Scaling the dynamic electron scattering in HAADF-imaging the graphene sheets

Free-standing graphene sheets (GS) with several to over a hundred layers are prepared by splitting the expandable graphite. The raw material is firstly transferred to a heating chamber for the thermal flashing at 1500 οC in a hydrogen chamber. The expanded powder (1 mg) is then dispersed in 10 ml of the PmPV/DCE suspension for high-power sonication and further centrifugation. After drop-cast onto a holey Formvar film, the GSs are suspended on the grid. The electron scattering is carried out in a Tecnai F20 TEM/STEM with a field emission gun. A high-angle annular dark field (HAADF) detector is fitted to collect the scattered electrons with a tunable collecting angle. The quantitative STEM image simulation is carried out using amultislice algorithm [4].