Absence of the rotator phase and evolution of dynamical motions in cluster monolayers

Alkyl chain dynamics in monolayer protected metal cluster systems has been studied by the quasielastic neutron scattering technique, using two different instruments having very different energy windows. Long chain thiolate protected clusters such as Au–ODT (Au–SC18H37) are rotationally frozen at room temperature ~RT! in a wide time scale of 10 to 10 s. The rotator phase is absent at RT even in much smaller chain length systems, Au–OT (Au–SC8H17) and Au–HT (Au–SC6H13). Dynamics was found to evolve upon increase in temperature. Alkyl chains in a metal cluster superlattice such as Ag–ODT (Ag–SC18H37) are also dynamically frozen at RT in the whole time scale range. Evolution of dynamics with temperature is found to be different in the superlattice and isolated cluster systems. In the former, the chains participating in the inter-cluster interaction were found to be dynamically different from those without. On heating just above the chain melting temperature (Tcm), chains not involved in the inter-cluster interaction become dynamic first. Other chains start softening subsequently and, on increasing temperature near to the superlattice melting, all the chains eventually become dynamic. The uniaxial diffusion motion about the chain axis consistently described the dynamics of the monolayers, both in isolated cluster and in superlattice systems. Direct information on the dynamics of cluster monolayers is obtained for the first time. © 2003 American Institute of Physics. @DOI: 10.1063/1.1544093#