Electrostatically driven adsorption of silica nanoparticles on functionalized surfaces.

Adsorption of nanoparticles on solid supports is a scientifically interesting and technologically important phenomenon that has been attracting ever-increasing attention. Formation of particle-based films onto surfaces from stable suspensions is at the center of the development of new devices that utilize the plethora of newly synthesized nanoparticles with exciting properties. In this study we exploit the attractive electrostatic interactions between silica (SiO(2)) nanoparticles and functionalized substrates that display an amine termination in order to devise a simple method for the fabrication of SiO(2) nanoparticle films. Electrostatically controlled adsorption allows for uniform coverage of nanoparticles over large areas. The Stöber method (a sol-gel approach) was employed to prepare uniformly sized SiO(2) nanoparticles with a diameter of 50-80 nm. Native oxide-covered silicon wafer substrates were amino-functionalized utilizing the self-assembled monolayer of 3-aminopropyltrimethoxysilane (APS). The adsorption of SiO(2) nanoparticle film onto the silicon wafer substrate was controlled by modulation of the electrostatic interaction between nanoparticles and the substrate. Modification of surface charge of either the SiO(2) NP or the substrate is a crucial step in the process. Thus the effect of APS adsorption time on the surface energy of the substrate was investigated. Also, process parameters such as NP concentration and solvent composition were varied in order to investigate the extent of NP adsorption. Moreover, NaCl was introduced to the SiO(2) suspension as a charge-screening agent to reduce the inter-particle repulsion in the suspension as well as interaction of the particles with the surface. This resulted in denser/thicker films.