Probing Light Atoms at Sub-nanometer Resolution: Realization of Scanning Transmission Electron Microscope Holography

Abstract

Atomic resolution imaging in transmission electron microscopy (TEM) and scanning TEM (STEM) of light elements in electron-transparent materials has long been a challenge. Biomolecular materials, for example, are rapidly altered when illuminated with electrons. These issues have driven the development of TEM and STEM techniques that enable the structural analysis of electron beam-sensitive and weakly scattering nano-materials. Here, we demonstrate such a technique, STEM holography, capable of absolute phase and amplitude object wave measurement with respect to a vacuum reference wave. We use an amplitude-dividing nanofabricated grating to prepare multiple spatially separated electron diffraction probe beams focused at the sample plane, such that one beam transmits through the specimen while the others pass through vacuum. We raster-scan the diffracted probes over the region of interest. We configure the post specimen imaging system of the microscope to diffraction mode, overlapping the probes to form an interference pattern at the detector. Using a fast-readout, direct electron detector, we record and analyze the interference fringes at each position in a 2D raster scan to reconstruct the complex transfer function of the specimen, t(x). We apply this technique to image a standard target specimen consisting of gold nanoparticles on a thin amorphous carbon substrate, and demonstrate 2.4 angstrom resolution phase images. We find that STEM holography offers higher phase-contrast of the amorphous material while maintaining Au atomic lattice resolution when compared with high angle annular dark field STEM.