Photothermal bending spectroscopy and photothermal deflection spectroscopy of C60 thin films

The effects of O2 incorporation into C60 films on their electronic structure have been investigated by measuring the optical absorption in the sub-bandgap energy region at 300 K and 77 K using photothermal deflection spectroscopy. It was found that the sub-bandgap absorption greatly increased when oxygen was incorporated, and that a shoulder absorption peak appeared when measured at 77 K. It was also clarified that the sub-bandgap absorption was closely related to the electron spin density. From the experimental results, we discussed the origin of unpaired electrons and suggested that the electron spin signal was originated from unpaired electrons in C60 which was induced by electron transfer between C60 molecule and oxygen. Further, we have also demonstrated a new photothermal technique, photothermal bending spectroscopy, developed particularly for the measurements of sub-bandgap absorption in environmentally sensitive materials such as C60.