Reflectance of conducting polypyrrole: Observation of the metal-insulator transition driven by disorder

Abstract

By controlling the extent of disorder through electrochemical synthesis at reduced temperatures, conducting polypyrrole (PPy) can be obtained in the metallic regime, in the insulating regime, and in the critical regime of the disorder-induced metal-insulator (M-I) transition. We present the results of reflectance measurements (0.002 -6 eV) of PPy carried out at room temperature on the metallic side and on the insulating side of the M-I transition. While the reflectance spectra obtained from samples on both sides of the M-I transition exhibit spectral features expected for a partially filled conduction band, the electronic states near the Fermi energy (EF) are different in the two regimes. The data obtained from metallic samples indicate delocalized electronic wave functions in the conduction band, whereas the spectral features which characterize the insulating regime indicate that the states near E+ are localized. Consistent with theoretical predictions for the metallic and insulating regimes, the optical conductivity o. (co) and the real part of the dielectric function E&(co) each show different frequency dependences in the far infrared. In the metallic regime o(co)~c o' for Am&600 cm ' and c&(co) ( &0) increases rapidly as co~0, as described by the "localization-modified Drude model, " leading to the conclusion that metallic polypyrrole is a disordered meta/ near the M-I transition. In contrast, the insulating regime is characterized as a Fermi glass as confirmed by o (co)~co for A~& 600 cm

Doping, density of states, and conductivity in polypyrrole and poly(p-phenylene vinylene)

“…The outcome of the sum-rule is somewhat arbitrary, because at energies of (∼ 3 eV) intraband excitations start playing a role as well. 13,19 By integrating the conductivity up to 3.2 eV, the results show that in PPy 4 (notation as in Fig 3) a carrier density of 2 holes/nm 3 is present while for PPy 1, see also Fig. 5, the carrier density equals 3 holes/nm 3 ; hence the values of carrier densities in all measured samples are rather close, though their σ(T )'s are widely different.…”

“…The polymerization was carried out at -40 0 C under nitrogen atmosphere to improve the structural order in the system, and the samples were systematically dedoped to attain the desired doping level. 12,13 Free-standing films (thickness ∼ 20 microns) were used for conductivity measurements; and the films on glass substrate, on which Au-contacts were evaporated before deposition, were used for electrochemical gated transistor (EGT) experiments. In the EGT measurements on PPV and PPy the hole charge was counterbalanced by PF − 6 anions from the electrolyte solution.…”

Metallic state in disordered quasi-one-dimensional conductors

“…Comparable trends are observed for the doped-semiconductors. The open symbols correspond to the "second" plasma frequency observed in conducting polymers [2,6], and single-walled carbon nanotubes [9]. The conducting polymers and single-walled carbon nanotubes are very unlike conventional metals and crystalline 1D conductors.…”

“…[2,[6][7][8][9][10] For conventional metals, the frequency (ω = 2πf ) dependence of the complex conductivity, σ * = σ + iωε 0 ε (ε 0 vacuum permittivity), is well explained in terms of the Drude free-electron model:…”

“…[2][3][4] Other studies suggest that the MIT is of the Anderson type [1] with disorder occurring on length-scales equal or less than the electronic correlation-length. [5,6] Then, extended and localized states are separated in energy by the mobility edge (E c ), and the MIT occurs when E F crosses E c .…”