Critical conditions for atomic resolution imaging of molecular crystals by aberration-corrected HRTEM.

Atomically resolved imaging of organic molecules consisting of thin crystals by aberration-corrected (AC) HRTEM was studied by experimental observations and image simulations. An atomically resolved image of the hexadecachlorophthalocyanatocopper (CuPcCl16) molecule was obtained under low-dose conditions. The conditions for imaging organic frameworks were found to be more restricted than those for heavier elements such as copper and chlorine. For the characterization of the benzene rings within CuPcCl16 molecules, the specimen thickness had to be less than ~5 nm. The effects of the defocus conditions were examined by changing the image according to the location of the inclined specimen. The optimal defocus range for atomic resolution imaging of organic molecules was limited to a narrow region around the Scherzer defocus. Compared with scanning transmission microscopy, AC-HRTEM is more suitable for low-dose imaging, but the optimum conditions were severely restricted.