Manipulation ordered and close-packed nanoparticle monolayers at air/liquid interface coupling Langmuir-Blodgett and self-assembly techniques

Langmuir monolayers of octadecylamine (ODA) and octadecylthiol (ODT) at the air/liquid interface were used as template layers to adsorb gold nanoparticles (AuNPs) dispersed in the subphase. A small quantity of ethanol was introduced into the aqueous subphase to regulate the stability of dispersed AuNPs and, thereby, enhance the adsorption of NPs to a template layer. The effects of monolayer density and ethanol concentration on the adsorption of AuNPs and the relevant behavior of mixed NP/ODA (or ODT) monolayers were studied using the pressure-area (π-A) isotherms and the observation of transmission electron microscopy (TEM). The results show that ODA monolayer has a much higher ability, than the ODT monolayer, in incorporatiing AuNPs, attributed to the electrostatic interaction between ODA and AuNPs. Furthermore, the ODA molecules also act as capping agents, modifying the adsorbed NPs, providing van der Waals interaction among NPs for self-organization into ordered domains. By using an ODA monolayer with appropriate molecular density, as well as the adjusting of the ethanol concentration, a close-packed particulate film of AuNPs can be prepared through the monolayer compression. The results also show that over loading of the ODA molecules increases the fractional ratio of free ODA molecules at the interface which obstruct the coalescence of particulate domains and a close-packed particulate monolayer cannot be approached. The presence of ethanol in the subphase not only enhances the particle adsorption to the interface but also triggers a more expanded ODA monolayer, which plays a key role in preparing a close-packed particulate monolayer.