A ReaxFF molecular dynamics study of hydrogen diffusion in ruthenium -- the role of grain boundaries

TL;DR

This study uses reactive molecular dynamics to explore how grain boundaries in ruthenium thin films influence hydrogen diffusion, revealing that grain boundaries act as sinks and pathways, affecting permeation and blistering.

Contribution

It introduces a new ReaxFF force field for Ru/H systems and demonstrates how grain boundary structures impact hydrogen diffusion in ruthenium films.

Findings

Grain boundaries serve as favorable sites for hydrogen accumulation.

Grain boundaries block cross-plane hydrogen transport and promote in-plane diffusion.

Tailoring ruthenium film morphology could reduce hydrogen permeation.

Abstract

Ruthenium thin films can serve as protective caps for multi-layer extreme ultraviolet mirrors exposed to atomic hydrogen. Hydrogen permeation through ruthenium is problematic as it leads to blisters on the mirrors. H has been shown to exhibit low solubility in bulk Ru, and rapidly diffuses in and out of Ru. Therefore, the underlying mechanisms of the blistering effect remains unknown. This work makes use of reactive molecular dynamics simulations to study the influence of imperfections in a Ru film on the behaviour of H. For the Ru/H system, a ReaxFF force field was parametrised which reproduces structures and energies obtained from quantum-mechanical calculations. Molecular dynamics simulations have been performed with the newly-developed force field, to study the effect of tilt and twist grain boundaries on the overall diffusion behaviour of H in Ru. Our simulations show the tilt and twist grain boundaries provide energetically favourable sites for hydrogen atoms and act as sinks and highways for H. They therefore block H transport across their planes, and favour diffusion along their planes. This results in the accumulation of hydrogen at the grain boundaries. The strong effect of the grain boundaries on the hydrogen diffusion suggests tailoring the morphology of ruthenium thin films as a means to curb the rate of hydrogen permeation.