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We investigate the chemical interaction of carbon monoxide (CO) and oxygen (O2) with kink atoms on steps of platinum crystal surfaces using a specially designed Pt curved sample. We aim at describing the fundamental stages of the CO oxidation reaction, i.e., CO-covered/poisoned stage and O-covered/active stage, at the poorly known kinked Pt facets by probing CO uptake/saturation and O2 saturation, respectively. Based on the systematic analysis that the curved surface allows, and using high-resolution X-ray photoemission, a diversity of terrace and step/kink species are straightforwardly identified and accurately quantified, defining a smooth structural and chemical variation across different crystal planes. In the CO-saturated case, we observe a preferential adsorption at step edges, where the CO coverage reaches a CO molecule per step Pt atom, significantly higher than their close-packed analogous steps with straight terrace termination. For the O-saturated surface, a significantly higher O coverage is observed in kinked planes compared to the Pt(111) surface. While the strong adsorption of CO at the kinked edges points toward a higher ignition temperature of the CO oxidation at kinks as compared to terraces, the large O coverage at steps may lead to an increased reactivity of kinked surfaces during the active stage of the CO oxidation.

Conventional spin-degenerate surface electrons have been effectively manipulated by means of self-organized nano-arrays. Of particular interest are one-dimensional, step superlattices at vicinal surfaces, because their fundamental behavior can be understood through simple model theory, and they can be utilized to imprint strong surface anisotropies in electron transport devices. In this work, the realization of periodic resonator arrays on the BiAg2 atom-thick surface alloy with atomic precision is presented, and their potential ability for tailoring the giant-split helical Rashba states is demonstrated. By employing curved crystals to select local vicinal planes, two sharply defined arrays of BiAg2 monoatomic steps with distinct spacing are fabricated, as experimentally determined from scanning tunneling microscopy and low-energy electron diffraction. The interaction of the Rashba helical states with the step arrays is assessed by scanning the photon beam across the BiAg2 curved surface in angle-resolved photoemission experiments and comparing these results with density functional theory. Remarkably, strong orbital-selective renormalization of bands perpendicular to the step superlattice, as well as spin mixing of Rashba bands, are induced by the coherent scattering of the periodic step potential. These results pave the way to fabricate atomically precise coupled arrays of electron resonators to engineer spin-orbital textures.

The ability to confine light down to atomic scales is critical for the development of applications in optoelectronics and optical sensing as well as for the exploration of nanoscale quantum phenomena. Plasmons in metallic nanostructures with just a few atomic layers in thickness can achieve this type of confinement, although fabrication imperfections down to the subnanometer scale hinder actual developments. Here, narrow plasmons are demonstrated in atomically thin crystalline silver nanostructures fabricated by prepatterning silicon substrates and epitaxially depositing silver films of just a few atomic layers in thickness. Specifically, a silicon wafer is lithographically patterned to introduce on-demand lateral shapes, chemically process the sample to obtain an atomically flat silicon surface, and epitaxially deposit silver to obtain ultrathin crystalline metal films with the designated morphologies. Structures fabricated by following this procedure allow for an unprecedented control over optical field confinement in the near-infrared spectral region, which is here illustrated by the observation of fundamental and higher-order plasmons featuring extreme spatial confinement and high-quality factors that reflect the crystallinity of the metal. The present study constitutes a substantial improvement in the degree of spatial confinement and quality factor that should facilitate the design and exploitation of atomic-scale nanoplasmonic devices for optoelectronics, sensing, and quantum-physics applications.

