2D PbS Quantum Dot Superlattices with Unprecedented Area Coverage and Homogeneity via Langmuir-Schaefer Deposition

Superlattices of lead chalcogenide colloidal quantum dots hold promise to revolutionise the field of infrared optoelectronics due to their unique combination of optical and transport properties. However, the main challenge remains to form a homogeneous thin-film with long-range order avoiding cracking upon ligand exchange. This problem is particularly evident in 2D superlattices where the interactions driving the self-assembly are limited to a single plane yielding very defective films. To overcome these issues, we introduce a novel approach where external lateral pressure is applied during the self-assembly and ligand exchange, forcing the quantum dots toward each other thus avoiding the formation of cracks due to the volume shrinking. Such films consist of a hexagonal monolayer superlattice with long-range order that are crack-free over several millimetres square. The mechanism beyond the formation and ordering of the samples under external pressure is elucidated by atomistic molecular dynamic simulations. Transport measurements in an ionic gel-gated field-effect transistor reveal that increasing the external pressure during the superlattice formation leads to higher electron mobilities above 25 cm ² /Vs thanks to better compactness, high ordering, and a higher number of nearest neighbours. These results demonstrate that colloidal quantum dot superlattices with high charge mobility can be fabricated over large areas with important implications for technological applications.