The Niezgoda lab researches new quantum dot (QD) systems in which chemically interesting ligands are incorporated into syntheses of QDs themselves as new “native” ligands. As a prototype example of this theme, we present recent findings concerning a project that details the on-particle modification of a surface bound ligand. In this first-of-its-kind work, we synthesized a ligand molecule that is purposefully designed to contain a functional group that can undergo chemical transformation, rendering the ligand shell less physically bulky and altering the QD solubility. Findings from this project are germane to thin film QD studies in which surface passivation, suppression of trap states, and short ligand length are of crucial importance. This ligand, 9,10-dihydroxystearic acid (DHSA) is a fatty acid vicinal diol (glycol) prepared from the careful oxidation of oleic acid by undergraduate researchers. We then designed a synthesis for CdSSe alloy QDs around this DHSA ligand, taking into account the heavily altered solubility and solution dynamics of the DHSA. This new solubility led us to develop a specific diphenylether/dimethyl formamide mixture that permitted growth and formation of solution stable QDs, which represents the first time such a solvent has been used in QD syntheses. This task required a rethinking of the possible solvent species—striving to achieve sufficiently high boiling point solvents with little to no neucleophilicity to form both the Cd2+ as well as the QDs themselves. In this way, we were able to obtain size-tunable and solution-stable CdSSe QDs utilizing only DHSA as a ligand. Absorption and PL data shows good size dispersity and tunable fluorescence. Finally, we are able to effectively decrease the length of this ligand in half, all while never removing or replacing the original binding group. This is achieved through oxidative cleavage of the glycol with lead (IV) acetate (LTA) to two terminal aldehydes, with the acid aldehyde remaining bound to the QD, proven through FTIR spectroscopy. This further alters the solubility of the QDs, however they still remain stable in solution. HR-TEM images show decreased particle separation in monolayer QD films, resulting from the shortened carbon chains in the ligand molecules. Finally, ongoing research projects are discussed for the introduction of new functional groups to as-synthesized QDs, including “clickable” moieties for substrate-specific linkage without surface alteration.

Changes without Exchanges: On-Particle Ligand Chemistry with Purpose-Designed, As-Synthesized Ligands

ACS Spring Meeting 2021

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