N. Sandhyarani et al., Monolayer-protected cluster superlattices: Structural, spectroscopic, calorimetric, and conductivity studies, CHEM MATER, 12(1), 2000, pp. 104-113
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.