Nos últimos 30 anos o crescimento paralelo das Ciências de Superfície tradicionais, em ambiente de ultra-alto vácuo (UHV), com a Eletroquímica levou ao nascimento de um novo campo interdisciplinar: Física de Superfície e Eletroquímica. Técnicas de ambas as áreas dão informações complementares e assim, quando realizadas em conjunto podem fornecer muitas respostas em nível atômico, estrutural e eletrônico quando o eletrodo está em contato com a solução eletrolítica. A intenção primordial dessa Dissertação foi o estudo fundamental das fases ricas em oxigênio presentes na superfície de Ru(0001) através de caracterizações eletroquímicas e morfológicas utilizando um sistema que permitiu o acoplamento de uma célula eletroquímica miniatura de fluxo a câmaras de UHV. Inicialmente exibi-se a modificação e a construção de equipamentos necessários para a preparação do sistema binário Au-Pt(111) e do óxido monocristalino Ru2O(110). Imagens de STM em escala morfológica mostraram o crescimento anisotrópico do filme de RuO2(110) sobre um substrato monocristalino de Ru(0001). Resultados obtidos através da técnica de Voltametria Cíclica na eletrooxidação de CO em RuO2(110) corroboraram cálculos teóricos sobre a estrutura da superfície quando esta em ambiente úmido. Superfícies modelos baseadas em ouro, crescido epitaxialmente sobre um substrato de Pt(111), foram preparadas no sistema de UHV. Dados eletroquímicos foram correlacionados às composições superficiais destas, mostrando o efeito do substrato prevalecendo sobre o efeito eletrônico. / In the last 30 years the parallel growth of the traditional Surface Science, under UHV environment, and Electrochemistry gave rise to a new interdisciplinary field: Surface Science and Electrochemistry. Techniques from both sciences give complementary information. Thus, in tandem, they are able to elucidate many atomic, structural and electronic phenomena, of an electrode in contact with a solution. The main goal of this Dissertation was the fundamental study of the Oxygen-rich Ru(0001) surface through electrochemical and morphologic characterizations using a coupled system which allowed the attachment of a miniature flow cell to UVH-chambers. Initially it is shown the construction and modifications of required equipments for the preparation of the binary system Au-Pt(111) and single crystal RuO2(110) oxide. Attainable morphological STM images demonstrated the anisotropic growth of the RuO2(110) over a Ru(0001) substrate. Results of the electrooxidation of CO on RuO2(110), obtained by means of Cyclic Voltammetry, corroborated theoretical calculations concerning the oxide superficial structure in a humid environment. Model surfaces based on Au, epitaxialy grown on a Pt(111) substrate, were prepared under UHV conditions. Electrochemical data and superficial composition were correlated, confirming that the substrate effect overcomes electronic strain effects.

The ability to isolate single atomic layers of van der Waals materials has led to renewed interest in the electronic and optical properties of these materials as they can be fundamentally different at the monolayer limit. Moreover, these 2D crystals can be assembled together layer by layer, with controllable sequence and orientation, to form artificial materials that exhibit new features that are not found in monolayers nor bulk. Twisted bilayer graphene is one such prototype system formed by two monolayer graphene layers placed on top of each other with a twist angle between their lattices, whose electronic band structure depends on the twist angle. This thesis presents the efforts to explore the electronic and optical properties of twisted bilayer graphene by Raman spectroscopy and scanning tunneling microscopy measurements. We first synthesize twisted bilayer graphene with various twist angles via chemical vapor deposition. Using a combination of scanning tunneling microscopy and Raman spectroscopy, the twist angles are determined. The strength of the Raman G peak is sensitive to the electronic band structure of twisted bilayer graphene and therefore we use this peak to monitor changes upon doping. Our results demonstrate the ability to modify the electronic and optical properties of twisted bilayer graphene with doping. We also fabricate twisted bilayer graphene by controllable stacking of two graphene monolayers with a dry transfer technique. For twist angles smaller than one degree, many body interactions play an important role. It requires eight electrons per moire unit cell to fill up each band instead of four electrons in the case of a larger twist angle. For twist angles smaller than 0.4 degree, a network of domain walls separating AB and BA stacking regions forms, which are predicted to host topologically protected helical states. Using scanning tunneling microscopy and spectroscopy, these states are confirmed to appear on the domain walls when inversion symmetry is broken with an external electric field. We observe a double-line profile of these states on the domain walls, only occurring when the AB and BA regions are gaped. These states give rise to channels that could transport charge in a dissipationless manner making twisted bilayer graphene a promising platform to realize controllable topological networks for future applications.

