Phase Transition in a Tetraaniline/Nanosilicon Composite Film Detected by Impedance Spectroscopy

Abstract

The composite films of phenyl/phenyl end-capped tetraaniline emeraldine salt (TANI-ES) and nanocrystalline silicon (nc-Si) TANI-ES/nc-Si were obtained using a drop-casting method by deposition from the solutions onto the substrates with metal electrodes. A comprehensive analysis of the impedance spectra of the TANI-ES/nc-Si composite film at the temperature range of 20−130 °C has been carried out. For the first time, it was found that there are two temperature ranges in which the impedance spectra are completely different. At the border of these ranges at temperature T* ≈ 101.5 °C, the impedance spectra exhibit a pronounced inductive character. Differential scanning calorimetry (DSC) measurements showed that two phase transitions occur in the TANI-ES/nc-Si composite at temperatures of 94 and 145 °C. An analysis of X-ray diffraction patterns revealed that the crystal structure of the TANI-ES/nc-Si composite is unstable upon heating to 140 °C in a nitrogen atmosphere, and the degree of this instability depends on the ratio of the number of TANI molecules per one nanoparticle. The temperature dependence of the DC conductivity changes sharply at a temperature of about 100 °C. The temperature dependences of the fitting parameters used to fit the experimental impedance curves also change significantly at 100 °C. Such changes in the electrical characteristics can be explained by a second-order phase transition in the TANI-ES/nc-Si composite at temperatures of about 100 °C. The Nyquist plot of the impedance of the TANI-ES/nc-Si composite has an inductive character at T* ≈ 101.5 °C, and we believe that such a character may indicate a phase transition in this composite. The conductivity spectra of the TANI-ES/nc-Si film at all investigated temperatures, except for 101.5 °C, are well approximated by the sum of the power law, polaron, and DC conductivities. However, the film conductivity at T* ≈ 101.5 °C has a local maximum at about 10 Hz, and this maximum cannot be approximated by standard functions used in impedance spectroscopy to fit conductivity curves in the RF region. It has been shown for the first time that a good approximation is the Drude−Smith formula, which has been successfully used to approximate conductivity spectra in the terahertz range of the spectrum. An exact approximation of the spectra ε′(ν) and ε″(ν) in the entire temperature range, except for temperature T*, is achieved by a combination of the Debye and Cole−Cole functions and the component determined by free charges. At temperature T*, in order to approximate the spectra ε′(ν) and ε″(ν), in addition to the indicated combination of functions, it is also necessary to use the Drude−Smith function.