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The local electronic structure of purified Mo 6 S (9 − x ) I x ( x = 6, 4.5) nanowires is investigated by X-ray absorption and fluorescence spectroscopies. The nanowires are found to be metallic with the density of states altered in both the conduction and the valence band depending on the stoichiometry. First-principle theoretical calculations suggest that the changes can be mainly attributed to the effect of strong Mo S hybridization in the nanowires. The atomic chain structure of the nanowires is confirmed and the effect of increasing sulphur content is investigated.

Moybdenum-based subnanometre diameter nanowires are easy to synthesize and disperse, and they exhibit a variety of functional properties in which they are superior to other one-dimensional materials. However, further progress in the understanding of physical properties and the development of new and specific applications have so far been impeded by the fact that their structure was not accurately known. Here we report on a combination of systematic x-ray diffraction and extended x-ray absorption fine structure experiments, and first-principles theoretical structure calculations, which are used to determine the atomic skeletal structure of individual Mo6S9−xIx (MoSIx) nanowires, their packing arrangement within bundles and their electronic band structure. From this work we conclude that the variations in functional properties appear to arise from different stoichiometry, not skeletal structure. A supplementary data file is available from http://stacks.iop.org/0957-4484/16/1578

Transverse nuclear spin relaxation measurements, employing the Carr-Purcell-Meiboom-Gill (CPMG) sequence, represent a powerful tool to study the dynamics of director fluctuations in liquid crystall...

Transverse nuclear spin relaxation measurements employing Carr-Purcell (CP) pulse sequences can provide detailed information on the slow-motional dynamics in biomembranes. In this paper, a comprehensive relaxation model is developed for the analysis of such experiments performed on unilamellar quasi-spherical vesicles. The basis of the model is the stochastic Liouville equation in which two different relaxation processes are considered (i.e., vesicle shape fluctuations and molecular translational diffusion). It is shown that for vesicle radii R 0 ≥ 200 nm, translational diffusion of the lipid molecules along the vesicle shell is too slow to contribute significantly to transverse spin relaxation in the kHz range, whereas vesicle shape fluctuations constitute the dominant transverse relaxation process. The theory is employed in model calculations for pulse frequency-dependent transverse 3 1 P nuclear spin relaxation rates. R C P 2 , ∞ (ω), from CP sequences. The analysis reveals that R C P 2 , ∞ (ω), induced by vesicle fluctuations, depends linearly on ω - 1 over a wide frequency range in the kHz regime. Notably, within this linear dispersion regime, the bending elastic modulus κ is the only relevant parameter because the magnitude of R C P 2 , ∞ (ω) does not depend on the size of the vesicle R 0 , the effective lateral tension σ, or the viscosity of the surrounding fluid η. On the other hand, R 0 , σ, ij, and K determine the frequency at which R C P 2 , ∞ (ω) levels off to a constant "plateau" value independent of ω. Thus, analysis of the R C P 2 , ∞ (ω) dispersion profiles is a direct way to determine the bending elastic modulus and other viscoelastic parameters of membrane vesicles.

Grazing-incidence X-ray diffraction data are combined with energy-minimization calculations to analyse the atomic structure of the Al-rich (3sqrt{3} x 3sqrt{3})R 30 deg reconstructed surface of sapphire alpha-Al_2O_3(0001). The experiments on the BM32 beamline of the ESRF provide the non-integer-order diffraction intensities and, after Fourier transform, an incomplete Patterson map. The computer simulations are implemented to obtain structural information from this map. In the simulations, the interactions between the Al overlayer atoms were described with the Sutton-Chen potential and the interactions between the overlayer and the sapphire substrate with a laterally modulated Lennard-Jones potential. We have shown that the hexagonal reconstructed unit cell is composed of triangles where the two layers of Al adatoms are FCC(111) ordered whereas between the triangles the stacking is FCC(001).

A scanning tunneling microscopy (STM) investigation of steps in the corrugated-iron structure of the missing row reconstructed Pt(110)-(1×2) surface is reported. We measured the terrace width distribution and found that it is indicative of a vanishing or, at most, slightly attractive interaction. To gain insight into the forces between the steps, we analyzed the fish-scale pattern of steps on vicinal Pt(110). The ratio between the local stretch and the local torque at the steps is found to be about r ≈1.3. We conclude that the simple dipolar model of interaction between the steps on a stressed surface is incompatible with the observed corrugated-iron structure and the fish-scale pattern on the Pt(110) surface. Our analysis suggests the importance of higher multipole forces acting between the steps.

The story of some not yet solved problems is presented, putting emphasis on the respective role of instrumentation, ab initio theories, phenomenology and simulations.

