In the present work we study transmission of electron through vibrat- ing molecular junction interacting with leads using a method of nonequilibrium Green functions. Our Green functions use combination of first quantization to treat vibrational levels and second quantization to treat electron processes. We treat the change of vibrational potential of junction induced by charging of junc- tion, we find a spectral function and a magnitude of current through the junction. We confirm the effect of Fermi sea and Frack-Condon factors on sharpening of peaks. We study these processes on model motivated by biphenyl molecule, in- troducing double well potential and study an effect of quasidegenerate energy levels. We present the shift of peaks, resulting from an increase of moment of inertia, to positions determined by Franck-Condon factors.

Carbon/molecule/Cu/Au molecular junctions were fabricated on 4-inch silicon wafers using microfabrication techniques common in commercial semiconductor manufacturing. Electron-beam deposited carbon films are introduced as substrates, and the junctions exhibited high yield and excellent reproducibility. Current density-voltage characteristics of the devices were area scaling, weakly dependent on temperature and exponentially on molecular film thickness, and quantitatively similar to those of devices made with other techniques reported previously in our group, which contained pyrolyzed photoresist films as substrates. Furthermore, the test of cycle life and thermal stability reveals that the devices can survive at least under several millions of potential cycles at room temperature in air, and elevated temperature up to 150 C in vacuum for >40 hours. Parallel fabrication, thermal stability, and high yield are required for practical applications of molecular electronics, and the reported results provide important steps toward integration of molecular electronic devices with commercial processes and devices.

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This thesis focuses on theoretical study of charge transportproperties in molecular systems. The understanding of the transportprocess and mechanism in molecular systems is essential forthe design of new functional molecular materials and molecularelectronic devices. The molecular junctions and organic molecularcrystals have been used as the model systems to highlight the usefulnessof theoretical modelling. A molecular junction is a system that consists ofone or several molecules sandwiched between two electrodes.The charge transport in molecular junctions is a very complex processthat is affected by the interaction between molecules and electrodes,the surroundings, as well as electron-electron (e-e) andelectron-phonon (e-p) couplings. When the molecule-electrode couplingis strong, the transport process can be very quick. If the e-p couplingis weak, the inelastic tunneling has only negligible contributions to thetotal current and the elastic electron tunneling plays the dominant role.Furthermore, the hopping process becomes dominant in the case of strong e-pcoupling, for which the geometric relaxation of the molecule needsto be considered. In this thesis, we have examined these three kinds oftransport processes separately. The first studied system is a molecular junction consisting of aromaticallycoupled bimolecules. Its elastic electron tunneling property is simulatedusing Green's functional theory at density functional theory level.The dependence of the conductance of bimolecular junctions on the vertical distances,horizontal distances and the tilt angles has been systematically studied. Theinelastic electron tunneling spectra (IETS) of molecular junctions have beencalculated for several systems that were experimentally measured with conflictingresults and controversial assignments. Our calculations provide the reliableassignments for the experimental spectra and revealed unprecedented detailsabout the molecular conformations within the junctions under different conditions.It demonstrates that a combined theoretical and experimental IETS study is capableof accurately determining the structure of a single molecule inside the junction.The hopping process is a dominant charge transfer process in organic molecularcrystals. We have studied the charge transport ability of four kinds of n-typeorganic semiconductor materials to find out the related structure-to-propertyrelationship. It is done by adopting the quantum charge transfer rate equationcombined with the random walk approach. / QC 20120515

