Study of structures at the boundary and defects in organic thin films of perchlorocoronene by high-resolution and analytical transmission electron microscopy.

Both the periodic and non-periodic structures of perchlorocoronene (C(24)Cl(12)) crystals were characterized by high-resolution transmission electron microscopy (HRTEM), electron diffraction (ED), electron energy-loss spectroscopy (EELS), and energy-filtered transmission electron microscopy (EFTEM). The HRTEM images at the boundary of the C(24)Cl(12) crystals exhibit the flexibility of defect structures, where molecules align to compensate for the discontinuity between two different domains. Emphasized by the filtered images, it was found that the non-periodic regions are created everywhere with a small electron beam irradiation (∼ 10(6)electrons nm(-2)) and then spread over the entire regions to completely destroy the periodic structures after a higher electron dose (∼ 2 × 10(6)electrons nm(-2)). The effect of the electron beam irradiation was monitored by ED, EELS, and EFTEM, where periodic structures and content elements are well preserved up to 10(6)electrons nm(-2), but chlorine atoms decreased with a much higher electron dose. This is explained by the breakage of the C-Cl bond to detach chlorine atoms, confirmed by energy-loss near the edge structures (ELNES) of carbon π * peaks and chlorine loss at the edge of the specimen, as well as by theoretical simulation. The detachment of chlorine is localized at the peripheral edge around a hole confirmed by core-loss EFTEM imaging.

Stability due to peripheral halogenation in phthalocyanine complexes.

The effect of peripheral halogenation is examined based on analytical transmission electron microscopy and thermal analyses of two chemical family structures, specifically the vanadyl-phthalocyanine family (VOPcX: X = H16, F14.5) and the copper-phthalocyanine family (CuPcX: X = H16, F16, Cl16, Cl8Br8), focusing on the process of molecular changes and crystalline disintegrations. To clarify the molecular transformations, electron energy-loss spectroscopy (EELS) is applied to two fluorinated phthalocyanines (VOPcF14.5 and CuPcF16), by monitoring mass changes as well as energy loss near edge structures (ELNES). The elemental mass of both VOPcF14.5 and CuPcF16 remain constant up to 0.5 C x cm(-2), except in the case of mass reduction attributed to oxygen loss occurring in VOPcF14.5. It is expected that the released oxygen will induce higher radiation damage in VOPcF14.5. Although mass variation is not observed in CuPcF16, it is found from ELNES that the pi resonant system of nitrogen is more radiation sensitive than that of carbon. These results imply that the electron sensitivity in VOPcX is triggered by eliminated oxygen or, thus, an induced larger empty space, whereas the sensitivity of CuPcX is dominated only by a large intermolecular empty space resulting in the following bond alterations. It is also found that the decomposition temperature (Td) measured by thermal analyses and the characteristic dose (D1/e) are exponentially correlated to the "effective molecular occupancy" (Oe) evaluated as a volume function of molecules in unit cells. By measuring Td and/or Oe, we discuss the durability of peripheral halogenation with respect to the radiation damage.

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 F(4)TCNQ. The characteristic doses, D(1/e), determined by the attenuation of the diffraction intensities are 0.08-0.11Ccm(-2) for TCNQ, and 0.04-0.06Ccm(-2) for F(4)TCNQ, respectively. It is found that F(4)TCNQ 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 F(4)TCNQ 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 F(4)TCNQ, it is found that the pi* peak of nitrogen in TCNQ decreases below the characteristic dose, while pi* loss of nitrogen in F(4)TCNQ, and pi* loss and sigma* 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 pi-bonding system of nitrogen or carbon turns into sigma-bonding. The difference in characteristic dose between TCNQ and F(4)TCNQ is explained by considering "effective molecular occupancy", where F(4)TCNQ has a larger intermolecular empty space than TCNQ.