In this thesis, the electronic properties in superconducting LiFeAs single crystal are investigated using low temperature scanning tunneling microscopy and spectroscopy (STM/S) at various temperatures. For this purpose, the differential conductance (dI/dV) measured by STS which is directly proportional to the local density of states (LDOS) of the sample to the sub-atomic precision, is used together with the topography information. The dI/dV spectra within the ±1 V energy range reveal a characteristic feature at around -350 mV to -400 mV in stoichiometric LiFeAs. This feature seems to be a universal property among all the Fe-based high temperature superconductors, because it is also found in Fe0.965Se1.035 and NaFe0.975Co0.025As single crystals at the energy of -210 mV and -200 mV, respectively. The temperature dependent spectroscopy data averaged over a spatially fixed clean area of 2 nm × 2 nm are successfully executed between 5 K and 20 K. The two distinct superconducting phases with critical temperatures Tc = 16 K and 18 K are observed. In addition, the distance between the dip position outside the superconducting gap and the superconducting coherence peak in the spectra remains temperature independent which confirms that it is not connected to an antiferromagnetic (AFM) spin resonance. The temperature dependent spectra have been measured between 5 K and 61 K within the energy range of ±100 mV as well. The hump structure at 42 mV tends to disappear around 60 K from unknown origin. The temperature dependent quasiparticle interference (QPI) has been studied within the temperature range between 6.7 K and 25 K and analyzed by the Fourier transformation of the measured spectroscopic maps. The dispersion plots in momentum space as a function of temperature show an enhancement of QPI intensity (±5.5 mV) within the superconducting gap at the Fermi level at 6.7 K near q ~ 0. This is interpreted on the basis of Andreev bound state. In both polarities outside of this, a depletion of QPI intensity is noticed between 5.5 mV and around 9 mV. At positive energies, the QPI intensity becomes very rich above 9 mV. The size of the enhanced QPI intensity near the Fermi level, and the edge of the rich QPI intensity beyond 9 mV are found to behave like superconducting order parameter with rising of temperature. Furthermore, an energy mode peaked at around 14 mV appears in the integrated QPI intensity below superconducting Tc (6.7 K). This is consistent with the observed peak at 1st derivative of the dI/dV spectra. In both of these cases, such 14 mV peak is suppressed at normal state (25 K). This mode is therefore directly related to superconductivity in LiFeAs. The off-stoichiometric LiFeAs single crystal with superconducting Tc of 6.5 K has a 10 mV rigid band shift of the Fermi level towards electron doping. The absence of the rich QPI intensity between 9 mV and 17 mV is found compared to the stoichiometric LiFeAs, and hence the 14 mV mode is absent here. This brings us to conclude once more time that such 14 mV energy mode is relevant for superconductivity in LiFeAs.

No description available.

