Getting the most sensitive samples to reveal their finest detail under the microscope

Prof. Boris Rybtchinski

Low-dose approaches to high-resolution microscopy are pursued in a number of projects with research groups at the WIS; these activities target at high-spatial resolution information about beam-sensitive organic materials or hybrid organic/inorganic materials, their emerging order upon condensation into the solid state from solution, and their ordering into a crystalline structure. Real-space imaging and order analysis from scanning nanobeam diffraction data with a finely focussed electron beam obtained on cutting-edge instrumentation helps us to obtain microstructure data up to sub-molecular resolution. 
Following the rapid advancement of aberration-corrected electron optics allowing for the highest spatial resolution, the pivotal aspect of beam current for beam-sensitive materials became undeniable for atomic-resolution microscopy. We address these challenges through a combination of new developments in collaboration between research groups at the WIS and the EM Unit.
The Boris Rybtchinski group and the EM Unit introduced low-dose focal-series reconstruction (LDFSR), a phase retrieval method implemented for the first time for counting mode detection of sparse image data. LDFSR takes full benefit of the high-duty cycle of a monolithic direct electron detector at electron counting mode to record a stack of a few hundred images in a few seconds under simultaneous fast-focus ramping to minimize radiation damage [1]. Using LDFSR on a conventional dedicated cryo TEM, such as a Titan Krios, a spatial resolution of 2 Angstrom for a dose budget of as little as 100 e-/Å2 is obtained on organic thin films. Figure 1 shows an example of a new polymorph in copper-phthalocyanine [1]. 
Scanning nanobeam diffraction is a complementary approach that is well suited for the nano-characterization of emerging or partial crystalline order in organics. Crystal phasing of nanoparticles of metal organic frameworks is part of the ongoing conjoint methodological development by the group of Prof. Milko van der Boom and the EM Unit. Here, the approach takes advantage of hybrid-pixel detectors and their single electron sensitivity. Figure 2 shows the characterization of metal-organic framework nanoparticles with a non-trivial morphology as an example. The data was obtained on an EMPAD hybrid-pixel detector with single-electron sensitivity and high DQE, quantitative evaluation reveals condensation into a tetragonal phase and a single crystal. Such methodology will be extended for the investigation of nucleation phenomena and the spatially sensitive polymorph identification in nanocrystalline organic thin films or nanoparticles. 
Our methodological achievements demonstrate unprecedented opportunities for dose-efficient characterization of beam-sensitive materials with organic components.

Figure 1: The Stars-and-stripes copper-phthalocyanine polymorph after exit-plane wave reconstruction [1]. (a) Phase images in the Stars-and-stripes orientation, reconstructed from a fast focal series taken in 4 s on a K3 camera in a Titan Krios with a total fluence of 340 e-/Å2. (b) Magnified area with a DFT-optimized model structure. (c) DFT-optimized model in <001> viewing direction. The inset in panels a is a forward-simulation based on the DFT-optimized model.

Figure 2: A pita-shaped MOF crystal built from achiral ligands coordinated by copper atoms in 4D STEM. The 4D-STEM datasets were obtained with a scanning nanobeam of 30 nm diameter with a total fluence of 0.5 e-/Å2 and recorded on an EMPAD hybrid-pixel detector in a Themis-Z instrument under cryo-conditions. (a) Virtual bright-field image of a single structure and region of interest diffraction images extracted from the 4D dataset. The diffraction data reveals identical zone-axis patterns and orientation. (b) Refined Debye-Scherrer pattern and inverse Bragg distance histogram plotted after peak refinement across the 4D-STEM diffraction patterns of approximately 80 structures. About 160000 reflections were detected, the histogram of inverse lattice spacings (green plot) shows fine detail despite the small sampling array on the EMPAD detector of 128x128 pixels. The histogram data matches formidably the X-ray powder diffraction plot and corresponds to a tetragonal crystal structure with  spacegroup 92 (simulated powder diffraction: black lines). E. Ben-Zikry, Michal Lahav, M. v.d. Boom, L. Houben. 


Further reading:
[1] Biran, I., Houben, L., Weissman, H., Hildebrand, M., Kronik, L., Rybtchinski, B., Real-Space Crystal Structure Analysis by Low-Dose Focal-Series TEM Imaging of Organic Materials with Near-Atomic Resolution. Adv. Mater. 2022, 34, 2202088.