Capping-ligand effect on the stability of CdSe quantum dot Langmuir monolayers

The stability of Langmuir monolayers of CdSe Qdots capped with dodecan-ethiol (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. Langmuir films of Qdots were created by depositing droplets of purified Qdots in chloroform at the air-water interface. While holding the Qdot films at 13 mN/m for 1 h in a Langmuir trough, the average monolayer areas decreased by roughly 9% for TOPO-capped Qdots, ∼15-18% for the three DTC-capped Qdot preparations, and ∼21% for DDT-capped Qdots. Using the model of Ter Minassian-Saraga, the relative stabilities 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 by Fourier-transform infrared spectroscopy (FTIR) revealed that the aliphatic tails of capping ligands were assembled on Qdot surfaces 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.