Metal oxide reactions at metal oxide surfaces or at metal-metal oxide interfaces are of exceptional significance in areas such as catalysis, micro- and nanoelectronics, chemical sensors, and catalysis. Such reactions are frequently complicated by the presence of high electric fields and/or H2O-containing environments. The focus of this research was to understand (1) the iron oxide growth mechanism on Fe(111) at 300 K and 500 K together with the effect of high electric fields on these iron oxide films, and (2) the growth of alumina films on two faces of Ni3Al single crystal and the interaction of the resulting films with water vapor under non-UHV conditions. These studies were conducted with AES, LEED, and STM. XPS was also employed in the second study. Oxidation of Fe(111) at 300 K resulted in the formation of Fe2O3 and Fe3O4. The substrate is uniformly covered with an oxide film with relatively small oxide islands, i.e. 5-15 nm in width. At 500 K, Fe3O4 is the predominant oxide phase formed, and the growth of oxide is not uniform, but occurs as large islands (100 - 300 nm in width) interspersed with patches of uncovered substrate. Under the stress of STM induced high electric fields, dielectric breakdown of the iron oxide films formed at 300 K occurs at a critical bias voltage of 3.8 ± 0.5 V at varying field strengths. No reproducible result was obtained from the high field stress studies of the iron oxide formed at 500 K. Ni3Al(110) and Ni3Al(111) were oxidized at 900 K and 300 K, respectively. Annealing at 1100 K was required to order the alumina films in both cases. The results demonstrate that the structure of the 7 Å alumina films on Ni3Al(110) is k-like, which is in good agreement with the DFT calculations. Al2O3/Ni3Al(111) (γ'-phase) and Al2O3/Ni3Al(110) (κ-phase) films undergo drastic reorganization and reconstruction, and the eventual loss of all long-range order upon exposure to H2O pressure > 10-5 Torr. Al2O3/Ni3Al(110) film is significantly more sensitive to H2O vapor than the Al2O3/Ni3Al(111) film, and this may be due to the incommensurate nature of the oxide/Ni3Al(110) interface. STM measurements indicate that this effect is pressure- rather than exposure- dependent, and that the oxide instability is initiated at the oxide surface, rather than at the oxide/metal interface. The effect is not associated with formation of a surface hydroxide, yet is specific to H2O (similar O2 exposures have no effect).

Hydrogen processing is becoming increasingly important not only in the production of electricity but also during its accumulation. One of the energy storage options are liquid organic hydrogen carriers (LOHC). The main drawback of LOHC is the need for a large amount of thermal energy to release molecular hydrogen. We can bypass this issue using heterogeneous catalysis by transferring hydrogen from LOHC to acetone and using the produced 2-propanol (IPA) in the fuel cell. This innovative strategy of getting electri- cal energy from LOHC can be potentially energetically neutral. In this work, we studied highly ordered Co3O4(111) model catalysts for IPA oxidation in the as-prepared state and enhanced with platinum (Pt) nanoparticles. Catalysts were prepared by Physical Vapour Deposition (PVD) and further investigated by means of Low Energy Electron Diffrac- tion (LEED), X-ray Photoelectron Spectroscopy (XPS), Scanning Tunneling Microscopy (STM) and Temperature Programmed Desorption (TPD). The nucleation process of Pt on the as-prepared Co3O4(111) surface was studied by depositing low amounts 0.04 and 0.13 monolayer (ML) of Pt, that create clusters as small as 2 or 3 atoms with no need for a special nucleation site. We have identified the formation of Pt-induced defects in the atomically flat cobalt oxide...

Hydrogen processing is becoming increasingly important not only in the production of electricity but also during its accumulation. One of the energy storage options are liquid organic hydrogen carriers (LOHC). The main drawback of LOHC is the need for a large amount of thermal energy to release molecular hydrogen. We can bypass this issue using heterogeneous catalysis by transferring hydrogen from LOHC to acetone and using the produced 2-propanol (IPA) in the fuel cell. This innovative strategy of getting electri- cal energy from LOHC can be potentially energetically neutral. In this work, we studied highly ordered Co3O4(111) model catalysts for IPA oxidation in the as-prepared state and enhanced with platinum (Pt) nanoparticles. Catalysts were prepared by Physical Vapour Deposition (PVD) and further investigated by means of Low Energy Electron Diffrac- tion (LEED), X-ray Photoelectron Spectroscopy (XPS), Scanning Tunneling Microscopy (STM) and Temperature Programmed Desorption (TPD). The nucleation process of Pt on the as-prepared Co3O4(111) surface was studied by depositing low amounts 0.04 and 0.13 monolayer (ML) of Pt, that create clusters as small as 2 or 3 atoms with no need for a special nucleation site. We have identified the formation of Pt-induced defects in the atomically flat cobalt oxide...

