Effect of the valence state on the band magnetocrystalline anisotropy in two-dimensional rare-earth/noble-metal compounds

[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.Discussions with the late J. I. Cerda are warmly thanked. Financial support from Spanish Ministerio deCiencia e Innovacion (projects MAT-2017-88374-P, PID2020-116093RB-C44 and PID2019-103910GB-I00 funded by MCIN/AEI/10.13039/501100011033/) , the Basque Govern-ment (Grants No. IT-1255-19 and No. IT1260-19) , and the University of the Basque Country UPV/EHU (Grant No. GIU18/138) is acknowledged. L.F. acknowledges funding from the European Union's Horizon 2020 research and in-novation programme through the Marie Skodowska-Curie Grant Agreement MagicFACE No. 797109. We acknowl-edge SOLEIL for provision of synchrotron radiation facilities at CASSIOPEE beamline under proposal 20181362. The XMCD experiments were performed at BOREAS beamline at ALBA Synchrotron with the collaboration of ALBA staff. Computational resources were provided by DIPC

A ferromagnetic Eu–Pt surface compound grown below hexagonal boron nitride

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

Near-Ambient Pressure Oxidation of Silver in the Presence of Steps: Electrophilic Oxygen and Sulfur Impurities

The oxidation of Ag crystal surfaces has recently triggered strong controversies around the presence of sulfur impurities that may catalyze reactions, such as the alkene epoxidations, especially the ethylene epoxidation. A fundamental challenge to achieve a clear understanding is the variety of procedures and setups involved as well as the particular history of each sample. Especially, for the often-used X-ray photoemission technique, product detection, or photoemission peak position overlap are problematic. Here we investigate the oxidation of the Ag(111) surface and its vicinal crystal planes simultaneously, using a curved crystal sample and in situ X-ray photoelectron spectroscopy at 1 mbar O2 near-ambient pressure conditions to further investigate surface species. The curved geometry allows a straightforward comparative analysis of the surface oxidation kinetics at different crystal facets, so as to precisely correlate the evolution of different oxygen species, namely nucleophilic and electrophilic oxygen, and the buildup of sulfur as a function of the crystal orientation. We observed that emission from both surface and bulk oxide contributes to the characteristic nucleophilic oxygen core-level peak, which arises during oxygen dosing and rapidly saturates below temperatures of 180 °C. The electrophilic oxygen peak appears later, growing at a slower but constant rate, at the expenses of the surface oxide. Electrophilic oxygen and sulfur core-levels evolve in parallel in all crystal facets, although faster and stronger at vicinal surfaces featuring B-type steps with {111} microfacets. Our study confirms the intimate connection of the electrophilic species with the formation of adsorbed SO4, and points to a higher catalytic activity of B-type stepped silver surfaces for alkene epoxidation or methane to formaldehyde conversion.The authors thank support from the project PID2020-116093RB-C44, funded by MCIN/AEI/10.13039/501100011033/and by “ERDF A way of making Europe”, by the Basque Government proposal IT-1591-22 and the Gipuzkoa Next program of the Diputación Foral de Gipukoa DFG- 2023-CIEN-000077. S.V.A. acknowledges funding from the European Union’s Horizon 2020 research and innovation program through the Marie Skłodowska-Curie grant agreement no. 101066965 CURVEO. This experiments were performed at CIRCE-NAPP beamline at ALBA, with collaboration of ALBA staff

A ferromagnetic Eu-Pt surface compound grown below hexagonal boron nitride

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.We acknowledge financial support from grants PID2020-116093RB-C44 funded by the Spanish MCIN/AEI/10.13039/501100011033 and the Basque Government (Grant IT-1591-22) as well as from the German BMBF (grant 05K19KER). We acknowledge the European Synchrotron Radiation Facility for provision of beam time on ID32. ESRF access was provided through proposal MA-5454.76 Part of the research leading to the results has been supported by the project CALIPSOplus under Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. Y. H. appreciates the support of Japan Society for the Promotion of Science (JSPS) Overseas Research Fellowships and I. P. and F. B. acknowledge financial support from EUROFEL project (RoadMap Esfri).Peer reviewe

Supplementary material of “A ferromagnetic Eu-Pt surface compound grown below hexagonal boron nitride”

The supplementary information file contains: Supplementary Figures S1-S8. Supplementary information and data are provided to further illustrate the intercalation process of Eu below the hBN/Pt substrate as well as magnetic easy axis determination of the Eu-Pt alloy below hBN.Peer reviewe

Room-temperature C–C σ-bond activation of biphenylene derivatives on Cu(111)

Activating the strong C–C σ-bond is a central problem in organic synthesis. Directly generating activated C centers by metalation of structures containing strained four-membered rings is one maneuver often employed in multistep syntheses. This usually requires high temperatures and/or precious transition metals. In this paper, we report an unprecedented C–C σ-bond activation at room temperature on Cu(111). By using bond-resolving scanning probe microscopy, we show the breaking of one of the C–C σ-bonds of a biphenylene derivative, followed by insertion of Cu from the substrate. Chemical characterization of the generated species was complemented by X-ray photoemission spectroscopy, and their reactivity was explained by density functional theory calculations. To gain further insight into this unique reactivity on other coinage metals, the reaction pathway on Ag(111) was also investigated and the results were compared with those on Cu(111). This study offers new synthetic routes that may be employed in the in situ generation of activated species for the on-surface synthesis of novel C-based nanostructures.The authors acknowledge financial support from MCIN/AEI/10.13039/501100011033 (Grants PID2019-107338RB-C62, PID2019-107338RB-C63, PID2019-109555GB-I00, and TED2021-132388B-C43), the Basque Government (IT1591-22 and PIBA19-0004), the Spanish Research Council (ILINKC20002), the European Union’s Horizon 2020 research and innovation program (Grant 863098 and Marie Skłodowska-Curie Actions Individual Fellowship 101022150), and the Xunta de Galicia (Centro Singular de Investigación de Galicia, 2019-2022, Grant ED431G2019/03). A.P.P. thanks the UAEU for an internal start-up grant (31S410).Peer reviewe

