High energy resolution EELS of copper phthalocyanine crystals

Abstract

Recent developments of a monochromator for transmission electron microscopy provide a new tool for chemical analysis in local area of materials. Owing to the improved energy resolution, the energy range probed by electron energy‐loss spectroscopy (EELS) has been extended to visible or infrared region, allowing vibrational spectroscopy in the electron microscope [1, 2]. Moreover, one can also measure the fine energy‐loss near‐edge structure (ELNES) by exciting electrons in relative shallow inner‐shells like a carbon K‐edge, being an advantage in the analysis of organic compounds. In the present work, we demonstrate the high energy resolution EELS study of copper phthalocyanine (Fig. 1) crystals performed by a JEM‐ARM200F equipped with a double Wien filter [3] and spherical aberration correctors. Figure 2 shows the carbon K‐edge ELNES measured from the thin film crystals of copper phthalocyanine (CuPc) and chlorinated one (Cl 16 CuPc). Since the radiation damage is serious for the CuPc crystal compared to the Cl 16 CuPc one, its ELNES is rather noisy due to the limited electron dosage (0.5 C/cm 2 ). However, the chlorination effects are clearly observed in the change of ELNES, which is attributed to the chemical shift of 1s level of carbon atoms because the molecule has four independent carbon atoms with a different 1s binding energy. From the orientation dependence of ELNES intensity, the peaks (a) to (d) can be assigned to the transitions from 1s to π* unoccupied molecular orbitals. The peak (a) is related to the excitations of the carbon atoms forming a benzene ring in CuPc, while the peak (c) in the spectrum of Cl 16 CuPc is assigned to the excitations of carbon atoms bonding to chlorine atoms having a large electronegativity. The intensity of peak (a) in the spectrum of CuPc rapidly decreased with the increase of electron dosage as shown in Fig. 3. This means that the primary damage process is C‐H bond scission. Figure 4 shows the low‐loss spectra of CuPc extended to the infrared region, in which the C‐H stretch excitation is observed at 376 meV as a shoulder peak. This peak also disappeared when an electron dosage increased. The low‐loss spectra obtained from these crystals will also be presented and compared to the optical measurements.