Promotion of plasma-induced deuterium uptake of ruthenium films by monolayer-thick tin layers

Surface impurities can have a significant influence on hydrogen uptake of materials. Examples such as the hydrogen spillover effect demonstrate that even very small surface impurity quantities can lead to order-of-magnitude changes in the total amount of hydrogen taken up by a material. In this work, we report the first experimental demonstration of promoted deuterium uptake in Ru thin films by Sn. Deuterium plasma exposures were carried out for Ru-capped targets covered by Sn up to a few atoms in thickness. After the exposure, the residual Sn content and the deuterium retention were measured to quantify the Sn etching and the deuterium uptake, respectively. By increasing the amount of Sn from zero to one atomic layer on Ru, we found after the exposure that the Sn content stays unchanged while the deuterium uptake rate severely increases with the Sn content by 2-3 orders of magnitude. These results can be understood by simulations using a reaction-diffusion model with multiple surface species and the lateral surface migration of deuterium. By contrast, as the as-deposited Sn content goes above one atomic layer, Sn removal takes place, and the deuterium uptake rate decreases with the as-deposited Sn content. Possible explanations are proposed by considering the interplay between Sn etching and deuterium uptake. In all, this work provides insights into interactions between multiple surface species in relation to plasma-induced hydrogen uptake. By further development, this could eventually lead to a potential mitigation method to circumvent the promoted hydrogen uptake in Ru-capped films.