Le graphène, un feuillet élémentaire de graphite, est un matériau très étudié par la communauté scientifique car ses propriétés physiques sont nouvelles et uniques. Il apparaît comme un matériau très prometteur pour des applications technologiques. Nous présentons une étude des propriétés structurelles et électroniques du graphène épitaxial sur 6H-SiC(0001) au moyen d’un microscope STM/AFM combiné basé sur un diapason en quartz avec une pointe conductrice en Pt/Ir ou en fibre de carbone. Les pointes fabriquées par attaque électrochimique présentent un rayon d’apex de quelques nanomètres et ont été caractérisées par SEM, TEM et émission électronique par effet de champ. On s’est d’abord focalisé sur les propriétés d’un échantillon qui présente des terrasses partiellement recouvertes de graphène. Dans ce cas, l’image STM ne donne pas la topographie de la surface. Celle-ci est donnée par l’AFM en mode répulsif. Les différentes propriétés électroniques de chaque terrasse sont confirmées par des mesures spectroscopiques I=f(V). Puis, l’étude à haute résolution par FM-AFM sur une terrasse lisse a révélé la structure ondulée et périodique de la reconstruction 6√3x6√3R30° du SiC(0001) recouverte de graphène. Nous montrons que les maxima des nappes d’iso-densité locale d’états électroniques au niveau de Fermi observés dans l’image STM ne se superposent pas avec les zones associées aux maxima des nappes d’iso-densité d’états totaux (Topographie AFM). Ils apparaissent décalés de ~1 nm le long de la direction [11] de la quasi-maille 6x6 de la reconstruction 6√3x6√3R30°. Comme l’amplitude mesurée des ondulations de la surface augmente avec le gradient de force appliqué, on montre que la surface du graphène est déformée par la pointe AFM. Cette déformation qui modifie le couplage électronique entre le graphène et la couche tampon influence fortement le contraste des images STM/AFM. Les conséquences de cette déformation sur les images STM résolvant le réseau du graphène sont aussi discutées. / The graphene, a basic sheet of graphite, is a new material intensively studied by the scientific community because of its new and unique physical properties. Furthermore it appears as a very promising material for technological applications. We present a study of structural and electronic properties of epitaxial graphene on 6H-SiC(0001) using a combined STM/AFM microscope based on a quartz tuning fork with a conductive tip. The tips made from electrochemical etched Pt/Ir wire or carbon fiber have an apex radius of few nanometers and were characterized by SEM, TEM and by field electron emission. First, we focused on the properties of a sample showing terraces partially covered with graphene. In this case, the STM images do not provide the real surface topography, which is given by the AFM topography working in repulsive mode. The electronic properties of each terrace are confirmed by local spectroscopic I=f(V) measurements. Then, the high-resolution FM-AFM study on a smooth terrace revealed the corrugated structure due to the periodic 6√3x6√3R30° reconstruction of SiC (0001) covered with graphene. We show that the maxima of the local density of electronic states at the Fermi level observed in STM image do not overlap with the zones associated with maxima of total states density (AFM Topography). They appear shifted by ~1 nm along the direction [11] of the 6x6 nanomesh of the 6√3x6√3R30° reconstruction. As the corrugation amplitude of the surface increases with the applied force gradient, we show that the surface of graphene is distorted by the AFM tip. This deformation modifies the electronic coupling between the graphene and the buffer layer and strongly influences the contrast in STM/AFM images. The consequences of this deformation are also discussed in the STM images showing the lattice of graphene.

Neste trabalho investigamos a dinâmica de crescimento de filmes de diamante sintetizados por meio de deposição química a vapor ativada por plasma de microondas (CVD). A caracterização foi feita utilizando, fundamentalmente, microscopia de força atômica (AFM). Analisamos o comportamento da rugosidade dos filmes como função da escala de observação e do tempo de deposição. Dessa maneira verificamos a existência de leis de potência para o crescimento e determinamos os expoentes críticos associados a essas leis. Os resultados obtidos estão em bom acordo com o processo de crescimento descrito pela equação estocástica KPZ. Os mecanismos principais são a deposição aleatória de partículas na superfície, o crescimento lateral e a dessorção. / Diamond films have been grown by Microwave Plasma assisted Chemical Vapor Deposition (CVD). The characterization has been made mainly by Atomic Force Microscopy (AFM). We could analyze the roughness behavior with the scale of observation and with the deposition time. We could determine the critical exponents associated with these laws. The results suggest that the growth process is in good agreement with the stochastic growth equation known as KPZ. The most important mechanisms are the random deposition, the lateral growth and the desorption.