Electronic devices, for logical and memory applications, are constructed based on bistable electronic units that can store binary information. Molecular electronics proposes the use of single molecules—with two distinctive states of conductance—as bistable units that can be used to create more complex electronic devices. The conductance of a molecule is strongly influenced by the contacts used to address it. The purpose of this work is to determine the electrical characteristics of several candidate molecular junctions, which are composed of a molecule and contacts. Specifically, we are interested in determining whether binary information, “0” or “1,” can be encoded in the low- and high-conductance states of the molecular junctions. First, we calculate quantum-mechanically the electronic structure of the molecular junction. Second, the continuous electronic states of the contacts, originated from their infinite nature, are obtained by solving the Schrödinger equation with periodic boundary conditions. Last, the electron transport through the molecular junctions is calculated based on a chemical interpretation of the Landauer formalism for coherent transport, which involves the information obtained from the molecule and the contacts. Metal-molecule-metal and metal-molecule-semiconductor junctions are considered. The molecule used is an olygo(phenylene ethynylene) composed of three benzene rings and a nitro group in the middle ring; this molecule is referred hereafter as the nitroOPE molecule. Gold, silicon, and metallic carbon nanotubes are used as contacts to the molecule. Results from the calculations show that the molecular junctions have distinctive states of conductance for different conformational and charge states. High conductance is found in the conformation in which all the benzene rings of the nitroOPE are coplanar. If the middle benzene ring is made perpendicular to the others, low conductance is found. Also, the negatively charged junctions (anion, dianion) show low conductance. Whenever a semiconducting contact is used, a flat region of zero current is found at low bias voltages. The results indicate that the use of Si contacts is possible; however, because of the flat region, the operating point of the devices needs to be moved to higher voltages.

Transport through nanostructures is studied at the many-body level using exact diagonalization and nonequilibrium Green's functions. Organic molecular junctions are a particular focus because of their technological promise. Work is presented regarding: (1) A π-electron model of organic molecular junctions developed using effective field theory; (2) series transmission and transmission node structure in interacting systems; (3) the effect of interactions on quantum interference and thermoelectricity in polycyclic junctions; and (4) nanoscale transport calculations using self-consistent statistical ensembles.

Les polyoxométallates (POMs) de type Keggin [XM12O40]n- (avec X=P… et M= W, Mo …) sont des oxydes moléculaires de métaux de transition à haut degré d’oxydation qui présentent des réductions successives et réversibles dans une gamme étroite de potentiel. Leur incorporation dans des dispositifs de mémoire moléculaire semble alors prometteuse. Nous avons développé leur intégration comme briques élémentaires via une approche “bottom-up” qui surmonte les limites de l’approche “top-down” plus commune. Cela nécessite un contrôle fin de leur greffage et de leur densité surfacique pour l’obtention de dispositifs performants. Nous avons donc développé le greffage covalent de POMs sur des surfaces à base de silicium et d’or. Des hybrides de POMs à terminaison diazonium BA3[PM11O39{SnC6H4C≡CC6H4N2}] (avec M=W or Mo) ont formé des monocouches complètes greffées sur des surfaces de silicium hydrogénées qui mettent en évidence l’influence du métal constitutif du fragment polyoxométallate sur les propriétés de transport de charges de la jonction. Le greffage d’hybrides de POM à terminaison acide carboxylique sur des substrats d’oxyde de silicium a aussi été développée. Un hybride de POM à terminaison aniline TBA4[PW11O39{SnC6H4C≡CC6H4NH2}] a été greffé en deux étapes via un couplage peptidique sur une monocouche à terminaison acide carboxylique sur surface d’or. Des monocouches compactes ont été obtenues mais pas de façon totalement reproductible et les premiers essais de dilution sont encourageants. Une nouvelle famille d’hybrides de POMs mixtes a été synthétisée : TBA4[PMoxW11-xO39{SnR}]. Cela permettra de combiner les propriétés redox du molybdène et la robustesse due au tungstène. / Keggin type polyoxometalates (POMs) [XM12O40]n- (with X=P… and M= W, Mo…) are molecular oxides of early transition metals with a high oxidation state. They present electrochemical successive reversible reduction waves in a narrow range of potential. This makes them good candidates to be incorporated into molecular memory devices. We chose a “bottom-up” approach where the POMs are the building blocks to overcome the limitation of the “top-down” process commonly used. A fine control of the POM grafting and of the surface density is essential to get better erase/writing time of the device. So, we developed POM hybrids for grafting them covalently onto silicon based and gold substrates. Diazonium-terminated POM hybrids (with M=W or Mo) lead to compact homogeneous monolayers onto hydrogenated silicon surfaces. Electrical measurements of the two analogous modified surfaces show different behaviour highlighting the role of the constituting POM metal into the charge transport. Carboxylic-terminated POM hybrids have also been grafted in a one-step process onto silicon oxide surface resulting in a smooth and dense monolayer. Then, an aniline-terminated POM hybrid has been grafted onto a carboxylic-terminated SAM of thiols onto gold thanks to a peptide coupling. Compact monolayers have been obtained without complete reproducibility and the first attempts of dilution are encouraging. A new family of POM hybrids have been synthesized: mixed-metal POM hybrids TBA4[PMoxW11-xO39{SnR}]. This will permit to combine the electrochemical properties of molybdenum and the robustness of tungsten.

