Crystalline Solids of Alloy Clusters

AuAg alloy clusters protected with octadecanethiol (ODT) monolayers form crystalline solids whose powder X-ray diffractograms can be indexed to single cubic unit cells. These solids can also be regarded as superlattices of alloy clusters. The bulk Au:Ag ratio is significantly different from the metal salt feed ratio used for the synthesis of the alloy clusters. The surface plasmon resonance band of the alloy clusters shift monotonically with change in composition and only one distinct band is seen in the composition range, indicating that the single metal core containing alloy clusters are not forming. The shifts observed can be linearly related to the compositions. Atomic emission spectroscopy gives similar compositions as determined from absorption spectroscopy. The monolayer coverage determined from thermogravimetry increases with an increase in silver concentration. Thermal stability of the monolayers is comparable to pure Au and Ag clusters and no change in desorption temperature is observed with composition. Both monolayer melting and superstructure melting transitions are seen in the differential scanning calorimetry of the solids. Infrared spectroscopy suggests that the monolayers are essentially solidlike and the conformations are similar to the monolayers on pure Ag clusters. Some of the bands exhibit factor group splitting in the Au-rich composition, which is observed in other compositions upon annealing.