Recent advances in spherical aberration (Cs) correction for TEM in combination with electron sources of low energy spread enable imaging of graphene with atomic resolution. Newly developed TEM techniques such as single atom or single atomic column spectroscopy and atomic resolution electron tomography stimulated the need for increased electron radiation doses to be applied to samples, while radiation damage started to be the limiting factor for high resolution TEM.

For graphene, the radiation dose limitation is particularly severe for several reasons. First, the knock-on damage cross section is higher for low atomic number elements. Secondly, carbon produces less contrast than heavier elements, so that even higher doses are needed to obtain a sufficient signal-to-noise ratio (SNR). Finally, graphene appears in the form of low dimensional allotropes that have only one or a few atoms in the typical projection of a high-resolution TEM image.

Optimization of the acquisition parameters of TEM systems allows minimizing electron dose and reducing possible sample damage. Here we present an extensive study of TEM tuning to obtain high quality HRTEM images of graphene. Special attention was paid to optimize the settings of the Cs corrector.

Tuning of Cs correctors is based on measurement of image defocus (df) and astigmatism while recording Zemlin tableau. It will be demonstrated that accounting for Cs of 3rd and 5th order results in more than double increase in contrast, i.e. more than 4 times less electron dose to obtain the same SNR in HRTEM pictures.