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Trabajo presentado al International Workshop on Spintronics, celebrado en San Carlos de Bariloche, Río Negro (Argentina) del 6 al 11 de noviembre de 2022., Topological insulators are a recently discovered class of materials exhibiting interesting properties for applications such as quantum computation devices. Recently, metal-organic frameworks (MOF) have been theoretically proposed as the organic counterpart of topological insulators. Despite the experimental achievement of several of the proposed MOFs, their edge state visualization, which is the fingerprint of 2D topological insulator, remains elusive. We have formed these proposed MOFs with expected topological insulator character using 9,10-dicyanoanthracene molecules and Cu, Fe and Co adatoms on a the three (111) noble metal substrates. The lattices generally extend throughout the surface as single-domain and exhibit an almost perfect order. By means of scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy we studied their electronic structure, and commonly find total absence of an edge state despite the excellent network order. Functionalized tip measurements and tight-binding calculations suggest that structural asymmetries drive the networks into a topologically trivial case that destroys its edge state. However, we observe an edge state emerging when the SOC of the coordination atom is significantly increased. Thus, we present the first experimental evidence compatible with the Organic Topological Insulator prediction., PID2019-107338RB-C64, RED2018-102833-T, financiado porMCIN/AEI /10.13039/501100011033 Regional Government of Aragon (E12-20R). European Research Council under European Union’s Horizon 2020 research and innovation programme of Marie Skłodowska-Curie grant agreementULTIMATE-I Nº 101007825 Access to STM instruments: LMA-University of Zaragoza

[EN] On-surface synthesis is emerging as a highly rational bottom-up methodology for the synthesis of molecular structures that are unattainable or complex to obtain by wet chemistry. Here, oligomers of meta-polyaniline, a known ferromagnetic polymer, were synthesized from para-aminophenol building-blocks via an unexpected and highly specific on-surface formal 1, 4 Michael-type addition at the meta position, driven by the reduction of the aminophenol molecule. We rationalize this dehydrogenation and coupling reaction mechanism with a combination of in situ scanning tunneling and non-contact atomic force microscopies, high-resolution synchrotron-based X-ray photoemission spectroscopy and first-principles calculations. This study demonstrates the capability of surfaces to selectively modify local molecular conditions to redirect well-established synthetic routes, such as Michael coupling, towards the rational synthesis of new covalent nanostructures., Spanish MINECO(MAT2017-85089-C2-1-R, RYC-2015–17730), European Research Council (ERC) under contract (ERC-2013-SYG-610256 NANOCOSMOS), Comunidad de Madrid via Programa de InvestigaciónTecnologías 2018 (FOTOART-CMS2018/NMT-4367), and the innovation program under grantagreements 785219 and 881603 (GrapheneCore2 and Gra-pheneCore3-Graphene-based disruptive technologies,re-spectively). CSS acknowledges MCIU for the “RamnyCajal” contract (RYC2018-024364-I). NRdA is grateful tothe Spanish MINECO for support from the FPI program(BES-2015–072642)

Resumen del trabajo presentado a la 64th Annual Conference on Magnetism and Magnetic Materials (MMM), celebrada en Las Vegas (USA) del 4 al 8 de noviembre de 2019.

Resumen del trabajo presentado a la XXXVII Reunión Bienal de la Real Sociedad Española de Física celebrada en Zaragoza del 15 al 19 de julio de 2019., The achievement of single atom arrays is key to new fundamental properties that lead to exciting physical properties, like for instance single-atom magnets. However, the resulting position of these atoms has a strong dependence on the supporting substrate and the system temperature. Liquid helium temperatures are required to freeze the atoms, which are often found random sites. A way to generate precise arrays of single atoms stable up to room temperature is the growth of self-assembled metalorganic coordination networks (MOCN) using organic molecules as spacers. Here, by means of STM/STS we probe the electronic properties of different single coordinating atoms embedded in an array di-cyano anthracene (DCA) molecules formed upon the three (111) noble metal surfaces. We have achieved these spontaneously formed MOCN using Fe, Co or Cu (see Fig. 1). We observe distinct spectroscopic fingerprints in the LDOS that leads to shifts of the molecular LUMO levels and relevant surface state pore confinements. We will discuss the effect that the different chemical species and the substrate have on the overall electronic properties of the system.

