105,881 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
Electron transfer with core-level excitations at hybrid interfaces
No full textElectron core-level spectroscopies have emerged as effective tools to investigate several aspects of the hybrid interface between organic molecules and a substrate. In particular, resonant photoemission spectroscopy can measure interfacial electron transfer times down to the femtosecond timescale. Furthermore, the strong perturbation induced by the core hole opens up the several questions on how the properties of the interface are modified, calling for a theoretical description of the core-excited system.
We adopt a theoretical framework based on density-functional theory (DFT), where the excitation is introduced explicitly in the core-level occupation of an atom in a molecule, to investigate the electronic structure and electron transfer from/to organic molecules adsorbed on metal, semimetal, and semiconducting substrates.
The perturbing potential lowers the energy of the molecular orbitals. Focusing on the lowest-unoccupied (LUMO), a filling of the core-excited LUMO* by substrate electrons may occur within the core-hole lifetime, as found for molecules on metals where the adsorption angle is also shown to influence the electron transfer rate [1,2]. In the case of a semimetal graphene substrate, a spin-polarized LUMO* pinned at the Fermi level can be determined for physisorbed molecules. In that case electron transfer would be suppressed given the low density of states of unsupported graphene at that energy, but still possible for graphene supported on a metal [3]. For molecules adsorbed on a semiconductor, the LUMO* may form a bound exciton in the gap [4]. Here, we found especially interesting to consider the influence of thermal motion on the energy-level alignment and the absorption coefficient [5,6].
References
[1] D. Cvetko, G. Fratesi, G. Kladnik, A. Cossaro, G.P. Brivio, L. Venkataraman, and A. Morgante, submitted.
[2] A. Baby, G. Fratesi, S.R. Vaidya, L.L. Patera, C. Africh, L. Floreano, G.P. Brivio, J. Phys. Chem. C 119 (2015) 3624.
[3] A. Ravikumar, A. Baby, H. Lin, G.P. Brivio, and G. Fratesi, Scientific Reports 6 (2016) 24603.
[4] G. Fratesi, C. Motta, M. I. Trioni, G. P. Brivio, and D. Sánchez-Portal, J. Phys. Chem. C 118 (2014) 8775
[5] H. Lin, G. Fratesi, S. Selçuk, G.P. Brivio, and A. Selloni, J. Phys. Chem. C, 120 (2016) 3899.
[6] M. Muller, D. Sànchez-Portal, H. Lin, G. Fratesi, G.P. Brivio, and A. Selloni, in preparation
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe 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
Acid/Base Controlled Size Modulation of Capsular Phosphates, Hydroxide Encapsulation, Quantitative and Clean Extraction of Sulfate with Carbonate Capsules of a Tripodal Urea Receptor
No full textA simple tris-(2-aminoethyl) amine based pentafluorophenyl substituted tripodal urea receptor L has been extensively studied as a versatile receptor for various anions. Combined H-1-NMR, Isothermal Titration Calorimetry (ITC) and single crystal X-ray diffraction studies reveal that mononegative anions like F-, OH- and H2PO4- are encapsulated into the pseudocapsular dimeric assemblies of L with 1 : 1 stoichiometry whereas dinegative anions like CO32-, SO42- and HPO42- form tight capsular dimeric assemblies of L with 1 : 2 stoichiometries. Single crystal X-ray diffraction study clearly depicts that the size of the dimer of H2PO4- encapsulated pseudocapsule is 13.8 angstrom whereas the size of the tight HPO42- encapsulated capsular assembly is only 9.9 angstrom. The charge dependent anion encapsulated capsular size modulation of phosphates has been demonstrated by simple acid/base treatment via solution state P-31-NMR and single crystal X-ray diffraction studies. L is also capable of encapsulating hydroxide in its C-3v-symmetric cavity that is achieved upon treating a DMSO solution of L with tetrabutylammonium (TBA) cyanide and characterized by single crystal X-ray diffraction study. To the best of our knowledge this is the first report on the encapsulation of hydroxide in a neutral synthetic receptor. The excellent property of L to quantitatively capture aerial CO2 in the form of CO32- capsules [L-2(CO3)][N(n-Bu)(4)](2) in basic DMSO solution has been utilized to study the liquid-liquid extraction of SO42- from water via anion exchange. Almost quantitative and clean extraction of SO42- from water (99% from extracted pure mass and > 95% shown gravimetrically) has been unambiguously demonstrated by NMR, FT-IR, EDX, XRD and PXRD studies. Selective SO42- extraction is also demonstrated even in the presence of H2PO4- and NO3-. On the other hand the mixtures of L and TBACl (to solubilize L in CHCl3) results impure sulfate extraction even when 1 : 1 L/TBACl is used. Similar impure SO42- extraction is also observed when organic layers containing [L(Cl)][N(n-Bu)(4)] are used as the extractant, obtained upon precipitating SO42- from the extracted mass, [L-2(SO4)][N(n-Bu)(4)](2) in the carbonate capsules method using aqueous BaCl2 solution.Department of Science and TechnologyCSIR, IndiaChemistr
Graphene Properties by Functionalization with Organic Molecules
No full textWe review first the unique band structure of graphene and explain how the linear dispersion near the Fermi level determines the so called Dirac cones and relativistic effects. Then we will deal with simple organic moieties on graphene and discuss the modification of the electronic structure of the pristine graphene by donor and acceptor chemisorbed radicals and physisorbed molecules. The peculiar magnetism induced by functionalization with chemisorbed molecules is accounted for. For excited organic molecules we will outline the charge transfer process to graphene. Electron core-level excitations to the valence shell will be considered as in the measurements of transfer times by resonant spectroscopies. The lifetimes for charge transfer are considered and possible applications outlined. Femtomagnetism in graphene with core-excited adsorbates is presented together with its implications for magnetic ordering. The electric conduction in single molecular switches with graphene electrodes is analyzed. Finally, the properties of functionalized graphene as sensor are summarized
Lattice mismatch and spectroscopy of buckybowls on Ag(111)
No full textWe investigated the adsorption of corannulene (C20H10) on the Ag(111) surface by experimental and simulated scanning tunneling microscopy (STM) and X-ray photoemission (XPS) and near-edge X-ray absorption fine structure (NEXAFS). Structural optimizations of the adsorbed molecules were performed by density functional theory (DFT) and spectra evaluated within the transition potential approach.
Corannulene is adsorbed in a bowl-up orientation displaying a very high mobility (diffusing, tilting, and spinning) at room temperature. At the monolayer saturation coverage, molecules order into a close compact phase with an average intermolecular spacing of ~10.5±0.5Å. The lattice mismatch drives a long wavelength structural modulation of the molecular rows, which however couldn't be identified with a specific superlattice periodicity.
We show that both the structural and spectroscopic properties are intermediate between those predicted for a simple on-hexagon geometry and a on-pentagon one, which can be accounted for by calculating a three-fold (~8.6Å spacing) and a four-fold (~11.5Å) phase, respectively. We suggest that molecules smoothly change their equilibrium configuration along the observed long wavelength modulation of the molecular rows by varying their tilt and azimuth in between the geometric constraints calculated for molecules in the three-fold and four-fold phases.
The good agreement in the peak sequence and energetic position between the experimental and calculated NEXAFS resonances allows us to interpret the measurements by virtue of the numerical analysis at the level of individual transitions with sigma and pi symmetry. In analyzing the spectra, we demonstrate that the curved aromatic backbone introduces a non-trivial dependence of the NEXAFS spectral intensities on the adsorption angle beyond the usual Stohr’s result
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 not magnetic anymore
Appropriate Similarity Measures for Author Cocitation Analysis
No full textWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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