180 p., Esta tesis se centra en el estudio de materiales ferromagnéticos bidimensionales. En concreto, en las aleaciones superficiales de Tierra Rara (RE) y Metal Noble (NM) y su interacción con monocapas (ML) de moléculas magnéticas. Las aleaciones superficiales de Tierra Rara (RE) y Metal Noble (NM) se han producido mediante la evaporación de RE sobre las superficies Au(111) y Ag(111). Posteriormente se han evaporado térmicamente las monocapas de CuPc y TbPc2. Su estructura tanto de las aleaciones superficiales como de las monocapas depositadas encima se ha estudiado mediante microscopia de efecto túnel (STM) y difracción de electrones de baja energía (LEED). La estructura electrónica tanto de las aleaciones superficiales como las capas de moléculas se ha estudiado mediante varias variante de photoemisión (PES): de rayos-X (XPS) resonante (res-PES) y resuelta en angulo (ARPES). El comportamiento magnético de las aleaciones se ha estudiado mediante dicroísmo magnético de Rayos X (XMCD). También se ha usado XMCD para el estudio del acoplo magnético entre las moléculas y las aleaciones superficiales

Magnetic hybrid metal–organic interfaces possess a great potential in areas such as organic spintronics and quantum information processing. However, tuning their carrier injection barriers on-demand is fundamental for the implementation in technological devices. We have prepared hybrid metal–organic interfaces by the adsorption of copper phthalocyanine CuPc on REAu2 surfaces (RE = Gd, Ho and Yb) and studied their growth, electrostatics and electronic structure. CuPc exhibits a long-range commensurability and a vacuum level pinning of the molecular energy levels. We observe a significant effect of the RE valence of the substrate on the carrier injection barrier of the hybrid metal–organic interface. CuPc adsorbed on trivalent RE-based surfaces (HoAu2 and GdAu2) exhibits molecular level energies that may allow injection carriers significantly closer to an ambipolar injection behavior than in the divalent case (YbAu2).

Magnetic hybrid metal–organic interfaces possess a great potential in areas such as organic spintronics and quantum information processing. However, tuning their carrier injection barriers on-demand is fundamental for the implementation in technological devices. We have prepared hybrid metal–organic interfaces by the adsorption of copper phthalocyanine CuPc on REAu2 surfaces (RE = Gd, Ho and Yb) and studied their growth, electrostatics and electronic structure. CuPc exhibits a long-range commensurability and a vacuum level pinning of the molecular energy levels. We observe a significant effect of the RE valence of the substrate on the carrier injection barrier of the hybrid metal–organic interface. CuPc adsorbed on trivalent RE-based surfaces (HoAu2 and GdAu2) exhibits molecular level energies that may allow injection carriers significantly closer to an ambipolar injection behavior than in the divalent case (YbAu2).

