Cette thèse présente une étude des propriétés électroniques de systèmes de basse dimension à base de métaux et de semiconducteurs. La première partie de l'étude traite le confinement de l'état de Shockley dans des nanostructures tridimensionnelles d'Ag(111), par des mesures STM/STS à très basse température (5 K). Nous avons d'abord analysé en détail la structure en énergie et la distribution spatiale des modes confinés. Nous avons ensuite mis à profit la nature discrète du spectre en énergie pour étudier le temps de vie des quasiparticules. Un comportement typique de liquide de Fermi est mis en évidence, et nous montrons que le mécanisme de diffusion dominant est associé au couplage électron-phonon. La contribution extrinsèque provenant du confinement partiel de l'onde électronique a également été obtenue. Une loi d'échelle est observée avec la taille des nanostructures, ce qui permet d'extraire un coefficient de réflexion plus important que dans de simples ilôts monoatomiques. La seconde partie de l'étude est consacrée aux couches ultra-minces semiconductrices obtenues par dépôts d'alcalins (K, Rb, Cs) sur la surface Si(111):B-[racine]3. Ce travail résout la controverse concernant la nature de l'état fondamental de ce système, et notamment l'origine de la reconstruction 2[racine]3 obtenue à la saturation du taux de couverture. La compréhension en amont de la structure cristallographique permet d'élucider les propriétés électroniques. Nous montrons qu'une approche à un électron, conduisant à un isolant de bandes, décrit le système de manière convaincante, malgré l'indication de forts effets polaroniques. Ce résultat est le fruit d'une étude approfondie combinant des techniques diverses et complémentaires (LEED, ARPES, XPS, STM/STS et calcul DFT) / This thesis is devoted to the electronic properties of low-dimensional systems based on metal and semiconducting materials. The first part deals with the Shockley state confinement in Ag(111) nanostructures, by means of very-low temperature (5 K) STM/STS measurements. We study the electronic structure and spatial distribution of the confined modes. Then the discrete nature of the electronic spectrum allows one to yield the quasiparticule lifetime. A Fermi-liquid behaviour is evidenced and we show that the dominant decay mechanism is attributed to the electron-phonon coupling. The extrinsic contribution arising from the partial confinement of the electronic wave is obtained as well. A scaling law with the nanostructure width is demonstrated, from which we deduce a higher reflection amplitude than in monoatomic islands. In the second part of the thesis, we study semiconducting ultra-thin films produced by alkali (K, Rb, Cs) deposition on the Si(111):B-[root of]3 surface. This work solves the controversy concerning the ground state of this system, and especially the nature of the 2[root of]3 surface recontruction obtained at saturation coverage. Prior understanding of the crystallographic structure allows to elucidate the electronic properties. We show that a one-electron picture, leading to a band insulator scenario, gives a good description of the system, in spite of strong polaronic effects. This conclusion results from an in-depth, combined study of complementary techniques (LEED, ARPES, XPS, STM/STS and DFT calculations).

In der vorliegenden Dissertation werden das Selbstaggregationsverhalten und die elektronischen Eigenschaften von vier expandierten pi-konjugierten Makrozyklen in geordnete supramolekulare Architekturen mit Hilfe von Rastertunnelmikroskopie (STM) und Tunnelspektroskopie (STS) an Fest-Flüssig-Grenzflächen zwischen organischen Moleküllösungen und der Basalfläche von Graphit untersucht. Zwei Makrozyklen sind die Fotoisomere Z,Z–8T6A und E,E–8T6A, in denen sechs Ethynylengruppen und zwei cis- bzw. trans-Ethylen erhalten sind. STM-Bilder zeigen 2-dimensionale hexagonale Gitter. Strom-Spannungs-Kennlinien bestätigten den erwarteten donor-artigen Charakter der Makrozyklen. Das Schalten von Z,Z–8T6A zu E,E–8T6A wird durch STS zyklische Messungen angezeigt, nachdem die stabilste kationische Spezies ausgebildet wurde. Diese Ergebnisse stellen das erste elektrochemische Schalten unter Standard STM Bedingungen dar. Außerdem wurden die Photoisomerisierungen zwischen Z,Z-8T6A und E,E-8T6A an der Fest-Flüssig-Grenzfläche beobachtet. Eine selbstorganisierte Monoschicht aus Wasserstoffbrücken-gebundenen Trimesinsäuren an der Fest-Flüssig-Grenzfläche bildet Wirtsstellen für die epitaktische Anordnung von Fullerenen mit E,E–8T6A Komplexen in Mono- und Doppelschichten aus. Mit Hilfe der STM-Tomographie wird die Bildung der Templatschicht überprüft. Die Konformationsstabilität und die Adsorptionsstellen der Monoschichten werden mit der Hilfe von Molekulardynamik-Simulation bestätigt. Die STS-Experimente zeigen die Modifikation der gleichrichtenden Eigenschaften der Makrozyklen durch die Bildung von Donor-Akzeptor-Komplexen in einer dicht gepackten, selbstorganisierten supramolekularen Nanostruktur. Die Kombination von Wirt-Gast-Komplexen mit der Schaltfähigkeit und den elektronischen Transporteigenschafte von makrozyklischen Oligothiophenen prädestinieren diese als Kandidaten für Anwendungen in supramolekular konstruierten Systemen mit gewünschten (opto)elektronischen Eigenschaften. / The present thesis concerns to the self-assembly and the electronic properties of four pi-expanded macrocycles into ordered supramolecular architectures, investigated by means of scanning tunneling microscopy (STM) and spectroscopy (STS) at the solid-liquid interface between organic solutions and the basal plane of graphite. Two macrocycles are the photoisomers Z,Z–8T6A and E,E–8T6A, which contain six ethynylenes and two cis- and trans-ethylenes in opposite positions of the ring, respectively. STM images reveal hexagonally ordered 2D-networks. Current–voltage characteristics confirm the expected donor-like character of the macrocycles. Cyclic STS measurements indicate that Z,Z–8T6A switches to E,E–8T6A after formation of a most stable cationic species. This result represents the first reported electrochemical switching experiment under standard STM conditions. Additionally, the reversible photoisomerization between Z,Z-8T6A and E,E-8T6A upon irradiation was recognized at the solid-liquid interface. Moreover, a self-assembled monolayer of hydrogen-bonded trimesic acid at the solid-liquid interface provides host-sites for the epitaxial ordering of Saturn-like complexes of fullerenes with E,E–8T6A macrocycles in mono- and bilayers. STM tomography verifies the formation of the templated layer, while molecular dynamics simulations corroborate the conformational stability and assign the adsorption sites of the adlayers. STS measurements reveal the modification of the rectifying properties of the macrocycles by the formation of donor-acceptor complexes in a densely packed all-self-assembled supramolecular nanostructure. The combination of host-guest complexation and the switching capabilities and electronics transport characteristics of cyclic oligothiophenes render them candidates for applications in the study of supramolecular engineered systems with desirable (opto)electronic properties.

