Search

We report the effects of deposition temperature on the evolution of defect and second phases and on the ferroelectric properties of BiFeO3 thin films. We grew BiFeO3 thin films on (001) SrTiO3 substrates by using pulsed laser deposition at temperatures in the range of 570–600 °C at intervals of 10 °C. We found that defects and a resulting second phase appeared at temperatures greater than 590 °C. The second phase led to significant changes in the optical absorption and to the appearance of impurity peaks in the X-ray diffraction data. An analysis of the X-ray diffraction data indicated that the second phase was Fe2O3. Atomic force microscopy measurements showed that the appearance of defects accompanied an abrupt increase in the surface roughness. Furthermore, the presence of the second phase significantly affected the ferroelectric hysteresis. Our results suggest that the evolution of defects and resulting second phase in BiFeO3 thin films depends strongly on the deposition temperature.

Two kinds of anthracene derivative thin films, 2, 6-bis((4-hexylphenyl)ethynyl)-9, 10-bis(phenyl ethynyl)anthracene (B-ant-THB) and 9, 10-bis((4-hexyl phenyl)ethynyl)-2, 6-bis (phenyl ethynyl) anthracene (HB-ant-TB), were synthesized to investigate their optical properties. The difference between the two anthracene derivatives consisted of the position of the 1-ethynyl-4-hexylbenzene group substitution into an anthracene ring. We measured the optical properties of the anthracene derivatives by using photoluminescence (PL), transmittance spectroscopy, and spectroscopic ellipsometry. The dielectric functions of the two films were similar with respect to transition energies, but were different in terms of amplitudes. The optical band gap energies of the B-ant-THB and the HB-ant-TB film states were estimated to be 2.45 eV and 2.34 eV, respectively, whereas density functional theory (DFT) calculations of isolated molecular states showed the same value of 2.42 eV. The large bathochromic shift of the HB-ant-TB film state compared to the DFT calculation is attributed to strong intermolecular coupling between the HB-ant-TB molecules in the crystalline film because the HB-ant-TB film has higher crystallinity than the B-ant-THB film. The symmetric structures of the transmittance and PL spectra of the anthracene derivative films were observed as distinct peaks having similar vibrational energy levels for the singlet ground state and the first excited state.

Today, most synchrotron facilities offer beamlines dedicated to infrared (IR) spectroscopy and IR microspectroscopy. The main advantage of synchrotron-based infrared spectroscopy is the brightness of the synchrotron radiation source which can provide a brightness two or three orders of magnitude higher than a thermal (black-body radiation) infrared source. Thus, synchrotron-based infrared spectroscopy allows a high spatial resolution (infrared microspectroscopy) and a high spectral resolution, especially for low-throughput technologies and far-infrared spectroscopy. In this article, a brief introduction to synchrotron-based infrared spectroscopy and its application will be given.

Using spectroscopic ellipsometry, we measure the dielectric functions of NiO and CoO single crystals, which are grown by using the Verneuil (flame fusion) process. Taking the 2nd derivative of the dielectric function, we identify several optical structures below and near the band gap. We fit the 2nd derivative of the dielectric function by using an exciton lineshape and the standard critical point (SCP) model. The band gap energies of NiO and CoO are estimated to be 3.87 eV and 5.43 eV, respectively. The optical structures below and near the band gaps are attributed to intra-atomic d-d transitions and are estimated to be 1.2, 1.3, 1.8, 2.6, 2.9, 3.2, 3.4, 3.7, and 4.2 eV for NiO and 1.61, 2.26, 2.72, and 3.6 eV for CoO. Both the band gap and the d-d transition energies of NiO and CoO are compared to the literature values. The derivative spectra show unambiguously the d-d transitions near and above the band gap and those below the band gap. The lineshape fitting using the SCP model provides accurate values for the band gap energy and the d-d transitions. In addition, the SCP analysis verifies the exciton nature of the band gap and the d-d transitions. We attribute this to the formation of Frenkel excitons in NiO and CoO transition metal oxides.