IT-2-P-2571 Optimization of imaging conditions for atomic resolution in Titan TEM to minimize radiation damage and to study low angle boundaries in graphene-like materials

   Recent advances in spherical aberration (Cs) correction for TEMs in a combination with monochromated electron sources enabled imaging of single and bilayer graphene with atomic resolution [1]. Newly developed TEM techniques such as a single atom or single-atomic-column spectroscopy [2, 3] and atomic resolution electron tomography [4] drive the need for increased electron radiation doses applied to samples. The radiation damage started to be the key limitation factor for high-resolution TEM [5].

   For graphene-like (light element) materials [6] the radiation dose limitation is particularly severe. First, the knock-on damage cross section is higher for low atomic number elements [7]. Second, light elements produce less contrast than heavier elements, so even higher doses are needed to obtain a sufficient signal-to-noise ratio (SNR). Finally, the graphene-like materials appear in the form of low dimensional allotropes that have only one or a few atoms in a typical projection of a high-resolution image.

   To minimize the electron dose the optimization of acquisition parameters is needed. Here we present an extensive study of TEM tuning to obtain high quality HRTEM images of graphene. We used a Titan TEM (FEI Co) equipped with a Cs image corrector, a super-high brightness gun and a monochromator (energy spread better than 0.15eV). Tuning of the Cs corrector is based on measurement of images defocus (df) and astigmatism while recording so-called Zemlin tableau [8]. It was demonstrated that proper accounting for Cs of 3rd and 5th order (C3 and C5) and systematic error of C3 measurement results in more than 2 times increase of contrast, meaning more than 4 times decrease in dose needed for the same SNR (Fig.1).

   The optimal settings found were applied to study low angle boundaries (LAB) in graphene. LAB is a row of edge dislocations, separation of those defining the boundary angle. LABs are not visible directly on the image but can be identified by methods such as geometrical phase analysis (GPA), see Fig.2. Physically LAB may be interesting as they represent a perfect discontinuous layer with periodically spaced singularities.

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