Operando atomic-scale study of graphene CVD growth at steps of polycrystalline nickel

An operando investigation of graphene growth on (100) grains of polycrystalline nickel (Ni) surfaces was performed by means of variable-temperature scanning tunneling microscopy complemented by density functional theory simulations. A clear description of the atomistic mechanisms ruling the graphene expansion process at the stepped regions of the substrate is provided, showing that different routes can be followed, depending on the height of the steps to be crossed. When a growing graphene flake reaches a monoatomic step, it extends jointly with the underlying Ni layer; for higher Ni edges, a different process, involving step retraction and graphene landing, becomes active. At step bunches, the latter mechanism leads to a peculiar \u2018staircase formation\u2019 behavior, where terraces of equal width form under the overgrowing graphene, driven by a balance in the energy cost between C\u2013Ni bond formation and stress accumulation in the carbon layer. Our results represent a step towards bridging the material gap in searching new strategies and methods for the optimization of chemical vapor deposition graphene production on polycrystalline metal surfaces

Enhancing electron correlation at a 3D ferromagnetic surface

Spin-resolved momentum microscopy and theoretical calculations are combined beyond the one-electron approximation to unveil the spin-dependent electronic structure of the interface formed between iron (Fe) and an ordered oxygen (O) atomic layer, and an adsorbate-induced enhancement of electronic correlations is found. It is demonstrated that this enhancement is responsible for a drastic narrowing of the Fe d-bands close to the Fermi energy (EF) and a reduction of the exchange splitting, which is not accounted for in the Stoner picture of ferromagnetism. In addition, correlation leads to a significant spin-dependent broadening of the electronic bands at higher binding energies and their merging with satellite features, which are manifestations of a pure many-electron behavior. Overall, adatom adsorption can be used to vary the material parameters of transition metal surfaces to access different intermediate electronic correlated regimes, which will otherwise not be accessible. The results show that the concepts developed to understand the physics and chemistry of adsorbate–metal interfaces, relevant for a variety of research areas, from spintronics to catalysis, need to be reconsidered with many-particle effects being of utmost importance. These may affect chemisorption energy, spin transport, magnetic order, and even play a key role in the emergence of ferromagnetism at interfaces between non-magnetic systems

Signatures of an Atomic Crystal in the Band Structure of a Molecular Thin Film

Transport phenomena in molecular materials are intrinsically linked to the orbital character and the degree of localization of the valence states. Here, we combine angle-resolved photoemission with photoemission tomography to determine the spatial distribution of all molecular states of the valence band structure of a C$_{60}$ thin film. While the two most frontier valence states exhibit a strong band dispersion, the states at larger binding energies are characterized by distinct emission patterns in energy and momentum space. Our findings demonstrate the formation of an atomic crystal-like band structure in a molecular solid with delocalized $\pi$-like valence states and strongly localized $\sigma$-states at larger binding energies

Physical Delithiation of Epitaxial LiCoO2 Battery Cathodes as a Platform for Surface Electronic Structure Investigation

We report a novel delithiation process for epitaxial thin films of LiCoO2(001) cathodes using only physical methods, based on ion sputtering and annealing cycles. Preferential Li sputtering followed by annealing produces a surface layer with a Li molar fraction in the range 0.5 < x < 1, characterized by good crystalline quality. This delithiation procedure allows the unambiguous identification of the effects of Li extraction without chemical byproducts and experimental complications caused by electrolyte interaction with the LiCoO2 surface. An analysis by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) provides a detailed description of the delithiation process and the role of O and Co atoms in charge compensation. We observe the simultaneous formation of Co4+ ions and of holes localized near O atoms upon Li removal, while the surface shows a (2 × 1) reconstruction. The delithiation method described here can be applied to other crystalline battery elements and provide information on their properties that is otherwise difficult to obtainThis work was supported by the Spanish MICINN (grant nos. PID2021-123295NB-I00 and PID2020-117024GB-C43), MAT2017-83722-R, “María de Maeztu” Programme for Units of Excellence in R&D (CEX2018-000805-M), within the framework of UE M-ERA.NET 2018 program under StressLIC Project (grant no. PCI2019-103594) and by the Comunidad Autónoma de Madrid (contract no. PEJD-2019- PRE/IND-15769 and S2108-NMT4321). The authors acknowledge Elettra Sincrotrone Trieste for providing access to its synchrotron radiation facilities. They thank Ignacio Carabias from the Diffraction Unit CAI UCM for his experimental support and helpful comments. The research leading to this result has been supported by the project CALIPSOplus under Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. M.J., P.M., I.P., and F.B. acknowledge funding from EUROFEL (RoadMap Esfri). The work at the University of Maryland was supported by ONR MURI (Award No. N00014-17-1-2661). The work at Sandia National Laboratories was supported by the Laboratory-Directed Research and Development (LDRD) Program and the DOE Basic Energy Sciences Award number DE-SC0021070. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the US Department of Energy’s National Nuclear Security Administration under contract DE-NA 000352

