Microscopy

Featured links

STEM group at ORNL

CdSe NanoHeart !!!

NanoHeartZ-STEM image of a twined, oleic acid-capped, CdSe nanocrystal, ~ 55 Angstroms in diameter.

Electron Microscopy is one of the most important tools used forcharacterizing nanomaterials. Specifically, Transmission Electron Microscopy (TEM) can be used to determine the crystal structure and size of the nanocrystals synthesized in our lab. Information of size and shape homogeneity can be readily obtained.

At Vanderbilt University we have access to a Phillips CM 20 ElectronMicroscope which operates at 200 kV and has a resolution of ~ 0.24 nm. Additionally, the microscope has been fitted with an EDS detector for elemental analysis. Images are basically obtained by capturing the interference pattern created by the coherent electron beam interacting with a crystal lattice. This imaging mode is called Phase Contrast Imaging due to the contrast seen in the image is created by the repeating crystal lattice.

Through a collaboration with Stephen J. Pennycook at Oak Ridge National Laboratories, the Rosenthal Group has access to one of the most powerful microscopes in the world. Operating at 300 kV, the Cs corrected VG 603U Scanning Transmission Electron Microscope has a achieved a world record probe size of 0.06 nm. The imaging technique commonly used with this instrument is called Atomic Number Contrast Scanning Transmission Electron Microscopy, or Z-STEM.

Images are formed by collected the transmitted electrons that are scattered at high angles. Since this resembles a Rutherford scattering process, atoms with larger atomic numbers more frequently scatter electrons at higher angles and will appear bright in the image, while lighter atoms will scatter electrons less and will exhibit a lower intensity. This allows for elemental identification directly from the images. Also, unlike phase contrast imaging, Z-STEM images can be directly related to the structure of the object being imaged, whether it is crystalline or amorphous. This technique has been used to directly image CdSe nanocrystal structures and their surfaces. The mass contrast afforded by this technique was applied to CdSe/ZnS nanocrystals to obtain the first Z-STEM images showing clear contrast between the heavier CdSe core and the lighter ZnS shell.

References

McBride, J. R.; Treadway, J.; Feldman, L. C.; Pennycook, S. J.; Rosenthal, S. J. Nano Letters 6 pp. 1496-501 (2006)

McBride, J. R.; Kippeny, T. C.; Pennycook, S. J.; Rosenthal, S. J. Nano letters 4 pp. 1279-1283 (2004)

McBride, J. R.; Kippeny, T. C.; Pennycook, S. J.; Rosenthal, S. J. Mat. Res. Soc. Symp. Proc. 818, M8, 15.1 - 15.6