Resumen del trabajo presentado a la XXXVII Reunión Bienal de la Real Sociedad Española de Física celebrada en Zaragoza del 15 al 19 de julio de 2019., Nanostructured Fe clusters whose atoms have a low average coordination number are expected to present enhanced orbital magnetic moment and anisotropy. Metal-organic interactions exhibit custom reversibility in bond formation that may result in large and extended self-assembled structures. Under ultra-high vacuum conditions, we use a Cu(111) monocrystal as substrate and generate an extended metalorganic network that fully covers its surface, which is stable at room temperature. When Fe is evaporated onto this molecular array, instead of the usual triangular bi-layer islands, single atomic height nanoclusters are formed (Fig. 1). The obtained structures are characterized as a function of coverage and temperature by means of Scanning Tunneling Microscopy. We find that both parameters affect the resulting nanodot sizes implying that a certain control can be exerted on its fabrication. Moreover, X-ray Magnetic Circular Dichroism experiments onto these samples show that the Fe nanostructures, which are stable at room temperature, significantly change the magnetic anisotropy compared to the case when the molecular layer is missing.

Resumen del póster presentado a la 7th European Conference on Molecular Magnetism (ECMM), celebrada en Florencia (Italia) del 15 al 18 de septiembre de 2019., Molecular overlayers on solid substrates have a broad field of application in catalysis, sensing, molecular electronics, light-to-energy conversion, etc. In particular, iron-phtalocyanines (FePc) are being investigated as viable substitutes for precious metals in catalysis of the Oxygen Reduction. Recent studies of FePc on Ag(110) have shown that sub-monolayer phases are catalitically active; remarkably, in oxygen-dosed phases O2 intercalates between the molecules and the surface, thereby substantially changing the Fe magnetic moment. Reversible switching of the Fe magnetic moment in some low-density (LD-R1/R2) FePc phases upon an oxygenation-annealing cycle has been demonstrated. In this contribution, we report on the oxygenation capabilities and associated magneto-structural changes of higher-density FePc/Ag(110) phases, the oblique (OB) and square (SQ)/quasi-squared (R3) high-density single monolayer phases, investigated by combining highresolution Scanning Tunneling Microscopy (STM), Scanning Tunneling Spectroscopy (STS), Density Functional Theory (DFT) simulations and x-ray magnetic circular dichroism (XMCD) at the Fe L2,3, edge. The STM images reveal the steric impediments of high-density phases to allocate O2 between the FePc and the substrate, which result in less efficient oxygenation / deoxydation processes compared to previously reported LD phases. In the SQ/R3 squared phases, only a fraction of the molecules are able to oxygenate, rotating and suffering a displacement. For the OB oblique phase, the oxygenation efficiency is smaller; and among the oxygenated molecules, only a minor fraction do rotate, while the majority allocate O2 without rotation. The XMCD spectra of the oxygenated phases reflect the different distribution of non-oxygenated, oxygenated-rotated and non-rotated species in the samples.

Trabajo presentado a la X Iberian Conference on Tribology y a la XI Iberian Vacuum Conference (IBERTRIVA), celebradas en Sevilla (España) del 26 al 28 de junio de 2019.

