The interactions between nanoparticles and colloids in suspensions are extremely important because they determine the system stability and suggest strategies for manipulation, processing, and controlled assembly. For example the formation of multiparticle clusters, two-dimensional particles arrays, or any other directed assembly will depend on the physical interaction between the particles, or with between the particles and a macroscopic surface. The most common interactions in aqueous systems are described by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, which considers the balance between the electrostatic repulsive and the van der Waals attractive forces. While being far from complete, the DLVO theory has been very successful in the analysis of the stability of dielectric colloidal particles dominated by electrostatic interactions.
Recently there has been a significant interest in suspensions of semiconductor colloids and their controlled manipulation in suspensions. Examples include fabrication of micro and nanostructured materials for electronic and optoelectronic applications. This prompted us to perform a theoretical analysis of the electrostatic interaction between such particles and examine the effect of semiconductor particle doping. Our hypothesis is that the interparticle charge mobile charges will respond to any electric field perturbation in the surrounding electrolyte solution phase. Hence, when two particles approach, their inner charge density will be redistributed, thus affecting the overall electrostatic potential distribution and interaction energy. We have performed a detailed theoretical analysis of this effect. It showed that the particle doping may have a very substantial effect on the interaction between particles (see the Figure), or between a particle and a macroscopic substrate. The kinetic of particle aggregation is particularly affected and in some cases a suspension of doped semiconductor nanoparticles may be more than 50 times faster than the undoped reference system. We are convinced that better understanding of the interface between semiconductor materials and electrolyte solutions will be instrumental in the effort to design novel “smart" materials at the micro and nanoscale.