Quenched growth of nanostructured lead thin films on insulating substrates

Lead island films were obtained via vacuum vapor deposition on glass and ceramic substrates at 80 K. Electrical conductance was measured during vapor condensation and further annealing of the film up to room temperature. The resistance behavior during film formation and atomic force microscopy of annealed films were used as information sources about their structure. A model for the quenched growth, based on ballistic aggregation theory, was proposed. The nanostructure, responsible for chemiresistive properties of thin lead films and the mechanism of sensor response are discussed.Comment: 2 figures; accepted to Thin Solid Film

Light-induced spiral mass transport in azo-polymer films under vortex-beam illumination

When an azobenzene-containing polymer film is exposed to a non-uniform illumination, a light-induced mass migration process may be induced, leading to the formation of relief patterns on the polymer free surface. Despite a research effort of many years and several proposed models many aspects of this phenomenon remain not well understood. Here we report the appearance of spiral-shaped relief patterns on the polymer under the illumination of focused Laguerre-Gauss beams, having helical wavefront and an optical vortex at their axis. The induced spiral reliefs are sensitive to the vortex topological charge and to the wavefront handedness. These findings are unexpected, because the "doughnut"-shaped intensity profile of Laguerre- Gauss beams contains no information about the wavefront handedness. We propose a model that explains the main features of this phenomenon from the surface-mediated interference of the longitudinal and the transverse components of the optical field. These results may find applications in optical micro- and nanolithography and optical-field.Comment: Open Access at Journal Reference homepag

Stimuli Thresholds for Isomerization-Induced Molecular Motions in Azobenzene-Containing Materials.

We use large-scale molecular dynamics simulations of the isomerizations of azobenzene molecules diluted inside a simple molecular material to investigate the effect of a modification of the cis isomer shape on the induced diffusion mechanism. To this end we simulate incomplete isomerizations, modifying the amplitude of the trans-to-cis isomerization. We find thresholds in the evolution of the host molecules mobility with the isomerization amplitude, a result predicted by the cage-breaking mechanism hypothesis (Teboul, V.; Saiddine, M.; Nunzi, J. M.; Accary, J. B. J. Chem. Phys. 2011, 134, 114517) and by the gradient pressure mechanism theory (Barrett, C. J.; Rochon, P. L.; Natansohn, A. L. J. Chem. Phys. 1998, 109, 1505–1516.). Above the threshold the diffusion then increases linearly with the variation of the chromophore size induced by the isomerization

Surface-immobilized PAMAM-dendrimers modified with cationic or anionic terminal functions: Physicochemical surface properties and conformational changes after application of liquid interface stress

Functionalization of surfaces with highly branched dendrimer mols. has gained attractiveness for various applications because the no. of functional groups exceeds those of surfaces functionalized with self-assembled monolayers. So far, little is known about the physicochem. properties of dendrimer functionalized surfaces, esp. if the flexibility of dendrimer structure remains after covalent immobilization. Therefore, the purpose of this study was to covalently immobilize polyamidoamine (PAMAM) dendrimer mols. exhibiting terminal amine and carboxyl groups to silicon model surfaces and to explore their properties and structure at the solid-air and solid-liq. interface. The authors' results show that the surface free energy is higher for PAMAM coatings than for analogously terminated SAMs and also higher for carboxyl than amine functionalized coatings. Also, several findings suggest that conformational freedom of the dendrimers was preserved after surface immobilization. Wet compared to dry PAMAM-NH2 surfaces show reduced hydrophilicity and increased contact angle hysteresis, whereas PAMAM-COOH surfaces become more hydrophilic and showed decreased hysteresis. Streaming current measurements showed an unexpected behavior for PAMAM-COOH surfaces in that they reveal a net pos. surface charge over a wide pH range in spite of the carboxylated periphery. All of these results indicate a certain degree of masking, burrowing, back-folding and unfolding of functional groups upon environmental changes