Alkyl-chain dynamics in monolayer protected metal cluster systems of gold have been studied by the quasielastic neutron scattering technique. Measurements have been carried out with 6, 12, and 18 carbon-long thiolate protected gold clusters of $4\mathrm{nm}$ core diameter. Dynamics of the monolayers in these systems, as indicated by the quasielastic broadening, were found to evolve continuously with increase in temperature from $270\text{to}400\mathrm{K}$. Analysis of the elastic incoherent structure factor showed that while only a small portion of each chain is dynamically active at low temperature, the dynamics extend to almost the whole chain at $400\mathrm{K}$. Although an improved model compared to our earlier reports, involving precessional motion of the chains on a cone is found to explain the observed data, a further improvement with reorientation of the chain around its axis with additional body axis fluctuation was most consistent with the experiment. The studies revealed the existence of complex dynamical motions of alkyl chains on the clusters. The studies suggest that repeated washings provide more space for the monolayer chains as a result of the removal of the phase transfer catalysts between the alkyl chains on the cluster surface. This provides space for greater orientational freedom, resulting in the observed dynamics even at lower temperatures.

• Received 3 October 2006

DOI:https://doi.org/10.1103/PhysRevB.75.075414