Suchergebnisse

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY). ; While the role of sampling of the electron momentum k in supercell calculations of the elastic electron transmission is well understood, its influence in the case of inelastic electron tunneling (IET) has not yet been systematically explored. Here we compare ab initio IET spectra of molecular monolayers in the commonly used Γ-point approximation to rigorously k-converged results. We study four idealized molecular junctions with either alkanedithiolates or benzenedithiolates, and explore variations due to varying molecular tilt angle, density, as well as chemical identity of the monolayer. We show that the Γ-point approximation is reasonable for a range of systems, but that a rigorous convergence is needed for accurate signal amplitudes. We also describe an approximative scheme which reduces the computational cost of the k-averaged calculation in our implementation. ; We acknowledge the support of the Basque Departamento de Educacion and the UPV/EHU (Grant No. IT-756-13), the Spanish Ministerio de Economía y Competitividad (MINECO Grants No. MAT2013-46593-C6-2-P and FIS2013-48286-C2-2-P), and the European Union Integrated Project PAMS (Contract No. 610446). ; Peer Reviewed

4 páginas, 3 figuras, 1 tabla.-- PACS numbers: 68.37.Ef, 72.10.-d, 72.25.-b, 79.20.Rf ; Density functional theory simulations of the vibrational inelastic electron tunneling spectroscopy (IETS) of O2 on Ag(110) permits us to solve its unexplained IETS data [ Hahn et al. Phys. Rev. Lett. 85 1914 (2000)]. When semilocal density functional theory is corrected by including static intra-atomic correlations, the IETS simulations are in excellent agreement with the experiment. The unforeseen consequence of our calculations is that when adsorbed along the [001] direction, molecular O2 on Ag(110) is a mixed-valent system. This analysis of IETS unambiguously reveals the paramagnetic nature of O2 on Ag(110). ; We acknowledge financial support from the Spanish MICINN (No. FIS2007-066711-CO2-00 and No. FIS2009-12721-C04-01), and the Basque Government—UPV/EHU (Grant No. IT-366-07). ; Peer reviewed

We present a generalization of the transfer Hamiltonian method to include inelastic currents due to electron-vibration coupling. This formalism is applied to CO vibrations on Cu(100). The method describes the changes in conductance across the vibrational threshold of both the inelastic (increase) and elastic (decrease) contributions, which are evaluated from electronic structure calculations of tip and sample. The most active modes are found to be the two frustrated rotations of the CO molecule. A comparison with previous results, based on the local density of states, shows that explicitly including the tip structure significantly improves the quantitative agreement with experiments. Finally, we discuss how different tip terminations affect the inelastic tunneling current. ; G.T. has been funded by EPRSC Grant No. EP/C541898/1. G.T. and N.L. also acknowledge support from the European Science Foundation, ESF exchange grant, Ref. 1280. M.P. and A.A. have been partially funded by the Spanish MEC, the UPV/EHU, and the Basque Government through the ETORTEK program. W.A.H. is supported by the Royal Society. ; Peer reviewed

This article is part of themed collection: 2015 Hot Papers in Nanoscale.-- arXiv:1412.7698v1.-- et al. ; We present a quantitative exploration, combining experiment and simulation, of the mechanical and electronic properties, as well as the modifications induced by an alkylthiolated coating, at the single nanoparticle (NP) level. We determined the response of the NPs to external pressure in a controlled manner using an atomic force microscope tip. We found a strong reduction in their Young's modulus, as compared to bulk gold, and a significant influence of strain on the electronic properties of the alkylthiolated NPs. Electron transport measurements of tiny molecular junctions (NP/alkylthiol/CAFM tip) show that the effective tunnelling barrier through the adsorbed monolayer strongly decreases by increasing the applied load, which translates in a remarkable and unprecedented increase in the tunnel current. These observations are successfully explained using simulations based on the finite element analysis (FEA) and first-principles calculations that permit one to consider the coupling between the mechanical response of the system and the electric dipole variations at the interface. ; P.L. is a Senior Research Associate from the Fund for Scientific Research of Belgium (F.R.S. – FNRS). K.S. has been supported by the NordPas-de Calais Council fund and the ANR project SAGE III-V (no. ANR11BS1001203) and S.D. by EU project I-ONE (FP7 no. 280772). The experiments were partly funded by the SINGLEMOL project supported by the Nord-Pas-de Calais council fund. We also acknowledge funding from the Basque Government (Grant No. IT-756-13) and the Spanish MINECO (Grant Nos. MAT2013-46593-C6-2-P and FIS2013-48286-C2-2-P). ; Peer Reviewed

