Shear Anisotropy\nDomains on Graphene Revealed by In-Plane\nElastic Imaging

Anisotropic domains with 180° periodicity are known\nto be\nuniversally present on graphene as well as on other two-dimensional\n(2D) crystals. The physical origin of the domains and the mechanism\nof its anisotropy are, however, still unclear. Here, by employing\nin-plane elastic imaging by torsional resonance atomic force microscopy\n(TR-AFM), we demonstrate that the observed domains on graphene are\nof in-plane elastic (shear) anisotropy but not of friction anisotropy\nas commonly believed. Our results also support that the anisotropic\ndomains originate from self-assembled environmental adsorbates on\ngraphene surfaces. The more densely packed backbone of the highly\nordered molecules within a domain defines the major axis of the shear\nanisotropy of the latter. This work suggests a quantitative understanding\nof the characteristics of anisotropic domains on 2D materials. It\nalso demonstrates TR-AFM as a powerful tool to study the in-plane\nelastic anisotropy of materials, including organic molecular crystals.