Three-Dimensional Self-Assembled Monolayer (3D SAM) of n-Alkanethiols on Copper Nanoclusters

Three-dimensional self-assembled monolayer (3D SAM) of 1-octanethiol, 1-decanethiol, and 1-dodecanethiol has been formed on copper nanoclusters. The morphology and spectroscopy of the nanoparticles were characterized while the conformation of the 3D SAMs was investigated with thermal and variable-temperature analyses. TEM results suggest the copper clusters consist of a spherical shape of ∼3−5 nm. IR, XPS, and 13 C CPMAS NMR results confirm that alkanethiols are chemisorbed via the −SH group and the packing density of the alkanethiols on copper nanoclusters increases with the alkyl chain lengths. The thiol chain is deduced to adopt the all-trans zigzag conformation through the analysis by IR and 13 C CPMAS NMR. Low-angle peaks observed in XRD further suggest strong interdigitation among these chains to form superlattice structures. On increasing temperature, VT-FTIR and VT- 13 C CPMAS NMR depict the disruption of lateral interaction between the chains. This melting of crystalline packing is endothermic but an exothermic peak was observed upon reheating in DSC. We attribute this exothermic “crystallization” to the conformational memory effect of the compact chains. We have found that much higher melting points and enthalpies are obtained on copper nanoclusters as compared to earlier reports on gold and silver clusters.