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STM Study of 2D Metal Chalcogenides and HeterostructuresZhang, Fan 31 January 2022 (has links)
In recent years, two-dimensional (2D) van der Waals (vdW) materials have aroused much interest for their unique structural, thermal, optical, and electronic properties and have become a hot topic in condensed matter physics and material science. Many research methods, including scanning tunneling microscopy (STM), transmission electron microscopy (TEM), optical and transport measurements, have been used to investigate these unique properties. Among them, STM stands out as a powerful characterization tool with atomic resolution and is capable of simultaneously revealing both atomic structures and local electronic properties. This dissertation focuses on scanning tunneling microscopy and spectroscopy (STM/S) investigation of 2D metal chalcogenides and heterostructures.
The first part of the dissertation focuses on the continuous interface in WS2/MoS2 heterostructures grown by the chemical vapor deposition (CVD) method. We observed a closed interface between the MoS2 monolayer and the heterobilayer with atomic resolution. Furthermore, our scanning tunneling spectroscopy (STS) results and density functional theory (DFT) calculations revealed band gaps of the heterobilayer and the MoS2 monolayer agree with previously reported values for MoS2 monolayer and MoS2/WS2 heterobilayer on SiO2 fabricated through the mechanical exfoliation method. The results could deepen our understanding of the growth mechanism, interlayer interactions and electronic structures of 2D transition metal dichalcogenides (TMD) heterostructures synthesized via CVD.
The second part of the dissertation focuses on phase transformation in 2D In2Se3. We observed that 2D In2Se3 layers with thickness ranging from single to ~20 layers stabilized at the beta phase with a superstructure at room temperature. After cooling down to around 180 K, the beta phase converted to a more stable beta' phase that was distinct from previously reported phases in 2D In2Se3. The kinetics of the reversible thermally driven beta-to-beta' phase transformation was investigated by temperature dependent transmission electron microscopy and Raman spectroscopy, combined with the expected minimum-energy pathways obtained from our first-principles calculations. Furthermore, DFT calculations reveal in-plane ferroelectricity in the beta' phase. STS measurements show that the indirect bandgap of monolayer beta' In2Se3 is 2.50 eV, which is larger than that of the multilayer form with a measured value of 2.05 eV. Our results on the reversible thermally driven phase transformation in 2D In2Se3 will provide insights to tune the functionalities of 2D In2Se3 and other emerging 2D ferroelectric materials and shed light on their numerous potential applications like non-volatile memory devices.
The third part of the dissertation focuses on domain boundaries in 2D ferroelectric In2Se3. The atomic structure of domain boundaries in two-dimensional (2D) ferroelectric beta' In2Se3 is visualized with scanning tunneling microscopy and spectroscopy (STM/S) combined with DFT calculations. A double-barrier energy potential across the 60° tail to tail domain boundaries in monolayer beta' In2Se3 is also revealed. The results will deepen our understanding of domain boundaries in 2D ferroelectric materials and stimulate innovative applications of these materials. / Doctor of Philosophy / Two-dimensional (2D) materials are materials consisting of a single layer or a few layers of atoms. They exhibit unique and interesting properties distinct from their bulk counterparts. Over the past decade, much effort has been devoted to a large family of 2D materials — 2D metal chalcogenides that exhibit fascinating structural and electronic properties. These 2D metal chalcogenides can also be stacked together to form various heterostructures. The scanning tunneling microscope (STM) is a powerful tool to study these materials with atomic resolution and is capable of simultaneously revealing both atomic structures and local electronic properties. It can also be used to manipulate nanometer-scale structures on the material surface. In this dissertation, we use scanning tunneling microscopy and spectroscopy (STM/S) to investigate 2D metal chalcogenides and heterostructures.
