Template-assisted generation of nanocavities within plasma polymer films

The generation of nanosized cavities within thin film layers is of interest for a number of fundamental and applied reasons. One challenge is to make such systems sufficiently robust mechanically. Plasma polymer (pp) films possess excellent mechanical stability if deposition conditions are selected such as to achieve a sufficient density of cross-linking and resistance to extraction of polymeric material by solvents. In this study, gold nanoparticles of 15 and 70 nm diameter were used as sacrificial templates to generate nanocavities in pp films of various thickness values in the tens of nanometers range. A first pp layer was deposited onto substrates using n-heptylamine (HA) to a thickness of 20 nm. Carboxy-thiolated gold nanoparticles were electrostatically bound onto the surface amine groups of the n-heptylamine plasma polymer (HApp) layer. A second HApp layer was then coated to various thicknesses onto the nanoparticle/HApp surface. The template particles embedded thus in-between the two HApp layers were then dissolved using aqueous KCN solution; monitoring of the plasmon resonance band of the gold nanoparticles enabled verification of template stripping and measurement of the kinetics of stripping. AFM topography images showed little change on extraction of the template nanoparticles, indicating that the plasma polymer layer maintained structural integrity upon template extraction and subsequent drying, and thereby prevented collapse of the empty nanocavities. The concept of template stripping to generate controlled size free volume in thin plasma polymer layers is thus shown to produce robust structures.