This first part of this thesis is focused on the search of novel low dimensional magnetic materials that consist on monolayers (MLs) of rare-earth (RE)/ Noble-Metal (NM) surface alloys, to be more precise RE-Au2 and RE-Ag2 monolayers (ML). The structure of the alloys has been investigated though low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). It will be shown that the new alloys present (√3×√3)R30° reconstructions with a Moiré structure. The electronic structure of the alloys has been studied through several forms of photoemission spectroscopy (PES). Resonant photoemission and X-ray photoemission has been used for determination of the valence state, while angle-resolved PES (ARPES) has been used for the study of the electronic structure near the Fermi level. It will be shown that the alloys share two types of electronic structure depending on the valence state of the RE. Finally, X-ray Magnetic Circular Dichroism (XMCD) has been used to study the magnetic behavior of the alloys, in special, the anisotropy, which is determined by the selected RE., The second part of this thesis is dedicated to the study of the metal-organic interface formed by adsorption of monolayers of organic molecules on RE noble-metal surface alloys by thermal evaporation. The specific molecules that have been adsorbed are Copper Phthalocyanines (CuPc) and Lanthanide bis-phthalocyaninato (TbPc2). The growth and structure of the molecular layers have been investigated by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) techniques. The CuPc layer reveals a well ordered growth that is commensurate with the substrate, while the molecule of TbPc2 presents a different scenario with some variation depending on the substrate. ARPES measurements have been performed in order to detect possible modifications in the band structure of the surface alloys upon adsorption of the molecular adlayers. In this thesis it will be shown that the molecular adsorption of these molecules is mainly by physisorption, which leaves the electronic structure of the substrate and the molecule (mostly) unaffected. Finally, XMCD measurements have been used to study the magnetic coupling of the molecule to the REAu2 substrates. It will be evidenced that the substrate can enhance the anisotropy of the molecule if there is a coincidence between both molecule and substrate easy-axis direction of anisotropy., [ES] En los últimos años, el desarrollo de la industria de semiconductores ha dejado de seguir la Ley Moore. Esta ley empírica predecía que el número de transistores en un circuito integrado se doblaría cada dos años. La desviación de esta tendencia se ha producido debido al continuo aumento de la dificultad de reducir el tamaños de los transistores. En particular, resulta complicada la reducción de la longitud del canal de los transistores MOSFET (transistor de efecto de campo de metal-óxido-semiconductor) llegando a encontrar problemas tales como la incapacidad de conseguir un aislamiento adecuado de la puerta. Para superar estas dificultades, se han propuesto varios métodos, como substituir el silicio por semiconductores de mayor movilidad como el GaAs23 o usar materiales de baja dimensionalidad como el grafeno. Otra opción que se ha considerado, es substituir la conducción de electrones por otro portadores, como excitaciones colectivas en plasmónica u ondas de espín en magnónica. Pero entre ellas, destaca la espintrónica, que se basa en el control de los espines y corrientes de espín de los electrones en vez de usar solamente la carga. Un dispositivo espintrónico se basa en el uso de corrientes de espín polarizadas, es decir que toda la corriente tiene una única dirección de espín, y el uso de sistemas sensibles a esas polarizaciones de espín. Por ese motivo, es posible la utilización de dos polarizaciones de espín como canales diferentes para doblar la cantidad de información transmisible por un único cable. El método mas simple para generar una corriente de espín polarizada es hacer pasar una corriente eléctrica por un material ferromagnético, el cual presenta una conductividad diferente dependiendo de su dirección de magnetización. Si se pone otro material ferromagnético en serie, desacoplado pero lo suficientemente cerca para que no se pierda la polarización de espín, el dispositivo resultante mostrara una fuerte diferencia de conductividad dependiendo de si

Electrolytic hydrogen appears as one of the most promising options to store renewable energy. In this water splitting process, the sluggish kinetics of the 4-electron oxygen evolution reaction (OER) with its high overpotentials have been widely regarded as the bottleneck to facilitate a fast, energy-efficient process. In alkaline media, numerous earth-abundant metal oxides are efficient OER catalysts, stabilized by the high concentration of hydroxide anions in the electrolyte. However, under acidic conditions, where the hydrogen evolution reaction (HER) is technologically preferred, only noble metal-based oxides (RuO2 and IrO2) are suitable OER catalysts, putting into question the scalability to wide-spread applications due to their scarcity and high cost. Most earth abundant metal oxides dissolve at high proton concentrations. A promising strategy to avoid this drawback consists of incorporating these catalysts into partially hydrophobic composite electrodes. Following this strategy, we have been able to conduct an extensive survey of the activity and stability of mono-, bi- and trimetallic earth-abundant transition metal oxides during the electrocatalytic OER under strongly acidic conditions. Our results confirm the general validity of the strategy by using a hydrophobic electrode to confer high stability to common metal oxides under these harsh conditions. Among all OER catalysts investigated, we found that simple manganese oxides appeared as the most active also exhibiting high, long-term stability. In particular, the stability of Mn2O3 oxide in the OER in acidic media was well confirmed by post-electrolysis characterization data.