The first part of the dissertation focuses on WS2/MoS2 heterostructures grown by the chemical vapor deposition (CVD) method. We observed a closed interface between the MoS2 monolayer and the heterobilayer with atomic resolution. Furthermore, our scanning tunneling spectroscopy (STS) results and density functional theory (DFT) calculations revealed band gaps of the heterobilayer and the MoS2 monolayer. The results could deepen our understanding of the growth mechanism, interlayer interactions and electronic structures of 2D transition metal dichalcogenides (TMD) heterostructures synthesized via CVD.
The second part of the dissertation focuses on phase transformation in 2D In2Se3. We observed that 2D In2Se3 layers transform from beta phase to a more stable beta' phase when the sample is cooled down from room temperature to 77 K. This thermally driven beta-to-beta' phase transformation was found to be reversible by temperature dependent transmission electron microscopy and Raman spectroscopy, corroborated with the expected minimum-energy pathways obtained from our first-principles calculations. Furthermore, DFT calculations reveal in-plane ferroelectricity in the beta' phase. Our results on the reversible thermally driven phase transformation in 2D In2Se3 will provide insights to tune the functionalities of 2D In2Se3 and other emerging 2D ferroelectric materials.
The third part of the dissertation focuses on domain boundaries in 2D ferroelectric In2Se3. The atomic structure of domain boundaries in 2D ferroelectric beta' In2Se3 is visualized by using STM/S combined with DFT calculations. A double-barrier energy potential across the 60° tail to tail domain boundaries in monolayer beta' In2Se3 is also revealed. The results will deepen our understanding of domain boundaries in 2D ferroelectric materials and stimulate innovative applications of these materials.
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Inelastic STM as a Tool for the Electronic Manipulation of Single MoleculesKühne, Tim 13 December 2021 (has links)
For the investigation of single molecules on surfaces, STM under UHV and at low temperatures is the experimental technique of choice. Inelastic STM is, furthermore, able to manipulate the target structures and to induce chemical reactions or to control single molecule mechanics precisely. This thesis presents inelastic STM experiments on three different molecules as a tool for the electronic manipulation at a single molecule level.
The first part of this work concerns the on-surface synthesis of dodecacene, the longest acene molecule obtained so far. Acenes as smallest zigzag edge graphene nanoribbons and model 1D electronic system play an important role in both experimental and theoretical science. Due to the high reactivity and low solubility of long acenes, precursor molecules were deoxygenated step wise in an on-surface reaction triggered by inelastic tunneling and through annealing at increasing temperatures.
The molecular structure was proven by high resolution STM employing a CO functionalized tip. Additionally, the electronic states of the molecule were observed in the energy spectrum by STS and their spatial distribution was measured in dI/dV maps. The increase in the band gap compared to shorter acenes was explained by increasing contributions of multiradical states to the electronic states and higher orbitals participating in virtual tunneling states. In the second part of this work, the inelastic tunneling effect was used to investigate the conversion of electrical into mechanical energy in azulene derivatives carrying a large dipole moment. Metal organic complexes consisting of gold adatoms and pristine as well as cleaved molecules were formed upon evaporation of BCA. These structures were identified with the aid of theoretical calculations. Voltage pulse experiments at different tunneling resistance revealed that the electric field in combination with the charge distribution of the structures is the origin of the motion. Metal organic complexes of cleaved molecules could be moved on the surface in a controlled way and driven along an arbitrarily chosen parcours. The third part of this work concerns the investigation of DMBI-P molecules as rotors for molecular machines. Demethylation during evaporation was used to create an open radical bond stably anchoring the molecule on the surface. This was utilized for a step wise rotation where the direction is controlled by the voltage sign and chirality of the molecule on the surface. A C-H stretch mode was identified as its origin, serving as energy entry channel excited by inelastic tunneling electrons. Temperature dependent measurements and theoretical calculations yielded the potential barrier for the rotation. / Rastertunnelmikroskopie (RTM) unter UHV Bedingungen und bei tiefen Temperaturen ist die experimentelle Methode der Wahl zur Untersuchung von Einzelmolekülen auf Oberflächen. Darüber hinaus ist inelastische RTM in der Lage, die Zielstrukturen zu manipulieren und chemische Reaktionen auszulösen oder die Mechanik der einzelnen Moleküle präzise zu kontrollieren. Diese Dissertation behandelt inelastische RTM-Experimente an drei verschiedenen Molekülen als Werkzeug zur elektronischen Manipulation einzelner Moleküle.
