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Alex Abelson1 Caroline Qian2 Jenna Wardini1 Matt Law1 2 3

1, Department of Materials Science and Engineering, University of California, Irvine, Irvine, California, United States
2, Department of Chemical Engineering, University of California, Irvine, Irvine, California, United States
3, Department of Chemistry, University of California, Irvine, Irvine, California, United States

Colloidal quantum dots (QDs) that are partially fused together to form highly-coupled, highly-ordered superlattices (SLs) are an exciting new class of materials for optoelectronics. These epitaxially-fused QD SLs promise to combine the band-like carrier transport of bulk semiconductors with the size-tunable photophysics and solution processability of QDs. However, the structure and formation mechanism of these epi-SLs are poorly understood, which limits our ability to fabricate high-performance QD epi-SLs with controlled electronic properties.
In this talk, I present a novel combination of X-ray scattering and correlative electron imaging and diffraction used to rigorously determine the superlattice unit cell of oleate-capped SLs (oleate-SLs) and epi-SLs made from PbSe QDs. This “dual space” method is a powerful and general way to unambiguously assign the structure of 3D colloidal SLs at the single-grain level. We find that oleate-SLs fabricated through self-assembly on a liquid substrate adopt a trigonally-distorted body-centered cubic structure with two major out-of-plane SL orientations. I show that the QDs have full orientational as well as positional order in this structure and that both the lateral size of the SL grains and their degree of distortion from true bcc depend on ligand coverage. The two orientations of oleate-SL grains convert to two distinct orientations of the epi-SL with a very different appearance in the electron microscope. The phase transition occurs almost purely via translation of the QDs with minimal rotational motion, resulting in a trigonally-distorted simple cubic epi-SL with fusion along the PbSe {100} facets. This simple translational phase transition is made possible by the trigonal distortion of the initial oleate-SL and results in >5 μm epi-SL grains free of linear or planar defects. The ligand content, elemental distribution, and impact of atomic layer deposition infilling on the structure of the epi-SLs will also be highlighted.
The powerful correlative imaging and diffraction methodology presented here will help enable detailed understanding of complex nanomaterial superstructures. Furthermore, we show that small distortions in the oleate-SL structure have important ramifications on the epi-SL phase transition and the ability to make highly-perfect epitaxially-fused QD superlattices.

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