Póster presentado a la XXXVII Reunión Bienal de la Real Sociedad Española de Física celebrada en Zaragoza del 15 al 19 de julio de 2019., Molecular overlayers on ordered substrates have a broad field of application in catalysis, sensing, molecular electronics, etc. Recent studies of FePc on Ag(110) have shown that sub-monolayer phases are catalitically active: in oxygen-dosed phases, O2 intercalates between the molecules and the surface, substantially changing the Fe magnetic moment. Reversible switching of this moment in low-density FePc phases upon an oxygenation - annealing cycle has been demonstrated. A continuous FePc monolayer on Ag(110) displays several phases, with different FePc density as a function of the molecular coverage. At low coverage, two degenerate but structurally distinct low-density superstructures simultaneously appear after a - 450 K annealing, namely a c(10x4) and a p(10x4)-phase. At higher coverages, a denser oblique phase O(1±4,4-+3) is stablished after annealing. At even higher coverage, denser phases do form, namely a >quasi-square> phase (R3) and a square phase (SQ). The different FePc phases have been oxygen-dosed (10-100 L). Remarkably, in oxygen-dosed phases O2 dissociates and intercalates between the molecules and the surface substantially changing the Fe magnetic moment. X-ray absorption combined with STM images allow us to determine the effectivity of the oxygenation procedure, as a function of the 20 structural phase, and to correlate it with the average magnetic moment as measured by XMCD at the Fe L2,3-edges. Sum rule analysis yield the spin and orbital magnetic moments for each sample. lt is concluded that all characterised phases display planar anisotropy, and the values of meff are about one order of magnitude larger than mL in clear contrast to the FePc molecule in bulk or in multilayer, wich shows a larger orbital moment. Our work shows how oxidation and therefore magnetic moment switching depends on the steric facility for the molecules to form the Ag(110)-0xygen-FePc ensemble, which is geometrically hampered in high density phases.

Resumen del póster presentado a la Conferencia bienal Fuerzas y Túnel, celebrada en Jaca (España) del 27 al 29 de junio de 2018., One of the key prerequisite for the optimal performance of organic opto-electronic devices is an efficient charge-carrier transport into and out of the organic semiconductor material. Carrier injection in turn strongly depends on the molecular level alignment of the organic semiconductor material against the fermi level of the conducting electrodes. In a recent work we studied the evolution of molecular levels of Fe-phthalocyanine with increasing layer thickness on a gold (111) surface. By means of STM and STS we found that beyond the strong hybridization of the electronic states of the 1st molecular monolayer with the metallic substrate, the gap between the frontier orbitals is still strongly influenced at the 5th mono layer away from the surface. This sequential decrease of the HOMO-LUMO gap towards the metallic electrode should strongly support the charge-carrier injection for this kind of materials. Accordingly, such a layer- wise STM/STS investigation can give a detailed atomistic insight into electronic processes at metal/organic interfaces towards the bulk material and might help to improve the design of organic opto-electronic devices.

Resumen del trabajo presentado a la 21st International Conference on Magnetism (ICM), celebrada en San Francisco (US) del 15 al 20 de julio de 2018., Molecular overlayers on ordered substrates have a broad field of application in catalysis, sensing, molecular electronics, etc. Recent studies of FePc on Ag(110) have shown that sub-monolayer phases are catalitically active: in oxygen-dosed phases, O2 intercalates between the molecules and the surface, substantially changing the Fe magnetic moment. Reversible switching of this moment in low-density FePc phases upon an oxygenation - annealing cycle has been demonstrated. A continuous FePc monolayer on Ag(110) displays several phases, with different FePc density as a function of the molecular coverage. At low coverage, two degenerate but structurally distinct low-density superstructures simultaneously appear after a ~ 450 K annealing, namely a c(10×4)- and a p(10×4)-phase. At higher coverages, a denser oblique phase O(1±4,4-+3) is stablished after annealing. At even higher coverage, denser phases do form, namely a “quasi-square” phase (R3) and a square phase (SQ). The different FePc phases have been oxygen-dosed (10-100 L). Remarkably, in oxygen-dosed phases O2 dissociates and intercalates between the molecules and the surface substantially changing the Fe magnetic moment. X-ray absorption combined with STM images allow us to determine the effectivity of the oxygenation procedure, as a function of the 2D structural phase, and to correlate it with the average magnetic moment as measured by XMCD at the Fe L2,3–edges. Sum rule analysis yield the spin and orbital magnetic moments for each sample. It is concluded that all characterised phases display planar anisotropy, and the values of mseff are about one order of magnitude larger than mL in clear contrast to the FePc molecule in bulk or in multilayer, wich shows a much larger orbital moment. Our work shows how oxidation and therefore magnetic moment switching depends on the steric facility for the molecules to form the Ag(110)-Oxygen-FePc ensemble, which is geometrically hampered in high density phases.

