Laser-Excited Elastic Guided Waves Reveal the Complex Mechanics of Nanoporous Silicon

TL;DR

This study uses laser-excited elastic guided waves to non-destructively analyze the complex mechanical properties of nanoporous silicon, revealing significant isotropic elasticity and stiffness reduction due to nanopore formation, with implications for various technological applications.

Contribution

It introduces a contactless laser ultrasonics method to characterize the mechanics of nanoporous silicon, uncovering isotropic elasticity and stiffness changes at wafer scale, advancing non-destructive analysis techniques.

Findings

Nearly isotropic elasticity perpendicular to pore axes

80% effective stiffness reduction in nanoporous silicon

Laser ultrasonics enables in-situ mechanical characterization

Abstract

Nanoporosity in silicon leads to completely new functionalities of this mainstream semiconductor. A difficult to assess mechanics has however significantly limited its application in fields ranging from nanofluidics and biosensorics to drug delivery, energy storage and photonics. Here, we present a study on laser-excited elastic guided waves detected contactless and non-destructively in dry and liquid-infused single-crystalline porous silicon. These experiments reveal that the self-organised formation of 100 billions of parallel nanopores per square centimetre cross section results in a nearly isotropic elasticity perpendicular to the pore axes and an 80% effective stiffness reduction, altogether leading to significant deviations from the cubic anisotropy observed in bulk silicon. Our thorough assessment of the wafer-scale mechanics of nanoporous silicon provides the base for predictive applications in robust on-chip devices and evidences that recent breakthroughs in laser ultrasonics open up entirely new frontiers for in-situ, non-destructive mechanical characterisation of dry and liquid-functionalised porous materials.