Suchergebnisse

A prerequisite to characterize magnetic materials is the capability to describe systems containing unpaired electrons. In this study, we benchmark the one-shot GW (G0W0) on top of different unrestricted mean-field solutions for open-shell molecules using Dunning's correlation-consistent basis sets expanded in terms of Gaussian functions. We find that the G0W0 correction to hybrid functionals provides reasonably accurate results for the ionization energies of open-shell systems when compared to those obtained from high-level ab initio methods. Moreover, the quality of the G0W0 exchange–correlation approximation is evaluated by the discrepancy between the ionization energy of the neutral molecules and the electron affinity of the corresponding cations. Furthermore, we assess the capability of the GW to reproduce the correct energy ordering of molecular spin–orbitals. To such an aim, we thoroughly discuss three open-shell molecules CN, NH2, and O2, for which approximate functionals fail to correctly capture the single-electron spectrum. Particularly, we demonstrate that the overestimation of the exchange energy in the studied spin–orbitals is reduced by the GW dynamic correlation term, restoring the molecular orbital ordering. Interestingly, we find that deviations of the exchange and correlation energies, in comparison with our ab initio reference, can be very different for molecular orbitals with different symmetry, e.g. σ and π-type orbitals. ; Authors acknowledge support from Spanish Agencia Estatal de Investigación (Grant Nos. PID2019-107338RB-C66, PID2019-109555GB-I00 and RTC-2016-5681-7), and the Eusko Jaurlaritza and UPV/EHU (Grant Nos. PIBA19-0004 and IT1246-19). DSP also acknowledges support from the European Union (EU) through Horizon 2020 (FET-Open project SPRING Grant No. 863098). ; Peer reviewed

arXiv:1512.02104v2 ; We present a study of the optical response of compact and hollow icosahedral clusters containing up to 868 silver atoms by means of time-dependent density functional theory. We have studied the dependence on size and morphology of both the sharp plasmonic resonance at 3–4 eV (originated mainly from sp-electrons), and the less studied broader feature appearing in the 6–7 eV range (interband transitions). An analysis of the effect of structural relaxations, as well as the choice of exchange correlation functional (local density versus generalised gradient approximations) both in the ground state and optical response calculations is also presented. We have further analysed the role of the different atom layers (surface versus inner layers) and the different orbital symmetries on the absorption cross-section for energies up to 8 eV. We have also studied the dependence on the number of atom layers in hollow structures. Shells formed by a single layer of atoms show a pronounced red shift of the main plasmon resonances that, however, rapidly converge to those of the compact structures as the number of layers is increased. The methods used to obtain these results are also carefully discussed. Our methodology is based on the use of localised basis (atomic orbitals, and atom-centered and dominant-product functions), which bring several computational advantages related to their relatively small size and the sparsity of the resulting matrices. Furthermore, the use of basis sets of atomic orbitals also allows the possibility of extending some of the standard population analysis tools (e.g. Mulliken population analysis) to the realm of optical excitations. Some examples of these analyses are described in the present work. ; This work is supported, in part, by the ORGAVOLT (ORGAnic solar cell VOLTage by numerical computation) Grant ANR-12-MONU-0014-02 of the French Agence Nationale de la Recherche (ANR) 2012 Programme Modèles Numériques. F Marchesin, P Koval and D Sánchez-Portal acknowledge support from the Deutsche Forschungsgemeinschaft (DFG) through the sfb1083 project, the Spanish mineco MAT2013-46593-C6-2-P project, the Euroregion Aquitaine-Euskadi program and from the Basque Departamento de Educación, upv/ehu (Grant No. IT-756-13). Peter Koval acknowledges financial support from the Fellows Gipuzkoa program of the Gipuzkoako Foru Aldundia through the FEDER funding scheme of the European Union. ; Peer reviewed

