title: Counterion-Mediated Ligand Exchange for PbS Colloidal Quantum Dot Superlattices.
abstract: In the past years, halide capping became one of the most promising strategies to passivate the surface of colloidal quantum dots (CQDs) in thin films to be used for electronic and optoelectronic device fabrication. This is due to the convenient processing, strong n-type characteristics, and ambient stability of the devices. Here, we investigate the effect of three counterions (ammonium, methylammonium, and tetrabutylammonium) in iodide salts used for treating CQD thin films and shed light on the mechanism of the ligand exchange. We obtain two- and three-dimensional square-packed PbS CQD superlattices with epitaxial merging of nearest neighbor CQDs as a direct outcome of the ligand-exchange reaction and show that the order in the layer can be controlled by the nature of the counterion. Furthermore, we demonstrate that the acidity of the environment plays an important role in the substitution of the carboxylates by iodide ions at the surface of lead chalcogenide quantum dots. Tetrabutylammonium iodide shows lower reactivity compared to methylammonium and ammonium iodide due to the nonacidity of the cation, which eventually leads to higher order but also poorer carrier transport due to incomplete removal of the pristine ligands in the QD thin film. Finally, we show that single-step blade-coating and immersion in a ligand exchange solution such as the one containing methylammonium iodide can be used to fabricate well performing bottom-gate/bottom-contact PbS CQD field effect transistors with record subthreshold swing.
authors.affiliation: Zernike Institute for Advanced Materials, University of Groningen , Nijenborgh 4, 9747AG Groningen, The Netherlands.
authors.forename: Daniel M
authors.initials: DM
authors.lastname: Balazs
authors.affiliation: Department of Chemistry and Applied Biosciences, ETH Zürich , Vladimir Prelog Weg 1, Zürich 8093, Switzerland.|Empa-Swiss Federal Laboratories for Materials Science and Technology , Überlandstrasse 129, Dübendorf 8600, Switzerland.
authors.forename: Dmitry N
authors.initials: DN
authors.lastname: Dirin
authors.affiliation: Zernike Institute for Advanced Materials, University of Groningen , Nijenborgh 4, 9747AG Groningen, The Netherlands.
authors.forename: Hong-Hua
authors.initials: HH
authors.lastname: Fang
authors.affiliation: Department of Chemistry and Applied Biosciences, ETH Zürich , Vladimir Prelog Weg 1, Zürich 8093, Switzerland.|Empa-Swiss Federal Laboratories for Materials Science and Technology , Überlandstrasse 129, Dübendorf 8600, Switzerland.
authors.forename: Loredana
authors.initials: L
authors.lastname: Protesescu
authors.affiliation: Zernike Institute for Advanced Materials, University of Groningen , Nijenborgh 4, 9747AG Groningen, The Netherlands.
authors.forename: Gert H
authors.initials: GH
authors.lastname: ten Brink
authors.affiliation: Zernike Institute for Advanced Materials, University of Groningen , Nijenborgh 4, 9747AG Groningen, The Netherlands.
authors.forename: Bart J
authors.initials: BJ
authors.lastname: Kooi
authors.affiliation: Department of Chemistry and Applied Biosciences, ETH Zürich , Vladimir Prelog Weg 1, Zürich 8093, Switzerland.|Empa-Swiss Federal Laboratories for Materials Science and Technology , Überlandstrasse 129, Dübendorf 8600, Switzerland.
authors.forename: Maksym V
authors.initials: MV
authors.lastname: Kovalenko
authors.affiliation: Zernike Institute for Advanced Materials, University of Groningen , Nijenborgh 4, 9747AG Groningen, The Netherlands.
authors.forename: Maria Antonietta
authors.initials: MA
authors.lastname: Loi
keywords: colloidal quantum dots; iodide-capped; lead sulfide; ligand exchange; transport