Gold Nanoparticle Films with Varying Surface Roughness Obtained by A Gas-Expanded Liquid Deposition Process

Alkanethiol-stabilized gold nanoparticles (AuNPs) were deposited via a gas-expanded liquid (GXL) technique utilizing CO₂-expanded hexane onto different substrates. The surfaces of the substrates were modified by coating silicon (100) substrates with various organic self-assembled monolayers (SAMs). The films were characterized and the rms surface roughness values were obtained to determine the nanoparticle film deposition and growth characteristics. Based on the critical surface tension of the SAMs, the film growth indicates a shift between layer-by-layer deposition and island formation, which also results in a change in the surface roughness. Moreover, the surface roughness is influenced by the concentration of the nanoparticle solution, the average size, the size distribution of AuNPs and the nature of the AuNP surface chemistry. The manner in which the surface roughness was affected by each of the influences was investigated and possible explanations for the results under different conditions were developed. By varying the surface roughness, the real contact surface area of the substrate will be changed as well. Therefore, the potential exists to engineer nanoparticle films to provide reduction in adhesion that occurs between surfaces due to interfacial forces of MEMS devices, and these aspects will be discussed in detail.