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; these results suggest that the main mechanism for beam damage in conjugated polymers is diffusion of free radicals. Thus, we show with both Dc experiments and high-resolution TEM that the addition of free radical scavengers such as butylated hydroxytoluene (BHT) mitigates beam damage at room temperature.