A microscopia de varredura por tunelamento (MVT) é uma nova maneira de se observar experimentalmente o efeito Kondo. Quando uma concentração de átomos é adicionada a um meio metálico (metal hospedeiro), a corrente de tunelamento passa a depender de fatores de origem não geométrica. O rearranjo das cargas dentro do volume metálico (oscilações de Friedel) e o espalhamento de spins eletrônicos (efeito Kondo), devido a introdução de impurezas, mudam o valor da corrente e influenciam o levantamento da topografia do espécime examinado. Esses fatores devem ser considerados para que a topografia gerada seja condizente com a topografia verdadeira. Utilizamos como modelo teórico para descrição desse sistema, o modelo de Anderson de uma impureza para simular o espécime examinado e uma banda de condução livre para representar os elétrons da agulha metálica do microscópio. Nossa abordagem usa a fórmula de Kubo para o cálculo da corrente de tunelamento, supondo Hamiltoniano de tunelamento como perturbação e o potencial elétrico no regime linear. Apresentamos inicialmente um estudo para o Modelo do Nível Ressonante, isto é, o modelo de Anderson sem correlação, com o objetivo de demonstrar a precisão do método do Grupo de Renormalização Numérico. Em seguida, analisamos o Modelo de Anderson correlacionado. Os resultados tanto para a condutância em função da distância entre ponta e impureza a temperatura fixa, como para condutância em função da temperatura e distância fixa, permitem interpretação física transparente desde que levem em conta a ressonância de Kondo na densidade espectral. / The scanning tunneling microscopy (STM) is a new way to observe experimentally the Kondo effect. When a concentration of atoms id added to a sample (host metal), the tunneling current begins to depend on other non-geometric factors. The rearrangement of charges in the metallic bulk (Friedel oscillations) and the electronic spin scattering (Kondo effect), due to the presence of impurities, change the current value and affect the sample´s topography. These factors must be considered in order to make a correspondence between the generated topography with the true one. As a theoretical description of the system, we use the single impurity Anderson model to simulate the examined sample and a free conduction band to represent the electrons of the microscope metallic tip. Our treatment uses the Kubo formula to calculate the tunneling current, assuming the tunneling Hamiltonian as a perturbation and the electric potential in the linear regime. We initially present a study of the Resonant Level Model, i.e, the Anderson model without correlaction, to show the accuary of the Numerical Renormalization Group procedure. In the next step, we analyse the correlated Anderson model. The dependence of the conductance on tip-impurity distance, at constant temperature, and its dependence on temperature for constant tip-impurity distance, allow a clear physical interpretation after taking into account the Kondo resonance in the spectral density.

