In this thesis I concentrate on the description of electron transport properties of microscopic objects, including molecular junctions and nano junctions, in particular, inelastic electron tunneling in surface-adsorbate systems are examined with more contemplations. Boosted by the rapid advance in experimental techniques at the microscopic scale, various electric experiments and measurements sprung up in the last decade. Electric devices, such as transistors, switches, wires, etc. are expected to be integrated into circuit and performing like traditional semiconductor integrated circuit (IC). On the other hand, detailed information about transport properties also provides new physical observable quantities to characterize the systems. For molecular electronics, which is in the state of growing up, its further applications demands more thorough understanding of the underlying mechanism, for instance, the effects of molecular configuration and conformation, inter- or intra-molecular interactions, molecular-substrate interactions, and so on. Inelastic electron tunneling spectroscopy (IETS), which reflects vibration features of the system, is also a finger print property, and can thus be employed to afford the responsibility of single molecular identification with the help of other experimental techniques and theoretical simulations.There are two parts of work presented in this thesis, the first one is devoted to the calculation of electron transport properties of molecular or nano junctions: we have designed a negative differential resistance (NDR) device based on graphene nanoribbons (GNRs), where the latter is a star material in scientific committee since its birth;The transport properties of DNA base-pair junctions are also examined by theoretical calculation, relevant experimental results on DNA sequencing have been explained and detailed issues are suggested.The second part focused on the simulation of scanning tunneling microscope mediated IETS (STM-IETS). We have implemented a numerical scheme to calculate the inelastic tunneling intensity based on Tersoff-Hamann approximation and finite difference method, benchmark results agree well with experimental and previous theoretical ones; Two applications of single molecular chemical identification are also presented following benchmarking. / QC20100630

Common to molecular electronics studies, nanoscale break junctions created through electromigration also naturally produce electroluminescent arrays of individual gold nanoclusters spanning the electrodes. Due to inelastic electron tunneling into cluster electronic energy levels, these several-atom nanoclusters (Au~18-22) exhibit bright, field-dependent, antibunched emission in the near infrared (650800 nm), acting as room-temperature electrically driven single-photon sources. AC electrical excitation with time-stamping of photon arrival times enables fast and local tracking of electrode-nanocluster coupling dynamics demonstrating that charge injection to the clusters is directly modulated by dynamic coupling to individual electrodes. The electrode-nanocluster coupling rate fluctuates by nearly an order of magnitude and, due to the asymmetry of the electromigration process, exhibits preferential charge injection from the anode. Directly reporting on (and often facilitating) nanoscale charge transport, time-tagged single-molecule electroluminescence reveals a significant mechanism for nanoscale charge transport in nanoscale gold break junctions, and offers direct readout of the electrode-molecule interactions that can be correlated with current flow. Single-molecule electroluminescence techniques for characterization of electrode heterogeneity and dynamics as well as implications for future research are discussed.

<p>A theoretical framework for calculating electron transport through molecular junctions is presented. It is based on scattering theory using a Green's function formalism. The model can take both elastic and inelastic scattering into account and treats chemical and physical bonds on equal footing. It is shown that it is quite reliable with respect to the choice of functional and basis set. Applications concerning both elastic and inelastic transport are presented, though the emphasis is on the inelastic transport properties. The elastic scattering application part is divided in two part. The first part demonstrates how the current magnitude is strongly related to the junction width, which provides an explanation why experimentalists get two orders of magnitude differences when performing measurements on the same type of system. The second part is devoted to a study of how hydrogenbonding affects the current-voltage (I-V) characteristics. It is shown that for a conjugated molecule with functional groups, the effects can be quite dramatic. This shows the importance of taking possible intermolecular interactions into account when evaluating and comparing experimental data. The inelastic scattering part is devoted to get accurate predictions of inelastic electron tunneling spectroscopy (IETS) experiments. The emphasis has been on elucidating the importance of various bonding conditions for the IETS. It is shown that the IETS is very sensitive to the shape of the electrodes and it can also be used to discriminate between different intramolecular conformations. Temperature dependence is nicely reproduced. The junction width is shown to be of importance and comparisons between experiment as well as other theoretical predictions are made.</p>

