Quantum dot thin film imaging enables in situ, benchtop analysis of ligand exchange at the solution-film interface

Abstract

This paper discusses experiments using benchtop video imaging to track the contraction of PbS quantum dot (QD) thin films supported on a liquid subphase during ligand exchange of the native oleate (C 17 H 33 O 2 − ) ligands with various primary carboxylic acids (C n H 2n+1 CO 2 H; n = 0–5), thiols (C n H 2n+1 SH; n = 2–6), and amines (C n H 2n+1 NH 2 ; n = 2–6). While widely used in the preparation of research-grade photovoltaics and thin-film transistors, this ligand exchange process at the thin film/solution interface has not been studied in situ and in real time with sufficient time resolution to distinguish fast ligand exchange processes from slower, destructive processes such as surface etching. Effective ligand shell lengths L e calculated from film areas were consistent with previous TEM studies, so film area measurements determined from video images effectively track changes in interparticle spacing on the nanoscale. L e values distinguished ligands that generated high degrees of oleate displacement (carboxylic acids and thiols) from those that generated low degrees of oleate displacement (amines). Displacement of the native ligand was found to limit the rate of film contraction, so contraction rates serve as a probe of underlying ligand exchange dynamics. When the entering ligand was in excess, the film contraction rate displayed a sublinear dependence on its concentration. This was explained in terms of a two-step mechanism involving the pre-equilibrium association of the entering ligand with the QD surface followed by dissociation of oleate. Thiols and carboxylic acids produced faster oleate displacement than did amines, but ligand length was not observed to have any consistent effect on the oleate displacement rate. The ability of carboxylic acids and thiols to promote oleate dissociation through proton transfer was proposed as an explanation of this rate difference as 1 H NMR spectra of colloidal-phase samples showed carboxylic acids and thiols generated protonated oleic acid while amines did not. These experiments demonstrate the utility of video imaging of QD thin films suspended on a liquid subphase as a simple and rapid benchtop technique for monitoring chemical and structural variables relevant for QD-based device preparation in situ . • PbS quantum dot thin film contraction is limited by the rate of oleate displacement. • Benchtop Video imaging can track dot reorganization and ligand exchange in situ. • The rate of oleate exchange increases with increasing entering ligand concentration. • Carboxylic acids and thiols displace oleate faster from PbS dots than do amines. • Amines displace less oleate from PbS dot films than do carboxylic acids or thiols.