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After evaporation of the organic solvents, benzene, toluene, and cyclohexane on gold substrates, Scanning Tunneling Microscope (STM) shows the presence of a remaining adsorbed layer. The different solvent molecules were individually observed at ambient conditions, and their electronic transport properties characterized through the STM in the Break Junction approach. The combination of both techniques reveals, on one hand, that solvents are not fully evaporated over the gold electrode and, secondly, characterize the electronic transport of the solvents in molecular electronics. This work was supported by the Spanish Government (MAT2016-78625-C2 and PID2019-109539 GB-C41) and the Generalitat Valenciana through PROMETEO/2017/139 and program CDEIGENT/2018/028.

This study describes the modulation of tunneling probabilities in molecular junctions by switching one of two parallel intramolecular pathways. A linearly conjugated molecular wire provides a rigid framework that allows a second, cross‐conjugated pathway to be effectively switched on and off by protonation, affecting the total conductance of the junction. This approach works because a traversing electron interacts with the entire quantum‐mechanical circuit simultaneously; Kirchhoff's rules do not apply. We confirm this concept by comparing the conductances of a series of compounds with single or parallel pathways in large‐area junctions using EGaIn contacts and single‐molecule break junctions using gold contacts. We affect switching selectively in one of two parallel pathways by converting a cross‐conjugated carbonyl carbon into a trivalent carbocation, which replaces destructive quantum interference with a symmetrical resonance, causing an increase in transmission in the bias window., Manipulating conductance in parallel molecular pathways is a crucial element in molecular electronics. We explore intramolecular parallel pathways with different bond topology, in form of a quantum circuit, in tunneling junctions comprising self‐assembled monolayers. We employ quantum‐interference switching/gating with an acid, without any structural changes, in single‐molecular junctions.

One of the principal aims of single-molecule electronics is to create practical devices out of individual molecules. Such devices are expected to play a particularly important role as novel sensors thanks to their response to wide ranging external stimuli. Here we show that the conductance of a molecular junction can depend on two independent stimuli simultaneously. Using a scanning tunnelling microscope break-junction technique (STM-BJ), we found that the conductance of 4,4′-vinylenedipyridine (44VDP) molecular junctions with Ni contacts depends on both the electrochemically applied gate voltage and the pH of the environment. Hence, not only can the Ni|44VDP|Ni junction function as a pH-sensitive switch, but the value of the pH at which switching takes place can be tuned electrically. Furthermore, through the simultaneous control of pH and potential the STM-BJ technique delivers unique insight into the acid-base reaction, including the observation of discrete proton transfers to and from a single molecule.

Using a scanning tunnelling microscope break-junction technique, we produce 4,4'-bipyridine (44BP) single-molecule junctions with Ni and Au contacts. Electrochemical control is used to prevent Ni oxidation and to modulate the conductance of the devices via nonredox gating--the first time this has been shown using non-Au contacts. Remarkably the conductance and gain of the resulting Ni-44BP-Ni electrochemical transistors is significantly higher than analogous Au-based devices. Ab-initio calculations reveal that this behavior arises because charge transport is mediated by spin-polarized Ni d-electrons, which hybridize strongly with molecular orbitals to form a "spinterface". Our results highlight the important role of the contact material for single-molecule devices and show that it can be varied to provide control of charge and spin transport.

Im Verlauf dieser Promotion wurden zwei unterschiedliche Themen mit unterschiedlichen wissenschaftlichen Fragestellungen bearbeitet. Teil I befasst sich mit den Vibrations-Eigenschaften von Membranen mit einer Dicke von einem bis hin zu wenigen hundert Nanometern und makroskopischen Abmessungen in lateraler Richtung. Es wird eine analytische Theorie zur Beschreibung von Moden-Form und Dispersions-Relation von Biege-Wellen hergeleitet, die daraufhin sowohl experimentell, als auch durch Finite-Elemente-Simulationen bestätigt wird.In Teil II geht es um den elektronischen Transport durch Gold-Kontakte aus wenigen Atomen oder im Tunnel-Bereich, die mit sichtbarem Licht beleuchtet werden. Die Kontakte werden mit Hilfe mechanisch kontrollierter Bruchkontakte erzeugt, die aus einer metallischen Leiterbahn mit einer Sollbruchstelle auf einer Silizium-Membran bestehen. Wird die Membran gedehnt, so bricht die Sollbruchstelle und es formt sich ein atomarer beziehungsweise ein Tunnel-Kontakt.Das Ziel dieser Experimente besteht darin, eine Änderung des Leitwerts nachzuweisen, die sich auf die Wechselwirkung zwischen Licht und Elektronen-Gas zurückführen lässt. Dazu ist es notwendig, einen solchen elektronischen Effekt von einer Leitwert-Änderung mit mechanischer Ursache, wie beispielsweise der thermischen Ausdehnung der Kontakt-Spitzen, unterscheiden zu können. Es wird ein Verfahren vorgestellt, wie sich diese Unterscheidung mit Hilfe gezielter Einkopplung von Vibrationen in den Kontakt experimentell realisieren lässt.Die wissenschaftlichen Fragestellungen der Teile dieser Arbeit, einerseits aus der klassischen Mechanik, andererseits aus dem Bereich des elektronischen Transport, sind voneinander unabhängig. Die Verwandtschaft beider Teile besteht in der experimentellen Methode: Während sich der erste Teil mit der Physik der Nanomembranen selbst beschäftigt, stellen sie für den zweiten Teil ein wichtiges Hilfsmittel dar, über das sich der metallische Kontakt kontrollieren lässt.

Molecular electronics tends to use a single molecule as a component of an electronic device. Such realization would merge the potential of chemistry potential with the most developed nanotechnologies. But there is still a long way to go before the electronics transport properties of a single molecule can be completely understood. Experimental realizations are difficult as they require contact of nanometer size electrodes. Our work is devoted to the development of this type of experimental technique which concerns not only molecular electronics but also electronic transport at nanometric scale. In the first part of the manuscript the experimental techniques are described. We use a modified scanning tunneling microscope to form nanosized electrodes (break junction technique). This approach combine two research domain namely charge transport in molecules and atomic point contact. More precisely, the formation and the electronic transport in atomic point contact are particularly studied. Afterwards we present our result on atomic point contact (Au-Au junction) and on molecular contact (Au-molecule-Au junction). Conductance quantization and ballistic transport are displayed. Our research shows that the electrical signature of a single molecule can be distinguished. But despite the progresses in single molecule conductance determination brought by theses experimental techniques, there is still a large disparities on reported results. At the end of the first part we concentrate on the importance of statistical studies on a large population of experimental data. The Au-Au junction is used in this study. In the second part we describe the specific statistical analysis tools which allow the extraction of the needed parameters (for example the lifetime) from each Au-Au junction conductance measurement. The statistical analyses of these parameters in different experimental conditions are discussed. They provide information on the mechanic of these nanosystems (i.e. on the breaking mechanism of the nanowire). The developed tools allow observing the subtle effects. It is shown that a little fraction of electron is not ballistic. Finally we show the existence of bistable fluctuations and discuss their effect on ballistic transport and their link with atomic movement.; 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.

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.

Tunneling measurements have been carried out on the Ba8Ga16Sn30. The break-junction was employed to obtain reliable tunneling spectra. The pseudo-gap like feature is obtained as 2Δ≈175 meV, at 4.2 K, which agrees with thermoelectric power data at low temperature. We have also obtained a small gap 2Δ≈30–40 meV.