Ligand length dependence of critical deviatoric stress required for the formation of ordered nanowire arrays in alkylthiol-capped gold superlattices

Deviatoric stress-induced coalescence of nanocrystal superlattices is a promising route for massively fabricating nanowires. We perform atomistic molecular dynamics simulations to characterize the deviatoric stress-induced fusion behavior of alkylthiol-capped gold superlattices and examine the influence of ligand length on the nanowire formation. The results show that a threshold deviatoric stress along the compression direction is essential for forming the ordered nanowire arrays, and it significantly increases with the ligand length. The ligand length dependence can be attributed to the fusion energy barrier between constituent gold nanocrystals as well as its alternation induced by the change in ligand length. We show that the ligands on neighboring gold nanocrystals abundantly interdigitate at the potential minimum but become highly splayed or bent at the repulsive maximum. Increasing the ligand length promotes ligand interdigitation at the potential minimum but causes a larger contact area between ligands on opposite nanocrystals at the repulsive maximum. This jointly results in a marked increase in fusion energy barrier with the increasing ligand length, thereby requiring a higher critical deviatoric stress to drive the gold nanocrystals into nanowires for longer ligands. This study reveals that reducing the ligand length can effectively decreases the operational stress required for the formation of nanowires, which can provide theoretical guidance for optimizing the stress-induced nanofabrication approaches.