Der erste Teil der Arbeit behandelt die Oberflächensynthese von Dodecacen, des längsten bisher erzeugten Acens. Als kleinste Graphen-Nanobänder mit Zickzack-Rand und Modell für eindimensionale elektronische Systeme spielen Acene sowohl in Theorie als auch Experimentalphysik eine wichtige Rolle. Aufgrund der hohen Reaktivität und geringen Löslichkeit langer Acene wurden Vorläufermoleküle sowohl durch inelastisches Tunneln als auch durch Heizen des Substrates schrittweise deoxygeniert. Die Molekülstruktur wurde durch hochaufgelöste RTM mittels einer CO-funktionalisierten Spitze nachgewiesen. Zusätzlich konnten die elektronischen Zustände des Moleküls im Energiespektrum identifiziert und ihre räumliche Verteilung in dI/dV-Karten festgehalten werden. Die Vergrößerung der Bandlücke im Vergleich zu kürzeren Acenen konnte hierbei durch zunehmenden Einfluss multiradikaler Zustände auf den Grundzustand des Moleküls und den Beitrag höherer Molekülorbitale zu den virtuellen Tunnelzuständen erklärt werden.
Im zweiten Teil dieser Arbeit wird die inelastische RTM dazu genutzt, um die Umwandlung von elektrischer Energie in mechanische mittels Azulen-Derivaten mit großem Dipolmoment zu untersuchen. Bei der Verdampfung bilden diese metallorganische Komplexe aus Goldatomen und sowohl intakten als auch gespaltenen Molekülen. Deren Strukturen wurden mit Hilfe von Berechnungen identifiziert. Experimente mit Spannungspulsen bei unterschiedlichen Tunnelwiderständen enthüllten das elektrische Feld in Kombination mit der Ladungsverteilung der Strukturen als Ursprung der Bewegung. Die metallorganischen Komplexe aus gespaltenen Molekülen konnten zielgerichtet auf der Oberfläche durch einen zufällig gewählten Parcours bewegt werden.
Der dritte Teil dieser Arbeit behandelt die Untersuchung von DMBI-P Molekülen zur Verwendung als Rotoren für molekulare Maschinen. Eine Demethylierung während der Verdampfung erzeugt eine offene Bindung, die das Molekül stabil auf der Oberfläche verankert. Dies wurde für eine schrittweise Rotation genutzt, deren Richtung durch das Vorzeichen der Spannung und die Chiralität auf der Oberfläche kontrolliert werden konnte. Eine C-H Streckschwingung dient hierbei als Eintrittskanal der durch inelastische Elektronen bereitgestellten Energie. Temperaturabhängige Messungen und theoretische Berechnungen lieferten die Potentialbarriere für die Rotation.
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Molecular tectonics : supramolecular 2D nanopatterning of surfaces by self-assemblyZhou, Hui January 2009 (has links)
Thèse numérisée par la Division de la gestion de documents et des archives de l'Université de Montréal.
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Local measurements of cyclotron states in grapheneKubista, Kevin Dean 04 April 2011 (has links)
Multilayer epitaxial graphene has been shown to contain "massless Dirac fermions" and is believed to provide a possible route to industrial-scale graphene electronics. We used scanning tunneling microscopy (STM) and spectroscopy (STS) in high magnetic fields to obtain local information on these fermions. A new STS technique was developed to directly measure graphene's energy-momentum relationship and resulted in the highest precision measurement of graphene's Dirac cone. STS spectra similar to ideal graphene were observed, but additional anomalies were also found. Extra peaks and an asymmetry between electron and hole states were shown to be caused by the work function difference between the Iridium STM tip and graphene. This tip effect was extracted using modeled potentials and performing a least square fit using degenerate perturbation theory on graphene's eigenstates solved in the symmetric gauge. Defects on graphene were then investigated and magnetic field effects were shown to be due to a mixture of potential effect from defects and the tip potential. New defect states were observed to localize around specific defects, and are believed to interact with the STM tip by Stark shifting in energy. This Stark shift gives a direct measurement of the capacitive coupling between the tip and graphene and agrees with the modeled results found when extracting the tip potential.
