Coulomb-blockade transport in quasi-one-dimensional polymer nanofibers

Abstract

We report the low temperature current-voltage (I-V) characteristics studies in quasi one-dimensional conducting polymer nanofibers. We find a threshold voltage V t below which little current flows at temperatures below 30 -40 K. For V > V t current scales as (V/V t -1) ζ , where ζ ~ 1.8 -2.1 at high biases. Differential conductance oscillations are observed whose magnitude increases as temperature decreases below 10 K. We attributed the observed low temperature I-V behavior to Coulomb blockade effects with a crossover to Luttinger liquid-like behavior at high temperature. We demonstrate that at low temperatures such a doped conjugated polymer fiber can be considered as an array of small conducting regions separated by nanoscale barriers, where the Coulomb blockade tunneling is the dominant transport mechanism.PACS numbers: 71.30.+h, 72.20.Ee, 72.80.Le Complex structures built of nanosize conducting objects provide model systems for investigation of transport phenomena on the mesoscopic scale, where quantum confinement and Coulomb charging play an important role [1]. Electronic transport through an array of metallic nanocrystals separated by nanobarriers is determined by the interplay between single-electron charging of an individual conducting region and tunneling between adjacent islands. In such systems both the tunneling resistance between neighboring regions is large, R T >> h/e 2 and the charging energy E C = e 2 /2C of an excess electron on a site is larger than k B T (C is the capacitance of the region). In the presence of both charge and structural disorder this interplay leads to highly nonOhmic current-voltage (I-V) characteristics. Coulomb blockade theory predicts that at low temperatures, there is no current below a specific threshold voltage, V t , while above V t the current follows a power law:with ζ ~ 1 in one dimensional (1D) and 5/3 or 2 in twodimensional (2D) systems [2]. V t depends on the nanocrystal number, the capacitance of the conducting regions and the capacitance between the each region and the back gate. Such a behavior with scaling exponents between 1 and 2.3 has been found, for example, in narrow chains of carbon nanoparticles [3], and in 100 nm wide multilayer of gold particles which exhibited ζ ~ 1.6 [4]. Coulomb-blockade effects have also been observed in multiwalled carbon nanotubes [5], organic thin-film transistors based on highly ordered molecular materials [6] as well as in single-molecular transistor structures [7]. It is evident that the nature of the individual regions: metallic, semiconducting, quantum dots -is irrelevant for this phenomenon to be observed [8]. In view of these results the question arises whether Coulomb-blockade effects can also be observed in such inherent quasi-1D systems as conducting polymer nanofibers. In this connection polyacetylene (PA) nanofibers are of particular interest as the model system for nanotransport studies because of the simple chemical structure, well defined polycrystallinity [9], and good ability for doping [10][11][12]. It is...

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