Les nanostructures auto-organisées sont essentielles dans le domaine des nanotechnologies, car elles fournissent un moyen simple de créer des structures périodiques avec des dimensions nanométriques. Dans ce travail, nous introduisons une nouvelle méthode de préparation de la surface Cu(110)-(2×1)O, qui permet un contrôle de sa morphologie. La périodicité varie de 6.5 à 11 nm pour recouvrement en oxygène entre 0.1 et 0.4 (saturation est 0.5). On a obtenu des périodicités allant jusqu'au 100 nm. La préparation est basée sur la pré-adsorption de faibles quantités de soufre. La présence de soufre change les propriétés élastiques et/ou électrostatiques de la surface et modifie ainsi son auto-organisation. Nous avons effectué une étude détaillée au moyen de la microscopie à effet tunnel (STM) et développé un modèle mathématique décrivant nos données expérimentales. Une surface auto-organisée accordable fournit un système idéal pour étudier l'interaction entre la réactivité et la structure. Nous avons étudié la sulfuration sur la surface nanostructurée, Cu(110)-(2×1)O. Le mécanisme réactionnel dépend de la largeur des bandes oxydées. Sulfuration d'une nanostructure à bandes CuO étroites conduit au détachement de chaînes reconstruites Cu-O et dans le cas de bandes larges, le mécanisme réactionnel est une combinaison du détachement de chaînes et de la formation d'îlots de la phase S-c(2×2) sur les bandes CuO. Nous présentons une étude STM de la formation de ces îlots, ainsi que leur stabilité sous ultra vide. Les îlots sont mobiles et subissent une maturation (Ostwald et Smoluchowski). Dans le cas d'une surface pas complètement saturé, les îlots disparaissent progressivement. / Self-organized nanostructures are essential for the field of nanotechnology, since they provide a simple way to create periodic structures with nanodimensions. In the present work, we have developed a new preparation method for the Cu(110)-(2×1)O nanostructure, which allows tuning of its morphology. For oxygen coverages between 0.1 and 0.4 (saturation coverage 0.5), the periodicity of the nanostructure varies from 6.5 to 11 nm. We have been able to expand the possibilities of the system and reach periodicities up to 100 nm for half oxygen coverage. The preparation method consists in co-adsorption of low amounts of sulfur. We have shown that the presence of sulfur influences the elastic and/or electrostatic properties of the surface and thus changes its self-organization. We present a detailed scanning tunneling microscopy (STM) study of this new preparation method and a mathematical model describing our experimental data. A tunable self-organized surface provides an ideal playground for testing the reactivity and structure interplay. We introduce a study of the sulfidation of the nanostructured Cu(110)-(2×1)O surface. The reaction mechanism has been found to depend on the width of the oxidized stripes. Sulfidation of narrow CuO stripes proceeds via Cu-O chain abstraction and in the case of wide CuO stripes, the reaction mechanism is a combination of the chain detachment and S-c(2×2) island formation on the CuO stripes. We present a thorough STM study of the S island formation and their stability under UHV conditions. The S islands are mobile and undergo ripening (Ostwald and Smoluchowski). Additionally, island decay has been observed for sub-saturation S coverages.

