Monolayer-protected cluster superlattices: Structural, spectroscopic, calorimetric, and conductivity studies

Alkanethiol-protected silver clusters of average diameter 4.0 +/- 0.5 nm form single-phase superlattice solids, and their X-ray powder diffractograms have been fully indexed to single cubic unit cells. Whereas alkanethiols with five or more carbon atoms form superlattices, the corresponding cluster with four carbons yield only separated clusters. The superlattice solids can be recrystallized from nonpolar solvents. No such superlattices are seen for the corresponding gold clusters. The superlattice collapses upon heating, but the solid retains the structure even at 398 K, much above the melting point of crystalline alkanes and the corresponding self-assembled monolayer. In situ variable-temperature X-ray diffraction investigations did not show any solid-state phase transitions in the superlattice. Temperature-dependent infrared spectroscopy reveals the melting of the alkyl chain, and it is seen that the chain as a whole achieves rotational freedom prior to the collapse oft;he superlattice. Calorimetric investigations show distinct monolayer and superlattice melting transitions. The chemical nature of the cluster-molecule interaction is similar to that of the previously investigated gold and silver systems, as revealed by NMR, mass, infrared, and X-ray photoelectron spectroscopies and thermogravimetry analyses. Conductivity measurements clearly manifest the superlattice melting transition. Diffusion constants in solution measured by NMR show that the relative decrease in the diffusion constant with increasing monolayer chain length is smaller for silver than for gold, suggested to be a signature of intercluster interaction even in solution. Corroborative evidence is provided by the variable-temperature UV/vis investigations of the clusters.