Conselho Nacional de Desenvolvimento CientÃfico e TecnolÃgico / O estudo reolÃgico do Cimento AsfÃltico de PetrÃleo (CAP) Ã de fundamental importÃncia para a determinaÃÃo de sua performance na fabricaÃÃo de estradas. Usualmente os parÃmetros reolÃgicos deste tipo de material sÃo obtidos em ensaios dinÃmicos atravÃs de um reÃmetro de cisalhamento. O presente trabalho tem por objetivo estudar as propriedades reolÃgicas do CAP atravÃs da tÃcnica de espectroscopia de forÃa utilizando um microscÃpio de forÃa atÃmica. Para isso, foi utilizada uma ferramenta computacional desenvolvida para o processamento das imagens de microscopia de forÃa chamada de FVLOAD. Cinco anÃlises foram realizadas com os dados das curvas de forÃa: anÃlise de slope, anÃlise do mÃdulo elÃstico ou mÃdulo de Young, anÃlise de adesÃo do filme de CAP, anÃlise FIEL (Force Integration to Equal Limits), Work Difference e AdesÃo. TrÃs amostras foram utilizadas neste estudo: CAP puro 50/70, CAP com 4% de EVA e CAP com 4,5% de SBS. Filmes dos trÃs tipo de CAP foram confeccionados em lamÃnulas de vidro e levados ao microscÃpio de forÃa atÃmica para obtenÃÃo dos dados das curvas de forÃa sobre uma Ãrea de 50 μm x 50 μm. Para cada imagem foram coletadas 1024 curvas de forÃa. As indentaÃÃes com a sonda AFM foram executadas em quatro frequÃncias: 0,5 Hz, 5 Hz, 14 Hz e 28 Hz. Finalizadas as aquisiÃÃes de dados, estes foram processados no programa FVLOAD. ExtraÃdo o mÃdulo elÃstico das amostras em cada frequÃncia, os dados foram comparados com aqueles obtidos em ensaios dinÃmicos atravÃs do reÃmetro de cisalhamento dinÃmico. / The rheological study of asphalt binders is of great importance for determining its performance in paving construction. Usually, rheological parameters are obtained by a Dynamic Shear Rheometer (DSR). The aim of this work is to study the rheological properties of bitumen using an Atomic Force Microscope (AFM). So, a computational tool caled FVLOAD was used for imaging processing. Five kinds of mathematical models were used in order to analise the force curves colected through AFM: the slope analysis, the YoungÂs model, the adesion model, the FIEL model (Force Integration to Equal Limits) and the work difference model. Three samples were studied: pure bitumen 50/70, bitumem 50/70 doped with 4% of EVA and bitumem 50/70 doped with 4,5% of SBS. Very thin lms of these materials were prepared on glass slides and lead to an AFM for imaging. The area studied on the sample surface was of 50 m x 50 m. There were colected 1024 force curves for each image. The indentations were carried out in four frequencies: 0,5 Hz, 5 Hz, 14 Hz e 28 Hz. The curves colected were analyzed with the FVLOAD program and once the elastic properties were calculated, they were compared to those obtained in the Dynamic Shear Rheometer.