Cette thèse présente une étude des propriétés électroniques et optoélectroniques de fils moléculaires de polythiophènes uniques par microscopie à effet tunnel (STM). Ces fils moléculaires sont obtenus par une polymérisation sur surface Au(111). Une étude spectroscopique (STS) est réalisée sur les fils adsorbés sur la surface. Cette étude met en évidence un phénomène de confinement électronique pour deux conformations de fils différentes (linéaire et cyclique). Ces molécules uniques sont ensuite suspendues par manipulation entre la pointe et la surface du STM pour obtenir une jonction moléculaire. Les propriétés de transport de cette jonction sont étudiées mettant notamment en évidence l’influence sur la conductance des orbitales moléculaires du fil, et des contraintes mécaniques. Enfin cette thèse présente une expérience inédite d’émission de photons de la jonction. Cette étude permet de détecter la fluorescence d’une molécule unique directement connectée à deux électrodes. / This thesis presents a study on electronic and optoelectronic properties of polythiophene molecular wires by scanning tunneling microscopy (STM). First, molecular wires are synthesized on a Au(111) surface . A spectroscopic study (STS) is realized on these molecular wires adsorbed on the surface. This study shows phenomena of electronic confinement for two different wire conformations (linear and cyclic). Then, by manipulation we manage to suspend a single polythiophenes wire between the tip and the surface of the STM. The transport properties of this molecular junction are investigated. We show that molecular orbitals of the wire are involved in the electronic transport. We observe an influence of the mechanical stress on the conductance, too. Finally, this thesis presents an original experiment of the photon emission of this molecular junction. With this study, we manage to detect the fluorescence of a single molecule directly bridging metallic electrodes.

Diese Arbeit präsentiert eine vollständige quantenmechanische Beschreibung eines Systems bestehend aus einem Molekül und einem metallischem Nanopartikel (MNP) in der Gegenwart eines Strahlungsfeldes. Zuerst wird ein System aus einem Molekül und einem Gold-MNP betrachtet. Das Emissionsund Absorptionsspektrum zeigt viele scharfe molekulare Schwingungssatelliten auf einem breiten Plasmonmaximum. Eine Verstärkung der Schwingungssatelliten um drei Größenordnungen ist auf effiziente Absorption und Emission durch die MNP zurückzuführen. Dann wird ein System aus einer molekularen Kette mit einem Gold-MNP untersucht. Alle zuvor genannten Phänomene treten auch hier auf, jedoch werden die Schwingungssatelliten durch das Exzitonenband der molekularen Kette ersetzt. Anschließend wird ein Nano-Laser aus vielen Molekülen und einem Gold-MNP betrachtet. Die Moleküle werden durch inkohärentes optisches Pumpen angeregt. Dabei wird eine starke Plasmonanregung durch die gemeinsame Kopplung an die Moleküle erreicht. Die Photonenemission des Lasers zeigt, dass die Intensität ansteigt, während die Linienbreite schmaler wird. Die Korrelationsfunktion in zweiter Ordnung für die Photonen in Verbindung mit der schmaler Emission könnte dabei sogar einen Hinweis auf Lasing geben. Zuletzt wird eine Nanoverbindung aus einem Molekül und zwei sphärischen metallischen Elektroden betrachtet. Das Molekül wird durch den sequentiellen Ladungstransfer angeregt. Durch die Kopplung an die Moleküle werden die Elektrodenplasmonen angeregt. Die Photonenemission der Verbindung zeigt, dass die scharfen molekularen Schwingungssatelliten um das Tausendfache verstärkt werden. Anschließend ist ein System aus zwei pyramidalen Elektroden, die seitlich von zwei Gold-MNP eingeschlossen werden, untersucht. Hier können die Schwingungssatelliten einzeln verstärkt werden, indem der Abstand zwischen den MNP variiert wird. Wir zeigen auch, dass das Lasing in einer Verbindung aus vielen Molekülen theoretisch möglich ist. / This thesis presents a unified quantum description of the combined molecule-metal nano-particle system in the presence of a radiation field. Firstly, a single molecule coupled to a gold nano-sphere is investigated. The emission and absorption spectrum show many sharp molecular vibrational satellites over one broad plasmon peak. The three orders of magnitude enhancement of the vibrational satellites is due to the great ability of the sphere to absorb and emit photons. Secondly, a molecular chain coupled to a gold nano-sphere is investigated. All the phenomena mentioned above appear also for such system, except that the vibrational satellites are replaced by the Frenkel exciton band of the molecular chain. Thirdly, a plasmonic nano-laser consisting of many dye molecules and a gold nano-sphere is considered. The molecules are initially excited by incoherent optical pump. The strong plasmon excitation of the sphere is achieved due to the concerted coupling with the molecules. The emission of the laser shows that the intensity is enlarged while the line-width is reduced. The second-order correlation function of photons together with the emission narrowing can be utilized to determine lasing operation. Finally, a nano-junction formed by a molecule and two spherical metallic leads is investigated. The molecule is excited through sequential electron transfer. The lead plasmons get excited due to the coupling with the excited molecule. The emission of the junction shows that the molecular vibrational satellites are about one thousand times enhanced by the lead plasmons. Then, a junction with two pyramidal metallic leads sandwiched by two gold nano-spheres is investigated. The simulations show that the molecular vibrational satellites can be selectively enhanced by varying the inter-sphere distance. It is also proved that the lasing can be realized by a junction with many molecules.

