Dissertations / Theses on the topic 'Break junction'

A mechanically controllable break junction setup was designed, constructed and characterized. The mechanically controllable break junction technique is commonly used for measurement of quantum conductance of metals and single molecule conductance. The technique relies on resistance to external vibrations disrupting the atomic or molecular junctions formed and should be in a low electronic noise environment. Through a series of experiments the setup was found to have high mechanical stability and low electronic noise. The quantum conductance of gold was measured repeatedly and a histogram was plotted showing good agreement with the literature. The results indicate that with modifications, the setup can be used to measure the conductance of single molecule junctions and single molecule thermoelectric properties.

Genomic instability and acquisition of invasiveness through the basement membrane extracellular matrix (ECM) are two major processes for epithelial cell malignancy in breast cancer. DNA double-strand break repair (DSBR) is one of the processes that get misregulated during breast cancer progression. In addition, radiation induced breaks such as those induced during radiation therapy to treat breast cancer patients are repaired by DSBR, rendering this pathway relevant for therapy as well. DSBR can occur either by homologous recombination (HR) or non-homologous end-joining (NHEJ). HR is accepted as the more error-free pathway. HR is regulated by the cell cycle status such that an increase is observed in G2/M, whereas NHEJ is observed throughout the cell cycle. Previous data show that ECM signaling regulates HR, as well as the kinetics of ionizing radiation (IR) induced complex formation at break sites, or foci kinetics. Both human breast epithelial cell lines and primary mouse mammary epithelial cells were used to show that the ECM receptor β1-integrin is necessary and sufficient in down regulating HR, as well as IR induced foci formation kinetics for the DSBR proteins RAD51, MRE11, and γ-H2AX in single mammary epithelial cells. RAD51 is required for most HR, whereas MRE11 and γ-H2AX function in HR as well as DNA damage signaling. Interestingly, ECM signaling up-regulates HR in cells that have “correct” in vivo-like cell-cell junctions. Based on the observation that single cells and junctioned cells respond to ECM in exact opposite manner, I hypothesized that ECM signaling may interact with cell-cell junction signaling pathways in regulating DNA repair. To test this hypothesis, I asked whether the main breast epithelial adherens junction cadherin, E-cadherin, is involved. I blocked E-cadherin function using a monoclonal antibody MB2. The function blocking was demonstrated by the loss of cell-cell junction interactions and observation of increased cell scattering using phase microscopy. I then asked whether blocking E-cadherin altered the expression and localization of proteins related to DNA repair. Indirect immuno-fluorescence showed that in the E-cadherin blocked non-tumorigenic breast epithelial cell line HMT-3522 S1 there is an up-regulation of nuclear γ-H2AX and RAD51, as well as an increase in the proliferation marker Ki67. In non-proliferative MB2 blocked cells there is an upregulation of γ-H2AX and reduced Ki67. Furthermore, in these proliferative and non-proliferative blocked cells we were able to see lower levels of β-catenin near the cell membrane and an increase in its levels inside the cell especially in the nucleus. The latter has been confirmed also by western blot technique comparing the nuclear and cytoplasmic fraction expression. In addition, western blots showed that total RAD51 level was down-regulated by E-cadherin blocking and γ-H2AX levels were found to be higher in proliferative and non-proliferative MB2 treated cells. MB2 treated cells have a higher frequency of HR in the absence of ECM and in the presence of ECM, MB2 blocking abolishes the ECM effect on HR. Furthermore, in the absence of ECM, RAD51 siRNA treated cells down-regulated HR but the absence of RAD51 did not down regulate HR in the presence of ECM. I was not able to see any difference in the phosphorylated forms of β-catenin such as Tyr-142, Ser-45 and Tyr-86 that has the ability to enter into the nucleus. Therefore, E-cadherin was found to block nuclear β-catenin, RAD51 and γ-H2AX in a proliferation-independent manner. E-cadherin also was necessary for ECM to up-regulate HR. The up-regulation of HR by ECM was only slightly dependent on RAD51 suggesting a novel E-cadherin-dependent and RAD51-independent HR component in breast epithelial cells in contact with ECM as they are in vivo in the normal breast tissue. These experiments will help us to understand the role of E-cadherin and β-catenin in DNA double-stand break repair directly, as well as in combination with ECM signaling. Both alterations in integrin mediated signaling and cell-cell junction integrity contribute to breast cancer progression by rendering breast epithelial cells more invasive. My project will shed light on whether these invasive processes also alter DNA repair and contribute to genome stability. Understanding of the interrelationships among integrin signaling, cell-cell junctions, and genome stability will contribute to understanding normal breast cell processes and open up investigations on how these may go awry in cancer progression.

