Academic literature on the topic '2D crystal structure'

Ce travail a pour objectif de contribuer à l'étude du magnétisme dans les oxydes isolants de type AB_2O_6 (A= Fe, Co, Ni; B= Ta, Sb). Ces composés cristallisent dans une structure trirutile et présentent une variété très riche de phases magnétiques. Tous les ordres observés peuvent être classés comme antiferromagnétiques mais diffèrent d'un ordre simple de type Néel. En outre, les substitutions comme Fe_xCo_{1-x}Ta_2O_6, Fe_xNi_{1-x}Ta_2O_6 et Co_xNi_{1-x}Ta_2O_6 conduisent à des domaines de coexistence de phases magnétiques et à des points bicritiques dans les diagrammes de phases T(x). Une caractéristique particulièrement intéressante de ces composés est le caractère quasi-bidimensionnel de leur magnétisme, puisque les ions de métaux de transitions A apparaissent dans des réseaux plans séparés par deux plans d'ions non magnétiques B. Une forte anisotropie de champ cristallin est observée sur les sites magnétiques, résultant de la distorsion axiale des octaèdres d'oxygènes qui entourent ces ions A. Nous avons abordé deux aspects complémentaires: théorique et expérimental. Du point de vue théorique, une reformulation du modèle bidimensionnel jusqu'à present utilisée pour décrire la susceptibilité paramagnétique nous a permi d'obtenir des constants d'échange compatibles avec les types d'ordre magnétique planaire observés. Considérant que le caractère tridimensionnel des structures magnétiques révélées par diffraction neutronique indique l'importance du couplage entre plans, quoique faible, à basse température, nous proposons un modèle tridimensionnel coherent avec les ordonnements observés aussi dans le plan ab qu'au long de l'axe c. Du point de vue expérimental, nous faisons des substitutions sur le site non magnétique pour modifier de façon systematique l'espacement entre les plans magnétiques et, en consequence, le couplage entre eux. En particulier, nous avons synthétisé des composés du type ANb_xTa_{2-x}O_6 avec A = Fe, Ni et Co, restant dans le domaine de stabilité de la phase quadratique, puisque la présence de Nb favorise une phase orthorhombique. Une caractérisation structurale et magnétique de ces systèmes est faite par diffraction de rayons X et de neutrons, complétées par des mesures de susceptibilité magnétique, chaleur spécifique et aimantation en fonction du champ appliqué. Nous étudions également des séries avec Sb à la place de Nb où nous avons également effectué des remplacements sur le site magnétique<br>This work aims to contribute to the study of magnetism in insulating oxides of the type AB_2O _6 (A = Fe, Co, Ni, Ta = B, Sb). These compounds crystallize in a trirutile structure and present a rich variety of magnetic phases. All the observed orderings can be classified as antiferromagnetic, but they differ from a simple Néel type order. Moreover, substitutions such as Fe_xCo_{1-x}Ta_2O_6, Fe_xNi_{1-x}Ta_2O_6 and Co_xNi_{1-x}Ta_2O_6 produce several regions of phase coexistence and bicritical points in the phase diagram T vs. x. A particularly interesting characteristic of these compounds is the quasi-two-dimensional nature of their magnetism, since the transition-metal ions A appear in layers separated by two planes of the non-magnetic ions B. They also show strong crystal-field anisotropy on the magnetic sites due to axial distortion of the oxygen octahedra surrounding the A ions. Our approach involves two complementary aspects: theoretical and experimental. From the theoretical point of view, a reformulation of the two-dimensional model used until now to describe the paramagnetic susceptibility allows us to obtain exchange parameters compatible with the observed planar magnetic orderings. Considering that the three-dimensional character the magnetic structures revealed by neutron diffraction indicates that the coupling between planes, although weak, plays an important role to low temperatures, we propose a three-dimensional model consistent with the observed orderings both in the ab plane and along the c axis. From the experimental point of view, we make substitutions on the non-magnetic sites, in order to systematically modify the spacing between planes and, consequently, the coupling between them. In particular, we have synthesized compounds of type ANb_xTa_{2-x}O_6 with A = Fe, Ni and Co, remaining in the stability region of tetragonal phase, since the presence of Nb favors an orthorhombic phase. The structural and magnetic characterization of the systems is done via the X-ray and neutron diffraction as well as measurements of magnetic susceptibility, specific heat, and magnetization as a function of the applied field. We also study series with Sb in place of Nb where we additionally include substitutions in the magnetic site