Orientador: Abner de Siervo / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-24T13:15:36Z (GMT). No. of bitstreams: 1 Fatayer_ShadiPassam_M.pdf: 3223190 bytes, checksum: f98805b3695907c808260ec6d392c1b8 (MD5) Previous issue date: 2014 / Resumo: O estudo de sistemas moleculares em cima de substratos metálicos tem atraído uma crescente atenção da comunidade científica. O melhor entendimento sobre as características de auto-organização e a habilidade de controlá-las em moléculas tem gerado formas mais baratas e rápidas de usar a abordagem bottom-up em nanociência. Dentre os diversos estudos feitos, podemos citar o desenvolvimento de sensores de gás que utilizam do sinal magnético de uma camada auto-organizada de moléculas e da ligação de pequenas moléculas como CO ou NO que promovem a emergência de magnetismo na amostra. Outro aspecto interessante do estudo de sistemas moleculares se encontra na similariedade das moléculas que podem ser utilizadas com moléculas encontradas nos processos recorrentes na natureza, por exemplo, as clorofilas e hemoglobinas. Isto significa que ao estudar moléculas simples é possível mimetizar um comportamento parecido com o das moléculas citadas. Neste sentido, em nosso trabalho estudamos dois tipos de moléculas ¿ Porfirinas e Ftalocianinas ¿ e as propriedades estruturais quando depositadas em diferentes substratos metálicos. As porfirinas foram analisadas em uma superfície de baixo índice de Miller, Cu(111), e tiveram seu comportamento comparado com o análogo em superfícies vicinais, Au(332) e Au(788). As porfirinas formam estruturas em 1D quando depositadas em pequenas quantidades, dependendo da natureza do substrato e a largura de seu terraço. Em maiores coberturas, as porfirinas formam diferentes estruturas de empacotamento fechado em 2D, de simetrias quadrada e paralelogrâmica. Eletronicamente observou-se a modificação do entorno químico do níquel quando a molécula de NiTPP é adsorvida no Cu(111). As ftalocianinas foram depositadas em diferentes substratos visando a produção de co-organização de dois tipos de moléculas num padrão tabuleiro de xadrez. Após a obtenção do padrão de tabuleiro de xadrez, nós realizamos experimentos para elucidar os mecanismos que possibilitam formar tais estruturas. Com o intuito de estudar auto-organização molecular, nós empregamos técnicas sensíveis a superfícies como a Microscopia de Tunelamento, Espectroscopia de Tunelamento e Espectroscopia de Fotoemissão por Raios-X. Tais técnicas possibilitam a obtenção das propriedades estruturais e eletrônicas das nanoestruturas formadas / Abstract: The study of molecular systems on top of metal substrates has gathered increased atten-tion of the scientific community. Better understanding over different self-assembly haracteristics and the ability to control them in molecules has led to the development of quicker and cheaper routes of the use of the bottom-up approach in nanoscience. Among the diverse studies, we can cite the development of gas sensors that use the mag-netic signal of a self-assembled layer of molecules and the eventual binding of small molecules such as CO or NO leading to the emergence of magnetism on the sample. Another interesting aspect of the study of molecular systems is the similarity of molecules commonly used with molecules found in nature processes, e.g. chlorophylls and hemeglobins. This means that by studying simple molecules one can try to mimic the natural processes of those natural molecules. In this sense, in our work we have studied two classes of molecules ¿ Porphyrins and Phthalocyanines ¿ and their structural properties when deposited on different metal substrates. The porphyrins were analyzed on a low-index miller surface, Cu(111) and compared to their be-havior when deposited on vicinal substrates, Au(332) and Au(788). The porphyrins were ob-served to form 1D structures when deposited in small quantities depending on the nature of the substrate and its terrace width. At higher coverages, porphyrins formed different close-packed 2D structures, with square and parallelogram symmetry. Electronically was observed the modifica-tion of the chemical environment of nickel when NiTPP is adsorbed on Cu(111). The phthalo-cyanines were deposited on different substrates as well, towards the goal of producing co-assembling of two types of molecules as chessboard arrays. After the chessboard array was obtained, we gathered knowledge about the mechanisms that formed such structures. Towards the goal of studying molecular self-assembly, we have employed proper surface sensitive techniques such as Scanning Tunneling Microscopy, Scanning Tunneling Spectroscopy and X-Ray Photoelectron Spectroscopy. Such techniques allowed us to obtain the structural and electronic properties of the nanostructures formed / Mestrado / Física / Mestre em Física

Electron transfer processes are investigated through conductance measurements of single molecules. Measurements are performed on metal-molecule-metal junctions using a modified scanning tunneling microscope technique. Through a series of experimental measurements, and accompanying theoretical models, the influence of the molecule on the measured current is explored. These explorations are presented in five separate chapters. In chapter two, the molecular orbitals of sp-hybridized carbon chains are discussed in detail. It is demonstrated that the molecular orbitals can assume an intriguing helical shape. In chapter three, the length-dependent conductance of sp²-hybridized carbon chains is investigated. Experiment and theory demonstrate that the conductance of odd-numbered chains is nearly uniform with length. In chapter four, a new theoretical scheme to calculate quantum interference is developed. Using this scheme, it is demonstrated that quantum interference yields the decay in conductance with length for molecular wires. In chapter five, current-voltage measurements of redox-active molecular clusters are shown to agree with a hopping transport model. In chapter six, a novel experimental setup is presented that can be used to investigate photoconductivity in single-molecule junctions. This thesis provides a broad, yet rigorous, survey of electron transfer processes in single-molecule junctions.

