Optically Modulated Propulsion of Electric-Field-Powered Photoconducting Janus Particles

Herein, we demonstrate the ability to optically tune the mobility of electrically powered Janus particles (JPs) that are half-coated with various zinc oxide (ZnO) semiconducting layers, i.e., polycrystalline, amorphous, and amorphous with a SiO2 passivation layer. The ZnO semiconductor photoresponse causes an increase in its electrical conductivity with light having wavelengths of sufficient photon energy with respect to the semiconductor band gap. This effect, which we name optically modulated electrokinetic propulsion (OMEP), can be harnessed to increase the contrast in polarizability between the dielectric and semiconducting hemispheres, which, in turn, results in an increased electrokinetic mobility. Optical control of electrical field propulsion enables an additional degree of control of JP mobility. We also demonstrate optical modulation of collective behavior and particle-particle interactions for dense semiconducting Janus particle populations.