Anisotropic structural dynamics of monolayer crystals revealed by femtosecond surface x-ray scattering

28 Nov 2018  ·  I-Cheng Tung, Aravind Krishnamoorthy, Sridhar Sadasivam, Hua Zhou, Qi Zhang, Kyle L. Seyler, Genevieve Clark, Ehren M. Mannebach, Clara Nyby, Friederike Ernst, Diling Zhu, James M. Glownia, Michael E. Kozina, Sanghoon Song, Silke Nelson, Hiroyuki Kumazoe, Fuyuki Shimojo, Rajiv K. Kalia, Priya Vashishta, Pierre Darancet, Tony F. Heinz, Aiichiro Nakano, Xiaodong Xu, Aaron M. Lindenberg, Haidan Wen ·

X-ray scattering is one of the primary tools to determine crystallographic configuration with atomic accuracy. However, the measurement of ultrafast structural dynamics in monolayer crystals remains a long-standing challenge due to a significant reduction of diffraction volume and complexity of data analysis, prohibiting the application of ultrafast x-ray scattering to study nonequilibrium structural properties at the two-dimensional limit. Here, we demonstrate femtosecond surface x-ray diffraction in combination with crystallographic model-refinement calculations to quantify the ultrafast structural dynamics of monolayer WSe$_2$ crystals supported on a substrate. We found the absorbed optical photon energy is preferably coupled to the in-plane lattice vibrations within 2 picoseconds while the out-of-plane lattice vibration amplitude remains unchanged during the first 10 picoseconds. The model-assisted fitting suggests an asymmetric intralayer spacing change upon excitation. The observed nonequilibrium anisotropic structural dynamics in two-dimensional materials agrees with first-principles nonadiabatic modeling in both real and momentum space, marking the distinct structural dynamics of monolayer crystals from their bulk counterparts. The demonstrated methods unlock the benefit of surface sensitive x-ray scattering to quantitatively measure ultrafast structural dynamics in atomically thin materials and across interfaces.

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Materials Science