In the last two decades, significant strides have been made towards utilizing the scanning tunneling microscope (STM) as a reaction chemistry tool, in addition to its primary use as an imaging instrument. Built off the STM, the STM-break junction (STM-BJ) technique was developed specifically for the reliable and reproducible measurement of properties of a single molecule suspended between two electrodes. These advances are crucial to the fields of molecular electronics and single-molecule reactivity, the latter also relating back to traditional bulk chemistry. By intelligently designing experiments and systems to probe with the STM and STM-BJ, we can begin to understand chemical processes on a deeper level than ever before. Chapter 1 provides an overview of the recent work using the STM and STM-BJ to effect chemical transformations which involve the making and breaking of bonds. We contextualize this progress in terms of single-molecule manipulation and synthetic chemistry, to understand the implications and outlook of this field of study. Seminal surface-based reactions are discussed, in addition to reactions that occur in both solution and within the single molecule junction. Differences between STM and STM-BJ capabilities and limitations are detailed, and the challenges of translating these fundamental experiments into functional reactions are addressed. Chapter 2 describes using the STM-BJ to study the binding of aryl iodides between gold electrodes. Important details regarding these binding modes, which were previously incompletely understood, are revealed via concrete experimental evidence. Our data suggests that this system, which is synthetically accessible, holds promise for forming the sought-after and highly conducting covalent gold-carbon bonds in situ and can be modulated with applied bias. Chapter 3 builds upon the knowledge gained in Chapter 2, and focuses on the reactivity of aryl iodides in the junction. We demonstrate a new in situ reaction of an Ullmann coupling, or dimerization, of various biphenyl iodides. By strategically designing the molecules studied, we are also able to gain mechanistic insight into this process, which in the bulk still remains debated, as well as demonstrate a cross-coupling reaction. This project is ongoing as of the submission of this dissertation, so other findings and continuing experiments are included. Chapter 4 transitions towards a different type of binder to gold, the cyclopropenylidene-based carbene. These amino-functionalized carbenes prove to be stronger linkers than N-heterocyclic carbenes, which are known binders to gold. Using a variety of surface analysis, imaging, and computational techniques, we explore the binding geometries and energies of cyclopropenylidenes, expanding the scope of carbene surface modifiers. Chapter 5 summarizes this body of PhD research, suggests directions for future work, and concludes the dissertation. These works explore the binding and reactivity of molecules on gold surfaces and within the single molecule junction, improving upon the understanding of this newly burgeoning field. This thesis seeks to encourage future work on these and related systems, to continue refining our comprehension of both junction and bulk reaction chemistry processes.