Copper-based hydrogen evolution electrocatalysts are promising materials to scale-up hydrogen production due to their reported high current densities; however, electrode durability remains a challenge. Here, we report a facile, cost-effective, and scalable synthetic route to produce Cu2-xS electrocatalysts, exhibiting hydrogen evolution rates that increase for ∼1 month of operation. Our Cu2-xS electrodes reach a state-of-the-art performance of ∼400 mA cm-2 at -1 V vs RHE under mild conditions (pH 8.6), with almost 100% Faradaic efficiency for hydrogen evolution. The rise in current density was found to scale with the electrode electrochemically active surface area. The increased performance of our Cu2-xS electrodes correlates with a decrease in the Tafel slope, while analyses by X-ray photoemission spectroscopy, operando X-ray diffraction, and in situ spectroelectrochemistry cooperatively revealed the Cu-centered nature of the catalytically active species. These results allowed us to increase fundamental understanding of heterogeneous electrocatalyst transformation and consequent structure-activity relationship. This facile synthesis of highly durable and efficient Cu2-xS electrocatalysts enables the development of competitive electrodes for hydrogen evolution under mild pH conditions.

Graphene is a light material for long-distance spin transport due to its low spin–orbit coupling, which at the same time is the main drawback for exhibiting a sizable spin Hall effect. Decoration by light atoms has been predicted to enhance the spin Hall angle in graphene while retaining a long spin diffusion length. Here, we combine a light metal oxide (oxidized Cu) with graphene to induce the spin Hall effect. Its efficiency, given by the product of the spin Hall angle and the spin diffusion length, can be tuned with the Fermi level position, exhibiting a maximum (1.8 ± 0.6 nm at 100 K) around the charge neutrality point. This all-light-element heterostructure shows a larger efficiency than conventional spin Hall materials. The gate-tunable spin Hall effect is observed up to room temperature. Our experimental demonstration provides an efficient spin-to-charge conversion system free from heavy metals and compatible with large-scale fabrication.

One of the fundamental applications for monolayer-thick 2D materials is their use as protective layers of metal surfaces and in situ intercalated reactive materials in ambient conditions. Here we investigate the structural, electronic, and magnetic properties, as well as the chemical stability in air of a very reactive metal, Europium, after intercalation between a hexagonal boron nitride (hBN) layer and a Pt substrate. We demonstrate that Eu intercalation leads to a hBN-covered ferromagnetic EuPt2 surface alloy with divalent Eu2+ atoms at the interface. We expose the system to ambient conditions and find a partial conservation of the di-valent signal and hence the Eu–Pt interface. The use of a curved Pt substrate allows us to explore the changes in the Eu valence state and the ambient pressure protection at different substrate planes. The interfacial EuPt2 surface alloy formation remains the same, but the resistance of the protecting hBN layer to ambient conditions is reduced, likely due to a rougher surface and a more discontinuous hBN coating.

Using a reactive molecular beam with high kinetic energy (Ekin), it is possible to speed gas–surface reactions involving high activation barriers (Eact), which would require elevated pressures (P0) if a random gas with a Maxwell–Boltzmann distribution is used. By simply computing the number of molecules that overcome the activation barrier in a random gas at P0 and in a molecular beam at Ekin = Eact, we establish an Ekin–P0 equivalence curve, through which we postulate that molecular beams are ideal tools to investigate gas–surface reactions that involve high activation energies. In particular, we foresee the use of molecular beams to simulate gas surface reactions within the industrial-range (>10 bar) using surface-sensitive ultra-high vacuum (UHV) techniques, such as X-ray photoemission spectroscopy (XPS). To test this idea, we revisit the oxidation of the Cu(111) surface combining O2 molecular beams and XPS experiments. By tuning the kinetic energy of the O2 beam in the range of 0.24–1 eV, we achieve the same sequence of surface oxides obtained in ambient pressure photoemission (AP-XPS) experiments, in which the Cu(111) surface was exposed to a random O2 gas up to 1 mbar. We observe the same surface oxidation kinetics as in the random gas, but with a much lower dose, close to the expected value derived from the equivalence curve.

189 p., Durante esta Tesis, se ha estudiado el efecto de átomos de baja coordinación (terrazas, escalones, escalones en zig-zag ¿) en la ignición de la oxidación de CO por parte de O2 en superficies de Pt y Rh. Para ello, hemos utilizado cristales curvos con una densidad de terrazas y escalones variable, para comparar superficies planas y escalonadas en una sola muestra.En las muestras de Pt, las superficies planas y escalonadas se encienden a la misma temperatura. Creemos que esto se debe a la acumulación de oxígeno debajo de la superficie.En la muestra de Rh, los escalones triangulares se encienden antes que los cuadrados. Creemos que esto se debe a que el CO adsorbido en los triangulares desorbe a una temperatura más baja, lo que permite que el O2 se adsorba en los escalones y empiece la reacción antes.

