Wafer-Scale Electroactive Nanoporous Silicon: Large and Fully Reversible Electrochemo-Mechanical Actuation in Aqueous Electrolytes

Abstract: Nanoporosity in silicon results in an interface-dominated mechanics, fluidics and photonics that are often superior to the ones of the bulk material. However, their active control, e.g. as a response to electronic stimuli, is challenging due to the absence of intrinsic piezoelectricity in the base material. Here, for large-scale nanoporous silicon cantilevers wetted by aqueous electrolytes, we show electrosorption-induced mechanical stress generation of up to 600 kPa that is reversible and adjustable at will by electrical potential variations of approximately 1 V. Laser cantilever bending experiments in combination with in-operando cyclic voltammetry and step-coulombmetry allow us to quantitatively trace this large electro-actuation to the concerted action of 100 billions of parallel nanopores per square centimeter cross section and to determine the capacitive charge-stress coupling parameter upon ion ad- and desorption as well as the intimately related stress actuation dynamics for perchloric and isotonic saline solutions. A comparison with planar silicon surfaces reveals mechanistic insights on the observed electrocapillarity (electrostatic Hellmann-Feynman interactions) with respect to the importance of oxide formation and pore-wall roughness on the single-nanopore scale. The observation of robust electrochemo-mechanical actuation in a mainstream semiconductor with wafer-scale, self-organized nanoporosity opens up entirely novel opportunities for on-chip integrated stress generation and actuorics at exceptionally low operation voltages.