Les azobenzènes sont des molécules photochromes très étudiées, leurs propriétés de photoisomérisation donnent notamment lieu à d’importants phénomènes de transport de matière photoinduit en milieu polymère. L’objectif de cette thèse est de mieux comprendre les différents paramètres régissant de tels effets photomécaniques. Pour ce faire, nous avons entrepris des études sur des monocouches de dérivés azobenzenes auto-assemblés ou SAMs (Self-Assembled Monolayers) sur substrats d’or atomiquement plans, de façon à pouvoir réaliser à la fois des caractérisations d’ensemble (propriétés de mouillage : mesures d’angles de contact) ou des caractérisations à l’échelle de la molécule individuelle (par microscopie à effet tunnel, STM). Nous avons choisi le système d'un alcane thiol fonctionnalisé par un azobenzène pour bénéficier de la facilité que possèdent les alcanes thiols à former des SAMs hautement organisées. L'autre avantage que propose ce système est de pouvoir jouer avec la longueur de la chaîne carbonée, ce qui permet d'espacer plus ou moins le groupe photo-actif de la surface métallique du substrat d'or. Ainsi ces systèmes paraissent bien adaptés pour identifier l’influence de paramètres tels que (1) les phénomènes de transferts d’énergie ou de charges pouvant exister entre molécules ou entre molécules et substrat et (2) les problèmes éventuels de gêne stérique.Nous avons étudié des azobenzenes greffés (1) sur une courte chaîne alcanethiol (3 atomes de C - "azoc3"), et (2), sur une chaîne plus longue (12 C - "azoc12"). Les caractérisations STM ont mis en évidence un réseau dense similaire aux alcanes thiols non substitués dans le cas des "azosc3" mais aucune modification sous éclairement n’a été observée. Des mesures d’angle de contact, utilisant le changement de polarité entre les photoisomères Trans et Cis, confirment cette absence de réaction pour ces SAMs. Une photo-réactivité a par contre été mise en évidence par des mesures d'angle de contact pour l’espaceur long ("azoc12") sur des substrats d’or polycristallins. Les raisons d'un tel comportement proviennent des propriétés intrinsèques d'une SAM d'alcanethiol : un réseau très dense implique un volume libre restreint et d'importantes interactions intermoléculaires qui ajoutent des voies supplémentaires de désexcitation et donc inhibent les changements de conformation photo-induits, à l’exception des zones de défauts. Ces conclusions ont pu être étayées par des caractérisations complémentaires de fluorescence de SAMs similaires. Nous avons pu démontrer qu’un espaceur alcane correspondant à 11 C permettait un découplage électronique suffisant pour observer de la fluorescence sur des SAMs non denses de dérivés thiolés de fluorescéine. Ces études ont cependant confirmé la difficulté de contrôler la densité surfacique de molécules photosensibles (photochromisme ou fluorescence) insérées au sein de SAMs de thiols. A ce titre, nous avons pu démontrer l’intérêt d’un système moléculaire original possédant une accroche en surface d’encombrement supérieur à celui de la base soufrée des alcanes thiols. Ces systèmes se physisorbent sur des surfaces de graphite, leurs propriétés d’auto-assemblage étant également conservées dans le cas de substrat de graphène sur Cuivre. Ces feuillets possèdent l’avantage d’être facilement transférables sur des substrats transparents. Ces travaux offrent ainsi de nouvelles perspectives pour la réalisation d'expériences in situ simultanées couplant des analyses topographiques à l'échelle moléculaire via l’utilisation de sondes locales (STM ou AFM) et mesures optiques à plus grande échelle (microscopie optique inversée), ouvrant la voie à des caractérisations plus approfondies. / One of the most widely studied type of photochromic molecules are azo-dye molecules whose photo-isomerization reaction entails important mass transport processes in polymer matrices.The aim of this PhD Thesis was to get a better understanding of the key parameters controling such photomechanical properties. For this purpose, we have driven some studies on Self-Assembled Monolayers (SAMs) formed by azobenzene derivatives grafted to alkanethiol linkers on atomically flat gold surfaces. Self-assembling offers the possibility to perform both scanning tunnelling microscopy (STM) experiments at the molecular scale and contact angle measurement at the macroscopic scale.A derivative alkanethiol system has been chosen to take benefit from the alkanethiol ability to form dense and regular self-assembled monolayers. Another interest of the alkanethiol linker is to easily adjust distance between the azo moiety and the gold surface through the length of the alkane chain. Then, this type of SAMs seems to be the good candidate to study the influence of (1) charges coupling effects between the neighbouring azo moieties and between the molecules and the metallic surface and (2) steric hindrance problems. Both a short (3 carbon atoms – “azoC3”) and a long (12 carbon atoms – “azo-C12”) alkane linker have more specifically been investigated. Although STM characterization have shown that azoC3 was organizing into a dense and regular packing showing a network quite similar to the one observed in the case of alkanethiols self-assembly, no modification could be evidenced upon illumination. Polarity modification between the trans and cis photoisomers, probed by wetting measurements, confirm the lack of photo-reactivity of these molecules grafted through a short linker to the metallic substrate. However, photo-isomerization effects have been observed at the macroscopic scale on polycristallines substrates for the long linker (“azo-C12”). The reasons for such behavior come from the intrinsic properties of alkanethiol SAMs: a dense network involves a restricted free volume and significant intermolecular interactions that add additional de-excitation channels thus leading to a quenching of the photoinduced conformational changes, except at surface defects areas. These findings have been supported by complementary characterizations of fluorescence of similar thiolates SAMs. We have demonstrated that an alkane spacer corresponding to 11 C was allowing electronic decoupling thus leading to fluorescent emission in the case of non-dense thiolated SAMs of fluorescein derivatives. However, these studies have also confirmed the difficulty to control the dilution of photosensitive molecules (photochromic or fluorescent) inserted within thiolated SAMs. In order to overcome this problem, we have demonstrated the interest of another original molecular system whose extended head group permits the formation of a network less densely-packed than those formed by alkanethiol derivatives. These molecular systems self-assemble by physisorption onto graphite (HOPG) substrates, their organization properties being also maintained for graphene sheets deposited onto copper substrates. Advantageously, these sheets can easily be transferred to transparent substrates. These studies open thus new perspectives for the realization of simultaneous in-situ experiments coupling molecular scale topography informations using local probe microscopy (AFM or STM) and larger scale optical measurents (inverted optical microscopy) towards more accurate characterizations.