La synthèse du graphène par traitement thermique d’un substrat de carbure de silicium (SiC) est une technique prometteuse pour l’intégration de ce nouveau matériau dans l’industrie, notamment dans les dispositifs électroniques. L’avantage de cette méthode réside dans la croissance de films minces de graphène de taille macroscopique directement sur substrat isolant. Toutefois, avant d’intégrer ce matériau, il convient d’en contrôler la synthèse et d’en moduler les propriétés. Dans ce travail de thèse, nous étudions les propriétés structurales et électroniques du graphène obtenu par la graphitisation des polytypes 3C-, 4H- et 6H-SiC. A partir de diverses méthodes de caractérisation, telles que la diffraction des électrons lents (LEED) ou la microscopie et spectroscopie à effet tunnel (STM/STS), nous avons vérifié, dans un premier temps, que le caractère discontinu du graphène sur les bords de marches peut introduire un confinement latéral supplémentaire des électrons dans le graphène. Dans un second temps, l’observation des singularités de Van Hove nous a permis de démontrer l’effet de confinement unidimensionnel dans les régions d’accumulations de marches du SiC. Enfin, l’introduction de désordre dans nos couches de graphène induit une réduction de la densité de porteurs de charges dans les couches. De même, ce désordre conduit à une transition de phase quantique entre le régime localisé et le régime d’effet Hall quantique. / The synthesis of graphene by thermal decomposition of silicon carbide (SiC) is a promising technique for the integration of this new material in the industry, especially in electronic devices. The advantage of this method lies in the growth of macroscopic graphene films directly on an insulator substrate. However, before using this material in electronic devices, it is advisable to control its synthesis and modulate its properties. In this thesis, we present the structural and electronic properties of graphene obtained by graphitization of 3C- , 4H - and 6H- SiC polytypes. Various characterization methods were used, including low energy electron diffraction (LEED) and microscopy and scanning tunneling spectroscopy (STM / STS). Based on STM / STS measurements, we show that the discontinuity of epitaxial graphene at the step edges may introduce an additional lateral confinement of electrons in graphene. The observation of Van Hove singularities in the STS spectra confirmed the one dimensional confinement of graphene in step bunching regions of SiC.Finally, we show that when disorder is introduced on our graphene samples, the charge carrier density is reduced. This disorder lead to the observation of a quantum phase transition from a localized regime to a quantum Hall effect regime.

As interessantes propriedades eletrônicas, mecânicas e óticas dos materiais orgânicos conjugados fizeram emergir diversas aplicações tecnológicas e comerciais em dispositivos baseados nesses materiais, tais como sensores, memórias, células solares e diodos emissores de luz poliméricos (LEDs). Neste sentido, o tema central desta tese é o estudo das propriedades elétricas e morfológicas e os mecanismos de transporte eletrônico de cargas no PEDOT:PSS, uma blenda polimérica que consiste de um policátion condutivo, o poli(3,4- etilenodioxitiofeno) (PEDOT) e do poliânion poli(estirenosulfonado) (PSS). PEDOT:PSS é amplamente usado como material de eletrodo em aplicações na área de eletrônica plástica, como mencionado anteriormente. Apesar da condutividade elétrica dos filmes finos de PEDOT:PSS possa variar várias ordens de grandeza, dependendo do método pela qual é processado e transformado em filme fino, as razões para este comportamento é essencialmente desconhecido. Esta tese descreve um estudo detalhado do transporte eletrônico de cargas anisotrópico e sua correlação com a morfologia, as condições e as dimensões da separação de fase entre os dois materiais, PEDOT e PSS. Antes de abordar as propriedades do PEDOT:PSS, uma camada de filme fino inorgânica usada para aumentar o tempo de vida de dispositivos orgânicos é descrita no Capítulo 2. Um importante mecanismo de degradação em LEDs poliméricos é a fotooxidação da camada ativa. Assim, isolar a camada ativa da água, oxigênio e luz, torna-se crucial para o aumento do tempo de vida. Um sistema de deposição química a partir da fase de vapor estimulada por plasma (PECVD) é usado para depositar filmes finos de nitreto de carbono em baixas temperaturas, menores que 100 °C, sobre PLEDs com a intenção de aumentar o tempo de vida destes dipositivos e diminuir a fotodegradação do poli[2-metoxi-5- (2-etil-hexiloxi)-p-fenileno vinileno] (MEH-PPV) em ambiente atmosférico. O filme fino de nitreto de carbono possui as características de um material que pode bloquear a umidade e que tem espessura e flexibilidade adequados para a nova geração de PLEDs flexíveis. As características dos filmes finos de nitreto de carbono e MEH-PPV foram investigadas usando-se técnicas de espectroscopia ótica, com particular ênfase no processo de degradação do MEHPPV sob iluminação. Os resultados mostraram que o filme fino de nitreto de carbono protege o filme polimérico e diminui consideravelmente a fotooxidação. Para avaliar o efeito do encapsulamento em dispositivos reais, LEDs poliméricos foram fabricados e pelas curvas de