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
新制・課程博士
博士(医学)
甲第23091号
医博第4718号
京都大学大学院医学研究科医学専攻
(主査)教授 篠原 隆司, 教授 増永 慎一郎, 教授 小川 誠司
学位規則第4条第1項該当
Doctor of Medical Science
Kyoto University
DFAM

This thesis focuses on the theoretical description of coherent electronic transport in organic molecular junctions. The ab-initio theoretical methods and the theory of quantum transport in nanoscale systems are presented. The Landauer theory of transport formulated in terms of Green's function is analyzed by means of the embedding theory for a simplified model in which electrons are considered as moving in a one-dimensional modulated potential introduced to simulate resonant tunneling junctions. Following the introductory section, relevant systems of interest from both basic and technology points of view are investigated. The transport properties of two-dimensional graphene/graphene-nanoribbon (GNR) heterojunctions are shown to critically depend upon the geometrical features of the GNR. Diarylethene junctions with graphene electrodes are comprehensively analyzed, with emphasis on the photoswitching properties of the system. The use of graphene electrodes can improve the performance of such switching junctions as compared with the use of other substrates. A full characterization of a platinum/pyrazine bistable junction studied in a recent experiment is then established. The switching mechanism has been determined as a result of a molecule-lead configurational rearrangement. A final section is devoted to the description of a new methodology to calculate the elastic lifetimes of electronic states of adsorbates on surfaces. The method has been applied to dye molecules on TiO2 substrates, which are relevant for photovoltaics applications. The effects of modification of the spacers between the acceptor and donor part of the dyes are analyzed.

Chemistry
Ph.D.
Molecular electronics based on bottom-up electronic circuit design is a potential solution to meet the continuous need to miniaturize electronic devices. The development of highly conductive molecular wires, especially for long distance charge transfer, is a major milestone in the molecular electronics roadmap. A challenge presented by single molecule conductance is to define the relative influence of the molecular "core" and the molecular "interconnects" on the observed currents. Much focus has been placed on designing conductive, conjugated molecules. However, the electrode-molecule contacts can dominate the responses of metal-molecule-metal devices. We have experimentally and theoretically probed charge transfer through single phenyleneethynylene molecules terminated with thiol and carbodithioate linkers, using STM break-junction and non-equilibrium Green's function methods. The STM break-junction method utilizes repeatedly formed circuits where one or a few molecules are trapped between two electrodes, at least one of which has nanoscale dimensions. The statistical analysis of thousands of measurements yields the conductance of single molecules. Experimental data demonstrate that the carbodithioate linker not only augments electronic coupling to the metal electrode relative to thiol, but reduces the barrier to charge injection into the phenyleneethynylene bridge. The theoretical analysis shows that sulfur hybridization provides the genesis for the order-of-magnitude increased conductance in carbodithioate-terminated systems relative to those that feature the thiol linker. Collectively, these data emphasize the promising role for carbodithioate-based connectivity in molecular electronics applications involving metallic and semi-conducting electrodes. One of the strategies for building molecular wires that can transfer charge over long distance is to incorporate metal ions into the conductive molecular core. Peptide nucleic acid (PNA) is a great candidate for this purpose. Studying the conductivity of PNA can not only contribute to a better understanding of charge transfer through biomolecules, but can also help develop better molecular wires and other building blocks of molecular electronics. We study the charge transfer of PNA molecules using the STM break-junction technique and compare with traditional macroscopic voltammetric measurements. By measuring the resistance of different PNA molecules, we hope to develop a deep understanding of how charge transport though PNA is affected by factors such as the number and type of natural and artificial bases, embedded metal ions, pH, etc. Self-assembled monolayers (SAMs) of porphyrins are of great interest due to their diverse applications, including molecular devices, nano-templates, electrocatalysis, solar cells, and photosynthesis. We combined a molecular level study of the redox reactions using electrochemical scanning tunneling microscopy (EC-STM) with a macroscopic electrochemical technique, cyclic voltammetry (CV), to study two redox active porphyrin molecules, TPyP (5,10,15,20-Tetra(4-Pyridyl)-21H,23H-Porphine) and 5, 10, 15, 20-tetrakis (4-carboxylphenyl)-21H, 23H-porphine (TCPP). We showed that the adsorbed oxidized TPyP molecules slowly change to brighter contrast, consistent with the appearance of the reduced form of TPyP, under reduction condition (0.0VSCE). The time scale of the slow reduction is in the order of tens of minutes at 0.0VSCE, but accelerates at more negative potentials. We propose that protonation and deprotonation processes play an important role in the surface redox reaction due to geometric restriction of the molecules adsorbed on the surface. EC-STM and CV experiments were performed at various pH values to investigate the mechanism of this anomalously slow redox reaction. Our results show that the increased concentration of H+ hinders the reduction of porphyrins, a feature that has not been reported preciously. This provides insight into the details of the surface redox reaction.
Temple University--Theses

Thin-film niobium mechanically controlled break junctions and resistively shunted niobium mechanically-controlled break junctions were developed and successfully microfabricated. Using these devices, high-stability atomic size contacts were routinely produced and investigated both in the normal and superconducting states. Investigations of the two-level conductance fluctuations in the smallest contacts allowed the calculation of their specific atomic structure. Embedding resistive shunts close to the superconducting atomic-sized junctions affected the coherence of the electronic transport. Finally, point contact spectroscopy measurements provide evidence of the interaction of conduction electrons with the mechanical degrees of freedom of the atomic-size niobium contacts.

