Dissertations / Theses on the topic '2D crystal structure'

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.

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

Ce travail de thèse porte sur des composants à cristaux photoniques (CP) bidimensionnels réalisés dans des matériaux à base d’InP pour un fonctionnement dans le domaine 1,55 μm. Au sein du CP, la périodicité de la constante diélectrique génère une bande interdite photonique, domaine de fréquence dans lequel la propagation des modes optiques est interdite. L’introduction de défauts dans le CP permet à certains modes optiques localisés d’exister. De telles structures peuvent alors être utilisées comme brique élémentaire d’un circuit intégré photonique. Nous avons étudié des adaptateurs de mode et des lasers monofréquences ainsi que des guides d’onde sur membrane InP. Les CP sont ici un réseau de trous fabriqués à l'aide de la gravure ionique réactive associée à un plasma à couplage inductif. Dans un plasma Cl2/Ar optimisé, nous avons obtenu une profondeur de gravure de 2,9 μm pour des trous de 250 nm diamètre. Nous avons montré que la présence de N2 dans un plasma contenant du chlore renforce la gravure anisotrope et supprime la rugosité des surfaces gravées, et que l’addition de BCl3 permet d’augmenter la verticalité des trous. Le plasma BCl3/N2 a permis d’obtenir les meilleurs profils et états de surface et une profondeur gravée de 1 μm. Plusieurs géométries d’adaptateurs de mode à CP. Ont été étudiées et leurs spectres de transmission ainsi que la divergence du mode émergent ont été caractérisés et comparés avec les résultats de simulation. La meilleure géométrie conduit à une amélioration de l’efficacité de transmission d’un facteur 4. Les guides W1 sur membrane InP présentent des pertes de propagation de 25 dB/cm pour des fréquences situées sous la ligne de lumière<br>This PhD work focuses on two-dimensional photonic crystals (PhC) devices based on InP materials for application around 1. 55 µm wavelength. PhC is a periodic structure in dielectric constant and is characterized by photonic band gap, a frequency domain in which the light propagation is inhibited for certain directions. Introducing defects in the periodicity offers another manner for light guiding and photon localization, which may provide a platform for photonic integrated circuits. The investigated devices include PhC taper waveguides and multiple-constricted-waveguide lasers on InP substrate, and PhC channel defect waveguides on InP suspended membrane. The perforated PhC structures are realized using reactive ion etching technique associated with inductive coupled plasma. A Cl2/Ar plasma has been optimized and demonstrated an etch depth of 1. 9~2. 9 µm for 110~250 nm-diameter holes. We have demonstrated that the addition of N2 into chlorine-containing plasmas can enhance the anisotropic etching and suppress the etched surfaces roughness. In addition, we have shown that adding BCl3 augments the feature verticality. Extremely smooth etched sidewall surfaces are obtained when the etching is performed under the BCl3/N2 plasma; in which an etch depth of 1 µm can be achieved. Several contour geometries of PhC tapers are studied and their transmission spectra and beam divergences are measured and compared with the simulation results. The transmission efficiency can be enhanced by a factor of 4 owing to the proper taper design. As for suspended membrane, a propagation loss of 25 dB/cm has been obtained for W1 PhC waveguide while operating below the air-light line

Research on two-dimensional (2D) materials currently occupies a sizeable fraction of the materials science community, which has led to the development of a comprehensive body of knowledge on such layered structures. However, the goal of this thesis is to deepen the understanding of the comparatively unknown heterostructures composed of different stacked layers. First, we utilise linear-scaling density functional theory (LS-DFT) to simulate intricate interfaces between the most promising layered materials, such as transition metal dichalcogenides (TMDC) or black phosphorus (BP) and hexagonal boron nitride (hBN). We show that hBN can protect BP from external influences, while also preventing the band-gap reduction in BP stacks, and enabling the use of BP heterostructures as tunnelling field effect transistors. Moreover, our simulations of the electronic structure of TMDC interfaces have reproduced photoemission spectroscopy observations, and have also provided an explanation for the coexistence of commensurate and incommensurate phases within the same crystal. Secondly, we have developed new functionality to be used in the future study of 2D heterostructures, in the form of a linear-response phonon formalism for LS-DFT. As part of its implementation, we have solved multiple implementation and theoretical issues through the use of novel algorithms.

