Formation and structural analysis of twisted bilayer graphene on Ni(111) thin flims

We synthesized twisted bilayer graphene on single crystalline Ni(111) thin films to analyze the statistical twist angle distribution on a large scale. The twisted bilayer graphene was formed by combining two growth methods, namely the catalytic surface reaction of hydrocarbons and carbon segregation from Ni. Low energy electron diffraction (LEED) investigations directly revealed dominant twist angles of 13 degrees, 22 degrees, 38 degrees, and 47 degrees. We show that the angle distribution is closely related to the sizes of Moire superlattices which form at commensurate rotation angles. In addition to the commensurate angles, quasi-periodic Moire structures were also formed in the vicinity of the dominant angles, confirmed by microscopic observations with low energy electron microscopy and scanning tunneling microscopy (STM). The quasi-periodic Moire patterns are presumably caused by insufficient mobility of carbon atoms during the segregation growth while cooling. Micro-LEED studies reveal that the size of single twisted domains is below 400 nm. Atomic-scale characterization by STM indicates that the twisted layer grown by segregation is located underneath the layer grown by surface reaction, i.e. between the Ni surface and the top single-crystal graphene layer. (C) 2014 Elsevier B.V. All rights reserved.