V.N. Prigodin

Low-dimensional variable range hopping in conducting polymers

Abstract Highly conducting polymers such as polyaniline and polypyrrole in a dielectrical state have unusually strong temperature of dependence of dynamic conductivity. We argue that this strong temperature dependence is due to the low dimensionality of charge transport in the polymers. The hopping network is formed by fast one-dimensional diffusion along the chains and rare interchain hops. Depending on the chain morphology the network is modeled by a quasi-one-dimensional system of randomly linked chains or by a quasi-one-dimensional chain fractal with dimensionality 1+s, where s≪1. We point out the experimental data for dielectric constant and ac-conductivity that supports these two models.

Magnetoresistance in an All‐Organic‐Based Spin Valve

Figure 1 . a) Device structure and b) schematic view of the energy levels of V[TCNE] x ( x ≈ 2). Spintronics is the new paradigm of electronics and utilizes the spin degree of freedom of the electron. [ 1 ] In addition to metalbased spintronic devices, which already have wide applications (e.g., read heads of hard disk drives), semiconductor spintronics provides the possibility to combine logic, communication, and storage operation using hybrid devices. [ 2 ] Recently, spintronic devices based on organic materials have attracted much attention because of the potential long spin lifetime due to low spin-orbit coupling and weak hyperfi ne interaction, as well as the ability to fabricate low-cost, light-weight, and mechanically fl exible devices. [ 3–9 ] Organic semiconductors have been used as the spacers in spin valve devices with ferromagnetic metal or transitional metal oxide contacts. One fundamental obstacle for spin injection from a ferromagnetic metal into a semiconductor is the conductivity mismatch. [ 10 ] The development of fully spinpolarized magnetic semiconductors offers a promising route to circumvent this problem. Organic-based magnets such as V[TCNE] x ( x ≈ 2, TCNE: tetracyanoethylene) provide advantages including high magnetic ordering temperature, fully spin-polarized semiconducting electronic structure, chemical tunability, and low-temperature processing. [ 11 ] Here, we show results for spin injection and detection in an all-organic-based spin valve using the organic magnetic semiconductor V[TCNE] x as both the spin injector and analyzer. We observed unusual inverted spin valve effect and propose that the negative magnetoresistance (MR) originates from the spin-dependent tunneling between highly spin-polarized bands split by Coulomb interaction. V[TCNE] x is the fi rst reported room-temperature moleculebased magnet with magnetic ordering temperature T C ≈ 400 K. [ 12 ] The magnetic order originates from the antiferromagnetic coupling between the three unpaired electrons of the V 2 +

Anomalous magnetoresistance in high-temperature organic-based magnetic semiconducting V(TCNE)x films

Anomalous positive magnetoresistance (MR) in high temperature organic-based magnet V(TCNE)x (TCNE=tetracynoethylene) thin films is reported. MR increases linearly with applied magnetic field and shows a maximum at the ferrimagnetic ordering temperature. The suggested roles of oppositely spin polarized π* electronic subbands and magnetization fluctuations due to the disordered nature of V(TCNE)x films are discussed.

Electric-field induced ion-leveraged metal–insulator transition in conducting polymer-based field effect devices

Abstract The field effect devices prepared completely from conducting polymers, especially poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonic acid) (PEDOT/PSS), were studied. Normally in a conductive “on” state, the transistor-like device has a transition to a substantially less conductive “off” state at an applied positive gate voltage, typically ∼15–25 V. The current ratio I off / I on can exceed 10 −4 at room temperature. We have found that the field effect is strongly temperature dependent and is substantially reduced upon decreasing the temperature by only a 10 °C. This loss of current reduction upon application of a gate voltage is not due to the temperature dependence of the electrical conductivity of polymers of which the devices are made. The temperature dependence of the dc conductivity of the PEDOT/PSS follows the variable range hopping law both before and after application of the gate voltage, though with an increased activation energy, T 0 . We suggest that the conducting polymer is near the metal–insulator transition and that the field effect in the device is related to the electric field modulating this transition in the region underneath the gate electrode. The transition is controlled and leveraged by ion motion. The time dynamics of the current with the gate modulation strongly supports our conjecture. We demonstrate the generality of the phenomena by presenting similar results for devices fabricated from the conducting polypyrrole doped with Cl.

Hopping conductivity of a nearly 1 D fractal: A model for conducting polymers

We suggest treating a conducting network of oriented polymer chains as an anisotropic fractal whose dimensionality

Room-temperature organic-based spin polarizer

D=1+\ensuremath{\epsilon}

Distribution of time constants for tunneling through a one-dimensional disordered chain

is close to 1. Percolation on such a fractal is studied within the real space renormalization group of Migdal and Kadanoff. We find that the threshold value and all the critical exponents are strongly nonanalytic functions of

New advances in organic spintronics

\ensuremath{\epsilon}

Electronic spin-driven resistance in organic-based magnetic semiconductor V[TCNE]X

as

Doped conducting polymer-based field effect devices

\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\epsilon}}0,