Spherical Aberration Corrected Z-STEM Characterization of CdSe and CdSe/ZnS Nanocrystals
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Spherical Aberration Corrected Z-STEM Characterization of CdSe and CdSe/ZnS Nanocrystals James McBride, Tadd C. Kippeny, Stephen J. Pennycook1, and Sandra J. Rosenthal Vanderbilt University, Nashville TN, 37235 U.S.A 1 Condensed Matter Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 U.S.A.
ABSTRACT Spherical aberration corrected Atomic Number Contrast Scanning Electron Microscopy (Z-STEM) has recently demonstrated an amazing ability to not only obtain sub-angstrom levels of detail but also yield chemical information at that level as well. With an optimal probe size of 0.8 Å, extremely detailed images of CdSe nanocrystals were obtained showing the lattice structure and surface morphology. As an example of the usefulness of this technique, a sample of CdSe nanocrystals prepared using trioctylphosphine oxide (TOPO) as the surfactant was compared to a sample of CdSe prepared using a mixture of TOPO and hexadecylamine (HDA) as the surfactant. The TOPO/HDA nanocrystals exhibit a narrower size distribution and several orders of magnitude greater fluorescence compared to that of the TOPO only nanocrystals. Interestingly, the Z-STEM images show a striking difference in nanocrystal morphology as the result of the addition of HDA to the reaction mixture. This result suggests surface morphology can be tuned through judicious choice of surfactant. A second example of ZSTEM imaging involves the characterization of CdSe/ZnS core/shell nanocrystals. The mass contrast afforded by Z-STEM can easily distinguish between core and shell. INTRODUCTION Semiconductor nanocrystals are of primary interest to several fields of research due to their unique properties. They exhibit quantum confinement effects such as size dependent absorption and emission, which allows for size tunable optical and electronic properties for applications in photovoltaics [1-3], photo-catalysis [4-7], bioassays [3, 812] and electronics [13-16]. These unique properties are sensitive to miniscule changes in size and shape requiring a characterization technique with sub-nanometer level of sensitivity and precision. Furthermore, information regarding surface structure would be an enormous asset for the development of nanocrystal-based devices and applications. For example, core/shell nanocrystals systems rely on a surface-passifying layer in order to obtain high fluorescent yields. Surface coverage on the order of a monolayer can significantly impact the fluorescent quantum yield[17-21]. Bulk characterization techniques such as XRD, absorption and fluorescence measurements only give detail about the ensemble of nanocrystals forming an average, yet incomplete, picture of the nanocrystal sample. Electron microscopy techniques such as high-resolution transmission electron microscopy (HRTEM) have been the primary technique used to characterize nanocrystal
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size and shape. HRTEM allows for analyzing individual nanocrystals and obtaining an average size distribution [22, 23]. Unfortunately, it can be very difficult to determine the p
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