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International audience; Chemical functionalization of graphene is one method pursued to engineer new properties into a graphene sheet. Graphene oxide is the most commonly used chemical derivative of graphene. Here we present experimental evidence for the formation of enolate moieties when oxygen atoms are added to the graphene basal plane. The exotic functional groups are stabilized by simultaneous bond formation between the graphene sheet and the underlying Ir(111) substrate. Scanning tunneling microscopy images demonstrate the patterned nature of CO bond formation and x-ray photoelectron spectroscopy and high resolution electron energy loss spectroscopy are used to characterize the enolate moiety. The results present a new mechanism for the formation of patterned graphene oxide and provide evidence of a functional group rarely considered for graphene oxide materials. Graphene is a 2D semi-metal in which all carbon atoms are sp 2 hybridized. 1 Chemical functionalization of graphene is driven by the desire to engineer the physical and chemical properties of pristine graphene sheets, and to introduce new means of incorporating graphene interfaces with other materials. 2-4 The addition of simple atomic species, such as hydrogen, 5 oxygen 6,7 or fluorine, 8 has been shown to change the electronic band structure of graphene from that of a semi-metal to that of a semiconductor or insulator. Graphene oxide is a term used to describe graphene materials that have been subjected to oxidation reactions. Supported graphene oxide materials have been studied as sensor materials, for their potential magnetic properties and as clusters to form graphene oxide quantum dots. 9-12 The growth of metallic nanoparticles on supported graphene sheets is also facilitated by the presence of CO bonds at the graphene basal plane, 4 with attachment or cleavage of particle growth initiators dependent on the nature of the CO bond. 13 This may be a viable method for atomic layer deposition on graphene films, allowing for integration of graphene sheets in more complex electronic devices. 14 Consequently, the nature of the CO bond formed during synthesis of graphene oxide is of general interest and importance. It is widely assumed that an oxygen atom bonding to the basal plane of a graphene sheet is initially covalently bound through the formation of an epoxy group. 15-18 The stability of different oxygen moieties on metal-supported graphene sheets was, however, recently explored for graphene islands on Ru(0001). 19 Those authors, building on theoretical results from Jung et al., 20 provided the first experimental evidence for the existence of the enolate moiety when oxygen atoms bind to a graphene sheet supported by a metal substrate. Their calculations indicate that interaction with an underlying metallic substrate can guide the formation of enolate groups when an oxygen atom binds to one carbon atom, and the adjacent carbon atom is suitably positioned to bind down to an underlying metal atom. A 370 meV gain in energy was reported for the formation of an enolate group, versus an epoxy group, in certain areas of the moiré pattern formed between the graphene lattice and atoms from the Ru(0001) surface. 19 Graphene on Ru(0001), however, is already a strongly coupled system with good evidence for the preexistence of a carbon-metal bond even before oxygen atoms are introduced. 21 Hence, it is timely to consider the nature of the CO bond for other, less strongly coupled systems. Graphene on Ir(111) (Gr/Ir(111)) demonstrates weak van der Waals interaction between the graphene sheet and the Ir(111) surface atoms. 22 Moreover, single crystal graphene sheets with a low concentration of defects and highly oriented domains are routinely prepared on Ir(111) substrates following chemical vapour deposition. 23 The attachment of oxygen atoms to Gr/Ir(111) has been reported previously but the possibility of enolate formation was not considered. 15,16 Here, we revisit this system adding new high-resolution electron energy loss spectroscopy (HREELS) data to demonstrate that at a low flux of oxygen atoms, enolate formation dominates and occurs only at select sites on the Gr/Ir(111) surface, generating a long-range pattern of graphene oxide nano-dots. The slight lattice mis-match between the carbon atoms in the graphene sheet and the Ir(111) surface atoms leads to a long-range repeating moiré pattern. 23 The moiré unit cell contains: TOP regions where Ir surface atoms are positioned directly beneath the center of a hexagonal ring in the graphene sheet; and HCP and FCC regions, where every second C atom is positioned directly above an Ir atom. HCP and FCC regions differ in the arrangement of Ir atoms in the second and third Ir layer. The remaining carbon atoms, in between

Monolayer hexagonal boron nitride (hBN) tunnel barriers investigated using conductive atomic force microscopy reveal moiré patterns in the spatial maps of their tunnel conductance consistent with the formation of a moiré superlattice between the hBN and an underlying highly ordered pyrolytic graphite (HOPG) substrate. This variation is attributed to a periodc modulation of the local density of states and occurs for both exfoliated hBN barriers and epitaxially grown layers. The epitaxial barriers also exhibit enhanced conductance at localized subnanometer regions which are attributed to exposure of the substrate to a nitrogen plasma source during the high temperature growth process. Our results show clearly a spatial periodicity of tunnel current due to the formation of a moiré superlattice and we argue that this can provide a mechanism for elastic scattering of charge carriers for similar interfaces embedded in graphene/hBN resonant tunnel diodes.