L’émergence d’une grande variété de structures photoniques, au cours des dernières décennies, a permis le développement de composants intégrés sur puce réalisant des fonctions optiques en espace libre de plus en plus complexes. Parmi elles, les structures diélectriques membranaires ont permis d’implémenter une large panoplie de composants optiques planaires, allant du filtrage spectral résonant à la mise en forme de faisceau avec de faibles pertes. Toutefois, si ces structures permettent d’obtenir un contrôle quasi-total du champ électromagnétique rayonné, ce contrôle est généralement statique et déterminé par la fabrication. Un nombre croissant d’applications – telles que les télécommunications en espace libre, les capteurs pour systèmes autonomes ou encore l’imagerie – nécessitent pourtant des composants photoniques agiles, motivant ainsi la recherche de moyens de contrôle actifs de la réponse optique à implémenter au sein des structures diélectriques. À cette fin, différentes propriétés du graphène s’avèrent prometteuses. En particulier, la possibilité de moduler dynamiquement son absorption ouvre de nombreuses perspectives pour le contrôle électrique et optique des structures photoniques intégrant du graphène. Des modulateurs électro-optiques et tout-optique ont ainsi pu être réalisés, s’appuyant sur le développement récent de procédés de transfert des matériaux 2D qui permettent aujourd’hui d’obtenir des structures hybrides graphène/diélectrique de grande qualité. Dans ce contexte, les travaux présentés dans cette thèse cherchent à exploiter l’absorption modulable du graphène pour obtenir une accordabilité dynamique de la réponse optique des composants adressables par la surface, dans le cas particulier de structures photoniques diélectriques travaillant dans le proche infrarouge. Un modèle générique de composant hybride diélectrique/ graphène est tout d’abord développé en théorie des modes couplés afin d’identifier les paramètres d’intérêt pour maximiser le contrôle permis par l’absorption du graphène. Dans le cas à une résonance, le comportement du système est principalement déterminé par la condition de couplage critique classiquement définie pour l’étude de l’absorption du graphène. Dans le cas à deux résonances en revanche, un nouveau paramètre de contrôle – associé à la différence d’absorption induite sur les résonances – permet d’obtenir un levier d’accordabilité supplémentaire. Différentes stratégies de maximisation de ce paramètre sont proposées et les procédés technologiques nécessaires à leur implémentation sont étudiés expérimentalement afin d’évaluer – par le biais de la spectroscopie Raman et de la spectroscopie de photoélectrons – leur effet sur la qualité structurelle et chimique du graphène, intégré dans de telles structures. La modulation spatiale de l’absorption du graphène – proposée pour différencier l’absorption induite sur différents modes optiques – est ensuite étudiée expérimentalement à l’aide de structures exploitant le transfert de charges entre le graphène et un oxyde à grand travail de sortie, à savoir l’oxyde de tungstène. Les dispositifs réalisés permettent d’obtenir une modulation du potentiel chimique du graphène de 0.1eV – caractérisée par nano-XPS (ligne ANTARES du synchrotron SOLEIL) et spectroscopie Raman – pouvant aboutir à une modulation de l’absorption supérieure à 70% pour certaines longueurs d’onde. Finalement, une architecture de composant hybride actif permettant d’obtenir un contrôle dynamique de l’émission laser est proposée. Cette architecture repose sur l’utilisation d’une membrane à brisure de symétrie verticale et permet, en principe, d’obtenir une commutation entre deux angles d’émission par la modulation de l’absorption du graphène. L’intérêt de ces structures pour parvenir à une accordabilité continue de l’angle d’émission est également exposé<br>The emergence of a wide variety of photonic structures over the past decades has enabled the realization of on-chip devices performing increasingly complex free-space optical functions. Among them, dielectric membrane structures have made it possible to implement a wide range of planar optical devices, ranging from resonant spectral filtering to beam shaping, with negligible losses. While these structures provide almost a full