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Z-Contrast Stem Imaging and Eels of CdSe Nanocrystals: Towards the Analysis of Individual Nanocrystal Surfaces

  • A.V. Kadavanich (a1) (a2), T. Kippeny (a1), M. Erwin (a1), S. J. Rosenthal (a1) and S. J. Pennycook (a2)...

Abstract

We have applied Atomic Number Contrast Scanning Transmission Electron Microscopy (Z-Contrast STEM) and STEM/EELS (Electron Energy Loss Spectroscopy) towards the study of colloidal CdSe semiconductor nanocrystals embedded in MEH-PPV polymer films. Z-Contrast images are direct projections of the atomic structure. Hence they can be interpreted without the need for sophisticated image simulation and the image intensity is a direct measure of the thickness of a nanocrystal. Our thickness measurements are in agreement with the predicted faceted shape of these nanocrystals.

Our unique 1.3Å resolution STEM has successfully resolved the sublattice structure of these CdSe nanocrystals. In [010] projection (the polar axis in the image plane) we can distinguish Se atom columns from Cd columns.

EELS measurements on individual nanocrystals indicate a significant amount (equivalent to 0.5–1 surface monolayers) of oxygen on the nanocrystals, despite processing in an inert atmosphere. Spatially resolved measurements at 7Å resolution suggest a surface oxide layer.

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