Phase transitions in octanethiol-capped Ag nanocluster microfilm assemblies

We describe phase transitions in microfilm assemblies of octanethiol (OT)-capped 2-4 nm-diameter Ag nanoclusters prepared from solutions with OT/Ag+ ratios of ∼50. Using DSC we observe two melting/crystallization-type reversible phase transitions: one at ∼61 °C due to interdigiated unattached octanethiol, and the other at ∼125 °C due to the phase comprised of the assembly of OT-capped nanoclusters. Increased thermal fluctuations weaken the inter-chain hydrophobic interactions between interdigitated OT molecules, leading to both phase transitions. The thiolate bond of OT-molecules bound to Ag nanoclusters are more rigid, thereby requiring a higher temperature to increase the flexibility of the alkyl chain of OT, and to melt the nanocluster assembly. The mobility of the nanoclusters in the melt is limited, and morphological features of the original assemblies are retained during recrystallization. No observable mass loss is detected up to ∼180 °C, above which OT molecules desorb from the Ag nanoclusters.