Melting of monolayer protected cluster superlattices

Melting of crystalline solids (superlattices) of octadecanethiol and octane thiol protected silver clusters has been studied with x-ray powder diffract ion (XRD), differential scanning calorimetry (DSC), and infrared (IR) spect roscopy. These solids have been compared with the silver thiolate layered c ompounds in view of their similarity in alkyl chain packing and x-ray diffr action patterns. Superlattice melting is manifested in XRD around 400 K as the complete disappearance of all the low angle reflections; only bulk silv er reflections due to the cluster cores are seen at 423 K. The superlattice structure is regained upon cooling from a temperature close to its melting point. However, cooling from a higher temperature of 473 K does not regain the superlattice order, whereas thiolate melting is repeatedly reversible even at these temperatures. Transmission electron microscopy suggests aggre gation of clusters during heating/cooling cycles. DSC shows two distinct tr ansitions, first corresponding to alkyl chain melting and the second corres ponding to superlattice melting. Only alkyl chain melting is observed in va riable temperature IR and increased order is manifested upon repeated heati ng/cooling cycles. Alkyl chain assembly shows strong interchain coupling le ading to factor group splitting in cluster superlattices upon annealing. In thiolates only one melting feature is seen in DSC and it produces gauche d efects, whereas significant increase in defect structures is not seen in su perlattices. Repeated heating/cooling cycles increase interchain interactio ns within a cluster and the superlattice order collapses. (C) 2000 American Institute of Physics. [S0021-9606(00)70245-5].