In this thesis, the electronic properties in superconducting LiFeAs single crystal are investigated using low temperature scanning tunneling microscopy and spectroscopy (STM/S) at various temperatures. For this purpose, the differential conductance (dI/dV) measured by STS which is directly proportional to the local density of states (LDOS) of the sample to the sub-atomic precision, is used together with the topography information. The dI/dV spectra within the ±1 V energy range reveal a characteristic feature at around -350 mV to -400 mV in stoichiometric LiFeAs. This feature seems to be a universal property among all the Fe-based high temperature superconductors, because it is also found in Fe0.965Se1.035 and NaFe0.975Co0.025As single crystals at the energy of -210 mV and -200 mV, respectively. The temperature dependent spectroscopy data averaged over a spatially fixed clean area of 2 nm × 2 nm are successfully executed between 5 K and 20 K. The two distinct superconducting phases with critical temperatures Tc = 16 K and 18 K are observed. In addition, the distance between the dip position outside the superconducting gap and the superconducting coherence peak in the spectra remains temperature independent which confirms that it is not connected to an antiferromagnetic (AFM) spin resonance. The temperature dependent spectra have been measured between 5 K and 61 K within the energy range of ±100 mV as well. The hump structure at 42 mV tends to disappear around 60 K from unknown origin. The temperature dependent quasiparticle interference (QPI) has been studied within the temperature range between 6.7 K and 25 K and analyzed by the Fourier transformation of the measured spectroscopic maps. The dispersion plots in momentum space as a function of temperature show an enhancement of QPI intensity (±5.5 mV) within the superconducting gap at the Fermi level at 6.7 K near q ~ 0. This is interpreted on the basis of Andreev bound state. In both polarities outside of this, a depletion of QPI intensity is noticed between 5.5 mV and around 9 mV. At positive energies, the QPI intensity becomes very rich above 9 mV. The size of the enhanced QPI intensity near the Fermi level, and the edge of the rich QPI intensity beyond 9 mV are found to behave like superconducting order parameter with rising of temperature. Furthermore, an energy mode peaked at around 14 mV appears in the integrated QPI intensity below superconducting Tc (6.7 K). This is consistent with the observed peak at 1st derivative of the dI/dV spectra. In both of these cases, such 14 mV peak is suppressed at normal state (25 K). This mode is therefore directly related to superconductivity in LiFeAs. The off-stoichiometric LiFeAs single crystal with superconducting Tc of 6.5 K has a 10 mV rigid band shift of the Fermi level towards electron doping. The absence of the rich QPI intensity between 9 mV and 17 mV is found compared to the stoichiometric LiFeAs, and hence the 14 mV mode is absent here. This brings us to conclude once more time that such 14 mV energy mode is relevant for superconductivity in LiFeAs.

The on-surface reactions of 10-bromo-10'-(2,6-dimethylphenyl)-9,9'-bianthracene on Au(111) surface have been investigated by a combination of bond-resolved scanning tunneling microscopy, scanning tunneling spectroscopy, and tightbinding and mean-field Hubbard calculations. The reactions afford the synthesis of two open-shell nanographenes (1a and 1b) exhibiting different scenarios of all-carbon magnetism. 1a, an allbenzenoid nanographene with previously unreported triangulenelike termini, contains a high proportion of zigzag edges, which endows it with an exceedingly low frontier gap of 110 meV and edge-localized states. The dominant reaction product (1b) is a non-benzenoid nanographene consisting of a single pentagonal ring in a benzenoid framework. The presence of this nonbenzenoid topological defect, which alters the bond connectivity in the hexagonal lattice, results in a non-Kekulé nanographene with a spin S = ½, which is detected as a Kondo resonance. Our work provides evidence of all-carbon magnetism, and motivates the use of topological defects as structural elements toward engineering agnetism in carbon-based nanomaterials for spintronics.

