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Charge transport and localization in atomically coherent quantum dot solids

Abstract

Epitaxial attachment of quantum dots into ordered superlattices enables the synthesis of quasi-two-dimensional materials that theoretically exhibit features such as Dirac cones and topological states, and have major potential for unprecedented optoelectronic devices. Initial studies found that disorder in these structures causes localization of electrons within a few lattice constants, and highlight the critical need for precise structural characterization and systematic assessment of the effects of disorder on transport. Here we fabricated superlattices with the quantum dots registered to within a single atomic bond length (limited by the polydispersity of the quantum dot building blocks), but missing a fraction (20%) of the epitaxial connections. Calculations of the electronic structure including the measured disorder account for the electron localization inferred from transport measurements. The calculations also show that improvement of the epitaxial connections will lead to completely delocalized electrons and may enable the observation of the remarkable properties predicted for these materials.

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Figure 1: Structural characterization of a nanocrystal superlattice.
Figure 2: Charge transport measurement by field-effect transistor.
Figure 3: Hopping behaviour of electrons and holes modulated by gate voltage.
Figure 4: Extent of delocalization in a disordered superlattice.
Figure 5: Complete delocalization in a quantum dot solid.

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Acknowledgements

This research was supported by the Cornell Center for Materials Research with funding from the NSF MRSEC program (DMR-1120296). K.W. and J.Y. were supported by the Basic Energy Sciences Division of the Department of Energy through Grant DE-SC0006647 ‘Charge Transfer Across the Boundary of Photon-Harvesting Nanocrystals’. B.H.S. was supported by the NSF IGERT grant DGE-0903653 and NSF GRFP grant DGE-1144153. This work was based on research conducted at the Cornell High Energy Synchrotron Source (CHESS), which is supported by the National Science Foundation and the National Institutes of Health/National Institute of General Medical Sciences under NSF award DMR-1332208. Charge transport measurements were performed in a facility supported by Award No. KUS-C1-018-02, made by King Abdullah University of Science and Technology (KAUST). The authors wish to thank CHESS staff scientist Detlef Smilgies for assistance with X-ray scattering experiments.

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K.W. prepared samples for electron microscopy and X-ray scattering, fabricated FET devices, performed electrical transport and X-ray scattering measurements, acquired bright-field TEM images and analysed the electron micrographs, X-ray data and electrical transport data. J.Y. performed calculations of electronic structure and localization length. B.H.S. acquired and analysed STEM micrographs. All authors contributed to the interpretation of results and preparation of the manuscript.

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Correspondence to Tobias Hanrath.

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The authors declare no competing financial interests.

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Whitham, K., Yang, J., Savitzky, B. et al. Charge transport and localization in atomically coherent quantum dot solids. Nature Mater 15, 557–563 (2016). https://doi.org/10.1038/nmat4576

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