Stabilization of Polar Step Edges on Calcite (10.4) by the Adsorption of Congo Red

In this work, we present the stabilization of polar step edges along the [010] direction of calcite (10.4) by the presence of a water-soluble organic molecule, namely Congo Red. While characteristic etch pits are observed on the surface in the absence of the additive, no etch pits can be found in the presence of the additive. Using atomic force microscopy, we can directly follow the restructuring of the surface. Upon addition of Congo Red, the charge-neutral step edges confining the characteristic etch pits vanish, while polar step edges along the [010] direction appear on the surface, which are entirely decorated by well-ordered molecular islands of the additive. After the restructuring has taken place, the surface exclusively exhibits these polar step edges. Our results give direct evidence of the fact that these polar step edges become thermodynamically favored when Congo Red is present.