Les nano-structures sont des systèmes physiques de premier intérêt pour les études de base et pour les applications, car elles montrent des effets quantiques comme le confinement, la discrétisation énergétique, la cohérence... Le comportement quantique des nano-dispositifs peut être cependant fortement influencé par le désordre, les effets thermiques et hors-équilibre. Dans cette Thèse, nous montrons, par exemple, comment la dissipation affecte le transport électronique dans les dispositifs supraconducteurs soumis aux fréquences micro-ondes.En utilisant un setup cryogénique AFM/STM fait maison, on peut étudier différents types de nano-structures. En premier, nous nous occupons de la réalisation d'un transistor à électron unique avec une sonde locale. Les nano-particules métalliques sont bien connues pour leur comportement comme boîtes quantiques zéro-dimensionnelles (QD), elles montrent du confinement quantique et des effets de charge, que l’on retrouve aussi dans nos mesures de microscopie à sonde locale à basse température. Nous démontrons comment un nouveau procédé de nano-fabrication peut être mis en œuvre avec l'introduction d' une électrode de grille suffisamment mince et sans-fuite, ce qui pourra fournir un réglage de précision des propriétés de la boîte quantique et permettre l'exploration résolue spatialement des phénomènes quantiques dans différents régimes de couplage. En deuxième, nous étudions le graphène épitaxial sur SiC comme un matériau 2D très prometteur pour l'électronique. En particulier, les nano-rubans de graphène obtenus par croissance épitaxiale sur des parois inclinées (GNRs) sont des nano-structures d'intérêt fondamental qui peuvent fournir un accès direct et contrôlable au graphène neutre. À cause du confinement quantique, ces systèmes peuvent montrer du transport balistique exceptionnel à température ambiante. Nous réalisons une technique novatrice de potentiométrie à sonde locale qui nous permet une résolution spatiale à l'échelle du nm et une résolution en tension à l'échelle du µV. Le potentiel locale et la résistance locale mesurés sur un dispositif unique basé sur des nano-rubans de graphène nous donnent des indications claires de transport non-diffusif.La physique explorée, les méthodes ainsi que les technique développées dans cette Thèse peuvent donc fournir des nouvelles visions aux nombreux (et assez divers) sujets en vogue. / Nanostructures are physical systems of paramount interest for both fundamental studies and applications, since they display quantum effects such as confinement, energy discretization, coherence…The quantum behavior of nano-devices can however be strongly influenced by disorder, thermal and non-equilibrium effects. In this Thesis, we show, for instance, how dissipation deeply affects the electron transport in superconducting nano-devices at microwave frequencies.By using a home-made cryogenic AFM/STM setup, we are able to investigate different kinds of nanostructures. First, we address the realization of a Single Electron Transistor with a Scanning Probe. Metallic nanoparticles are well known for their behavior as 0D-Quantum Dots (QD), and they display quantum confinement and charging effects, which are evidenced in our low-temperature SPM measurements as well. We demonstrate how a novel nanofabrication process can be implemented with the addition of gate electrodes sufficiently thin and leakage-proof, which in the future can provide a fine-tuning of the QD's properties and allow spatially-resolved exploration of quantum phenomena in a variety of different coupling regimes. Second, we study epitaxial graphene on SiC as a very promising 2D material for electronics. In particular, epitaxial sidewalls graphene nanoribbons (GNRs) are nanostructures of fundamental interest which can provide direct and controllable access to charge neutral graphene. Due to quantum confinement, these systems can display exceptional ballistic transport at room temperature. We implemented an innovative Scanning Tunneling Potentiometry technique allowing for nm-scale spatial resolution and μ V-scale voltage resolution. Measured local potential and resistance of single GNRs devices provide clear indication of non-diffusive transport.The physics investigated and the methods and the techniques developed in this Thesis can thus provide a new insight on several (and quite diverse) on-trend topics.

