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