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Molecular tectonics : supramolecular 2D nanopatterning of surfaces by self-assemblyZhou, Hui January 2009 (has links)
Thèse numérisée par la Division de la gestion de documents et des archives de l'Université de Montréal
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Engineering 2D organic nanoarchitectures on Au(111) by self-assembly and on-surface reactions / Elaboration de nanoarchitectures organiques bidimensionnelles par auto-assemblage et réactions sur surfacePeyrot, David 06 January 2017 (has links)
Ces dernières années ont été marquées par de grandes évolutions technologiques à travers notamment une course à la miniaturisation. De gros efforts de recherche se concentrent en particulier sur le domaine de l’électronique organique mais aussi sur de nouveaux matériaux bidimensionnels comme le graphène. Ces matériaux 2D présentent des propriétés physiques exceptionnelles et sont des candidats prometteurs pour le développement de futurs dispositifs électroniques. Au cours de cette thèse, l’approche ascendante, qui consiste à assembler ensemble des petites briques élémentaires, a été utilisée pour élaborer des nanostructures bidimensionnelles originales sur des surfaces. Des états électroniques localisés dus à un couplage électronique latéral particulier entre les molécules ont été observés. Quatre nanoarchitectures hybrides ioniques-organiques différentes ont été réalisées en faisant varier la température de la surface. Des nanostructures organiques covalentes ont aussi été élaborées par une réaction de couplage d’Ullmann sur la surface. Deux précurseurs différents en forme d’étoile avec des substituants iodés et bromés respectivement, ont été étudiés. De grandes nanostructures carbonées hexagonales poreuses ont notamment été synthétisées en faisant varier la température du substrat. Ces travaux ouvrent de nouvelles perspectives pour la réalisation de matériaux organiques bidimensionnels aux propriétés contrôlées. / Over the last few years, important technological developments were made following a trend towards miniaturization. In particular, lots of research efforts are put into the research on organic electronics and on 2D materials like graphene. Such 2D materials show great physical properties and are promising candidates for the development of future electronic devices.In this project, bottom-up approach consisting in assembling elementary building blocks together, was used to engineer novel twodimensional nanostructures on metal surfaces. The properties of these two-dimensional nanostructures were investigated using Scanning Tunneling Microscopy (STM) and X-ray Photoemission Spectroscopy (XPS). Two-dimensional nanostructures based on the self-assembly of organic building blocks stabilized by intermolecular interactions were engineered. In particular, nanostructures stabilized by hydrogen bonds, halogen bonds and ionic-organic interactions were investigated. Localized electronic states due to specific molecular lateral electronic coupling were observed. Four different ionic-organic nanoarchitectures were engineered varying the substrate temperature. Covalent organic nanostructures were also engineered by onsurface Ullmann coupling reaction. Two different star-shaped precursors with iodine and bromine substituents respectively, were investigated. Large periodic porous 2D covalent hexagonal carbon nanostructures weresuccessfully engineered by temperature driven hierarchal Ullmann coupling. These results open new perspectives for the development of 2D organic materials with controlled structures and properties.
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Graphen auf Siliziumcarbid: elektronische Eigenschaften und Ladungstransport / Graphene on silicon carbide: electronic properties and charge transportDruga, Thomas 07 March 2014 (has links)
In dieser Arbeit werden die lokalen elektronischen Eigenschaften sowie der Ladungstransport bis auf atomare Längenskalen von epitaktischem Graphen auf der SiC(0001)-Oberfläche charakterisiert. Dazu wird neben den etablierten Rastersondenverfahren erstmals bei 6 K und unter UHV-Bedingungen die Methode der Rastertunnelpotentiometrie (STP) eingesetzt.