The scanning tunneling microscope-break junction (STM-BJ) technique is an ideal platform for single-molecule studies related to the design of molecular electronics. STM-BJ is particularly advantageous for molecular junctions for characterizing key properties of molecular conductance as well as many other related properties, which contribute to a growing understand of the mechanisms of electron transport on the single-molecular level. Prior STM-BJ studies have generally focused on simple systems with only one type of molecule forming one type of junction. However, some systems (such as those involve in-situ chemical reactions) are intrinsically complex with multiple molecules and junction structures that can be accessed in the experiment. The analysis of such complex systems requires more powerful analytical methods that can distinguish different junction types. Machine learning has been demonstrated as a powerful tool for the analysis of such large datasets. In this work, we develop tools to analyze, with a high-accuracy, individual junction characteristics using machine learning to classify the data and provide mechanistic understanding of the STM-BJ method.We start our work by investigating the imidazolyl linker. Imidazole is a five-member aromatic heterocycle with two nitrogen atoms, in which its pyridinic nitrogen can bind to gold electrodes. We study a series of alkanes of different lengths with two terminal 1-imidazolyl linker groups. While the intramolecular transmission across these molecules gives the pyridinic double peak, we find and prove that π-stacking between two imidazole rings is strong enough to form a third intermolecular conductance peak with higher conductance. This behavior is a good example where multiple types of junction are formed with just one molecule. Then, we focus on developing a trace-wise classification method using deep learning to resolve the data from such complicated systems of special molecules, mixture solutions, or in-situ¬ chemical reactions. Compared to existing methods, ours reduces the loss of information during the data preprocessing and demonstrates better performance by employing a convolutional neural network structure with larger capacity. Benchmarking with several commercially available molecules, we show that our model reaches up to 97% accuracy and outruns all the existing methods significantly. Nevertheless, we also demonstrate that our model can retain high accuracy when two essential parameters, the average conductance and the length of the molecular conductance plateau, are removed. Importantly, this capability has not been seen for the other algorithm designs. We then apply our method to an in-situ chemical reaction to realize the monitoring of the reaction process. This excellent performance of our model on the trace classification task demonstrates the capability of machine learning methods on STM-BJ data analysis. Finally, we also explore the feasibility of utilizing the machine learning toolkit in other types of analysis on molecular junctions. We study the relaxation of gold electrodes after junction rupture (termed “snapback”) and its relation to pre-rupture evolution of gold contact. With the assistance of machine learning tools, we reveal that while the snapback can be well explained by this evolution history, the length of molecular conductance plateau is not related to either the snapback or this history. We also discover that the junction formation probability for short molecules is negatively correlated to the extension of single-atomic gold contact. Based on these findings, we conclude that the major mechanism for a molecular junction formation involves a molecule bridging across the junction prior to the rupture of the gold contact, in contrast to the previously-accepted picture where the molecule is captured immediately following the rupture. As a conclusion, we apply machine learning/deep learning on STM-BJ data analysis by developing a model to efficiently classify STM-BJ traces with high accuracy, which is important for measuring complex systems containing multiple species. We also demonstrate the feasibility of analyzing junction formation mechanisms with the help of machine learning tools.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19318号 / 工博第4115号 / 新制||工||1634(附属図書館) / 32320 / 京都大学大学院工学研究科合成・生物化学専攻 / (主査)教授 松田 建児, 教授 北川 進, 教授 杉野目 道紀 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

付記する学位プログラム名: 充実した健康長寿社会を築く総合医療開発リーダー育成プログラム / 京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第21798号 / 工博第4615号 / 新制||工||1719(附属図書館) / 京都大学大学院工学研究科合成・生物化学専攻 / (主査)教授 松田 建児, 教授 杉野目 道紀, 教授 浜地 格 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

This thesis investigates the unique properties of 2D devices such as twisted semiconducting bilayers and a-RuCl₃ heterostructures employing scanning tunneling microscopy (STM) and spectroscopy (STS) probes. The research presented here sheds light on the vast opportunities that 2D materials provide in condensed matter systems as well as future device applications. Among 2D materials, transition metal dichalcogenide (TMD) heterobilayers provide a promising platform to study many quantum phenomena such as excitonic states due to their tunability of band gap. In addition, TMDs are excellent candidates to achieve localized states and carrier confinement, crucial for single photon emitters used in quantum computation and information. We begin this thesis with a brief overview of STM/STS and utilizing these techniques on 2D materials in the first and second chapters.The third chapter of this work investigates the twisted bilayer of WSe₂ and MoSe₂ in the H-stacking configuration using STM/STS which was