Encapsulation of nanoparticles within hexaniobate nanoscrolls presents interesting advances in the formation of nanocomposites exhibiting unique multi-dimensional properties. Building upon previous successes, facile yet versatile wet-chemical and microwave-irradiation synthetic protocols for the fabrication of a series of hexaniobate composites are presented herein. Solvothermal and, more recently, microwave-assisted methods have been developed that allow for the fabrication of peapod-like structures. During solvothermal treatment, exfoliated hexaniobate nanosheets scroll around highly ordered chains of preformed nanoparticles (NPs) to produce nanopeapods (NPPs). This approach offers versatility and high yields, in addition to the potential for advanced functional device fabrication. For the characterization of these materials, advanced techniques in atomic force microscopy (AFM) were used for investigating the surface of materials at the nanometer scale. Extensive physical, dynamic, and force modulation studies were performed on novel oxide nanocomposites by implementing particular scanning techniques to determine information such as topology, stress-induced behavior at the nanoscale, magnetic behavior, and frictional forces of the nanoscale materials. These composites were then analyzed by topological intermittent contact studies in tapping and contact mode, as well as with derivative techniques of these commonly used scanning probe approaches. In addition to studying surfaces using conventional modes of AFM, the mechanical properties of these nanocomposites were measured via dynamic lateral force modulation (DLFM) and magnetic properties of functionalized magnetic nanosheets were mapped via magnetic sampling modulation (MSM). By utilizing the capabilities of the DLFM imaging mode, elastic properties such as Young’s Modulus were measured from force-distance curves. In addition to this modulation mode, MSM was used to selectively map the vibrating magnetic nanomaterials from a modulated electromagnetic field. The information obtained from these AFM techniques can be helpful in determining the relative structural behavior of these nanocomposites and gauge their use in various applications such as structural engineering of nanoarchitectures as well as studying magnetic characteristics of metal oxide nanocomposites that exhibit characteristics different from their bulk counterparts.

Complete understanding of the physics underlying the changes in viscoelasticity, relaxation time, and phase transitions that mesoscopic fluid-like systems undergo at solid-liquid interfaces or under confinement remains one of the major challenges in condensed matter physics. Moreover, studies of confined mesoscopic fluid films are relevant to technological areas like adhesion, wetting processes and nanotribology. This thesis addresses the interaction between two sliding solids interfaces separated by a nanometer sized gap, with emphasis on the role of the mesoscopic fluid film trapped between them. For this purpose we integrated two acoustic techniques, recently introduced by our group, into a sub-nanometer precision and thermal drift corrected scanning probe microscope (SPM): the shear-force/acoustic near-field Microscope (SANM) and the whispering gallery acoustic sensing (WGAS). The SANM monitors the sound waves originating in the probe-layer interaction while the motion of the probe is monitored by the WGAS. Additionally, we decouple the interaction forces by using frequency modulation and measure the local tunneling current to help establish the location of the substrate. Our results show a strong correlation between the elastic component of the probe's interaction and the SANM amplitude, as well as between the phase lag response of the fluid relative to the probe's excitation (represented by the SANM phase) and the onset of the probe-sample contact region. Frequency modulation SANM-WGAS brings a new acoustic sensing mechanism to the challenging characterization of fluid-like physical systems at the nanometer scale.

Le travail de thèse présenté dans ce manuscrit concerne d’une part l’étude théorique des résonances plasmon de nanoparticules métalliques, et d’autre part une étude expérimentale d’inscription de nanostructures métalliques basée sur l’utilisation d’un Microscope à Force Atomique. La partie théorique présente une nouvelle approche phénoménologique permettant l’analyse des modes de résonance propres de particules uniques ainsi que de leur couplage dans des structures simples. Des algorithmes numériques ont été développés afin d’extraire les différents paramètres phénoménologiques à partir du calcul rigoureux du champ diffusé par les particules. Cette méthodologie a été appliquée à divers cas allant de la particule unique à des réseaux à deux dimensions de particules. La partie expérimentale développe une méthode d’inscription de nanostructures métalliques basée sur une réduction électrolytique d’ions métalliques présents dans une couche de silice méso-poreuse, en appliquant une différence de potentiel entre une pointe AFM conductrice et le substrat conducteur supportant la couche. Des structures sont formées de part et d’autre de la couche de silice, avec la possibilité de commuter leur position par simple inversion du potentiel appliqué. De plus, il apparait que cette commutation est accompagnée de modifications dans la conductivité locale de la couche de silice. Une conséquence du processus d’inscription est la formation de filaments métalliques à l’extrémité des pointes AFM. En particulier, des filaments d’or sont obtenus avec des dimensions allant de quelques dizaines à quelques centaines de nanomètres de long pour une épaisseur de quelques nanomètres / The thesis presented in this manuscript concerns firstly the theoretical study of plasmon resonances of metal nanoparticles, and also an experimental study metallic nanostructures inscription based on the use of an Atomic Force Microscope. The theoretical part presents a new phenomenological approach for analyzing the resonant modes of unique particles and their coupling in simple structures. Numerical algorithms have been developed to extract the phenomenological parameters from the rigorous calculation of the field scattered by the particles. This methodology has been applied to various cases from the single particle to two dimensional particle arrays. The experimental section develops a metallic nanostructures inscription method based on electrolytic reduction of metal ions in meso-porous silica thin film, by applying a voltage between a conductive AFM tip and the conductive substrate supporting the film. Structures are formed on both sides of the silica layer, with the possibility to switch their position by a simple reversal of the applied potential. Moreover, it appears that this switching is accompanied by changes in the local conductivity of the silica layer. A consequence of the inscription process is the formation of metal filaments at the ends of AFM tips. In particular, gold filaments are obtained with sizes ranging from tens to hundreds of nanometers long with a few nanometers thick