A theoretical framework for calculating electron transport through molecular junctions is presented. It is based on scattering theory using a Green's function formalism. The model can take both elastic and inelastic scattering into account and treats chemical and physical bonds on equal footing. It is shown that it is quite reliable with respect to the choice of functional and basis set. Applications concerning both elastic and inelastic transport are presented, though the emphasis is on the inelastic transport properties. The elastic scattering application part is divided in two part. The first part demonstrates how the current magnitude is strongly related to the junction width, which provides an explanation why experimentalists get two orders of magnitude differences when performing measurements on the same type of system. The second part is devoted to a study of how hydrogenbonding affects the current-voltage (I-V) characteristics. It is shown that for a conjugated molecule with functional groups, the effects can be quite dramatic. This shows the importance of taking possible intermolecular interactions into account when evaluating and comparing experimental data. The inelastic scattering part is devoted to get accurate predictions of inelastic electron tunneling spectroscopy (IETS) experiments. The emphasis has been on elucidating the importance of various bonding conditions for the IETS. It is shown that the IETS is very sensitive to the shape of the electrodes and it can also be used to discriminate between different intramolecular conformations. Temperature dependence is nicely reproduced. The junction width is shown to be of importance and comparisons between experiment as well as other theoretical predictions are made. / QC 20101118

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Pour la première fois, en associant un microscope à effet tunnel (STM) et un microscope optique inversé,nous avons imagé les plasmons de surface excités électriquement sur un film d’or avec la pointe d’un STM.Par microscopie de fuite radiative, en observant l’image de l’interface air/or et celle du plan de Fourierassocié, nous avons distingué les plasmons propagatifs des plasmons localisés sous la pointe. Les plasmonspropagatifs sont caractérisés par une distance de propagation et une direction d’émission en accord aveccelles de plasmons propagatifs créés par excitation laser sur des films d’or de mêmes épaisseurs. Les fuitesradiatives des plasmons localisés s’étalent jusqu’à l’angle maximum d’observation. Plasmons propagatifs etlocalisés ont une large bande spectrale dans le visible. Si la pointe est plasmonique (en argent), lesplasmons localisés ont une composante supplémentaire due au couplage associé. Pour différents types depointe, nous avons déterminé les intensités relatives des plasmons localisés et propagatifs. Nous trouvonsque chaque mode plasmon (propagatif ou localisé) peut être préférentiellement sélectionné en modifiant lematériau de la pointe et sa forme. Une pointe en argent produit une intensité élevée de plasmons localisés,tandis qu’une pointe fine de tungstène (rayon de l’apex inférieur à 100 nm) produit essentiellement desplasmons propagatifs. Nous avons étudié la cohérence spatiale des plasmons propagatifs excités par la pointe du STM. Avec un film d’or opaque (épaisseur 200 nm) percé de paires de nanotrous nous avons réalisé une expérienceanalogue à celle des fentes d’Young. Des franges d’interférences sont observées. La mesure de leurvisibilité en fonction de la distance des nanotrous donne une longueur de cohérence des plasmons de 4.7±0.5 μm. Cette valeur, très proche de la valeur 3.7± 1.2 μm déduite de la largeur de la distribution spectraledes plasmons, indique que l’élargissement spectral des plasmons propagatifs est homogène.Nous avons aussi étudié la diffusion des plasmons propagatifs excités par la pointe du STM par desnanoparticules d’or déposées sur un film d’épaisseur 50 nm. Nous observons une diffusion élastique et unediffusion radiative. Des franges d’interférences sont observées dans la région d’émission lumineuseinterdite du plan de Fourier, dont la période est inversement proportionnelle à la distancepointe-nanoparticule d’or avec un facteur de proportionnalité égal à la longueur d’onde moyenne desplasmons. Il y a donc interférence entre la radiation des plasmons localisés et la radiation provenant de ladiffusion des plasmons propagatifs sur les nanoparticules d’or. Ceci indique que les plasmons localisés etpropagatifs excités électriquement par la pointe du STM sont différentes composantes du plasmon uniqueproduit par effet tunnel inélastique avec la pointe du STM. Ces résultats originaux sur les plasmons créés sur film d’or par un effet tunnel inélastique localisé à l’échelle atomique (i) élargissent la compréhension du processus et (ii) offrent des perspectives intéressantes pour une association de la nanoélectronique et de la nanophotonique. / For the first time, using a equipment combining a scanning tunneling microscope (STM) and an invertedoptical microscope, we excite and directly image STM-excited broadband propagating surface plasmons ona thin gold film. The STM-excited propagating surface plasmons have been imaged both in real space andFourier space by leakage radiation microscopy. Broadband localized surface plasmons due to the tip-goldfilm coupled plasmon resonance have also been detected. Quantitatively, we compare the intensities ofSTM-excited propagating and localized surface plasmons obtained with different STM tips. We find that the intensity of each plasmon mode can be selectively varied by changing the STM tip shape or material composition. A silver tip produces a high intensity of localized surface plasmons whereas a sharp (radius < 100 nm) tungsten tip produces mainly propagating surface plasmons. We have investigated the coherence of STM-excited propagating surface plasmons by performingexperiments on a 200 nm thick (opaque) gold film punctured by pairs of nanoholes. This work is analogousto Young’s double-slit experiment, and shows that STM-excited propagating surface plasmons have acoherence length of 4.7±0.5 μm. This coherent length is very close to the value 3.7±1.2 μm expected fromthe spectrum, which indicates that the spectrum broadening of STM-excited surface plasmons ishomogeneous. We have also studied the in-plane and radiative scattering of STM-excited propagating surface plasmons bygold nanoparticles deposited on a 50 nm thick gold film. In the Fourier space images, interference fringesare observed in the forbidden light region. This interference occurs between STM-excited localized surfaceplasmons (radiating at large angles from the tip position) and the radiative scattering by the goldnanoparticle of STM-excited propagating surface plasmons. This indicates that STM-excited localized andpropagating surface plasmons are different components of the same single plasmon produced by inelasticelectron tunneling with the STM tip. These results not only broaden the understanding about the excitation process of STM excited surface plasmons but also offer interesting perspectives for the connection between nanoelectronics andnanophotonics.