corrente-tensão um aumento no tempo de vida é confirmado quando a camada de nitreto de carbono é presente. O tempo de vida desejado, maior que 10.000 horas, para aplicações comerciais não foi atingido, entretanto, o encapsulamento pode ser melhorado otimizando as propriedades da camada de nitreto de carbono e combinando-as com camadas de outros materiais orgânicos e inorgânicos. Os capítulos seguintes deste trabalho aborda os estudos realizados com o PEDOT:PSS, uma vez que é amplamente usado em eletrônica orgânica, mas relativamente tem recebido pouca atenção com respeito ao transporte eletrônico de cargas, bem como sua correlação com a morfologia. No Capítulo 3, experimentos com microscopia de varredura por sonda (SPM, Scanning Probe Microscopy) e medidas de condutividade macroscópica são utilizados para estudar e obter um modelo 3D morfológico completo que explica, qualitativamente, a condutividade anisotrópica observada nos filmes finos de PEDOT:PSS depositados pela técnica de spin coating. Imagens topográficas de microscopia de varredura por tunelamento (STM) e imagens da seção transversal observadas com o microscópio de forca atômica (X-AFM) revelaram que o filme fino polimérico é organizado em camadas horizontais de partículas planas ricas em PEDOT, separadas por lamelas quasi-contínuas de PSS. Na direção vertical, lamelas horizontais do isolante PSS reduzem a condutividade e impõe o transporte eletrônico a ser realizado por saltos em sítios vizinhos próximos (nn-H, nearest-neighbor hopping) nas lamellas de PSS. Na direção lateral, o transporte eletrônico via saltos 3D em sítios a longas distâncias (3D-VRH, variable range hopping) ocorre entre as ilhas ricas em PEDOT que são separadas por barreiras muito mais finas de PSS, causando um aumento da condutividade nesta direção. Esta discussão é estendida ao Capítulo 4 com uma descrição quantitativa do transporte eletrônico de cargas predominantes. Particularmente, é demonstrado que o transporte de cargas via saltos 3D em sítios a longas distâncias ocorre entre ilhas ricas em PEDOT e não entre segmentos isolados de PEDOT ou dopantes na direção lateral, enquanto que na direção vertical o transporte de cargas via saltos em sítios vizinhos próximos ocorre dentro das lamelas do quasi-isolante PSS. Em algumas aplicações, faz-se necessário usar PEDOT:PSS com alta condutividade elétrica. Isso pode ser feito adicionando-se sorbitol à solução aquosa de PEDOT:PSS. Após um tratamento térmico, e dependendo da quantidade de sorbitol adicionado, a condutividade aumenta várias ordens de grandeza e as causas e consequências de tal comportamento foram investigadas neste trabalho. O Capítulo 5 investiga as várias propriedades tecnológicas do PEDOT:PSS altamente condutivo tratado com sorbitol, tais como a própria condutividade, os efeitos dos tratamentos térmicos e exposição à umidade. É observado que o aumento da condutividade elétrica, devido à adição de sorbitol na solução aquosa, é acompanhado por uma melhoria na estabilidade da condutividade elétrica em condições atmosféricas. Surpreendentemente, a condutividade elétrica do PEDOT:PSS, sem tratamento com sorbitol (~ 10-3 S/cm), aumenta mais de uma ordem de grandeza sob ambiente úmido de 30-35 % umidade relativa. Este efeito é atribuido a uma contribuição iônica à condutividade total. Análise Temogravimetrica (TGA), espectrometria de massa com sonda de inserção direta (DIP-MS) e análise calorimétrica diferencialmodulada (MDSC) foram usadas como técnicas adicionais para o entendimento dos estudos deste Capítulo. No Capítulo 6, microscopia de varredura por sonda-Kelvin (SKPM) foi empregada para medir o potencial de superfície dos filmes finos de PEDOT:PSS tratados com diferentes concentrações de sorbitol. Mostra-se que a mudança no potencial de superfície é consistente com uma redução de PSS na superfície do filme fino. Para estudar o transporte eletrônico nos filmes finos de PEDOT:PSS altamente condutivos tratados com sorbitol, o Capítulo 7 usa medidas de temperatura e campo elétrico em função da conduvitidade correlacionados com analises morfológicas realizadas por STM. É observado que o transporte eletrônico por saltos, na direção lateral, muda de 3D-VRH para 1D-VRH quando o PEDOT:PSS é tratado com sorbitol. Esta transição é explicada por uma auto-organização das ilhas ricas em PEDOT em agregados 1D, devido ao tratamento com sorbitol, tornando-se alinhadas em domínios micrométricos, como observado pelas imagens de STM. / Employing the unique mechanical, electronic, and optical properties of the conjugated organic and polymer materials several technological and commercial applications have been developed, such as sensors, memories, solar cells and light-emitting diodes (LEDs). In this respect, the central theme of this thesis is the electrical conductivity and mechanisms of charge transport in PEDOT:PSS, a polymer blend that consists of a conducting poly(3,4-ethylenedioxythiophene) polycation (PEDOT) and a poly(styrenesulfonate) polyanion (PSS). PEDOT:PSS is omnipresent as electrode material in plastic electronics applications mentioned above. Although the conductivity of PEDOT:PSS can vary by several orders of magnitude, depending on the method by which it is processed into a thin film, the reason for this behavior is essentially unknown. This thesis describes a