Hierzu wurden epitaktisch gewachsene Graphenproben auf der 6H-Si(0001)-Oberfläche unter UHV-Bedingungen durch resistives Heizen präpariert und anschließend elektrisch kontaktiert. Mit Hilfe des Rasterkraftmikroskopie und niederenergetischen Elektronenbeugung wird die Morphologie der Proben untersucht. Es können heterogene Proben mit einer Bedeckung von einlagigem und zweilagigem Graphen präpariert werden, die eine direkte vergleichende Untersuchung mit dem Rastertunnelmikroskop ermöglichen. Ergänzend wird zur Bestimmung der Lagenanzahl der gebildeten Graphenschichten die Differenz des Oberflächenpotentials von ein- und zweilagigem Graphen an Atmosphäre durch die Raster-Kelvin-Mikroskopie (KPFM) ermittelt.
Für Transportexperimente und zukünftige Anwendungen spielt der Kontaktwiderstand zwischen epitaktisch gewachsenem Graphen und den kontaktierenden Elektroden eine entscheidende Rolle. Es wird erstmals demonstriert, wie durch räumlich aufgelöste Messungen mit Hilfe der Raster-Kelvin-Mikroskopie am Gold-Graphen-Interface auf semi-isolierendem SiC(0001) eine obere Grenze des Kontaktwiderstandes von ρ_c=1×10^(-6) Ωcm² abgeschätzt werden kann.
Die Untersuchung der epitaktisch gewachsenen Graphenproben mit der Methode der Rastertunnelmikroskopie (STM) ermöglichen die eindeutige Identifizierung von ein- und zweilagigem Graphen und deren hexagonale atomare Struktur, die über mehrere 100 nm² keine Punktdefekte zeigen. Die unter der Graphenschicht liegende Zwischenschicht zeigt eine stark ungeordnete quasiperiodische Struktur mit zahlreichen Trimeren, die ebenso bei einer Bedeckung der Zwischenschicht mit ein- und zweilagigem Graphen abgebildet werden können. Einlagiges Graphen ist auf atomaren Längenskalen elektronisch stark inhomogen. Es können im Energiebereich von E_F±100 mV zahlreiche lokalisierte, räumlich variierende Zustände identifiziert werden, die selbst bei der Fermienergie auf Längenskalen von 5 nm² zu Variationen in der Zustandsdichte führen. Auf zweilagigem Graphen fallen Variationen in der lokalen Zustandsdichte geringer aus.
Um den für den elektronischen Transport relevanten Energiebereich bei E_F zu spektroskopieren, wird die Thermospannung im Tunnelkontakt ausgenutzt, welche sich mit der STP-Methode bestimmen lässt. Diese liefert neue Einblicke in die elektronische Struktur der Graphenoberfläche bei E_F. Die räumliche Variation der Thermospannung bei abgeschätzten Temperaturdifferenzen von einigen 10 bis 100 K zwischen Spitze und Probe liegt bei einigen 10 bis 100 µV sowohl auf atomarer Skala als auch zwischen ein- und zweilagigem Graphen und ist sehr empfindlich auf die atomaren Eigenschaften der eingesetzten STM-Spitze. Die hohe laterale und energetische Auflösung des Verfahrens ermöglicht die Analyse von Streuprozessen wie der Intra- und Intervalley-Streuung und zeigt im Gegensatz zu bisherigen Annahmen, dass auch noch zweilagiges Graphen elektronisch von der Zwischenschicht beeinflusst wird.