La Spectroscopie par Effet Tunnel Inélastique (IETS) avec un Microscope à Effet Tunnel (STM) est une nouvelle technique de spectroscopie vibrationnelle, qui permet de caractériser des propriétés très fines de molécules adsorbées sur des surfaces métalliques. Des règles de selection d’excitation vibrationnelle basées sur la symétrie ont été proposées, cependant, elles ne semblent pas exhaustives pour expliquer la totalité du mécanisme et des facteurs en jeu; elles ne sont pas directement transposables pour les propriétés d'un adsorbat et sont lourdes d'utilisation. Le but de cette thèse est donc d'améliorer ces règles de selection par une étude théorique. Un protocole de simulation de l'IETS a été développé, paramétré, et évalué, puis appliqué pour calculer des spectres IETS pour différentes petites molécules, qui sont systématiquement liées, sur une surface de cuivre. Des principes additifs de l'IETS ont été developpés, notamment concernant l’extension dans le vide de l’état de tunnel, l'activation/ quench sélectif de certains modes du aux propriétés électroniques de certains fragments moléculaires, et l'application de certaines règles d'addition de signaux IETS. De plus, des empreintes vibrationnelles par des signaux IETS ont été determinées pour permettre de différentier entre les orientations des adsorbats, la nature chimique des atomes et les isomères de structures. Une stratégie simple utilisant les propriétés de distribution de la densité électronique de la molécule isolée pour prédire les activités IETS sans des couts importants de calculs a aussi été développée. Cette expertise a été utilisée pour rationaliser et interpréter les mesures expérimentales des spectres IETS pour des métalloporphyrines et métallophtalocyanines adsorbées. Ces études sont les premières études IETS pour des molécules aussi larges et complexes. L'approche expérimentale a permis de déterminer les limitations actuelles des simulations IETS. Les défauts associés à l'identification ont été résolus en faisant des simulations d'images STM complémentaires. / Inelastic Electron Tunneling Spectroscopy (IETS) with the Scanning Tunneling Microscope (STM) is a novel vibrational spectroscopy technique that permits to characterize very subtle properties of molecules adsorbed on metallic surfaces. Its proposed symmetry-based propensity selection rules, however, fail to fully capture its exact mechanism and influencing factors; are not directly retraceable to an adsorbate property and are cumbersome. In this thesis, a theoretical approach was taken to improve them. An IETS simulation protocol has been developed, parameterized and benchmarked, and consequently used to calculate IETS spectra for a set of systematically related small molecules on copper surfaces. Extending IETS principles were deduced that refer to the tunneling state’s vacuum extension, the selective activating/quenching of certain types of modes due to the moieties’ electronic properties, and the applicability of a sum rule of IETS signals. Also, fingerprinting IETS-signals that enable discrimination between adsorbate orientations, the chemical nature of atoms and structural isomers were determined and a strategy using straightforward electronic density distribution properties of the isolated molecule to predict IETS activity without (large) computational cost was developed. This expertise was used to rationalize and interpret experimentally measured IETS spectra for adsorbed metalloporphyrins and metallophthalocyanines, being the first IETS studies of this large size. This experimental approach permitted to determine the current limitations of IETS-simulations. The associated identification shortcomings were resolved by conducting complementary STM-image simulations.