Dans cette thèse, nous avons étudié les propriétés de deux matériaux 2D constitués de germanium : le germanène, l'équivalent du graphène, et un empilement multicouche de germanane à terminaison méthyle. En raison d’une structure atomique gauchie et d’un couplage spin orbite important, ces matériaux se démarquent du graphène et du graphite. Bien que très étudiés théoriquement, leurs propriétés physiques restent encore peu caractérisées. Dans le cas du germanène, l’étude de ce matériau a été réalisée en déposant du germanium sur une surface d’aluminium (111) sous ultravide. Pour des températures de croissance relativement basses, autour de 100°C, le germanène est épitaxié avec deux structures : la phase (3x3) et la phase (√7x√7). La microscopie à effet tunnel a été utilisée pour approfondir notre connaissance de ces phases. Dans un premier temps nous nous sommes intéressés aux propriétés électroniques. Des mesures spectroscopiques par microscopie à effet tunnel ont été réalisées à des températures de 77K et 5K. Elles n’ont malheureusement pas permis de conclure quant à la véritable nature du germanène en raison du fort couplage électronique de ce matériau avec la surface d’aluminium. Toutefois, au travers d’une diversité inattendue de spectres, cette analyse a révélé la faible adhésion du germanène à la surface Al(111), ce qui conduit à une contamination fréquente de l’apex de la pointe du microscope par les atomes de la surface. Parallèlement aux mesures spectroscopiques, la croissance de feuillets aux dimensions réduites a permis d’étudier la structure des bords des feuillets. Les observations par microscopie à effet tunnel montrent que ces feuillets croissent dans le plan des terrasses atomiques d’aluminium. Leurs bords présentent généralement un contraste plus clair que le reste du feuillet. Pour mieux comprendre ce changement de contraste, des calculs ab-initio basés sur la théorie de la fonctionnelle de la densité (DFT), ont été développés. Ils ont montré le rôle clé des atomes d’aluminium dans la formation des bords possédant quelque soit la structure zigzag ou « armchair ». A l’inverse du germanène mono-feuillet qui requiert une épitaxie, des cristaux de germanane peuvent être synthétisé par voie chimique, ce qui assure un découplage électronique du matériau avec son environnement. Nous avons réalisé une analyse multiphysique de tels cristaux passivés par des groupements méthyles, qui révèlent deux types de cristaux. Les plus gros, autour de 10 m de dimension latérale, sont polycristallins, recèlent des molécules intercalées entre les feuillets ou possèdent des surfaces oxydés et se chargent sous irradiation électronique en raison de la présence d’isopropanol à l’interface avec le substrat hôte. Les plus petits, identifiés comme les plus purs, sont les plus enclins à être caractérises par des mesures de transport à quatre pointes en ultravide. Ces mesures ont montré un transport de trous, qui se produit en volume. Ce résultat inattendu pour un matériau lamellaire suggère la présence de défauts et d’imperfection dans le plan des feuillets qui appellent à un meilleur contrôle de la synthèse des cristaux pour rendre possible l’étude des propriétés physiques fondamentales de ces cristaux<br>In this thesis, we have studied the properties of two 2D materials made of germanium: germanene, the equivalent of graphene, and a multilayer stack of germanene terminated with methyl groups. Due to a buckled atomic structure and a strong spin orbit coupling, these materials stand out from graphene and graphite. Although much studied in theory, their physical properties remain little characterized. In the case of germanene, the study of this material was carried out by depositing germanium on an aluminum (111) surface in ultra-high vacuum. For relatively low temperatures, around 100° C, the growth of germanene is epitaxial with two structures: the (3x3) reconstruction and the (√7x√7) reconstruction. Scanning tunneling microscopy has been used to deepen our knowledge of these phases. First of all, we were interested in the electronic properties. Spectroscopic measurements were carried out at temperatures of 77K and 5K. Unfortunately, they did not reveal the true nature of germanene due to the strong electronic coupling of this material with the aluminum surface.Throughout an unexpected diversity of spectra, this analysis showed the weak adhesion of germanene to the Al(111) surface, which leads to frequent contamination of the apex of the tip of the microscope by atoms of the area. In addition to spectroscopic measurements, the growth of small-sized sheets enabled the