control of the radiated electromagnetic field, this control is usually static and determined by manufacturing. An increasing number of applications - such as free-space telecommunications, sensors for autonomous systems or imaging - require agile photonic devices, thus motivating the search for means of active control of the optical response to be implemented within the dielectric structures. To this purpose, various properties of graphene are proving promising. In particular, the capability to modulate its absorption opens up numerous prospects for the electrical and optical control of photonic structures that integrate graphene. This has led to the demonstration of various electro-optic and all-optical modulators, by leveraging the recently developed 2D material transfer processes, which have made it possible to obtain high-quality hybrid graphene/dielectric structures. In this context, the work presented in this thesis seeks to exploit graphene’s tunable absorption to achieve dynamic control of surface-addressable device’s optical response, in the special case of dielectric photonic structures operating in the near infrared. A generic coupled mode theory model is first developed and adapted to hybrid dielectric/ graphene structures in order to identify the key parameters for maximising the control allowed by graphene absorption. In the single resonance case, the system’s response is mainly determined by the critical coupling condition classically defined for the study of graphene’s absorption. In the two-resonance case however, a new control parameter – associated with the absorption difference between the resonances – provides an additional tunability factor. Different strategies for maximising this parameter are therefore proposed and the technological processes underlying their implementation are studied experimentally in order to assess - by means of Raman spectroscopy and photoelectron spectroscopy - their effect on the structural and chemical quality of graphene. The spatial modulation of graphene’s absorption – here proposed to differentiate the absorption induced on different optical modes – is then studied experimentally using structures exploiting the charge transfer effect at the interface between graphene and an oxide with high workfunction, namely tungsten oxide. The devices developed here allow to obtain a graphene’s chemical potential modulation of 0.1eV - characterized by nano-XPS (ANTARES beamline of the SOLEIL synchrotron) and Raman spectroscopy - which can lead to an absorption modulation higher than 70% for certain wavelengths. Ultimately, an active hybrid device architecture enabling dynamic control of the laser emission is proposed. This architecture is based on a vertical symmetry breaking membrane and allows us, in principle, to switch between two emission angles by modulating graphene’s absorption. The interest of these structures in achieving continuous tunability of the emission angle is also presented

碩士<br>國防大學中正理工學院<br>應用物理研究所<br>94<br>In this paper, the photonic crystal with two-dimensional hexagonal pipes structures is investigated. This photonic crystal includes the special case that with the two-dimensional hexagonal cylinders structure. In this investigation, we know that the energy band of the hexagonal cylinders structure photonic crystal will shift to lower frequency when the filling rate and dielectric coefficient increasing. And the width of complete gap will increase then decreasing. The structure of the Complete Gap is not effected by the alteration of dielectric coefficient. It would effect the width of band gap and frequencies when the dielectric coefficient been changed. In the different materials of hexagonal cylinders structure photonic crystal, the energy band gaps shift to lower frequencies when the dielectric coefficient increasing. When the cylinders radii are equal, the conditions of maximum gap been found. And in the hexagonal pipe structure photonic crystals, when the fitting materials are changed from cylinders to pipes, the energy band shift to higher frequencies just as the filling rate is decreasing. In this condition, it would be Complete Gape that it should not be Complete Gape. The Complete Gap of the hexagonal pipes structure photonic crystals would be found by this way.

