Invited Paper TF-MoM10
PACSURF 2016 PLENARY LECTURE: Detecting the Invisible with Electron Beams: The Hidden Secrets of Nanocrystals, Interfaces and Surfaces at Atomic Resolution
Monday, December 12, 2016, 11:00 am, Room Makai
As heterogeneous materials scale below 10 nm, a suitable combination of single digit nanocrystals with their rich variety of tunable surfaces and interfaces allows tailoring unprecedented materials with novel structure-function relationships. The design of new catalysts [1] or investigations of polymers at atomic resolution [2] may serve as examples. This contribution describes research that aims at exploiting the emerging ability to analyze and understand such materials by directly determining their atom arrangement in three-dimensions using aberration-corrected transmission electron microscopy [3]. Attempts to unravel the atomic structure of such nanoscale composites in this manner must explicitly address their pronounced sensitivity to the probing radiation that can unintentionally alter their pristine structure, often beyond recognition. We address this challenge by applying low dose-rate in-line holography [4], which allows operating electron microscopes with dose rates as low as 5-10 e/Å2s that help maintaining structural integrity at atomic resolution to an unexplored end. The approach mimics best practices in biological research but achieves atomic resolution with single atom sensitivity by the acquisition of large image series. In essence, the method captures series of entirely noise dominated single images that are reconstructed to obtain electron exit wave functions of the radiation sensitive matter with unprecedented contrast and resolution. We observe a variety of previously unknown atom configurations in surface proximity of CoOx nanocrystals and coatings that are hidden behind unusually broadened diffraction patterns but become visible in real space images because the phase problem is solved. The observed structures are drastically altered by an exposure of the material to water vapor or other gases, which is investigated at atomic resolution in environmental electron microscopy. It is shown for Rh/W catalysts that electron beam-induced atom dynamics can be entirely suppressed even for atom clusters made from less than 10 atoms. Resultantly, chemical compositions can be determined by contrast measurements alone and functional processes can be triggered and tracked in real time at atomic resolution.
[1] J. A. Haber et al., Multiphase Nanostructure of a Quinary Metal Oxide Electrocatalyst Reveals a New Direction for OER Electrocatalyst Design, Advanced Energy Materials 5 (2015) 1402307
[2] D. Lolla et al., Polyvinylidene fluoride molecules in nanofibers, imaged at atomic scale by aberration corrected electron microscopy, Nanoscale 8 (2016) 120 - 128
[3] F.R. Chen et al., In-line three-dimensional holography of nanocrystalline objects at atomic resolution,Nature Commun. 7:10603 doi: 10.1038/ ncomms10603 (2016)
[4] C. Kisielowski, Observing atoms at work by controlling beam-sample interactions, Advanced Materials27 (2015) 5838-5844