Self assembled monolayer of 10.0 nm Co nanoparticles were formed on OH-passivated Si(111) via covalent linkage. The Co nanoparticles were capped with 11-bromoundecanoic acid to be terminated with Br. These Co nanoparticles can react with OH-passivated Si(111) by $S_N2$ mechanism between the alkyl bromide of the nanoparticle surfactants and OH on the surface. Binding energy shift of Si(2p) electron of the surface Si due to this covalent linkage was conformed in X-ray photoelectron spectroscopy (XPS) analysis. It was also proved that surface number density of nanoparticles depends on the concentration of the particles through the analysis of the field-emission scanning electron microscopy (FE-SEM) image. At low concentrations, nanoparticles were assembled forming islands. As increasing the concentration of nanoparticles, the area of these islands became larger and near the saturation coverage, there were formed networks of the islands. On the other hand, in harsh reaction conditions by reflux reaction, Co nanoparticles aggregated each other and multilayer stacking was observed.
