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Hua Zhou1 Zhonghou Cai1 Zhan Zhang1 I-Cheng Tung1 Haidan Wen1

1, Argonne National Laboratory, Lemont, Illinois, United States

Ever since the storming rise of graphene, the expanding list of two dimensional material family as predicted by theorists has been experimentally verified almost in every few months in the last years. Most fundamental properties of 2D atomic thin crystals, such as morphology/geometric profiles, electronic/magnetic transports and optoelectronic responses can be investigated by various optically excited and surface force sensitive techniques like Raman/IR spectroscopy and AFM/STM probes. However, determining atomic structures of versatile 2D crystal surfaces and interfaces in the burgeoning 2D heterostructure materials remains very challenging. So far, high-resolution cross-section TEM is still the most popular and viable method to map out surface/interface atomic structures of 2D crystal and other derivative materials although the delicate interface bonding can be undesirably vulnerable to electron-beam effects. Synchrotron-based surface X-ray diffraction, in particular crystal truncation rod (CTR) technique, can render a complete and precise atomic structure of single crystals and high quality epitaxial thin films/heterostructures in non-destructive manner. Nevertheless, the miniature lateral dimension (e.g. less than a few to tens of microns) of most 2D flakes and heterostructures makes conventional surface X-ray diffraction almost impractical to map out the complete Bragg rod so as to extract the complete atomic structures. Moreover, structural and electronic phases of some unique 2D crystals are strikingly controllable by strain applied by the underlying substrate or support when it has a large surface curvature, which for certain throws another big technical barrier for any surface-sensitive X-ray techniques.

High-brilliance, high flux synchrotron source and state-of-art focusing optics capable of routinely realizing nanobeam below 100 nm makes X-ray nanodiffraction, even surface X-ray nanodiffraction become practical and user-accessible. In this talk, I will discuss the feasibility of surface X-ray nanodiffraction measurements, and then demonstrate two most recent intriguing practices on investigating 2D atomic thin crystal and Lego-style 2D heterstructures. In one case, surface nanodiffraction helps to map out the complete specular CTR of a high quality graphene-hexagon BN heterostructure. The resolved interfacial atomic structures suggest a subtle variation of interfacial van-der-waals bonding between exfoliated and CVD grown 2D thin crystals. In another example, surface nanodiffraction allowed for precise determining in-plane lattice expansion of miniature MoS2 2D flakes vapor grown on highly curved glass spheres, which provides an excitingly new approach to effectively manipulate electronic band valley structures. In summary, surface X-ray nanodiffraction brings about significant opportunities for us to explore new two-dimensional materials, unravel emergent phenomena, and develop novel functionalities.

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