Controlled Vertical Transfer of Individual Au Atoms Using a Surface Supported Carbon Radical for Atomically Precise Manufacturing.

To explore a proof-of-concept for atomically precise manufacturing (APM) using scanning probe microscopy (SPM), first principle theoretical calculations of atom-by-atom transfer from the apex of an SPM tip to an individual radical on a surface-bound organic molecule have been performed. Atom transfer is achieved by spatially controlled motion of a gold terminated tip to the radical. Two molecular tools for SPM-based APM have been designed and investigated, each comprising an adamantane core, a radical end group, and trithiol linkers to enable strong chemisorption on the Au(111) surface: ethynyl-adamantane-trithiol and adamantyl-trithiol. We demonstrate the details of controlled Au atom abstraction during tip approach toward and retraction from the radical species. Upon approach of the tip, the apical Au atom undergoes a transfer toward the carbon radical at a clearly defined threshold separation. This atomic displacement is accompanied by a net energy gain of the system in the range -0.5 to -1.5 eV, depending on the radical structure. In the case of a triangular pyramidal apex model, two tip configurations are possible after the tip atom displacement: (1) an Au atom is abstracted from the tip and bound to the C radical, not bound to the tip base anymore, and (2) apical tip atoms rearrange to form a continuous neck between the tip and radical. In the second case, subsequent tip retraction leads to the same final configuration as the first, with the abstracted Au atom bound to radical carbon atom of the molecular tool. For the less reactive adamantyl-trithiol radical molecular tool, Au atom transfer is less energetically favored, but this has the advantage of avoiding other apex gold atoms from rearrangement.