Zener-like electrical transport in polyaniline-graphene oxide nanocomposites.

The present study includes the fabrication and characterization and an investigation of the electrical transport properties of nanocomposites of n-PANI and graphene oxide (GO). The samples were prepared by loading different weight percentages D of GO during the chemical oxidative in situ polymerization of aniline monomers. Structural characterization by XRD, FTIR, FESEM, etc. confirmed that the nanocomposites exhibited superior morphology and thermal stability. The transport properties were studied by measuring the variation of conductivity with temperature T, V-I characteristics and the fundamental response V f at different temperatures T. The dc conductance Σ showed a transition from insulator type behavior to weakly temperature dependent behavior at temperature T D, which decreased with increasing D. The V-I characteristics were generally nonlinear and the nonlinearity increased with decreasing temperature. Moreover, at temperatures T ≥ T D, the characteristics showed saturation of voltage for higher values of current, similar to Zener diodes. At lower temperatures (T ≤ T D), a voltage maximum occurred, similar to thyristors. This behavior leads to the possibility of fabricating devices containing these nanocomposites. We have tried to analyze these results using the framework of scaling theory and the concept of inter-chain hopping conduction and tunneling between conducting grains separated by insulating regimes in the nanocomposite.

Nonlinearity exponent of ac conductivity in disordered systems.

We measured the real part of ac conductance Σ(x,f) or Σ(T,f) of iron-doped mixed-valent polycrystalline manganite oxides LaMn(1-x)Fe(x)O(3) as a function of frequency f by varying initial conductance Σ(0) by quenched disorder x at a fixed temperature T (room) and by temperature T at a fixed quenched disorder x. At a fixed temperature T, Σ(x,f) of a sample with fixed x remains almost constant at its zero-frequency dc value Σ(0) at lower frequency. With increase in f, Σ(x,f) increases slowly from Σ(0) and finally increases rapidly following a power law with an exponent s at high frequency. Scaled appropriately, the data for Σ(T,f) and Σ(x,f) fall on the same universal curve, indicating the existence of a general scaling formalism for the ac conductivity in disordered systems. The characteristic frequency f(c) at which Σ(x,f) or Σ(T,f) increases for the first time from Σ(0) scales with initial conductance Σ(0) as f(c) ~ Σ(0)(x(f)), where x(f) is the onset exponent. The value of x(f) is nearly equal to one and is found to be independent of x and T. Further, an inverse relationship between x(f) and s provides a self-consistency check of the systematic description of Σ(x,f) or Σ(T,f). This apparent universal value of x(f) is discussed within the framework of existing theoretical models and scaling theories. The relevance to other similar disordered systems is also highlighted.