previously challenging to measure. The spectroscopic results obtained from the heterobilayer indicate that a combination of structural rippling and electronic coupling generates unexpectedly large \moire potentials, in the range of several hundred meV. Our analysis reveals that the \moire structure and internal strain, rather than interlayer coupling, are the main factors of the moire potential. Large moire potentials lead to deeply trapped carriers such as electron-hole pairs, so-called excitons. Our findings open new routes toward investigating excitonic states in twisted TMDs. In the next chapter, we investigate the ultralocalized states of twisted WSe₂/MoSe₂ nanobubbles. Mechanical and electrical nanostructurings are expected to modify the band properties of transition metal dichalcogenides at the nanoscale. To visualize this effect, we use STM and near-field photoluminescence to examine the electronic and optical properties of nanobubbles in the semiconducting heterostructures. Our findings reveal a significant change in the local bandgap at the nanobubble, with a continuous evolution towards the edge of the bubble. Moreover, at the edge of the nanobubble, we show the formation of in gap bound states. A continuous redshift of the interlayer exciton on entering the bubble is also detected by the nano-PL. Using self-consistent Schrodinger-Poisson simulations, we further show that strong doping in the bubble region leading to band bending is responsible for achieving ultralocalized states. Overall, this work demonstrates the potential of 2D TMDs for developing well-controlled optical emitters for quantum technologies and photonics. We next turn to the effect of the electric field in band gap tuning of WSe₂/WS₂ heterobilayer. The tunability of band gap is a crucial element in device engineering to achieve quantum emitters. The electrostatic gate generates doping and an electric field giving access to continuous tunability, higher doping level, and integration capability to nanoelectronic devices. We employ scanning tunneling microscopy (STM) and spectroscopy (STS) to probe the band properties of twisted heterobilayer with high energy and spatial resolution. We observe continuous band gap tuning up to several hundreds of meV change by sweeping the back gate. We introduced a capacitance model to take into account the finite tip size leading to an enhanced electric field. The result of our calculation captures well the band gap change observed by STS measurements. Our study offers a new route toward creating highly tunable semiconductors for carrier confinement in quantum technology. In the next chapters, we focus on a-RuCl₃ heterointerfaces. We first explore the nanobubble of graphene/a-RuCl3 to create sharp p-n junctions. The ability to create sharp lateral p-n junctions is a critical requirement for the observation of numerous quantum phenomena. To accomplish this, we used a charge-transfer based heterostructure consisting of graphene and a-RuCl₃ to create nanoscale lateral p-n junctions in the vicinity of nanobubbles. Our approach relied on a combination of scanning tunneling microscopy (STM) and spectroscopy (STS), as well as scattering-type scanning near-field optical microscopy (s-SNOM), which allowed us to examine both the electronic and optical responses of these nanobubble p-n junctions. Our results showed a massive doping variation across the nanobubble with a band offset of 0.6 eV. Further, we observe the formation of an abrupt junction along nanobubble boundaries with an exceptionally sharp lateral width (<3 nm). This is one order of magnitude smaller length scale than previous lithographic methods. Our work paves the way toward device engineering via interfacial charge transfer in graphene and other low-density 2D materials. In chapter 7, we describe the use of low-temperature scanning tunneling microscopy (STM) measurements to observe the \moire pattern in graphene/a-RuCl3 heterostructure to validate the InterMatch method. This method is effective in predicting the charge transfer, strain, and stability of an interface. The InterMatch method was applied to moire patterns of graphene/a-RuCl3 to predict the stable interface structure. STM topographs show three regions with distinct moire wavelengths due to atomic reconstructions. Using the InterMatch method, we perform a comprehensive mapping of the space of superlattice configurations and we identify the energetically favorable superlattices that occur in a small range of twist angles. This range is consistent with the STM results. Moreover, the spectra on these regions exhibit strong resonances with the spacing between resonances following the expectation from Landau levels on a Dirac spectrum due to strain and doping. The results of our scanning tunneling microscopy (STM) measurements confirm that the InterMatch method is effective in predicting the charge transfer and stability of interfaces between materials. We next investigate WSe₂/a-RuCl₃ heterostructure through a multi-faceted approach. Our exploration encompassed diverse techniques such as STM, and optical measurements. We detect a significant charge transfer between the two layers by STM measurements, leading to a shift in the Fermi level towards the valence band of WSe₂. Our findings are supported by optical measurements and DFT calculations, which confirm the p-doped WSe₂ observed through STM. The results of this work highlight a-RuCl₃ potential for contact engineering of TMDs and unlocking their functionalities for the next generation optoelectronic devices. In the last chapter of this thesis, I provide a brief conclusion as well as a few future directions and insights for investigating 2D materials.