Propulsion mechanism of miniature diodes in aqueous solution under AC or light stimuli*

Semiconductor nanowires in suspension have been shown to be controllable under electric or light stimuli thanks to their asymmetrical response to these fields, making them excellent candidates for nanomotors that could be deployed in biomedical applications. However, their propulsion mechanism is not very well understood. Here, we study the propulsion of miniature diodes floating in aqueous solution under AC or light activation. These diodes share similar rectifying behavior with nanowire diodes but are much larger, thus allowing us to readily visualize their motion as well as perform direct-contact electrical measurements. We observe that the propulsion of the diode is driven by the forward current in an AC field, and by photocurrent under illumination. This is because the current creates an imbalance of ions at the two ends of the diode, which induces an electric field and drives an electrokinetic flow around the diode. In both cases, we find that the propelling velocities scale linearly with the current. Moreover, we measure the propelling velocity of diodes that are intentionally damaged by very high reverse biases. Our measurements show the velocity is substantially reduced for the damaged diodes, which have a smaller difference between forward and reverse currents. Our findings reveal potential applications in manipulating, characterizing, and separating nanowire diodes based on a key device property, their reverse-saturation current. This method may also be applicable to other electronic devices, such as transistors and logic gates.