In this work, the crystal growth as well as structural and magnetic investigations of several metal trichalcogenides compounds with a general formula M2X2Ch6 are presented. M stands for a main group metal or transition metal, X is an element of the IV or V main group and Ch is a chalcogen. In particular, these compounds are the phosphorus sulfides Fe2P2S6, Ni2P2S6 as well as intermediate compounds of the substitution regime (Fe1-xNix)2P2S6, the quarternary phosphorus sulfides CuCrP2S6 and AgCrP2S6 and the germanium tellurides Cr2Ge2Te6 and In2Ge2Te6. As members of the metal trichalcogenides, all these compounds have a van der Waals layered honeycomb structure in common. This layered structure in combination with their magnetic properties makes these compounds interesting candidate materials for the production of magnetic monolayers by exfoliation from bulk crystals. Crystals of the phosphorus sulfides were grown by the chemical vapor transport technique and, for the growth of the germanium tellurides, the self-flux growth technique was used. Crystals of all phases were extensively characterized regarding their morphology, chemical composition and homogeneity as well as regarding their crystal structure. The structural analysis, especially for Ni2P2S6, yields insight into details of the stacking order and disorder of the corresponding quasi-two-dimensional layers in the bulk. Regarding the magnetic properties, both Fe2P2S6 and Ni2P2S6 order antiferromagnetically but exhibit different magnetic anisotropies (i.e. Ising-like anisotropy for Fe2P2S6 and XYZ anisotropy for Ni2P2S6). In this context, it is surprising to find that compounds in the solid solution regime of (Fe1-xNix)2P2S6 up to x = 0.9 exhibit an anisotropic magnetic behavior that is comparable to Fe2P2S6 and, thus, indicative of Ising-like anisotropy. For CuCrP2S6 and AgCrP2S6, the ordering of the two different transition elements on the honeycomb sites yields more complex magnetic structures. The magnetic Cr3+ atoms in CuCrP2S6 order in a triangular arrangement and form an antiferromagnetic ground state with notable ferromagnetic interactions. AgCrP2S6 exhibits pronounced features of low dimensional magnetism resulting from the (quasi-)one-dimensional stripe-like arrangement of magnetic Cr3+ atoms and no onset of long-range magnetic order is unambiguously observed. Cr2Ge2Te6 exhibits ferromagnetic order and an anisotropic feature in the temperature dependence of the magnetization. Based on the magnetic phase diagrams for two orientations between the magnetic field and the crystallographic directions, the temperature dependence of the magnetocrystalline anisotropy constant as well as the critical exponents of the magnetic phase transition are extracted. Concluding from this, the magnetic interactions in Cr2Ge2Te6 are dominantly of two-dimensional nature and the anisotropy is uniaxial with the before mentioned anisotropic feature resulting from the interplay between magnetocrystalline anisotropy, magnetic field, and temperature. In2Ge2Te6 is diamagnetic as to be expected for a closed-shell system. Additional to the investigations on single crystals, the quasi-binary phase diagram of (Cu1-xAgx)CrP2S6 was investigated for regimes of solid solution behavior based on polycrystalline samples. Accordingly, isostructural substitution is most likely possible in the composition range of (Cu0.25Ag0.75)CrP2S6 to AgCrP2S6, potentially allowing to tune the magnetic interactions of the Cr sublattice indirectly by substitution on the Cu/Ag sublattice.:1. Introduction 1.1. M2X2Ch6 Class of Materials 1.2. Magnetism in Solid State Materials 1.2.1. Diamagnetism 1.2.2. Paramagnetism 1.2.3. Cooperative Magnetism 1.2.4. Magnetic Anisotropy 1.2.5. Magnetism in D < 3 1.2.6. Critical Exponents 2. Methods 2.1. Synthesis and Crystal Growth 2.1.1. Solid State Synthesis 2.1.2. Crystal Growth via the Liquid Phase 2.1.3. Crystal Growth via the Vapor Phase 2.2. X-ray Diffraction 2.2.1. Single Crystal X-ray Diffraction 2.2.2. Powder X-ray Diffraction 2.3. Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy 2.3.1. Scanning Electron Microscopy 2.3.2. Energy Dispersive X-ray Spectroscopy 2.4. Magnetometry 2.5. Nuclear Magnetic Resonance Spectroscopy 2.6. Specific Heat Capacity 3. M2P2S6 3.1. Ni2P2S6 3.1.1. Crystal Growth 3.1.2. Characterization 3.1.3. Magnetic Properties 3.1.4. 31P-NMR Spectroscopy 3.1.5. Stacking (Dis-)Order in Ni2P2S6 3.2. (Fe1-xNix)2P2S6 3.2.1. Synthesis and Crystal Growth 3.2.2. Characterization 3.2.3. Evolution of Magnetic Properties 3.3. Summary and Outlook 4. M1+CrP2S6 4.1. CuCrP2S6 4.1.1. Crystal Growth 4.1.2. Characterization 4.1.3. Magnetic Properties 4.2. AgCrP2S6 4.2.1. Crystal Growth 4.2.2. Characterization 4.2.3. Magnetic Properties 4.3. Polycrystalline (Cu1-xAgx)CrP2S6 4.3.1. Synthesis 4.3.2. Phase Analysis 4.4. Summary and Outlook 5. M2(Ge,Si)2Te6 5.1. Cr2Ge2Te6 5.1.1. Crystal Growth 5.1.2. Characterization 5.1.3. Magnetic Properties 5.1.4. Analysis of the Critical Behavior 5.2. In2Ge2Te6 5.2.1. Crystal Growth 5.2.2. Characterization 5.2.3. Magnetic Properties 5.2.4. Specific Heat 5.3. Summary and Outlook 6. Conclusion Bibliography List of Publications Acknowledgements Eidesstattliche Erklärung A. Appendix A.1. Scanning Electron Microscopic Images A.1.1. (Fe1-xNix)2P2S6 A.2. scXRD A.2.1. (Fe1-xNix)2P2S6<br>In dieser Arbeit werden die Kristallzüchtung sowie strukturelle und magnetische Untersuchungen an mehreren Metalltrichalkogenid-Verbindungen mit der allgemeinen Summenformel M2X2Ch6 vorgestellt. M steht für ein Hauptgruppen- oder Übergangsmetall, X ist ein Element der IV- oder V-Hauptgruppe und Ch ein Chalkogen. Insbesondere handelt es sich bei diesen Verbindungen um die Phosphorsulfide Fe2P2S6, Ni2P2S6 sowie um Verbindungen der Substitutionsreihe (Fe1-xNix)2P2S6, die quaternären Phosphorsulfide CuCrP2S6 und AgCrP2S6 sowie die Germaniumtelluride Cr2Ge2Te6 und In2Ge2Te6. Als Mitglieder der Metalltrichalkogenide haben alle diese Verbindungen eine van-der-Waals-Schichtstruktur mit Honigwabenmotiv gemein. Diese Schichtstruktur in Kombination mit ihren magnetischen Eigenschaften macht diese Verbindungen zu interessanten Kandidaten für die Herstellung von magnetischen Monolagen durch Exfoliation aus Volumenkristallen. Kristalle der Phosphorsulfide wurden mit der chemischen Dampfphasentransporttechnik gezüchtet und für die Züchtung der Germaniumtelluride wurde die Selbstflusstechnik verwendet. Die Kristalle aller Phasen wurden sowohl hinsichtlich ihrer Morphologie, chemischen Zusammensetzung und Homogenität als auch hinsichtlich ihrer Kristallstruktur umfassend charakterisiert. Die Strukturanalyse, insbesondere für Ni2P2S6, gibt Aufschluss über Details der Stapelordnung und -unordnung der entsprechenden quasizweidimensionalen Schichten im Volumen. Bezüglich der magnetischen Eigenschaften ordnen sowohl Fe2P2S6 als auch Ni2P2S6 antiferromagnetisch, zeigen aber unterschiedliche magnetische Anisotropien (d.h. Ising-artige Anisotropie für Fe2P2S6 und XYZ-Anisotropie für Ni2P2S6). In diesem Zusammenhang ist es überraschend, dass Verbindungen im Mischkristallregime von (Fe1-xNix)2P2S6 bis x = 0.9 ein anisotropes magnetisches Verhalten zeigen, das mit dem von Fe2P2S6 vergleichbar ist und daher auf Ising-artige Anisotropie hindeutet. Bei CuCrP2S6 und AgCrP2S6 führt die Anordnung der beiden unterschiedlichen Übergangselemente auf den Gitterplätzen der Wabenstruktur zu komplexeren magnetischen Strukturen. Die magnetischen Cr3+ Atome in CuCrP2S6 ordnen sich in einer Dreiecksanordnung an und bilden einen antiferromagnetischen Grundzustand mit ausgeprägten ferromagnetischen Wechselwirkungen. AgCrP2S6 weist deutliche Merkmale von niederdimensionalem Magnetismus