study of the edge structure. Observations by tunneling microscopy showed that these sheets grow in the plane of the aluminum atomic terraces. Their edges generally present a clearer contrast than the rest of the sheet. To better understand this change of contrast, ab-initio calculations based on density functional theory (DFT) have been performed. They showed the key role of aluminum atoms in the formation of edges, with both zigzag or armchair structures. Unlike single-sheet germanene which requires an epitaxial growth, germanane crystals can be chemically synthesized, which ensures the electronic decoupling of the material from its environment. We carried out a multi-physics analysis of such crystals passivated by methyl groups, which revealed two types of crystals. The largest, around 10 micrometres in lateral dimension, are polycrystalline, contain water molecules intercalated between the layers or have oxidized surfaces and become charged under electron irradiation due to the presence of isopropanol at the interface with the host substrate. The smallest, identified as the purest, are the most prone to being characterized by ultra-high vacuum four-probe transport measurements. These measurements showed a transport of holes, which occurs in the volume of the microstructure. This unexpected bulk transport for a lamellar material suggests the presence of defects and imperfection in the plane of the layers, which calls for a better control of the synthesis of these crystals

Le nitrure de bore hexagonal (hBN) est un semi-conducteur à grand gap (&gt;6 eV) appartenant à la nouvelle famille des cristaux 2D. Ses propriétés isolantes et sa structure cristalline font de lui un matériau stratégique dans la réalisation d’hétérostructures 2D à base de graphène. L’objectif de cette thèse a été d’étudier les propriétés optiques et structurales des feuillets de hBN.Après une description des méthodes expérimentales, les propriétés du matériau massif - loin des interfaces - sont étudiées sur le cristal de référence synthétisé par croissance haute-pression haute-température au Japon. L’étude en microscopie électronique à transmission a permis d’identifier l’empilement AA’, caractéristique du hBN. Les 3 principales régions d’émission de luminescence du hBN sont identifiées et analysées dans le détail : excitons libres, excitons piégés et défauts profonds. L’efficacité radiative excitonique a été analysée sur des cristaux issus de différentes voies de synthèse mettant en évidence des qualités dispersées. L’origine des processus de luminescence est discutée en regard des différentes interprétations actuelles, théoriques et expérimentales.Le cœur de la thèse porte sur les propriétés des cristaux 2D de faibles épaisseurs obtenus par clivage mécanique, ceci jusqu’à la monocouche atomique. Les expériences réalisées en spectroscopie Raman basse fréquence, en spectroscopie de pertes d’énergie et en cathodoluminescence ont mis en évidence une série d’effets de basse dimensionnalité sur les propriétés vibrationnelles, diélectriques et excitoniques du hBN. L’étude des défauts introduits lors de l’étape d’exfoliation et leur impact sur les émissions de luminescence ont permis d’isoler les propriétés intrinsèques des cristaux 2D de hBN. Les premiers résultats obtenus sur des feuillets suspendus dans le vide sont présentés et les effets de déformation élastique et plastique sur la luminescence de hBN discutés.La dernière partie de cette thèse porte sur des cristaux de nitrure de bore rhomboédrique (rBN) où les feuillets atomiques forment un empilement ABC. Ces cristaux ont permis d’aborder l’effet de l’empilement des plans atomiques sur la luminescence du BN en hybridation sp²<br>Hexagonal boron nitride (hBN) is a wide bandgap semi-conductor (&gt;6 eV) which belongs to the 2D crystals family. Its structure and insulating properties make him as a strategic component towards the conception of graphene-based 2D heterostructures. This thesis focuses on the structural and optical properties of hBN layers.After a brief description of experimental methods, bulk material properties have been investigated on the reference HPHT-grown crystal fabricated in Japan. The characteristic stacking AA’ sequence of the hexagonal BN phase has been identified by transmission electron microscopy. Characteristics features of the 3 main luminescence regions have been identified and analyzed into details: free excitons, bound excitons and deep defects. The radiative efficiency of excitons recombinations in hBN has been studied on crystals obtained through various synthesis routes. The origin of hBN luminescence processes is discussed on the basis of current theoretical and experimental