Intracrystalline Diffusion in Zeolites Studied by Neutron Scattering Techniques

Neutron scattering techniques can be used to study the different motions of molecules adsorbed in zeolites: vibrations, rotations and translation. Using combined instruments, it is now possible to measure diffusivities ranging from 10-7 to 10-14 m2s-1. Furthermore, incoherent scattering, using hydrogenated molecules, allows to determine the self-diffusivity; while coherent scattering, studying deuterated molecules, gives access to the transport diffusivity. Several applications of the neutron techniques are described in this review. They are illustrated by the case study of xylenes in X-type zeolites. Other examples are related to alkanes in MFI-type zeolites, n-alkanes in 5A zeolite, and water in X and A-type zeolites

The interaction between a protein and a ligand molecule studied by linear and non-linear optical spectroscopies

peer reviewedWe use infrared reflection-adsorption (IRRAS) and sum-frequency generation (SFG) spectroscopies to study the interaction between a protein (avidin) and a ligand molecule (biocytin or its thiol derivative) adsorbed on metals (Au and Ag) and insulators (CaF2 in two separate spectral ranges 2800-3500 cm-1 and 1400-1800 cm-1, respectively. No specific interaction is detected by SFG when the measurements are carried out on metals although IRRAS measurements attest the presence of a protein ad-layer. This is explained by the disordered character of the adsorbed film of avidin which is therefore SFG inactive. When the experiments are performed on an insulating substrate, no change of the biocytin/CaF2 SFG spectrum is detected in the low spectral range (1400-1800 cm-1) after immersion in an avidin solution in contrast with what was clearly demonstrated in the 2800-3500 cm-1 spectral range. Finally, we report, for the first time, the observation by SFG of the CH2 scissor vibration mode at 1465 cm-1

Semiconductor quantum dots reveal dipolar coupling from exciton to ligand vibration

Within semiconductor quantum dots (QDs), exciton recombination processes are noteworthy for depending on the nature of surface coordination and nanocrystal/ligand bonding. The influence of the molecular surroundings on QDs optoelectronic properties is therefore intensively studied. Here, from the converse point of view, we anlayze and model the influence of QDs optoelectronic properties on their ligands. As revealed by sum-frequency generation spectroscopy, the vibrational structure of ligands is critically correlated to QDs electronic structure when these are pumped into their excitonic states. Given the different hypotheses commonly put forward, such a correlation is expected to derive from either a direct overlap between the electronic wavefunctions, a charge transfer, or an energy transfer. Assuming that the polarizability of ligands is subordinate to the local electric field induced by excitons through dipolar interaction, our classical model based on nonlinear optics unambiguously supports the latter hypothesis

Sum-frequency generation spectroscopy applied to model biosensors systems

Vibrational information recorded by infrared-visible sum frequency generation spectroscopy was used to study the adsorption of a derivated vitamin (biocytin) on different substrates and its subsequent reaction with a protein (avidin). No reaction is observed on metallic substrates. When the experiments are carried out with a CaF2 substrate in the total internal reflection configuration, significant changes of the CH and NH vibrations can be related to the specific bonding of avidin to biocytin

Charge transport through redox active [H 7 P 8 W 48 O 184 ] 33− polyoxometalates self-assembled onto gold surfaces and gold nanodots

Paper and supporting informationInternational audiencePolyoxometalates (POMs) are redox-active molecular oxides, which attract growing interest for their integration into nano-devices, such as high-density data storage non-volatile memories. In this work, we investigated the electrostatic deposition of the negatively charged [H7P8W48O184]33- POM onto positively charged 8-amino-1-octanethiol self-assembled monolayers (SAMs) preformed onto gold substrates or onto an array of gold nanodots. The ring-shaped [H7P8W48O184]33- POM was selected as an example of large POMs with high charge storage capacity. To avoid the formation of POM aggregates onto the substrates, which would introduce variability in the local electrical properties, special attention has to be paid to the preformed SAM seeding layer, which should itself be deprived of aggregates. Where necessary, rinsing steps were found to be crucial to eliminate these aggregates and to provide uniformly covered substrates for subsequent POM deposition and electrical characterizations. This especially holds for commercially available gold/glass substrates while these rinsing steps were not essential in the case of template stripped gold of very low roughness. Charge transport through the related molecular junctions and nanodot molecule junctions (NMJs) has been probed by conducting-AFM. We analyzed the current-voltage curves with different models: electron tunneling though the SAMs (Simmons model), transition voltage spectroscopy (TVS) method or molecular single energy level mediated transport (Landauer equation) and we discussed the energetics of the molecular junctions. We concluded to an energy level alignment of the alkyl spacer and POM lowest occupied molecular orbitals (LUMOs), probably due to dipolar effects