High-resolution electron tomography on beam-sensitive carbon materials

In this work a protocol for bright-field electron tomography in the transmission electron microscope (TEM) is developed. This protocol provides high resolution in 3D better than 1nm. It enables to study materials prone to radiolysis effects such as carbon materials. For the latter an aberration corrected TEM operated at a low acceleration voltage of 80kV is necessary to reduce the knock-on related beam damage. This protocol is exemplified on commercially available Vulcan XC-72 carbon soot particles which serve as a support for catalytically active nanoparticles (here platinum). This approach enables to visualize and quantify the internal three-dimensional graphitic nanostructure with ~0.35nm graphitic interplanar distance by layer distance, fringe length and tortuosity providing statistical measures for the 3D carbon volume. The protocol ensures that the sample withstands the entire tilt series acquisition by a minimum dose protocol. The appropriate imaging conditions are specified and experimentally preserved by an automatic focus and stigmation (AFS) module correcting focus and astigmatism intermittently at each tilt angle with a high accuracy required for aberration corrected high-resolution TEM. A new iterative self-consistent alignment strategy ensures high resolution in the three-dimensional reconstruction. The quantification of the entire three-dimensional carbon volume is shown to be superior to a two-dimensional projection analysis and making calibrations with x-ray data redundant.