interpretations.The main part of the thesis is dedicated to the study of nanometer-thick hBN crystals obtained by mechanical cleavage, down to the monolayer. Experiments carried out by low-frequency Raman spectroscopy, energy loss spectroscopy and cathodoluminescence demonstrated a series of low-dimensionality effects on the vibrational, dielectric and excitonic properties of hBN. Defects introduced during the exfoliation step have been studied, their impact on luminescence emissions allowed us to isolate the intrinsic properties of 2D hBN flakes. Preliminary results obtained on hBN layers suspended in vacuum are presented and the effects of elastic and plastic deformation on BN luminescence are discussed.The last part of the work focuses on rhombohedral boron nitride (rBN) crystals where the BN stacking sequence follows the ABC type. Studying these crystals made possible the investigation of the influence of the stacking sequence on sp² BN luminescence

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2005.<br>Includes bibliographical references (p. 227-232).<br>Dielectric reflectors that are periodic in one or two dimensions, also known as 1D and 2D photonic crystals, have been widely studied for many potential applications due to the presence of wavelength-tunable photonic bandgaps. However, the unique optical behavior of photonic crystals is based on theoretical models of perfect analogues. Little is known about the practical effects of dielectric imperfections on their technologically useful optical properties. In order to address this issue, a finite-difference time-domain (FDTD) code is employed to study the effect of three specific dielectric imperfections in 1D and 2D photonic crystals. The first imperfection investigated is dielectric interfacial roughness in quarter-wave tuned ID photonic crystals at normal incidence. This study reveals that the reflectivity of some roughened photonic crystal configurations can change up to 50% at the center of the bandgap for RMS roughness values around 20% of the characteristic periodicity of the crystal. However, this reflectivity change can be mitigated by increasing the index contrast and/or the number of bilayers in the crystal. In order to explain these results, the homogenization approximation, which is usually applied to single rough surfaces, is applied to the quarter-wave stacks.<br>(cont.) The results of the homogenization approximation match the FDTD results extremely well, suggesting that the main role of the roughness features is to grade the refractive index profile of the interfaces in the photonic crystal rather than diffusely scatter the incoming light. This result also implies that the amount of incoherent reflection from the roughened quarter-wave stacks is extremely small. This is confirmed through direct extraction of the amount of incoherent power from the FDTD calculations. Further FDTD studies are done on the entire normal incidence bandgap of roughened ID photonic crystals. These results reveal a narrowing and red-shifting of the normal incidence bandgap with increasing RMS roughness. Again, the homogenization approximation is able to predict these results. The problem of surface scratches on ID photonic crystals is also addressed. Although the reflectivity decreases are lower in this study, up to a 15% change in reflectivity is observed in certain scratched photonic crystal structures. However, this reflectivity change can be significantly decreased by adding a low index protective coating to the surface of the photonic crystal. Again, application of homogenization theory to these structures confirms its predictive power for this type of imperfection as well.<br>(cont.) Additionally, the problem of acircular pores in 2D photonic crystals is investigated, showing that almost a 50% change in reflectivity can occur for some structures. Furthermore, this study reveals trends that are consistent with the D simulations: parameter changes that increase the absolute reflectivity of the photonic crystal will also increase its tolerance to structural imperfections. Finally, experimental reflectance spectra from roughened D photonic crystals are compared to the results predicted computationally in this thesis. Both the computed and experimental spectra correlate favorably, validating the findings presented herein.<br>by Karlene Rosera Maskaly.<br>Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2012.<br>"June 2012." Vita. Cataloged from PDF version of thesis.<br>Includes bibliographical references.