auf, welche aus der (quasi-)eindimensionalen, streifenartigen Anordnung der magnetischen Cr3+ Atome resultieren, und das Einsetzen von langreichweitiger magnetischer Ordnung kann nicht eindeutig beobachtet werden. Cr2Ge2Te6 weist ferromagnetische Ordnung und einen anisotropen Verlauf der Temperaturabhängigkeit der Magnetisierung auf. Anhand von magnetischen Phasendiagrammen für zwei Orientierungen zwischen Magnetfeld und kristallographischen Richtungen wurden die Temperaturabhängigkeit der magnetokristallinen Anisotropiekonstante sowie die kritischen Exponenten des magnetischen Phasenübergangs extrahiert. Hieraus ergibt sich, dass die magnetischen Wechselwirkungen in Cr2Ge2Te6 überwiegend zweidimensionaler Natur sind und die Anisotropie uniaxial ist, wobei der zuvor erwähnte anisotrope Verlauf aus dem Zusammenspiel von magnetokristalliner Anisotropie, Magnetfeld und Temperatur resultiert. In2Ge2Te6 ist diamagnetisch, wie es für ein System mit geschlossener Schale zu erwarten ist. Zusätzlich zu den Untersuchungen an Einkristallen wurde das quasibinäre Phasendiagramm von (Cu1-xAgx)CrP2S6 anhand von polykristallinen Proben auf Bereiche mit Mischkristallverhalten hin untersucht. Folglich ist eine isostrukturelle Substitution höchstwahrscheinlich im Zusammensetzungsbereich von (Cu0.25Ag0.75)CrP2S6 bis AgCrP2S6 möglich, was es erlauben könnte, die magnetischen Wechselwirkungen des Cr-Untergitters indirekt durch Substitution auf dem Cu/Ag-Untergitter zu beeinflussen.:1. Introduction 1.1. M2X2Ch6 Class of Materials 1.2. Magnetism in Solid State Materials 1.2.1. Diamagnetism 1.2.2. Paramagnetism 1.2.3. Cooperative Magnetism 1.2.4. Magnetic Anisotropy 1.2.5. Magnetism in D < 3 1.2.6. Critical Exponents 2. Methods 2.1. Synthesis and Crystal Growth 2.1.1. Solid State Synthesis 2.1.2. Crystal Growth via the Liquid Phase 2.1.3. Crystal Growth via the Vapor Phase 2.2. X-ray Diffraction 2.2.1. Single Crystal X-ray Diffraction 2.2.2. Powder X-ray Diffraction 2.3. Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy 2.3.1. Scanning Electron Microscopy 2.3.2. Energy Dispersive X-ray Spectroscopy 2.4. Magnetometry 2.5. Nuclear Magnetic Resonance Spectroscopy 2.6. Specific Heat Capacity 3. M2P2S6 3.1. Ni2P2S6 3.1.1. Crystal Growth 3.1.2. Characterization 3.1.3. Magnetic Properties 3.1.4. 31P-NMR Spectroscopy 3.1.5. Stacking (Dis-)Order in Ni2P2S6 3.2. (Fe1-xNix)2P2S6 3.2.1. Synthesis and Crystal Growth 3.2.2. Characterization 3.2.3. Evolution of Magnetic Properties 3.3. Summary and Outlook 4. M1+CrP2S6 4.1. CuCrP2S6 4.1.1. Crystal Growth 4.1.2. Characterization 4.1.3. Magnetic Properties 4.2. AgCrP2S6 4.2.1. Crystal Growth 4.2.2. Characterization 4.2.3. Magnetic Properties 4.3. Polycrystalline (Cu1-xAgx)CrP2S6 4.3.1. Synthesis 4.3.2. Phase Analysis 4.4. Summary and Outlook 5. M2(Ge,Si)2Te6 5.1. Cr2Ge2Te6 5.1.1. Crystal Growth 5.1.2. Characterization 5.1.3. Magnetic Properties 5.1.4. Analysis of the Critical Behavior 5.2. In2Ge2Te6 5.2.1. Crystal Growth 5.2.2. Characterization 5.2.3. Magnetic Properties 5.2.4. Specific Heat 5.3. Summary and Outlook 6. Conclusion Bibliography List of Publications Acknowledgements Eidesstattliche Erklärung A. Appendix A.1. Scanning Electron Microscopic Images A.1.1. (Fe1-xNix)2P2S6 A.2. scXRD A.2.1. (Fe1-xNix)2P2S6

博士<br>元智大學<br>電機工程學系<br>95<br>In recent years, it is very popular for applying photonic crystal (PC) structure to design optical components. In this thesis, we designed a two-dimensional photonic crystal optical modulator by using the general complementary metal-oxide semiconductor (CMOS) integrated circuit (IC) processing parameters. Chips of optical polarizer have been successfully designed by adopting the technologies of semiconductor fabrication. This study extends the fabrication of ICs to that of PCs and makes the shrinkage of device dimension from the order of centimeter to