Seeing structural evolution of organic molecular nano-crystallites using 4D-scanning confocal electron diffraction (4D-SCED)

Abstract text (incl. figure legends and references)

Direct observation of organic molecular nanocrystals using electron microscopy is highly challenging, due to their radiation sensitivity and complex structure. 4D-STEM nano-beam diffraction (NBD) using small convergence α has been demonstrated to reveal the rich structural information in soft materials [1]. In this context, homogeneous beam-specimen interaction is desired to maximize dose/information efficiency before total structural damage, and spot diffraction signals are preferred for sensitivity and accuracy of Bragg peaks detection in dense reciprocal lattices typical in organic crystals. In NBD, the beam illuminates the sample with an Airy disk profile, and diffraction disks size is coupled to α, not optimized for dose efficiency and Bragg peaks detection due to spread signals and possible disks overlapping. Here, we introduce 4D-scanning confocal electron diffraction (4D-SCED) which combines high dose efficiency and high angular resolution. 4D-SCED applies defocused pencil beam illumination (Fig. 1b) on the sample and combines a confocal electron optic setup with a camera to record spot diffraction patterns (Fig. 1c). The defocused illumination reduces the dose with a homogenous beam sample interaction, and the confocal optics transfer spot-like diffraction signals, optimized for Bragg peaks detection. The spatial and angular resolution can be estimated using geometric considerations (Fig. 1d), which is largely decoupled from α.

We first compare SCED and NBD with a single crystal thin film of 2D molecular α,ω-DH6T bilayer (fig. 1e-h). An order-of-magnitude higher diffraction signal is seen in SCED compared to NBD, enabling accurate Bragg peak detection for structural studies, e.g. layer rotation relation. We apply 4D-SCED to study an active layer in organic solar cells, DRCN5T:PC71BM BHJ thin films after solvent vapor annealing (Fig. 2a-c). With careful balancing spatial and angular resolution for the given dose budget, structural details of DRCN5T nano-crystallites oriented both in- and out-of-plane are imaged at ~5 nm resolution and dose budget of ~5 e-/Å2. Finally, we use 4D-SCED to study the structural evolution during thermal annealing by in situ heating the thin film in the TEM (Fig. 2d). The evolution of crystallite size (coarsening) and texture, as well as the progressive enrichment of PC71BM at interfaces are directly revealed [2].

The unique combination of high dose efficiency and high angular resolution makes 4D-SCED an ideal technique for studying beam-sensitive soft materials. The new possibilities of the technique are currently employed to explore further soft materials.
Refs:
[1] Bustillo, K., et. al., Acc. Chem. Res. 11 (2021) 2543
[2] Wu, M., et. al. Nat. Commun. 13 (2022) 2911

The authors acknowledge financial support from DFG via projects GRK1896 and SFB953. Work at the Molecular Foundry has been supported by U.S. DoE under contract No. DE-AC02-05CH11231.

Fig 1. (a) STEM setup. (b) Defocusing the probe mitigates dose for beam-sensitive samples. (c) Scheme of SCED. (d) A geometric scheme for considering spatial and angular resolution. (e-h) Comparison of NBD and SCED applied to an α,ω-DH6T bilayer.
Fig 2. Visualizing (a) edge-on (with color wheel) domain orientation and (b) face-on (grayscale) domain location of the donor crystals in DRCN5T:PCBM blends. (c) Mapping the whole area with 4D-SCED dataset. (d) The structural evolution of donor nano-crystallites during an in situ annealing experiment in the TEM.