Electrochemical and optical properties of two dimensional electrostatic assembly of Au nanocrystals
The spectroscopic and electrochemical properties of two-dimensional electrostatic assembly of Au nanocrystals are examined on poly-L-lysine (pLys) modified gold electrodes. The surface preparation for the nanoparticle deposition involved the self-assembly of a monolayer of 11-mercaptoundecanoic acid on the electrode surface, followed by the electrostatic deposition of pLys from aqueous solution. The polyelectrolyte layer acts as the electrostatic anchor for the Au particles. Electrostatically stabilised Au particles were prepared by homogeneous reduction in the presence of citrate, yielding monodispersed colloidal suspension with an average diameter of 18 ± 2 nm. After 4 h of deposition, the citrate-stabilised particles reach a maximum surface density of (8.2 ± 0.1) × 1010 particles cm−2, with an average edge-to-edge distance of 25 nm. The particle surface density was estimated from scanning electron micrographs. Kelvin probe measurements were employed for examining changes in surface dipole introduced by the 2D array of nanocrystals. From simple electrostatic arguments, the apparent static dipole moment per particle was estimated of the order of 2700 D. The strong interaction between the nanocrystals and the pLys layer is responsible for the surface charge displacement, leading to changes in the surface dipole of 0.35 eV. These electrostatic interactions also manifest itself by the red shift of the plasmon resonance of the assembly with respect to the aqueous colloidal suspension. Analysis of the spectral broadening was attempted within the framework of the so-called coherent-potential approximation. Finally, electrochemical studies in 1,2-dichloroethane show a large electronic overlap between the nanocrystals and the metal substrate. Results obtained from electrochemical impedance spectroscopy strongly suggest that the electrostatic assembly of nanocrystal behaves like a 2D array of randomly distributed spherical nanoelectrodes.