Abstract

Size and shape tunability and low-cost solution processability make colloidal lead chalcogenide quantum dots (QDs) an emerging class of building blocks for innovative photovoltaic, thermoelectric and optoelectronic devices. Lead chalcogenide QDs are known to crystallize in the rock-salt structure, although with very different atomic order and stoichiometry in the core and surface regions; however, there exists no convincing prior identification of how extreme downsizing and surface-induced ligand effects influence structural distortion. Using forefront X-ray scattering techniques and density functional theory calculations, here we have identified that, at sizes below 8 nm, PbS and PbSe QDs undergo a lattice distortion with displacement of the Pb sublattice, driven by ligand-induced tensile strain. The resulting permanent electric dipoles may have implications on the oriented attachment of these QDs. Evidence is found for a Pb-deficient core and, in the as-synthesized QDs, for a rhombic dodecahedral shape with nonpolar {110} facets. On varying the nature of the surface ligands, differences in lattice strains are found.

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Acknowledgements

F.B. acknowledges University of Insubria for Junior Fellowship Grant 2013, M.V.K. acknowledges the European Union for financial support via FP7 ERC Starting Grant 2012 (Project NANOSOLID, GA No. 306733), D.N.D. thanks the European Union for Marie Curie Fellowship (PIIF-GA-2012-330524) and M.I. thanks AGAUR for her Beatriu i Pinós post-doctoral grant (2013 BP-A 00344). Synchrotron XRPD data were collected at the X04SA-MS Beamline of the Swiss Light Source. M. Döbeli is gratefully acknowledged for taking RBS spectra. Electron microscopy was performed at the Scientific Center for Optical and Electron Microscopy (ScopeM) at ETH Zürich. Computations were performed using the BlueGene/Q supercomputer at the SciNet HPC Consortium provided through the Southern Ontario Smart Computing Innovation Platform (SOSCIP). We thank N. Stadie and J. Mason for reading the manuscript.

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Affiliations

  1. Dipartimento di Scienza e Alta Tecnologia and To.Sca.Lab, Università dell’Insubria, via Valleggio 11, I-22100 Como, Italy

    • Federica Bertolotti
    •  & Norberto Masciocchi
  2. Department of Chemistry and Applied Biosciences, ETH, Vladimir-Prelog-Weg 1, CH-8093 Zürich, Switzerland

    • Dmitry N. Dirin
    • , Maria Ibáñez
    • , Frank Krumeich
    •  & Maksym V. Kovalenko
  3. Empa-Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland

    • Dmitry N. Dirin
    • , Maria Ibáñez
    •  & Maksym V. Kovalenko
  4. SLS, Laboratory for Synchrotron Radiation—Condensed Matter, Paul Scherrer Institut, CH-5232 Villigen, Switzerland

    • Antonio Cervellino
  5. Department of Chemistry, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland

    • Ruggero Frison
  6. Istituto di Cristallografia and To.Sca.Lab, CNR, via Valleggio 11, I-22100 Como, Italy

    • Ruggero Frison
    •  & Antonietta Guagliardi
  7. The Edward S. Rogers Department of Electrical and Computer Engineering, University of Toronto, 10 King’s College Road, Toronto, Ontario M5S 3G4, Canada

    • Oleksandr Voznyy
    •  & Edward H. Sargent

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Contributions

A.G., N.M. and M.V.K. formulated the project. D.N.D. and M.I. synthesized the compounds and performed the optical properties characterization. A.C., F.B., R.F., A.G. and N.M. collected and analysed the X-ray total scattering data. F.K. collected and analysed the electron microscopy images. O.V. and E.H.S. performed DFT calculations. A.G. and N.M. wrote the manuscript, with the contribution of all authors.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to Antonietta Guagliardi or Norberto Masciocchi.

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DOI

https://doi.org/10.1038/nmat4661