Center for Functional Nanomaterials Seminar

Atomic resolution scanning transmission electron microscopy (STEM) has become an essential tool in materials science due to its ability to determine which atoms are where. However the Z-contrast annular dark field (ADF) imaging that is synonymous with STEM has its limitations. The method is relatively insensitive to light elements, and makes inefficient use of the scattered electrons. For locating light elements near heavy elements one can turn to spectroscopic imaging, but this is slow and requires high electron doses, making it susceptible to contamination, distortion from sample drift, and challenging to perform for materials that are susceptible to beam damage. Ptychography provides highly dose efficient imaging that is sensitive to both heavy and light elements, but was until recently held back by the slow scan speeds imposed by the cameras used to record the diffraction patterns it requires. By introducing event driven 4D STEM with nanosecond time resolution we have overcome this limitation, allowing us to perform simultaneous Z-contrast and ptychography without any loss of scan speed. This is a powerful combination, providing the interpretability of Z-contrast with the sensitivity of phase imaging. With its resilience to temporal incoherence, ptychography can exceed the dose efficiency of conventional TEM, opening opportunities for beam sensitive materials ranging from energy applications to proteins. I will present ultralow dose imaging of a halide perovskite, including the first atomic resolution imaging of a halide perovskite edge. Using dose fractionation we are able to capture dynamics and correlate damage rates with different defect types, providing important insights that could help stabilize these fragile materials. For more robust materials, ptychography can provide higher precision imaging. Superresolution can be used to resolve finer spacings, or as I will show, the phase sensitivity and accuracy can be used to detect subtle electronic effects such as charge transfer due to bonding. Key to this is the ability to correct the residual aberrations that would otherwise obscure the charge transfer even on an aberration corrected microscope. Bio: Timothy Pennycook is a specialist in advanced scanning transmission electron microscopy, currently focusing on 4D STEM and electron ptychography development and applications. He received his PhD from Vanderbilt University in 2012 and has held positions with the Universities of Oxford, Vienna, Antwerp, and the Max Planck Institute for Solid State Research.