Resumen del trabajo presentado a la 21st International Conference on Magnetism (ICM), celebrada en San Francisco (US) del 15 al 20 de julio de 2018., Metal-organic interactions are weaker than the covalent ones, allowing the bond formation and breaking necessary to self-assemble, but are stronger than the purely intermolecular interactions. This is why metalorganic networks own so interesting properties. In this work, we grow and characterize Fe upon a selected molecular network under ultra-high vacuum conditions. The resulting self-assembled structures are characterized by means of Scanning Tunnelling Microscopy and Low Energy Electron Diffraction. We use a Cu(111) monocrystal as substrate and generate a self-assembled, extended metalorganic network which is stable at room temperature. It will be shown that depending on the molecular coverage the porous network evolves into a compact structure detectable by LEED. Once the porous network is formed, Fe atoms are evaporated on the surface. Instead of he usual bi-layer islands, the Fe forms single atomic height nanoclusters. We have performed a STM study of the Fe nanostructures as a function of coverage and deposition temperature. In addition, X-ray Magnetic Circular Dichroism experiments at the European Synchrotron Radiation Facility show that the Fe nanostructures (stable also at room temperature) significantly change their magnetic anisotropy compared to the case when the molecular layer is missing.

Resumen del trabajo presentado a la Conferencia bienal Fuerzas y Túnel, celebrada en Jaca (España) del 27 al 29 de junio de 2018., Metal-organic interactions are weaker than covalent ones, allowing the bond breaking and formation necessary to generate large and extended self-assembled structures. Under ultra-high vacuum conditions, we grow and characterize Fe clusters using a selected metalorganic network as a buffer layer. The resulting self-assembled structures are characterized by means of Scanning Tunnelling Microscopy and Low Energy Electron Diffraction. We use a Cu(111) monocrystal as substrate and generate an extended metalorganic network, which is stable at room temperature. It will be shown that depending on the molecular coverage the porous network evolves into a compact structure detectable by LEED. Once the porous network is formed, Fe atoms are evaporated on the surface. Instead of the usual bi-layer islands, the Fe forms single atomic height nanoclusters. We have performed a STM study of such Fe nanostructures as a function of coverage and deposition temperature. In addition, X-ray Magnetic Circular Dichroism (XMCD) experiments at the ESRF show that the Fe nanostructures (stable at room temperature) significantly change the magnetic anisotropy compared to the case when the molecular layer is missing.

Resumen del póster presentado a la Conferencia bienal Fuerzas y Túnel, celebrada en Jaca (España) del 27 al 29 de junio de 2018., Polyaniline (PANI) has been studied since 1835 because of its interest as inherently conducting polymer due to its wide range of potential applications that range from batteries to biosensors. The interest of PANI lies on its capability to express different properties depending on its nitrogen oxidation state: leucoemeraldine (the fully reduced form), emeraldine (the half-oxidized form), and pernigraniline (the fully oxidized form). There are two conventional synthesis methods, electrochemical processing and chemical oxidative polymerization of aniline in a wet environment. Although doped emeraldine structure present good conductivity, it has been shown that the size, the preparation method and the existence of a metal-polymer interface, influence the physical properties of the final PANI. Here, we present a new route to synthesize 1D oligomers of PANI based on a specific on-surface synthesis process. The synthesis was carried out using p-aminophenol (p-Ap) molecules as building blocks on a Pt(111) single-crystal in a UHV environment. At 200°C p-Ap molecules are activated inducing a shift base reaction resulting in oligomer chains with pernigraniline structure. The mechanism of the chemical reaction was followed by XPS, LT-STM/STS and nc-AFM and confronted with theoretical calculations. XPS show that oxygen is removed from the p-AP molecule resulting in a coupling of the precursors via the nitrogen. STM and nc-AFM reveals that oligomers are formed by ca. 10 molecular precursors. STS shows the presence of two broad unoccupied electronic states around 200 mV and 650 mV above the Fermi level that we are able to resolve spatially. Finally, DFT calculations show that the atomic structure is influenced by the substrate crystallography.

