Colloidal quantum dots (QDs) that are epitaxially interconnected (necked or partially fused) to form highly-ordered, highly-coupled superlattices are an exciting new class of materials for optoelectronics. These epitaxially-fused QD superlattices (epi-SLs), typically made by self-assembly and ligand exchange on a liquid surface followed by stamp transfer to a solid substrate (the Langmuir-Schaefer technique), promise to combine the size-tunable photophysics and solution processability of QDs with delocalized, bulk-like carrier transport; but several types of structural defects conspire to reduce order within the epi-SL grains and localize charge carriers.<br/>    In this talk, I will first explain the molecular mechanism of epi-SL formation, and then show how this mechanistic understanding was leveraged to develop a contactless photochemical process to make epi-SLs that are markedly more uniform and structurally perfect than is possible using conventional approaches based on point injection of a chemical trigger. Our photochemical process employs a photobase generator dissolved in the liquid substrate that, upon illumination, releases an amine that triggers uniform epi-fusion of the floating QD film, thus eliminating lateral ligand exchange gradients and mechanical disturbances that decrease the structural order of the epi-SL. Comparative chemical, structural, and electronic characterization of both polycrystalline epi-SL films and individual epi-SL grains made by photobase illumination versus amine injection will be presented, and the key structure-property relationships discussed. This light triggering process produces more uniform, structurally-perfect epi-SLs and creates new capabilities to scale and pattern 3D epi-SLs for next-generation optoelectronics.

Photochemically-triggered formation of 3D epitaxially-fused colloidal quantum dot superlattices

ACS Spring 2022

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