Crossover in Electrical Frequency Response through an Insulator-Metal Transition

Abstract

The insulator-metal transition (IMT) for a model quasi-one-dimensional (quasi-1D) conducting polymer (polyaniline) is probed at room temperature (RT) over an unusually broad frequency range (2 meV-6 eV) and also via T -dependent dc conductivity (s dc ). We determine that the IMT is not monotonic with increasing s dc ͑RT͒. The RT far infrared scattering time (t) becomes unusually long ($ 10 213 s) as s dc ͑RT͒ increases, even for samples on the insulating side of the IMT. We conclude that the IMT is due to percolation in the presence of inhomogeneous disorder and quasi-1D localization rather than 3D Anderson localization. [S0031-9007(96)01283-5] PACS numbers: 71.30.+h, 72.30.+q, 72.60.+g, 78.66.Qn The study of insulator-metal transitions (IMTs) has provided insight into a wide variety of phenomena in heavily doped semiconductors, metal ammonia solutions, and conducting polymers [1][2][3][4][5][6][7][8][9][10]. Generally, these studies focused on dc transport properties [1,2,5,[8][9][10], and the effects of pressure [1,2,5,9], magnetic field [1,2,4,5,9], and composition [1,2,4,5]. Systematic probes of the electronic response away from the Fermi level (E F ) would provide new insight into IMTs. High frequency studies are particularly useful for discerning the essential differences between a three-dimensional (3D) Anderson transition and an IMT due to percolation in the presence of inhomogeneous disorder and quasi-1D localization.A 3D Anderson transition occurs in the presence of large homogeneous (uniform statistical) disorder. A mobility edge (E C ) separates localized from extended states [1][2][3][4], and the transport scattering time (t) varies slowly with energy in the vicinity of E C [11]. At the IMT, the electronic localization length (L loc ) diverges. A monotonic development of the transport properties occurs as E F crosses E C into the delocalized states [1]. In particular, s dc grows in magnitude and weakens in temperature dependence [1], and the slope of the quantity W [ϵ d ln s dc ͑T͒͞d ln T ] [12] vs T changes sign from negative to positive. However, the large disorder required to localize electronic wave functions leads to the slowly varying t being short (typically ϳ10 215 s). The IoffeRegel condition [13] requires k F l ϳ 1, where k F is the Fermi wave vector and l is the mean free path. Approaching the IMT from the metallic side, the optical conductivity [s͑v͒] is monotonically suppressed due to the short t beneath the free carrier s Drude ͑v͒ at low energy [2,14]. For short t, localization corrections to the metallic Drude response result in a dielectric function [e͑v͒] that is positive in the far infrared [2,4,11,14], in contrast to the negative low energy e Drude ͑v͒ for usual metals [15].In contrast, a very different IMT is presented by an array of 3D (spatially [16,17] and electronically [18] anisotropic) metallic ellipsoids with an open Fermi surface [6,7,10,19] separated by a disordered quasi-1D medium (inhomogeneous disorder model). Conduction electrons in isolated 1D systems are localiz...

Electrical Conductivity Studies of Polyaniline Nanotubes Doped with Different Sulfonic Acids

“…Figure 4 shows the UV-Vis spectra of doped PANI nanotubes dispersed in N-methyl-2-pyrrolidone by using ultrasonic bath. The UV-Vis absorption patterns of the nanotubes obtained are consistent with the previously reported work [29][30][31]. The two absorption peaks for all three samples are indicating that the samples are in doped state, and the second peak nearly about 600 nm for all three samples assigns the polaron band [32].…”

Electric-field-controlled conductance of “metallic” polymers in a transistor structure

“…Generally, a doped polymer system can be viewed as ordered regions embedded in disordered regions. 11,14,17,49 The size and the fraction of the ordered regions vary from system to system. Polymer chains are parallel to each other and well packed in the ordered regions while others are misoriented or coil-like or both in the disordered regions.…”

“…13 Heavily doped CPs are conductors with the finite densities of states at the Fermi level. 12,[14][15][16] The conductivities for doped CPs range from 10 −2 S/cm-10 5 S / cm. 12 However, the charge transport of this class of organic conductor behaves like neither conventional metals nor semiconductors due to their morphological structure.…”

“…12 However, the charge transport of this class of organic conductor behaves like neither conventional metals nor semiconductors due to their morphological structure. 14,15,17 An electrical field effect in devices using the conjugated polymer ͓poly͑3,4-ethylenedioxythiophene͒ fully doped with poly͑styrene sulfonic acid͒ ͑PEDOT:PSS͔͒ as the active channel was recently reported. [18][19][20][21] The chemical structure for PEDOT:PSS is shown in Fig.…”

Polyaniline Nanofibers Prepared by Dilute Polymerization

“…After purification by dialysis against deionized water, polyaniline nanofibers presented three absorption peaks at ca. 338 nm, 430 nm, and 960 nm (free carrier tail), [30,31] as shown by the dotted line of Figure 4. After adding one drop of dilute 70 wt.-% HClO 4 (aq) to the above polyaniline nanofiber dispersion (dotted line), the absorption intensity of the peaks at ca.…”