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Gold nanoparticle deposition using CO2 expanded liquids: effect of pressure oscillation and surface-particle interactions.

Postsynthesis processing of nanoparticles to obtain mesoscale hierarchal nanostructures is the key for the development of nanotechnology and smart composites/coatings from these materials. We have utilized gas-expanded liquid deposition of alkyl-coated gold nanoparticles to study the effects of variable process flowrates, variable flow oscillation and variable interaction potential of the substrate on nanoparticle array quality. Array quality is measured here as completeness of area surface coverage of approximately a monolayer of nanoparticles. Quantitative values for surface coverage are averages obtained from multiple TEM photomicrographs using Image J digital analysis. The process was modified using higher CO2 addition rate outside of the pressure range necessary for deposition, and this modified process produced an excellent film quality while reducing overall processing time by 45%. The effects of pressure oscillation during deposition appeared to anneal the film at the lower flow rates, 0.5 and 1.0 mL/min, but a reduction in area coverage was observed with pressure oscillation at 3.0 mL/min. Pressure oscillation has emerged as a useful tool for researchers to tune the film uniformity and therefore the surface roughness. Calculations based on Hamaker theories for surface-particle interactions on various substrates were performed, and better surface coverage was predicted for C-based surfaces compared to Si3N4 and SiO2 surfaces. Indeed, experimental studies verified this general ordering, indicating that if surface interactions with the particles are strong deposition directly on the surface rather than on pre-existing nanoparticle islands may govern uniform deposition.