Resumen del póster presentado a la 10th Conferencia Fuerzas y Túnel, celebrada en Girona (España) del 5 al 7 de septiembre de 2016.-- et al., Molecular overlayers on ordered substrates have a broad field of applications in catalysis, sensors, molecular electronics, light-to-energy conversion, etc. In particular, bio-inspired oxygen-binding metalated macrocycles, such as iron-phtalocyanines (FePc), are being investigated as viable substitutes for precious metals in catalysis of the Oxygen Reduction Reaction in low-temperature fuel cells. Recent studies of FePc on Ag(110) have shown that sub-monolayer (sub-ML) phases are catalytically active. Remarkably, in oxygen-dosed phases O2 intercalates between the molecules and the surface, thereby substantially changing the Fe magnetic moment. Reversible switching of the Fe magnetic moment in some low-density FePc phases upon an oxygenation - annealing cycle has been demonstrated. In this contribution we report on the structural and magnetic changes along a catalytic cycle of a new dense, “quasi-squared” FePc sub-ML phase (R3) evaporated on Ag(110). Structural changes were monitored by Scanning Tunneling Microscopy (STM) and Low Energy Electron Diffraction (LEED). X-ray absorption spectroscopy (XAS), x-ray lineal polarized absorption (XLPA) and x-ray magnetic circular dichroism (XMCD) experiments at the Fe L2,3 edge were performed on four samples: the as-evaporated phase (R3), two differently oxygenated samples (OX1 and OX2) and the annealed phase (R3ANN). XLPA and XMCD analysis evidence the new R3 phase is catalytically active, however, oxygenation/reduction is less effective than in the previously characterized low-density phase. Sum rules were used to determine the effective spin (mseff/nh) and orbital (mL/nh) magnetic moments per hole as a function of the incidence angle. It is concluded that all characterized phases display planar anisotropy, and the values of mseff/nh are one order of magnitude larger than mL/nh. By oxidation, the isotropic moment undergoes an increase from 7.2x10-2 μB/hole to 1.8x10-1 μB/hole, which is about a factor of 2 smaller than the increase achieved for low-density phase. Our spectroscopic results, in agreement with a combined study of STM and non-contact tuning fork AFM microscopies, show that oxidation of FePc on Ag(110) requires a rotation and displacement of the FePc molecule, which is more difficult to fulfill in denser phases.

Resumen del trabajo presentado a la 10th Conferencia Fuerzas y Túnel, celebrada en Girona (España) del 5 al 7 de septiembre de 2016.-- et al., The magnetic properties of magnetic atoms on metals are inherently connected with many body interactions between the localized magnetic moments and the supporting surface. This is because the electronic orbitals of atoms couple with the continuum of conduction electrons of the metal host. But this influence is reciprocal, since the properties of the substrate also change locally due to the presence of an atom. The understanding of the behavior of the spin moments of the atoms on a metal surface is one of the pillars of spintronics for the development of prototypes at the atomic scale. In particular, the study of the Kondo resonance appearing under particular conditions when a magnetic adatom lies on a metal surface provides deep insight into the involved magnetic interactions. We have chosen Co/Ag(111) as a model system to study the influence of the host LDOS in the Kondo effect of Co atoms. Combining STS and lateral manipulation techniques, we have carried out a study of the Kondo resonance as a function of the contacting lead LDOS of the Ag(111) surface with accurate control of the Co position. This has enabled us to probe a high degree of inhomogeneity in the surface state of Ag(111). Here we unambiguously identify the relevant contribution of surface state electrons to the relevant energy scale of the many-body Kondo sate (the Kondo temperature TK), in contrast to other previous works. This entails that the coupling strength between a magnetic impurity and its foremost environment can be tuned through the electronic properties of the metal host, leading to important fundamental and technological implications.

