Disorder and Defect Healing in Graphene on Ni(111)

The structural evolution of graphene on Ni(111) is investigated as a function of growth temperature by scanning tunneling microscopy (STM). Low temperature (400−500 °C) growth results in a continuous but highly defective film with small ordered graphene domains and disordered domains composed of Stone−Wales (SW)-like defects. As the growth temperature is increased, the disordered domains shrink leaving small clusters of defects alongside epitaxially matched graphene. Density functional theory (DFT) calculations indicate the crucial role of the metallic support for the healing of SW defects, as the interaction with the substrate leads to a stabilization of the reaction intermediate. This work highlights the effect of the graphene−substrate interaction on the temperature dependence of the defect concentration in epitaxial graphene on Ni(111). SECTION: Surfaces, Interfaces, Catalysis T prototypical two-dimensional material graphene exhibits a unique suite of electronic properties resulting from its Dirac fermions. These exotic properties were first observed using mechanically exfoliated graphene transferred onto SiO2, whereas graphene grown by chemical vapor deposition (CVD) on metals then transferred to an insulator exhibits less robust performance. Structural defects in graphene may strongly influence the local electronic structure and mechanical properties resulting in conflicting measurements of physical properties. Transmission electron microscopy (TEM) and scanning tunneling microscopy (STM) studies of CVD-grown graphene reveal polycrystalline films, resulting from the coexistence of numerous rotational domains. The atomic rearrangement at grain boundaries is typified by the formation of Stone−Wales (SW) defects consisting of pentagon−heptagon pairs. While the structure and properties of grain boundaries is currently of great interest, defects extending beyond the grain boundary have rarely been studied because no reliable production method has been identified. Freestanding graphene has recently been transformed into amorphous graphene by electron bombardment in a TEM, but such methods are not amenable to widespread study. In this Letter we describe the evolution of defect structures in graphene grown by CVD on Ni(111) as a function of growth temperature. Growth at low temperatures results in an amorphous carbon film with defects similar to the SW variety, while growth at sequentially higher temperatures results in smaller defective areas and domains in registry with the substrate. This process culminates at 650 °C, where a 1 × 1 epitaxial film is formed. Density functional theory (DFT) calculations are used to reveal the relevant mechanisms of defect healing for supported and unsupported graphene sheets. Figure 1 shows STM images of the graphene layer as a function of growth temperature. At 400 °C (Figure 1a), the film is highly defective, with graphene present in 1 nm patches. The honeycomb structure and SW-type defect structures are clearly visible (Figure 1a,e). In the more defective regions, the major features are bright protrusions and dark holes 2−3 Å higher or lower than the surrounding region. As the growth temperature is increased to 450 °C (Figure 1b), there is an increase in the area covered by epitaxial graphene. Highly disordered regions are still observed, but the protrusions are now less than 1 Å high. When the growth temperature is increased to 500 °C (Figure 1c) the heavily disordered patches and dark holes largely disappear, but the film still contains a high density of protrusions (<1 Å) in small defective clusters. Figure 1d shows 1 × 1 epitaxial graphene grown at 650 °C. Graphene grown under these conditions exhibits a few defects removing the 1 × 1 epitaxy, but otherwise displays excellent long-range order. To further probe the effect of initial growth conditions, a graphene layer grown at 450 °C was subsequently annealed to 650 °C for 15 min (Figure 1f). This annealed film resembles the films grown at 450 and 500 °C (Figure 1b,c). At the lowest growth temperature, 400 °C, the graphene honeycomb and defect structures are clearly observed. The dark features in the defective areas correspond to irregular ring structures, which expose the Ni(111) substrate, and are thus potential sites for further hydrocarbon decomposition. Some structures in Figure 1a are similar to SW defects, which consist of neighboring fiveand seven-membered rings, but this area Received: November 14, 2011 Accepted: December 19, 2011 Published: December 19, 2011 Letter