Silica Nanoparticle Layer-by-Layer Assembly on Gold: Dielectric Effects
Feng Wang, University of Cincinnati, Cincinnati, OH 45221 and Anastasios Angelopoulos, Chemical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221.
Layer-by-layer (LBL) assembly of silica nanoparticles has been investigated as a means of altering the water wetting properties of gold surfaces on electronic components. The strong polyelectrolyte acrylamide/β-methacryl-oxyethyl-trimethyl-ammonium copolymer was used as the cationic binder. The impact of silica nanoparticle size and zeta potential, ζ, from a range of -56.8 mV to 5.0 mV on the thickness, wettability, and electrical contact resistance of the final LBL-assembled coatings is presented. Parameter ζ was varied by altering both the pH and alcohol content of the silica suspensions and polymer solution. Nanoparticle adsorption was found to occur rapidly, with equilibration of surface coverage obtained after only 1 minute and uptake which is nearly linear with respect to the number of bilayers deposited. An increase in the absolute value of (negative) ζ in silica suspension was observed upon the addition of alcohol and a decrease in the dielectric constant of the suspending media, consistent with the Debye-Huckel approximation. Such an increase in ΙζΙ was found to increase the bilayer thickness to an average value as high as 99% of the individual nanoparticle diameter for the smaller nanoparticles investigated, suggesting that nearly complete surface coverage may be achieved after the application of only a single nanoparticle-polymer bilayer (a coating thickness as low as 18.9 nm). A more porous bilayer structure was observed with larger silica nanoparticles and if the zeta potential of the previously deposited nanoparticles is increased while immersed in the cationic polymer solution. Wetting measurements demonstrate that silica LBL assembly results in a substantial reduction in contact angle from 84o on the bare substrate surface to as low as 15o after the application of a single bilayer and 7o after the application of 8 bilayers. Monotonic increase in coating contact resistance is observed with an increase in the thickness with a characteristic volumetric electrical through-plane resistivity as low as 1.63 kΩ-cm. This value is within the acceptable range for the electrostatic discharge of electronic components.