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Kinetics of the self-assembly of nanocrystal superlattices measured by real-time in situ X-ray scattering

Abstract

On solvent evaporation, non-interacting monodisperse colloidal particles self-assemble into a close-packed superlattice. Although the initial and final states can be readily characterized, little is known about the dynamic transformation from colloid to superlattice. Here, by using in situ grazing-incidence X-ray scattering, we tracked the self-assembly of lead sulfide nanocrystals in real time. Following the first appearance of an ordered arrangement, the superlattice underwent uniaxial contraction and collective rotation as it approached its final body-centred cubic structure. The nanocrystals became crystallographically aligned early in the overall self-assembly process, showing that nanocrystal ordering occurs on a faster timescale than superlattice densification. Our findings demonstrate that synchrotron X-ray scattering is a viable method for studying self-assembly in its native environment, with ample time resolution to extract kinetic rates and observe intermediate configurations. The method could be used for real-time direction of self-assembly processes and to better understand the forces governing self-organization of soft materials.

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Figure 1: Characterization of nanocrystals.
Figure 2: X-ray scattering from the colloidal form (initial state) and the self-assembled superlattice (final state).
Figure 3: Time-resolved X-ray scattering reveals the transition from a disordered colloid to a highly ordered superlattice.
Figure 4: Kinetics of structural rearrangement during self-assembly.
Figure 5: Overall depiction of nanocrystal self-assembly highlighting the transition from fcc to bcc superlattice states.

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Acknowledgements

We thank R. Li for assistance with the experimental set-up and measurements. This work was supported as part of the Center for Excitonics, an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences under award DE-SC0001088 (MIT). This work is based on research conducted at the Cornell High Energy Synchrotron Source (CHESS), which is supported by the National Science Foundation and the National Institutes of Health/National Institute of General Medical Sciences under NSF award DMR-1332208. This work made use of the MRSEC Shared Experimental Facilities at MIT, supported by the National Science Foundation under award number DMR-08-19762. M.C.W. was supported by the National Science Foundation Graduate Research Fellowship under grant number 1122374.

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M.C.W. and W.A.T. conceived and designed the project. D.-M.S. conceived and designed the in situ X-ray scattering system. M.C.W. synthesized materials and performed all experiments. M.C.W. and D.-M.S. indexed and analysed the X-ray scattering patterns. All authors discussed the results and interpretation. M.C.W. wrote the manuscript with contributions from the other authors.

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Correspondence to William A. Tisdale.

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The authors declare no competing financial interests.

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Weidman, M., Smilgies, DM. & Tisdale, W. Kinetics of the self-assembly of nanocrystal superlattices measured by real-time in situ X-ray scattering. Nature Mater 15, 775–781 (2016). https://doi.org/10.1038/nmat4600

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