(191f) Inorganic Organic Ligand Exchange On the Surface of Zinc Sulfide Nanocrytals

Inorganic ligand exchange on the surface of zinc sulfide nanocrystals

Qudus Lawal, Steve M. Herron and Stacey F. Bent

Department of Chemical Engineering, Stanford University, Stanford, CA 94305

Nanocrystals (NCs) have been the subject of intense interest because of their current and potential applications in optical and opto-electronic devices, sensing, and medical therapeutics.  Control over the surface chemistry of NCs is crucial to the performance of the NCs in opto-electronic devices, and the NC surface properties play a major role in thin-film formation from nanocrystal inks.  Specifically, small inorganic species such as ammonium sulfide can provide colloidal stability in polar solvents and may facilitate charge transport between individual nanocrystals, creating more opportunities for device integration of these materials. In these studies, zinc sulfide nanocrystals were synthesized with long-chain hydrophobic ligands including oleylamine.  Subsequently, a series of ligand exchanges were performed on these nanocrystals to replace the nascent nonpolar ligands with a metal-free inorganic species, ammonium sulfide. This was done in a two-step process, in which the first step was to reduce the length of the bulky oleylamine ligand to butyl amine, to facilitate transport of ammonium sulfide to the nanocrystal surface.  The NC’s were dispersed in the polar solvent formamide to stabilize this matrix before finally extracting excess hydrophobic ligand with hexanes. Different mass ratios (0.1:1, 1:1, 1:10) of NC to sulfide ion were used to determine the optimal ratio for a desirable exchange. The NCs before and after ligand exchange were characterized using FTIR spectroscopy to determine the efficacy of ligand exchange by monitoring the change in surface chemical environment, and UV-Vis spectroscopy to examine the optical characteristics of the NCs before and after ligand exchange. The effect of ligand exchange on the NC properties will be discussed.