Using time-dependent density-functional theory we calculate from first principles the rate of energy transfer from a moving proton or antiproton to the electrons of an insulating material, LiF. The behavior of the electronic stopping power versus projectile velocity displays an effective threshold velocity of ∼0.2a.u. for the proton, consistent with recent experimental observations, and also for the antiproton. The calculated proton/antiproton stopping-power ratio is ∼2.4 at velocities slightly above the threshold (v∼0.4a.u.), as compared to the experimental value of 2.1. The projectile energy loss mechanism is observed to be extremely local. ; This work has been funded by the EC's Marie Curie program, UK's BNFL and NERC, the Spanish MEC, the UPV/EHU and the Basque Government through the Etortek program. ; Peer reviewed

12 páginas, 11 figuras.-- PACS number(s): 73.20.At, 73.20.Hb, 73.22.Dj, 68.37.Ef ; We report on a joint scanning tunneling microscopy (STM) and theoretical wave packet propagation study of field emission resonances (FER's) of nanosized alkali metal clusters deposited on a Cu(100) surface. In addition to FER's of the pristine Cu(100) surface, we observe the appearance of island-induced resonances that are particularly well resolved for STM bias voltage values corresponding to electron energies inside the projected band gap of the substrate. The corresponding dI/dV maps reveal island-induced resonances of different nature. Their electronic densities are localized either inside the alkali cluster or on its boundaries. Our model calculations allow us to explain the experimental results as due to the coexistence and mixing of two kinds of island-induced states. On the one side, since the alkali work function is lower than that of the substrate, the nanosized alkali metal clusters introduce intrinsic localized electronic states pinned to the vacuum level above the cluster. These states can be seen as the FER's of the complete alkali overlayer quantized by the cluster boundaries. On the other side, the attractive potential well due to the alkali metal cluster leads to two-dimensional (2D) localization of the FER's of the Cu(100) surface, the corresponding split component of the resonances appearing below the bottom of the parent continuum. Our main conclusions are based on the attractive nature of the alkali ad-island potential. They are of general validity and, therefore, significant to understand electron confinement in 2D. ; We acknowledge partial financial support from the European Research Council (StG 203239), the Spanish Ministerio de Ciencia e Innovación (MAT2010-15659 and FIS2010-19609-C02-01), the Basque Government-University of the Basque Country G.V.-UPV/EHU (IT-366-07), and the Catalan Agència de Gestió d'Ajuts Universitaris i de Recerca (2009 SGR 695). A.M. acknowledges the spanish Ministerio de Ciencia e Innovacion for financial support through the RyC program. ; Peer reviewed

Resumen del trabajo presentado al Symposium on Surface Science (3S), celebrado en Baqueira Beret, Lleida, Spain del 6 al 12 de marzo de 2011. ; Support by the Spanish Ministerio de Ciencia e Innovación (ref. FIS2010-19609-C02-01) and Basque Government – UP/EHU (No. IT-366-07) is gratefully acknowledged. ; Peer reviewed

The accurate determination of electronic temperatures in metallic nanostructures is essential for many technological applications, like plasmon-enhanced catalysis or lithographic nanofabrication procedures. In this Letter, we demonstrate that the electronic temperature can be accurately measured by the shape of the tunnel electroluminescence emission edge in tunnel plasmonic nanocavities, which follows a universal thermal distribution with the bias voltage as the chemical potential of the photon population. A significant deviation between electronic and lattice temperatures is found below 30 K for tunnel currents larger than 15 nA. This deviation is rationalized as the result of a two-electron process in which the second electron excites plasmon modes with an energy distribution that reflects the higher temperature following the first tunneling event. These results dispel a long-standing controversy on the nature of overbias emission in tunnel junctions and adds a new method for the determination of electronic temperatures and quasiparticle dynamics. ; R.M. and R.O. acknowledge financial support from the Spanish Ministry for Economy and Competitiveness (Grants PGC2018-098613-B-C21, PGC2018-096047-B-I00), the regional government of Comunidad de Madrid (Grant S2018/NMT-4321), Universidad Autónoma de Madrid (UAM/48) and IMDEA Nanoscience. Both IMDEA Nanoscience and IFIMAC acknowledge support from the Severo Ochoa and Maria de Maeztu Programmes for Centres and Units of Excellence in R&D (MINECO, Grants SEV-2016-0686 and CEX2018-000805-M). R.O. acknowledges support from the excellence programme Echegaray, funded by the regional government of Madrid. A.A. and V.M.S. acknowledge support from the Spanish Ministry of Science and Innovation (Grants Nos. PID2019-103910GB-I00 and PID2019-105488GB-I00, respectively) and from the Projects of the Basque Government for consolidated groups of the Basque University, through the Department of Universities (Grant Nos. IT-1246-19 and IT-1164-19, respectively). ; Peer reviewed

