Site- and orbital-dependent charge donation and spin manipulation in electron-doped metal phthalocyanines

Chemical doping offers promise as a means of tailoring the electrical characteristics of organic molecular compounds. However, unlike for inorganic semiconductors used in electronics applications, controlling the influence of dopants in molecular complexes is complicated by the presence of multiple doping sites, electron acceptor levels, and intramolecular correlation effects. Here we use scanning tunnelling microscopy to analyse the position of individual Li dopants within Cu- and Ni-phthalocyanine molecules in contact with a metal substrate, and probe the charge transfer process with unprecedented spatial resolution. We show that individual phthalocyanine molecules can host at least three distinct stable doping sites and up to six dopant atoms, and that the ligand and metal orbitals can be selectively charged by modifying the configuration of the Li complexes. Li manipulation reveals that charge transfer is determined solely by dopants embedded in the molecules, whereas the magnitude of the conductance gap is sensitive to the molecule-dopant separation. As a result of the strong spin-charge correlation in confined molecular orbitals, alkali atoms provide an effective way for tuning the molecular spin without resorting to magnetic dopants. © 2013 Macmillan Publishers Limited. All rights reserved.This work was supported by the European Research Council (StG 203239 NOMAD), Ministerio de Economía y Competitividad (MAT2010-15659), and Agència de Gestió d'Ajuts Universitaris i de Recerca (2009 SGR 695). A.M. acknlowledges the Spanish Ministerio de Ciencia e Innovación for a Ramon y Cajal Fellowship.Peer Reviewe

Determination of the photoelectron reference plane in nanostructured surfaces

In angle-resolved photoemission (ARPES) from crystalline solids, wave-vector conservation applies to the two-dimensional (2D) surface, which may thus be defined as the reference plane in ARPES. We investigate whether such reference varies for photoemitted electrons in nanometer-sized systems that expose different crystal planes. To this aim, we exploit the structural tunability of the Ag/Cu(223) system which is capable of offering surfaces with periodic arrays of nanofacets of varying size and orientation. A thorough, photonenergy- dependent analysis of the surface states confined to such nanostructures is performed comparing different reference planes for photoemitted electrons. Assuming the premise that k|| must be a good quantum number for 2D states, we conclude that the (final state) photoelectron reference direction is not the average optical direction but the local facet that confines the (initial state) surface electrons. Moreover, in the general case of nanostructured systems with uneven surfaces, we show how the photoelectron reference plane can be empirically determined through such a photon-energy-dependent ARPES analysisJLC and AM acknowledge the Spanish Ministerio de Ciencia e Innovación (MICINN) for financial support through the research program Ramón y Cajal. JEO acknowledges support from the Spanish MICINN (MAT2010-21156-C03-01), the Basque Government (IT-257-07) and the DIPC (sabbatical program). EGM acknowledges support from the Spanish MICINN (FIS2008-00399

Measurement of electron wave functions and confining potentials via photoemission

Wave functions and electron potentials of laterally confined surface states are determined experimentally by means of photoemission from stepped Au(111) surfaces. The confined nature of the wave function makes it possible to use an iterative formalism borrowed from x-ray diffraction where real-space wave functions are retrieved from the absolute values of their Fourier transforms. The latter are measured by angle-resolved photoemission. The effective confining potential is then obtained by introducing the wave functions into Schrödinger’s equation.A.M. and J.E.O. are supported by the Universidad del Pais Vasco Grant No. 9/UPV 00057.240-13668/2001 and the Max Planck Research Institute. J.G.A. gratefully acknowledges support by the Basque Departamento de Educacion, Universidades e Investigacion, the University of the Basque Country UPV/EHU Contract No. 00206.215-13639/2001, and the Spanish Ministerio de Ciencia y Tecnologia Contract No. MAT2001-0946. F.J.H. acknowledges support by the NSF under Grants Nos. DMR-9815416 and DMR0084402.Peer reviewe

Water adsorption on O (2×2 )/Ru (0001): STM experiments and first-principles calculations