Indium phosphide (InP) is an important III-V compound, with a variety of applications, for example, in light emitting diodes (LED), InP based photonic crystals and in semiconductor lasers, heterojunction bipolar transistors in integrated circuit applications and in transistors for microwave and millimeter-wave systems. The optical and electrical properties of this compound can be further tailored by ion implantation or prospectively by swift heavy ion beams. ¶ Thus knowledge of ion-induced disorder in this material is of important fundamental and practical interest. However, the disorder produced during heavy ion irradiation and the subsequent damage accumulation and recovery in InP is far from being completely understood. In terms of the damage accumulation mechanisms, the conclusions drawn in the numerous studies performed have often been in conflict with one another. A factor contributing to the uncertainties associated with these conflicting results is a lack of information and direct observation of the “building blocks” leading to the ultimate damage created at high ion fluences as an amorphous layer. These building blocks formed at lower fluence regimes by single ion impacts can be directly observed as isolated disordered zones and ion tracks for low energy and swift heavy ion irradiation, respectively. ¶ The primary aim of this work has thus been to obtain a better understanding of the disorder in this material through direct observations and investigation of disorder produced by individual heavy ions in both energy regimes (i.e. elastic and inelastic energy deposition regimes) especially with low ion fluence irradiations. In this thesis the heavy ion induced disorder introduced by low energy Au ions (100 keV Au+) and high energy Au (200 MeV Au+16) ion irradiation in InP were investigated using Transmission Electron Microscopy (TEM), Rutherford Backscattering Spectrometry (RBS/C) and Atomic Force Microscopy (AFM). ¶ The accumulation of damage due to disordered zones and ion tracks is described and discussed for both low energy and swift ion irradiation respectively. ¶ The in-situ TEM annealing of disordered zones created by 100 keV Au+ ion irradiation shows that these zones are sensitive to electron beam irradiation and anneal under electron energies not sufficient to elastically displace lattice atoms, i.e. subthreshold energies for both constituent atoms In and P. ¶ Ion tracks due to swift heavy ion irradiation were observed in this material and the interesting track morphology was described and discussed. The surface nanotopographical changes due to increasing fluence of swift heavy ions were observed by AFM where the onset of large increase in surface roughness for fluences sufficient to cause complete surface amorphization was observed. ¶ In addition to InP, the principle material of this project, a limited amount of TEM observation work has been performed on several other important compounds (apatite and monazite) irradiated by 200 MeV Au+ ions for comparative purposes. Again the observed segmental morphology of ion tracks were shown and possible track formation scenario and structure were discussed and similarities were drawn to the previously observed C60 cluster ion tracks in CaF2 as more knowledge and data base exist about defect dynamics and formation in that material.

Ce travail a porté sur l'étude des propriétés des monocouches de copolymères diblocs neutres-chargés de poly(tert-butylstyrène)/poly(styrène sulfonate) à l'interface eau/air. Conformément aux modèles théoriques, l'étude des isothermes de compression des monocouches a permis de montrer que les chaînes chargées ancrées à l'interface ont une conformation de type brosse osmotique aux aires moléculaires étudiées. Nous montrons aussi l'importance du solvant d'étalement sur les isothermes de pression de surface et une restructuration de l'interface à forte compression qui apparaît sous la forme d'une relaxation de la pression de surface. Les observations de la monocouche par microscopie à l'angle de Brewster et diffusion de rayons X ont révélé l'existence d'une instabilité de flambage de l'interface. Lors de sa compression, l'interface se déforme et laisse apparaître des structures de grandes dimensions (5 à 35 micromètres), aisément observables malgré leur faible amplitude (quelques nanomètres). Nous avons montré que l'apparition du flambage résulte d'un échange de molécules entre l'interface et la sous-phase. Nous avons proposé différents mécanismes de déformation de la monocouche qui permettent de rendre compte qualitativement des différentes caractéristiques du flambage comme la dépendance de la taille des structures avec l'épaisseur de la monocouche, la dépendance de la pression d'apparition des structures avec la concentration en copolymère dans la sous-phase et l'insensibilité au sel. L'impossibilité de mesurer directement l'amplitude des structures à l'interface nous a conduit à développer une nouvelle méthode d'observation des monocouches à l'interface eau/air au moyen d'un microscope à force atomique. Les résultats obtenus sur différentes couches à l'interface eau/air montrent ainsi toutes les potentialités de cette nouvelle technique dans l'observation de monocouches présentant des domaines de taille inférieure à 100 nm.