Nesta tese apresentamos um estudo sobre as propriedades estruturais e eletrônicas de heteroestruturas semicondutoras epitaxiais livres do substrato original, conhecidas como nanomembranas semicondutoras. Nosso enfoque encontra-se na versatilidade dada a estes sistemas, uma vez que a sua fabricação possibilita-nos investigar diversas propriedades presentes em materiais epitaxiais, apenas depositando-os sobre substratos específicos de interesse. Os resultados observados mostram que Utilizamos técnicas de microscopia eletrônica de transmissão (TEM), difração de raio-X e microscopia/espectroscopia de varredura por tunelamento (STM/S) para a investigação destes pontos quânticos. A investigação das propriedades estruturais, que faz uso das duas primeiras técnicas citadas, permitiu-nos observar diretamente um relaxamento anisótrópico nestas nanoestruturas tridimensionais a partir de uma análise estatística de 164 ilhas observadas por TEM. Além das ilhas completamente coerentes ou completamente relaxadas, encontramos um conjunto de ilhas formadas pelo relaxamento parcial e anisotrópico do parâmetro de rede do material tensionado, que nos levou a conclusões acerca dos processos de relaxamento da deformação (strain) durante o crescimento de ilhas III-V. O segundo trabalho, no qual estudamos pontos quânticos de InAs, descobertos e cobertos por GaAs, a partir da técnica de STM/S, permitiu-nos observar estados de energia discretos da heteroestrutura, compatíveis com os presentes na literatura, através de uma metodologia extremamente simples de praparação da amostra (nanomembrana), transferida para um substrato de Au. Níveis de energia foram observados em ilhas cobertas, mesmo com a presença de uma camada de óxido nativo. Cálculos das energias destes niveis validam a intepretação dos resultados obtidos experimentalmente. In this thesis we present a study of the structural and electronic properties of semiconductor heterostructures, released from their original substrate. The investigated systems were prepared with InAs quantum points grown on GaAs. Our focus is on to explore the versatility of nanomembrane manufacturing, providing new possibilities for the study of structural and electronic characteristics of epitaxially grown materials. Transmission electron microscopy (TEM), X-ray diffraction and scanning tunneling microscopy/spectroscopy (STM/S) techniques were used to investigate the islands of InAs / GaAs transferred to different substrates of interest. The study of structural properties, which makes use of the first two techniques mentioned, allowed us to directly observe anisotropic relaxation in these three-dimensional nanostructures from the statistical analysis of 164 islands observed by TEM. In addition to the completely coherent or completely relaxed islands we have observed a set of islands formed by the partial and anisotropic relaxation of the lattice parameter of the strained material, which led us to infer mechanisms of the strain relaxation processes that take place during the growth of III-V islands . The second work, in which we studied islands of InAs uncapped and capped by GaAs using STM/S, allowed the observation of energy states of the heterostructure that are compatible with those previously reported in the literature. The main advantage here is the use of an extremely simple method of sample preparation (nanomembrane), which is transferred to a Au substrate. Discrete energy levels were observed in capped islands, even with the presence of a layer of native oxide. Calculations of the energies of these levels validate the interpretation of the results obtained experimentally.

The recently observed superconductivity in twisted bilayer graphene emerges from insulating states believed to arise from electronic correlations. While there have been many proposals to explain the insulating behaviour, the commensurability at which these states appear suggests that they are Mott insulators. Here we focus on the insulating states with $\pm 2$ electrons or holes with respect to the charge neutrality point. We show that the theoretical expectations for the Mott insulating states are not compatible with the experimentally observed dependence on temperature and magnetic field if, as frequently assumed, only the correlations between electrons on the same site are included. We argue that the inclusion of non-local (inter-site) correlations in the treatment of the Hubbard model can bring the predictions for the magnetic and temperature dependencies of the Mott transition to an agreement with experiments and have consequences for the critical interactions, the size of the gap, and possible pseudogap physics. The importance of the inter-site correlations to explain the experimental observations indicates that the observed insulating gap is not the one between the Hubbard bands and that antiferromagnetic-like correlations play a key role in the Mott transition., 8 pages (including appendix), 5 figures

We analyze the doubly differential electron momentum distribution in above-threshold ionization of atomic hydrogen by a linearly-polarized mid-infrared laser pulse. We reproduce side rings in the momentum distribution with forward-backward symmetry previously observed by Lemell \textit{et al.} in Phys. Rev. A \textbf{87}, 013421(2013), whose origin, as far as we know, has not been explained so far. By developing a Fourier theory of moir\'{e} patterns, we demonstrate that such structures stems from the interplay between intra- and intercycle interference patterns which work as two separate grids in the two-dimensional momentum domain. We use a three dimensional (3D) description based on the saddle-point approximation (SPA) to unravel the nature of these structures. When the periods of the two grids (intra- and intercycle) are similar, principal moir\'{e} patterns arise as concentric rings symmetrically in the forward and backward directions at high electron kinetic energy. Higher order moir\'{e} patterns are observed and characterized when the period of one grid is multiple of the other. We find a scale law for the position (in momentum space) of the center of the moir\'{e} rings in the tunneling regime. We verify the SPA predictions by comparison with time-dependent distorted wave strong-field approximation (SFA) calculations and the solutions of the full 3D time-dependent Schr\"{o}dinger equation (TDSE)., Comment: 22 pages, 6 figures

Being the basic building blocks for nano-magnetic and nano-catalytic devices, regular arrays of nano-clusters play a crucial role in modern nanotechnology. One of the possible fabrication methods of periodic nanostructures consists in exploiting nano-patterned substrates as templates for the self-assembly of the deposited atoms. Here, we have investigated the templating properties of a Moire superlattice formed at the interface between a FeO(111)-like ultrathin film and a Ni/Fe(001) substrate. Co and Au, representative of elements with high and low oxygen affinity, respectively, have been deposited on the iron oxide film. Scanning tunneling microscopy reveals that Co nucleates preferentially along the corrugated regions of the Moire superstructure, forming stripes with high aspect ratio. On the other hand, Au atoms nucleate randomly distributed three-dimensional islands on the FeO(111) surface.

