Dynamics of alkyl chains in monolayer-protected Au and Ag clusters and silver thiolates: a comprehensive quasielastic neutron scattering investigation

Abstract

Temperature-dependent dynamics of monolayer-protected Au and Ag nanoclusters and silver thiolates have been investigated with quasielastic neutron scattering. The simplest motion in these systems is the uniaxial rotation of the chain, which evolves slowly with temperature. While longer chain monolayers (above C₈) on Au clusters are rotationally frozen at room temperature, dynamic freedom exists in lower chain lengths. In the superlattice solids of Ag clusters, the dynamics evolve slowly, and at superlattice melting, all the chains are dynamic. The data are consistent with a structure in which the monolayers form bundles on the planes of metal clusters and such bundles interdigitate, forming the cluster assemblies. In thiolates, the dynamics is distinctly different in long- and short-chain systems. It arises abruptly at the melting temperature in C₁₂ but a bit sluggishly in C₁₈, whereas in C₆ and C₈, it evolves with temperature. The data are correlated with temperature-dependent infrared spectroscopy, which preserves some of the progression bands even after the bulk melting temperature, but loses them completely above 498 K, suggesting a possible partially ordered phase in this temperature window. Our studies have established the fact that (a) no rotational freedom exists in several of the alkyl chain monolayers on metal cluster solids at room temperature, (b) simple uniaxial rotation explains the dynamics of these systems, (c) the dynamics evolves slowly, and (d) such motions arise abruptly in long-chain layered thiolates which are similar to planar thiolates. We find that longer chains can possess conformational defects at higher temperatures, which slow the rotational dynamics.