La bande transporteuse est utilisée dans l’industrie pour acheminer de la matière ou des objets dans de multiples domaines d’applications tels que les mines souterraines ou aériennes, les carrières, l’industrie alimentaire, l’industrie agricole ou les supermarchés. Ses composants lui permettent de travailler dans des conditions extrêmement difficiles. L’intérêt est porté sur les bandes transporteuses de mines de fond dont les contraintes de fonctionnement et d’acheminement sont particulières. En effet, la bande est acheminée sur rouleaux et jonctionnée sur le site de l’utilisateur pour former une bande sans fin. La jonction est le point faible de la bande transporteuse car sa résistance représente 50% seulement de la force de rupture nominale de la bande. L’objet de cette étude est de proposer une nouvelle solution de jonctionnement.Après avoir analysé les différentes contraintes mécaniques et chimiques auxquelles la bande transporteuse est soumise, une étude comparative sur les différents types de jonctions a été menée.Il y a les jonctions chimiques et les jonctions mécaniques. Chacune a des avantages et des inconvénients en termes de coût ou de longévité, qui ont été présentés. Dans un troisième temps, une étude a été menée sur la compréhension du comportement de rupture de la jonction mécanique. L’étude sur la jonction a été réalisée à l’aide d’un montage expérimental conçu spécifiquement. L’influence des paramètres de la jonction a pu être évaluée grâce à un nouvel indicateur adimensionnel représentant l’efficacité de la jonction par rapport à la bande normale (Junction Tensile Strength Efficiency : JTSE). Dans un quatrième temps, une étude a été faite sur un système de jonction innovant par couture. La jonction réalisée a été testée sur un banc de traction pour en vérifier l’efficacité, puis des essais ont été effectués sur un banc dynamique pour en étudier la fatigue. La jonction cousue est une innovation qui permet de dégager de nouvelles perspectives dans le jonctionnement des bandes transporteuses
The conveyor belt is used as a material and object transportation tool in a lot of industries such as underground mines, quarries, food industries, agribusinesses or supermarkets. The studied subject is the underground mine conveyor belt, because of their transportation and use difficult conditions.Indeed, the conveyor belt must be cut to be dispatched using belt reeling and joined afterwards in the underground mine forming an endless belt. However, the junction is the weakest part of the conveyor belt due to the 50% belt weakening. The aim of this study is to propose a new junction solution. After analysing the underground belt mechanical and chemical use conditions, the different junction types have been analysed. A junction advantage and inconvenience comparison has been presented. Then, the junction mechanical behaviour has been studied using an original mechanical setup reproducing the mechanical junction. The different parameter influence has been evaluated using a new adimensional indicator called junction tensile strength efficiency. In the last part, a new junction solution made by a sewn has been explored. The sewn junction tensile efficiency has been tested on a tensile strength machine and its fatigue efficiency has been verified through dynamic tests. New outlines in the conveyor belt joining are open as a result of the sewn junction solution development

