Porous Film Deposition by Electrohydrodynamic Atomization of Nanoparticle Sols

Abstract

Film deposition by electrohydrodynamic atomization of nanoparticle sols was studied experimentally using zinc oxide nanoparticles in ethanol and using sequential Monte Carlo simulations, in which proper statistics and particle transport to the substrate were accounted for. Experimentally produced and simulated films were in good agreement, and simulations were used to determine the influences of deposition time, nanoparticle concentration in the sol, mean droplet size, and droplet polydispersity on the film thickness, porosity, surface roughness, and lateral feature size. Film growth rates were non-constant due to decreases in the film porosity early in the deposition process. Over time, the thickness of films increased, while the porosity decreased. After sufficient deposition time, the porosity reached a constant value and consequently the film growth rate was constant. The use of high concentration sols or large droplets resulted in the deposition of agglomerated particles, which gave rise to thicker, more porous films. Control over film morphology using droplets from size distributions with geometric standard deviations greater than 2.0 was not possible and the film thickness and air to particle volume ratio for all films produced from polydisperse droplets were convergent regardless of the number concentration of nanoparticles in the sol. Film surface roughnesses were less than the volume equivalent diameter of the agglomerated particles deposited, indicating that films were relatively uniform in thickness. Radial distribution functions were calculated for both experimentally deposited and simulated films and were used to determine lateral feature sizes. Lateral feature sizes increased with increasing deposition time and deposited agglomerate size until the deposition surface was completely covered. This work provides the necessary link between electrohydrodynamic atomization parameters and the resulting morphology of deposited films.

Acknowledgments

This work was partially funded by an NSF-NIRT (NSF-0304649). C.J.H. Jr. acknowledges support from a National Science Foundation Graduate Research Fellowship. The authors thank Luis B. Modesto-Lopez, Elijah Thimsen, and Jingkun Jiang for their thoughtful comments on this manuscript, and Matthew R. Herndon for assistance in the conversion of digital images to a two dimensional data array.