We present a combined theoretical and experimental study of water adsorption on Ru(0001) precovered with 0.25 ML (monolayer) of oxygen forming a (2×2) structure. Several structures were analyzed by means of density functional theory calculations for which scanning tunneling microscope (STM) simulations were performed and compared with experimental data. Up to 0.25 ML, the molecules bind to the exposed Ru atoms of the 2×2unit cell via the lone pair orbitals. The molecular plane is almost parallel to the surface with its H atoms pointing toward the chemisorbed O atoms of the 2×2 unit cell forming hydrogen bonds. The existence of these additional hydrogen bonds increases the adsorption energy of the water molecule to approximately 616meV, which is ∼220meV more stable than on the clean Ru(0001) surface with a similar configuration. The binding energy shows only a weak dependence on water coverage, with a shallow minimum for a row structure at 0.125 ML. This is consistent with the STM experiments that show a tendency of the molecules to form linear rows at intermediate coverage. Our calculations also suggest the possible formation of water dimers near 0.25 ML.This work has been supported by the Basque Departamento de Educación, the UPV/EHU Grant No. 9/UPV 00206.215-13639/2001, the Spanish Ministerio de Educación y Ciencia Grant No. FIS2004-06490-C3-00, the European Network of Excellence FP6-NoE NANOQUANTA Grant No. 500198-2, and the projects NANOMATERIALES and NANOTRON funded by the Basque Departamento de Industria, Comercio y Turismo within the ETORTEK program, and the Departamento para la Innovación y la Sociedad del Conocimiento from the Diputación Foral de Guipúzcoa. The experimental work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. A.M. was financed by the Marie Curie Outgoing International Foundation, Project No. 514412.Peer reviewe

Cross-linguistic differences in case marking shape neural power dynamics and gaze behavior during sentence planning

Languages differ in how they mark the dependencies between verbs and arguments, e.g., by case. An eye tracking and EEG picture description study examined the influence of case marking on the time course of sentence planning in Basque and Swiss German. While German assigns an unmarked (nominative) case to subjects, Basque specifically marks agent arguments through ergative case. Fixations to agents and event-related synchronization (ERS) in the theta and alpha frequency bands, as well as desynchronization (ERD) in the alpha and beta bands revealed multiple effects of case marking on the time course of early sentence planning. Speakers decided on case marking under planning early when preparing sentences with ergative-marked agents in Basque, whereas sentences with unmarked agents allowed delaying structural commitment across languages. These findings support hierarchically incremental accounts of sentence planning and highlight how cross-linguistic differences shape the neural dynamics underpinning language use.This work was funded by Swiss National Science Foundation Grant Nr. 100015_160011 (B.B. and M.M.), the NCCR Evolving Language, Swiss National Science Foundation Agreement Nr. #51NF40_180888 (B.B. and M. M.), and the PhD Program in Linguistics and the Graduate Research Campus of the University of Zurich (A.E.). DEB is supported by a grant from the Harvard Data Science Initiative and the Branco Weiss Foundation. I.B.-S. is supported by an Australian Research Council Future Fellowship (FT160100437). I.L. is supported by grants from the Spanish Ministry of Economy and Competitiveness (Grant No. FFI2015-64183-P) and the Basque Government (IT1169-19). The authors thank Anne-Lise Giraud for the suggestion to include beta-band analyses, Vitória Piai for advice on EEG data processing, Giuachin Kreiliger for statistical consultation, Andrina Balsofiore and Edurne Petrirena for help recording the lead-in fragments, Nathalie Rieser and Debora Beuret for help with data collection and processing, and the Phonogram Archives of the University of Zurich for technical support. The authors also thank two anonymous reviewers for their helpful comments on an earlier version of the manuscript

Fieldlike and antidamping spin-orbit torques in as-grown and annealed Ta/CoFeB/MgO layers

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY).-- et al.We present a comprehensive study of the current-induced spin-orbit torques in perpendicularly magnetized Ta/CoFeB/MgO layers. The samples were annealed in steps up to 300 °C and characterized using x-ray-absorption spectroscopy, transmission electron microscopy, resistivity, and Hall effect measurements. By performing adiabatic harmonic Hall voltage measurements, we show that the transverse (fieldlike) and longitudinal (antidampinglike) spin-orbit torques are composed of constant and magnetization-dependent contributions, both of which vary strongly with annealing. Such variations correlate with changes of the saturation magnetization and magnetic anisotropy and are assigned to chemical and structural modifications of the layers. The relative variation of the constant and anisotropic torque terms as a function of annealing temperature is opposite for the fieldlike and antidamping torques. Measurements of the switching probability using sub-μs current pulses show that the critical current increases with the magnetic anisotropy of the layers, whereas the switching efficiency, measured as the ratio of magnetic anisotropy energy and pulse energy, decreases. The optimal annealing temperature to achieve maximum magnetic anisotropy, saturation magnetization, and switching efficiency is determined to be between 240 and 270°C.This work was supported by the the European Commission under the Seventh Framework Programme (GA 318144, SPOT), the European Research Council (StG 203239 NOMAD), the Ministerio de Economía y Competitividad (MAT2010-15659), and the Swiss Competence Centre for Materials Science and Technology (CCMX).Peer Reviewe