Chemistry
Ph.D.
Understanding charge transport through molecular junctions and factors affecting the conductivity at the single molecule level is the first step in designing functional electronic devices using individual molecules. A variety of methods have been developed to fabricate metal-molecule-metal junctions in order to evaluate Single Molecule Conductance (SMC). Single molecule junctions usually are formed by wiring a molecule between two metal electrodes via anchoring groups that provide efficient electronic coupling and bind the organic molecular backbone to the metal electrodes. We demonstrated a novel strategy to fabricate single molecule junctions by employing the stabilization provided by the long range ordered structure of the molecules on the surface. The templates formed by the ordered molecular adlayer immobilize the molecule on the electrode surface and facilitate conductance measurements of single molecule junctions with controlled molecular orientation. This strategy enables the construction of orientation-controlled single molecule junctions, with molecules lacking proper anchoring groups that cannot be formed via conventional SMC methods. Utilizing Scanning Tunneling Microscopy (STM) imaging and STM break junction (STM-BJ) techniques combined, we employed the molecular assembly of mesitylene to create highly conductive molecular junctions with controlled orientation of benzene ring perpendicular to the STM tip as the electrode. The long range ordered structure of mesitylene molecules imaged using STM, supports the hypothesis that mesitylene is initially adsorbed on the Au(111) with the benzene ring lying flat on the surface and perpendicular to the Au tip. Thus, long range ordered structure of mesitylene facilitates formation of Au-π-Au junctions. Mesitylene molecules do not have standard anchoring groups providing enough contact to the gold electrode and the only assumable geometry for the molecules in the junction is via direct contact between Au and the π system of the benzene ring in mesitylene. SMC measurements for Au/mesitylene/Au junctions results in a molecular conductance value around 0.125Go, two orders of magnitude higher than the measured conductance of a benzene ring connected via anchoring groups. We attributed this conductance peak to charge transport perpendicular to the benzene ring due to direct coupling between the π system and the gold electrode that happens in planar orientation. The conductance we measured for planar orientation of benzene ring is two order of magnitude larger than conductance of junctions formed with benzene derivatives with conventional linkers. Thus, altering the orientation of a single benzene-containing molecule between the two electrodes from planar orientation to the upright attached via the linkers, results in altering the conductivity in a large order. Based on these findings, by utilizing STM imaging and STM-BJ in an electrochemical environment including potential induced self-assembly formation of terephthalic acid, we designed an electrochemical single molecule switch. Terephthalic acid forms large domains of ordered structure on negatively charged Au(111) surface under negative electrochemical surface potentials with the benzene ring lying flat on the surface due to hydrogen bonding between carboxylic acid groups of neighboring molecules. Formation of long range ordered structure facilitates direct contact between the π system of the benzene ring and the gold electrodes resulting in the conductance peak. On positively charged Au(111), deprotonation of carboxylic acid groups leads to absence of long range ordered structure of molecules with planar orientation and absence of the conductance peak. In this case alternating the surface (electrode) potential from negative to positive charge densities induces a transition in the adlayer structure on the surface and switches conductance value. Hence, electrochemical surface potential can, in principle, be employed as an external stimulus to switch single molecule arrangement on the surface and the conductance in the junction. The observation of conductance switching due to molecule’s arrangement in the junction lead to the hypothesis that for any benzene derivative, an orientation-dependent conductance in the junction due to the contact geometry (i.e. electrode-anchoring groups versus direct electrode-π contact) should be expected. Conventional techniques in fabricating single molecule junctions enable accessing charge transport along only one direction, i.e., between two anchoring groups. However, molecules such as benzene derivatives are anisotropic objects and we are able to measure an orientation-dependent conductance. In order to systematically study anisotropic conductivity at single molecule level, we need to measure the conductance in different and well-controlled orientations of single molecules in the junction. We employed the same EC-STM-BJ set up for SMC measurements and utilize electrochemical potential of the substrate (electrode) as the tuning source to variate the orientation of the single molecule in the junction. We investigated single molecule conductance of the benzene rings with carboxylic acid functional groups in two orientations: one with the benzene ring bridging between two electrodes using carboxylic acids as anchoring groups (upright); and one with the molecule lying flat on the substrate perpendicular to the STM tip (planar). Physisorption of these species on the Au (111) single crystal electrode surface at negative electrochemical potentials results in an ordered structure with the benzene ring in a planar orientation. Positive electrochemical potentials cause formation of the ordered structure with molecules standing upright due to coordination of a deprotonated carboxyl groups to the electrode surface. Thus, formation of the single molecule junction and consequently conductivity measurements is facilitated in two directions for the same molecule and anisotropic conductivity can be studied. In engineering well-ordered two-dimensional (2-D) molecular structures with controlled assembly of molecular species, pH can be employed as another tuning source for the molecular structures and adsorption in experiments conducted in aqueous solutions. Based on simple chemical principles, amine (NH2) groups are hydrogen bond acceptors and donors. Amines are soluble in water and protonation results in protonated (NH3+) and unprotonated (NH2) amine groups in acidic and moderately acidic/neutral solutions, respectively. Thus, amines are suitable molecular building blocks for fabricating 2-D supramolecular structures where pH is employed as a knob to manipulate intermolecular hydrogen bonding leading to phase transitions. We investigated pH induced structural changes in the 1,3,5–triaminobenzene (TAB) monolayer and the formation/disruption of hydrogen bonds between neighboring molecules. Our STM images indicate that in the concentrated acidic solution, the protonated amine groups of TAB are not able to form H-bonds and long range ordered structure of TAB does not form on the Au(111) surface. However, in moderately acidic solution (pH ~ 5.5) at room temperature, protonation on the ring carbon atom generates species capable of forming H-bonds leading to the formation of the long range ordered structures of TAB molecules. Utilizing EC-STM set up, we investigated the controllable fabrication of a TAB 2-D supramolecular structure based on amine-amine hydrogen bonding and effect of pH in formation of ordered/disordered TAB network.
Temple University--Theses

SYNOPSIS OF THIS THESIS The aim of this thesis is to more fully understand and explain the binding mechanism of organic molecules to the Au(111) surface and to explore the conduction of such molecules. It consists of five discreet chapters connected to each other by the central theme of “The Single Molecule Device: Conductance and Binding”. There is a deliberate concentration on azine linkers, in particular those with a 1,10-phenanthroline-type bidentate configuration at each end. This linker unit is called a “molecular alligator clip” and is investigated as an alternative to the thiol linker unit more commonly used. Chapter 1 places the work in the broad context of Molecular Electronics and establishes the need for this research. In Chapter 2 the multiple break-junction technique (using a Scanning Tunnelling Microscope or similar device) was used to investigate the conductance of various molecules with azine linkers. A major finding of those experiments is that solvent interactions are a key factor in the conductance signal of particular molecules. Some solvents interfere with the molecule’s interaction with and attachment to the gold electrodes. One indicator of the degree of this interference is the extent of the enhancement or otherwise of the gold quantized conduction peak at 1.0 G0. Below 1.0 G0 a broad range for which the molecule enhances conduction indicates that solvent interactions contribute to a variety of structures which could bridge the electrodes, each with their own specific conductance value. The use of histograms with a Log10 scale for conductance proved useful for observing broad range features. vi Another factor which affects the conductance signal is the geometric alignment of the molecule (or the molecule-solvent structure) to the gold electrode, and the molecular alignment is explored in Chapters 3 for 1,10-phenanthroline (PHEN) and Chapter 4 for thiols. In Chapter 3 STM images, electrochemistry, and Density Functional Theory (DFT) are used to determine 1,10-phenanthroline (PHEN) structures on the Au(111) surface. It is established that PHEN binds in two modes, a physisorbed state and a chemisorbed state. The chemisorbed state is more stable and involves the extraction of gold from the bulk to form adatom-PHEN entities which are highly mobile on the gold surface. Surface pitting is viewed as evidential of the formation of the adatom-molecule entities. DFT calculations in this chapter were performed by Ante Bilic and Jeffery Reimers. The conclusions to Chapter 3 implicate the adatom as a binding mode of thiols to gold and this is explored in Chapter 4 by a timely review of nascent research in the field. The adatom motif is identified as the major binding structure for thiol terminated molecules to gold, using the explanation of surface pitting in Chapter 3 as major evidence and substantiated by emergent literature, both experimental and theoretical. Furthermore, the effect of this binding mode on conductance is explored and structures relevant to the break-junction experiment of Chapter 2 are identified and their conductance values compared. Finally, as a result of researching extensive reports of molecular conductance values, and having attempted the same, a simple method for predicting the conductance of single molecules is presented based upon the tunneling conductance formula.

