J.G. Park

Field-effect transistor made of individual V2O5 nanofibers

A field-effect transistor (FET) with a channel length of ∼100 nm was constructed from a small number of individual V2O5 fibers of the cross section 1.5 nm×10 nm. At low temperature, the conductance increases as the gate voltage is changed from negative to positive values, characteristic of a FET with n-type enhancement mode. The carrier mobility, estimated from the low-field regime, is found to increase from 7.7×10−5 cm2/V s at T=131 K to 9.6×10−3 cm2/V s at T=192 K with an activation energy of Ea=0.18 eV. The nonohmic current/voltage dependence at high electric fields was analyzed in the frame of small polaron hopping conduction, yielding a nearest-neighbor hopping distance of ∼4 nm.

Nanotransport in polyacetylene single fiber: Toward the intrinsic properties

The electrical transport properties of iodine doped polyacetylene (PA) nanofibers were measured as function of temperature in micron and sub-micron scale. Polyacetylenc fiber network measured in micron scale shows weaker temperature dependence of resistivity and smaller negative magnetoresistance (MR) at T=1.5K compared to those of hulk PA film. The reaction of Au electrodes with dopant became serious in submicron experiment, so that stripes of Pt electrodes with 100nm separation were patterned on top of the SiO 2 substrate to prevent the reaction. Non-ohmic I-V charactersitics are observed in PA nanofiber. The gate dependence shows the charge carrier to be hole with mobility μ FET ∼ 4.4×10 -5 cm 2 /Vs at 233K. The non-ohmic I-V dependence at high electric fields could be originated from the soliton tunneling conduction in PA nanofiber.

Gating effect in the I-V characteristics of iodine doped polyacetylene nanofibers

The I-V characteristics of iodine doped polyacetylene (PA) nanofibers were measured as function of temperature. Platinum electrodes on top of SiO 2 substrates were used to prevent reaction with iodine dopant. The distance between the two electrodes is approximately 100 nm. Upon iodine doping, non-ohmic I-V characteristics are observed. The gate dependence shows the charge carrier to be hole with a mobility μ FET ∼ 4.4×10 -5 cm 2 /Vs at 233K.

Simple technique for the simultaneous measurements of the four-probe resistivity and the thermoelectric power

We propose simple configurations to measure the thermoelectric power and the four-probe resistivity simultaneously for different types of samples.

Tunneling conduction in polyacetylene nanofiber

We report the first low temperature measurements of I-V characteristics of polyacetylene nanofibers. Our data provide (i) no temperature dependence below 10-30K, (ii) no significant change of MR in magnetic field up to H=6T, to within the accuracy of the data. These are signatures of a novel conduction mechanism: the creation of charged soliton-antisoliton pairs by quantum-mechanical tunneling.

Quantum transport in low-dimensional organic nanostructures

We have studied three low-dimensional systems with sub-micron dimensions: a single polyacetylene (PA) nanofiber; a single-walled carbon nanotube (SWNT)-rope; and a lithographically prepared stripe of poly(2-methoxy-5-(2-ethyl hexyloxy)-p-phenylene vinylene) (MEH-PPV). In each case, the sample was contacted to four-probe electrodes, with 100-nm spacing and various electronic transport properties such as the I-V characteristics, the temperature dependence of resistivity and the gate voltage dependence of the transport current were measured. The PA nanofiber was found to be non-ohmic with a room temperature conductivity of ∼ 0.1 S/cm. Its carriers were found to be hole-like with charge carrier mobility of μ = 7.76 × 10 -2 cm 2 /Vs. For the SWNT-rope, the temperature-dependence of resistivity exhibited signatures of a Luttinger liquid for temperatures below 30 K. With varying gate voltage, periodic peaks were seen in the nanotube current which would normally be attributed to the effects of Coulomb blockade. Interestingly, these peaks show three-way splitting, similar to observations in triple quantum dot experiments. The MEH-PPV stripe, which was produced using electron beam lithography, had I-V characteristics similar to that of a large band-gap semiconductor. In the high field region, these characteristics could be explained in terms of a single carrier device model which considers the field-dependent mobility along with space charge limited conduction (SCLC). All three samples can be considered as field-effect transistors (FETs), with potential use in future high density integrated electronic devices.

