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.

Highly Passivated n‐Type Colloidal Quantum Dots for Solution‐Processed Thermoelectric Generators with Large Output Voltage

“…It can clearly be seen that the IBAI-CQD films exhibit a noisy and low source-drain current, while a significant improvement in the current was measured in the FI-CQD films. [41,49] These results support our hypothesis that counterion-deprotonated OA leftovers may contribute to charge trapping, which is detrimental for electronic transport. A further current improvement was measured in the MAI-CQD films, confirming a more favorable charge transport.…”

“…[37][38][39][40] For example, hydrazine treatments employed in the past have left the CQD surface unpassivated and disconnected, leading to poor electrical conductivity (<10 −2 S cm −1 ) and an inefficient n-type thermoelectric power factor (<1 µW m −1 K −2 ). [41][42][43][44][45][46][47][48][49] We hypothesize that this all-inorganic halide-based ligand strategy could be deployed successfully in forming efficient n-type thermoelectrics. [41][42][43][44][45][46][47][48][49] We hypothesize that this all-inorganic halide-based ligand strategy could be deployed successfully in forming efficient n-type thermoelectrics.…”

Electron Mobility of 24 cm2 V−1 s−1 in PbSe Colloidal‐Quantum‐Dot Superlattices

“…Orientation of lead‐chalcogenide CQDs in a superlattice can be controlled by adjusting the reactivity of the subphase through the choice of solvent, by adding chemical species or by changing the temperature . This possibility stems from the faceted nature of the CQDs; crystal orientation‐specific interactions and different binding energy of the ligands at the main crystallographic facets drive the orientation process . While much work has been done on the formation mechanism and properties of the superlattices on solid substrates, fewer studies investigated liquid interface‐grown layers, and even fewer have been devoted to study the structure–property relation in these samples .…”

“…This possibility stems from the faceted nature of the CQDs; crystal orientation‐specific interactions and different binding energy of the ligands at the main crystallographic facets drive the orientation process . While much work has been done on the formation mechanism and properties of the superlattices on solid substrates, fewer studies investigated liquid interface‐grown layers, and even fewer have been devoted to study the structure–property relation in these samples . Whitham et al measured the effect of disorder on charge localization in PbSe CQD superlattices, they deduced a carrier localization over two to three quantum dots in their system and calculated a disorder limit below which band‐like, coherent transport is expected to occur .…”

Lead‐Free Halide Perovskite Nanocrystals: Crystal Structures, Synthesis, Stabilities, and Optical Properties

“…[97] It is interesting to note that Cs 2 SnI 6 NCs with no ligands are colloidally stable up to 12 hand can be conveniently deposited into high-quality thin films using asimple drop-cast procedure,dispensing with the need to remove the excess organic materials (e.g., ligands) that was afeature of previous approaches. [53,[97][98][99][100][101] There were no large holes and cracks in these films,and they also possess as mooth substrate-film interface (Figure 8b), which are advantageous for their applications in electronic devices. [97] Recently,Cs 2 SnI 6 NCs with different shapes were synthesized by areverse injection process.…”