This dissertation investigates the influence of surface heterogeneities on colloid deposition. First, deposition of colloidal particles on a nanofiltration membrane during cross flow membrane filtration was studied under different operating pressures and solution chemistries. An atomic force microscope (AFM) was then used to observe the deposit morphology formed on the membrane. At the initial stages of fouling, more particles preferentially accumulate near the peaks than in the valleys of the rough nanofiltration membrane surface. This study demonstrates that it is difficult to isolate, correlate and assess the effects that physical (roughness) heterogeneity and chemical heterogeneity has on colloid deposition based on experiments involving surfaces where the physical and chemical heterogeneities are uncorrelated or randomly distributed. In the second phase of the study, the deposition of model colloidal particles onto patterned charge-heterogeneous surfaces was studied both experimentally and theoretically. Controlled charge heterogeneity was created experimentally employing self assembled monolayers of alkanethiols patterned onto gold substrates using a soft lithographic technique. Model colloidal particles and fluorescent nanoparticles were sequentially deposited onto the patterned substrate under no flow (quiescent) conditions, and the deposited structures and the micro-patterns were imaged in situ using a combination of phase contrast and fluorescence microscopy. This study indicates that particles tend to preferentially deposit at the edges of the chemically favourable stripes. The theoretical investigation involved the formulation of a mathematical model based on Random Sequential Adsorption (RSA). This study showed that a simple binary probability distribution assumed in the model is able to predict the experimental deposit morphology adequately, particularly the periodicity of the underlying patterns on the substrate. Furthermore, the effect of charge heterogeneity on the electrostatic double layer interaction between a particle and a charge heterogeneous planar surface was studied numerically employing a 3D finite element model. In this system, significant lateral forces at close separation distances were observed, and found to be appreciably higher when the particle is near the edge of a heterogeneous region of the substrate. From the above studies, it can be concluded that by altering/controlling the chemical heterogeneity of the substrate, it is possible to achieve significant control on the resulting deposit morphology.

<p>Die vorliegende Arbeit beschäftigte sich mit zwei Themengebieten. Es ging zum einen um die mechanischen Eigenschaften von Polyelektrolythohlkapseln und zum anderen um die Adhäsion von Polyelektrolythohlkapseln.</p> <p>Die mechanischen Eigenschaften wurden mit der AFM „colloidal probe” Technik untersucht. Dabei zeigte sich, dass die Kraftdeformationskurven für kleine Deformationen den nach der Schalentheorie vorhergesagten linearen Verlauf haben. Ebenso wurde die quadratische Abhängigkeit der Federkonstanten von der Dicke bestätigt. Für PAH/PSS findet man einen E-Modul von 0.25 GPa. Zusammen mit der Tatsache, dass die Deformationskurven unabhängig von der Geschwindigkeit sind und praktisch keine Hysterese zeigen, sowie der Möglichkeit die Kapseln plastisch zu deformieren, kann man schließen, dass das System in einem glasartigen Zustand vorliegt.</p> <pt>Erwartungsgemäß zeigte der pH einen starken Einfluss auf die PEM. Während in einem pH-Bereich zwischen 2 und 11.5 keine morphologischen Änderungen festgestellt werden konnten, vergrößerte sich der Radius bei pH = 12 um bis zu 50 %. Diese Radienänderung war reversibel und ging einher mit einem sichtbaren Weicherwerden der Kapseln. Eine Abnahme des E-Moduls um mindestens drei Größenordungen wurde durch Kraftdeformationsmessungen bestätigt. Die Kraftdeformationskurven zeigen eine starke Hysterese. Das System befindet sich nun nicht mehr in einem glasartigen Zustand, sondern ist viskos bis gummiartig geworden.