many other lowenergy properties.

In 1992 Du and Hwang published a paper confirming the correctness of a well known 1968 conjecture of Gilbert and Pollak suggesting that the Euclidean Steiner ratio for the plane is 2/3. The original objective of this thesis was to adapt the technique used in this proof to obtain results for other Minkowski spaces. In an attempt to create a rigorous and complete version of the proof, some known results were given new proofs (results for hexagonal trees and for the rectilinear Steiner ratio) and some new results were obtained (on approximation of Steiner ratios and on transforming Steiner trees). The most surprising result, however, was the discovery of a fundamental gap in the proof of Du and Hwang. We give counter examples demonstrating that a statement made about inner spanning trees, which plays an important role in the proof, is not correct. There seems to be no simple way out of this dilemma, and whether the Gilbert-Pollak conjecture is true or not for any number of points seems once again to be an open question. Finally we consider the question of whether Du and Hwang's strategy can be used for cases where the number of points is restricted. After introducing some extra lemmas, we are able to show that the Gilbert-Pollak conjecture is true for 7 or fewer points. This is an improvement on the 1991 proof for 6 points of Rubinstein and Thomas. / Mathematical Sciences / Ph. D. (Mathematics)

La problématique des condensateurs céramiques à très hautes performances est récurrente dansde nombreux secteurs de l’énergie électrique. Elle prend toute son acuité en microélectronique « nomade», où les circuits doivent être de plus en plus miniaturisés. Ces condensateurs, peuvent occuperjusqu’à 50 % de l’espace sur le circuit imprimé, réduire leurs dimensions devient donc un impératif.Dans ce contexte de nouveaux matériaux à permittivité colossale ont été découverts, commepar exemple CaCu3Ti4O12 (CCTO). Le mécanisme IBLC (Internal Barrier Layer Capacitance),décrivant des grains semi-conducteurs et des joints de grains isolants, permet aujourd’hui d’expliquerces fortes permittivités. Cependant l’origine de la semi-conductivité des grains ainsi quele caractère isolant des joints de grains donne encore matière à controverse. Ce travail a donc undouble objectif, d’une part concevoir un matériau possédant des propriétés diélectriques similairesà CCTO, et d’autre part comprendre les différents mécanismes responsables de ces permittivitésexceptionnelles. La première partie de ce manuscrit présente les caractéristiques d’usages descondensateurs, elle à également pour objectif de lister les mécanismes qui ont été proposés parla communauté scientifique pour tenter d’expliquer la forte permittivité de CCTO. La deuxièmepartie présente d’une part les techniques de caractérisations qui ont été utilisées pour analyserles composés présentés dans ce manuscrit, en insistant plus particulièrement sur la spectroscopied’impédance, largement utilisée dans cette étude. Elle présente d’autre part les techniques de synthèsesqui ont été expérimentées pour synthétiser un nouveau matériau à permittivité colossale :Ba4YMn3O11,5±δ. Les caractérisations structurale et micro-structurale de Ba4YMn3O11,5±δ serontévoquées dans la troisième partie de ce manuscrit. Enfin, les diverses optimisations qui ontété réalisées sur Ba4YMn3O11,5±δ, pour tenter d’améliorer ces propriétés diélectriques, serons évoquées.Nous détaillerons dans cette dernière partie les substitutions ayant été réalisées sur le sitedu baryum et du manganèse, ainsi que la création d’une phase possédant un cation différent del’actuel yttrium et présentant également des propriétés diélectriques très intéressantes. / The problem of ceramic capacitors with very high performance is recurrent in many areas ofElectrical Energy. It takes all its acuteness in « nomadic » microelectronics (cell phone, tablet, mp3player...) where the circuits must be increasingly miniaturized. These capacitors can take up to50 % of the space on the PCB, reducing their size becomes an imperative. In this context newmaterials with colossal permittivity were discovered, such CaCu3Ti4O12 (CCTO). The mechanismIBLC (Internal Barrier Layer Capacitance), describing semiconductor grains and insulating grainboundaries can now explain these high permittivities. However, the origin of the semiconductivitygrains and the insulating character of grain boundaries give even controversial. The aim is thus twofold,on one hand to synthesize a material possessing similar dielectric properties to CCTO, and onthe other hand to understand the various mechanisms responsible for these exceptional permittivity.The first part of this thesis presents various characteristics of capacitors uses, it also aims to listthe mechanisms that have been proposed by the scientific community to try to explain the highpermittivity of CCTO. The second part presents firstly the characterization techniques whichwere used to analyze all the compounds described in this manuscript, with particular emphasison impedance spectroscopy. It presents on the other hand synthesis techniques that have beentried to synthesize a new material with colossal permittivity : Ba4YMn3O11,5±δ. Structural andmicro-structural characterizations of Ba4YMn3O11,5±δ will be discussed in the third part to themanuscript. Finally, the various optimizations that were performed on Ba4YMn3O11,5±δ, to try toimprove the dielectric properties, will be discussed. We detail in this last part substitutions havingbeen made on the site of barium and manganese and the creation of a phase having a differentcation in the present yttrium site and having dielectric properties very interesting also.