Development of STEM-Holography

Abstract

We have developed a new technique we call STEM-holography, inspired by J. M. Cowley [1], to measure the phase imparted on an electron after transmission through thin, low-atomic number specimens via phase contrast. Low-atomic number materials play a crucial role in life sciences, medicine, and the carbon energy cycle. However, our ability to image these materials at the atomic length scale is limited because they do not scatter electrons at high-angles in the same way a crystalline or high atomic number material does. Additionally, these materials are easily damaged under electron beam illumination. To get around these issues, bold efforts have been made in the fields of electron holography [2] and ptychography [3, 4], leading to myriad techniques that can potentially achieve sub-nanometer resolution. Additionally, off-axis electron holography has been developed and applied in many research groups [5 -8], pushing the boundaries of electron microscopy with unprecedented feats such as the atomic resolution electrostatic potential mapping of graphene sheets [9]. We utilize a different configuration of electron holography, inspired by a 1990 proposal by J. M. Cowley. As illustrated in Fig.1, the electron beam travels down the microscope column to the Condenser 2 aperture, where it diffracts through a highly efficient phase grating into electron probes with tens of nanometers spatial separation. The specimen is positioned such that all three diffraction probes initially pass through vacuum. The probes are then scanned such that the +1 order probe interacts with the specimen while the 0 and -1 order probes pass through vacuum. They are then recombined through the post specimen optics and interfere at the CCD camera. We performed this experiment on an FEI 80-300 Titan TEM at 300 kV in STEM mode. A 50 μm diameter, 100 nm pitch grating is positioned in the C2 aperture plane. The test sample we used was graphitic carbon and gold nanoparticles on amorphous carbon. The interference fringes formed on the CCD camera created from such a setup should have the form í µí°¼ ! (í µí±¥ !) ≡ í µí¼“ í µí±˜ ! = í µí° !