Current–Voltage Characteristics in Individual Polypyrrole Nanotube, Poly(3,4-ethylenedioxythiophene) Nanowire, Polyaniline Nanotube, and CdS Nanorope
© to the authors 2008
Received: 16 September 2008
Accepted: 29 October 2008
Published: 20 November 2008
In this paper, we focus on current–voltage (I–V) characteristics in several kinds of quasi-one-dimensional (quasi-1D) nanofibers to investigate their electronic transport properties covering a wide temperature range from 300 down to 2 K. Since the complex structures composed of ordered conductive regions in series with disordered barriers in conducting polymer nanotubes/wires and CdS nanowires, all measured nonlinearI–V characteristics show temperature and field-dependent features and are well fitted to the extended fluctuation-induced tunneling and thermal excitation model (Kaiser expression). However, we find that there are surprisingly similar deviations emerged between theI–V data and fitting curves at the low bias voltages and low temperatures, which can be possibly ascribed to the electron–electron interaction in such quasi-1D systems with inhomogeneous nanostructures.
KeywordsConducting polymers I–V characteristics FIT model Nanowires/tubes
Recently, low-dimensional materials especially nanowires and nanotubes have attracted considerable attention in view of their novel features and electronic device applications in future [1–3]. The unusual electronic transport properties in conducting chemically doped polymers have been widely investigated and reported owing to their unique structural features, which are known to be very inhomogeneous; namely, in some regions, the polymer chains are ordered, and in other regions, the chains are disordered [4–6]. This complex structure in disordered materials including doped polymer and inorganic nanofibers is generally considered as conduction regions or long conducting pathways separated by small insulating barriers. Especially, as a key indicator to electrical behavior, novel nonlinear current–voltage (I–V) characteristics with temperature and field-dependent features are observed in such quasi-one-dimensional (quasi-1D) inhomogeneous structures [7–13]. In particular, Kaiser et al. [11, 12] recently proposed a generic expression (extended fluctuation-induced tunneling (FIT)  and thermal excitation model) for the nonlinear I–V characteristics based on numerical calculations for metallic conduction interrupted by small barriers and showed that the expression can give a very good description to the temperature and field-dependent nonlinearities of I–V curves in polyacetylene nanofibers, vanadium pentoxide nanofibers, etc.
The electronic density of states (DOS) near the Fermi energy E F is known as an important physical quantity for understanding the electronic transport mechanism in strongly localized systems  where the electron–electron interaction (EEI) is first showed with created depletion in DOS near E F by Pollak  and Srinivasan . Efros and Schklovskii [17, 18] called this depletion “Coulomb gap,” which can strongly affect the transport properties. Furthermore, it is reported that the electron states in doped nanofibers are more localized by disorder at low temperature . However, most of conduction electrons are considered as delocalized and free to move over a very large distance compared to atomic dimension in FIT regime , the EEI is not considered. Thus, if taking the EEI into account, is Kaiser expression still generic for nonlinear I–V characteristics of quasi-1D material? In this paper, the I–V characteristics of a series of doped polymer nanofibers and helically twisted CdS nanowire ropes are measured by a standard two-probe method covering a wide temperature range to investigate the transport behavior and figure out this open question. We find all these I–V characteristics show similar nonlinear features and are well fitted to Kaiser expression. However, the surprisingly similar deviations between the I–V data and fitting curves emerge in low-field region at low temperatures, which have not been reported and discussed before.
The Pt microleads attached on isolated nanofibers were fabricated by focused-ion beam deposition. The detailed procedure can be found in previous publications [6, 21–24]. The I–V characteristics of individual polypyrrole nanotube, helically twisted CdS nanowire rope, PEDOT nanowire and polyaniline nanotube were measured by scanning the voltage from −6 to 6 V with a step of 0.03 V using a Physical Property Measurement System from Quantum Design and a Keithley 6487 picoammeter/voltage source over a wide temperature range from 300 to 2 K. Here, it is noted that since the low-temperature resistance of the measured polypyrrole tube and CdS nanorope is very large, and thus, the corresponding current is very small (~ pA), their I–V curves are only measured above 15 and 60 K, separately.
where G 0 h, and V 0 are parameters. G 0 is the temperature-dependent low-field (V → 0) conductance. The parameter h = G 0/G h (h < 1) yields a decrease of G below the exponential increase at higher voltages V (G h is the saturated conductance at a high-field value). V 0 is a voltage scale factor, which gives an exponential increase in conductance as V increases depending strongly on the barrier energy. More details can be found in Refs. [11, 12].
Comparison with Experiment
Fitting parameters to Eq. 3for polypyrrole nanotube at different temperatures
V 0 (V)
Fitting parameters to Eq. 3for CdS nanorope from 200 down to 60 K
Fitting parameters to Eq. 3for PEDOT nanowire from 80 down to 2 K
Fitting parameters to Eq. 3for polyaniline nanotube from 250 down to 3 K
V 0 (V)
Thus, based on our experimental results as shown from Figs. 1 2 3, and 4, we conclude that the FIT model and Kaiser expression can give a very good explanation to the electronic transport properties and the nonlinear I–V characteristics in quasi-1D materials in accordance with previous reports [11, 12]. In FIT regime, most of delocalized and free conduction electrons compared to atomic dimension in disordered materials transfer across the insulating gaps in the conducting pathways . In terms of Kaiser expression, considering the complex structures composed of ordered metallic regions in series with disordered conduction barriers in such quasi-1D systems, essentially, the nonlinear I–V behavior corresponds to tunneling through barriers with thermal fluctuations considerably smaller than the barrier height. As temperature increase, the thermal energy becomes comparable to the barrier height and linearity becomes dominant. Besides temperature, the nonlinearity also shows field-dependent feature. As the bias voltage increases, the difference in Fermi levels between two sides of barriers is comparable to the barrier energy, then the conductance will saturate at a value G h and the I–V curves will become linear.
In summary, the electronic transport properties in several kinds of individual polymer nanofibers and CdS nanoropes were measured and investigated covering a wide temperature range. All these quasi-1D materials show similar temperature and field-dependentI–V characteristics, which are well fitted to the extended FIT and thermal activation conduction model (Kaiser expression) consistent with the complex structures composed of ordered metallic region in series with disordered conduction barriers in such quasi-1D systems. We conclude that Kaiser expression is a possible way to explain the electrical behavior at relatively high temperatures and propose that the deviations emerged in low-field region at low temperatures are possibly due to the enhanced EEI in quasi-1D nanofibers with nanoscale inhomogeneous structures.
This work was financially supported by the National Natural Science Foundation of China (Grant No 10604038) and the Program for New Century Excellent Talents in University of China (Grant No NCET-07-0472).
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