RESUMEN
We demonstrate that the conductance switching of benzo-bis(imidazole) molecules upon protonation depends on the lateral functional groups. The protonated H-substituted molecule shows a higher conductance than the neutral one (Gpro > Gneu), while the opposite (Gneu > Gpro) is observed for a molecule laterally functionalized by amino-phenyl groups. These results are demonstrated at various scale lengths: self-assembled monolayers, tiny nanodot-molecule junctions and single molecules. From ab initio theoretical calculations, we conclude that for the H-substituted molecule, the result Gpro > Gneu is correctly explained by a reduction of the LUMO-HOMO gap, while for the amino-phenyl functionnalized molecule, the result Gneu > Gpro is consistent with a shift of the HOMO, which reduces the density of states at the Fermi energy.
RESUMEN
The localized deformation of molecular monolayers constrained between the spherical surfaces of Au nanoparticles is studied by means of molecular dynamics simulations. Alkyl or polyethylene glycol long-chain molecules were homogeneously distributed over the curved Au surface, pushed against each other by repeated cycles of force relaxation and constant-volume equilibration at temperatures increasing from 50 to 300 K before being slowly quenched to near-zero temperature. Plots of minimum configurational energy can be obtained as a function of the nanoparticle distance, according to different directions of approach; therefore, such simulations describe a range of deformations, from perfectly uniaxial compression to a combination of compression and shear. Despite the relative rigidity of molecular backbones, the deformation is always found to be localized at the interface between the opposing molecular monolayers. We find that shorter ligands can be more densely packed on the surface but do no interdigitate upon compression; they respond to the applied force by bending and twisting, thus changing their conformation while remaining disjointed. On the contrary, longer ligands attain lower surface densities and can interprenetrate when the nanoparticles are compressed against each other; such molecules remain rather straight and benefit from the increased overlap to maximize the adhesion by dispersion forces. The apparent Young's and shear moduli of a dense nanostructure, composed of a triangular arrangement of identical MUDA-decorated Au nanoparticles, are found to be smaller than estimates indirectly deduced by atomic-force experiments but quite close to previous computer simulations of molecular monolayers on flat surfaces and of bulk nanoparticle assemblies.
RESUMEN
by performing electrodeless time-resolved microwave conductivity measurements, the efficiency of charge carrier generation, their mobility, and the decay kinetics on photoexcitation were studied in arrays of Si nanowires grown by the vapor-liquid-solid mechanism. Large enhancements in the magnitude of the photoconductance and charge carrier lifetime are found depending on the incorporation of impurities during the growth. They are explained by the internal electric field that builds up, due to higher doped sidewalls, as revealed by detailed analysis of the nanowire morphology and chemical composition.