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Study of colloidal quantum-dot surfaces using an innovative thin-film positron 2D-ACAR method

Nature Materials volume 5, pages 23–26 (2006)Cite this article

Abstract

Nanosized inorganic particles are of great interest because their electronic properties can be easily tailored, providing a tremendous potential for applications in optoelectronic devices, light-emitting diodes1, solar cells2,3,4 and hydrogen storage5. Confinement of electrons and holes to dimensions comparable to their wavelength leads to quantum-well states with modified wavefunctions and density of states6. Surface phenomena are crucial in determining nanoparticle properties in view of their large surface-to-volume ratio. Despite a wealth of information, many fundamental questions about the nature of the surface and its relationship with the electronic structure remain unsolved. Ab initio calculations on CdSe nanocrystals7 suggest that passivating the ligands does not produce the ideal wurtzite structure and that Se atoms relax outwards irrespective of passivation. Here we show that implanted positrons are trapped at the surface of CdSe nanocrystals. They annihilate mostly with the Se electrons, monitor changes in composition and structure of the surface while hardly sensing the ligand molecules, and we thus unambiguously confirm the predicted strong surface relaxation.

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Figure 1: S–W plot as a function of size for colloidal CdSe nanocrystals over the diameter range 2.5–6 nm, and for a bulk CdSe single crystal.
Figure 2: Isotropic electron–positron momentum distribution.
Figure 3: Isotropic electron–positron momentum distribution ρ2γ(p) obtained by first principles band structure calculations on zincblende CdSe.
Figure 4: Cd(4d) contribution, ICd, to the electron–positron momentum density (relative to bulk CdSe) versus Doppler S parameter.
Figure 5: Schematic drawing of the shell-like positron density |ψ+(r)|2 in the trapped state at the surface of the quantum dot.

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Acknowledgements

This work is supported by the USDOE contract DE-AC03-76SF00098, and benefited from the allocation of supercomputer time at NERSC, Northeastern University’s Advanced Scientific Computation Center (ASCC) and the NCF (Foundation National Computer Facilities) with support from NWO. We thank N. Zaitseva for the generous supply of CdSe quantum-dot samples and A. Houtepen and D. A. M. Vanmaekelbergh for the pyridine-capped CdSe quantum-dot sample.

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  1. Anton (Tom) van Veen: Deceased

Authors and Affiliations

  1. Department of Radiation, Radionuclides & Reactors, Faculty of Applied Sciences, Delft University of Technology, 2629 JB, Delft, The Netherlands

    Stephan W. H. Eijt, Henk Schut & Peter E. Mijnarends

  2. Physics Department, Northeastern University, Boston, Massachusetts, 02115, USA

    Peter E. Mijnarends, Bernardo Barbiellini & Arun Bansil

  3. Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California, 94551, USA

    Art B. Denison

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Correspondence to Bernardo Barbiellini.

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Eijt, S., van Veen, A., Schut, H. et al. Study of colloidal quantum-dot surfaces using an innovative thin-film positron 2D-ACAR method. Nature Mater 5, 23–26 (2006). https://doi.org/10.1038/nmat1550

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