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

Under the terms of the Creative Commons Attribution License 3.0 (CC-BY). ; The Bethe-Salpeter equation (BSE) is currently the state of the art in the description of neutral electronic excitations in both solids and large finite systems. It is capable of accurately treating charge-transfer excitations that present difficulties for simpler approaches. We present a local basis set formulation of the BSE for molecules where the optical spectrum is computed with the iterative Haydock recursion scheme, leading to a low computational complexity and memory footprint. Using a variant of the algorithm we can go beyond the Tamm-Dancoff approximation. We rederive the recursion relations for general matrix elements of a resolvent, show how they translate into continued fractions, and study the convergence of the method with the number of recursion coefficients and the role of different terminators. Due to the locality of the basis functions the computational cost of each iteration scales asymptotically as O(N3) with the number of atoms, while the number of iterations typically is much lower than the size of the underlying electron-hole basis. In practice we see that, even for systems with thousands of orbitals, the runtime will be dominated by the O(N2) operation of applying the Coulomb kernel in the atomic orbital representation. ; We acknowledge support from the Deutsche Forschungsgemeinschaft (DFG) through the SFB 1083 project, the ANR ORGAVOLT project, and the Spanish MINECO MAT2013-46593-C6-2-P project. P.K. acknowledges financial support from the Fellows Gipuzkoa program of the Gipuzkoako Foru Aldundia through the FEDER funding scheme of the European Union, Una manera de hacer Europa. F.F. acknowledges support from the EXTRA programme of the "Universita degli Studi di Milano-Bicocca" and from the Erasmus Placement programme for student mobility. ; 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 analyze theoretically four-terminal electronic devices composed of two crossed graphene nanoribbons (GNRs) and show that they can function as beam splitters or mirrors. These features are identified for electrons in the low-energy region where a single valence or conduction band is present. Our modeling is based on p z orbital tight binding with Slater-Koster-type matrix elements fitted to accurately reproduce the low-energy bands from density functional theory calculations. We analyze systematically all devices that can be constructed with either zigzag or armchair GNRs in AA and AB stackings. From Green's function theory the elastic electron transport properties are quantified as a function of the ribbon width. We find that devices composed of relatively narrow zigzag GNRs and AA-stacked armchair GNRs are the most interesting candidates to realize electron beam splitters with a close to 50:50 ratio in the two outgoing terminals. Structures with wider ribbons instead provide electron mirrors, where the electron wave is mostly transferred into the outgoing terminal of the other ribbon, or electron filters where the scattering depends sensitively on the wavelength of the propagating electron. We also test the robustness of these transport properties against variations in the intersection angle, stacking pattern, lattice deformation (uniaxial strain), inter-GNR separation, and electrostatic potential differences between the layers. These generic features show that GNRs are interesting basic components to construct electronic quantum optical setups. ; This work was supported by the project Spanish Ministerio de Economía y Competitividad (MINECO) through the Grants no. FIS2017-83780-P (Graphene Nanostructures "GRANAS") and no.MAT2016-78293-C6-4R, the Basque Departamento de Educación through the PhD fellowship no. PRE_2019_2_0218 (S.S.), the University of the Basque Country through the Grant no. IT1246-19, and the European Union (EU) through Horizon 2020 (FET-Open project SPRING Grant no. 863098). ; Peer reviewed

The correlation between transport properties across subnanometric metallic gaps and the optical response of the system is a complex effect that is determined by the fine atomic-scale details of the junction structure. As experimental advances are progressively accessing transport and optical characterization of smaller nanojunctions, a clear connection between the structural, electronic, and optical properties in these nanocavities is needed. Using ab initio calculations, we present here a study of the simultaneous evolution of the structure and the optical response of a plasmonic junction as the particles forming the cavity, two Na380 clusters, approach and retract. Atomic reorganizations are responsible for a large hysteresis of the plasmonic response of the system, which shows a jump-to-contact instability during the approach process and the formation of an atom-sized neck across the junction during retraction. Our calculations demonstrate that, due to the quantization of the conductance in metal nanocontacts, atomic-scale reconfigurations play a crucial role in determining the optical response of the whole system. We observe abrupt changes in the intensities and spectral positions of the dominating plasmon resonances and find a one-to-one correspondence between these jumps and those of the quantized transport as the neck cross-section diminishes. These results reveal an important connection between transport and optics at the atomic scale, which is at the frontier of current optoelectronics and can drive new options in optical engineering of signals driven by the motion and manipulation of single atoms. ; We acknowledge financial support from Projects FIS2013-41184-P and MAT2013-46593-C6-2-P from MINECO. M.B., P.K., F.M., and D.S.P. also acknowledge support from the ANR-ORGAVOLT project and the Euroregion Aquitaine-Euskadi program. M.B. acknowledges support from the Departamento de Educacion of the Basque Government through a Ph.D. grant. P.K. acknowledges financial support from the Fellows Gipuzkoa program of the Gipuzkoako Foru Aldundia through the FEDER funding scheme of the European Union. J.A. also acknowledges support from Grant 70NANB15H321, "PLASMOQUANTUM", from the US Department of Commerce (NIST). ; Peer Reviewed

