Synthesis of Phosphonic Acid Ligands for Nanocrystal Surface Functionalization and Solution Processed Memristors

Here we synthesize 2-ethylhexyl, 2-hexyldecyl, 2-[2-(2-methoxyethoxy)ethoxy]ethyl, oleyl and n-octadecyl phosphonic acid and use them to functionalize CdSe and HfO₂ nanocrystals. In contrast to branched carboxylic acids, post-synthetic surface functionalization of CdSe and HfO₂ nanocrystals is readily achieved with branched phosphonic acids. A simple flow coating process is used to deposit ribbons of individual phosphonic acid capped HfO₂ nanocrystals, which are subsequently evaluated as a memristor using conductive atomic force microscopy (c-AFM). We find that 2-ethylhexyl phosphonic acid is a superior ligand, combining a high colloidal stability with a compact ligand shell that results in a record-low operating voltage that is promising for application in flexible electronics.