Monolayer hexagonal boron nitride (hBN) tunnel barriers investigated using conductive atomic force microscopy reveal moire? patterns in the spatial maps of their tunnel conductance consistent with the formation of a moire? superlattice between the hBN and an underlying highly ordered pyrolytic graphite (HOPG) substrate. This variation is attributed to a periodc modulation of the local density of states and occurs for both exfoliated hBN barriers and epitaxially grown layers. The epitaxial barriers also exhibit enhanced conductance at localized subnanometer regions which are attributed to exposure of the substrate to a nitrogen plasma source during the high temperature growth process. Our results show clearly a spatial periodicity of tunnel current due to the formation of a moire? superlattice and we argue that this can provide a mechanism for elastic scattering of charge carriers for similar interfaces embedded in graphene/hBN resonant tunnel diodes.

The speckle interferometry technique has been used in this work in order to determine the thermomechanical behaviour of Printed Wiring Board (PWB) (circuits) of a radio integrated with tape player and speakers. A preliminary experiment of such technique has been carried out on a single electronic component (silicon transistor), during the thermal transient and at the steady state. The thermal deformation and stresses on PWB have been obtained through related experimental analyses on both cases. The results showed a very good applicability of speckle technique on the irregular object surface as PWB.

In this dissertation, I focus on grains of cylindrical microdomains in block copolymers. The dissertation consists of three chapters. In Chapter 1, I applied an image processing technique on the basis of the Fourier transform (FT) / inverse FT method to evaluate the grain size observed by transmission electron microscopy (TEM) for the block copolymer in which grains of perpendicularly oriented cylinders exit with slightly different orientation mismatch of the hexagonal lattice. The method reported herein also provided insights of defects and dislocations in such a regularly ordered microdomains. I eventually ascribed the region where such defect-like patterns due to moiré effects are observed to the inclined grain boundary. This grain boundary is considered to be accommodated by chance in the thin section for the TEM observation. In Chapter 2, the quantitative method of evaluation of the grain size using small-angle X-ray scattering (SAXS) was presented. For this purpose, collagen taken from a chicken leg tendon was used as a standard sample to estimate the extent of the instrumental broadening. Then, this method was applied for block copolymer sample in which cylindrical microdomains are oriented perpendicular to the surface of the film specimen. The evaluated value of the grain size was in sufficient accord with the result of the TEM observation as described in Chapter 1, implying the validity of the method proposed in this chapter. In Chapter 3, characterization of the surface morphology was performed by atomic force microscopic (AFM) observation for the thin film specimen of block copolymer sample in which the perpendicularly oriented cylinders are existing. It is found that the cylinder orientation at the surface is also perpendicular. Furthermore, the grain size was successfully evaluated by AFM with the image analysis (as described in Chapter 1). The grain size in the interior of the specimen was also evaluated by SAXS (as described in Chapter 2), and it is found that the grain is slightly larger on the surface than in the interior of the sample. However, this difference might be due to difference in the evaluation methods. Both results for the grain growth on the surface and in the interior of the sample exhibited power-law behavior with the same extent of the growth exponent (~0.45).

Colloque avec actes et comité de lecture. Internationale.; International audience; CEA-LITEN is developing an original process called Boostream to manipulate, assemble and connect micro- or nano-particles of various origins, sizes, shapes and functions [1]. This process uses the upper surface of a liquid film flowing down a ramp to assemble particles in a manner that is close to the horizontal situation of a Langmuir -Blodgett film constructions. The strong potentials of our new process rely on its advantages: (i) collective handling of micro- or nano-sized particles, (ii) safe handling of powder materials, (iii) numerous potential industrial applications. The process uses a laminar water film on an inclined plane slope flowing down to a horizontal reservoir. Particles are deposited on the top of the water/air interface and gradually accumulate on the upper surface of the reservoir; once the reservoir surface is filled by single-layer film of particles, this film continues to form on the feeding inclined surface. Thus, a film composed of a single layer of particles is formed (typical size ranging from 280 nm to 10 µm) and can be transferred on any kind of solid support, forming a micro- or nano-structured surface on it and allowing new physical, chemical or optical functions [2], [3]. The film down-flow configuration exhibits two hydraulic jumps, the first being upstream to the border of the particle film, the second located at the connection of the slope and the reservoir surface. The second jump is a classical one and not worthy to us. The first jump is more unusual; it results from the fluid flow accommodation to the change of the film upper surface boundary condition: upstream, the film has a free surface; downstream, the closed-packed particles result in a mixed (or Robin) boundary condition, with vanishing tangential velocity components, whereas the film thickness remains free. It is worth managing this hydraulic jump to control the film construction process, especially when non-isotropic particles are used, such as micro-fibers. Indeed, unlike spherical particles, micro-fibers are far more sensitive to flow singularities, such as a hydraulic jump that tends to induce recirculating surface patterns upstream of them and lets the particles rotate on the free surface, close to the film border. In order to master our process, the fluid flow has been modeled and experimentally characterized by optical means, such as with the moiré technic that consists in observing the reflection of a succession of periodic black-and-red fringes on the liquid surface mirror. The fringe images are deformed when reflected by the curved liquid surface associated with the hydraulic jump, the fringe deformation being proportional to the local slope of the surface. This original experimental setup allowed us to get the surface profile in the jump region and to measure it along with the main process parameters (liquid flow rate, slope angle, temperature sensitive fluid properties such as dynamic viscosity or surface tension, particle sizes). Our oral presentation will depict the experimental setup and its modeling, the different experimental characterization techniques used and will focus on the way the hydraulic jump relies on the process parameters. Some of the applications of our process are also to be presented. [1] O. Delléa et al., Control methods in microspheres precision assembly for colloidal lithography, Proc. IPAS Conf., Chamonix, France, 2014. [2] O. Shavdina, O. Delléa, et al., Large Area Fabrication of Periodic TiO2 Nanopillars Using Microsphere Photolithography on a Photopatternable Sol-Gel Film, Langmuir, 31 (28), pp. 7877-7884, 2015. [3] O. Shavdina, « Micro-nanostructuration de surface par renforcement local du flux électromagnétique », thèse, Université Jean Monnet, soutenue le 20 Décembre 2016.