We combine x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD) and x-ray magnetic linear dichroism (XMLD) data with first principles density functional theory (DFT) calculations and a multiorbital many-body Hamiltonian approach to understand the electronic and magnetic properties of Co atoms adsorbed on h-BN/Ru(0001) and h-BN/Ir(111). The XAS line shape reveals, for both substrates, an electronic configuration close to 3d , corresponding to a spin S = 1. Magnetic field dependent XMCD data show large (14 meV) out-of-plane anisotropy on h-BN/Ru(0001), while it is almost isotropic (tens of μeV) on h-BN/Ir(111). XMLD data together with both DFT calculations and the results of the multiorbital Hubbard model suggest that the dissimilar magnetic anisotropy originates from different Co adsorption sites, namely atop N on h-BN/Ru(0001) and 6-fold hollow on h-BN/Ir(111). ; IG and AA acknowledge financial support by the Spanish Ministerio de Economia y Competitividad (MINECO Grant No. FIS2016-75862-P). The calculations were performed using the Computer Center at the Donostia International Physics Center (DIPC). FD acknowledges funding from the Ministerio de Ciencia e Innovación, grant MAT2015-66888-C3-2-R and FEDER funds; Basque Government, grant IT986-16, and Canary Islands program Viera y Clavijo (Ref. 2017/0000231). We gratefully acknowledge funding by the Swiss National Science Foundation (SNSF) through Grants No.200021_146715/1 (RB), No. 200020_157081/1 (AS), and No. PZ00P2_142474 (CW and JD).

A combined scanning tunneling microscopy, x-ray photoelectron spectroscopy, angle-resolved photoemission spectroscopy, and density functional theory study of graphene on a Fe-Ir(111) alloy with variable Ir concentration is presented. Starting from an intercalated Fe layer between the graphene and Ir(111) surface we find that graphene-substrate interaction can be fine-tuned by Fe-Ir alloying at the interface. When a critical Ir-concentration close to 0.25 is reached in the Fe layer, the Dirac cone of graphene is largely restored and can thereafter be tuned across the Fermi level by further increasing the Ir content. Indeed, our study reveals an abrupt transition between a chemisorbed phase at small Ir concentrations and a physisorbed phase above the critical concentration. The latter phase is highly reminiscent of the graphene on the clean Ir(111) surface. Furthermore, the transition is accompanied by an inversion of the graphene's induced magnetization due to the coupling with the Fe atoms from antiferromagnetic when chemisorbed to weakly ferromagnetic in the physisorption regime, with spin polarizations whose magnitude may be tuned with the amount of Fe content. ; This work has been funded by the Spanish MINECO under contract Nos. FIS2013-48286-C2-1-P, MAT2013-47878-C2-R, MAT2015-66888-C3-1R, and MAT2013-46593-C6-4-P as well as the Basque Government Grants IT621-13 and IT756-13. ; Peer Reviewed

4 páginas, 4 figuras.-- PACS numbers: 73.20.-r, 68.37.Ef, 68.55.-a, 73.22.Pr.-- et al. ; We explore the spatial variations of the unoccupied electronic states of graphene epitaxially grown on Ru(0001) and observed three unexpected features: the first graphene image state is split in energy; unlike all other image states, the split state does not follow the local work function modulation, and a new interfacial state at þ3 eV appears on some areas of the surface. First-principles calculations explain the observations and permit us to conclude that the system behaves as a self-organized periodic array of quantum dots. ; Financial support by the Spanish MICINN through project CONSOLIDER-INGENIO 2010 on Molecular Nanoscience and Grants No. FIS2007-61114 and No. FIS2007-6671, Comunidad de Madrid through the program NANOBIOMAGNET S-2009/MAT1726, the Basque Government (Grant No. IT-366-07 and inanoGUNE projects) is gratefully acknowledged. ; S-2009/MAT-1726/Nanobiomagnet ; Peer reviewed

