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

TL;DR

This paper demonstrates large, reversible electrochemo-mechanical actuation in wafer-scale nanoporous silicon, driven by electrosorption in aqueous electrolytes, enabling novel on-chip stress generation at low voltages.

Contribution

It introduces a method to achieve and quantify significant reversible mechanical stress in nanoporous silicon via electrochemical control, revealing mechanistic insights at the nanoscale.

Findings

Electrosorption induces up to 600 kPa stress reversible with 1 V potential change.

Nanopores (10^11 per cm^2) enable large-scale, controllable actuation.

Mechanistic insights into electrocapillarity effects and pore-wall influences.

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.