Ionic strength effects: Tunable nanocrystal distribution in colloidal gold films

The self-assembly of colloidal particles into disordered structures or highly ordered superlattices can be achieved in different ways. Hydrodynamic forces during controlled drying allow control over the deposition process by varying the solvent vapor pressure and temperature, or, more physically, by structuring the substrate. Impressive results have been obtained, for example, with nanocolloidal magnetic particles. In addition, the application of electric and magnetic fields during drying leads to modifications of the deposited structures. To study deposition processes without the influence of hydrodynamic interactions, colloidal superstructures need to be formed prior to evaporation of the solvent. One way to achieve this is by chemical modification of the substrate to induce a specific affinity for the colloidal particles. For a number of different particles, chemical modifications by using either polymers, amino-functionalized and thiol-functionalized monolayers, or even DNA have been described. For example, for gold nanocrystals, there is a strong attractive electrostatic interaction between positively charged NH2 groups on the derivatized substrate and negatively charged citrate groups on the surface of the suspended gold particles. The high surface affinity and the resulting strong bonding of the deposited gold particles to the surface give rise to negligible surface mobility and sticking probability of one. Deposition processes governed by these characteristics can be adequately described as a random sequential adsorption (RSA) process. In RSA simulations, the adsorbing particles are treated as "hard" disks, which are randomly placed on a planar surface. Placement of such a disk is only successful when it does not overlap with any other previously deposited disks. The maximum particle density after an RSA event is markedly lower than that of a hexagonal close-packed monolayer of spherical particles. The saturation coverage has been determined from extensive numerical calculations and is equal to the jamming limit 0jam = 54.7%. However, strong repulsive interactions, for example, because of electrostatic forces, can lead to a considerably lower maximum attainable coverage, in agreement with the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. This will be described in more detail in Electrostatic Interactions in Particle Deposition. In previous work, many investigations into the importance of electrostatic interactions in colloid deposition processes have been described based on results using large particles (diameter > 100 nm). In this paper, we use scanning electron microscopy (SEM) to study ex situ the deposition of colloidal gold nanocrystals (≈13 nm) on Si/SiO2 substrates, derivatized with aminopropyltriethoxysilane (APTES). Spatial distributions after saturation of the deposition experiments are analyzed in terms of radial distribution functions, and indicate that the interparticle distance is tunable by varying the ionic strength. The results are shown to be in good quantitative agreement with other experiments on markedly larger particles.