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Diffraction imaging of nanocrystalline structures in organic semiconductor molecular thin films

Abstract

The properties of organic solids depend on their structure and morphology, yet direct imaging using conventional electron microscopy methods is hampered by the complex internal structure of these materials and their sensitivity to electron beams. Here, we manage to observe the nanocrystalline structure of two organic molecular thin-film systems using transmission electron microscopy by employing a scanning nanodiffraction method that allows for full access to reciprocal space over the size of a spatially localized probe (~2 nm). The morphologies revealed by this technique vary from grains with pronounced segmentation of the structure—characterized by sharp grain boundaries and overlapping domains—to liquid-crystal structures with crystalline orientations varying smoothly over all possible rotations that contain disclinations representing singularities in the director field. The results show how structure–property relationships can be visualized in organic systems using techniques previously only available for hard materials such as metals and ceramics.

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Fig. 1: Schematic of the diffraction imaging technique.
Fig. 2: Comparison of grain morphology between DIO samples.
Fig. 3: Comparison of grain morphology between PBTTT samples.
Fig. 4: Autocorrelation of the primary lattice vectors over distance Δr and relative orientation Δθ, for the films characterized in Figs. 2 and 3.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Acknowledgements

Primary funding for the work was provided by the Electron Microscopy of Soft Matter Program from the Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division of the US Department of Energy under contract no. DE-AC02-05CH11231. Work at the Molecular Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the US Department of Energy under contract no. DE-AC02-05CH11231. L.B. and A.S. acknowledge funding from the National Science Foundation DMR Award no. 1808401. The authors also thank H. Yan for providing materials and M. Toney for useful discussions.

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A.M.M. and N.B. conceived of the project. C.J.T., L.B. and O.P. prepared the samples. O.P. and K.C.B. designed the experiment and acquired the data. C.O., O.P., K.C.B., C.J.T., L.B., A.S. and A.M.M, analysed the data and all authors contributed to writing the manuscript.

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Correspondence to Andrew M. Minor.

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Panova, O., Ophus, C., Takacs, C.J. et al. Diffraction imaging of nanocrystalline structures in organic semiconductor molecular thin films. Nat. Mater. 18, 860–865 (2019). https://doi.org/10.1038/s41563-019-0387-3

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  • DOI: https://doi.org/10.1038/s41563-019-0387-3

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