Step patterns on vicinal $(2\times1)$ reconstructed surfaces of noble metals Au(110) and Pt(110), miscut towards the (100) orientation, are investigated. The free energy of the reconstructed surface with a network of crossing opposite steps is calculated in the strong chirality regime when the steps cannot make overhangs. It is explained why the steps are not perpendicular to the direction of the miscut but form in equilibrium a network of crossing steps which make the surface to look like a fish skin. The network formation is the consequence of competition between the -- predominantly elastic -- energy loss and entropy gain. It is in agreement with recent scanning-tunnelling-microscopy observations on vicinal Au(110) and Pt(110) surfaces., 11 pages with 5 eps figures in text. Uses psfig and elsart.sty (ELSEVIER Science). To be published in Surf. Sci

Optical reflectivity and absorbance measurements of oriented ${\mathrm{Mo}}_{6}{\mathrm{S}}_{3}{\mathrm{I}}_{6}$ nanowire thin films and dispersions in different solvents are presented extending from the far infrared to the ultraviolet. In spite of the highly one-dimensional character of the nanowire material and narrow electronic valence and conduction subbands, as predicted by the density-functional theory calculations, sharp Van Hove features in the optical absorption spectra are not observed, partly because of the large density of interpenetrating electron subbands and partly due to damping and disorder. The optically measured electrical conductivity extrapolated to zero frequency ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega}\ensuremath{\rightarrow}0)$ and the calculated conductivity are significantly higher than the typical dc value from resistance measurements, indicating that disorder limits electron transport, a feature characteristic of strongly one-dimensional systems.

The properties of ${\mathrm{Mo}}_{6}{\mathrm{S}}_{3}{\mathrm{I}}_{6}$ nanowires were investigated with ab initio calculations based on the density-functional theory. The molecules build weakly coupled one-dimensional chains with three sulfur atoms in the bridging planes between the Mo octahedra, each dressed with six iodines. Upon uniaxial strain along the wires, each bridging plane shows two energy minima, one in the ground state with the calculated Young modulus $Y=82\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, and one in the stretched state with $Y=94\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$. Both values are at least four times smaller than the experimental values and the origin of the discrepancy remains a puzzle. The ideal tensile strength is about $8.4\phantom{\rule{0.3em}{0ex}}\mathrm{GPa}$, the chains break in the Mo-Mo bonds within the octahedra and not in the S bridges. The charge-carrier conductivity is strongly anisotropic and the ${\mathrm{Mo}}_{6}{\mathrm{S}}_{3}{\mathrm{I}}_{6}$ nanowires behave as quasi-one-dimensional conductors in the whole range of investigated strains. The conductivity is extremely sensitive to strain, making this material very suitable for stain gauges. Very clean nanowires with good contacts may be expected to behave as ballistic quantum wires over lengths of several $\ensuremath{\mu}\mathrm{m}$. On the other hand, with high-impedance contacts they are good candidates for the observation of Luttinger liquid behavior. The pronounced one-dimensional (1D) nature of the ${\mathrm{Mo}}_{6}{\mathrm{S}}_{3}{\mathrm{I}}_{6}$ nanowires makes them a uniquely versatile and user-friendly system for the investigation of 1D physics.

The magnetoconductive properties of ultrathin Pb films deposited on Si(111) are measured and compared with density-functional electronic band-structure calculations on two-dimensional, free-standing, 1 to 8 monolayers thick Pb(111) slabs. A description with free-standing slabs is possible because it turned out that the Hall coefficient is independent of the substrate and of the crystalline order in the film. We show that the oscillations in sign of the Hall coefficient observed as a function of film thickness can be explained directly from the thickness dependent variations of the electronic bandstructure at the Fermi energy., 4 pages incl. 3 figures, RevTeX, to appear in Phys. Rev. B

Nuclear-spin relaxation rates resulting from shape fluctuations of unilamellar quasispherical vesicles are calculated. We show that in the kHz range these fluctuations yield-in contrast to previous conclusions on planar membranes - a relaxation rate proportional to the inverse Larmor frequency and provide direct information on the bending rigidity of membranes.

The structure of the $(\sqrt{31}\times \sqrt{31})R\pm9^\circ$ reconstructed phase on sapphire (0001) surface is investigated by means of a simulation based on the energy minimization. The interaction between Al adatoms is described with the semi-empirical many-body Sutton-Chen potential, corrected for the charge transfer between the metallic overlayer and the substrate. The interactions between the Al adatoms and sapphire substrate are described with a simple three-dimensional potential field which has the hexagonal periodicity of sapphire surface. Our energy analysis gave evidence that the structure which is observed at room temperature is in fact a frozen high-temperature structure. In accordance with the X-ray scattering, a hexagonal domain pattern separated by domain walls has been found. The Al adatoms, distributed in two monolayers, are ordered and isomorphic to metallic Al(111) in the domains and disordered in the domain walls. The main reason for the rotational reconstruction is the lattice misfit between the metallic Al and sapphire., Comment: 15 pages with 4 eps figures in text. Uses psfig and elsart.cls (ELSEVIER Science). Submitted to Surf. Sci

The interplay between the surface phonons and different missing-row reconstructions of (110) noble-metal surfaces is investigated. The phonon dispersion curves and densities of states are calculated for a slab model with 24 layers of atoms, using the Sutton–Chen many-body potential. The phonon contribution to the surface free energy is evaluated for Au, Pt and Ag surfaces. The results show that surface phonons play an important role in stabilizing a surface reconstruction when the energy difference between different reconstructions is of the order 1 meV/(surface unit cell) or smaller, and in particular in the case of Au(110) when the (1×2) and (1×3) reconstructions are close in energy. We also observe that the Sutton–Chen potential reproduces the phonon dispersion curves better for Ag (4d metal) than for Au and Pt (5d metal).