Moiré patterns arise from the superposition of two (or more) periodic structures (one-dimensional or two-dimensional) defined over flat (or curved) surfaces in which one (or more) is rotated, translated, or any form of altercation from the initial position. This results in a series of periodic dark/bright fringes which characteristics are determined by the period, orientation, and shape of the patterned samples. When two arrays of similar periods are superposed, the period of the resultant pattern acts like a magnifier of the original period. The Moiré effect has been used in different areas which includes microscopy, cryptography, profilometry, and material stress evaluation. Most of the applications, however, were carried out at optical frequencies. In this work, we performed studies on the generation of Moiré patterns at thermal frequencies (f ~ 20-40 THz) using the superposition of metallic arrays of two-dimensional structures with different periods and lattice configurations. Thermal imaging experiments revealed that the period of the superposed structures can be significantly modified as the angle between patterns is varied. A combination of samples with different features and dimensions resulted in Moiré patterns with period and lattice geometries very different from the individual patterns. The obtained results can be prospectively used to deploy reconfigurable metamaterials frequency selective filters for operation at thermal frequencies.

The recent advances in technology, in particular the nanotechnology, lead to the new investigations in the cientific enviroment and contribute toe new ways to investigate the nature. In the present work, we show some investigations that are consequences of theses advances. First of all, we investigate the behavior between two nanocones that have interactions. Initially we study the carbon nanocones, that derived from fullerene and used as nanotube caps, but with many applications when sinthetize individually. The chapter about carbon nanocones is composed by investigation of the energetic stability between two nanocones with same disclination angle and a composition of two nanocones with different disclination swith the goal to find the ideal distance between this cones overlapping. Beyond that, we rotated the upper cone with specific angles, that generate the Moire pattern to investigate the influence of rotation over the electronic states. It was identified that the rotated configurations rather most stable than not rotated. This occurs because this cones are similar to the Bernal stacked graphite. In the chapter about the hybrid nanocones, we calculated the sames properties from carbon nanocones, however, with the specific features of the nancones that involve the boron,nitrogenand carbon atoms, what gives the insulating character to the cones. All configurations showed different atoms on the defect line and this factor is responsible to change the formation energy, interaction energy and the electronic states, mainly on the gap region. The double-walled nanocones with only nitrogen or boron atoms on the defect line as most stable when we rotated the upper cone. This is due the greater distance between the atoms located on the defect line. To the Otherwise, the configurations with carbon and the composition between boron and nitrogen has most stability without rotation, and this garantee more interaction between these atoms. The following chapter approaches the effects of dopping with gallium atoms in graphene monolayers. We use different concentrations and the stability showed a different behavior when odd numbers or even numbers of gallium atoms was putted in the structure. The presence of different numbers of gallium atoms in each sub-lattice of graphene deforms the different way the supercell. This affect directly the electronic properties. The odd numbers of gallium atoms keeps the metallic character, with states crossing the Fermi Level, while the even numbers to dopping open the gap, leading to the semiconductor behavior. To the calculated optical properties, the dopping with gallium atoms reduces the optical absorption and conductivity on the infrared and ultraviolet region, what garantee the control with a few atoms. Finally, we dopped a monolayer ofh-BN with gallium atoms. Through the simulated annealing algorithm, each process with different gallium atoms, monotomic bonds like B-B and N-N arises and this bonds affects directly the properties. The distortions suffered with the dopping affects mainly the substitution the nitrogen and the B-N pair to the gallium atoms, that leads the dissociation of the monolayer. When we substitute the gallium to the boron, we growing linearly the numbers of the Ga. The electronic properties was affected with reduction of the gap due the states that arise on this region due the monotomic bonds. For the optical properties, states on the visible region arises, what shows the influence of this bonds due the simulated annealing algorithm. Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES Os recentes avanços na tecnologia, em particular, a nanotecnologia levaram à novas investigações no meio científico e contribuem para novas descobertas sobre a natureza. No presente trabalho, apresentamos algumas investigações que são consequência desses avanços. Inicialmente nós investigamos o comportamento entre dois nanocones que interagem entre si sobrepostos. Primeiramente nós estudamos nanocones de carbono, que são derivados do fulereno e usados como tampas de nanotubos, mas com diversas aplicações quando sintetizados individualmente. O capítulo sobre os nanocones de carbono é composto pela investigação da estabilidade energética entre dois nanocones com desclinações iguais e uma composição entre dois nanocones de desclinações diferentes com o objetivo de encontrar a distância ideal entre esses cones sobrepostos. Além disso, rotacionamos o cone superior em ângulos específicos, que geram o padrão de Moiré para entendermos a influência da rotação sobre os estados eletrônicos. Foi identificado que as configurações rotacionadas são mais estáveis que as que não apresentam rotação. Isso ocorre devido à semelhança dos cones rotacionados à configuração Bernal do grafite, o que leva a uma maior estabilidade. No capítulo sobre os nanocones híbridos, calculamos as mesmas propriedades para os nanocones de carbono, porém, com as características específicas dos nanocones que envolvem átomos de boro, nitrogênio e carbono, o que dá um caráter de isolante aos cones. Todas configurações apresentam diferentes átomos na linha de defeito dos cones e esse fator é responsável por alterar sua energia de formação, de interação e seus estados eletrônicos, principalmente na região do gap. Os nanocones de paredes duplas com defeitos formados totalmente por átomos de boro ou nitrogênio são mais estáveis quando estas linhas estão mais afastadas, ou seja, quando há rotação. De forma contrária, para os que possuem em sua linha de defeito carbono e nitrogênio e uma composição entre boro e nitrogênio possuem maior estabilidade aos sem rotação, o que garante uma maior interação entre esses átomos. O capítulo seguinte aborda os efeitos da dopagem de átomos de gálio em monocamadas de grafeno. Nós dopamos com diferentes concentrações e vimos que em relação a estabilidade, há uma diferença entre a dopagem com números ímpares e pares de átomos de gálio. A presença de diferentes números de gálio em cada sub-rede do grafeno distorce de forma diferente a super-célula, o que afeta diretamente as propriedades eletrônicas. A dopagem com números ímpares mantém o caráter metálico, com estados cruzando o nível de Fermi, enquanto que a dopagem com números pares causa a abertura do gap, dando o caráter de semicondutor. Para as propriedades ópticas calculadas, a dopagem com átomos de gálio reduz a absorção e condutividade nas regiões infravermelhoeultravioleta,oquegaranteumaformadecontroledessaspropriedadescompoucos átomos. Por fim, dopamos com átomos de gálio uma monocamada de nitreto de boro. Por meio do algoritmo de recozimento simulado, a cada substituição de átomos de gálio, ligações monoatômicas B-B e N-N surgiram e essas ligações afetam diretamente as propriedades das monocamadas. As distorções causadas pela dopagem afetam principalmente quando substituímos os átomos de nitrogênio e os pares B-N por átomos de gálio, já que para números maiores que 1 átomo, houve dissociação do gálio da monocamada. Quando a substituição ocorreu com átomos de boro, foi possível aumentarmos linearmente a dopagem com átomos de gálio. As propriedades eletrônicas foram afetadas com a redução do gap devido ao surgimento de estados nesta região devido a essas ligações monoatômicas. Para as propriedades ópticas, estados surgiram na região do visível, o que mostra a influência dessas ligações refeitas devido ao algoritmo de recozimento simulado

