Langmuir 2007, 23, 2843-2850
2843
Two-Dimensional Self-Organization of Polystyrene-Capped Gold Nanoparticles He´le`ne Yockell-Lelie`vre, Jessie Desbiens, and Anna M. Ritcey* De´ partement de Chimie and Centre de Recherche en Sciences et Inge´ nierie des Macromole´ cules (CERSIM), UniVersite´ LaVal, Que´ bec, Que´ bec, Canada, G1K 7P4 ReceiVed October 2, 2006. In Final Form: December 6, 2006 Thiol end-functionalized polystyrene chains have been introduced onto the surface of gold nanoparticles via a two-step grafting-to method. This simple grafting procedure is demonstrated to be efficient for gold nanoparticles of different sizes and for particles initially dispersed in either aqueous or organic media. The method has been applied successfully for a relatively large range of polystyrene chain lengths. Grafting densities, as determined by thermogravimetric analysis, are found to decrease with increasing chain length. In all cases, the grafting density indicates a dense brush conformation for the tethered chains. The resulting functionalized nanoparticles self-organize into hexagonally ordered monolayers when cast onto solid substrates from chloroform solution. Furthermore, the distance between the gold cores in the dried monolayer is controlled by the molecular weight of the grafted polystyrene. Optical absorption spectra recorded for the organized monolayers show the characteristic plasmon absorption of the gold particles. Importantly, the plasmon resonance frequency exhibits a distinct dependence on interparticle separation that can be attributed to plasmon coupling between neighboring gold cores.
Introduction Metallic structures of nanometric dimensions have been a center of interest in materials science for over a decade due to the unique physical properties that they exhibit.1-10 In particular, the electronic and optical properties of the nanostructured materials differ from those of the bulk and represent a very attractive avenue to new devices in fields such as photonics6,7 and biosensing.11 The characteristic optical extinction spectrum of noble metal nanoparticles (NPs) is dominated by the collective surface electronic oscillation called the localized surface plasmon (LSP) resonance, which takes the form of a very sharp and intense absorption band in the visible region of the spectrum. The exact frequency of this resonance is affected not only by the size and shape of the NPs, but also by the dielectric function of the surrounding matrix and thus by the presence of nearby NP neighbors. The LSPs of neighboring small (
