Dual beam system

Cross-sectional lamellae for scanning transmission electron microscopy (STEM) investigations were prepared on a Dual-Beam System (FEI Helios Nanolab). Lamella preparation starts with the beam-induced deposition of a Pt ridge to protect the surface. After Ga ion beam milling, the lamella is lifted out using an Omniprobe micromanipulator and attached to a Cu transmission electron microscope (TEM) grid. The lamella is then thinned to electron transparency using several polishing steps first at 30 keV and finally at 2 keV ion energy. Transfer to the TEM was carried out ex situ, exposing the lamella to ambient air for several minutes.
STEM measurements were taken in an uncorrected FEI Titan 80-300 microscope operating at 300 kV acceleration voltage, a beam current of 100–120 pA, a convergence semiangle of 10 mrad and dwell time of 10-20 μs per pixel. The STEM image in Fig. 2 and Supplementary Note 2 were taken in high angle annular dark field (HAADF) mode (scattering angles between 40 and 250 mrad). The spatial resolution under this condition is on the order of 140 pm. A gamma filter with g = 0.5 was applied to the HAADF Signal in order to enhance the weak signal of the graphene layer.

Images of the lattice structure of the calcite particles were obtained using HRTEM imaging of thin sections prepared by FIB Milling . FIB to electron transparency was performed using an FEI Dual Beam system equipped with a 30 kV Ga beam and a field emission electron gun operated at 5 kV. The samples were then analysed with a FEI Tecnai TF20 FEG-TEM fitted with an Oxford Instruments INCA 350 EDX system/80 mm X-Max SDD detector and a Gatan Orius SC600A CCD camera operating at 200 kV. In order to minimize electron beam related damage during the recording of images and diffraction patterns recording, a 10-μm condenser lens and the smallest spot size were used.