We have studied the incommensurate moir\'e structure of epitaxial graphene grown on iridium(111) by dynamic low-energy electron diffraction [LEED $I$($V$)] and noncontact atomic force microscopy (AFM) with a CO-terminated tip. Our LEED $I$($V$) results yield the average positions of all the atoms in the surface unit cell and are in qualitative agreement with the structure obtained from density functional theory. The AFM experiments reveal local variations of the moir\'e structure: The corrugation varies smoothly over several moir\'e unit cells between 42 and 56 pm. We attribute these variations to the varying registry between the moir\'e symmetry sites and the underlying substrate. We also observe isolated outliers, where the moir\'e top sites can be offset by an additional 10 pm. This study demonstrates that AFM imaging can be used to directly yield the local surface topography with pm accuracy even on incommensurate two-dimensional structures with varying chemical reactivity.

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.

Moir\'e superlattices in graphene supported on various substrates have opened a new avenue to engineer graphene's electronic properties. Yet, the exact crystallographic structure on which their band structure depends remains highly debated. In this scanning tunneling microscopy and density functional theory study, we have analysed graphene samples grown on multilayer graphene prepared onto SiC and on the close-packed surfaces of Re and Ir with ultra-high precision. We resolve small-angle twists and shears in graphene, and identify large unit cells comprising more than 1,000 carbon atoms and exhibiting non-trivial nanopatterns for moir\'e superlattices, which are commensurate to the graphene lattice. Finally, a general formalism applicable to any hexagonal moir\'e is presented to classify all reported structures., Comment: 14 pages, 6 figures

