Layer‐By‐Layer Self‐Assembly of Metal Nanoparticles on Planar Substrates: Fabrication and Properties

Abstract

Self-assembly processes involve non-covalent interactions based on electrostatics, charge-transfer, hydrophilicity-hydrophobicity, H-bonding, chelation, metal coordination, p±p coupling, etc. The term ªselfº in self-assembly accounts for the fact that each interactive building block unit carries information that inherently defines its binding properties and is relevant enough to determine a spontaneous interaction at a receptive site or surface. The building block under consideration can be any molecule, ion, cluster, particle, polymer, etc., that participates in the film growth. The idea of using the binding capabilities (especially through electrostatic interactions) of micrometric objects to sequentially build-up structures on a support was first presented in 1966 [1]. Since then micrometer-sized objects have been designed and functionalized in such a way that they can self-organize into complex patterns [2]. Sequential electrostatic self-assembly was further extended to the build-up of biphosphonate anions with polycationic species (1988) [3] and to the adsorption of polyelectrolyte multilayers (1991) [4] as an alternative to the Langmuir-Blodgett monolayer transfer technique for film formation. The layer-bylayer assembly technique relies on the alternate immersion of a derivatized substrate, S, into a solution of building blocks A and a solution of building blocks B, both bearing chemical functions directing their mutual spontaneous interaction. Upon repeating adsorption cycles a periodic film grows at the substrate surface, S±(AB) n . The consecutive adsorption of particles with a binder on a planar substrate defines layers whose properties integrate over the entire film. The sequences of layering along with the choice of building blocks to be used determine a certain synergy between the layers. The versatility of this technique is not limited to the build-up of binary systems, and complex arrangements can be designed to form three-dimensional nanostructures with interesting properties [5]. Very often one building block is used as an inert binder to facilitate the adsorption of an element that brings the desired functionality upon assembly (Fig. 13.1). A given functionality is not only ªpackagedº within a discrete entity, say a nanoparticle, with specific magnetic, optical or electrical properties, but also localized within a three-dimensional nanostructure. Ideally, the layer-by-layer self-assembly