Low-energy electron microscopy (LEEM) uses an electron optics similar to a standard transmission electron microscope (TEM). In contrast to TEM, the high-energy electrons are decelerated to a few electron volts in front of the sample. Because of the low penetration depth of such electrons they are diffracted at the sample surface, after which they are re-accelerated in the electron optics. The method gives real-space images of surfaces with a resolution of the order of 10 nm. Time-dependent processes can also be monitored. Photo-emitted electrons can also be used for imaging by the same apparatus (PEEM). We use these methods in the DFG-funded priority programm SPP 1459 "Graphene" in collaboration with T. Seyller (TU Chemnitz). They are also applied in a joint project with S. Günther (TUM) and A. Locatelli (ELETTRA, Trieste).
The movie shows the CVD growth of graphene (bright) at 653°C on a Ni(111) single crystalline film (dark) observed by LEEM (E = 3.61 eV). In the first half of the movie the ethylene pressure was 4.0 × 10−9 mbar, while it was switched off in the second half. The stopping of the ethylene results in a much slower growth rate together with a higher structural quality. The graphene layer follows the well-known "carpet mode" and grows in a coherent fashion across atomic steps and step bunches (visible as dark lines). For more information see: "Single crystalline metal films as substrates for graphene growth", P. Zeller, M. Weinl, F. Speck, M. Ostler, A.-K. Henß, T. Seyller, and J. Wintterlin, Ann. Phys. 529, 170023 (2017).