Nonohmic Ionic Conductivity and Bulk Current Rectification in Nanoparticle Superlattices

Nanoparticle superlattices (NPSLs) are self-assembled materials formed by periodically ordered monodisperse nanoparticles (NPs). This work theoretically studies the mechanisms of ion transport in NPSLs made of polyelectrolyte-modified NPs, which are relevant in chemoelectronics and nanofluidic applications. The ion fluxes in the NPSLs are obtained by solving the three-dimensional Poisson–Nernst–Planck equations. An analytic perturbation theory is also presented to prove the generality of some of the numerical results. We consider superlattices made of charged NPs and their monovalent counterions and systematically study the effects of the lattice parameter, supercrystalline structure, and added salt on the bulk ionic conductivity of the lattice. Nonohmic current–potential responses obtained for large NP separations, low dielectric constants, and absence of added salt are explained in terms of the reorganization of the ionic concentrations in the nonequilibrium steady state. It is also shown that NPs with asymmetric charge distribution (Janus NPs) give rise to bulk current rectification. This result provides a proof of concept for the fabrication of nanofluidic and chemoelectronic elements based on NPSLs that works as the ionic equivalents of electronic diodes.