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Brown R.D.,James Franck Institute | Hund Z.M.,James Franck Institute | Campi D.,University of Milan Bicocca | O'Leary L.E.,Beckman Institute and Kavli Nanoscience Institute | And 5 more authors.
Journal of Chemical Physics | Year: 2014

A combined helium atom scattering and density functional perturbation theory study has been performed to elucidate the surface phonon dispersion relations for both the CH3-Si(111)-(1 × 1) and CD 3-Si(111)-(1 × 1) surfaces. The combination of experimental and theoretical methods has allowed characterization of the interactions between the low energy vibrations of the adsorbate and the lattice waves of the underlying substrate, as well as characterization of the interactions between neighboring methyl groups, across the entire wavevector resolved vibrational energy spectrum of each system. The Rayleigh wave was found to hybridize with the surface rocking libration near the surface Brillouin zone edge at both the M̄-point and K̄-point. The calculations indicated that the range of possible energies for the potential barrier to the methyl rotation about the Si-C axis is sufficient to prevent the free rotation of the methyl groups at a room temperature interface. The density functional perturbation theory calculations revealed several other surface phonons that experienced mode-splitting arising from the mutual interaction of adjacent methyl groups. The theory identified a Lucas pair that exists just below the silicon optical bands. For both the CH3- and CD3-terminated Si(111) surfaces, the deformations of the methyl groups were examined and compared to previous experimental and theoretical work on the nature of the surface vibrations. The calculations indicated a splitting of the asymmetric deformation of the methyl group near the zone edges due to steric interactions of adjacent methyl groups. The observed shifts in vibrational energies of the -CD 3 groups were consistent with the expected effect of isotopic substitution in this system. © 2014 AIP Publishing LLC.

Garcia-Berrios E.,Beckman Institute and Kavli Nanoscience Institute | Theriot J.C.,Beckman Institute and Kavli Nanoscience Institute | Woodka M.D.,Beckman Institute and Kavli Nanoscience Institute | Woodka M.D.,U.S. Army | Lewis N.S.,Beckman Institute and Kavli Nanoscience Institute
Sensors and Actuators, B: Chemical | Year: 2013

Thin-film chemiresistive composites of octaethylporphine-based transition-metal complexes (Ph(M), M = Co, Cu and Zn) and carbon black (CB) have been fabricated and tested as chemical vapor sensors. The sensing performance of such sensor composites was compared to the sensing performance of composites of metallophthalocyanines (Phtc(M)) and CB. The relative differential resistance response of Ph(M)/CB sensor films upon exposure to organic vapors, such as n-hexane, n-heptane, n-octane, iso-octane, cyclohexane, toluene, ethyl acetate and ethanol, was dependent on the nature of the metal center. An array of chemiresistive Ph(M)/CB vapor sensors therefore provided discrimination between the organic vapor analytes that had different polarities, specifically classifying non-polar vapors, aprotic polar vapors and protic polar vapors. However, discrimination was not observed for analytes that had mutually similar polarities. The Ph(M)/CB sensors showed reversible responses toward ammonia, NH3(g), at concentrations below the 8 h permissible exposure level (50 ppm). Ph(M)/CB composites exhibited a slightly larger resistance response than Phtc(M)/CB composites, consistent with the Ph(M) species having less π-stacked molecular aggregates, resulting in an increase in the number of adsorption sites relative to the Phtc(M)/CB composites. Resistance responses with a signal-to-noise ratio value of ∼900 were obtained upon exposure to vapor pulses saturated with 2,4,6-trinitrotoluene. © 2013 Published by Elsevier B.V.

Audesirk H.A.,Beckman Institute and Kavli Nanoscience Institute | Warren E.L.,Beckman Institute and Kavli Nanoscience Institute | Warren E.L.,National Renewable Energy Laboratory | Ku J.,Beckman Institute and Kavli Nanoscience Institute | And 2 more authors.
ACS Applied Materials and Interfaces | Year: 2015

Silicon microwires grown by the vapor-liquid-solid process have attracted a great deal of interest as potential light absorbers for solar energy conversion. However, the research-scale techniques that have been demonstrated to produce ordered arrays of micro and nanowires may not be optimal for use as high-throughput processes needed for large-scale manufacturing. Herein we demonstrate the use of microimprint lithography to fabricate patterned templates for the confinement of an electrodeposited Cu catalyst for the vapor-liquid-solid (VLS) growth of Si microwires. A reusable polydimethylsiloxane stamp was used to pattern holes in silica sol-gels on silicon substrates, and the Cu catalyst was electrodeposited into the holes. Ordered arrays of crystalline p-type Si microwires were grown across the sol-gel-patterned substrates with materials quality and performance comparable to microwires fabricated with high-purity metal catalysts and cleanroom processing. © 2015 American Chemical Society.

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