Selective Removal of Ligands from Colloidal Nanocrystal Assemblies with Non-Oxidizing He Plasmas

Helium plasmas are attractive reagents for the removal of organics from hybrid materials because of their minimal ablative power and relative inertness, compared to oxidizing feed gases such as O₂ and highly ablative inert gases such as Ar. This work describes the use of dilute helium plasmas to selectively remove the organic ligands from films of colloidal nanoparticles (i.e., colloidal nanoparticle assemblies). We determine the relative contribution to etching of different plasma species in a model system consisting of films of ZrO₂ nanoparticles capped with trioctylphosphine oxide. Unexpectedly, we find that the strong ultraviolet radiation of He plasma is only a minor contributor to etching (25% of the etched carbon). Excited He species are responsible for most of the etching (75% of the etched carbon). Carbon concentrations as low as 3.5 atom % can be achieved under non-optimized plasma processing conditions.