detailed study of the anisotropic charge transport of PEDOT:PSS and its correlation with the morphology, the shape, and the dimension of the phase separation between the two components, PEDOT and PSS. Before addressing the properties of PEDOT:PSS, a new barrier layer is described in Chapter 2 that enhances the lifetime of organic devices. An important degradation mechanism in polymer LEDs is photo-oxidation of the active layer. Hence, isolating the active layer from water and oxygen is crucial to the lifetime. Plasma-enhanced chemical vapor deposition (PECVD) is used to deposit a thin layer of carbon nitride at low deposition temperatures, below 100 °C, on a polymer LED that uses poly[2-methoxy-5-(2´-ethylhexyloxy)-1,4- phenylene vinylene] (MEH-PPV) as active layer. A thin layer of carbon nitride acts as barrier for humidity, but is still sufficiently bendable to be used in flexible polymer LEDs. The characteristics of carbon nitride and MEH-PPV films have been investigated using optical spectroscopy, with particular emphasis on the degradation process of MEH-PPV under illumination. The measurements show that the carbon nitride coating indeed protects the polymer film and diminishes the photo-oxidation considerably. To study the effect of the encapsulation in real devices, polymer LEDs were made and their current-voltage characteristics confirm the enhanced lifetime in the presence of a carbon nitride barrier layer. However, the target, a lifetime of more than 10,000 hours for commercial applications, was not achieved. The remaining chapters of this thesis describe the investigations of PEDOT:PSS. PEDOT:PSS is widely used in organic electronics. So far, relatively little attention has, been paid to the mechanisms of charge transport in this material and the correlation of those properties to the morphology. In Chapter 3, scanning probe microscopy (SPM) and macroscopic conductivity measurements are used to obtain a full 3D morphological model that explains, qualitatively, the observed anisotropic conductivity of spin coated PEDOT:PSS thin films. Topographic scanning probe microscopy (STM) and cross-sectional atomic force microscopy images (X-AFM) reveal that the thin film is organized in horizontal layers of flattened PEDOT-rich particles that are separated by quasi-continuous PSS lamella. In the vertical direction, the horizontal PSS insulator lamellas lead to a reduced conductivity and impose nearest-neighbor hopping (nn-H) transport. In the lateral direction, 3D variable-range hopping (3D-VRH) transport takes place between PEDOT-rich clusters which are separated by much thinner barriers, leading to an enhanced conductivity in this direction. This discussion is extended in Chapter 4, where a quantitative description of the length scales of the predominant transport is obtained. Particularly, it is demonstrated that the hopping process takes place between PEDOT-rich islands and not between single PEDOT segments or dopants in the lateral direction, whilst in the vertical direction the current limiting hopping transport occurs between dilute states inside the quasi-insulating PSS lamellas. By a post-treatment it is possible to modify PEDOT:PSS to raise its conductivity, by orders of magnitude. Typically, the addition of sorbitol to the aqueous dispersion of PEDOT:PSS that is used to deposit thin films via spin coating leads to an enhancement of the conductivity after thermal annealing. The causes and consequences of such behavior were investigated in detail. Chapter 5 describes the various properties of the highly conductive sorbitol-treated PEDOT:PSS, such as the conductivity itself, and the effects of thermal annealing and exposure to moisture. It is found that the conductivity enhancement upon addition of sorbitol is accompanied by a better environmental stability. Surprisingly, the electrical conductivity of PEDOT:PSS thin films without sorbitol treatment is increased by more than one order of magnitude in an environment with more than 30-35 % relative humidity. This effect is attributed to an ionic contribution to the overall conductivity. Thermal gravimetric analysis (TGA), direct insert probe-mass spectrometry (DIP-MS) and modulation differential scanning calorimetry (MDSC) were used as additional tools to demonstrate that, after thermal treatment, the concentration of sorbitol in the final PEDOT:PSS layer is negligibly small. In Chapter 6, scanning Kelvin probe microscopy (SKPM) is employed to measure the surface potential and work function of this PEDOT:PSS films that were deposited from water with different sorbitol concentrations. It is shown that work function of PEDOT:PSS is reduced with increasing sorbitol concentration. This shift can be explained by and is in agreement with- a reduction in the surface enrichment with PSS of the film. To study the charge transport properties of the highly conductive sorbitoltreated PEDOT:PSS films, temperature dependent and electric field dependent measurements are correlated with morphological analysis by STM in Chapter 7. It is found that by sorbitol treatment the hopping transport changes from 3DVRH to 1D-VRH. This transition is explained by a sorbitol-induced selforganization of the PEDOT-rich grains into 1D aggregates that are aligned within micrometer sized domains, as observed in STM images.