that of micrometer possible. Moreover, it is achievable to use the IC processes for the PC production. As to the research for the optical properties of biological eyes, this is the first study to investigate the firefly’s eyes by replacing its crystalline cone with the periodic indices of refraction and constructing a configuration of 190 parabolic layers. It was treated as a one-dimensional periodic photonic crystal on the optical axis. By using the principles of geometric optics, we simulated its dioptric portion and realized its behavior of the visual output according to the optical ray-tracing. We further applied the transfer matrix to observe its transmission and found that there exists the filtering capability on its optical axis. On the contrary, there is no such filtering effect but with the capability of receiving other visible light for the retinal cell nuclei around the axis. It might be deduced that all the retinal cell nuclei of the firefly’s eyes can receive the different range of light wave. Besides, we also discussed the focusing efficiency of the firefly’s eyes. Based on the input of collimating light, the efficiency of an overlapped image focused by a compound eye is higher than that of a parallel image by nine hundred times. Furthermore, there is radial gradient index existing by using such a configuration proposed in this thesis.

Perovskite oxides have provided a wide variety of exotic functionalities based on their unique physical and chemical properties. By combining different perovskite oxides, interesting physical phenomena have been observed at the interfaces of perovskite heterostructures. The most interesting among these phenomena is the formation of two dimensional electron gas at the interface of two perovskite materials SrTiO3 and LaAlO3 which led to a number of fascinating physical properties such as metal-insulator transition, super-conductivity, large negative magnetoresistance and so on. This has raised the interest in exploiting the interface of various hybrids structures built on the perovskite oxide backbone. On the other hand, the two dimensional (2D) van der Waals materials such as graphene, MoS2, boron nitride etc. represent a new paradigm in the 2D electron-ics. The functionalities of these individual materials have been combined to obtain new enriched functionalities by stacking different materials together forming van der Waals heterostructures. In this work, we present a detailed study of the interface in hybrid structures made of vander Waals materials (graphene and MoS2) and their hybrids with a perovskite material namely, SrTiO3 which is known as the building block of complex oxide heterostructures. In graphene-MoS2 vertical heterostructure, we have carried out a detailed set of investigations on the modulation of the Schottky barrier at the graphene-MoS2 interface with varying external electric field. By using different stacking sequences and device structures, we obtained high mobility at large current on-off ratio at room temperature along with a tunable Schottky barrier which can be varied as high as ∼ 0.4 eV by applying electric field. We also explored the interface of graphene and SrTiO3 as well as MoS2 and SrTiO3 by electrical transport and low frequency 1/f noise measurements. We observed a hysteretic feature in the transfer characteristics of dual gated graphene and MoS2 field effect transistors on SrTiO3. The dual gated geometry enabled us to measure the effective capacitance of SrTiO3 interface which showed an enhancement indicating the possible existence of negative capacitance developed by the surface dipoles at the interface of SrTiO3 and the graphene or MoS2 channel. Our 1/f noise study and the analysis of higher order statistics of noise also support the possibility of electric field-driven reorient able surface dipoles at the interface.