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.

Crystallographic image processing (CIP) is a technique first used to aid in the structure determination of periodic organic complexes imaged with a high-resolution transmission electron microscope (TEM). The technique has subsequently been utilized for TEM images of inorganic crystals, scanning TEM images, and even scanning probe microscope (SPM) images of two-dimensional periodic arrays. We have written software specialized for use on such SPM images. A key step in the CIP process requires that an experimental image be classified as one of only 17 possible mathematical plane symmetry groups. The current methods used for making this symmetry determination are not entirely objective, and there is no generally accepted method for measuring or quantifying deviations from ideal symmetry. Here, we discuss the crystallographic symmetries present in real images and the general techniques of CIP, with emphasis on the current methods for symmetry determination in an experimental 2D periodic image. The geometric Akaike information criterion (AIC) is introduced as a viable statistical criterion for both quantifying deviations from ideal symmetry and determining which 2D Bravais lattice best fits the experimental data from an image being processed with CIP. By objectively determining the statistically favored 2D Bravais lattice, the determination of plane symmetry in the CIP procedure can be greatly improved. As examples, we examine scanning tunneling microscope images of 2D molecular arrays of the following compounds: cobalt phthalocyanine on Au (111) substrate; nominal cobalt phthalocyanine on Ag (111); tetraphenoxyphthalocyanine on highly oriented pyrolitic graphite; hexaazatriphenylene-hexacarbonitrile on Ag (111). We show that the geometric AIC procedure can unambiguously determine which 2D Bravais lattice fits the experimental data for a variety of different lattice types. In some cases, the geometric AIC procedure can be used to determine which plane symmetry group best fits the experimental data, when traditional CIP methods fail to do so.

Extending electronic devices beyond the limitations of current micro-electronics manufacturing will require detailed knowledge of how to make contacts to semiconductor surfaces. In this work, we investigated several methods by which such connections to silicon surfaces could be achieved. Scanning tunneling microscopy (STM) was our main experimental tool, allowing direct imaging of the surfaces at the atomic level. First, the growth of self-forming linear nanostructures of organic molecules on silicon surfaces offers a possibility of creating devices with hybrid organic-silicon functionality. We have studied the growth of many different molecules on a variety of hydrogen-terminated silicon surfaces: H-Si(100)-2x1, H-Si(100)-3x1, and H-Si(111)-1x1. We found molecular growth patterns affected by steric crowding, by sample doping level, or by exposure to ion-pump created radicals. We formed the first contiguous "L-shaped" molecular lines, and used an external electric field to direct molecular growth. We attempted to study a novel method for nanoscale information transfer along molecular lines based on excitation energy transfer. The second part of the work focuses on the development and use of a new multiple-probe STM instrument. The design and the custom STM control software written for it are described. Connections to Si surfaces were achieved with a combination of lithographically defined metal contacts and STM tips. Two-dimensional surface conductivity of the Si(111)-7x7 surface was measured, and the effect of modifying the surface with organic molecules was investigated. A novel method, scanning tunneling fractional current imaging (STFCI), was developed to further study surface conductance. This method allowed us to determine, for the first time, that the resistance of steps on the Si(111)-7x7 surface is significantly higher than that of the surface alone.