L’objectif de cette thèse est d’explorer le contrôle de processus électroniquement induits dans diverses molécules fonctionnalisées adsorbées sur la surface du Si(100). Ce travail s’inscrit dans le contexte des nanosciences moléculaires et a été réalisé à l’aide d’un microscope à effet tunnel (STM) à basse température (9K). Nous avons utilisé une approche combinant étude statistique et modélisation théorique afin de pouvoir explorer la physique des divers processus observés. Cette thèse débute par l’étude de la molécule d’hexaphényle benzène (HPB) dont les phényles latéraux permettent un découplage électronique entre la molécule et la surface du silicium. Grâce à cet effet, nous avons pu contrôler la diffusion directive et réversible de la molécule d’HPB physisorbée le long des marches de type SA à la surface du Si(100)−2×1 à travers un processus combinant l’action des électrons tunnels et celle du champ électrostatique induit par la pointe du STM. Ces premiers résultats ont permis d’envisager l’étude d’un couple de molécules de tétraphényles porphyrines métalliques adsorbées à la surface du Si(100)−2×1. Il s’agit de NiTPP et de CuTPP qui, comme pour l’HPB, possèdent des cycles phényles latéraux. Plusieurs conformations d’adsorption de ces deux molécules ont été caractérisées et leurs réponses à des excitations électroniques étudiées. Ceci nous a permis, pour la molécule de NiTPP, d’aboutir au contrôle de l’activation réversible d’un bistable intra-moléculaire en dépit de la chimisorption partielle de la molécule sur le silicium. L’étude de la molécule de CuTPP, quant à elle, montre des courbes de conductance I(V) en forme d’hystérésis associées à des changements réversibles de conformations réalisant ainsi une fonction mémoire. Dès lors, nous avons pu étudier la co-adsorption des molécules de NiTPP et de CuTPP sur le Si(100) afin de réaliser un binôme moléculaire. Divers couples de molécules ont pu être étudiés. Sur l’un d’entre eux, nous avons pu activer des processus d’excitations inter-moléculaires en excitant électroniquement l’une des molécules afin d’observer un changement de conformation de la seconde molécule du binôme. Ce résultat réalise ainsi le contrôle électronique d’un dispositif bi-moléculaire en s’affranchissant des processus électroniques induits via le substrat. Enfin, à titre de perspective, ce travail de thèse présente un procédé novateur permettant le contrôle local de l’hydrogénation de la surface de Si(100). Ceci est réalisé grâce à la passivation de la pointe du STM par l’hydrogène moléculaire à 9K. Les électrons tunnels sont ensuite utilisés pour induire la dissociation intra-dimer des molécules d’H2 sur la surface du Si(100). Cette technique peut être envisagée pour la passivation du Si(100) ou pour agir localement sur des circuits moléculaires. / The objective of this thesis is to explore the control of electronically induced processes in various functionalized molecules adsorbed on the surface of silicon (100). In the context of molecular nanoscience, this work has been carried out using a scanning tunneling microscope operating at low temperature (9K). We used an approach combining statistical study and theoretical modelling in order to explore the physics of the various observed processes. This thesis begins with the study of the Hexaphenylbenzene (HPB) molecule for which the lateral phenyl rings enable the molecule-silicon surface electronic decoupling. Thanks to this effect, we could achieve a directive and reversible diffusion control of physisorbed HPB molecules along the SA silicon step edge through a process combining the joint actions of tunnel electrons and the local STM tip induced electrostatic field. These first results allowed considering the study of a couple of metaltetraphenyl porphyrin molecules adsorbed on the Si(100)-2x1 surface. Similarly to the HPB molecules, the two chosen metalloporphyrins: NiTPP and CuTPP, have lateral phenyl rings. Several adsorption conformations for these molecules were characterized and their response to electronic excitation has been studied. In the case of NiTPP, this led to the control of the reversible activation of an intra-molecular bistable despite the partial chemisorption of the molecule on the silicon surface. As for CuTPP molecule, our study revealed hysteresis behavior on the I(V) conduction curves associated with reversible conformation changes which represents the realization of a memory function. Following the study of each molecule apart, we performed the co-adsorption of the two molecules on the Si(100) surface to study molecular pairs. Various pairs of molecules have been studied. On one of them, we were able to activate an inter-molecular excitation transfer process by locally exciting one molecule and observing a conformation change of the second molecule of the pair. This result thus shows the electronic control of a bi-molecular device getting rid of substrate mediated electronic process. Finally, as a perspective, this thesis presents a novel technique allowing the controlled local hydrogenation of the Si(100) surface. This is achieved thanks to the passivation of the STM tip by molecular hydrogen at 9K. The tunnel electrons are then used to induce the intra-dimer dissociative adsorption of H2 molecules on the Si(100) surface. This technique could be considered for the passivation of Si(100) or to locally modify molecular circuits.