Perovskite oxides have provided a wide variety of exotic functionalities based on their unique physical and chemical properties. By combining different perovskite oxides, interesting physical phenomena have been observed at the interfaces of perovskite heterostructures. The most interesting among these phenomena is the formation of two dimensional electron gas at the interface of two perovskite materials SrTiO3 and LaAlO3 which led to a number of fascinating physical properties such as metal-insulator transition, super-conductivity, large negative magnetoresistance and so on. This has raised the interest in exploiting the interface of various hybrids structures built on the perovskite oxide backbone. On the other hand, the two dimensional (2D) van der Waals materials such as graphene, MoS2, boron nitride etc. represent a new paradigm in the 2D electron-ics. The functionalities of these individual materials have been combined to obtain new enriched functionalities by stacking different materials together forming van der Waals heterostructures. In this work, we present a detailed study of the interface in hybrid structures made of vander Waals materials (graphene and MoS2) and their hybrids with a perovskite material namely, SrTiO3 which is known as the building block of complex oxide heterostructures. In graphene-MoS2 vertical heterostructure, we have carried out a detailed set of investigations on the modulation of the Schottky barrier at the graphene-MoS2 interface with varying external electric field. By using different stacking sequences and device structures, we obtained high mobility at large current on-off ratio at room temperature along with a tunable Schottky barrier which can be varied as high as ∼ 0.4 eV by applying electric field. We also explored the interface of graphene and SrTiO3 as well as MoS2 and SrTiO3 by electrical transport and low frequency 1/f noise measurements. We observed a hysteretic feature in the transfer characteristics of dual gated graphene and MoS2 field effect transistors on SrTiO3. The dual gated geometry enabled us to measure the effective capacitance of SrTiO3 interface which showed an enhancement indicating the possible existence of negative capacitance developed by the surface dipoles at the interface of SrTiO3 and the graphene or MoS2 channel. Our 1/f noise study and the analysis of higher order statistics of noise also support the possibility of electric field-driven reorient able surface dipoles at the interface.

碩士<br>國立臺灣大學<br>光電工程學研究所<br>106<br>Blue phase liquid crystal display is a popular research topic for the liquid crystal display. Because of its sub-millisecond response time and its display without alignment layer that make it be fabricated easier. However, the primary problems of blue phase liquid crystal are low transmittance and high operating voltage. Many theses aim to design different electrode structures to improve the high operating voltage and low transmittance. Most theses aim to design two-dimension electrode structures about parameters on X-Z direction. Therefore, we design the three-dimension electrode structures and hope that new structures can reduce the dead zone on y-direction. At first, we refer to three kinds of 2D electrode structures to design three kinds of 3D electrode structures. We discover that 3D electrodes have the problem of central dead zone and the problem influence the transmittance of the display. After the analysis, the 2D electrodes still have the highest transmittance. However, 3D electrodes also have their advantage. 3D electrodes have higher transmittance at low operating voltage if the electrode gap is small. Then we refer to Diamond-shape electrodes to design continuous electrodes to reduce the central dead zone on expectation so we compare continuous electrodes with Diamond-shape electrodes first. The result is that the pointed diamond electrodes have higher transmittance in comparison with Diamond-shape electrodes generally ( ). The enhanced Diamond electrodes have higher transmittance and lower operation voltage in comparison with Diamond-shape electrodes. In the end, we compare continuous electrodes with 2D and 3D electrodes. Among 2D, 3D triangle and pointed diamond electrodes, 2D triangle electrodes have the highest transmittance with large gap but 3D triangle electrodes have the higher transmittance at low voltage with small gap. Pointed diamond electrodes have high transmittance over 75% with most electrode gaps. Among 2D, 3D enhanced trapezoid and enhanced diamond electrodes, 2D enhanced trapezoid electrodes have the highest transmittance with large gap but 3D enhanced trapezoid electrodes have the higher transmittance at low voltage with small gap. Enhanced diamond electrodes have high transmittance over 80% with most electrode gaps. In terms of transmittance vs electrode gaps, continuous electrodes combine the features of 2D and 3D electrodes. Continuous electrodes have less sensitivity on electrode gaps compared to 2D and 3D electrodes with reasonably high transmittance.