Pushing the resolution limits for imaging conjugated polymers in the transmission electron microscope

Transmission electron microscopy (TEM) of conjugated polymers has remained a challenge because resolution is limited by the electron dose the sample can handle. We have characterized beam damage in poly(3-hexylthiophene) (P3HT), poly(3-dodecylthiophene-2,5-diyl) (P3DDT), and poly[(5,6-difluoro-2,1,3-benzothiadiazol-4,7-diyl)-alt-(3,3’’’-di(2-octyldodecyl)-2,2’;5’,2’’;5’’,2’’’-quaterthiophene-5,5’’’-diyl)] (PffBT4T-2OD) via electron diffraction and electron energy-loss spectroscopy (EELS). Critical dose D_C values were calculated from the decay of diffraction and low-loss EELS peaks as functions of dose rate, temperature, and beam size. At room temperature, D_C first increases then decreases with increasing dose rate, whereas at cryogenic conditions this dose rate dependence is less pronounced and the overall D_C increases; these results suggest that a significant mechanism for beam damage in conjugated polymers is diffusion of secondary free radicals. D_C also increases with decreasing beam size. These new concepts in beam damage revealed strategies to push the resolution in the TEM, allowing us to image 3.6 Å π-π stacking in PffBT4T-2OD with 4D STEM, high-resolution TEM (HRTEM) at cryogenic conditions, and HRTEM at room temperature with antioxidants.