(Invited) 3D Epitaxially-Connected Quantum Dot Superlattices: Formation, Structure, and Transport

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), which are 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. In this talk, I describe recent progress in making epi-SLs of sufficient spatial and energetic order to delocalize carriers into mini-bands. I discuss the fabrication of high-quality 3D epi-SL films, along with the determination of their structure (the complete superlattice unit cell) and physical formation pathway from the parent oleate-capped SL using a combination of X-ray scattering and correlative electron microscopy and diffraction of single SL grains. The oleate- to epi-SL phase transition occurs by nearly-pure translation of the QDs with minimal rotational motion, resulting in a rhombohedrally-distorted simple cubic epi-SL with fusion along the PbSe {100} facets. This simple translational phase transition is facilitated by the rhombohedral distortion of the initial oleate-capped SL and results in >5 μm epi-SL grains free of linear or planar defects. While large epi-SL grains are useful, several types of intra-grain structural defects conspire to reduce order within the grains and localize charge carriers. Quantitative characterization of these intra-grain defects and a better understanding of the molecular processes involved in epi-SL formation have enabled fabrication improvements that enhance intra-grain order and overall film uniformity. Measurements probing mini-band formation and the impact of SL surface chemistry on charge transport will be discussed.