</p> <p>Messungen an Kapseln, die mit Glutardialdehyd behandelt wurden, zeigten, dass die Behandlung das pH-abhängige Verhalten verändert. Dies kann darauf zurückgeführt werden, dass das PAH durch den Glutardialdehyd quervernetzt wird. Bei einem hohen Quervernetzungsgrad, zeigen die Kapseln keine Änderung des mechanischen Verhaltens bei pH = 12. Schwach quervernetzte Kapseln werden immer noch signifikant weicher bei pH = 12, jedoch ändert sich der Radius nicht.</p> <p>Außerdem wurden Multilagenkapseln untersucht, deren Stabilität nicht auf elektrostatischen Wechselwirkungen sondern auf Wasserstoffbrückenbindungen beruhte. Diese Kapseln zeigten eine deutlich höhere Steifigkeit mit E-Modulen bis zu 1 GPa. Es wurde gefunden, dass auch dieses System für kleine Deformationen ein lineares Kraft-Deformationsverhalten zeigt, und dass die Federkonstante quadratisch von der Dicke abhängt. Die Kapseln lösen sich praktisch sofort bei pH = 6.5 auf. In der Nähe dieses pHs konnte das Abnehmen der Federkonstanten verfolgt werden.</p> <p>Außerdem wurde das Adhäsionsverhalten von PAH/PSS Kapseln auf mit PEI-beschichtetem Glas untersucht. Die Adhäsionsflächen waren zu einem großen Teil rund und ließen sich quantitativ auswerten. Der Adhäsionsradius nimmt mit dem Kapselradius zu und mit der Dicke ab. Das Verhalten konnte mit zwei Modellen, einem für die große und einem für die kleine Deformation beschrieben werden. Das große Deformationsmodell liefert um eine Größenordung niedrigere Adhäsionsenergien als das kleine Deformationsmodell, welches mit Werten von ‑0.2 mJ/m² Werte in einem plausiblen Bereich liefert. Es wurde gefunden, dass bei einem Verhältnis von Dicke zu Deformation von etwa eins "buckling" auftritt. Dieser Punkt markierte zugleich den Übergang von der großen zur kleinen Deformation.</p> / <p>This work had two objectives. The first was to study the mechanical properties of polyelectrolyte hollow capsules depending on the pH and the wall composition utilizing the AFM colloidal probe technique. The second objective was to study the adhesion of these capsules varying the radius and thickness.</p> <p>It was found that the AFM colloidal probe technique can be utilized to measure the shells spring constant. The acquired deformation curves for small deformations showed the linear force deformation relation predicted by the shell theory. The stiffness of capsules composed of polyallylamine and polystyrenesulfonate scales quadratically with the thickness giving Young’s modulus of 0.25 GPa.</p> <p>Changing the pH had no effect concerning the morphology and the elastic properties of the afore mentioned capsules up to pH = 11.5. At pH = 12 the capsules’ radius increased between 30 % and 50 % and the elastic modulus decreased by more than one order of magnitude. This pH-change can be viewed as the glass transition of the material, being in a glassy state below pH = 11.5 and becoming rubbery at pH = 12. Introducing crosslinking into the capsule wall inhibited any morphological changes at pH = 12, while leaving the capsule stiffness sensitive for small degrees of crosslinking.</p> <p>It was possible to prove the quadratic thickness dependency of the stiffness for a system where the internal forces keeping together the multilayer were hydrogen bonds (poly(methacrylic acid) / poly-4-vinylpyrrolidone). These apsules are significantly stiffer than the afore mentioned ones (Young’s modulus 0.6 to 1 GPa) and show a strong reaction towards increasing the pH by dissolving within seconds. Close to the dissolution pH these capsules also exhibit a transition from a glassy to a rubbery state.</p> <p>Capsules composed of polyallylamine / polystyrene sulfonate adhered to polyethylene imine coated glass. The radius of the adhesion disc increased with the radius of the capsule and decreased with increasing thickness. This behaviour could be semi quantitatively described with a strong and a weak deformation model giving a value for the adhesion energy of ‑0.02 mJ/m² for the small deformation model. At deformations of the order of the thickness, the theoretically predicted buckling was observed.</p>