We employ a variant of optical absorption spectroscopy, namely in situ differential reflectance spectroscopy (DRS), for an analysis of the structure-properties relations of thin epitaxial organic films. Clear correlations between the spectra and the differently intense coupling to the respective substrates are found. While rather broad and almost structureless spectra are obtained for a quaterrylene (QT) monolayer on Au(111), the spectral shape resembles that of isolated molecules when QT is grown on graphite. We even achieve an efficient electronic decoupling from the subjacent Au(111) by inserting an atomically thin organic spacer layer consisting of hexa-peri-hexabenzocoronene (HBC) with a noticeably dissimilar electronic behavior. These observations are further consolidated by a systematic variation of the metal substrate (Au, Ag, and Al), ranging from inert to rather reactive. For this purpose, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) is chosen to ensure comparability of the molecular film structures on the different metals, and also because its electronic alignment on various metal surfaces has previously been studied with great intensity. We present evidence for ionized PTCDA at several interfaces and propose the charge transfer to be related to the electronic level alignment governed by interface dipole formation on the respective metals. / Zur Analyse der Struktur-Eigenschafts-Beziehungen dünner, epitaktischer Molekülfilme wird in situ differentielle Reflexionsspektroskopie (DRS) als Variante der optischen Absorptionsspektroskopie verwendet. Klare Zusammenhänge zwischen den Spektren und der unterschiedlich starken Kopplung zum jeweiligen Substrat werden gefunden. Während man breite und beinahe unstrukturierte Spektren für eine Quaterrylen (QT) Monolage auf Au(111) erhält, ist die spektrale Form von auf Graphit abgeschiedenem QT ähnlich der isolierter Moleküle. Durch Einfügen einer atomar dünnen organischen Zwischenschicht bestehend aus Hexa-peri-hexabenzocoronen (HBC) mit einem deutlich unterschiedlichen elektronischen Verhalten gelingt sogar eine effiziente elektronische Entkopplung vom darunter liegenden Au(111). Diese Ergebnisse werden durch systematische Variation der Metallsubstrate (Au, Ag und Al), welche von inert bis sehr reaktiv reichen, untermauert. Zu diesem Zweck wird 3,4,9,10-Perylentetracarbonsäuredianhydrid (PTCDA) gewählt, um Vergleichbarkeit der molekularen Filmstrukturen zu gewährleisten, und weil dessen elektronische Anordnung auf verschiedenen Metalloberflächen bereits eingehend untersucht worden ist. Wir weisen ionisiertes PTCDA an einigen dieser Grenzflächen nach und schlagen vor, dass der Ladungsübergang mit der elektronischen Niveauanpassung zusammenhängt, welche mit der Ausbildung von Grenzflächendipolen auf den entsprechenden Metallen einhergeht.

No description available.

No description available.

The Phthalocyanines (Pc’s) are a group of macro-cyclic molecules, widely investigated due to the possibility to use them in a variety of applications. Electronic and geometrical structure investigations of molecular model systems of Pc’s adsorbed on surfaces are important for a deeper understanding of the functionality of different Pc-based devices. Here, Pc’s monolayers and films, deposited on different surfaces, were investigated by X-ray Photoelectron Spectroscopy (XPS), X-ray Absorption Spectroscopy (XAS) and Scanning Tunneling Microscopy (STM). In addition Density Functional Theory (DFT) simulations were performed. For molecular films of Metal-free (H2Pc) and Iron (FePc) Pc’s, on surfaces, it is found that the intermolecular interaction is weak and the molecules arrange with their molecular plane mainly perpendicular to the surface. Several monolayer systems were characterized, namely H2Pc and FePc adsorbed on Graphite, ZnPc on InSb(001)-c(8x2), H2Pc on Al(110) and on Au(111). For all the studied monolayers it was found that the molecules are oriented with their molecular plane parallel to the surface. The electronic structure of the molecules is differently influenced by interaction with the surfaces. For H2Pc adsorbed on Graphite the nearly negligible effect of the surface on the molecular electronic structure allowed STM characterization of different molecular orbitals. A strong interaction is instead found in the case of H2Pc on Al(110) resulting in molecules strongly adsorbed, and partly dissociated. Modifications of the electronic and geometrical structure induced by alkali doping of H2Pc films and monolayers were characterized. It is found both for the H2Pc film on Al(110) and monolayer adsorbed on Au(111), that the molecular arrangement is changed upon doping by Potassium and Rubidium, respectively. Potassium doping of the H2Pc films results in a filling of previously empty molecular orbitals by a charge transfer from the alkali to the molecule, with significant modification of the molecular electronic structure.

