Electronic and transport properties of artificial gold chains

We study the electronic and transport properties of artificial Au atomic chains on a NiAl(110) surface template using state-of-the-art first principles calculations. Au chains display remarkable one-dimensional electronic properties that can be tuned by the selective adsorption of small molecules: a single CO group is shown to modulate the electronic wave functions, acting as a "chemical scissor" along the chain, to strongly modify the coherent transport properties of the system, and to help design one-dimensional nanodevices through artificial profiling of energy barriers.     

Isomerization kinetics of small hydrocarbons in confinement

Chemical reactions are often carried out in nano-structured materials, which can enhance reactions due to their large specific surface area, their interactions with the reacting mixture and confinement effects. In this work, we present a systematic study of the effect that the geometrical restrictions imposed by the pore walls can have on reactions that involve a three dimensional rearrangement of the atoms in a molecule. In particular, we consider the isomerization of three 4-membered hydrocarbons—n-butane, 1-butene and 1,3-butadiene confined in carbon nanopores of slit geometry. Our results illustrate the fact that, in the molecular sieving limit, the reaction rates change as the double exponential of the pore size (Santiso et al., in J. Chem. Phys., 2007a, submitted), and therefore the transition rates in nanopores can be many orders of magnitude different from the corresponding bulk values. These results can be used as a guideline for the molecular-level design of improved catalytic materials.     

Hexagon versus trimer formation in in nanowires on Si(111): energetics and quantum conductance

The structural and electronic properties of the quasi-one-dimensional In/Si(111) surface system are calculated from first principles. It is found that the symmetry lowering of the In chains is energetically favorable, provided neighboring nanowires are correlated, giving rise to a doubling of the surface unit cell both along and perpendicular to the chain direction. The recently suggested formation of hexagons within the In nanowires [C. González, F. Flores, and J. Ortega, Phys. Rev. Lett. 96, 136101 (2006)]--in clear contrast to the trimer formation proposed earlier-drastically modifies the electron transport along the In chains, in agreement with experiment.     

A study of the 75As(n,n'γ)75As reaction

The de-excitation gammas following the inelastic scattering of neutrons from ⁷⁵As have been studied for incident neutron energies from 1300 to 2800 keV in steps of 100 keV. The energy levels and the branching ratios of their decays have been deduced from excitation function measurements: 58 energy levels have been found in the excitation energy region below 2.8 MeV, twelve of which are reported for the first time in this work. The experimental excitation functions and angular distributions have been compared with the theoretical predictions based on the statistical theory of the compound nucleus. Spin and parity assignments for the levels and multipolarities for the decays are proposed for excitation energies of levels up to 2300 keV.     

Hg ION implantation in silicon: Lattice disorder created and hg atom lattice location

Abstract

The lattice disorder produced by 42-keV and 75-keV Hg ions implanted in Silicon at room temperature and the lattice location of the Hg atoms were studied by means of the channeling technique with a 2.0 MeV ⁴He⁺ beam. The damage produced was found to increase linearly with ion dose until a saturation value, connected to the ion range, is reached. The number of Si atoms displaced for Hg ion implanted was evaluated and compared with the theoretical expectation. The substitutional Hg fraction is connected to the disorder produced: the replacement mechanism is discussed.     

Onset of ferrielectricity and the hidden nature of nanoscale polarization in ferroelectric thin films

Using calculations from first principles and the concept of layer polarization, we have elucidated the nanoscale organization and local polarization in ferroelectric thin films between metallic contacts. The profile of the local polarization for different film thicknesses unveils a peculiar spatial pattern of atomic layers with uncompensated dipoles in what was originally thought to be a ferroelectric domain. This effectively ferrielectric behavior is induced by the dominant roles of the interfaces at such reduced dimensionality and can be interpreted using a simple classical model where the latter are explicitly taken into account.     

Crystal and molecular structure of 1-(2-aminoethyl)biguanide-isothiocyanato-copper(II) thiocyanate

The crystal structure of 1-(2-aminoethyl)biguanide-isothiocyanato-copper(II)thiocyanate, [Cu(aebg)(NCS)] (SCN) (aebg = NH₂. CH₂. CH₂. N=C(NH₂). NH. C(NH). NH₂) has been determined and refined by Fourier methods using three-dimensional X-ray data collected on a Weissenberg automatic diffractometer. The unit cell parameters are:a = 11·41(2),b = 8·81(2),c = 7·45(2) Å; = 87·6° (2); = 67·2° (2); = 70·1° (2);Z = 2.Assuming space group P¯1, the structure was refined down toR = 8·5 %. The coordination around Cu is slightly distorted square planar and concerns three N atoms from the aebg ligand (Cu-N = 1·988(8), 2·019(7), 1·963(5) Å) and the terminal N of an isothiocyanate group (Cu-N = 1·941(5) Å). Two unsymmetrical, long interactions (Cu-N = 3·367(7) and 2·947(7) Å), with the N atoms belonging to two NCS groupstrans with respect to the coordination plane, complete the coordination to a distorted octahedron. In the aebg ligand the double bonds are localized on the C-N groups, whose nitrogen atoms coordinate to the metal.This work was carried out with the aid of financial support from the Consiglio Nazionale delle Ricerche, Roma.