High-frequency dielectric response of polyaniline pellets as nanocomposites of metallic emeraldine salt and dielectric base

• IR and THz permittivity and conductivity spectra of conducting polyaniline salt and nonconducting base compared.
• Vibrational spectra agree with the absorbance on powder in KBr pellet and thin film.
• Broad absorption in the mid- and far-IR assigned to conductivity of localized carriers and Boson peak, respectively.
• Spectra of salt modeled as a nanocomposite of base and metallic islands using effective medium approach.
• Permittivity below the THz range increases with lowering frequency, in contrast to some published data.

Conducting polyaniline (PANI) salt and dielectric PANI base pellets were studied in the infrared (IR) and terahertz (THz) range at room temperature and compared with the IR transmission spectra of powdered samples dispersed in potassium bromide and with those of thin films. The IR reflectivity combined with THz transmission was fitted to calculate the permittivity and IR conductivity spectra. In addition to vibration modes and Drude contribution of the free carriers in the conducting PANI, strong and broad mid-IR absorption was detected and assigned to localized carriers. Additional absorption in the far IR, much stronger in case of the conducting PANI, was assigned to Boson peak. Good agreement between the IR conductivity peaks and those from the absorbance evaluated from the transmission measurements was obtained and differences between the spectra of PANI salt and base are discussed. The dielectric and conductivity spectra of PANI salt were modeled as the spectra of a composite of PANI base and metallic PANI reported by Lee et al., Nature 441, 65 (2006). Effective medium approach using three different models was applied and discussed. The best results, including the semiconductor-like temperature dependence of the low-frequency conductivity, were obtained with the generalized Lichtenecker model. The strong permittivity increase on decreasing frequency below the THz range is discussed and compared with the literature data.

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