Academic literature on the topic 'STM-IETS'

AbstractThe adsorption behavior of single CO molecules at 4 K bound to Au adatoms on a Ag(001) metal surface is studied with scanning tunneling microscopy (STM) and inelastic electron tunneling spectroscopy (IETS). In contrast to earlier observations two different binding configurations are observed—one on top of a Au adatom and the other one adsorbed laterally to Au on Ag(001). Moreover, IETS reveals different low-energy vibrational energies for the two binding sites as compared to the one for a single CO molecule bound to Ag(001). Density functional theory (DFT) calculations of the adsorption energies, the diffusion barriers, and the vibrational frequencies of the CO molecule on the different binding sites rationalize the experimental findings.

Mes travaux théoriques concernent les potentialités du microscope STM pour induire deux types d’excitations locales, soit vibrationnelle soit électronique, sur des molécules aromatiques isolées adsorbées sur des surfaces. La spectroscopie vibrationnelle sous le microscope à effet tunnel (IETS pour «Inelastic Electron Tunneling Spectroscopy») est une technique récente qui mesure le spectre vibrationnel d’une seule molécule déposée sur une surface conductrice. A partir d’une tension seuil, on peut exciter une vibration moléculaire qui se traduit par un pic dans la dérivée de la conductance par rapport à la tension et la perte d’énergie de l’électron tunnel: c’est le signal vibrationnel inélastique. Pour des spectres mesurés sur Cu(100) de radicaux issus de la perte d’hydrogènes du benzène comme le phényl et le benzyne (groupe de W. Ho, Irvine), nos simulations du signal inélastique atteignent un niveau de comparaison quantitative permettant l’identification du radical. D’autre part, en dupliquant l’étude pour la déshydrogénation de la pyridine, nous montrons que l’hétéroatome en substitution ne modifie que l’intensité des signaux inélastiques. Enfin, nous montrons que les règles de sélection se rationalisent en tenant compte de la symétrie des états électroniques avant et après couplage électron-vibration et de l’état vibrationnel. Ensuite, nous nous intéressons à la réactivité électro-induite du biphényl sur Si(100) étudiée par STM (groupe de G. Dujardin, Orsay) qui montre une sélectivité régie par la polarité de l’impulsion de tension. D Par un calcul exhaustif de tous les chemins possibles, nous montrons que les barrières cinétiques de diffusion et de déshydrogénation fournissent la preuve que la sélectivité observée en fonction de la polarité d’impulsion de tension ne peut pas être attribuée directement à différentes énergies d'activation
The subject of my theoretical work deals with the capabilities of the STM tool to induce two types of local excitations, either vibrational or electronic on single adsorbed aromatic molecules on surfaces. Concerning vibrational excitations, the changes in tunneling conductance at vibrational thresholds have recently been used as an Inelastic Electron Tunneling Spectroscopy (IETS). We study the STM-induced dehydrogenation of benzene on Cu(100), where the reaction products could be either phenyl or benzyne fragments (group of W. Ho, Irvine). We demonstrate that they are solely identified with their theoretical IETS fingerprints being in quantitative agreement with the IETS measurements. We similarly investigate the dehydrogenation of pyridine and show that one heteroatom in the aromatic ring affects the magnitude of the IETS signatures. Conversely, we rationalize our findings in terms of inelastic propensity rules that couple the symmetry of the electronic scattering states and the molecular vibrators. In a second part, we study the electron-induced reactions of individual biphenyl molecules on a Si(100) surface, which have been investigated by using the tip of the STM as an atomic size source of electrons (group of G. Dujardin, Orsay). Selected types of molecular reactions are produced, depending on the polarity of the surface voltage during STM excitation. We determine all possible reaction pathways on the silicon surface, providing evidence that the observed selectivity as a function of the surface voltage polarity cannot be ascribed to different activation energies

