Lattice rotation and incommensurability of epitaxial graphene on transition metal surfaces

Abstract

The growth and defect structure of graphene (g) on Ru(0001) and Ir(111) surface is investigated using low energy electron microscopy (LEEM) and micro-low energy electron diffraction (μLEED). The aim of this work is to gain a deeper understanding of the origin of small angle lattice rotations that were recently discovered in g/Ru(0001) by similar techniques and to explore if they are present on other metal substrate surfaces. Since small angle rotation domains may alter the electron properties of g/Ru(0001) from first principle calculation, it is also the goal to find growth conditions that produce more uniform graphene layer with high orientation uniformity. μLEED measurements from small a 250nm size areas and shadow dark-field (SDF) imaging, are used to characterize local rotation features in graphene grown by chemical vapor deposition with ethylene. In LEED mode, superstructure spots are is related to the moiré superlattice of graphene on metal surface, which has high sensitivity to the lattice rotation angle and lateral periodicity change in graphene. We find that the small angle rotation domain defect structure in g/Ru(0001) is diminished but cannot be completely eliminated using higher CVD growth temperature. However, uniformly oriented graphene can be grown by CVD if the crystal is first pre-loaded with C atoms by bulk segregation. Although the uniform rotation domain can be extended to 10 μm size, it is also found that the lateral periodicity within these uniformly oriented graphene varies considerably. A strain energy calculation using the Keating's model predicts a correlation between the corrugation and periodicity of graphene. In studies of g/Ir(111) using similar techniques, significant rotational disorder is observed in graphene that is nominally aligned with the Ir(111) substrate. On the other hand, graphene exhibits significantly better rotational uniformity with it is misaligned from the substrate by larger angles. μLEED measurements are used to record the relationship between the size of moiré superlattice and moiré rotation angle. Fitting this data with the known relationship is used to determine the graphene lattice constant. Interestingly, the data points are bounded by two sharp boundaries corresponding to two different lattice constants of graphene, which suggested that there may exist a spatial variation of lateral periodicity of graphene in this system as well. Compared to the small rotation domains in g/Ru(0001), which has fixed lateral periodicity of graphene, uniform rotation domain has variable lateral periodicities of graphene. This suggests that there is a correlation between small rotation angles and the periodicities of graphene. These finding may help to understand the controversies in corrugation and lateral periodicities of g/Ru(0001) system. The change oflateral periodicities found in g/Ir(111) can act as a further evidence that the graphene in graphene on metal substrates system in general has a certain range of lateral periodicities.

Date of Award	2013
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology