The kinetics of charge transfer across a metal−insulator−metal architecture is investigated by electrochemical
impedance spectroscopy. The insulating component of the architecture is composed by a self-assembled
monolayer of 11-mercaptoundecanoic acid (MUA), polyelectrolyte multilayers, and a monolayer of 22 nm
SiO2 nanoparticles. The charge transfer to the hexacyanoferrate couple is strongly hindered by the MUA
monolayer. The blocking properties effectively vanish with the adsorption of a diluted monolayer of Au
nanoparticles (19 nm). Atomic force microscopy and scanning electron microscopy analyses demonstrate
that the Au nanoparticles are physically separated from the Au surface by the SiO2 monolayer. The strong
electronic communication between the metal nanoparticles and the electrode is rationalized by a nonthermalized
transport process involving redox species trapped in the multilayer assembly.
We study the surface roughness of adhesive tape peeled in stick-slip. We find two regions: a smooth surface region associated with fast crack propagation and a rough surface region associated with slow crack propagation. In both regions the surfaces may be self-affine fractal-like with the fractal dimension D f ≈ 2.3. This fractal dimension is typical for surfaces produced by crack propagation, but unexpected in the present case.
SummaryAmplitude-modulation atomic force microscopy (AM-AFM) is used to determine the retention properties of CaF2 nanoparticles adsorbed on mica and on tooth enamel in liquid. From the phase-lag of the forced cantilever oscillation the local energy dissipation at the detachment point of the nanoparticle was determined. This enabled us to compare different as-synthesized CaF2 nanoparticles that vary in shape, size and surface structure. CaF2 nanoparticles are candidates for additives in dental care products as they could serve as fluorine-releasing containers preventing caries during a cariogenic acid attack on the teeth. We show that the adherence of the nanoparticles is increased on the enamel substrate compared to mica, independently of the substrate roughness, morphology and size of the particles.
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