Photoswitching electron transport properties of an azobenzene containing thiol-SAM.

The influence of conformational and electrical properties of azobenzene molecules on the electron transfer barrier properties of their SAMs was studied by SECM and ellipsometry.

Cellulose nanocrystal submonolayers by spin coating.

Dilute concentrations of cellulose nanocrystal solutions were spin coated onto different substrates to investigate the effect of the substrate on the nanocrystal submonolayers. Three substrates were probed: silica, titania, and amorphous cellulose. According to atomic force microscopy (AFM) images, anionic cellulose nanocrystals formed small aggregates on the anionic silica substrate, whereas a uniform two-dimensional distribution of nanocrystals was achieved on the cationic titania substrate. The uniform distribution of cellulose nanocrystal submonolayers on titania is an important factor when dimensional analysis of the nanocrystals is desired. Furthermore, the amount of nanocrystals deposited on titania was multifold in comparison to the amounts on silica, as revealed by AFM image analysis and X-ray photoelectron spectroscopy. Amorphous cellulose, the third substrate, resulted in a somewhat homogeneous distribution of the nanocrystal submonolayers, but the amounts were as low as those on the silica substrate. These differences in the cellulose nanocrystal deposition were attributed to electrostatic effects: anionic cellulose nanocrystals are adsorbed on cationic titania in addition to the normal spin coating deposition. The anionic silica surface, on the other hand, causes aggregation of the weakly anionic cellulose nanocrystals which are forced on the repulsive substrate by spin coating. The electrostatically driven adsorption also influences the film thickness of continuous ultrathin films of cellulose nanocrystals. The thicker films of charged nanocrystals on a substrate of opposite charge means that the film thickness is not independent of the substrate when spin coating cellulose nanocrystals in the ultrathin regime (<100 nm).

Optical switching of coupled plasmons of Ag-nanoparticles by photoisomerisation of an azobenzene ligand.

The optical switching of coupled plasmons of silver nanoparticles derivatised with a photoisomerisable azobenzene ligand is presented. It is shown that nanoparticle clusters, linked with an azobenzene dithiol molecule, display switchable optical properties. The photoisomerisation of the linker molecule was used to vary the separation between nanoparticles, which was monitored by changes in the UV-Vis-spectra of the plasmon band of adjacent nanoparticles. A red-shift due to the appearance of a coupled longitudinal plasmon band was observed resulting from the formation of nanoparticle clusters. The maximum absorbance wavelength of this secondary plasmon band was altered by isomerisation of the linker and the spectral changes observed were in good agreement with theory and earlier measurements for gold. Evidence of energy transfer between a nanoparticle and an azobenzene terminated monothiol attached to it was also observed in the UV-Vis spectra.

Stability and electrostatics of mercaptoundecanoic acid-capped gold nanoparticles with varying counterion size.

The solubility of charged nanoparticles is critically dependent on pH. However, the concentration range available with bases such as NaOH is quite narrow, since the particles precipitate due to compression of the electric double layer when the ionic strength is increased. The stability of mercaptoundecanoic acid-capped Au nanoparticles is studied at a set pH using the hydroxide as base and different cations of various sizes. The counterions used are sodium (Na(+)), tetramethylammonium (TMA(+)), tetraethylammonium (TEA(+)), and tetrabutylammonium (TBA(+)). The particles precipitate in the 70-90 mM range with Na(+) as the counterion, but with quaternary ammonium hydroxides the particles are stable even in concentrations exceeding 1 M. The change in solubility is linked to a strongly adsorbed layer on the surface of the ligand shell of the nanoparticles. The increased concentration range obtained with TEAOH is further used to facilitate thiol exchange which occurs at a greater extent than would be achieved in NaOH solution.

Formation of stable Ag-nanoparticle aggregates induced by dithiol cross-linking.

Aggregation of thiol-stabilized silver nanoparticles induced by 1,6-hexane dithiol was studied in situ by dynamic light scattering. The aggregates were observed to reach a finite size in the 100-200 nm range depending on the applied conditions. Growth kinetics were shown to be linked to the dynamics of the thiol-exchange reaction. A model for the aggregation process was developed on the basis of a simple diffusion-kinetic approach assuming an elementary kinetic reaction at the surfaces and a spherical diffusion field surrounding the aggregates. The rate constant for the thiol exchange reaction was found to vary between 0.6 and 4.0 x 10(-4) s(-1), and the activation energy was 46 +/- 10 kJ mol(-1).