Radiation damage analysis of 7,7,8,8,-tetracyanoquinodimethane (TCNQ) and 2,3,5,6,-tetrafluoro-7,7,8,8,-tetracyanoquinodimethane (F4TCNQ) by electron diffraction and electron energy loss spectroscopy

The electron irradiation sensitivity is compared between TCNQ and F4TCNQ. The characteristic doses, D1/e, determined by the attenuation of the diffraction intensities are 0.08-0.11 C cm−2 for TCNQ, and 0.04-0.06 C cm−2 for F4TCNQ, respectively. It is found that F4TCNQ is more sensitive to radiation damage than TCNQ in spite of the substitution of hydrogen with fluorine. From electron energy-loss spectroscopy (EELS), it is found that the damaging process for the two materials begins in a similar way, as seen from mass loss and spectrum changes observed for doses which exceed the characteristic dose. Although sensitive to the sample orientation, the carbon K-edge fine structures of TCNQ are almost preserved below the critical dose. Theoretical calculation predicts that the scission of hydrogen contributes to the spectrum shape very little compared to nitrogen scission. Beyond the characteristic dose, fluorine loss from F4TCNQ occurs faster than nitrogen loss but little loss of carbon is observed. In a similar way, nitrogen loss from TCNQ occurs beyond the characteristic dose, while carbon appears constant. From detailed analysis of the carbon and nitrogen K-edge fine structures of TCNQ and F4TCNQ, it is found that the π* peak of nitrogen in TCNQ decreases below the characteristic dose, while π* loss of nitrogen in F4TCNQ, and π* loss and σ* increase of carbon in both materials are observed beyond the characteristic dose. The changes in the fine structures are believed to be due to the chemical alteration such as cross-linking, in which the π-bonding system of nitrogen or carbon turns into σ-bonding. The difference in characteristic dose between TCNQ and F4TCNQ is explained by considering 'effective molecular occupancy', where F4TCNQ has a larger intermolecular empty space than TCNQ.