1,721,011 research outputs found
Femtomagnetism in graphene induced by core level excitation of organic adsorbates
No full textWe predict the induction or suppression of magnetism in the valence shell of physisorbed and chemisorbed organic molecules on graphene occurring on the femtosecond time scale as a result of core
level excitations. For physisorbed molecules, where the interaction with graphene is dominated by van der Waals forces and the system is non-magnetic in the ground state, numerical simulations based on density functional theory show that the valence electrons relax towards a spin polarized configuration upon excitation of a core-level electron. The magnetism depends on efficient electron transfer from graphene on the femtosecond time scale. On the other hand, when graphene is covalently functionalized, the system is magnetic in the ground state showing two spin dependent mid gap states localized around the adsorption site. At variance with the physisorbed case upon core-level excitation, the LUMO of the molecule and the mid gap states of graphene hybridize and the relaxed valence shell is no more magnetic. [1]
[1] A. Ravikumar, A. Baby, H. Lin, G. P. Brivio, and G. Fratesi, Scientific Reports 6, Article number: 24603
(2016) doi:10.1038/srep2460
Substrate induced ultrafast electron injection dynamics at organic-graphene interfaces
No full textElectron core-level spectroscopies can effectively be used to investigate electron transfer rates at organic/inorganic interfaces occurring within few femtoseconds. The core-level excitation at an adsorbed molecule strongly perturbs the system and calls for a proper theoretical description. On the other hand it induces novel phenomena such as backward electron transfer (substrate-to-molecule) as we measure by X-ray resonant photoemission and calculate by a theoretical framework based on density-functional theory (DFT) [1]. The rates can be controlled by varying molecular properties like the adsorption angle [2], as well as by tailoring the substrate like we show here for molecules on graphene.
N1s core excitation induces ultrafast electron transfer (τ=4fs) for bipyridine molecules on epitaxial graphene/Ni(111), which is characterized by a strong hybridization between C and metal states. We demonstrate that this interface can be decoupled by the addition of a second layer of graphene, so that the one in contact with the molecule is less hybridized with Ni underneath. In that case, transfer rates decrease by about one order of magnitude in the experiments and in the simulations, whereas no transfer is in principle expected for molecules on freestanding graphene within the current description.
[1] G. Fratesi, C. Motta, M. I. Trioni, G. P. Brivio, and D. Sánchez-Portal, J. Phys. Chem. C 118 (2014) 8775
[2] D. Cvetko, G. Fratesi, G. Kladnik, A. Cossaro, G.P. Brivio, L. Venkataraman, and A. Morgante, Phys. Chem. Chem. Phys. 18 (2016) 2214
Collective diffusion in a lattice-gas: application to O/W(110)
No full textIn this paper the diffusion of oxygen on the W(110) surface for coverages theta < 1/8 nd at various temperatures T = 600, 800, 1000 and 1200 K is studied. In particular the energy width of the coherent dynamic structure factor and the chemical diffusion coefficient, which are computed by using the Mori projection operator technique and neglecting memory effects, are presented. The energy width displays a relative minimum at the border of the first Brillouin zone in the direction q = q_x (1, 1/sqrt(2))
Erratum to “Collective diffusion in a lattice gas: application to O/W(110)” [Surface Science 307–309 (1994) 565]
No full textNo description supplie
Azimuthal dichroism in near-edge X - ray absorption fine structure spectra of planar molecules
No full textAzimuthal Dichroism in Near-Edge X - ray Absorption Fine Structure Spectra of Planar Molecules:
The dependence of the near-edge x-ray absorption fine structure (NEXAFS) spectrum of molecules on the photon electric field direction is investigated by means of first principles simulations based on density-functional theory with the transition-potential approach. In addition to the well-known dependence of the NEXAFS resonances on the orientation of the electric field with respect to the molecular plane, we demonstrate that for planar molecules with sufficient in-plane anisotropy such as pentacene, a dichroic effect is computed with a splitting of the σ* resonance as a function of the azimuthal orientation of the photon electric field in the molecular plane. The σ* splitting is investigated as a function of the length of acenes and closely related molecules. A proper assignment of such spectral features guided by theory together with variable polarization experiments, may allow one to completely determine the orientation of molecules at interfaces.
[1] G. Fratesi, V. Lanzilotto, L. Floreano, and G. P. Brivio, J. Phys. Chem. C (2013), vol. 117, pages 6632-663
Selectivity of Auger decays to the local surface environment
No full textThe line shape of the Auger decay of adatoms is studied by a joint theoretical and experimental effort, the former within a DFT framework, and the latter with synchrotron radiation measurements. We investigate the KL2,3V Auger deexcitation of Na on Al(111), a system with different adsorption geometries. In particular, we study the (root3 x root3)R30degrees phase at 1/3 ML (monolayer) and the more complex (2x2) structure at 1/2 ML coverage. From the comparison between theory and experiment, we unambiguously determine features that allow for the determination of the adsorption environment from the adatom Auger spectrum
Ab initio adiabatic He and Ne interaction on Ag: An all-electron calculation
No full textWe report an ab initio calculation of the adiabatic electronic properties of He and Ne atoms interacting with an Ag-like jellium metal surface, using the embedding method and the density functional theory in the local density approximation (LDA). Differently from previous results obtained in this framework, the noble atoms are described with their full potential. The linearized augmented plane-wave basis set is introduced to tackle the Kohn-Sham equation. Attention is focused on the atom-surface potential in the repulsive regime, i.e., at distances from the jellium edge smaller then the adsorption one, where the LDA has been shown to work well, and which are relevant in elastic and inelastic scattering experiments of He and Ne on metals. For incident atoms with initial kinetic energies in the experimental range of interest, it is shown that Ne gets closer to the metal than He as previously found with an Al substrate. An analysis of the shift of the atomic core levels by varying the atom-metal distance is also presented. [S0163-1829(98)01539-2]
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Lattice-gas model of diffusion of NH3 on Re(0001)
No full textA lattice-gas model for the chemical diffusion of NH3 on Re(0001) is presented. The system is modelled as a triangular lattice gas with nearest-neighbour repulsive lateral interactions. The average jump rate and the static structure factor are calculated within the cluster variation method framework by using a cluster of seven sites. Then the chemical diffusion coefficient is evaluated in the Darken approximation. This results in the jump rate and diffusion coefficient strongly increasing with coverage in agreement with experimental measurements
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