Epitaxial graphene on carbon-terminated face (C-face) of SiC substrates consists of graphene layers rotated from each other. This rotation of layers grants this material single-layer like properties, such as a linear dispersion band structure and a ballistic transport. As discussed in chapter 2, the full-wafer size and the insulating SiC substrate are two of many advantages of graphene films grown on SiC compared to those prepared differently (exfoliation method and chemical vapour deposition method). These two advantages allow experimentalists to perform scanning tunneling microscopic (STM) experiments and to study graphene properties easily.In this PhD work, we grew graphene on C-face of SiC substrates to investigate the induced superconducting proximity effect in ballistic regime. The physics of this phenomenon is explained in chapter 3 as the formation of time-reversed pairs of electrons and holes. Concerning the superconducting materials, we relied on vanadium and niobium carbide to induce the proximity effect. These two approaches are discussed in detail in chapter 3 (for V) and chapter 4 (for NbC). STM characterizations performed on fabricated samples show a superconducting gap in V and a part of the NbC surface, but no induced gap in graphene. Several possible reasons, like a poor interface between superconductors and graphene, the unability of the STM to reach the true graphene-superconductor interface, and the degradation of the surface of NbC, were suggested and discussed. However, our high-quality epitaxial NbC films meet the requirements for hot-electron bolometers.Besides their single-layer like properties, the rotation of layers also leads to tunable van Hove singularities and the localization of states, which are thoroughly discussed in chapter 5 and 6. Once one of these singularities stays at the Fermi level, graphene is predicted to gain intrinsic superconductivity and magnetic properties. This condition can be achieved by reducing the rotation angle towards zero, as these singularities converge to the Dirac point or the Fermi level for undoped graphene. In addition to the intrinsic superconductivity, the localization of states also appears for layers rotated with a small angle, as observed in several STM experiments. Experimentally, we found regions in rotated layers, which appear as periodic Moiré patterns in our STM images. The rotation angles were estimated from the Fast Fourier Transform of the recorded STM images. Comparing our experimental results with tight-binding calculations for disorder-free layers rotated with the same angles leads to a qualitatively good agreement for the positions of van Hove peaks. However, the appearance of new peaks in proximity to the Dirac point for layers rotated with θ=1.5º and a spatial evolution of of spectroscopic features for the small rotation angles cannot be explained by the calculations for disorder-free layers. In order to explain these two phenomena, we considered the influence of disorder. This indeed improved the agreement between theoretical and experimental results. But, since no electronic disorder could be evidenced from our STM images, other explanations, like strain, need to be considered too.; Les couches de graphene épitaxiées sur la face carbone du carbure de silicium sont tournées les unes par rapport aux autres. Cette rotation préserve la structure de bande linéaire du graphene mono-couche et permet un transport balistique des porteurs de charge. Parmi les propriétés intéressantes développées dans le chapitre 2, la possibilité de former de pleines couches de graphene sur le substrat isolant qu'est le SiC est un avantage majeur de cette technique comparé aux autres méthodes de croissance du graphene (exfoliation et épitaxie en phase vapeur sur métaux). Les grandes surfaces produites permettent aux expérimentateurs de faire facilement des mesures STM car la localisation de la partie utile de l’échantillon n'est pas un problème dans ce cas.Dans ce travail de thèse, j'ai réalisé la croissance de graphene sur la face carbone du SiC dans le but d'étudier la supraconductivité induite dans le graphene par la proximité d'un supraconducteur. Cette supraconductivité induite dont le principe expliqué dans le chapitre 3 se développe d'autant plus loin de l'interface que le matériau non supraconducteur possède un grand libre parcours moyen. D'où notre choix du graphene. Dans le chapitre 3 je présente aussi les efforts que j'ai mené pour fabriquer des jonctions graphene/supraconducteur par une technique de lithographie propre : la lithographie par microsphères. Cette méthode utilise des micro-sphères de silice comme masque dur durant le dépôt par évaporation d'un matériaux supraconducteur tel le vanadium. Malgré la propreté de cette méthode telle qu'avérée par les images STM des échantillons, nous n'avons pas réussi à induire la supraconductivité dans le graphene. Suite à ce résultat négatif, nous avons développé une seconde approche décrite dans le chapitre 4. Un matériau supraconducteur réfractaire, le niobium, est cette fois-ci déposé sur le substrat avant la croissance du graphene. A l'issue de la croissance, nous avons eu la surprise de constater que la température critique du matériaux supraconducteur s'était élevée de 7 à 12 K. Cela s'explique par la carburation du Niobium lors du recuit. Par ailleurs, nous avons bien démontré que des couches graphitiques sont aussi crues sur le NbC permettant ainsi de réaliser des jonctions. Néanmoins, nous n'avons à nouveau pas réussi à observer de supraconductivité induite dans le graphene.Outre les propriétés intéressantes pour l'étude de la supraconductivité induite, les couches de graphene en rotation constituent en elle même un sujet d'étude intéressant. En effet, la densité d'état de ce système présente des singularités de van Hove dont la position en énergie dépend de l'angle de rotation. Ce système ouvre donc la porte à l'étude de la physique associée à ces singularités (supraconductivité, magnétisme) à des énergies accessibles par dopage électrostatique. De plus, une localisation des fonctions d'onde électroniques a été prédite pour les faibles angles de rotation et cette localisation a été confirmée par des résultats expérimentaux préliminaires. Cependant, il manquait une étude systématique des propriétés électriques des systèmes à faible angle de rotation. Les mesures que j'ai réalisé dans ce régime sont présentées dans la dernière partie de ce mémoire. Ces mesures de spectroscopie sont comparées à un modèle de liaison fortes. Le modèle sans désordre et en présence de désordre ne permettent pas de décrire correctement les expériences menées pour des angles inférieurs à 2°. Mon travail souligne qu'une physique riche existe aux faibles angles de rotation et qu'il reste encore beaucoup de travail à faire pour la comprendre.