Ferroelectricity at atomic scale has been recently shown in ultrathin layers of NaCl on Cu2N/Cu(001). The origin was suppossed to be the strain of NaCl. In this thesis, the two rocksalt compounds KBr and LiCl were investigated with respect to their growth behaviour on Cu(001) and the polar insulator Cu2N/Cu(001). Furthermore the electronic structure and the ferroelectric behaviour of KBr is described. In gap-states due to the hybridisation with Cu(001) were found at KBr/Cu(001), which are suppressed due to the adding of Cu2N. Surprisingly, also without a strain, ultrathin layers of KBr on Cu2N/Cu(001)shows ferroelectrism. In this thesis we applied the following experimental techniques: Scanning Tunneling Microscopy(STM), Scanning Tunneling Spectroscopy(STS), Atomic Force Microscopy(AFM), Kelvin-Probe-Spectroscopy and Low Energy Electron Refraction(LEED). LiCl was investigated at ultra high vacuum and room temperature, KBr at ultra high vacuum and 4K.

The scope of this work is the investigation of the interactions that appear in molecular switches on noble metal surfaces. The adsorption of two different types of switching molecules is studied by means of near edge X-ray absorption fine structure (NEXAFS) and X-ray photoelectron spectroscopy (XPS). Samples of molecular powders and evaporated molecular multilayers are used as references in which the molecules are decoupled from the surface. The interpretation of the measured data is supported by quantum-chemical calculations based on density-functional theory (DFT), resulting in a deep understanding of the interaction mechanisms. In the first part of this work dimetacyano azobenzene and dimetacarboxymethylester azobenzene are chosen as simple model systems representing the class of conformal switches. These two compounds exhibit the same adsorption behavior and provide complementary information about the adsorption state. Au(111) and Cu(001) are used as substrates with different surface reactivities. On Au(111) at room temperature, azobenzene physisorbs flat in its trans configuration up to a saturation coverage of one monolayer, while the electronic structure of the adsorbate resembles the one calculated for the free molecule. In contrast, on Cu(001) we find that the substrate temperature and the molecule coverage have an influence on the adsorption state. Below half a monolayer evaporated on Cu(001) at 150 K, the majority of azobenzene molecules are found to be in the same physisorbed state as on Au(111). After annealing the substrate above 250 K most of the molecules chemisorb via their azobenzene center, where the frontier orbitals at the azo bridge rehybridize with the substrate orbitals. This interaction forces the molecule to a butterfly-like bent molecular geometry in which the outer aromatic groups are tilted out of the surface plane. In this conformation the lone-pair electrons can participate in the chemical binding, leading to a higher stabilization of the bent structure. The structural reorientation is accompanied by the deoccupation of the bonding and the occupation of the antibonding molecular π orbital, which is described by the Dewar-Chatt- Duncanson model. At higher coverages the increased intermolecular interaction provokes a larger tilt, which increases the stronger lone-pair – surface interactions and finally leads to the dissociation at the central N–N azo bond. The second part examines the adsorption of nitro-spiropyran (SP) as a member of the class of ring-opening/ring-closing switches in multi- and monolayers on Au(111). In multilayered films, the SP molecules order in a crystalline structure. UV irradiation causes half of the amount of molecules to undergo the ring-opening reaction. The resulting merocyanine (MC) is stabilized in the crystalline confinement such that a back reaction is not