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

Abstract

Porous silicon has been attracting much attention since the discovery of self-organized porosity in a monolithic semiconductor. [1][2][3][4][5][6][7] It provides a single-crystalline medium with anisotropic pores for the fundamental study of nanoconfinement on the structure and dynamics of matter. [7][8][9][10][11][12] Interface-dominated optical, electrical, and thermal properties attract the attention of the applied sciences in fields ranging from in vivo [4] and opto-electronics [13] via biosensing [6,14] to energy storage [15,16] and harvesting. [17] Silicon is one of the most abundant elements in the earths crust and is available in outstanding qualities. An integration of wafer-scale porous silicon into electrical circuit designs already existing can reasonably be achieved, as semiconductor devices are predominantly fabricated out of bulk silicon. [6] Furthermore, porous silicon has been found to be biocompatible [18] and a lot of functionalization schemes exist, which are using subsequent treatments to change, extend, or enhance its properties by incorporating additional functional materials [4,[19][20][21][22][23][24] or, as recently shown, by laser writing directly in pore space. [25] In particular, functionalization with liquids offers a plethora of opportunities to tune properties and to create adaptive hybrids, where the soft, dynamic filling, affected by confinement, provides novel functionalities, but the rigid, semiconducting scaffold structure provides mechanical robustness on the macroscopic scale. [24,26] An aspect that has not yet prompted significant research is the mechanics and particularly a functionalization of porous silicon as an actuator material. Porous silicon among other, different porous media has been investigated with a focus on humidity and gas sorption-induced actuation. [27][28][29] Also liquid adsorption-induced deformation of mesoporous silicon has been explored [9,[30][31][32] to study the mechanical properties of mesoporous silicon or its functionalization with artificial muscle molecules. [23] Another promising direction of research could be actuation mechanisms due to an electrochemical process within the material or on its surface. [33][34][35] Given the absence of intrinsic piezoelectricity in silicon, an actuator functionality of silicon has been so far achieved either by piezo-ceramic thin-film on-silicon coatings Nanoporosity in silicon results in interface-dominated mechanics, fluidics, and photonics that are often superior to the ones of the bulk material. However, their active control, for example, by 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, electrosorption-induced mechanical stress generation of up to 600 kPa that is reversible and adjustable at will by potential variations of ≈1 V is shown. Laser cantilever bending experiments in combination with in operando voltammetry and step coulombmetry allow this large electr...

Orientation order of a nonpolar molecular fluid compressed into a nanosmall space

“…Confinement of the matter in prose matrix delivers materials with new properties important for the application in nanotechnology and electronic industry. 61,62 The properties of those materials are averaged over the size distribution and quality of the pore assemble in the confining matrix. The experimental and theoretical investigation in a single gap can be considered as local (single pore) structure characterization.…”

Wafer‐Scale Fabrication of Hierarchically Porous Silicon and Silica by Active Nanoparticle‐Assisted Chemical Etching and Pseudomorphic Thermal Oxidation

“…[23,24] Nanoporous Si as a nanostructured scaffold offers a wide range of applications for the exploration of the fundamentals of spatially confined condensed-matter systems. [25][26][27][28][29][30] As a nanoporous mainstream semiconductor np-Si is interesting for sensorics, actuorics, (bio-)photonics, and drug delivery [16,[31][32][33][34][35][36][37][38] as well as energy storage and conversion. [39][40][41][42][43] Filling the pore space with soft or hard materials, for example, electrically conductive polymers can improve properties such as electrical conductivity and mechanical robustness.…”

“…However, in the extreme spatial confinement of np-Si, the pure spatial restriction and the interactions with the pore walls can hamper significantly hydrodynamic flow and self-diffusion, which results in slow process kinetics. [32,47,[49][50][51][52][53][54] One approach to address this challenge is the synthesis of hierarchically porous silicon (hp-Si). Pore diameters in the order of micrometers result in bulk-like flow properties and a low diffusion inhibition.…”

Pore size effects on surface charges and interfacial electrostatics of mesoporous silicas

“…Furthermore, it was reported that migration of ions in mesoporous silicon was retarded due to high viscosity of water near hydrophilic surface and surface roughness. 25 From these literatures, a study on the dependence of surface charge density on pore diameter will be required to link the experimental and simulation studies and provide direct evidence on the effects of confinement on interfacial reactions.…”