| AVS 58th Annual International Symposium and Exhibition | |
| Graphene and Related Materials Focus Topic | Wednesday Sessions |
| Session GR-WeA |
| Session: | Graphene Characterization including Microscopy and Spectroscopy |
| Presenter: | Michael Altman, Hong Kong University of Science and Technology, China |
| Authors: | K.L. Man, Hong Kong University of Science and Technology, China M.S. Altman, Hong Kong University of Science and Technology, China |
| Correspondent: | Click to Email |
The strength of the interaction between graphene and a supporting metal substrate is revealed in its influence on electronic structure and properties. Knowledge of structure is of elementary importance for understanding the nature of this interaction. The structure of a single layer of graphene (g) on the Ru(0001) surface has been controversial, beginning with the elementary matter of its lateral periodicity. It was eventually shown using surface x-ray diffraction (SXRD) that a superstructure forms from the moiré-like superposition of (25x25) graphene on (23x23) Ru units [1]. Although corrugation was detected within the supercell, the predominant origin of corrugation in this system, whether physical or electronic, has been disputed. Intriguing evidence from SXRD was also put forth that g/Ru(0001) exhibits chirality, whereby the weakly bound, protruding regions of a physically corrugated graphene layer are rotated in-plane by up to two degrees [2]. We have investigated single layer g/Ru(0001) using low energy electron microscopy (LEEM) and micro-low energy electron diffraction (μLEED) in order to verify the existence of chirality. Chirality should give rise to differences between the intensities of diffraction spots mirrored across the high symmetry directions. However, this broken mirror symmetry will be very difficult to detect using laterally averaging techniques such as SXRD because of the presence of two chiral enantiomers and two terminations of the hcp substrate. Using LEEM and μLEED we have successfully obtained diffraction information from an area with a diameter of 250 nm within a single surface termination. While no evidence is found with such high spatial resolution diffraction measurements that confirms chiral features within the unit cell, we do in fact observe rotation or twist of the moiré-like superposition over long length scales. Moiré twist causes the entire ensemble of satellite diffraction spots around each integer order spot to rotate as a group about their respective stationary foci. When the μLEED beam is scanned across the surface, the rotation angle undulates randomly. The data also suggest that the ground state configuration comprises a graphene layer that is slightly rotated with respect to the underlying substrate surface. Conceivably, the orientational variations that are observed here over sub-micron length scales might produce a signature in laterally averaging measurements that could be construed as evidence of chiral features on the short length scale within the unit cell.
[1] D. Martoccia et al., Phys. Rev. Lett. 101, 126102 (2008).
[2] D. Martoccia et al., New J. Phys. 12, 043028 (2010)