PhD
SYNOPSIS OF THIS THESIS The aim of this thesis is to more fully understand and explain the binding mechanism of organic molecules to the Au(111) surface and to explore the conduction of such molecules. It consists of five discreet chapters connected to each other by the central theme of “The Single Molecule Device: Conductance and Binding”. There is a deliberate concentration on azine linkers, in particular those with a 1,10-phenanthroline-type bidentate configuration at each end. This linker unit is called a “molecular alligator clip” and is investigated as an alternative to the thiol linker unit more commonly used. Chapter 1 places the work in the broad context of Molecular Electronics and establishes the need for this research. In Chapter 2 the multiple break-junction technique (using a Scanning Tunnelling Microscope or similar device) was used to investigate the conductance of various molecules with azine linkers. A major finding of those experiments is that solvent interactions are a key factor in the conductance signal of particular molecules. Some solvents interfere with the molecule’s interaction with and attachment to the gold electrodes. One indicator of the degree of this interference is the extent of the enhancement or otherwise of the gold quantized conduction peak at 1.0 G0. Below 1.0 G0 a broad range for which the molecule enhances conduction indicates that solvent interactions contribute to a variety of structures which could bridge the electrodes, each with their own specific conductance value. The use of histograms with a Log10 scale for conductance proved useful for observing broad range features. vi Another factor which affects the conductance signal is the geometric alignment of the molecule (or the molecule-solvent structure) to the gold electrode, and the molecular alignment is explored in Chapters 3 for 1,10-phenanthroline (PHEN) and Chapter 4 for thiols. In Chapter 3 STM images, electrochemistry, and Density Functional Theory (DFT) are used to determine 1,10-phenanthroline (PHEN) structures on the Au(111) surface. It is established that PHEN binds in two modes, a physisorbed state and a chemisorbed state. The chemisorbed state is more stable and involves the extraction of gold from the bulk to form adatom-PHEN entities which are highly mobile on the gold surface. Surface pitting is viewed as evidential of the formation of the adatom-molecule entities. DFT calculations in this chapter were performed by Ante Bilic and Jeffery Reimers. The conclusions to Chapter 3 implicate the adatom as a binding mode of thiols to gold and this is explored in Chapter 4 by a timely review of nascent research in the field. The adatom motif is identified as the major binding structure for thiol terminated molecules to gold, using the explanation of surface pitting in Chapter 3 as major evidence and substantiated by emergent literature, both experimental and theoretical. Furthermore, the effect of this binding mode on conductance is explored and structures relevant to the break-junction experiment of Chapter 2 are identified and their conductance values compared. Finally, as a result of researching extensive reports of molecular conductance values, and having attempted the same, a simple method for predicting the conductance of single molecules is presented based upon the tunneling conductance formula.

The use of single molecules as circuit elements represents the ultimate in device miniaturization. The mechanically controlled break junction (MCBJ) technique is a commonly used method for the formation of metal-molecule-metal junctions. Using MCBJs, the electrical properties of single molecules can be investigated. In this thesis, an MCBJ experimental setup for the study of charge transport is described. Early investigations began with gold wire break junctions, eventually progressing to lithographically fabricated break junctions. In particular, the measurement electronics, LabVIEW programming and mechanical setup underwent significant revisions, leading to improvements in noise floor, sensitivity, and speed. The resulting setup is capable of measuring the charge transport properties of single molecule junctions accurately and at a high rate.

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
新制・課程博士
博士(医科学)
甲第23114号
医科博第125号
京都大学大学院医学研究科医科学専攻
(主査)教授 斎藤 通紀, 教授 篠原 隆司, 教授 滝田 順子
学位規則第4条第1項該当
Doctor of Medical Science
Kyoto University
DFAM