Near edge X-ray absorption, valence and core-level photoemission, and density functional theory calculations are used to study molecular levels of tetracyano-2,3,5,6-tetrafluoroquinodimethane (FTCNQ) deposited on Ag(111) and BiAg/Ag(111). The high electron affinity of FTCNQ triggers a large static charge transfer from the substrate, and, more interestingly, hybridization with the substrate leads to a radical change of symmetry, shape, and energy of frontier molecular orbitals. The lowest unoccupied molecular orbital (LUMO) shifts below the Fermi energy, becoming the new highest occupied molecular orbital (n-HOMO), whereas the n-LUMO is defined by a hybrid band with mixed π∗ and σ∗ symmetries, localized at quinone rings and cyano groups, respectively. The presence of Bi influences the way the molecule contacts the substrate with the cyano group. The molecule/surface distance is closer and the bond more extended over substrate atoms in FTCNQ/Ag(111), whereas in FTCNQ/BiAg/Ag(111) the distance is larger and the contact more localized on top of Bi. This does not significantly alter molecular levels, but it causes the respective absence or presence of optical excitations in FTCNQ core-level spectra. ; We acknowledge financial support from the Spanish Ministry of Economy (Grants MAT2013-46593-C6-4-P, MAT2016-78293-C6-6-R, and FIS2016-75862-P) and Basque Government (Grants IT-621-13 and IT-756-13). P.B. acknowledges financial support from the EU-Horizon 2020 Research and Innovation Programme through the Marie Sklodowska-Curie grant agreement No 658056. ; Peer Reviewed

Trabajo presentado al 32nd Symposium on Surface Science (3S'19), celebrado en Baqueira Beret, Lleida (España) del 10 al 16 de marzo de 2019. ; Support by the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, Grant No. MAT2016-78293-C6-6-R) from the regional Government of Aragón (RASMIA project) and from the European Regional Development Fund (ERDF) under the program Interreg V-A España-Francia-Andorra (Contract No. EFA 194/16 TNSI) is thankfully acknowledged.

Quantum dot arrays in the form of molecular nanoporous networks are renowned for modifying the electronic surface properties through quantum confinement. Here we show that, compared to the pristine surface state, the band bottom of the confined states can exhibit downward shifts accompanied by a lowering of the effective masses simultaneous to the appearance of tiny gaps at the Brillouin zone boundaries. We observed these effects by angle resolved photoemission for two self-Assembled homothetic (scalable) Co-coordinated metal-organic networks. Complementary scanning tunneling spectroscopy measurements confirmed these findings. Electron plane wave expansion simulations and density functional theory calculations provide insight into the nature of this phenomenon, which we assign to metal-organic overlayer-substrate interactions in the form of adatom-substrate hybridization. To date, the absence of the experimental band structure resulting from single metal adatom coordinated nanoporous networks has precluded the observation of the significant surface state renormalization reported here, which we infer to be general for low interacting and well-defined adatom arrays. ; We acknowledge Prof. J. García de Abajo for providing the EPWE code and the financial support from the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, Grant No. MAT2016-78293-C6 and FIS2016-75862-P), from the Basque Government (Grant No. IT-1255-19 and IT-756-13), from the Regional Government of Aragon (RASMIA project), from the European Regional Development Fund (ERDF) under the program Interreg V-A España-Francia-Andorra (Contract No. EFA 194/16 TNSI) and from the European Research Council (ERC-2012-StG 307760-SURFPRO).

Quantum dots are known to confine electrons within their structure. Whenever they periodically aggregate into arrays and cooperative interactions arise, novel quantum properties suitable for technological applications show up. Control over the potential barriers existing between neighboring quantum dots is therefore essential to alter their mutual crosstalk. Here we show that precise engineering of the barrier width can be experimentally achieved on surfaces by a single atom substitution in a haloaromatic compound, which in turn tunes the confinement properties through the degree of quantum dot intercoupling. We achieved this by generating self-assembled molecular nanoporous networks that confine the two-dimensional electron gas present at the surface. Indeed, these extended arrays form up on bulk surface and thin silver films alike, maintaining their overall interdot coupling. These findings pave the way to reach full control over two-dimensional electron gases by means of self-assembled molecular networks. ; This work was supported in part by the Spanish Ministry of Economy (grants MAT2013-46593-C6-4-P, MAT2016-78293-C6-6-R and FIS2013-48286-C2-1-P), by the Spanish Research Council (CSIC- 201560I022), by the Basque Government (grants IT621-13 and IT-756-13), by the Japan Science and Technology Agency (JST), 'Precursory Research for Embryonic Science and Technology' (PRESTO) for a project of 'Molecular technology and creation of new function', by JSPS KAKENHI Grant Number 15K21765, by the Swiss National Science Foundation, by the Swiss Nanoscience Institute, and by COST Action (European Cooperation in Science and Technology) MP1303. Swiss National Supercomputing Center in Lugano (Project s499 and s621) is acknowledged. ; Peer Reviewed