Tailoring ZnSe-CdSe Colloidal Quantum Dots via Cation Exchange: From Core/Shell to Alloy Nanocrystals

Esther Groeneveld, Leon Witteman, Merel Lefferts, Xiaoxing Ke, Sara Bals, Gustaaf Van Tendeloo, Celso de Mello Donega*

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

Abstract

We report a study of Zn2+ by Cd2+ cation exchange (CE) in colloidal ZnSe nanocrystals (NCs). Our results reveal that CE in ZnSe NCs is a thermally activated isotropic process. The CE efficiency (i.e., fraction of Cd2+ ions originally in solution, Cd-sol, that is incorporated in the ZnSe NC) increases with temperature and depends also on the Cd-sol/ZnSe ratio. Interestingly, the reaction temperature can be used as a sensitive parameter to tailor both the composition and the elemental distribution profile of the product (Zn,Cd)Se NCs. At 150 degrees C ZnSe/CdSe core/shell hetero-NCs (HNCs) are obtained, while higher temperatures (200 and 220 degrees C) produce (Zn1-xCdx)Se gradient alloy NCs, with increasingly smoother gradients as the temperature increases, until homogeneous alloy NCs are obtained at T >= 240 degrees C. Remarkably, sequential heating (150 degrees C followed by 220 degrees C) leads to ZnSe/CdSe core/shell HNCs with thicker shells, rather than (Zn1-xCdx)Se gradient alloy NCs. Thermal treatment at 250 degrees C converts the ZnSe/CdSe core/shell HNCs Into (Zn1-xCdx)Se homogeneous alloy NCs, while preserving the NC shape. A mechanism for the cation exchange in ZnSe NCs is proposed, in which fast CE takes place at the NC surface, and is followed by relatively slower thermally activated solid-state cation diffusion, which is mediated by Frenkel defects. The findings presented here demonstrate that cation exchange in colloidal ZnSe NCs provides a very sensitive tool to tailor the nature and localization regime of the electron and hole wave functions and the optoelectronic properties of colloidal ZnSe-CdSe NCs.

Original languageEnglish
Pages (from-to)7913-7930
Number of pages18
JournalACS Nano
Volume7
Issue number9
DOIs
Publication statusPublished - Sept 2013

Funding

The authors acknowledge financial support from the European Union under the 7th Framework Program, FP7 (Integrated Infrastructure Initiative No. 262348 European Soft Matter Infrastructure, ESMI). X.K. and G.V.T. are grateful to the European Research Council for funding under the 7th Framework Program (FP7), ERC grant No. 246791 "COUN-TATOMS". The authors are also grateful to F. Pietra (Utrecht University, The Netherlands) for kindly providing a sample of CdSe/CdS dot core/rod shell nanorods and to X. Zhong and W. Knoll (Max Planck Institute for Polymer Research, Mainz, Germany) for kindly supplying a sample of (Zn,Cd)Se homogeneous alloy NCs.

Keywords

  • cation exchange
  • ZnSe
  • ZnSe-CdSe
  • core-shell nanocrystals
  • gradient alloy nanocrystals
  • quantum dots
  • SEMICONDUCTOR NANOCRYSTALS
  • HETEROSTRUCTURED NANOCRYSTALS
  • HIGH LUMINESCENCE
  • SIZE DISPERSION
  • EMISSION
  • HETERONANOCRYSTALS
  • STABILITY
  • SYSTEM
  • LOCALIZATION
  • SPECTROSCOPY

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