The intermolecular interaction among phthalocyanine molecules deposited on Ag(111) has been investigated at submonolayer coverages. By means of infrared absorption spectroscopy and spot-profile analysis low-energy electron diffraction, unambiguous evidence for an attractive interaction between neighboring titanyl-phthalocyanine (TiOPc) molecules is found for dilute layers, which is in contrast to the intermolecular repulsion reported for the copper-phthalocyanine (CuPc)/Ag(111) system. Accordingly, the formation of dense two-dimensional islands of TiOPc molecules with a well-defined long-range order (commensurate phase) is observed upon cooling to temperatures below 75 K, while a disordered arrangement is retained for adsorbed CuPc. Using density functional theory (DFT) calculations as a guide, these differences have been traced to the competition of attractive van der Waals and repulsive electrostatic interactions. Specifically, the vertically oriented molecular dipoles are substantially smaller for adsorbed TiOPc as compared to CuPc due to a compensation of the oppositely oriented dipole moments arising from the Pauli push-back effect on the one hand and the internal molecular dipole associated with the axial Ti–O group on the other. A detailed account and comparison of the molecule–substrate interaction, molecular binding geometries, and intermolecular interactions of TiOPc and CuPc on the Ag(111) surface are provided in a theoretical analysis based on DFT. ; We gratefully acknowledge support from the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation), Project ID 223848855-SFB 1083 "Structure and Dynamics of Internal Interfaces". L.F. acknowledges financial support from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement MagicFACE no. 797109. D.S.-P. and A.X.B.-R. acknowledge the Spanish Agencia Estatal de Investigación (grant nos. MAT2016-78293-C6-4-R and PID2019-107338RB-C66) and Dep. Educación of the Basque Government and UPV/EHU (grant no. IT1246-19) for support. ; 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

Plasmonic gaps are known to produce nanoscale localization and enhancement of optical fields, providing small effective mode volumes of about a few hundred nm. Atomistic quantum calculations based on time-dependent density functional theory reveal the effect of subnanometric localization of electromagnetic fields due to the presence of atomic-scale features at the interfaces of plasmonic gaps. Using a classical model, we explain this as a nonresonant lightning rod effect at the atomic scale that produces an extra enhancement over that of the plasmonic background. The near-field distribution of atomic-scale hot spots around atomic features is robust against dynamical screening and spill-out effects and follows the potential landscape determined by the electron density around the atomic sites. A detailed comparison of the field distribution around atomic hot spots from full quantum atomistic calculations and from the local classical approach considering the geometrical profile of the atoms' electronic density validates the use of a classical framework to determine the effective mode volume in these extreme subnanometric optical cavities. This finding is of practical importance for the community of surface-enhanced molecular spectroscopy and quantum nanophotonics, as it provides an adequate description of the local electromagnetic fields around atomic-scale features with use of simplified classical methods. ; Financial support from Project No. FIS2016-80174-P and MAT2016-78293-C6-4-R of MINECO, grant of Consolidated Groups at UPV/EHU (IT-756-13) of the Basque Government, and project 70NANB15H32 from U.S. Department of Commerce, National Institute of Standards and Technology, is acknowledged. M.U. acknowledges support from the University of the Basque Country through a Ph.D. grant, as well as DIPC and CFM at the initial stages of this work. M.B. acknowledges support from the Departamento de Educacion of the Basque Government through a Ph.D. grant, as well as from Euskampus and the DIPC at the initial stages of this work. P.K. acknowledges financial support from the Fellows Gipuzkoa program of the Gipuzkoako Foru Aldundia through the FEDER funding scheme of the European Union, "Una manera de hacer Europa". ; Peer Reviewed

Pentacene molecules have recently been observed to form a well-ordered monolayer on the (110) surface of rutile TiO, with the molecules adsorbed lying flat, head to tail. With the geometry favorable for direct optical excitation and given its ordered character, this interface seems to provide an intriguing model to study charge-transfer excitations where the optically excited electrons and holes reside on different sides of the organic-inorganic interface. In this work, we theoretically investigate the structural and electronic properties of this system by means of ab initio calculations and compute its excitonic absorption spectrum. Molecular states appear in the band gap of the clean TiO surface, which enables charge-transfer excitations directly from the molecular HOMO to the TiO conduction band. The calculated optical spectrum shows a strong polarization dependence and displays excitonic resonances corresponding to the charge-transfer states, which could stimulate new experimental work on the optical response of this interface. ; This work was financed by the Deutsche Forschungsgemeinschaft (DFG) through the SFB 1083 project. MPL, DSP and PK also acknowledges support from Spanish MINECO (Grants No. MAT2013-46593-C6-2-P and MAT2016-78293-C6-4-R) and PK also acknowledges financial support from Fellows Guipuzcoa program from Gipuzkoako Foru Aldundia through the FEDER funding scheme of the European Union. ; Peer Reviewed

