Electron tomography is widely used to understand the 3D structure of materials by acquiring a series of images with different tilt angles. The Analytical Instrumentation Facility (AIF) at NC State has recently developed the STEM tomography technique using a Fischione Instruments Model 2020 Single Tilt Tomography holder.
- Acquisition of tilt series up to ±70°.
- 3D structure determination and visualization.
- High spatial resolution (on the order of nanometers) of 3D structure.
- Complementary structural and chemical information from conventional TEM techniques can be obtained in the same session.
- Compatible with all samples prepared by conventional sample preparation methods.
This technique can be extended in the following ways:
- EDS tomography is possible by acquiring element maps at different tilt angles. Correlation of the EDS datasets to the STEM tomography reconstruction can reveal the 3D chemical information of the materials.
- STEM images acquired using different acceptance angle range and combination of different detectors to enhance certain contrast can reveal structural information such as 3D distribution of defects within materials.
A Recent Electron Tomography Example
Using the Titan scanning transmission electron microscopy, we are able to achieve nanometer-level spatial resolution in the reconstructed 3D structure of porous carbon spheres with impregnated Pt nanoparticles. Spatial distribution of extremely small Pt nanoparticles (1-3 nm) inside the carbon sphere and larger Pt nanoparticles (5-10 nm) can be visualized and analyzed from tomography reconstruction.
Industry usage of porous carbons include support membranes, gas adsorbents, electrodes for fuel cells, electrochemical capacitors, and catalyst supports. Impregnated Pt catalyst particle distribution is clearly observed in the tomographic reconstruction. Knowledge of the location for these catalyst particles will allow for improved synthesis methods, which will improve both coverage and penetration of impregnated catalyst particles .
This example is from carbon spheres derived from the pyrolysis of a polymer blend mixed from polyfurfuryl alcohol and polyethylene glycol diacid precursors. Platinum nanoparticles on the order of 1-10 nm were impregnated. The TEM sample was prepared by crushing the carbon powder using a mortar and pestle, followed by dispersion in isopropanol and sonication for one hour. Twenty microliters of solution was then drop cast onto a Cu Lacey-Carbon TEM grid. The image (left) shows a representative frame of experimental tomography dataset acquired between tilt angles from -70.8 to 64 degree. The large white carbon sphere is decorated with small Pt nanoparticles. The image (right) is a representative tomography reconstruction showing the carbon sphere matrix (red), impregnated nanoparticles (blue) and nanoparticles on the surface (yellow). Full movies showing all frames are available on the AIF website. (Courtesy of Center for Dielectrics and Piezoelectrics: David Monroe and Beth Dickey: NCSU, Ramakrishnan Rajagopalan: PSU)
- Peer, Maryam, Ali Qajar, Ramakrishnan Rajagopalan, and Henry C. Foley. “On the Effects of Emulsion Polymerization of Furfuryl Alcohol on the Formation of Carbon Spheres and Other Structures Derived by Pyrolysis of Polyfurfuryl Alcohol.” Carbon 51 (2013): 85-93.