As interessantes propriedades eletrônicas, mecânicas e óticas dos materiais orgânicos conjugados fizeram emergir diversas aplicações tecnológicas e comerciais em dispositivos baseados nesses materiais, tais como sensores, memórias, células solares e diodos emissores de luz poliméricos (LEDs). Neste sentido, o tema central desta tese é o estudo das propriedades elétricas e morfológicas e os mecanismos de transporte eletrônico de cargas no PEDOT:PSS, uma blenda polimérica que consiste de um policátion condutivo, o poli(3,4- etilenodioxitiofeno) (PEDOT) e do poliânion poli(estirenosulfonado) (PSS). PEDOT:PSS é amplamente usado como material de eletrodo em aplicações na área de eletrônica plástica, como mencionado anteriormente. Apesar da condutividade elétrica dos filmes finos de PEDOT:PSS possa variar várias ordens de grandeza, dependendo do método pela qual é processado e transformado em filme fino, as razões para este comportamento é essencialmente desconhecido. Esta tese descreve um estudo detalhado do transporte eletrônico de cargas anisotrópico e sua correlação com a morfologia, as condições e as dimensões da separação de fase entre os dois materiais, PEDOT e PSS. Antes de abordar as propriedades do PEDOT:PSS, uma camada de filme fino inorgânica usada para aumentar o tempo de vida de dispositivos orgânicos é descrita no Capítulo 2. Um importante mecanismo de degradação em LEDs poliméricos é a fotooxidação da camada ativa. Assim, isolar a camada ativa da água, oxigênio e luz, torna-se crucial para o aumento do tempo de vida. Um sistema de deposição química a partir da fase de vapor estimulada por plasma (PECVD) é usado para depositar filmes finos de nitreto de carbono em baixas temperaturas, menores que 100 °C, sobre PLEDs com a intenção de aumentar o tempo de vida destes dipositivos e diminuir a fotodegradação do poli[2-metoxi-5- (2-etil-hexiloxi)-p-fenileno vinileno] (MEH-PPV) em ambiente atmosférico. O filme fino de nitreto de carbono possui as características de um material que pode bloquear a umidade e que tem espessura e flexibilidade adequados para a nova geração de PLEDs flexíveis. As características dos filmes finos de nitreto de carbono e MEH-PPV foram investigadas usando-se técnicas de espectroscopia ótica, com particular ênfase no processo de degradação do MEHPPV sob iluminação. Os resultados mostraram que o filme fino de nitreto de carbono protege o filme polimérico e diminui consideravelmente a fotooxidação. Para avaliar o efeito do encapsulamento em dispositivos reais, LEDs poliméricos foram fabricados e pelas curvas de corrente-tensão um aumento no tempo de vida é confirmado quando a camada de nitreto de carbono é presente. O tempo de vida desejado, maior que 10.000 horas, para aplicações comerciais não foi atingido, entretanto, o encapsulamento pode ser melhorado otimizando as propriedades da camada de nitreto de carbono e combinando-as com camadas de outros materiais orgânicos e inorgânicos. Os capítulos seguintes deste trabalho aborda os estudos realizados com o PEDOT:PSS, uma vez que é amplamente usado em eletrônica orgânica, mas relativamente tem recebido pouca atenção com respeito ao transporte eletrônico de cargas, bem como sua correlação com a morfologia. No Capítulo 3, experimentos com microscopia de varredura por sonda (SPM, Scanning Probe Microscopy) e medidas de condutividade macroscópica são utilizados para estudar e obter um modelo 3D morfológico completo que explica, qualitativamente, a condutividade anisotrópica observada nos filmes finos de PEDOT:PSS depositados pela técnica de spin coating. Imagens topográficas de microscopia de varredura por tunelamento (STM) e imagens da seção transversal observadas com o microscópio de forca atômica (X-AFM) revelaram que o filme fino polimérico é organizado em camadas horizontais de partículas planas ricas em PEDOT, separadas por lamelas quasi-contínuas de PSS. Na direção vertical, lamelas horizontais do isolante PSS reduzem a condutividade e impõe o transporte eletrônico a ser realizado por saltos em sítios vizinhos próximos (nn-H, nearest-neighbor hopping) nas lamellas de PSS. Na direção lateral, o transporte eletrônico via saltos 3D em sítios a longas distâncias (3D-VRH, variable range hopping) ocorre entre as ilhas ricas em PEDOT que são separadas por barreiras muito mais finas de PSS, causando um aumento da condutividade nesta direção. Esta discussão é estendida ao Capítulo 4 com uma descrição quantitativa do transporte eletrônico de cargas predominantes. Particularmente, é demonstrado que o transporte de cargas via saltos 3D em sítios a longas distâncias ocorre entre ilhas ricas em PEDOT e não entre segmentos isolados de PEDOT ou dopantes na direção lateral, enquanto que na direção vertical o transporte de cargas via saltos em sítios vizinhos próximos ocorre dentro das lamelas do quasi-isolante PSS. Em algumas aplicações, faz-se necessário usar PEDOT:PSS com alta condutividade elétrica. Isso pode ser feito adicionando-se sorbitol à solução aquosa de PEDOT:PSS. Após um tratamento térmico, e dependendo da quantidade de sorbitol adicionado, a condutividade aumenta várias ordens de grandeza e as causas e consequências de tal comportamento foram investigadas neste trabalho. O Capítulo 5 investiga as várias propriedades tecnológicas do PEDOT:PSS altamente condutivo tratado com sorbitol, tais como a própria condutividade, os efeitos dos tratamentos térmicos e exposição à umidade. É observado que o aumento da condutividade elétrica, devido à adição de sorbitol na solução aquosa, é acompanhado por uma melhoria na estabilidade da condutividade elétrica em condições atmosféricas. Surpreendentemente, a condutividade elétrica do PEDOT:PSS, sem tratamento com sorbitol (~ 10-3 S/cm), aumenta mais de uma ordem de grandeza sob ambiente úmido de 30-35 % umidade relativa. Este efeito é atribuido a uma contribuição iônica à condutividade total. Análise Temogravimetrica (TGA), espectrometria de massa com sonda de inserção direta (DIP-MS) e análise calorimétrica diferencialmodulada (MDSC) foram usadas como técnicas adicionais para o entendimento dos estudos deste Capítulo. No Capítulo 6, microscopia de varredura por sonda-Kelvin (SKPM) foi empregada para medir o potencial de superfície dos filmes finos de PEDOT:PSS tratados com diferentes concentrações de sorbitol. Mostra-se que a mudança no potencial de superfície é consistente com uma redução de PSS na superfície do filme fino. Para estudar o transporte eletrônico nos filmes finos de PEDOT:PSS altamente condutivos tratados com sorbitol, o Capítulo 7 usa medidas de temperatura e campo elétrico em função