2011/2012
The present work pertains to the surface science approach to heterogeneous catalysis. In particular model systems for CO2 hydrogenation to methanol, and NO selective catalytic reduction, are investigated by means of a combined approach, where the molecular-level insight provided by a low-temperature scanning tunneling microscope is complemented by density functional theory (DFT) calculations of their electronic structure. To this end, the Inelastic Electron Tunneling Spectroscopy (STM-IETS) technique was introduced for the first time in our laboratory, a recent development which allows to measure the vibrational spectrum of individual molecules adsorbed on a surface. Regarding CO2, we provide single molecule imaging and characterization of CO2/Ni(110), chemisorbed with high charge transfer from the substrate, in an activated state that plays a crucial role in the hydrogenation process. We obtain a detailed characterization of the adsorption geometries and an estimate of the energies corresponding to the different adsorbed states. A consistent picture of CO2 chemisorption on Ni(110) is provided on the basis of the newly available information, yielding a deeper insight into the previously existing spectroscopic and theoretical data. In the Selective Catalytic Reduction (SCR) process, nitrogen oxide is selectively transformed to N2 by reductants such as ammonia. The specificity of this reaction was tentatively attributed to the formation of NH3-NO coadsorption complexes, as indicated by several surface science techniques. Here we characterize the NH3-NO complex at the atomic scale on the (111) surface of platinum, investigating the intermolecular interactions that tune the selectivity. The structures that arise upon coadsorption of NH3 and NO are analyzed in terms of adsorption sites, geometry, energetics and charge rearrangement. An ordered 2 × 2 adlayer forms, where the two molecules are arranged in a configuration that maximizes mutual interactions. In this structure, NH3 adsorbs on top and NO on fcc-hollow sites, leading to a cohesional stabilization of the extended layer by 0.29 eV/unit cell. The calculated vibrational energies of the individually-adsorbed species and of the coadsorption structure fit the experimental values found in literature within less than 6%. The characterizations and optimizations that had to be tackled in order to successfully perform STM-IETS measurement are eventually presented, focusing in particular on an original method which allows to increase the achieved resolution. Namely, the modulation broadening associated to phase-sensitive detection is reduced by employing a tailored modulation function, different from the commonly-used sinusoid. This method is not limited to STM-IETS, but can be easily applied whenever a lock-in amplifier is used to measure a second derivative.
XXV Ciclo
1984

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

Organic molecules such as porphyrins and alkanethiols are currently being investigated for applications such as sensors, light-emitting diodes and single electron transistors. Porphyrins are stable, highly conjugated compounds and the choice of metal ion and substituents bound to the macrocycle as well as other effects such as chemical surrounding and cluster size modulate the electronic and photonic properties of the molecule. Porphyrins and their derivatives are relatively non-toxic and their very rich photo- and electro-chemistry, and small HOMO-LUMO gaps make them outstanding candidates for use in molecularly-enhanced electronic applications. For these studies, self-assembled tri-pyridyl porphyrin thiol derivatives have been fully characterized on Au(111) surfaces. A variety of surface characterization techniques such as Atomic Force Microscopy (AFM), Scanning Tunneling Microscopy (STM), FT-IR spectroscopy and X-ray photoelectron spectroscopy (XPS) have been implemented in order to obtain information regarding the attachment orientation based on the angle and physical height of the molecule, conductivity which is determined based on the apparent height and current-voltage (I-V) measurements of the molecule, conductance switching behavior due to conformational or other effects as well as the stability of the molecular ensembles. Specifically, the transport properties of free base and zinc coordinated tri-pyridyl porphyrin thiol molecular islands inserted into a dodecanethiol matrix on Au(111) were investigated using STM and cross-wire inelastic electron tunneling spectroscopy (IETS). The zinc porphyrin thiol islands observed by STM exhibited reversible bias induced switching at high surface coverage due to the formation of Coulomb islands of ca. 10 nm diameter driven by porphyrin aggregation. Low temperature measurements (~ 4 K) from crossed-wire junctions verified the appearance of a Coulomb staircase and blockade which was not observed for single molecules of this compound or for the analogous free base. Scanning probe lithography via nanografting has been implemented to directly assemble nanoscale patterns of zinc porphyrin thiols and 16-mercapotohexadecanoic acid on Au surfaces. Matrix effects during nanopatterning including solvent and background SAMs have been investigated and ultimately ~ 10 nm islands of zinc porphyrins have been fabricated which is the optimal size for the observed switching effect.

This chapter discusses various aspects of scanning tunneling spectroscopy (STS). It is an extension of the classical tunneling spectroscopy experiment to nanometer-scale or atomic-scale features on the sample surface. First, the electronics for STS is presented. The nature of STS as a convolution of tip DOS and sample DOS is discussed. Special tip treatment for the STS experiment, often different from the atomic-resolution STM, is described. The purpose is to produce tips with flat DOS, instead of special tip orbitals. Experimental methods to determine tip DOS is discussed. A detailed account of the inelastic scanning tunneling spectroscopy, or STM-IETS, is then discussed. It includes the principles, the electronics, and the instrumental broadening of the features. This chapter concludes with the STS study of superconductors, especially High-Tc supercondoctors.