Spontaneous assembly of a monolayer of charged gold nanocrystals at the water/oil interface.

Abstract

Novel materials based on 2D and 3D arrays of nanocrystalline building blocks can be prepared from colloidal suspensions by controlled processing. There is a large effort in this field due to the exciting electrical, magnetic 5] and optical properties of such systems, which can be tuned by the size of the building blocks and the interactions between them. Ordered arrays of uncharged, sterically stabilized nanocrystals have been successfully obtained by a subtle destabilization of the suspension or by assembly at a liquid/air or liquid/liquid interface. There is, however, an important class of suspensions that are stable owing to the nanocrystal surface charge. Assembly of uncapped, charged nanocrystals into arrays is considerably more difficult as destabilization of a suspension usually leads to uncontrolled coagulation. Herein, we report surprising results that may lead to novel routes for the controlled fabrication of materials from chargestabilized nanocrystal colloids. We have observed that gold nanocrystals spontaneously form a monolayer at the water/oil interface if the surface charge of the nanocrystals is gradually reduced. The separation between the nanocrystals in the layer is smaller than the width of the diffuse electrical double layer. Nevertheless, coagulation of the particles into clumps does not occur. The monolayers are remarkably robust and can be easily transferred to substrates, opening the way to technological applications. The spontaneous 2D assembly of charged nanocrystals is qualitatively described in terms of a reduction of the water/oil interfacial energy upon particle adsorption counteracted by electrostatic repulsion in the film. Suspensions of sterically stabilized and supposedly uncharged gold nanocrystals have been used as model systems for the study of self-assembly. A wealth of gold nanocrystal structures have been reported and their optoelectrical properties are still under investigation. The synthesis of charge-stabilized gold sols is well established. The processing of such sols has been focused on the capping of charged nanocrystals with organic molecules to allow transfer to a