Influence of 4f filling on electronic and magnetic properties of rare earth-Au surface compounds

Altres ajuts: This work was supported in part by the Basque Government Project IT-1255-19, and University of the Basque Country (UPV/EHU) grant GIU18/138 and the European Regional Development Fund (ERDF) under the program Interreg V-A España-Francia-Andorra (Contract No. EFA 194/16 TNSI).One-atom-thick rare-earth/noble metal (RE-NM) compounds are attractive materials to investigate two-dimensional magnetism, since they are easy to synthesize into a common RE-NM2 structure with high crystal perfection. Here we perform a comparative study of the GdAu2, HoAu2, and YbAu2 monolayer compounds grown on Au(111). We find the same atomic lattice quality and moiré superlattice periodicity in the three cases, but different electronic properties and magnetism. The YbAu2 monolayer reveals the characteristic electronic signatures of a mixed-valence configuration in the Yb atom. In contrast, GdAu2 and HoAu2 show the trivalent character of the rare-earth and ferromagnetic transitions below 22 K. Yet, the GdAu2 monolayer has an in-plane magnetic easy-axis, versus the out-of-plane one in HoAu2. The electronic bands of the two trivalent compounds are very similar, while the divalent YbAu2 monolayer exhibits different band features. In the latter, a strong 4f-5d hybridization is manifested in neatly resolved avoided crossings near the Fermi level. First principles theory points to a residual presence of empty 4f states, explaining the fluctuating valence of Yb in the YbAu2 monolayer

Observation of termination-dependent topological connectivity in a magnetic Weyl Kagome lattice

The research leading to these results has received funding from the European Union’s Horizon 2020 research and innovation program under Marie Skłodowska-Curie Grant Agreement 897276. The authors gratefully acknowledge the Gauss Centre for Supercomputing e.V. (https://www.gauss-centre.eu) for funding this project by providing computing time on the GCS Supercomputer SuperMUC-NG at Leibniz Supercomputing Centre (https://www.lrz.de). The authors are grateful for funding support from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy through the Würzburg-Dresden Cluster of Excellence on Complexity and Topology in Quantum Matter ct.qmat (EXC 2147, Project 390858490), through FOR 5249-449872909 (Project P5), and through the Collaborative Research Center SFB 1170 ToCoTronics (Project 258499086). The authors greatly acknowledge the Diamond Light Source that supported the entire micro-ARPES experiment and corresponding costs. The Flatiron Institute is a division of the Simons Foundation. P.D.C.K. and C.B. gratefully acknowledge support from The Leverhulme Trust via Grant RL-2016-006.Engineering surfaces and interfaces of materials promises great potential in the field of heterostructures and quantum matter designers, with the opportunity to drive new many-body phases that are absent in the bulk compounds. Here, we focus on the magnetic Weyl kagome system Co3Sn2S2 and show how for the terminations of different samples the Weyl points connect differently, still preserving the bulk-boundary correspondence. Scanning tunneling microscopy has suggested such a scenario indirectly, and here, we probe the Fermiology of Co3Sn2S2 directly, by linking it to its real space surface distribution. By combining micro-ARPES and first-principles calculations, we measure the energy-momentum spectra and the Fermi surfaces of Co3Sn2S2 for different surface terminations and show the existence of topological features depending on the top-layer electronic environment. Our work helps to define a route for controlling bulk-derived topological properties by means of surface electrostatic potentials, offering a methodology for using Weyl kagome metals in responsive magnetic spintronics.Publisher PDFPeer reviewe

