Capping-Ligand Effect on the Stability of CdSe Quantum Dot Langmuir Monolayers

The stability of Langmuir monolayers of CdSe Qdots capped with dodecanethiol (DDT), with dithiocarbamates having one, two, or three long alkyl chains (DTC-1, DTC-2 and DTC-3) Or with tri-n-octylphosphine oxide (TOPO), Was investigated and linked to the transport of Qdots into the subphase via a dissolution and diffusion mechanism. Langmur films of Qdots were created by depositing droplets of purified Qdots chloroform at the air-water interface. While holding the Qdot films at 13 mN/m for 1 h in Langmuir trough, the average monolayer areas: decreased by roughly 9% for TOPO-capped Qdots, similar to 15-18%, for the three DTC-capped Qdot preparations, and similar to 21% for DDT-capped Qdot. Using the model of Ter Minassian-Saraga the relative stabililties of the Qdot films studied were related to differences in equilibrium partitioning into the subphase and to apparent Qdot diffusivities within the subphase. An analysis of the Qdot preparations with similar packing densities for all ligand chemistries. A combined analysis of the film-area contraction and FTIR data suggested that, for the chemistries examined in this study, both the capping-ligand headgroup and the aliphatic tail groups impact Qdot Langmuir film stability through their joint influence on nanoparticle wettability and the tendency to aggregate upon partitioning into the subphase.