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Sökning: LAR1:uu > Ahuja Rajeev > Övrigt vetenskapligt

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  • Emanuelsson, Rikard, et al. (författare)
  • Configuration- and Conformation-Dependent Electronic Structure Variations in 1,4-Disubstituted Cyclohexanes Enabled by a Carbon-to-Silicon Exchange
  • 2014
  • Ingår i: Chemistry - A European Journal. - 0947-6539. ; 20:30, s. 9304-9311
  • Tidskriftsartikel (övrigt vetenskapligt)abstract
    • Cyclohexane, with its well-defined conformers, could be an ideal force-controlled molecular switch if it were to display substantial differences in electronic and optical properties between its conformers. We utilize sigma conjugation in heavier analogues of cyclohexanes (i.e. cyclohexasilanes) and show that 1,4-disubstituted cyclohexasilanes display configuration-and conformation-dependent variations in these properties. Cis- and trans-1,4-bis(trimethylsilylethynyl)-cyclohexasilanes display a 0.11 V difference in their oxidation potentials (computed 0.11 V) and a 0.34 eV difference in their lowest UV absorption (computed difference between first excitations 0.07 eV). This is in stark contrast to differences in the corresponding properties of analogous all-carbon cyclohexanes (computed 0.02 V and 0.03 eV, respectively). Moreover, the two chair conformers of the cyclohexasilane trans isomer display large differences in electronic-structure-related properties. This enables computational design of a mechanically force-controlled conductance switch with a calculated single-molecule ON/OFF ratio of 213 at zero-bias voltage.
  • Isaev, Eyvaz I., et al. (författare)
  • Phonon related properties of transition metals, their carbides, and nitrides : A first-principles study
  • 2007
  • Ingår i: Journal of Applied Physics. - 0021-8979. ; 101:12
  • Forskningsöversikt (övrigt vetenskapligt)abstract
    • Lattice dynamics of body-centered cubic (bcc) V-b-VIb group transition metals (TM), and B1-type monocarbides and mononitrides of IIIb-VIb transition metals are studied by means of first-principles density functional perturbation theory, ultra soft pseudopotentials, and generalized gradient approximation to the exchange-correlation functional. Ground state parameters of transition metals and their compounds are correctly reproduced with the generated ultrasoft pseudopotentials. The calculated phonon spectra of the bcc metals are in excellent agreement with results of inelastic neutron scattering experiments. We show that the superconductivity of transition metal carbides (TMC) and transition metal nitrides (TMN) is related to peculiarities of the phonon spectra, and the anomalies of the spectra are connected to the number of valence electrons in crystals. The calculated electron-phonon interaction constants for TM, TMC, and TMN are in excellent agreement with experimentally determined values. Phonon spectra for a number of monocarbides and mononitrides of transition metals within the cubic NaCl- and hexagonal WC-type structures are predicted. Ideal stoichiometric B1 crystals of ScC, YC, and VC are predicted to be dynamically stable and superconducting materials. We also conclude that YN is a semiconductor.
  • Löfås, Henrik, et al. (författare)
  • Computational Study of the Chaotic Behavior in Single-molecule Conduction
  • 2013
  • Ingår i: 2013 MRS Spring Meeting : Electrical Contacts to Nanomaterials and Nanodevices.
  • Konferensbidrag (övrigt vetenskapligt)abstract
    • Recently we have seen great advances in synthesis and fabrication of nanostructures. However, there is still no consensus on the conductance of small organic molecules, where different values of the conductance are often attributed to differences in metal-molecule interface structure or different molecular conformations[1,2]. Control and characterization of the metal-molecule interface during formation of the junction is in practice an impossible task. To get insight into this highly dynamic process, computer simulations are needed; here we are going to show a combination of ab-initio molecular dynamics (MD)-simulations and conductance calculations to address this problem.The conductance of a junction is mainly determined by the relative position of the energy level closest to the Fermi level of the electrodes and by the coupling of the corresponding electronic state to the electrodes[2]. These parameters are greatly influenced by the nature of the interaction and/or chemical bond between electrodes and the molecule. Information about the nature of this interaction and its variation with different binding sites can be extracted from the conduction spectra. Here we are using MD-simulations to get an unbiased set of geometries, thus mimicking the randomness of a real junction under thermal fluctuations. From the obtained geometries the zero-bias conductance is calculated and used for histograms to investigate the statistics of the junction.The obtained histograms for the thiol-bonded molecules are fitted with probability distributions for different Gaussian ensembles and we show that the interaction between the electrode and the molecule gives rise to quantum chaos in the junction. The effect of quantum chaos have earlier been found experimentally for quantum dots[3] and nanowires[4]. By removing the symmetry in the junction the chaotic behavior can be increased. We also compare the thiol anchoring groups with amines and we can see that the weaker coupling to the gold for the amines increases the conductance fluctuations in the junctions by one to two orders of magnitude. By tuning the ratio of the coupling between the electrodes and the molecular state we show, that the junction can be switched from a chaotic behavior to a case with a normal distributed conductance spectrum where only temperature fluctuations are present.[1] S. L. Bernasek, Angew. Chem. Int. Ed. 51, 9737 (2012).[2] A. Nitzan and M. A. Ratner, Science 300, 1384 (2003).[3] L. A. Ponomarenko, F. Schedin, M. I. Katsnelson, R. Yang, E. W. Hill, K. S. Novoselov, and A. K. Geim, Science 320, 356 (2008).[4] J. L. Costa-Krämer, N. García, P. García-Mochales, P. A. Serena, M. I. Marqués, and A. Correia, Phys. Rev. B 55, 5416 (1997).
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