da conduvitidade correlacionados com analises morfológicas realizadas por STM. É observado que o transporte eletrônico por saltos, na direção lateral, muda de 3D-VRH para 1D-VRH quando o PEDOT:PSS é tratado com sorbitol. Esta transição é explicada por uma auto-organização das ilhas ricas em PEDOT em agregados 1D, devido ao tratamento com sorbitol, tornando-se alinhadas em domínios micrométricos, como observado pelas imagens de STM. / Employing the unique mechanical, electronic, and optical properties of the conjugated organic and polymer materials several technological and commercial applications have been developed, such as sensors, memories, solar cells and light-emitting diodes (LEDs). In this respect, the central theme of this thesis is the electrical conductivity and mechanisms of charge transport in PEDOT:PSS, a polymer blend that consists of a conducting poly(3,4-ethylenedioxythiophene) polycation (PEDOT) and a poly(styrenesulfonate) polyanion (PSS). PEDOT:PSS is omnipresent as electrode material in plastic electronics applications mentioned above. Although the conductivity of PEDOT:PSS can vary by several orders of magnitude, depending on the method by which it is processed into a thin film, the reason for this behavior is essentially unknown. This thesis describes a detailed study of the anisotropic charge transport of PEDOT:PSS and its correlation with the morphology, the shape, and the dimension of the phase separation between the two components, PEDOT and PSS. Before addressing the properties of PEDOT:PSS, a new barrier layer is described in Chapter 2 that enhances the lifetime of organic devices. An important degradation mechanism in polymer LEDs is photo-oxidation of the active layer. Hence, isolating the active layer from water and oxygen is crucial to the lifetime. Plasma-enhanced chemical vapor deposition (PECVD) is used to deposit a thin layer of carbon nitride at low deposition temperatures, below 100 °C, on a polymer LED that uses poly[2-methoxy-5-(2´-ethylhexyloxy)-1,4- phenylene vinylene] (MEH-PPV) as active layer. A thin layer of carbon nitride acts as barrier for humidity, but is still sufficiently bendable to be used in flexible polymer LEDs. The characteristics of carbon nitride and MEH-PPV films have been investigated using optical spectroscopy, with particular emphasis on the degradation process of MEH-PPV under illumination. The measurements show that the carbon nitride coating indeed protects the polymer film and diminishes the photo-oxidation considerably. To study the effect of the encapsulation in real devices, polymer LEDs were made and their current-voltage characteristics confirm the enhanced lifetime in the presence of a carbon nitride barrier layer. However, the target, a lifetime of more than 10,000 hours for commercial applications, was not achieved. The remaining chapters of this thesis describe the investigations of PEDOT:PSS. PEDOT:PSS is widely used in organic electronics. So far, relatively little attention has, been paid to the mechanisms of charge transport in this material and the correlation of those properties to the morphology. In Chapter 3, scanning probe microscopy (SPM) and macroscopic conductivity measurements are used to obtain a full 3D morphological model that explains, qualitatively, the observed anisotropic conductivity of spin coated PEDOT:PSS thin films. Topographic scanning probe microscopy (STM) and cross-sectional atomic force microscopy images (X-AFM) reveal that the thin film is organized in horizontal layers of flattened PEDOT-rich particles that are separated by quasi-continuous PSS lamella. In the vertical direction, the horizontal PSS insulator lamellas lead to a reduced conductivity and impose nearest-neighbor hopping (nn-H) transport. In the lateral direction, 3D variable-range hopping (3D-VRH) transport takes place between PEDOT-rich clusters which are separated by much thinner barriers, leading to an enhanced conductivity in this direction. This discussion is extended in Chapter 4, where a quantitative description of the length scales of the predominant transport is obtained. Particularly, it is demonstrated that the hopping process takes place between PEDOT-rich islands and not between single PEDOT segments or dopants in the lateral direction, whilst in the vertical direction the current limiting hopping transport occurs between dilute states inside the quasi-insulating PSS lamellas. By a post-treatment it is possible to modify PEDOT:PSS to raise its conductivity, by orders of magnitude. Typically, the addition of sorbitol to the aqueous dispersion of PEDOT:PSS that is used to deposit thin films via spin coating leads to an enhancement of the conductivity after thermal annealing. The causes and consequences of such behavior were investigated in detail. Chapter 5 describes the various properties of the highly conductive sorbitol-treated PEDOT:PSS, such as the conductivity itself, and the effects of thermal annealing and exposure to moisture. It is found that the conductivity enhancement upon addition of sorbitol is accompanied by a better environmental stability. Surprisingly, the electrical conductivity of PEDOT:PSS thin films without sorbitol treatment is increased by more than one order of magnitude in an environment with more than 30-35 % relative humidity. This effect is attributed to an ionic contribution to the overall conductivity. Thermal gravimetric analysis (TGA), direct insert probe-mass spectrometry (DIP-MS) and modulation differential scanning calorimetry (MDSC) were used as additional tools to demonstrate that, after thermal treatment, the concentration of sorbitol in the final PEDOT:PSS layer is negligibly small. In Chapter 6, scanning Kelvin probe microscopy (SKPM) is employed to measure the surface potential and work function of this PEDOT:PSS films that were deposited from water with different sorbitol concentrations. It is shown that work function of PEDOT:PSS is reduced with increasing sorbitol concentration. This shift can be explained by and is in agreement with- a reduction in the surface enrichment with PSS of the film. To study the charge transport properties of the highly conductive sorbitoltreated PEDOT:PSS films, temperature dependent and electric field dependent measurements are correlated with morphological analysis by STM in Chapter 7. It is found that by sorbitol treatment the hopping transport changes from 3DVRH to 1D-VRH. This transition is explained by a sorbitol-induced selforganization of the PEDOT-rich grains into 1D aggregates that are aligned within micrometer sized domains, as observed in STM images.