<br>Membrane proteins mediate critical functions in biological systems and are important drug targets for a number of diseases. Determining the three-dimensional structure and function of membrane proteins under physologically relevant conditions is a long sought-after goal in structural biology. In pursuit of this objective we have applied Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) to investigate the structure and function of the 32 kDa human integral membrane protein VDAC, the Voltage Dependent Anion Channel. Our results on VDAC have relied on a number of improvements in NMR methodology described in this thesis. These new methods include '3C-' 3C and 5N- 3C recoupling experiments optimized for measuring long-range homonuclear and heteronuclear restraints at high magnetic field strengths and spinning frequencies. We also describe precise 13C-33C distance measurements used to define a protein hydrophobic core. Novel methods for processing nonuniform sampling are presented as well as optimized sampling schedules for faster data acquisition. This new methodology is extended first from small peptides to a microcrystalline preparation of GB 1, a 56 residue globular protein. Methods are then extended to the much larger membrane protein VDAC. This thesis then presents structural and functional characterization of VDAC in lipid bilayer two-dimensional (2D) crystals. A strategy combining specific isotopic labeling with advanced 3D heteronuclear experiments enabled assignment of a large extent of the primary sequence. Extensive assignments of VDAC in lipid bilayers permitted comparison of VDAC's secondary structure between lipid bilayers and detergent micelles. MAS NMR experiments were paired with temperature dependent 2H NMR and DSC experiments to investigate protein-lipid interactions in the 2D crystals. Functional studies performed on the same samples used for NMR showed proper channel gating. To assess the function of VDAC we characterize its pH dependence as well as a phosphorylation mimetic.<br>by Matthew T. Eddy.<br>Ph.D.

2D-Photonic crystal (PC) structures have enabled the fabrication of a wide variety of nanophotonic components. In perfect PCs, the exploitation of the enhanced local density of states at critical points of the band diagram has attracted considerable attention. Near these points, where the group velocity vanished, low curvature flat bands give rise to delocalized and stationary optical slow Bloch modes (or slow light modes). Properties of slow light make them good candidates to enhance Purcell or various non-linear effects or to design low-threshold lasers. Among these modes, slow Bloch modes (SBMs) emitting in the vertical direction, i.e. located at the Γ- point of the Brillouin zone are particularly interesting for integrating 2D PC architectures with free space optics. In particular, some SBMs proved to be suitable for achieving strong vertical emission with peculiar polarization properties. Other promising applications concern disorder: by introducing a controlled randomness into the PC structure, it is possible to induce a transition from slow Bloch mode (in ordered PC) to Anderson's localization (in disordered PC) as a function of disorder degree. In this PhD dissertation, Slow Bloch modes have been studied and characterized by the means of Near-field Scanning Optical microscopy (NSOM). We particularly focused on Slow Bloch laser mode at Γ- point of a honeycomb 2DPC. This NSOM technique enables to visualize the evanescent component of the mode with a spatial resolution below the diffraction limit. In this work, we showed that the far-field and the near-field image of the mode at the 2D-PC surface are different and that near-field results yield a better insight in the real mode structure inside the PC slab in agreement with theoretical prediction. The importance of the probe selection (bare silica, metallized tip and bow-tie aperture nanoantenna) for studying III-V photonic crystal structures was also demonstrated. Besides intensity measurement of the electromagnetic field, the polarization of the electric field has been measured at the nanoscale for the first time by using a bow-tie nano-antenna probe. These results enable the unambiguous identification of the modes with the 3D-FDTD simulations.In this work is also reported the first observation of two-dimensional localization of light in two types of 2D random photonic crystal lasers, where Slow Bloch Mode (SBM) is scattered by artificial structural randomness in triangular PCs. The structural randomness is introduced whether by nanometer displacements in the positions of lattice elements (air holes), whether by variation of the hole diameters. The direct near-field imaging of the lasing mode by use of NSOM for the first time, allowed us to observe the transition of the extended planar SBM to be Anderson localized.