Morphology and electronic structure of graphene supported by metallic thin films

The increasing demand for data storage capacity and the environmental sustainability of electronic storage devices ask for the use of innovative technologies. Extensive production of such devices encounters an economical barrier, where a low production cost is fundamental for a sustainable production chain. The use of graphene both as functionalizing and as passivating layer emerged as a solution matching the demands listed above. It shifted the interest of the scientific community in the past decade towards the optimization of graphene growth, using a variety of different approaches. In this thesis, a multi-technique characterization of single-layer graphene growth on top of ferromagnetic supports is reported. Preceding the graphene growth, characterization of the temperature-dependent thickness behavior of thin metallic films allowed for the optimization of their quality, followed by the investigation of the electronic properties of the metal films. The substrate was chosen as cobalt both from geometrical reasons, i.e. lattice mismatch, as well as better suitability to the experimental setup used. Using spatially resolved techniques, the well-known Co martensitic phase transition as a function of temperature has been observed and characterized. On top of the cobalt support, the chemical vapor deposition growth has been used for the formation of a graphene monolayer, using ethylene as the carbon supply. The graphene crystallographic quality varies as a function of growth temperature showing different azimuthal alignments with respect to the substrate. However, in this thesis, it is demonstrated that a transformation involving carbon exchange with the substrate allows reverting the different configurations in an epitaxially aligned graphene monolayer. The subsequent characterization of the electronic structure reveals that the single spin-polarized feature near the Fermi level, forming upon graphene adsorption on cobalt, is a general characteristic of the interface, independent on the relative orientation at the graphene-cobalt interface. Having control over the epitaxial relation between the graphene and the cobalt substrate, modification of graphene-substrate interaction can be achieved either by controlled substitutional implantation of exospecies into the C lattice mesh or by intercalation of foreign species. Therefore, in this thesis the nitrogen substitution within the graphene lattice as well as oxygen and gold intercalation at the graphene-Co interface have been studied. The momentum mapping unravels that the modification of the graphene-cobalt interaction leads to the disappearance of the single spin-polarized band in graphene

Doping of epitaxial graphene by direct incorporation of nickel adatoms

Direct incorporation of Ni adatoms during graphene growth on Ni(111) is evidenced by scanning tunneling microscopy. The structure and energetics of the observed defects is thoroughly characterized at the atomic level on the basis of density functional theory calculations. Our results show the feasibility of a simple scalable method, which could be potentially used for the realization of macroscopic practical devices, to dope graphene with a transition metal. The method exploits the kinetics of the growth process for the incorporation of Ni adatoms in the graphene network

Principal component analysis: Reveal camouflaged information in x-ray absorption spectroscopy photoemission electron microscopy of complex thin oxide films

Principal component analysis (PCA) has become a standard tool in spectromicroscopy and hyperspectral imaging to handle large spectral data sets and to decompose raw data into relevant and residual information. In particular in studies of complex compounds, PCA can be used to disentangle chemical information and thereby deepen the understanding of chemical and physical material properties. Surprisingly, in photoemission electron spectromicroscopy (PEEM), PCA is rarely used. This paper serves to demonstrate how powerful PCA can be to detect hidden chemical information in PEEM data. We demonstrate the capability of PCA in PEEM spectromicroscopy for the case of a thin film of a complex quaternary oxide, Pr0.5Ba0.5CoO3-δ (PBCO) which is a main contender catalyst material for electrocatalytic water splitting. Upon annealing in air, PBCO decomposes into different phases at the surface. Two of them become obvious from the raw PEEM images, but one is revealed only after PCA