Thiocarboxylate functionalization of silver nanoparticles : effect of chain length on the electrical conductivity of nanoparticles and their polymer composites

Thiocarboxylate-functionalized silver nanoparticles (NPs) were synthesized and characterized in terms of their thermal and electrical properties. Thiocarboxylic acids with chain lengths of 3 and 11 carbons were used to functionalize silver NPs, leading to the formation of NPs smaller than 5 nm. We found that silver NPs, functionalized with a short-chain acid, were twice smaller than those with a long-chain acid. Electrical conductivity measurements showed that the short-chain functionalized NPs were electrically conductive while the long-chain functionalized ones were nonconductive. The short-chain functionalized NPs were incorporated into a conventional electrical conductive adhesive (ECA) composite, consisting of micron-sized silver particles and an epoxy matrix to form a hybrid-designed ECA. The electrical conductivity measurements and SEM images of the cross-section of the cured hybrid composite showed that lower contents of the NPs (<20 wt% of the silver microparticles) increased the electrical conductivity due to the filling of NPs into the interstices of the micron-sized particles as well as their low sintering temperature, whereas higher NPs contents reduced the electrical conductivity because they clustered and increased the gaps between the microparticles. We attributed the positive effect of synthesized NPs on the electrical conductivity of the nanocomposite to their small size (<5 nm) and the specific design of the covering layer over the surface of NPs. These research findings provide insights into the relationship between the overall properties of the nanocomposites and the nanoparticles' surface chemistry, size and weight fractions.