Cette thèse présente les résultats de mesures expérimentales effectués à basse température par les techniques de microscopie tunnel à balayage et de spectroscopie par tunnel à balayage (STS) sur les métaux de transitions phthalocyanines déposées sur les surfaces de métaux nobles. Les mesures STM/STS ont été effectuées pour les molécules MnPc et CuPc adsorbées sur les surfaces Ag (111) et Au (111) à la température expérimentale de travail de 4,5 K. Ces deux types de molécules présentent une différence substantielle de configurations d'adsorption, des comportements électroniques et magnétiques et des structures vibratoires moléculaire. Les études STM/STS ont principalement porté sur les propriétés magnétiques de ces molécules à travers l’effet Kondo et une attention particulière a été accordée à la molécule de MnPc en raison de son comportement magnétique plus intéressant issu de l'atome Mn central. Particulièrement, nous avons étudié l'évolution spectrale des structures électroniques et magnétiques du MnPc partant d'une molécule unique jusqu'à la structure bicouche ordonnée sur la surface Ag (111). En outre, les études STM/STS ont montré une preuve de couplage magnétique entre les moments magnétiques de l'atome de Co et de la molécule de MnPc ainsi que sa forte dépendance vis-à-vis du site d'adsorption de l'atome de Co. Ces études STM/STS sur ce système nous ont permis de comprendre l'effet des interactions molécule-substrat, molécule-molécule et molécules-atome sur les propriétés électroniques et magnétiques des molécules de MnPc. / This thesis presents the results of low-temperature scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) studies on transition-metal phthalocyanines molecules on the noble metal surfaces. The STM/STS measurements have been performed for MnPc and CuPc molecules adsorbed on Ag(111) and Au(111) surfaces at the experimental working temperature of 4.5 K. These two types of molecules exhibit substantially different adsorption configurations, the electronic and magnetic behaviors and the molecule vibrational structures. The STM/STS studies have focused mainly on the magnetic properties of these molecules by means of Kondo effect, and special attention has been paid to MnPc molecule due to its more interesting magnetic behavior arising from the central Mn atom. Particularly we investigated the spectral evolution of electronic and magnetic structures of MnPc starting from a single molecule up to the ordered bilayer structures on Ag(111) surface. In addition, the STM/STS investigations showed an evidence of magnetic coupling between the magnetic moments of the Co atom and MnPc molecule and its strong dependence on the adsorption site of Co atom. These STM/STS investigations on this system allowed us to understand the effect of molecule-substrate, molecule-molecule and molecule-atom interactions on the electronic and magnetic properties of MnPc molecules.

L’évolution de la reconstruction de surface de l’Au(111) sous contrainte-déformation a été étudiée dans le cadre d’une approche, à la fois expérimentale par microscopie à effet tunnel sous environnement ultra-vide couplée à un dispositif en compression, et numériquement par simulations en dynamique moléculaire. Dans un premier temps, nous avons étudié l’interaction entre les marches atomiques (vicinales ou traces de glissement) et la reconstruction. Nous avons notamment montré expérimentalement une forte dépendance de la longueur de la reconstruction avec la largeur des terrasses, en très bon accord avec les simulations atomistiques. Nous avons démontré de manière quantitative que ce comportement provenait de la relaxation des contraintes de surface, à la fois le long et perpendiculairement aux marches atomiques. Par la suite, nous avons montré que l’apparition d’une trace de glissement, résultant de l’émergence d’une dislocation à la surface, induit une réorganisation de la reconstruction, caractérisée par la formation d’un motif en forme de U. Nous avons par ailleurs observé expérimentalement la présence de décrochements le long de la trace. Les simulations ont confirmé que ces décrochements étaient corrélés avec la modification de la reconstruction. Dans un second temps, l’étude s’est axée sur l’évolution de la reconstruction en chevrons sous contrainte-déformation appliquée. Les observations expérimentales ont montré qu’une contrainte de compression macroscopique était à l’origine d’une modification de la structure en chevrons. Les simulations en dynamique moléculaire ont permis d’analyser l’influence de l’orientation de la contrainte sur les dislocations perçant la surface. Nous avons montré qu’une réorganisation irréversible de la structure en chevrons a lieu, se caractérisant par l’annihilation des dislocations perçant la surface et la suppression de la structure en chevrons. / The evolution of the surface reconstruction of the Au(111) under stress-strain has been studied in the context of an experimental approach, both by tunneling microscopy under ultra-vacuum environment coupled to a compression device, and numerically by molecular dynamics simulations. At first, we studied the interaction between atomic steps (vicinal or slip traces) and reconstruction. In particular, we showed experimentally a strong dependence of the length of the reconstruction with the width of the terraces, in very good agreement with the atomistic simulations. We have quantitatively demonstrated that this behavior is originated from the release of surface stress, both along and perpendicular to the atomic steps. Subsequently, we have shown that the appearance of a slip traces, resulting from the emergence of dislocations at the surface, induce a reorganization of the reconstruction, characterized by the formation of a U-shaped pattern. We also observed experimentally the presence of kinks along the trace. The simulations confirmed that these kinks are correlated with the modification of the reconstruction. At last, the study focused on the evolution of the chevron pattern under applied stress-strain. Experimental observations have shown that a macroscopic compressive strain involved a modification of the herringbone structure. Molecular dynamics simulations allowed to analyze the influence of stress orientation on surface threading dislocations. We have shown that an irreversible reorganisation of the herringbone structure takes place, characterized by the annihilation of the surface threading dislocations and the removal of the herringbone structure.