La mesure de couplage dipolaire permet d’accéder à la structure tridimensionnelle d’un composé solide. Cependant, en présence d’une forte densité de spins couplés, le phénomène de troncature dipolaire rend difficile l’obtention de ces informations par RMN du solide. Ce problème peut être affranchi par l’étude de spins rares en abondance naturelle. En effet, avec une abondance naturelle de 1.1 %, la probabilité que trois 13C soient couplés, et avec elle la troncature dipolaire, devient négligeable. Une méthodologie basée sur la séquence de recouplage dipolaire POST-C7 permet d’accéder à des informations structurales d’échantillons en abondance naturelle sensibles à la fois à la conformation moléculaire et à l’empilement cristallin par mesure de couplages dipolaires 13C-13C. La sensibilité de détection des signaux RMN 13C est augmentée à l’aide la polarisation dynamique nucléaire ce qui permet de réduire considérablement les temps d’expériences. De plus, la séquence de recouplage R20_9_2 aidée de supercycles s’est montrée être plus robustes que POST-C7 face à de fortes anisotropies de déplacement chimique ou de forts couplages hétéronucléaires 1H-13C. La seconde problématique abordée concerne l’attribution de signaux 13C. En effet, il existe seulement quelques exemples de détermination de connectivités 13C -13C en abondance naturelle. Nous montrons ici que des spectres de corrélations dipolaires 13C-13C peuvent être obtenus en quelques jours à l’aide de la séquence de recouplage R20_9_2. Contrairement aux méthodologies basées sur le couplage J, notre séquence requiert un temps d’excitation DQ plus court ce qui la rend adaptée à l’étude de solides désordonnés<br>Measurment of dipolar coupling provides 3D structural information of powder samples. However, in practice, the high density of spins in organic compounds prevents the measurements of long-range dipolar couplings in solid-state NMR by the so-called dipolar truncation effect. The study of rare spins on natural abundance allows to overcome this problem. In fact, with a natural abundance of 1.1 %, the probability for three 13C to be coupled is negligible. We developed a methodology based either on the dipolar recoupling NMR pulse sequence POST-C7 or on the dramatic increase in sensitivity provided by dynamic nuclear polarization. We demonstrated that its methodology provides a measure of 13C-13C dipolar couplings in natural abundance powder samples and that the so-obtained distance information is sensitive to both molecular conformation and crystal packing of powder samples. Moreover, we show that the recoupling pulse sequence R20_9_2 is more robust to strong chemical shift anisotropy and also to strong 1H-13C heteronuclear dipolar couplings than POST-C7. The second challenge involves 13C signal assignment for natural abundance. In fact, there are only a few examples of 13C-13C correlation spectra obtained for natural abundance samples. Here, we show that 13C-13C correlation spectra sequence based on the reintroduction of 13C−13C dipolar couplings can be obtained with standard MAS probe and within few days using R20_9_2 pulse sequence. Contrary to pulse sequences based on 13C-13C J coupling, our pulse sequence requires shorter DQ excitation time and hence, is more suitable for samples having short T2 relaxation times such as amorphous solids

Nanomaterials have attracted great interest due to the unique physical properties and great potential in the applications of nanoscale devices. Two dimensional atomic crystals, which are atomic thickness, especially graphene, have triggered the gold rush recently due to the fascinating high mobility at room temperature for future electronics. The crystal structure of nanomaterials will have great influence on their physical properties. Thus, this thesis is focused on developing the methods to control the crystal structure of nanomaterials, namely quantum dots as semiconductor, boron nitride (BN) as insulator, graphene as semimetal, with low cost for their applications in photonics, structural support and electronics. In this thesis, firstly, Mn doped ZnSe quantum dots have been synthesized using colloidal synthesis. The shape control of Mn doped ZnSe quantum dots has been achieved from branched to spherical by switching the injection temperature from kinetics to thermodynamics region. Injection rates have been found to have effect on controlling the crystal phase from zinc blende to wurtzite. The structural-property relationship has been investigated. It is found that the spherical wurtzite Mn doped ZnSe quantum dots have the highest quantum