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In this work, we analyze a 1-dimensional, atomically defined, phosphonate-capped CdSe nanomaterial using in-situ SAXS/WAXS at the P62 beamline at DESY combined with anomalous SAXS and x-ray absorption (XAS) close to the Se edge. The structure is related to the class of magic-size clusters that form in the early stages of nanocrystal synthesis and play a pivotal role in anisotropic growth of colloidal nanorods [1, 2].
The material forms a colourless gel at room temperature and melts into individual fibrils that are stable up to 310 °C. The change in the aggregation state from aggregated bundles of fibrils at room temperature to individual fibrils was studied in real time by SAXS/WAXS measurements. With this we could reveal the separation of the bundles above 60 °C and could follow the transformation to separated 1D fibrils with a diameter of only ~2 nm and above 100 nm in length. This is achieved, e.g., by analysing the slope of the SAXS intensity at low q as a function of temperature.
The small diameter of the fibrils is related to the formation of magic size clusters [3] that merge along the rotational axis into a continuous, inorganic material with a helical structure. This rod formation is only initiated by the use of a phosphonate based ligand, whereby a pure oleic acid ligand does not lead to a homogenous rod formation.
From standard SAXS analysis we found, however, that the diameter determination of the CdSe fibrils is influenced by the phosphonate shell. Thus, anomalous SAXS (ASAXS) measurements are required for retrieving the pure CdSe dimensions that can be then compared to magic-sized CdSe clusters [4].
[1] D. Wurmbrand, at al., & K. Boldt, Chem. Commun. 2018, 54, 7358.
[2] D. Fischli, F. Enders, K. Boldt, J. Phys. Chem C 2020, 124, 12774.
[3] C. B. Williamson, at al., & R. D. Robinson, Science 2016, 363, 731.
[4] K. Boldt, et al., & P. Schwarz, R.T. Lechner, to be submitted 2026