Magnetoresistance of the metallic polyacetylene

Abstract The temperature dependence of the zero field resistivity, ρ ( T ), and the magnetoresistance (MR) of polyacetylene (PA) doped with iodine, metal-halide (AuCl 3 , FeCl 3 ) and perchlorate are measured. The results of doped PA are different for each dopant and for the degree of aging. The ρ ( T ) of the metal-halide and the perchlorate doped PAs show a resistivity minimum near T *≈200 K and the weak temperature dependence at low temperature. In particular, some of the perchlorate doped PA samples show the positive temperature coefficient of resistivity (TCR) from T =1.5 K to T =300 K with resistivity ratio ρ ( T =1.5 K)/ ρ ( T =300 K)=0.5∼0.7 depending on the samples. The MR is negative for all of the metallic PA samples. Both transverse ( J ⊥ H ) and longitudinal ( J ‖ H ) MR show similar magnetic field dependencies, although the magnitude of the transverse MR is larger than that of the longitudinal MR. For the aged samples, the ρ ( T ) increase more rapidly upon cooling than that of the fresh samples. The MR of the aged samples is positive. The localization–interaction theory is examined for the observed ρ ( T ) and MR data of doped PA. But the model of interchain charge transfer bridged by the dopants seems to explain the negative MR as well as the metallic temperature dependence of resistivity for the perchlorate doped polyacetylene.

Conductivity and magnetoresistance of polyacetylene fiber network

Abstract The four-probe conductivity and magnetoresistance (MR) in micron-scale for the iodine-doped polyacetylene (PA) fiber network were measured. A remarkably weaker temperature dependence of resistivity compared to that of the bulk PA film was observed. The transverse MR at T =1.5 K was negative and its relative magnitude was approximately 0.1% at H =10 T. The slow oscillatory tendency with small amplitude was suggested in MR. The results suggest the importance of contact barriers in the nanosize PA fiber network.

Orientation dependence of magneto-resistance behaviour in a carbon nanotube rope

The orientation dependence of magneto-resistance behaviour for a single-walled carbon nanotube (SWCN) rope is reported.A clear delineation of behaviours is observed between applying a magnetic field perpendicular or parallel to the rope axis.For a perpendicular field, monotonic negative magneto-resistance is observed due to two-dimensional weak localisation within the rope. By contrast, for a parallel field, complicated oscillatory behaviour is observed due to the Altshuler–Aronov–Spivak effect around closed electron trajectories on discrete cylinders within the SWCN rope.A dominant oscillatory mode can be identified which corresponds to closed paths around the outer circumference of the SWCN rope.However, due to the composite filamentary nature of the rope, the overall oscillatory behaviour is rather complicated and can be classified as universal conductance fluctuations. With a backgate voltage applied to the sample, Coulomb blockade peaks are observed in the transport current with additional peak structure superimposed due to resonant tunnelling.We find an interesting effect whereby these peaks are suppressed in the presence of a magnetic field. 2002 Elsevier Science B.V. All rights reserved.

Frequency and field dependent conductivity of carbon nanotube networks

Abstract The measured resistance of carbon nanotube networks is often dominated by defects, inter-tube and inter-rope contacts. We show that the peak reported in the frequency-dependent conductivity of single-wall carbon nanotube networks is consistent with metallic conduction interrupted by nonmetallic defects that act as barriers. Such barriers also contribute to the electric field dependence of the conductivity. Using Shengs model, we calculate the field dependence of fluctuation-assisted tunnelling conduction between metallic regions separated by an insulating barrier, obtaining nonlinearities consistent with our experimental data on carbon nanotube networks.