Sub-nanometer Replica Molding of Molecular Steps on Ionic Crystals

Replica molding with elastomeric polymers has been used routinely to replicate features less than 10 nm in size. Because the theoretical limit of this technique is set by polymer-surface interactions, atomic radii and accessible volumes, replication at sub-nm length scales should be possible. Using PDMS to create a mold and polyurethane to form the replica, we demonstrate replication of elementary steps 3-5 Å in height that define the minimum separation between molecular layers in the lattices of the ionic crystals potassium dihydrogen phosphate (KDP) and calcite (CaCO 3 ). This work establishes the operation of replica molding at the molecular scale.