Die starke elektronische Inhomogenität der Proben bei der Fermienergie spiegelt sich auch in den Transportexperimenten mit dem STP-Verfahren wider. Es zeigen sich signifikante Spannungsabfälle auf ein- und zweilagigen Graphenflächen und an lokalisierten Defekten wie Übergängen zwischen einlagigen Graphenflächen und Übergängen zwischen ein- und zweilagigen Graphenflächen. Der Potentialverlauf kann gut durch ein klassisches ohmsches Transportmodell mit spezifischen Widerständen beschrieben werden. Die quantitative Analyse liefert spezifische Widerstände der einzelnen Defekte, die in der Größenordnung bisheriger Transportuntersuchungen liegen. Dabei zeigt sich, dass ein- und zweilagiges epitaktisches Graphen nahezu identische Mobilitäten von ~1000 cm²/Vs bzw. mittlere freie Weglängen von ~40 nm bei 6 K aufweisen. Diese Werte liegen weit unter den theoretisch erwarteten einer defektfreien Graphenoberfläche.
Im Zuge der Transportmessungen wird ebenso der Einfluss der Thermospannung im Tunnelkontakt untersucht. Für Ladungstransportmessungen stellt sie einen zunächst unerwünschten Nebeneffekt dar, da die Variationen in der Thermospannung in derselben Größenordnung wie die Variationen im lokalen elektrochemischen Potential im Fall der durchgeführten Transportexperimente sind. Dies kann zu Fehlinterpretationen bei der Bestimmung von Spannungsabfällen führen. Jedoch wird im Rahmen der experimentellen Auflösung gezeigt, dass sich die Thermospannung rein additiv verhält und für Messungen des lokalen elektrochemischen Potentials mit entgegensetzten Stromrichtungen eliminieren lässt.
Des Weiteren wird der Verlauf des elektrochemischen Potentials in der unmittelbaren Umgebung von Übergängen zwischen ein- und zweilagigem sowie einlagigem Graphen untersucht. Die Spannungsabfälle sind auf einen Bereich kleiner λ_F/2 lokalisiert. Im Bezug auf den topographischen Verlauf zeigt sich für den Spannungsabfall am Übergang zwischen ein- und zweilagigem Graphen ein lateraler Versatz hin zum zweilagigen Graphen. Als Ursache wird ein kombinierter Streumechanismus aus einer lokalen Änderung der Dotierung und Fehlanpassung der Wellenfunktionen am Übergang zwischen ein- und zweilagigem Graphen vorgeschlagen.
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Organisation moléculaire dirigée par le groupe CONH2 en 2D et 3DLacatus, Monica Elena 10 1900 (has links)
Notre étude a pour objet la conception, la synthèse ainsi que l’étude structurale
d’architectures supramoléculaires obtenues par auto-assemblage, en se basant sur les
concepts de la tectonique moléculaire. Cette branche de la chimie supramoléculaire
s’occupe de la conception et la synthèse de molécules organiques appelées tectons, du grec tectos qui signifie constructeur. Le tecton est souvent constitué de sites de reconnaissance
branchés sur un squelette bien choisi. Les sites de reconnaissance orientés par la géométrie du squelette peuvent participer dans des interactions intermoléculaires qui sont suffisamment fortes et directionnelles pour guider la topologie du cristal résultant. La stratégie envisagée utilise des processus d'auto-assemblage engageant des interactions réversibles entre les tectons. L’auto-assemblage dirigé par de fortes interactions intermoléculaires directionnelles est largement utilisé pour fabriquer des matériaux dont les composants doivent être positionnés en trois dimensions (3D) d'une manière prévisible.
