Superlattices of lead chalcogenide colloidal quantum dots (QDs) hold promise to revolutionise the field of infrared optoelectronics due to their unique combination of optical and transport properties. The main challenge remains to form a homogeneous thin-film with long-range order, avoiding the formation of macroscopic cracks caused by the 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 an external lateral pressure during the self-assembly and subsequent ligand exchange, forces the PbS QDs closer compensating crack formation due to volume shrinking. We achieve this with a Langmuir-Schaefer deposition where the degree of compression can be controlled from differential surface pressure measurements and tuned to obtain increasingly compact films.

The Langmuir-Schaefer superlattices present a hexagonal arrangement, long-range order, and partial collective alignment of the nanocrystals. After ligand exchange, the hexagonal arrangement is preserved, resulting in a highly compact film, contrarily to the square superlattices in uncompressed assembly. The highly-compressed superlattices are crack-free over several millimetres square which, to our knowledge, has never been demonstrated. The superlattice formation mechanism is elucidated by atomistic molecular dynamic simulations supporting the positive effect of the external pressure. Transport measurements in an ionic gel-gated field-effect transistor reveal that electron mobilities increase up to 37 cm²V^-1s^-1 with increasing surface pressure  thanks to enhanced compactness, higher number of nearest-neighbours, and degree of ordering.

External compression during fabrication results in crack-free, highly ordered superlattices. The samples’ electron mobilities are on par with the state-of-the-art 2D superlattices obtained with pressure-free methods but with unprecedently higher large-area coverage. These results demonstrate that QD superlattices with high charge mobility can be fabricated over millimetre-square areas. Further extension of this deposition method to 3D superlattices is highly relevant for application in optoelectronic devices like short wavelength infrared photodetectors.