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Nature Materials, Oct 14, 2019: doi:10.1038/s41563-019-0485-2

Nature Materials, Oct 14, 2019: doi:10.1038/s41563-019-0485-2

Collective topo-epitaxy in the self-assembly of a 3D quantum dot superlattice

Alex Abelson1,6, Caroline Qian2,6, Trenton Salk3, Zhongyue Luan1, Kan Fu1, Jian-Guo Zheng4, Jenna L. Wardini1 and Matt Law 1,2,5*

1Department of Materials Science and Engineering, University of California, Irvine, Irvine, CA, USA.
2Department of Chemical and Biomolecular Engineering, University of California, Irvine, Irvine, CA, USA.
3Department of Physics and Astronomy, University of California, Irvine, Irvine, CA, USA.
4Irvine Materials Research Institute, University of California, Irvine, Irvine, CA, USA.
5Department of Chemistry, University of California, Irvine, Irvine, CA, USA.
6These authors contributed equally: Alex Abelson, Caroline Qian.
*e-mail: matt.law@uci.edu

Epitaxially fused colloidal quantum dot (QD) superlattices (epi-SLs) may enable a new class of semiconductors that combine the size-tunable photophysics of QDs with bulk-like electronic performance, but progress is hindered by a poor understanding of epi-SL formation and surface chemistry. Here we use X-ray scattering and correlative electron imaging and diffraction of individual SL grains to determine the formation mechanism of three-dimensional PbSe QD epi-SL films. We show that the epi-SL forms from a rhombohedrally distorted body centred cubic parent SL via a phase transition in which the QDs translate with minimal rotation (~10°) and epitaxially fuse across their {100} facets in three dimensions. This collective epitaxial transforma-tion is atomically topotactic across the 103–105 QDs in each SL grain. Infilling the epi-SLs with alumina by atomic layer deposi-tion greatly changes their electrical properties without affecting the superlattice structure. Our work establishes the formation mechanism of three-dimensional QD epi-SLs and illustrates the critical importance of surface chemistry to charge transport in these materials.

Films of quantum dots are promising for next-generation electronic and optoelectronic devices, including photodetectors and solar cells, but they usually suffer from poor order and inefficient charge transport. To improve transport, researchers have recently focused on making highly-ordered quantum dot “superlattices” (crystals of quantum dots) in which the quantum dots are fused to each other to form porous single crystals. These fused superlattices feature exceptional coupling and spatial order and may enable a new class of materials that combines the size-tunable photophysics of quantum dots with the excellent charge transport of bulk semiconductors. However, progress toward this goal has been hindered by a lack of understanding as to how the superlattices form. In this study, we used X-ray scattering and electron microscopy to establish the formation mechanism of the fused superlattices. We show that the superlattices form from an unfused parent superlattice in which the quantum dots are rotationally aligned to fuse with their six neighbors by a sliding motion involving only a slight rotation of each dot. Our results are important because knowledge of the formation mechanism is the basis for the rational fabrication of fused superlattices with improved spatial order and electrical properties.

 

 

Quotes:

“Now that we’ve established the pathway of the phase transition, we should be able to intelligently manipulate the system to achieve more perfect fused superlattices, which hopefully show much more efficient charge transport.”

 

“UCI’s new Center for Transmission Electron Microscopy was crucial to the success of this project. Access to the new campus TEMs made this work possible.”

 

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