It is by now routine to achieve sub-atomic resolution (a few 10’s of pm) by aberration-corrected TEM and by STEM. However, reducing the signal to noise ratio sufficient for the highest resolution requires exposing the sample to fairly high electron dose. The electron irradiation causes sample damage while the image is being acquired. Several mechanisms may contribute: electron knock-on (in which an imaging electron transfers enough momentum to an atom to displace the atom from its original position), radiolysis (in which chemical bonds are rearranged, leading the target material to change into something else), heating (including sample melting), and Coulomb explosion (due to sample charging).
In viewing two-dimensional materials, one cannot average the imaging over depth; a two-dimensional material is by definition all surface. So any damage immediately and irreversibly degrades the image.
Two groups, one at Manchester, the other at Ulm, have independently discovered that it is possible to stablize MoS2 during (S)TEM imaging. This is done making a sandwich of graphene/MoS2/graphene. The Ulm group imaged with aberration-corrected TEM, while at Manchester, they used STEM. Additions to the image from the graphene layers, being periodic, can be subtracted out, leaving only the image of the study material, MoS2. It is an interesting coincidence that the two groups compared identical material stacks.
“The pristine atomic structure of MoS2 monolayer protected from electron radiation
damage by graphene”, G Algara-Siller, et al. [arXiv:1310.2411]
“Control of Radiation Damage in MoS2 by Graphene Encapsulation”, R Zan et al. [ACS Nano (DOI: 10.1021/nn4044035), see also arXiv:1310.4012]