Circumventing the stability problems of graphene nanoribbon zigzag edges

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.Research was supported by MCIN/AEI/10.13039/501100011033 (grant nos PID2019-107338RB-C62 (D.P.), PID2019-107338RB-C63 (M.C. and D.G.d.O.) and FJC2019-041202-I (F.S.)); the European Union’s Horizon 2020 programme (grant nos 863098 (D.P.) and 635919 (D.G.d.O.), and Marie Skłodowska-Curie Actions Individual Fellowship no. 101022150 (T.W.)); the Gobierno Vasco (grant no. PIBA_2020_1_0036 (D.G.d.O.)); the Xunta de Galicia (Centro Singular de Investigación de Galicia, 2019–2022, grant no. ED431G2019/03 (D.P.)); the European Regional Development Fund; the Praemium Academie of the Academy of Science of the Czech Republic (GACR project no. 20-13692X (P.J.)); the Czech Nanolab Research Infrastructure supported by MEYS CR (project no. LM2018110 (P.J.)); and the Operational Programme for Research, Development and Education of the European Regional Development Fund (project no. CZ.02.1.01/0.0/0.0/16_019/0000754 (P.J.)).Peer reviewe

Análisis del efecto del procesado en la estructura celular de materiales micro y nanocelulares basados en PMMA

El principal objetivo de este TFG ha consistido en analizar el efecto que el tipo de procesado, al que se ve sometido la matriz polimérica previo a la étapa de espumado, tiene sobre la estructura celular. Para ello se ha trabajado principalmente con dos grados de polimetilmetacrilato (PMMA). Estos polímeros han sido procesados empleando diferentes técnicas: extrusión y mezclado con mezclador interno. Además los parámetros de espumado se han modificado y los materiales celulares se han producido empleando dos presiones de saturación diferentes. Para poder cumplir con el objetivo establecido se han realizado diversas caracterizaciones tanto de los polímeros sólidos como de los plímeros celulares que han permitido entender los efectos observdos tras porcesar los materiales poliméricos.Grado en Físic

Rare-Earth-Noble-Metal Surface Alloys and its Interaction with Phthalocyanine Molecules

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 ambos materiales ferromagnéticos están magnetizados en la misma dirección o en opuestas. Este efecto se llama Magneto Resistencia Gigante (GMR), y el dispositivo resultante es la válvula de espín, el cual representó el primer gran éxito de la espintrónica. A día de hoy, la sensibilidad y versatilidad de las válvulas de espín son la base de aplicaciones como discos duros, MRAMs (memoria no volátil magnetorresistiva), brújulas electrónicas, sensores magnéticos, etc. La espintrónica ha empezado a promover el desarrollo de la electrónica, integrando la estabilidad y la capacidad de reescritura casi-infinita de la grabación magnética. Esto resulta prometedor para el desarrollo de la electrónica de bajo consumo, nuevas arquitecturas especializadas, e incluso computación no convencional (como la computación estocástica y computación cuántica), así como sistemas de detección y medidas más sensibles basadas en el magnetismo. Para el desarrollo de todas estas ideas futuras, es necesario un mejor entendimiento de los sistemas magnéticos, y el desarrollo de nuevos materiales con propiedades a medida. Es más, esos materiales se tienen que poder llevar a la nanoescala, lo que introduce un nuevo desafío para el desarrollo de la espintrónica. En este marco, las moléculas orgánicas semiconductoras ofrecen una alternativa prometedora para la creación de dispositivos spintronicos. El uso de moléculas orgánicas ofrece nuevas características no existentes en materiales inorgánicos, como mayor sensibilidad a estímulos eléctricos o ópticos externos o funcionalidades físico-químicas intrínsecas. Los dispositivos espintrónicos moleculares se basan en la generación, transporte y detección de portadores con espín polarizado mediante la combinación de un material ferromagnético con un semiconductor orgánico. En estos sistemas la interfaz entre los distintos materiales gobierna la transferencia de carga y la conducción de espín entre el nivel de Fermi del metal y los orbitales moleculares, actuando como un filtro adicional de espín que solamente permite el paso de electrones de una determinada orientación de espín. Varios trabajos ya han mostrado las ventajas de una interfaz ferromagnéticosemiconductor orgánico y su relevancia para la inyección de espín. De este modo, el acoplo y enlaces químicos entre el semiconductor orgánico y el metal ferromagnético da lugar a nuevos estados electrónicos híbridos con polarización de espín frecuentemente llamados “spinterface”. Estos nuevos estados híbridos determinan el transporte de carga entre el metal y los orbitales moleculares solo dejando pasar electrones con una dirección de espín determinada, y actuando por tanto como un filtro de espín. 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). Posteriormente, se enfoca en la absorción de moléculas orgánicas magnéticas sobre estas aleaciones y los efectos de su interacción con la aleación.Peer reviewe

Rare-Earth-Noble-Metal Surface Alloys and its Interaction with Phthalocyanine Molecules

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 superficiale