Ces dernières années ont été marquées par de grandes évolutions technologiques à travers notamment une course à la miniaturisation. De gros efforts de recherche se concentrent en particulier sur le domaine de l’électronique organique mais aussi sur de nouveaux matériaux bidimensionnels comme le graphène. Ces matériaux 2D présentent des propriétés physiques exceptionnelles et sont des candidats prometteurs pour le développement de futurs dispositifs électroniques. Au cours de cette thèse, l’approche ascendante, qui consiste à assembler ensemble des petites briques élémentaires, a été utilisée pour élaborer des nanostructures bidimensionnelles originales sur des surfaces. Des états électroniques localisés dus à un couplage électronique latéral particulier entre les molécules ont été observés. Quatre nanoarchitectures hybrides ioniques-organiques différentes ont été réalisées en faisant varier la température de la surface. Des nanostructures organiques covalentes ont aussi été élaborées par une réaction de couplage d’Ullmann sur la surface. Deux précurseurs différents en forme d’étoile avec des substituants iodés et bromés respectivement, ont été étudiés. De grandes nanostructures carbonées hexagonales poreuses ont notamment été synthétisées en faisant varier la température du substrat. Ces travaux ouvrent de nouvelles perspectives pour la réalisation de matériaux organiques bidimensionnels aux propriétés contrôlées. / Over the last few years, important technological developments were made following a trend towards miniaturization. In particular, lots of research efforts are put into the research on organic electronics and on 2D materials like graphene. Such 2D materials show great physical properties and are promising candidates for the development of future electronic devices.In this project, bottom-up approach consisting in assembling elementary building blocks together, was used to engineer novel twodimensional nanostructures on metal surfaces. The properties of these two-dimensional nanostructures were investigated using Scanning Tunneling Microscopy (STM) and X-ray Photoemission Spectroscopy (XPS). Two-dimensional nanostructures based on the self-assembly of organic building blocks stabilized by intermolecular interactions were engineered. In particular, nanostructures stabilized by hydrogen bonds, halogen bonds and ionic-organic interactions were investigated. Localized electronic states due to specific molecular lateral electronic coupling were observed. Four different ionic-organic nanoarchitectures were engineered varying the substrate temperature. Covalent organic nanostructures were also engineered by onsurface Ullmann coupling reaction. Two different star-shaped precursors with iodine and bromine substituents respectively, were investigated. Large periodic porous 2D covalent hexagonal carbon nanostructures weresuccessfully engineered by temperature driven hierarchal Ullmann coupling. These results open new perspectives for the development of 2D organic materials with controlled structures and properties.