X-ray electron density investigation of chemical bonding in van der Waals materials

Abstract

Van der Waals (vdW) solids have attracted great attention ever since the discovery of graphene, with the essential feature being the weak chemical bonding across the vdW gap. The nature of these weak interactions is decisive for many extraordinary properties, but it is a strong challenge for current theory to accurately model long-range electron correlations. Here we use synchrotron X-ray diffraction data to precisely determine the electron density in the archetypal vdW solid, TiS2, and compare the results with density functional theory calculations. Quantitative agreement is observed for the chemical bonding description in the covalent TiS2 slabs, but significant differences are identified for the interactions across the gap, with experiment revealing more electron deformation than theory. The present data provide an experimental benchmark for testing theoretical models of weak chemical bonding.

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Fig. 1: Structure of 1T-TiS2.
Fig. 2: Static deformation density and Laplacian for intralayer Ti−S interaction.
Fig. 3: Static deformation density and Laplacian for interlayer S···S interaction.

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Acknowledgements

This work was supported by the Danish National Research Foundation (DNRF93), the Danish Center for Synchrotron and Neutron Science (DanScatt), the JSPS Bilateral Open Partnership Joint Research Projects for 2015−2017 and 2017−2019, the International Education and Research Laboratory Program and the International Tenure Track system of Univ. Tsukuba. The synchrotron experiment was performed at SPring-8 BL02B1 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) as a Partner User (Proposal No: 2015A0078). The theoretical calculations were performed at the Center for Scientific Computing, Aarhus.

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B.B.I designed and coordinated this study. M.Ø.F. synthesized the samples; H.K., K.T., M.S., V.R.H. and K.S. performed the experiments; H.K. and K.T. analyzed the data, with discussions with M.S., V.R.H., J.O., E.N. and B.B.I.; J. Z. and S.C. performed the theoretical calculations. H.K. and B.B.I. wrote the manuscript. All authors discussed the experimental results and contributed to the manuscript.

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Correspondence to Bo B. Iversen.

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Supplementary Tables 1–10, Supplementary Figures 1–12, Supplementary References 1–23

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Kasai, H., Tolborg, K., Sist, M. et al. X-ray electron density investigation of chemical bonding in van der Waals materials. Nature Mater 17, 249–252 (2018). https://doi.org/10.1038/s41563-017-0012-2

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