Enhanced Dielectric Relaxation in Self-Organized Layers of Polypeptides Coupled to Platinum Nanoparticles: Temperature Dependence and Effect of Bias Voltage

Abstract

Using alternative current impedance spectroscopy, we investigate the dynamical conductivity of hybrid nanomaterials composed of helical polypeptide layers containing platinum nanoparticles (PtNP). The electrical characteristics of the self-organized poly(γ-benzyl-L-glutamate) (PBGL) in bidimensional lamellar assembly in the presence of Pt nanoparticles are well modeled and described by a single equivalent circuit of parallel resistance and capacitance. The latter are determined using a comparison between the measured and calculated Nyquist plots, which allows extracting the characteristic relaxation time and frequency of the dipolar relaxation process. We found that the relaxation frequency in the PBLG−PtNP hybrid materials is enhanced by 4 orders of magnitudes compared to pure PBLG, which indicates a much faster dielectric relaxation in PBLG−PtNP due to dipole orientation and dipole−dipole interactions. The temperature dependence of the relaxation time is analyzed using Arrhenius plots, from which the activation energy of the relaxation process is found to be around 0.1 eV. Such a value close to the peptide vibration energy of the PBLG indicates a vibration-assisted relaxation process and a polaronic charge transport mechanism. An advantage of the PBLG−PtNP nanocomposite material is that the activation energy can be finely tuned by the PBLG degree of polymerization. Finally, an important outcome of this work is the investigation of the dielectric relaxation process in PBLG−PtNP under applied DC bias. We found that the activation energy decreases with increasing bias voltage for all degrees of polymerization of the PBLG molecule. This effect is interpreted in terms of electric field-induced alignment of the dipoles and of increased mobility of the polaronic charge carriers. The presence of piezoelectricity in the hybrid material gives the possibility to use the DC bias as a simple mean of monitoring the dynamical conductivity involving polaronic states.