In order to examine mutually overlapped flakes of two-dimensional crystals such as graphene with an electron microscope we need to obtain a contrast contribution from a single layer of carbon atoms. This task requires increasing the scattering rate of incident electrons by means of a drastic lowering of their energy to hundreds of eV or less.

The effect of airborne contaminants, mainly hydrocarbons, on graphene layers distinguishing in low energy S(T)EMs can be significant and due to their removing from the surface by appropriate in-situ methods is very important from practical point of view. During the scanning of the surfaces by electrons, the image usually gradually darkens because the hydrocarbon layer is deposited on the top. This effect can be described as an electron stimulated deposition, the thermal diffusion of organic molecules around the irradiated area is use as a source of building atoms, precursor. On the other hand, the effect of electron stimulated desorption occurred at the same time, especially at low observation energies, and then depends which process, deposition or desorption, is dominated. Our experiments have showed the fact that prolonged bombardment with electrons in a range of hundreds or even tens eV gradually increases the transmissivity (and decreases the reflectivity) of graphene due to the removal of adsorbed gas molecules providing an ultimate cleaning procedure evidently leading to an atomically clean surface. A sample such as graphene enables one to distinguish this kind of cleaning from radial damage, so this result opens up new possibilities for certain surface studies performed without ultrahigh vacuum.

Experiments have been performed in a FEI SEM/STEM microscope equipped by beam deceleration mode on free standing graphene (Ted Pella^®) and the effects of landing electron energy, samples biasing, electron dose, heating, etc. on the cleaning efficiency of electrons was studied in details. Moreover, the structural damage of the graphene after extremely high electron doses was observed, and the critical values for selected impacted energies were also calculated.

Acknowledgment

The work was financially supported from the European Commission for the Marie Curie Initial Training Network (ITN) SIMDALEE2: Grant No. 606988 under FP7- PEOPLE-2013-ITN.

Figures:

Fig. 1 Low voltage STEM image of free standing multilayers graphene observed at 100 eV

Fig. 2 Low voltage SEM image of CVD multi-layered graphene on copper observed at 100 eV

Fig. 3 The influence of the electron energy (500 eV, 5000 eV) and sample biasing (0 V, -4000 V) on the monolayer graphene transmissivity observed at 300 eV

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EMC Abstracts - https://emc-proceedings.com/abstract/low-energy-electron-beam-induced-cleaning-of-graphene-layers-in-sems/