Symmetry forbidden morphologies and domain boundaries in nanoscale graphene islands

The synthesis of graphene nanoislands with tailored quantum properties requires an atomic control of the morphology and crystal structure. As one reduces their size down to the nanometer scale, domain boundary and edge energetics, as well as nucleation and growth mechanisms impose different stability and kinetic landscape from that at the microscale. This offers the possibility to synthesize structures that are exclusive to the nanoscale, but also calls for fundamental growth studies in order to control them. By employing high-resolution scanning tunneling microscopy we elucidate the atomic stacking configurations, domain boundaries, and edge structure of graphene nanoislands grown on Ni(1 1 1) by CVD and post-annealed at different temperatures. We find a non-conventional multistep mechanism that separates the thermal regimes for growth, edge reconstruction, and final stacking configuration, leading to nanoisland morphologies that are incompatible with their stacking symmetry. Whole islands shift their stacking configuration during cooling down, and others present continuous transitions at the edges. A statistical analysis of the domain structures obtained at different annealing temperatures reveals how polycrystalline, ill-defined structures heal into shape-selected islands of a single predominant stacking. The high crystallinity and the control on morphology and edge structure makes these graphene nanoislands ideal for their application in optoelectronics and spintronics

Atomically sharp lateral superlattice heterojunctions built-in nitrogen-doped nanoporous graphene

Altres ajuts: this research was funded by the CERCA Programme/Generalitat de Catalunya.Nanometer scale lateral heterostructures with atomically sharp band discontinuities can be conceived as the 2D analogues of vertical Van der Waals heterostructures, where pristine properties of each component coexist with interfacial phenomena that result in a variety of exotic quantum phenomena. However, despite considerable advances in the fabrication of lateral heterostructures, controlling their covalent interfaces and band discontinuities with atomic precision, scaling down components and producing periodic, lattice-coherent superlattices still represent major challenges. Here, a synthetic strategy to fabricate nanometer scale, coherent lateral superlattice heterojunctions with atomically sharp band discontinuity is reported. By merging interdigitated arrays of different types of graphene nanoribbons by means of a novel on-surface reaction, superlattices of 1D, and chemically heterogeneous nanoporous junctions are obtained. The latter host subnanometer quantum dipoles and tunneling in-gap states, altogether expected to promote interfacial phenomena such as interribbon excitons or selective photocatalysis

Fieldlike and antidamping spin-orbit torques in as-grown and annealed Ta/CoFeB/MgO layers

We present a comprehensive study of the current-induced spin-orbit torques in perpendicularly magnetized Ta/CoFeB/MgO layers. The samples were annealed in steps up to 300 degrees C and characterized using x-ray absorption spectroscopy, transmission electron microscopy, resistivity, and Hall effect measurements. By performing adiabatic harmonic Hall voltage measurements, we show that the transverse (field-like) and longitudinal (antidamping-like) spin-orbit torques are composed of constant and magnetization-dependent contributions, both of which vary strongly with annealing. Such variations correlate with changes of the saturation magnetization and magnetic anisotropy and are assigned to chemical and structural modifications of the layers. The relative variation of the constant and anisotropic torque terms as a function of annealing temperature is opposite for the field-like and antidamping torques. Measurements of the switching probability using sub-{\mu}s current pulses show that the critical current increases with the magnetic anisotropy of the layers, whereas the switching efficiency, measured as the ratio of magnetic anisotropy energy and pulse energy, decreases. The optimal annealing temperature to achieve maximum magnetic anisotropy, saturation magnetization, and switching efficiency is determined to be between 240 degrees and 270 degrees C