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

Alkanethiol-protected silver clusters of average diameter 4.0 +/- 0.5 nm fo rm single-phase superlattice solids, and their X-ray powder diffractograms have been fully indexed to single cubic unit cells. Whereas alkanethiols wi th five or more carbon atoms form superlattices, the corresponding cluster with four carbons yield only separated clusters. The superlattice solids ca n be recrystallized from nonpolar solvents. No such superlattices are seen for the corresponding gold clusters. The superlattice collapses upon heatin g, but the solid retains the structure even at 398 K, much above the meltin g point of crystalline alkanes and the corresponding self-assembled monolay er. In situ variable-temperature X-ray diffraction investigations did not s how any solid-state phase transitions in the superlattice. Temperature-depe ndent 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 mon olayer and superlattice melting transitions. The chemical nature of the clu ster-molecule interaction is similar to that of the previously investigated gold and silver systems, as revealed by NMR, mass, infrared, and X-ray pho toelectron spectroscopies and thermogravimetry analyses. Conductivity measu rements clearly manifest the superlattice melting transition. Diffusion con stants in solution measured by NMR show that the relative decrease in the d iffusion constant with increasing monolayer chain length is smaller for sil ver than for gold, suggested to be a signature of intercluster interaction even in solution. Corroborative evidence is provided by the variable-temper ature UV/vis investigations of the clusters.