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Crystal symmetry breaking and vacancies in colloidal lead chalcogenide quantum dots

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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Figure 1: Lattice distortion of the rock-salt structure in PbS QDs.
Figure 2: Size-dependent lattice expansion, ligands-induced tensile stress and dipole moment.
Figure 3: Morphology evolution of PbS QDs.
Figure 4: STEM analysis of the PbS QD structure and morphology.
Figure 5: Organic-to-inorganic ligand exchange and induced lattice strain.
Figure 6: Stoichiometry of homo-core-shell PbS QDs.

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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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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.

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Correspondence to Antonietta Guagliardi or Norberto Masciocchi.

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The authors declare no competing financial interests.

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Bertolotti, F., Dirin, D., Ibáñez, M. et al. Crystal symmetry breaking and vacancies in colloidal lead chalcogenide quantum dots. Nature Mater 15, 987–994 (2016). https://doi.org/10.1038/nmat4661

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