A moiré pattern is observed by dividing the incident beam into two and formed by multiple Bragg–Laue interference fringes corresponding to the two incident beams. The coherency of X-rays from a bending-magnet beamline is evaluated using the moiré pattern., In X-ray section topography of Si 220 diffraction in a multiple Bragg–Laue mode, a moiré pattern is observed when the incident beam is divided into two parts by inserting a platinum wire in the middle of the beam. The moiré pattern can be explained by the summation of two interference fringes corresponding to the two incident beams. The coherency of the X-rays from the bending-magnet beamline is estimated using the moiré pattern.

Iron-phthalocyanine (FePc) molecules have been adsorbed on a graphene sheet prepared on the Ir(111) surface. The FePc molecules are flat-lying on graphene, as determined by near-edge X-ray absorption fine-structure, constituting a sub-nanometer thick molecular array at the single-layer coverage. The flat FePc single-layer presents a weak interaction of the organic macrocycle with the graphene surface and Ir subsurface substrate. Further FePc deposition on top of the first flat single-layer determines a three-dimensional island growth with varying molecular orientation.

With α irrational the graph of [jα] against integer j, displays interesting patterns of self-matching. This is best seen by comparing the Bernoulli (characteristic Sturmian) or difference sequence 〈βj〉, term by term with the Bernoulli sequence displaced by k terms 〈βj−k〉, where βj=[(j+1)α]−[jα].It is shown that the fraction of such self-matching is the surprisingly simple M(k)=max(|1−2{α}|,|1−2{kα}|).Of particular interest is the graph of M(k) against k as it is seen to exhibit an unexpected Moiré pattern obtained simply by folding the lower half of the graph of {kα} over the upper half.

This paper presents a hybrid procedure for mechanical characterization of hyper-elastic materials based on moire, finite element analysis and global optimization. The characterization process is absolutely general because does not require any assumption on specimen geometry, loading or/and boundary conditions. The novel experimental approach followed in this research relies on a proper combination of intrinsic moire and projection moire which allows 3D displacement components to be measured simultaneously and independently using always the same experimental setup and just one single camera. In order to properly compare experimental data and finite element predictions, 3D displacement information encoded in moire patterns which are relative to the deformed configuration taken by the specimen are expressed in the reference system of the unloaded state. A global optimization algorithm based on multi-level and multi-point simulated annealing which keeps memory of all best records generated in the optimization is used in order to find the unknown material properties through the minimization of the Ω functional built by summing over the differences between displacements measured experimentally and those predicted numerically. Feasibility, efficiency and robustness of the proposed methodology are demonstrated for both isotropic and anisotropic specimens subject to increasing pressure loads: a natural rubber membrane and a glutaraldehyde treated bovine pericardium patch, respectively. Remarkably, the results of the characterization process are in very good agreement with target data independently determined. For the isotropic specimen, the maximum error on hyper-elastic constants is less than 1% and the residual error on displacements is less than 3.5%. For the anisotropic specimen, the maximum error on material properties is about 3.5% while the residual error on displacements is less than 3%. The identification process fails or becomes less reliable if “local” displacement values are considered.

The parallax barrier of step structure (step barrier) technology with multiple parallax images has overcome the problem of conventional parallax barrier system that the image quality of each image deteriorates only in the horizontal direction. The step barrier distributes the resolution reduction both to the horizontal and the vertical directions. The system has a simple structure, which consists of a flat-panel display and a step barrier....

Structural components may contain cracks, and for these components a crack tip represents a highly singular stress. The evaluation of the associated strain gradient is difficult to achieve with experimental discrete methods. An efficient alternative are the optical techniques which are non-contact, and so delivering continuous information of the displacement fields and associated derivatives for strain evaluation. This paper describes some experimental methods to fully characterize the displacement field near a crack tip existing in flat plates. Three optical field techniques based on image analysis were used in the present work; respectively, Electronic Speckle Pattern Interferometry, Moir ´e Interferometry, and Digital Image Correlation. These methods allow different resolutions which can be adjusted according to the expected strain gradient. While the first method depends on the laser wavelength and position of the illumination sources, the second depends on the grating pitch and the last on the surface texture or painted speckle. Algorithms to derivate and filtering the displacement field are developed to compute the strain field and will be used for further works. info:eu-repo/semantics/publishedVersion

