π - ADDI

38 Chapter 1. IntroductionFigure 1.16: TEM image of a graphene monolayer with Pt atoms at a substitutional site. An enlarged view of thearea arrowed in the left panel is seen on right inset. Adapted from Ref. [78].results.1.7.3 Graphene and carbon nanotubes with substitutional transition metalsWe commented in the previous sections that defects and dopants severely affect some theproperties of graphenic systems and can be used to tune their response. Here we focus onsubstitutional impurities in graphene, in which a single metal atom substitutes one or severalcarbon atoms in the layer. Direct experimental evidence of the existence of these kind ofdefects has been recently provided by Gan et al. [78]. Using HRTEM, these authors wereable to visualise individual Au and Pt atoms incorporated into a very thin graphitic layerprobably consisting of one or two graphene layers as we shown in Figure 1.16. From thereal-time evolution and temperature dependence of the dynamics they obtained informationabout the diffusion of these atoms. Large diffusion barriers (∼2.5 eV) were observed forin-plane migration, which indicates the large stability of these defects and the presence ofstrong carbon-metal bonds. These observations indicate that the atoms occupy substitutionalpositions.In another experiment using double-wall CNT (DWCNT) [63], Fe atoms were trappedat vacancies likewise that observed in graphene layers. Figure 1.17(A)-(B) shows the STEMimages before and after the electron beam was directed onto a predefined position and kept stationaryfor few seconds in order to create a lattice defect 4 . After irradiation, a bright spot in thedark-field image was observed. A quantitative analysis showed an increase of the scattered in-4 Fe atoms were previously deposited on the nanotube surface before the defect formation