[EN] In intermetallic compounds with zero orbital momentum (L = 0) the magnetic anisotropy and the electronic band structure are interconnected. Here, we investigate this connection in divalent Eu and trivalent Gd intermetallic compounds. We find by x-ray magnetic circular dichroism an out-of-plane easy magnetization axis in two-dimensional atom-thick EuAu2. Angle-resolved photoemission spectroscopy and density-functional theory prove that this is due to strong f-d band hybridization and Eu2+ valence. In contrast, the easy in-plane magnetization of the structurally equivalent GdAu2 is ruled by spin-orbit-split d bands, notably Weyl nodal lines, occupied in the Gd3+ state. Regardless of the L value, we predict a similar itinerant electron contribution to the anisotropy of analogous compounds.

189 p., Durante esta Tesis, se ha estudiado el efecto de átomos de baja coordinación (terrazas, escalones, escalones en zig-zag ¿) en la ignición de la oxidación de CO por parte de O2 en superficies de Pt y Rh. Para ello, hemos utilizado cristales curvos con una densidad de terrazas y escalones variable, para comparar superficies planas y escalonadas en una sola muestra.En las muestras de Pt, las superficies planas y escalonadas se encienden a la misma temperatura. Creemos que esto se debe a la acumulación de oxígeno debajo de la superficie.En la muestra de Rh, los escalones triangulares se encienden antes que los cuadrados. Creemos que esto se debe a que el CO adsorbido en los triangulares desorbe a una temperatura más baja, lo que permite que el O2 se adsorba en los escalones y empiece la reacción antes.

It is undeniable that catalysis is one of the pillars of modern society, since almost all processes occurring in the industry and combustion vehicles are based on such phenomenon. Within this context, the catalytic carbon monoxide (CO) oxidation is one of the most studied topics. This is not surprising, since we must understand this reaction in order to improve the performance of catalytic converters and other processes in the petrochemical industry involving CO. In addition, this reaction is a pillar for investigating more complex catalytic processes, from which we can cement the knowledge for tailoring new and efficient catalysts. Rhodium (Rh), Platinum (Pt) and Palladium (Pd) are among the best CO oxidation catalysts. Typically, particles of these metals are placed in the catalytic converter of combustion cars to oxidize the dangerous CO to the less harmful carbon dioxide (CO2), among other processes. Since the reaction takes place at the surface of such nanoparticles, we must explore how these behave under reaction conditions. With the aid of Surface Science, we may study the composition and/or structure of well-defined surfaces when exposed to both reactants, namely CO and oxygen (O2). Thereafter, we may extend such investigations to stepped surfaces and eventually to nanoparticles. The catalytic CO oxidation requires, at least at low pressures, the coadsorption of both reactants, in order for them to collide and form CO2. In the case of O2, the molecule will dissociate on the surface to form atomic oxygen (O), which will later react with coadsorbed CO. The resulting CO2 molecule will readily desorb from the surface of the catalyst. On one hand, chemisorbed CO will quench the O2 adsorption, and will prevent the reaction. Therefore, the CO-covered surface is called poisoned stage (no/low CO2 production). On the other hand, chemisorbed O does not hinder the CO adsorption. Typically, O-covered surfaces usually feature a large catalytic CO2 turnover, for which O-co, Three curved samples were employed throughout this Thesis. Two of them have the (111) plane located at the center, and an increasing density of either A-type ({100} microfacets, square) or B-type ({111} microfacets, triangular) straight steps as one departs from the position of the (111) surface at the apex of the crystal. These are the c-Pt(111) and c-Rh(111) samples, and their design is ideal for studying A-B asymmetries. The other sample has the kinked Pt(645) plane at the center of the crystal [k-Pt(645)]. At kinked surfaces, steps feature alternating {111} and {100} microfacets. On the k-Pt(645) sample, the (111) plane is located between the center and one of the edges, enabling to probe Pt vicinal surfaces with kinked steps up to the densely stepped (312) plane. The following Chapters comprise three different, well-distinguished sets of experiments. First of all, the bare CO chemisorption was investigated in Ultra-High Vacuum (UHV) with X-Ray Photoemission Spectroscopy (XPS), in order to have a proper reference to characterize the CO-poisoning layer found at higher pressures. Later, the very same systems were studied under reaction conditions at several temperatures to study their composition and CO2 turnover. These CO oxidation experiments were conducted close to operando conditions, in order to mimic the environment to which real catalysts are submitted. Working with curved substrates demands for sufficient spatial resolution (< 0.5 mm) to probe the different facets across the curved surface. During this Thesis, two different techniques conducted at relatively high pressures (millibar range) were employed. On one hand, Planar Laser-Induced Fluorescence (PLIF) allowed probing the CO2(g) cloud arising from the CO oxidation all over the curved sample simultaneously. This enabled to easily track differences in the CO ignition across the curved sample. However, since this technique provides only information about the gas phase above the sample, we resorted to Nea, For the Pt crystals with either close-packed and kinked steps, PLIF revealed a surprising simultaneous ignition of the stepped surfaces: the CO2 production explodes at the same time across the curved surface. The NAPXPS measurements point to CO-depleted terraces while the steps remain CO-saturated prior to the ligth-off of the reaction. The consistent analysis of the XPS spectra during a slow ignition cycle at the (111) surface points towards the existence of an additional, unresolved oxygen species prior to ligth-off. With the aid of theoretical calculations, we postulate that such species is subsurface oxygen that builds up before ignition, and vanishes afterwards. This subsurface oxygen would increase the adsorption energy of CO anchored at terraces to values close to its step analogous, explaining the simultaneous ignition at Pt vicinals. PLIF measurements on the c-Rh(111) sample showed that, as compared to its A-type analogous, the B-steps ignite at lower temperature. Therefore, the ignition at A- and B-type Rh vicinal surfaces is asymmetric around the (111) plane. Complementary NAP-XPS measurements showed that both Aand B-steps are CO-saturated after exposing them to high CO pressures and room temperature, away from the ignition temperature. However, as the light-off approaches the B-steps become partially CO-depleted and covered by chemisorbed O, while the A-steps remain CO-saturated. We believe that this is the reason for the asymmetry in the ignition, since coexistence of CO and O at the B-steps would tentatively increase the catalytic CO2 production. For the c-Rh(111) surface we examined the active stage in more detail. We found markedly different oxygen-related species for the (111) plane and the A- and B-type microfacets. While trilayers of RhO2 readily form in the A-steps after the surface transits to the active stage, an increase in the temperature is needed to develop the oxide also at the (111) plane. We did not observe the formation of RhO2 in the B

Carbon nanostructures with zigzag edges exhibit unique properties—such as localized electronic states and spins—with exciting potential applications. Such nanostructures however are generally synthesized under vacuum because their zigzag edges are unstable under ambient conditions: a barrier that must be surmounted to achieve their scalable integration into devices for practical purposes. Here we show two chemical protection/deprotection strategies, demonstrated on labile, air-sensitive chiral graphene nanoribbons. Upon hydrogenation, the chiral graphene nanoribbons survive exposure to air, after which they are easily converted back to their original structure by annealing. We also approach the problem from another angle by synthesizing a form of the chiral graphene nanoribbons that is functionalized with ketone side groups. This oxidized form is chemically stable and can be converted to the pristine hydrocarbon form by hydrogenation and annealing. In both cases, the deprotected chiral graphene nanoribbons regain electronic properties similar to those of the pristine nanoribbons. We believe both approaches may be extended to other graphene nanoribbons and carbon-based nanostructures.