De manière ultime, l'électronique moléculaire aspire à utiliser une molécule unique comme partie active d'un composant. Une telle réalisation fusionnerait l'énorme potentiel de la chimie aux technologies les plus avancées des nanosciences. Cependant, les propriétés de transport électronique d'une seule molécule restent, aujourd'hui, l'enjeu de débats animés qui s'appuient sur des calculs et sur de trop rares expériences. Les expériences sont, en effet, difficiles car elles nécessitent de pouvoir fabriquer des électrodes de contact dont l'écartement correspond à la taille de la molécule. Notre travail contribue au développement de telles techniques instrumentales dont l'intérêt dépasse celui de l'électronique moléculaire et englobe, plus généralement, le transport électronique à l'échelle nanométrique. Dans la première partie, nous décrivons d'abord la technique. Elle fait appel à un microscope à effet tunnel modifié pour fabriquer des électrodes nanométriques (technique des jonctions brisées). Cette approche combine en fait deux domaines de recherche qui sont d'une part, les mesures de conductance moléculaire et, d'autre part, les contacts atomiques ponctuels. Plus précisément, la physique de la formation et du transport d'électrons dans ces derniers est particulièrement étudiée. Après avoir décrit l'instrumentation développée, nous présentons donc des résultats à la fois sur des contacts atomiques ponctuels (jonction Au-Au) et sur des contacts moléculaires (jonction Au-molécule-Au). Notamment, la quantification de la conductance et le transport balistique sont mis en évidence. Cela montre que la présence d'une seule molécule peut être décelée électriquement. Nous soulignons qu'en dépit des énormes progrès apportés par cette technique à la détermination de la conductance d'une molécule, la disparité des résultats expérimentaux reportés reste importante. Nous clôturons la première partie en insistant sur l'impérieuse nécessité d'études statistiques rigoureuses à partir des nombreuses données expérimentales. Nous effectuons ce travail pour les jonctions Au-Au. Dans la seconde partie nous développons des outils d'analyse statistique. Ils permettent d'extraire de chaque mesure de conductance d'une nanojonction d'Au les paramètres indispensables à leur étude (le temps de vie par exemple). La statistique de ces paramètres sur des dizaines de milliers de mesures dans différentes conditions expérimentales est discutée et, outre les aspects de transport, donne des informations sur la mécanique de ces nanosystèmes (i.e. sur des mécanismes de rupture de la nanojonction). Les outils développés permettent d'observer des effets fins. Il est montré qu'une petite fraction des électrons échappe au transport balistique. Enfin, nous montrons l'existence de fluctuations bistables et discutons de leur effet sur le transport balistique et de leur rapport avec les mouvements atomiques.

In our pursuit of ever smaller transistors, with greater computational throughput, many questions arise about how material properties change with size, and how these properties may be modelled more accurately. Metallic nanocontacts, especially those for which magnetic properties are important, are of great interest due to their potential spintronic applications. Yet, serious challenges remain from the standpoint of theoretical and computational modelling, particularly with respect to the coupling of the spin and lattice degrees of freedom in ferromagnetic nanocontacts in emerging spintronic technologies. In this thesis, an extended method is developed, and applied for the first time, to model the interplay between magnetism and atomic structure in transition metal nanocontacts. The dynamic evolution of the model contacts emulates the experimental approaches used in scanning tunnelling microscopy and mechanically controllable break junctions, and is realised in this work by classical molecular dynamics and, for the first time, spin-lattice dynamics. The electronic structure of the model contacts is calculated via plane-wave and local-atomic orbital density functional theory, at the scalar- and vector-relativistic level of sophistication. The effects of scalar-relativistic and/or spin-orbit coupling on a number of emergent properties exhibited by transition metal nanocontacts, in experimental measurements of conductance, are elucidated by non-equilibrium Green’s Function quantum transport calculations. The impact of relativistic effects during contact formation in non-magnetic gold is quantified, and it is found that scalar-relativistic effects enhance the force of attraction between gold atoms much more than between between atoms which do not have significant relativistic effects, such as silver atoms. The role of non-collinear magnetism in the electronic transport of iron and nickel nanocontacts is clarified, and it is found that the most-likely conductance values reported for these metals, at first- and lastcontact, are determined by geometrical factors, such as the degree of covalent bonding in iron, and the preference of a certain crystallographic orientation in nickel.
Physics
Ph. D. (Physics)

碩士
輔仁大學
物理學系
92
Abstract Room-temperature conductance quantization at a mechanically controlled break junction We have studied conductance quantization through point-contact junctions of metal wires using a mechanically controlled wire-breaking device in ambient condition. This experiment demonstrates that the conduction through a nano constriction between two electron reservoirs can be treated by Landauer's formula,where G0 is the unitary conductance 2e^2/h , n denotes each possible conduction channel, and T is the transmission coefficient for each channel at the junction. The Landauer’s formula states a specific quantum size effect: T either equal to 1 or 0, in a ballistic constriction, e.g., the physical dimension of the confinement is much smaller than the electron mean free path. We have tested quantized conductance with various metals such as Au, Pt, Cu, and Ag. For Pt junctions, the standard deviation of a quantized conductance was found to be the smallest. This could be explained by a lower Pt diffusivity at room temperature. The junctions of Au and Pt also behaved differently as stretched. The conductance of Au tended to drop stepwise to the lowest quantized G whereas the Pt junctions often drifted back to a higher G value. Keyword：conductance、ballistic transport、Landauer's formula、 Mechanically Controlled Break Junction、nanowire

