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Summary
This project develops STEM-in-SEM methods or low-energy transmission electron diffraction, imaging, and spectroscopy in the scanning electron microscope, to enable determinations of microscopic structure, defect types, and interface character in ultrathin films, nanoparticles, and nano-bio material systems, to overcome imaging and analytical challenges faced by conventional SEM and TEM methods.
Description
Transmission electron diffraction patterns obtained in a scanning electron microscope.
Established imaging and diffraction techniques for measuring structure of nanomaterials and soft matter do not show both good contrast and high resolution, and they can cause significant material damage. This is particularly the case for isolated nanostructures such as individual nanoparticles and ultrathin films. For example, identifying the crystallographic phase of an unknown nanoparticle of diameter < 5 nm is extremely difficult, even in the most powerful high-resolution TEMs commercially available today. The problem centers on the generation of electron scattering within small volumes. For structures in the size regime of 10 nm and smaller, electrons with energies of the order of 200 keV exhibit a mean free path for scattering that can easily approach an order of magnitude larger than the particle itself. Decreasing those energies to those typical of an SEM (~ 20 keV) decreases the mean free path to values commensurate with the particle size. As a result, more electrons will scatter, to provide the information-rich content needed to measure crystal phase, crystal orientation, defects, and internal order within nanostructures.
A fully analytical transmission-SEM, or STEM-in-SEM, would not only fuel more thorough characterization of nanoscale structures, enabling more precise process control and material reliability, but it would make available many powerful TEM-like capabilities to a large population of SEM users worldwide, in a diverse range of applications.
Present activities include novel electron detector development, interpretation of image contrast, and application to low-dimensional material systems.
Major Accomplishments
- Invented t-EBSD (aka transmission Kikuchi diffraction in the SEM), and demonstrated that the important Kikuchi scattering responsible for diffraction appears to occur within a few nanometers of the exit surface of the specimen.
- Obtained t-EBSD patterns from several types of nanomaterials, including HfO2 films of thickness < 5 nm, Fe-Co particles of diameter ~ 10 nm, Pt particles of diameter ~ 2 nm, Fe3O4 particles of diameter ~ 8 nm, Ag particles of diameter ~ 10 nm, Al2O3 particles of diameter ~ 20 nm.
- Demonstrated that t-EBSD measurements can be made over a broad range of film thickness, ranging from < 5 nm to > 3 μm. This can be explained by considering effects of mass-thickness on electron penetration.
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Developed a modular aperture system that significantly improves angular selectivity in the collection of electrons transmitted through a material, as compared to state of the art commercial detectors available today. Acceptance half-angles can be varied from zero to 1350 mrad, with spread controllable to single milliradians, depending on aperture fabrication methods.
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Demonstrated imaging under bright-field, annular bright-field, low-angle annular dark-field, medium-angle annular dark-field, and high-angle annular dark-field conditions – imaging modes typically reserved for the TEM, and until now unattainable as distinct contrast-inducing mechanisms in the SEM.
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Developed a programmable-aperture STEM detector capable of displaying a full diffraction pattern as well as synchronizing a portion of a diffraction pattern with the SEM scan – this results in the ability to generate images associated with any combination of scattering angles, including bright-field, annular bright-field, low-angle annular dark-field, medium-angle annular dark-field, and high-angle annular dark-field conditions. The detector is capable of defining any arbitrary subset of acceptance angles as the basis for imaging.
B.W. Caplins, J.D. Holm, and R. R. Keller, "Orientation mapping of graphene in a scanning electron microscope," Carbon 149, pp. 400-406 (2019). https://doi.org/10.1016/j.carbon.2019.04.042
B.W. Caplins, J. D. Holm, and R. R. Keller, "Transmission imaging with a programmable detector in a scanning electron microscope," Ultramicroscopy 196, pp. 40-48 (2019). https://doi.org/10.1016/j.ultramic.2018.09.006
J. D. Holm and R. R. Keller, "Angularly-selective transmission imaging in a scanning electron microscope," Ultramicroscopy 167, pp. 43-56 (2016). https://doi.org/10.1016/j.ultramic.2016.05.001
R. R. Keller and R. H. Geiss, "Transmission EBSD from 10 nm domains in a scanning electron microscope," Journal of Microscopy vol. 245, pp. 245-251 (2012). https://doi.org/10.1111/j.1365-2818.2011.03566.x
National Research Council Post-Doctoral Research Opportunities: (you will be leaving NIST website when selecting this link)
Transmission Scanning Electron Microscopy for Materials Characterization
Nanometer-Scale Characterization and Reliability of 2D Materials