Molecular insights into the thermal stability of gold superlattices

Atomistic molecular dynamics simulations are performed to study the thermal stability of bulk superlattices consisting of alkylthiol-coated gold nanocrystals. Using nanocrystals passivated by dodecanethiol chains, we show that the gold superlattice possesses a remarkable high-temperature stability, in agreement with experiment. When heated from room temperature, the superlattice expands slightly at lower temperature (<500 K) and then exhibits a considerable lattice contraction above 500 K, while maintaining the intact crystal structure up to 710 K. Once the temperature increases above 720 K, the gold superlattice becomes structurally unstable due to the local sintering of adjacent nanocrystals. Continuous heating to 750 K drives a large number of gold nanocrystals to coalesce and finally results in a tremendous destruction of the superstructure. The structural change and instability of superlattice are mainly attributed to the ligand desorption from nanocrystal surface induced by the variation in temperature. Furthermore, longer ligand length can effectively improve the thermal stability of gold superlattices. These findings are expected to provide a deep microscopic understanding of the thermal stability of superlattice materials.