碩士
國立清華大學
物理學系
88
Abstract The superconducting proximity effect can be used to produce Josephson junctions in HTS structures with a normal conducting barrier. We describe a work on "break-film" junctions made by Au bridge to link the upper and lower layer of a YBa2Cu3O7(YBCO)-SrTiO3(STO)-YBa2Cu3O7 (YBCO) trilayer. The structures are grown in situ by pulse laser deposition. In our early structure we observed no supercurrent. That is because the exposure of the film in air before N-layer evaporation gives rise to an interface potential barrier. When T increases from 4.2K, the gap feature broadens until it is barely resolved around 65K. Such behavior is in general agreement with the BTK theory. We observed zero-bias anomaly (ZBA) spectra in some of our "break-film" junctions, which can also be explained by the BTK theory with the s-wave symmetry of the superconducting order parameter of YBCO. Our junction has a critical current density about 1.6*104 A/cm2 at 4.2K, IcR-n=0.125mV. The characteristic of the junction implies a high potential for applications.

abstract: Charge transport in molecular systems, including DNA (Deoxyribonucleic acid), is involved in many basic chemical and biological processes. Studying their charge transport properties can help developing DNA based electronic devices with many tunable functionalities. This thesis investigates the electric properties of double-stranded DNA, DNA G-quadruplex and dsDNA with modified base. First, double-stranded DNA with alternating GC sequence and stacked GC sequence were measured with respect to length. The resistance of DNA sequences increases linearly with length, indicating a hopping transport mechanism. However, for DNA sequences with stacked GC, a periodic oscillation is superimposed on the linear length dependence, indicating a partial coherent transport. The result is supported by the finding of delocalization of the highest occupied molecular orbitals of Guanines from theoretical simulation and by fitting based on the Büttiker’s theory. Then, a DNA G4-duplex structures with a G-quadruplex as the core and DNA duplexes as the arms were studied. Similar conductance values were observed by varying the linker positions, thus a charge splitter is developed. The conductance of the DNA G-tetrads structures was found to be sensitive to the π-stacking at the interface between the G-quadruplex and DNA duplexes by observing a higher conductance value when one duplex was removed and a polyethylene glycol (PEG) linker was added into the interface. This was further supported by molecular dynamic simulations. Finally, a double-stranded DNA with one of the bases replaced by an anthraquinone group was studied via electrochemical STM break junction technique. Anthraquinone can be reversibly switched into the oxidized state or reduced state, to give a low conductance or high conductance respectively. Furthermore, the thermodynamics and kinetics properties of the switching were systematically studied. Theoretical simulation shows that the difference between the two states is due to a difference in the energy alignment with neighboring Guanine bases.
Dissertation/Thesis
Doctoral Dissertation Chemistry 2016

碩士
國立臺灣大學
化學研究所
101
Measuring the conductance of metal-molecule-metal (MMM) junctions is the most fundamental and direct way to investigate molecular electronics, which correlates the structure of molecules to their electrical behaviors under nanometer-scale. To create suitable widths of the electrode-gaps for the formation of MMM junctions, most experimental techniques start from the breaking events of metallic point contacts and monitor the current variation to detect the signal caused by the bridging of molecules. Devices with wide current windows are required to conduct the procedure mentioned above. A dynamic signal acquisition device with high resolving ability is connected to the commercial scanning tunneling microscopy (STM). The dynamic range of data acquired during STM break junction (STM-bj) is widened from 3 orders to 6 orders of magnitude, in the range of which it is made possible to record the conductance distributions of gold point contacts and MMM junctions in one experiment setup. The conductance of a series of porphyrin derivatives were measured in the narrower dynamic range of the commercial STM before. In previous results, two sets of conductance distribution are shown in STM I(s) method, during which no gold point contacts occur; but the higher one is absence in STM-bj, which form gold point contacts in each cycle. However, the noise level of the commercial STM is much higher than the range of the lower one in the setup of STM-bj. It is unsure whether the gold point contacts hinder the more conductive junctions only. With the improvement of the data acquisition, the conductance and junction geometry of these porphyrin derivatives are probed by STM I(s) method and STM-bj again. In STM-bj, the conductance distributions of both gold point contacts and the less conductive junctions are recorded. It is clearly confirmed that only the more conductive junctions are hinders by gold point contacts and the possibility of possessing a tilted angle-confined geometries in these junctions is raised. The limits of the junction stretching distances are close to the distance between the two nitrogen atoms in the anchoring groups. This result further proves that the less conductive junctions bridge the electrodes by anchoring. In STM I(s) method, three set of conductance distribution are observed. The highest one is above the upper limit of the previous current windows and is attributed to the junction by lying porphyrins on the gold surfaces.

abstract: Understanding charge transport in single molecules covalently bonded to electrodes is a fundamental goal in the field of molecular electronics. In the past decade, it has become possible to measure charge transport on the single-molecule level using the STM break junction method. Measurements on the single-molecule level shed light on charge transport phenomena which would otherwise be obfuscated by ensemble measurements of groups of molecules. This thesis will discuss three projects carried out using STM break junction. In the first project, the transition between two different charge transport mechanisms is reported in a set of molecular wires. The shortest wires show highly length dependent and temperature invariant conductance behavior, whereas the longer wires show weakly length dependent and temperature dependent behavior. This trend is consistent with a model whereby conduction occurs by coherent tunneling in the shortest wires and by incoherent hopping in the longer wires. Measurements are supported with calculations and the evolution of the molecular junction during the pulling process is investigated. The second project reports controlling the formation of single-molecule junctions by means of electrochemically reducing two axial-diazonium terminal groups on a molecule, thereby producing direct Au-C covalent bonds in-situ between the molecule and gold electrodes. Step length analysis shows that the molecular junction is significantly more stable, and can be pulled over a longer distance than a comparable junction created with amine anchoring bonds. The stability of the junction is explained by the calculated lower binding energy associated with the direct Au-C bond compared with the Au-N bond. Finally, the third project investigates the role that molecular conformation plays in the conductance of oligothiophene single-molecule junctions. Ethyl substituted oligothiophenes were measured and found to exhibit temperature dependent conductance and transition voltage for molecules with between two and six repeat units. While the molecule with only one repeat unit shows temperature invariant behavior. Density functional theory calculations show that at higher temperatures the oligomers with multiple repeat units assume a more planar conformation, which increases the conjugation length and decreases the effective energy barrier of the junction.
Dissertation/Thesis
Ph.D. Materials Science and Engineering 2013

