Size-Dependent Formation of Au-NPs Monolayer Under High Electric Field

Nanoparticles (NPs) are considered functional materials due to their fundamental properties of high surface-to-volume ratio, high conductivity, and diversified surface functionalization. We conducted experiments on NPs of different sizes and calculated their UV-Vis absorption and zeta potential when they were suspended as a colloid solution of DI water with some traces of citrate. NPs are stabilized by citrate ions and rendered with a net negative surface potential, which can be confirmed by using a zeta-sizer for zeta potential measurement. Citrate ions acting as ligands form a coating on the surface of NPs and induce electrostatic repulsion due to their overall negative charges, whereas NP's inner cores attract each other due to the van der Waals (vdW) force of attraction. The balance of repulsion and attraction keeps NPs stabilized in the aqueous solution for a prolonged period. Different sizes of NPs were subjected to an electric field to form the monolayer on a silicon substrate. The NPs of smaller sizes form an incomplete monolayer of disintegrated islands, due to their lower core: core attractions, these NPs fail to make a uniform and continuous monolayer. NPs of sizes greater than 40 nm show aggregation when placed in a high electric field environment due to their large core size and hence elevated core: core attraction. These large NPs aggregate irreversibly and form the clumps of NPs called nanoclusters. Interestingly, the optimal size of 20nm NPs leads to the formation of a perfectly hexagonally packed monolayer on the substrate when subjected to an electric field of
$\approx 7.2\mathrm{x}10^{4}\mathrm{V}/\mathrm{m}$
,