Dans une première partie, nous avons étudié quelques propriétés de molécules magnétiques impliquant des radicaux organiques (seuls ou conjointement avec des terres rares). Nous avons ainsi pu interpréter l'évolution de la susceptibilité magnétique et de l'aimantation en fonction de la température en évaluant par des approches ab initio fonctions d'onde les constantes d'échange ou le tenseur g au sein de ces matériaux. De plus, nous avons chercher à définir les conditions pour que des matériaux à base de radicaux organiques présentent simultanément des propriétés magnétiques et conductrices. Nous avons ainsi examiné différentes familles de composés et l'influence de la structure géométrique et chimique des radicaux organiques utilisés. Pour cette partie, nous avons extrait les intégrales physiques pertinentes par la méthode des Hamiltoniens effectifs.Dans une deuxième partie, nous avons utilisés ces quantités physiques (intégrale de saut, répulsion sur site, échange) pour décrire le phénomène de transport dans des jonctions pour lesquelles les effets de la corrélation électronique ne peuvent être écartés. Munis de ces paramètres ab initio, nous avons développé un modèle phénoménologique permettant de décrire la conduction moléculaire à l'aide d'un jeu d'équations maîtresses. Nous avons ainsi cherché à mettre en évidence l'intérêt des approches post Hartree-Fock empruntant une fonction d'onde corrélée et de spin adapté dans la description du transport électronique. Que ce soit dans le cas de transport polarisé en spin ou non, l'approche utilisée (mono ou multi-déterminentale) conditionne qualitativement et quantitativement la caractéristique courant/tension. / In a first part, we studied the magnetic properties of organic radicals (coupled with rare earth or between each other). We calculated the magnetic exchange and the g-tensor of these compounds to understand their magnetic susceptibility and thei magnetization curves via ab initio calculations based on the wave-function. We studied how the chemistry and the crystal stacking affect meaningful parameters linked to magnetism and conduction. Those parameters were extracted with the thory of effective Hamiltonians fo various families of organic radicals. From the observed trends for the different parameters, we predicted some ways to obtain multifunctional compounds. In a second part, we used the same parameters (hoping integral, coulombic repulsion, magnetic exchange) to describe transport properties through highly correlated molecular junctions. From the ab initio parameters, we developed a phenomenological model based on master equations to describe the electronic transport. We stressed the importance of a multiconfigurational description to reproduce properly the transport properties for spin unpolarized and spin polarized situations. In both cases, the mono- or multi-configurational description affects qualitatively and quantitatively the predicted conductance curve.