Minimizing beam damage with antioxidants to enable high resolution imaging of conjugated polymers in the 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 the effects of beam damage on 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 addition of antioxidants. 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. D_C is also higher for polymers with higher T_g. Altogether, these results suggest that the main mechanism for beam damage in conjugated polymers is diffusion of free radicals. Thus, we show that the addition of free radical scavengers such as butylated hydroxytoluene (BHT) mitigates beam damage at room temperature. We predict that the addition of BHT is important to enable high-resolution experiments without cryogenic conditions.