Electromagnetic field localization in nanoantennas is one of the leitmotivs that drives the development of plasmonics. The near-fields in these plasmonic nanoantennas are commonly addressed theoretically within classical frameworks that neglect atomic-scale features. This approach is often appropriate since the irregularities produced at the atomic scale are typically hidden in far-field optical spectroscopies. However, a variety of physical and chemical processes rely on the fine distribution of the local fields at this ultraconfined scale. We use time-dependent density functional theory and perform atomistic quantum mechanical calculations of the optical response of plasmonic nanoparticles, and their dimers, characterized by the presence of crystallographic planes, facets, vertices, and steps. Using sodium clusters as an example, we show that the atomistic details of the nanoparticles morphologies determine the presence of subnanometric near-field hot spots that are further enhanced by the action of the underlying nanometric plasmonic fields. This situation is analogue to a self-similar nanoantenna cascade effect, scaled down to atomic dimensions, and it provides new insights into the limits of field enhancement and confinement, with important implications in the optical resolution of field-enhanced spectroscopies and microscopies. ; We acknowledge financial support from projects FIS2013-14481-P and MAT2013-46593-C6-2-P from MINECO. M.B., P.K., F.M., and D.S.P. also acknowledge support from the ANR-ORGAVOLT project and the Euroregion Aquitaine-Euskadi program. M.B. acknowledges support from the Departamento de Educacion of the Basque Government through a PhD grant, as well as from Euskampus and the DIPC at the initial stages of this work. R.E. and P.K. acknowledge financial support from the Fellows Gipuzkoa program of the Gipuzkoako Foru Aldundia through the FEDER funding scheme of the European Union, "Una manera de hacer Europa". ; Peer Reviewed

arXiv:1502.00771v1 ; We combine experimental observations by scanning tunneling microscopy (STM) and density functional theory (DFT) to reveal the most stable edge structures of graphene on Ni(111) as well as the role of stacking-driven activation and suppression of edge reconstruction. Depending on the position of the outermost carbon atoms relative to hollow and on-top Ni sites, zigzag edges have very different energies. Triangular graphene nanoislands are exclusively bound by the more stable zigzag hollow edges. In hexagonal nanoislands, which are constrained by geometry to alternate zigzag hollow and zigzag top edges along their perimeter, only the hollow edge is stable, whereas the top edges spontaneously reconstruct into the (57) pentagon–heptagon structure. Atomically resolved STM images are consistent with either top-fcc or top-hcp epitaxial stacking of graphene and Ni sites, with the former being favored by DFT. Finally, we find that there is a one-to-one relationship between the edge type, graphene stacking, and orientation of the graphene islands. ; We acknowledge support from the Basque Departamento de Educacion, UPV/EHU (Grant No. IT-756-13), the Spanish Ministerio de Ciencia e Innovación (Grant Nos. MAT2013-46593-C6-2-P and MAT2013-46593-C6-5-P, MAT2007-62732), the ETORTEK program funded by the Basque Departamento de Industria, the European Union FP7-ICT Integrated Project PAMS under Contract No. 610446, the European Research Council (StG 203239 NOMAD), and Agència de Gestió d'Ajuts Universitaris i de Recerca (2014 SGR 715). ; Peer Reviewed

3 pages, 3 figures.-- PACS nrs.: 68.35.Np, 61.46.Fg, 61.46.Df. ; Our high resolution transmission electronic microscopy studies of multiwall carbon nanotubes show, after the growth of zirconia nanoparticles by a hydrothermal route, the presence of surface Zr, forming an atomically thin layer. Using first-principles calculations we investigate the nature of the Zr–C interaction, which is neither ionic nor covalent, and the optimal coverage for the Zr metal in a graphene flake. This preferred coverage is in agreement with that deduced from electron energy loss spectra experiments. We show also that the amount of charge transferred to the C layer saturates as the Zr coverage increases and the Zr–C bond becomes weaker. ; We want to acknowledge the support by the ETORTEK (NANOMAT) program of the Basque government, the Intramural Special Project (Reference No. 2006601242), the Spanish Ministerio de Ciencia y Tecnología (MCyT) of Spain (Grant No. Fis 2007-66711-C02-C01), and the European Network of Excellence NANOQUANTA (NM4-CT-2004-500198). Y.S.P. gratefully acknowledges his DIPC grant. ; Peer reviewed