Our group has developed a family of single molecules termed nanocars, which are aimed at performing controllable motion on surfaces. In this work, a series of light-activated motorized nanomachines incorporated with a MHz frequency light-activated unidirectional rotary motor were designed and synthesized. We hope the light-activated motor can serve as the powering unit for the nanomachines, and perform controllable translational motion on surfaces or in solution. A series of motorized nanovehicles intended for scanning tunneling microscopy (STM) imaging were designed and synthesized. A p-carborane-wheeled motorized nanocar was synthesized and monitored by STM. Single-molecule imaging was accomplished on a Cu(111) surface. However, further manipulations did lead to motor induced lateral motion. We attributed this result to the strong molecule-surface interactions between the p-carborane-wheeled nanocar and the Cu(111) surface. To fine-tune the molecule-surface interactions, an adamantane-wheeled motorized nanocar and a three-wheel nanoroadster were designed and synthesized. In addition, the STM substrates will be varied and different combinations of molecule-surface interactions will be studied. As a complimentary imaging method to STM, single-molecule fluorescence microscopy (SMFM) also provides single-molecule level resolution. Unlike STM experiment requires ultra-high vacuum and conductive substrate, SMFM experiment is conducted at ambient conditions and uses non-conductive substrate. This imaging method allows us to study another category of molecule-surface interactions. We plan to design a fluorescent motorized nanocar that is suitable for SMFM studies. However, both the motor and fluorophore are photochemically active molecules. In proximity, some undesired energy transfer or interference could occur. A cyanine 5- (cy5-) tagged motorized nanocar incorporated with the MHz motor was designed and synthesized in order to minimize the potential energy transfer or interference between the motor and the fluorophore. The SMFM study of this cy5-tagged motorized nanocar is currently undergoing. The design of light-activated motorized nanocar inspired the design of nanosubmarines. We used fluorescence quenching and fluorescence correlation spectroscopy (FCS) to study the diffusion of single molecules. The fluorescence quenching experiments of Ru(bpy)3+2 by a quenching nanosubmarine was conducted, but no motor induced acceleration of the molecule were observed. Another fluorescent nanosubmarine was monitored by FCS, and no increase of diffusion coefficient was found. Finally, a 1-D channel approach was adopted for decreasing the effects of Brownian motion, and acceleration of nanosubmarine was observed.

This work investigates the electronic properties of very small gold and semiconductor particles using Scanning Tunneling Microscopy/Spectroscopy (STM/STS) and Ballistic Electron Emission Spectroscopy (BEES). Complementary theoretical works were also performed. The first theoretical work was to calculate the quantized states in the CdS/HgS/CdS quantum-well-quantum-dot nanocrystals. An eight-band envelope function method was applied to this system. This method treats exactly the coupling between the conduction bands, the light-hole bands, the heavy-hole bands, and the spin-orbit split bands. The contributions of all other bands were taken into account using second order perturbation theory. Gold nanocrystals with diameters of 1.5 nm have discrete energy levels with energy spacings of about 0.2 eV. These values are comparable to the single electron charging energy, which was about 0.5 eV in our experimental configuration. Since bulk gold doesnt have an energy gap, we expect the electron levels both below and above the Fermi level should be involved in the tunneling. Measured spectroscopy data have rich features. In order to understand and relate these features to the electronic properties of the nanocrystals, we developed a tunneling model. This model includes the effect of excited states that have electron-hole pairs. The relaxation between discrete electron energy levels can also be included in this model. We also considered how the nanocrystals affect the BEES current. In this work an ultra-high vacuum and low-temperature STM was re-designed and rebuilt. The BEEM/BEES capabilities were incorporated into the STM. We used this STM to image gold nanocrystals and semiconductor nanocrystals. STS and BEES spectra of gold nanocrystals were collected and compared with calculations.

Elastic and inelastic interactions are studied in tunnel junctions of a superconducting nanoelectromechanical setup and in response to resent experimental superconducting scanning tunneling microscope findings on a paramagnetic molecule. In addition, the electron density of molecular graphene is modeled by a scattering theory approach in very good agreement with experiment. All studies where conducted through the use of model Hamiltonians and a Green function formalism. The nanoelectromechanical system comprise two fixed superconducting leads in-between which a cantilever suspended superconducting island oscillates in an asymmetric fashion with respect to both fixed leads. The Josephson current is found to modulate the island motion which in turn affects the current, such that parameter regions of periodic, quasi periodic and chaotic behavior arise. Our modeled STM setup reproduces the experimentally obtained spin excitations of the paramagnetic molecule and we show a probable cause for the increased uniaxial anisotropy observed when closing the gap distance of tip and substrate. A wider parameter space is also investigated including effects of external magnetic fields, temperature and transverse anisotropy. Molecular graphene turns out to be well described by our adopted scattering theory, producing results that are in good agreement with experiment. Several point like scattering centers are therefore well suited to describe a continuously decaying potential and effects of impurities are easily calculated.

No description available.