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

Abstract

Surface rippling has been observed when atoms of X and A are mixed on 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 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 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 voids in the (100) surface. Application of machine learning in predicting rippling amplitude also suggests an importance of the electronegativity. The behaviors of small atoms are further explored by studying (i) lateral displacements of the first layer in the A substrate, (ii) a formation of the X dimer above, below, and across the first layer, and (iii) surface adsorption of X atom on the top, bridge, and hollow sites in Cu(100) surface. The present work provides an example to understand when atoms are not hard spheres.