BYU Physics and Astronomy

Carbon nanotube adhesion force measurements were performed on single-walled nanotubes grown over lithographically defined trenches. An applied vertical force from an atomic force microscope (AFM), in force distance mode, caused the tubes to slip across the 250-nm-wide silicon dioxide trench tops at an axial tension of 8 nN. The nanotubes slipped at an axial tension of 10 nN after being selectively coated with a silicon dioxide layer.

Significant enhancements in ion yields in time-of-flight secondary ion mass spectrometry (TOF-SIMS) are observed when water-soluble analytes are mixed with a polyelectrolyte, e.g., poly(diallyldimethylammonium chloride) or poly(sodium 4-styrenesulfonate), and then deposited in the layer-by-layer method on a surface. This previously unobserved effect is demonstrated for 5-chloro-8-methoxyquinoline appended diaza-18-crown-6, 5-(2-aminoethoxy)methyl-5-chloro-8-methoxyquinoline appended diaza-18-crown-6, acridine, 9-anthracenecarboxylic acid, and ferrocenecarboxylic acid. By optical ellipsometry film thicknesses range from ca. 5-20 angstrom. X-ray photoelectron spectroscopy shows significantly less analyte in the polyelectrolyte-analyte films than in the neat analytes. However, TOF-SIMS generally shows significant enhancements in ion yields from the polyelectrolyte films compared with either the neat compounds or the compounds solubilized with acid or base and then dried on a surface. These significant enhancements in ion yields also appear to extend to analyte fragments and cationized molecular species. Some enhancement is also observed for dried droplets of analytes mixed with a polyelectrolyte on surfaces.

Extended abstract of a paper presented at Microscopy and Microanalysis 2005 in Honolulu, Hawaii, USA, July 31--August 4, 2005

We describe a technique which allows for atomic force microscopy to be used to make a physical measurement of the thickness of thin film samples. When dealing with a film which is ultrathin (<100 nm), standard measurement techniques may become difficult to apply successfully. The technique developed involves the fabrication of a distinct, abrupt step on the film surface, using a device we call the Abruptor. This step can be scanned with an atomic force microscope, revealing the height of the step. Films from 6-15 nm are now routinely measured in this way, though it is possible to apply this measurement technique to thinner and thicker films. The thinnest film we measured was 3.6 nm.

Horse spleen ferritin (HoSF) containing 800−1500 cobalt or 250−1200 manganese atoms as Co(O)OH and Mn(O)OH mineral cores within the HoSF interior (Co−HoSF and Mn−HoSF) was synthesized, and the chemical reactivity, kinetics of reduction, and the reduction potentials were measured. Microcoulometric and chemical reduction of HoSF containing the M(O)OH mineral core (M = Co or Mn) was rapid and quantitative with a reduction stoichiometry of 1.05 ± 0.10 e/M forming a stable M(OH)2 mineral core. At pH 9.0, ascorbic acid (AH2), a two-electron reductant, effectively reduced the mineral cores; however, the reaction was incomplete and rapidly reached equilibrium. The addition of excess AH2 shifted the reaction to completion with a M3+/AH2 stoichiometry of 1.9−2.1, consistent with a single electron per metal atom reduction. The rate of reaction between M(O)OH and excess AH2 was measured by monitoring the decrease in mineral core absorbance with time. The reaction was first order in each reactant with second-order rate constants of 0.53 and 4.74 M-1 min-1, respectively, for Co− and Mn−HoSF at pH 9.0. From the variation of absorbance with increasing AH2 concentration, equilibrium constants at pH 9.0 of 5.0 ± 1.9 for Co−HoSF and 2.9 ± 0.9 for Mn−HoSF were calculated for 2M(O)OH + AH2 = 2M(OH)2 + D, where AH2 and D are ascorbic acid and dehydroascorbic acid, respectively. Consistent with these equilibrium constants, the standard potential for the reduction of Co(III)−HoSF is 42 mV more positive than that of the ascorbic acid reaction, while the standard potential of Mn(III)−HoSF is 27 mV positive relative to AH2. Fe2+ in solution with Co− and Mn−HoSF under anaerobic conditions was oxidized to form Fe(O)OH within the HoSF interior, resulting in partial displacement of the Co or Mn by iron.