The end-member types of shallow magmatic intrusions comprise cone sheets and dykes. These propagate from their source magma chamber towards the Earth’s surface. The propagation of magma within the shallow part of the volcano plumbing system produces surface deformation and seismicity, signs of volcanic unrest. Studying surface displacement using GPS and InSAR, geodetic modelling, fitting modelled data to real displacements, and inversion is regularly used to understand the processes in the subsurface. There is no possibility in nature to validate the assumptions of these models. However, using analogue modelling, it is possible to reproduce cone sheets and dykes in a controlled environment and study the associated surface deformation. This thesis tests the hypothesis that the two end-member types of magmatic sheet intrusions produce specific patterns of surface deformation. The analogue model surface is documented using two different monitoring setups: moiré monitoring and photogrammetry. The moiré method (dataset 1) consists of 43 experiments, 19 of which were dykes, 22 were cone sheets, and 2 were classified as hybrids. Photo- grammetry (dataset 2) was applied in 8 experiments, 3 of which produced cone sheets and 5 dykes. Dataset 1 successfully identified surface deformation patterns specific to the two intrusion types. Cone sheets develop in a gradual linear fashion, while the dykes exhibit a two-phase behaviour. The first phase shows little deformation until about halfway through the experiment duration when rapid deformation starts to occur. The point of maximum uplift, in both intrusion types, indicates the area of eruption from an early stage. Dataset 2 primarily evaluated the benefits of using the photogrammetric method. It successfully resolved horizontal components of displacement making it possible to study brittle defor- mation. As Dataset 2 consists of a limited amount of experiments, the results cannot be considered conclusive. However, they indicate that characteristic patterns of brittle deformation exist for the two types. Future improvements in this field include studying effects of topography and anisotropy on the surface deformation of the analogue models. Improvements in temporal and spatial resolution in the monitoring methods used to study surface displacement in nature is needed to perform analyses, similar to the ones presented here, on real surface deformation. The misfit of the surface deformation seen in numerical models compared to analogue models indicate that sheet intrusion propagation is not yet fully understood. När magma rör sig från magmakammare mot jordytan i den ytliga delen av det underjordiska system av magma som finns under vulkaner, bildas ytdeformation. Genom att studera ytdeformationen med hjälp av GPS och InSAR samt numerisk modellering försöker forskare förstå dessa underjordiska processer för att förutspå framtida utbrott. Dessvärre finns ingen möjlighet att undersöka huruvida dessa numeriska modeller faktiskt överensstämmer med naturen. I gamla utdöda vulkaner som eroderats ner kan man se vulkanens inre där det finns stelnade, magmafyllda sprickor, så kallade gångar. Tyvärr ger detta dock ingen inblick i hur gångarnas bildning återspeglades i ytan. Detta kan man undersöka med hjälp av analoga modeller där det är möjligt att skapa likartade intrusioner i en kontrollerad labb-miljö och studera den tillhörande ytdeformationen. Denna avhandling undersöker hypotesen att de två huvudsakliga typerna av ytliga magmatiska gångar skapar specifika identifierbara mönster av ytdeformation. Detta testas genom att dokumentera den analoga modellens yta alltjämt som intrusionerna bildas med hjälp av två olika övervaknings- metoder. Den första metoden lyckades identifiera mönster som är specifika för de två olika typerna. Den högsta punkten på den deformerade ytan kan användas för att förutspå den plats där framtida utbrott kommer att ske i båda typerna. Den andra metoden lyckades urskilja dem horisontella komponenterna av förskjutning som gör det möjligt att studera den spröda deformation som utvecklas på ytan. Resultaten tyder på att karakteristiska mönster av spröd deformation existerar för de två typerna. För att förbättra de analoga modellerna måste man undersöka hur en mer topografiskt varierad yta påverkar den bildade deformation samt en skorpa som inte är helt homogen. Vid jämförelse mellan numeriska modeller och analoga modeller sågs en stor skillnad som tyder på att vi ännu inte förstår hur magma rör sig genom jordskorpan. Framtida användning av analyserna presenterade i denna avhandling kräver en förbättring av upplösningen på systemen som används till vulkanövervakning i naturen.

U ovome radu opisana je metoda Moire korištena u građevinarstvu sa svim svojim vrstama i podvrstama te su ispitane njezine mogućnosti za određivanje naprezanja, deformacija i pomaka. Na kraju je prikazan primjer upotrebe ove metode preko proste grede opterećene čistim savijanjem. Izvršena je usporedba eksperimentalnih i numeričkih rezultata te su iskazani određeni zaključci.

The moiré hole drilling method in a biaxially loaded infinite plate in plane stress is an inverse problem that exhibits a dual nature: the first problem results from first drilling the circular hole and then applying the biaxial loads, while the other problem arises from doing the opposite, i.e., first applying the biaxial load and then drilling the circular hole. The first problem is hardly ever addressed in the literature but implies that either separation of stresses or material property identification may be achieved from interpreting the moiré signature around the hole. The second is the well-known problem of determination of residual stresses from interpreting the moiré fringe orders around the hole. This paper addresses these inverse problem solutions using the finite element method as the means to model the plate with a hole, rather than the typical approach using the Kirsch solution, and a least-squares optimization approach to resolve for the quantities of interest. To test the viability of the proposed method three numerical simulations and one experimental result in a finite width plate are used to illustrate the techniques. The results are found to be in excellent agreement. The simulations employ noisy data to test the robustness of this approach. The finite-element-method-based inverse problem approach employed in this paper has the potential for use in applications where the specimen shape and boundary conditions do not conform to symmetric or well-used shapes. Also, it is a first step in testing similar procedures in three-dimensional samples to assess the residual stresses in materials.

This paper presents a new hybrid technique for mechanical characterization of hyperelastic materials. The research is motivated by the fact that standard identification procedures based on the fitting of strain–stress curves determined experimentally from planar biaxial tests may be inaccurate for non-uniform states of deformation. Therefore, we propose an alternative approach where the difference Ω between the displacement field measured with Projection Moire and its counterpart predicted by FEM is minimized using non-linear optimization algorithms that finally find unknown material properties. In order to check the feasibility of the new procedure, we considered a thin latex membrane modelling it as a two-parameter Mooney–Rivlin (MR) hyperelastic material. The Ω function is minimized either using optimization routines available in a commercial finite element package and by implementing a global optimizer able to deal with non-linearity and non-convexity included in the identification process. In order to check accuracy of optimization results, target values of MR constants for the latex specimen tested have previously been determined by fitting experimental stress–strain data gathered from a standard planar biaxial tension test. Results indicate that the present hybrid identification procedure can determine accurately properties of the hyperelastic material under investigation. In fact, the average residual error on displacements was less than 1% while the difference between the MR constants found with optimization and their target values was less than 3.5%.