Small atoms fall into bulk from non-close-packed surfaces?

TL;DR

This study combines first-principles calculations and theoretical analysis to show that small atoms like H, C, N, O, and F do not penetrate bulk through the (100) surface due to electronic effects, challenging the hard sphere model assumptions.

Contribution

The paper provides a first-principles investigation of surface rippling and atomic penetration, highlighting the role of electronic effects in atom-surface interactions.

Findings

Small atoms attract electrons from the substrate, preventing penetration.

Electronic effects inhibit small atom passage through the (100) surface.

Hard sphere models are insufficient to describe atomic behavior at surfaces.

Abstract

Surface rippling has been observed when atoms of $X$ and $A$ are mixed on the $A$ substrate surface. The rippling amplitude has been estimated using hard sphere models. We present a gedanken experiment predicting a penetration of small atoms into bulk through the (100) surface. To understand how the electronic effects alter this picture, we investigate the surface rippling of $X/A(100)$ from first-principles, assuming $X=$ H to Bi except for noble gases and $A=$ Cu, Ag, and Au. We show that the small atoms (such as H, C, N, O and F) attract electrons from the substrate due to the large electronegativity, which prevent them from passing through the void in the (100) surface. The behaviors of small atoms are further explored by studying lateral displacements of the top layer in the $A$ substrate and a formation of the $X$ dimer above, below, and across the top layer. The present work provides an example to understand when atoms are not hard spheres.