Dealloying of Platinum-Aluminum Thin Films Part I. Dynamics of Pattern Formation

TL;DR

This paper investigates the physical mechanisms behind porosity formation in platinum-aluminum thin films during dealloying, revealing a reaction-diffusion process characterized by a linearly propagating diffusion front modeled by FKPP equations.

Contribution

It provides an in-depth analysis of the dealloying dynamics using FIB nanotomography and RBS, linking porosity evolution to a reaction-diffusion system and FKPP modeling.

Findings

Porosity formation is driven by a linearly propagating diffusion front.

The front moves at approximately 42 nm/s in 4M NaOH solution.

Porosity gradually increases with enhanced surface near regions.

Abstract

Applying focused ion beam (FIB) nanotomography and Rutherford backscattering spectroscopy (RBS) to dealloyed platinum-aluminum thin films an in-depth analysis of the dominating physical mechanisms of porosity formation during the dealloying process is performed. The dynamical porosity formation due to the dissolution of the less noble aluminum in the alloy is treated as result of a reaction-diffusion system. The RBS analysis yields that the porosity formation is mainly caused by a linearly propagating diffusion front, i.e. the liquid/solid interface, with a uniform speed of 42(3) nm/s when using a 4M aqueous NaOH solution at room temperature. The experimentally observed front evolution is captured by the normal diffusive Fisher-Kolmogorov-Petrovskii-Piskounov (FKPP) equation and can be interpreted as a branching random walk phenomenon. The etching front produces a gradual porosity with an enhanced porosity in the surface-near regions of the thin film due to prolonged exposure of the alloy to the alkaline solution.