The aim of my Thesis was to develop a general synthetic methodology for the preparation of long helicenes equipped with suitable functional groups that control their solubility or serve as anchoring groups for attachment to metallic surfaces, especially gold. The well-established transition metal catalyzed [2+2+2] cyclotrimerization of triynes was selected as the key scaffold-forming transformation in the synthesis of long helicenes because of its high regioselectivity, atom efficiency, functional group tolerance and general robustness. A modular approach was used for the preparation of the starting oligoynes, thus enabling a high level of their structural diversity. Individual resorcinol- based aromatic building blocks were interconnected by Sonogashira cross-coupling reactions, providing complex cyclization precursors encompassing up to twelve alkyne units pre-arranged for the multiple [2+2+2] cycloisomerization to produce three six- membered rings from each set of three neighboring alkyne units. Thus, a small series of long helicenes with up to 19 rings constituting the helical scaffold was synthesized. The quadruple cyclization leading to the longest oxahelicene prepared to date was performed in a high-temperature-high-pressure flow reactor at 250 řC in the presence of CpCo(CO)2. The set of...

abstract: Studying charge transport through single molecules tethered between two metal electrodes is of fundamental importance in molecular electronics. Over the years, a variety of methods have been developed in attempts of performing such measurements. However, the limitation of these techniques is still one of the factors that prohibit one from gaining a thorough understanding of single molecule junctions. Firstly, the time resolution of experiments is typically limited to milli to microseconds, while molecular dynamics simulations are carried out on the time scale of pico to nanoseconds. A huge gap therefore persists between the theory and the experiments. This thesis demonstrates a nanosecond scale measurement of the gold atomic contact breakdown process. A combined setup of DC and AC circuits is employed, where the AC circuit reveals interesting observations in nanosecond scale not previously seen using conventional DC circuits. The breakdown time of gold atomic contacts is determined to be faster than 0.1 ns and subtle atomic events are observed within nanoseconds. Furthermore, a new method based on the scanning tunneling microscope break junction (STM-BJ) technique is developed to rapidly record thousands of I-V curves from repeatedly formed single molecule junctions. 2-dimensional I-V and conductance-voltage (G-V) histograms constructed using the acquired data allow for more meaningful statistical analysis to single molecule I-V characteristics. The bias voltage adds an additional dimension to the conventional single molecule conductance measurement. This method also allows one to perform transition voltage spectra (TVS) for individual junctions and to study the correlation between the conductance and the tunneling barrier height. The variation of measured conductance values is found to be primarily determined by the poorly defined contact geometry between the molecule and metal electrodes, rather than the tunnel barrier height. In addition, the rapid I-V technique is also found useful in studying thermoelectric effect in single molecule junctions. When applying a temperature gradient between the STM tip and substrate in air, the offset current at zero bias in the I-V characteristics is a measure of thermoelectric current. The rapid I-V technique allows for statistical analysis of such offset current at different temperature gradients and thus the Seebeck coefficient of single molecule junctions is measured. Combining with single molecule TVS, the Seebeck coefficient is also found to be a measure of tunnel barrier height.
Dissertation/Thesis
Ph.D. Electrical Engineering 2012

abstract: This dissertation describes the work on two projects which involves measuring molecular conductance and studying their properties on the nanoscale using various Scanning Tunneling Microscopy (STM) techniques. The first molecule studied was a porphyrin-fullerene moiety known as a molecular Dyad for photovoltaic applications. This project is further divided into two section, the first one involving the characterization of the Dyad monolayers and conductance measurement in the dark. The Dyads are designed to form charge separated states on illumination. The lifetime of the charged states have been measured efficiently but the single-molecule conductance through the molecules have yet to be characterized. The second part of the project describes the set-up of a novel sample stage which enables the study of molecular conductance under illumination. This part also describes the subsequent study of the molecule under illumination and the observation of a unique charge-separated state. It also contains the verification of the presence of this charge-separated using other characterization techniques like transient absorption spectroscopy. The second project described in the dissertation was studying and comparing the predicted rectifying nature of two molecules, identical in every way except for one stereocenter. This project describes the formation of monolayers of the molecule on gold and then studying and analyzing the current-voltage characteristics of the molecules and looking for rectification. Both the molecules proved to be rectifying, one more than the other as predicted by theoretical calculations.
Dissertation/Thesis
Ph.D. Chemistry 2011