2D Materials Surface Crystal Structure Characterization via Dynamical µLEED/LEEM

Two-dimensional (2D) materials have attracted much attention over the past decade due to their novel mechanical, optical and electronic properties with many potential applications in photovoltaics, quantum computing, and other modern electronics. However, the detailed atomic structural information has been rarely experimentally investigated due to the following difficulties: (i) the limited sample size of 2D materials prepared through mechanical exfoliation of a few µm, and (ii) easy oxidation and (iii) surface instability of various 2D materials under high energy probing techniques. Selected area low-energy electron diffraction analysis (µLEED-IV) performed in a low-energy electron microscope (LEEM), is a highly surface sensitive and non-intrusive surface characterization technique, which also has the advantage of µm-size sampling selectivity. Here I present the first detailed experimental characterizations of atomic crystal structures of a series of technologically promising 2D materials: MoS₂, black phosphorus (BP), the topological crystalline insulator (TCI) SnSe, and tungsten doped MoTe₂. Furthermore, α-RuCl₃, a new member of the 2D material family has recently been reported to be a candidate for hosting the quantum spin liquid state (QSLs). Experimental reports of neutron scattering and thermal quantum Hall effect have provided indirect but promising evidence for the existence of QSLs in α-RuCl₃. However, numerous controversies still remain regarding its crystal and electronic structure. In our preliminary measurements we have observed an extra set of diffraction spots originated from the surface of α-RuCl₃, which indicates intrinsic surface reconstruction that may have a significant impact on its ability to host QSLs