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Using first-principles calculations,we investigated the thermoelectricproperties of a newly synthesized Si-based ternary clathrate K8Al8Si38, composed of ∼1 nm hollowcages with a metal atom inside. This compound contains solely Earthabundant elements. We found that, similar to other nanostructuredtype I clathrates, this system is a semiconductor and has a low thermalconductivity (∼1 W/mK). It was long believed that the merepresence of rattling centers was responsible for the low lattice thermalconductivity of type I clathrates. We found instead that the cagestructural disorder induced by atomic substitution plays a crucialrole in determining the conductivity of these materials, in additionto the dynamics of the guest atoms. Our calculations showed that thelatter is substantially affected by the charge transfer between themetal and the cages. Our results provide design rules for the searchof new types of promising nanocage structured thermoelectric materials. [ABSTRACT FROM AUTHOR]

Abstract: We carry out a computational investigation to show how the exponential concentration dependence of hydrogen diffusion, which was recently verified in a combined experimental and analytical study, could affect the characteristics of hydrogenation and dehydrogenation of a vanadium-coated magnesium nanoblade. A reaction model is built that separates hydrogen surface sorption and interior diffusion during the hydrogenation/dehydrogenation process. For the hydrogenation process, the hydrogen surface adsorption is much faster than the hydrogen diffusion, resulting in high hydrogen concentration buildup at the surface at a relatively low temperature. With increasing temperature, the hydrogen diffusion time decreases more rapidly than the hydrogen surface adsorption time. This leads to a relatively low-gradient diffusion field in the nanoblade during most time of the hydrogenation process, and no shell–core structure with a finite hydride layer is observed. However, for the dehydrogenation process, when hydrogen molecules are released at the surface, a hydride core is formed inside the nanoblade and the interface recedes gradually. The receding rate of the hydride core is determined by the hydrogen molecule release rate. In a two-dimensional simulation with decorated vanadium catalyst islands on the surface, isolated interior hydride islands are sometimes observed before the hydride core entirely fades away. The hydride core boundary is sharper at lower temperature when the surface reaction rate is high relative to the interior diffusion rate. [Copyright &y& Elsevier]

During the oblique angle co-deposition of Si and Cu, the nanoculumn tilting angle β changes systematically with the relative composition of Si and Cu when the vapor incident angle θ is fixed. This result demonstrates that the β-θ relationship does depend closely on the material property and composition, which is very different from the predictions of the well-known 'universal' models, such as tangent or cosine rules. A statistic model based on the size, spacing, and stacking of Si and Cu atoms has been proposed and the predicted result agrees well with the experimental data. [ABSTRACT FROM AUTHOR]

Helical silver nanorod arrays with different arm numbers are designed by oblique angle deposition and their surface-enhanced Raman scattering properties are characterized. Assuming that the hot spots are located at the bends between the arms, and considering the optical absorbance of different layers, the SERS behavior can be predicted qualitatively. [ABSTRACT FROM AUTHOR]

The effect of underlayer thin films on the sensitivity of Ag nanorod surface-enhanced Raman scattering (SERS) substrates was studied both theoretically and experimentally. With the same Ag nanorod film, different materials (Ag, Al, Si, and Ti) with different thicknesses (25, 100, and 400 nm) were used as underlayers to alter the reflectivity systematically. The SERS intensity was found to increase linearly with the underlayer reflectivity, which can be explained by a modified Greenler's model due to the contribution of reflected electric field from the substrate. This finding can be used to design high enhancement SERS substrates. [ABSTRACT FROM AUTHOR]