Cette stratégie peut également être utilisée pour contrôler l’association moléculaire en deux dimensions (2D), ce qui permet la construction de monocouches organisées et
prédéterminées sur différents types des surfaces, tels que le graphite.Notre travail a mis l’accent sur le comportement de la fonction amide comme fonction de reconnaissance qui est un analogue du groupement carboxyle déjà utilisé dans
plusieurs études précédentes. Nous avons étudié le comportement d’une série de composés contenant un noyau plat conçu pour faciliter l'adsorption sur le graphite et modifiés par l'ajout de groupes amide pour favoriser la formation de liaisons hydrogène entre les molécules ainsi adsorbées. La capacité de ces composés à former de monocouches organisées à l’échelle moléculaire en 2D a été examinée par microscopie à effet tunnel, etleur organisation en 3D a également été étudiée par cristallographie aux rayons X. Dans notre étude, nous avons systématiquement modifié la géométrie moléculaire et d'autres paramètres afin d'examiner leurs effets sur l'organisation moléculaire. Nos résultats
suggèrent que les analyses structurales combinées en 2D et 3D constituent un important atout dans l'effort pour comprendre les interactions entre les molécules adsorbées et l’effet de l’interaction avec la surface du substrat. / Our study involves the design, synthesis and structural analysis of supramolecular
architectures obtained by self-assembly, based on the concepts of molecular tectonics. This branch of supramolecular chemistry explores the properties of molecules called tectons,from the Greek word tectos, meaning builder. Tectons typically incorporate sites of recognition connected to well-chosen skeletons with defined geometries. The sites of recognition, oriented by the geometry of the skeleton, can participate in intermolecular
interactions that are sufficiently strong and directional to control the topology of the resulting assembly. This strategy is thereby based on self-assembly processes involving reversible interactions between tectons. Self-assembly directed by strong directional intermolecular interactions is widely used to produce materials whose components must be positioned in three dimensions (3D) in a predictable way. This strategy can also be used to
control molecular association in two dimensions (2D), thereby allowing the construction of predictably organized and predetermined nanopatterns on various surfaces, such as
graphite.Our work has focused on the behavior of the amide groups as primary sites of intermolecular interaction. These groups are analogues of carboxyl groups, which have been widely used in previous studies of directed molecular assembly. We have studied the 3D and 2D association of compounds with flat cores designed to favor the formation of sheets and to facilitate adsorption on graphite, modified by the addition of amide groups to promote the formation of intermolecular hydrogen bonds. The ability of these compounds to form predictably ordered 2D nanopatterns has been examined by scanning tunneling
microscopy, and their organization in 3D has also been investigated by X-ray
crystallography. In our study, we have systematically altered molecular geometry and other parameters to examine their effect on molecular organization. Our results suggest that combined structural analyses in 2D and 3D are an important asset in the effort to understand why molecules aggregate in particular ways and how these preferences can be altered by underlying surfaces.
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Alguns aspectos acerca da adsorção de alcanotióis e bases nitrogenadas em ouro via espectroscopia não linear e microscopia de tunelamento de elétrons / Some aspects concerning the adsorption of alkanethiols and uracil derivates on Au via nonlinear spectroscopy and scanning tunneling microscopyAguiar, Hilton Barbosa de 15 February 2007 (has links)
Estudos de interface têm presenciado um recente crescimento devido as novas propriedades físico-químicas, que puderam ser explorados com o advento de técnicas com resolução ao nível molecular/atômico. Dessas técnicas, dois ramos que merecem destaque são as Microscopias de Varredura por Ponta de Prova e Espectroscopias Óticas. Neste trabalho fazemos uso de algumas destas técnicas (o Microscópio de Tunelamento de Elétrons e Espectroscopia Vibracional por Geração de Soma de Freqüências) para estudar a adsorção de moléculas alifáticas e aromáticas em Au. Dois casos são abordados: como a rugosidade do substrato influencia no mecanismo de automontagem de monocamadas de alcanotióis e a automontagem de derivados de uracil em interfaces eletroquímicas. No primeiro caso, mostra-se que a quantidade de defeitos moleculares na monocamada adsorvida e extremamente sensível a rugosidade do substrato utilizado. Unem-se os resultados das técnicas acima aos resultados