yield comparing with other shape or crystal phase of the dots. Then, the Mn doped ZnSe quantum dots were deposited onto the BN sheets, which were micron-sized and fabricated by chemical exfoliation, for high resolution imaging. It is the first demonstration of utilizing ultrathin carbon free 2D atomic crystal as support for high resolution imaging. Phase contrast images reveal moiré interference patterns between nanocrystals and BN substrate that are used to determine the relative orientation of the nanocrystals with respect to the BN sheets and interference lattice planes using a newly developed equation method. Double diffraction is observed and has been analyzed using a vector method. As only a few microns sized 2D atomic crystal, like BN, can be fabricated by the chemical exfoliation. Chemical vapour deposition (CVD) is as used as an alternative to fabricate large area graphene. The mechanism and growth dynamics of graphene domains have been investigated using Cu catalyzed atmospheric pressure CVD. Rectangular few layer graphene domains were synthesized for the first time. It only grows on the Cu grains with (111) orientation due to the interplay between atomic structure of Cu lattice and graphene domains. Hexagonal graphene domains can form on nearly all non-(111) Cu surfaces. The few layer hexagonal single crystal graphene domains were aligned in their crystallographic orientation over millimetre scale. In order to improve the alignment and reduce the layer of graphene domains, a novel method is invented to perform the CVD reaction above the melting point of copper (1090 ºC) and using molybdenum or tungsten to prevent the balling of the copper from dewetting. By controlling the amount of hydrogen during the growth, individual single crystal domains of monolayer over 200 µm are produced determined by electron diffraction mapping. Raman mapping shows the monolayer nature of graphene grown by this method. This graphene exhibits a linear dispersion relationship and no sign of doping. The large scale alignment of monolayer hexagonal graphene domains with epitaxial relationship on Cu is the key to get wafer-sized single crystal monolayer graphene films. This paves the way for industry scale production of 2D single crystal graphene.

The manipulation of photons, especially the control of spontaneous emission, has become a core area of photonics research in the 21st century. One of the key challenges is the control of the broadband emission profile of thermal emitters. Recently, attention has focused on resonant nanophotonic structures to control the thermal emission with most of the work concentrating on the mid-infrared wavelength range and/or based on metallic nanostructures. However, the realisation of a high temperature, single wavelength, narrowband and efficient thermal source, remains a challenge. In this project, four individual nanophotonic resonant structures are presented for the control of thermal emission, all operating in the near-infrared (≈ 1.5 μm) wavelength range. The work is split over two different emission materials; gold and doped silicon. While I present two successful designs of narrowband thermal emitters from gold, the main backbone of the research is concentrated on doped silicon as the emission material. By combining the weak broadband absorption of doped silicon with a photonic crystal resonator, resonantly enhanced narrowband absorption is achieved. Using Kirchhoff's law of thermal radiation which equates the absorptivity and emissivity, narrowband absorption leads to narrowband emission, which is the underlying principle used throughout the work presented in this thesis to achieve narrowband thermal emission. One common oversight in many of the presented thermal emitter designs is the angular emission dependence, i.e. how the emission wavelength behaves away from surface normal. Typically, since the majority of the devices are based on periodic structures, the resonant emission wavelength changes with emission angle, which is not ideal. Here, the angular sensitivity is considered and addressed, by constructing a device that is based on localised confined resonances and not on propagating resonances, it is possible to achieve a truly monochromatic source i.e. one with the same emission wavelength in all directions, all the way up to an angle of 90°. Finally, the devices presented here demonstrate that weak absorption together with photonic resonances can be used as a wavelength-selection mechanism for thermal emitters, both for the enhancement and the suppression of emission away from the resonant wavelength.