possible. For monolayer coverages, the molecule adsorbs on the substrate at 150 K with its nitro moiety flat on the surface. Annealing the substrate to 330 K provokes the ring-opening reaction. The resulting planar merocyanine molecule lies flat on the surface arranged in dimers that are liked by hydrogen bonds. The latter stabilize the ring-opened form and prevent the back reaction., Ziel dieser Arbeit ist es, die Wechselwirkungen molekularer Schalter auf und mit verschiedenen Edelmetalloberflächen zu untersuchen. Zu diesem Zweck wird das unterschiedliche Adsorptionsverhalten von zwei Typen schaltbarer Moleküle mit Hilfe von Röntgen-Nahkanten-Absorptionsspektroskopie (NEXAFS) und Röntgen- Photoelektronenspektroskopie (XPS) charakterisiert. Proben der Moleküle in Pulverform und adsorbiert in Multilagen dienen als Referenz zu den an die Oberfläche gekoppelten Adsorbaten. Quantenchemische Berechnungen auf Basis von Dichtefunktionaltheorie (DFT) ermöglich dabei eine Interpretation der experimentellen Daten in Hinsicht auf ein grundlegendes Verständnis der auftretenden Wechselwirkungen. Im ersten Teil der Arbeit werden Dimetacyano- and Dimetacarboxymethylesterazobenzol als prototypische Vertreter der Klasse konformeller Schalter untersucht. Beide Verbindingen zeigen dasselbe Adsorptionsverhalten und liefern dennoch unterschiedliche, sich ergänzende Informationen über ihren chemischen Zustand auf der Oberfläche. Au(111) und Cu(001) dienen als Oberflächen mit unterschiedlicher Reaktivität. Die Azobenzole physisorbieren auf Au(111) bei Raumtemperatur, wobei die Moleküle in ihrer planaren trans-Form parallel zur Oberfläche ausgerichtet sind. Im Gegensatz dazu ist ein Teil der Moleküle auf Cu(001) für Bedeckungen unterhalb einer halben Monolage schon bei Temperaturen von 150 K chemisorbiert. Oberhalb von 250 K ist die Chemisorption der gesamten Submonolage annähernd vollständig abgeschlossen. In diesem Zustand rehybridisieren die π- Orbitale der Azo- Brücke mit denen des Substrats. Diese Wechselwirkung verursacht eine leichte Deformation der planaren Struktur des trans-Azobenzols, bei der sich die äußeren aromatischen Einheiten von der Oberfläche wegbiegen. In dieser Geometrie können sich die freien Elektronenpaare der Stickstoffatome an der Bindung zum Substrat beteiligen, was zu einer erhöhten Stabilität der Struktur führt. Die Ausbildung der Bindungen zur Oberfläche wird begleitet von einer teilweisen Entvölkerung des bindenden π- Orbitals und einer teilweisen Besetzung des antibindenden π- Orbitals, so wie es im Rahmen des Dewar-Chatt- Duncanson-Modells beschrieben wird. Für höhere Bedeckungen nahe einer Monolage nehmen die zwischenmolekularen Wechselwirkungen an Bedeutung zu und bewirken eine noch stärkere Verbiegung der adsorbierten Moleküle. Dies führt zu einer teilweisen Dissoziation der Adsorbatmoleküle. Der zweite Teil der Arbeit behandelt die Adsorption von Nitro-Spiropyran (SP), einem Mitglied der Klasse der Ringöffnungs-/Ringschlussschalter, in Multi- und Monolagen auf Au(111). In Multilagen ordnen sich die SP-Moleküle zu einkristallinen Strukturen. Die Beleuchtung dieser Ensembles führt bei etwa der Hälfte der Moleküle zur Ringöffnung. Das entstandene Merocyanin ist so in den Kristall eingebunden, dass eine Rückreaktion nicht möglich ist. Auch hier kann ein temperaturabhängiges Verhalten beobachtet werden. Nach der Adsorption auf einer Au(111)-Oberfläche bei 150 K ist die Nitrogruppe des Moleküls parallel zur Oberfläche orientiert. Durch Erhöhung der Substrattemperatur auf 330 K öffnet sich der Pyranring und das resultierende planare Merocyanin adsorbiert flach auf der Oberfläche. Dabei formen sich Dimere, die durch Wasserstoffbrücken stabilisiert sind. Letzteres ist unter anderem verantwortlich für die Unterdrückung der entgegengesetzten Ringschlussreaktion.