Layer-by-layer deposition of all-nanoparticle multilayers in confined geometries

Nanofluidic arrays containing high-aspect-ratio nanochannels were used as a platform for the deposition of all nanoparticle multilayers. LbL assembly of 6 nm titania and 15 nm silica nanoparticles resulted in conformal multilayers of uniform thickness throughout the nanochannels. These multilayers are inherently nanoporous with void volume fractions of about 0.5. Compared to unconfined assembly of the same materials on flat substrates, thinner multilayer films were observed for the case of deposition within confined channel geometries because of surface charge-induced electrostatic depletion of the depositing species. Additionally, systematic and reproducible bridging of the nanochannels occurred as multilayer assembly progressed, a phenomenon not seen in our earlier work involving polyelectrolytes. This behavior was attributed to relatively weak nanoparticle adsorption and the resulting formation of large aggregates. These results demonstrate a new route by which confined geometries can be coated and even bridged with a nanoporous multilayer without the need for calcination or other postassembly steps to introduce porosity into the conformal coating.