Imaging photoexcited states at the 1 nanometer scale using electron microscopy
- Photons are the primary carriers of energy that drive life on earth. Photo-absorption of visible light involving an electronic transition forms the first step in ubiquitous processes such as photosynthesis, atmospheric chemistry processes, and is harnessed by us in photovoltaic devices. Imaging photoexcited states at the nanometer (nm) scale therefore offers fundamental insights about photo-excitation and recombination pathways, with numerous applications in photocatalysis, semiconductor physics, quantum emitters and so on. However, imaging photo-excited states at the 1 nm scale is nontrivial. As was discovered in the 19th century, traditional optical imaging techniques are limited by the Abbe diffraction limit to around 0.5 μm for visible light. Therefore, while optical microscopes have uncovered the microbial world, their resolution limit does not permit the observation of photoexcited states in systems like viruses (100 nm), proteins (10 nm), quantum dots (few nm) and atomic chemical bonds (1 Angstrom). Various optical super-resolution techniques have therefore been developed to address this problem. Some techniques employ clever work-arounds in Abbe diffraction limited systems such as centroid localization of stochastic emission to get a higher resolution reconstruction, an example of which is PhotoActivated Localization Microscopy (PALM) which won the 2014 Chemistry Nobel Prize. A second set of approaches use a near field evanescent optical source, such as a sharp metallic tip that scatters confined light near the sample. However, tip convolution artefacts make image contrast difficult to interpret. Because of all of these limitations, 1 nm scale photoexcited state imaging is not yet routine. Here we developed a new nanometer scale imaging technique called 'Photo Absorption Microscopy using Electron Analysis', or PAMELA, which uses high energy electrons to probe optically excited states of matter with nanometer resolution. We developed a conceptual basis for PAMELA in a Scanning Electron Microscope (SEM) and a Transmission Electron Microscope (TEM). We then implemented PAMELA in our in-house SEM showing, achieving about 10-20 nm resolution and deducing that surface photovoltage was the dominant contrast mechanism. We then attempted to improve the spatial resolution to 1 nm using TEM and STEM, but were limited by various instrumental limitations. Finally, we using aberration corrected SEM in collaboration with the Nion company (Kirkland, WA), we improved upon our previous result by 10x and achieved 1 nm resolution photo-absorption imaging. We then explored the resolution limits of secondary electron imaging using a sub-Angstrom probe and showed experimentally that the contrast mechanism behind atomic scale SE was core electron excitation events. We believe that these results are the beginning for PAMELA, and we envision a long and productive road ahead.
|Type of resource
|electronic resource; remote; computer; online resource
|1 online resource.
|Degree committee member
|Degree committee member
|Stanford University, School of Engineering
|Stanford University, Department of Mechanical Engineering
|Statement of responsibility
|Submitted to the Department of Mechanical Engineering.
|Thesis Ph.D. Stanford University 2023.
- © 2023 by Joel Martis
- This work is licensed under a Creative Commons Attribution Non Commercial Share Alike 3.0 Unported license (CC BY-NC-SA).
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