de sondas eletroquímica para se chegar a um modelo. Também e estudado a dependência das etapas de fisisorção e quimisorção em função da concentração da solução de alcanotiól. Para o segundo caso, um derivado halogenado do uracil (5-fluorouracil), mostra-se como a substituição química na base nitrogenada leva a diferentes mecanismos de formação de monocamadas na interface eletroquímica. Imagens de Microscopia de Tunelamento de Elétrons com resolução atômica e molecular mostram que em densidade de cargas negativas as moléculas estão fisisorvidas, porém não formam estruturas periódicas em contraste com uracil e timina, entretanto em densidades de cargas positivas formam estruturas periódicas quimisorvidas, assim como uracil e timina. E discutido como são diferentes os mecanismos de interação intermolecular: no caso dos alcanotiois preponderando às interações de van der Waals e no caso dos derivados de uracil pelas ligações via pontes de hidrogênio são dominantes. / Interface science has experienced a new rebirth since the development of new probes with atomic/molecular resolution, giving new insights about the physical-chemical properties, which differ substantially from the bulk. Among these techniques, two branches deserve special attention: the Scanning Probe Microscopies and Optical Spectroscopy. In this work, two derivatives of theses techniques (the Scanning Tunnelling Microscopy and Vibrational Spectroscopy by Sum-Frequency Generation) are combined giving new insights about the molecular adsorption onto Au. Two examples are focused: how roughness plays a key role in the structure of self-assembled alkanethiol monolayer and the uracil derivatives self-assembling at electrochemical interfaces. For the former, it has been shown that the amount of defects on the adsorbed monolayer is highly sensitive to substrate roughness. Combining the results of each technique with a well-known electrochemical probe, a physical model is proposed. The physisorbed and chemisorbed states are studied as a function of alkanethiol solution concentration as well. For the later case, the chemical substitution of uracil leads to drastically different results for the physisorbed phase (negative charge densities), compared to uracil and thymine. In the chemisorbed phase (positive charge densities) imaging with molecular resolution is achieved showing a quasi-hexagonal structure, similar to the structure of thymine and uracil. It is discussed what are the main driving forces for the self-assembling mechanism: van der Waal interactions for the alkanethiols and hydrogen bonding for uracil derivatives.
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Electronic and Geometrical Structure of Phthalocyanines on Surfaces : An Electron Spectroscopy and Scanning Tunneling Microscopy StudyÅhlund, John January 2007 (has links)
Core- and Valence Photoelectron Spectroscopy (PES), X-ray- and Ultraviolet-Visible Absorption Spectroscopy (XAS and UV-Vis), Scanning Tunneling Microscopy (STM) and Density Functional Theory (DFT) calculations are used to study the electronic and geometrical structure of a class of macro-cyclic molecules, Phthalocyanines (Pc), on surfaces. These molecules are widely studied due to their application in many different fields. Multilayer and monolayer coverages of Iron Phthalocyanine (FePc) and metal-free Phthalocyanine (H2Pc) deposited on different surfaces are investigated in order to get insight in the electronic and geometrical structure of the obtained overlayers, of crucial importance for the understanding of the film functionality. Sublimation of molecular thick films on Si(100) and on conducting glass results in films with molecules mainly oriented with their molecular plane orthogonal to the surface. Ex-situ deposited H2Pc films on conductive glass show different molecular orientation and morphology with respect to the vacuum sublimated films. We study the monolayer adsorption structure of FePc and H2Pc and compare our results with other Pc’s adsorbed on graphite. We find that the molecular unit cell and the superstructure is characteristic for each Pc adsorbed on graphite, even if the geometrical size of the compared molecules is the same. The PE- and XA- spectra of FePc on graphite are essentially identical for the mono- and multilayer preparations, evidencing weak intermolecular and molecular-substrate interactions of van der Waals nature. Furthermore, we characterize Pc’s on InSb (001)-c(8x2). The substrate In rows are observed to be the adsorption site for Pc’s. We find that the growth of the two-dimensional islands of FePc is prolonged in the [-110] direction, in contrast to ZnPc adsorbed on the same substrate at room temperature. We interpret this result as an indication that the adsorption is controlled by the substrate corrugation observed at 70 K.
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