| The nature of the electronic states of charge carriers and the origin of metal to insulating (MI) transition, in highly interconnected conducting polymer nanostructure network, notably Polyaniline nanofibers (PAN-N), were determined via temperature dependent DC conductivity, optical reflectance (300 cm-1--50,000 cm-1), electron paramagnetic resonance (EPR) and X-ray diffraction probes.;The nanostructured network has a room temperature (RT) conductivity of ∼1 S/cm, similar to that of conventional disordered Polyaniline in the Emeraldine Salt (ES) form, but this value is small when compared to the Mott minimum for metallic conductivity (∼100 S/cm). Therefore, the signature of the temperature dependent DC conductivity (sigmadc (T)) of PAN-N should be dominated by phonon activation, with very little or no metallic contribution. In fact, the signature of the charge transport of PAN-N films shows a large "metallic" contribution from RT to ∼235 K, with the change in sigma dc(T) from RT to the peak of the maximum conductivity ∼200%. We attribute this large metallic contribution to a "mechanically-induced" crossover from metallic to insulating behavior, due to the "fragile" nature of the conductance at interfiber interfaces. By mechanically modifying the nanostructure morphology via applied pressure, the signature of the charge transport resembles that of conventional disordered Polyaniline, having a broad MI peak and moderate change in conductivity(< 15%) from RT to the peak conductivity. The reduced energy activation W[≡ d lnsigmadc(T)/ d lnT] of PAN-N has a negative slope at low temperatures, which suggest that the charge carriers are localized by disorder.;Similarly, the dielectric functions for all measured temperature reveal that the charge carriers within the network are localized. EPR measurements show a temperature dependent Pauli susceptibility between 300 K and 130 K, and below 130 K, we see the onset of a Curie-like contribution to the magnetic susceptibility. The nanostructured network has a low magnetic susceptibility, dominated by a weak Curie component, with the density of Curie spins of ∼1 spin per 200 (2-ring) repeat unit. This suggests that a significant fraction of the spins are paired up as bipolarons, implying that most of the charge carriers are localized.;Structural studies indicate that the nanostructure films are ∼50% crystalline with a coherence length of ∼2 nm. This coherence length is similar to the values reported earlier for conventional disordered Polyaniline with higher conductivity. This suggests that the nature of conductance within the interfiber interfaces affects the effective conductivity of the network.;The data of other charge dynamics including optical, magnetic, and structural probes suggest that the role of interfiber contacts within the network contributes largely to the "metallic" signature of thes sigma dc(T). We conclude that the MI behavior is due to the "fragile" nature of the conductance at the nanostructure interface, while disorder and localization dominate the charge dynamics. |
