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Structural basis for near unity quantum yield core/shell nanostructures

James McBride,† Joe Treadway,§ L. C. Feldman,‡,| Stephen J. Pennycook,| and Sandra J. Rosenthal*,†,|

Nanomaterials are destined to be the building blocks for future innovative devices because of their promise of revolutionary advances in photovoltaics, fuel cells, material composites, catalysis, and even drug discovery. Fluorescence imaging is one such field that has been advanced with the recent introduction of highly fluorescent quantum dots. Aberration-corrected Z-contrast images of core/shell nanocrystals show clear correlations between structure and quantum efficiency. Uniform shell coverage is obtained only for a graded CdS/ZnS shell material and is found to be critical to achieving near 100% quantum yield. The sublattice sensitivity of the images confirms that preferential growth takes place on the anion-terminated surfaces, resulting in the three-dimensional “nanobullet” shape observed in the case of core/shell nanorods.


Preferential facet coverage. The high magnification view of a nanobullet (A) definitively reveals the sublattice polarity in the CdS and CdSe regions from the relative intensity, shown by line traces (C). A model structure commonly assumed for nanocrystals (B) can be directly compared to the image. Comparing the image with the model illustrates how the shell must be growing at a higher rate only on the Se rich facets to form the bullet shape, as shown schematically in (D). The red arrows indicate the c axis while blue arrows indicate the Se-rich (101) facets. The scale bar in (A) corresponds to a length of 3 nm.
 

 

 

† Department of Chemistry, Vanderbilt University
‡ Department of Physics and Astronomy, Vanderbilt University
§ Quantum Dot Corporation
| Oak Ridge National Laboratory Condensed Matter Sciences Division

   
 

 Oak Ridge National Laboratory