One-dimensional transport in polymer nanofibers

Multiscale Charge Transport in van der Waals Thin Films: Reduced Graphene Oxide as a Case Study

Large area van der Waals (vdW) thin films are assembled materials consisting of a network of randomly stacked nanosheets. The multiscale structure and the two-dimensional (2D) nature of the building block mean that interfaces naturally play a crucial role in the charge transport of such thin films. While single or few stacked nanosheets (i.e., vdW heterostructures) have been the subject of intensive works, little is known about how charges travel through multilayered, more disordered networks. Here, we report a comprehensive study of a prototypical system given by networks of randomly stacked reduced graphene oxide 2D nanosheets, whose chemical and geometrical properties can be controlled independently, permitting to explore percolated networks ranging from a single nanosheet to some billions with room-temperature resistivity spanning from 10^-5 to 10^-1 Ω·m. We systematically observe a clear transition between two different regimes at a critical temperature T*: Efros-Shklovskii variable-range hopping (ES-VRH) below T* and power law behavior above. First, we demonstrate that the two regimes are strongly correlated with each other, both depending on the charge localization length ξ, calculated by the ES-VRH model, which corresponds to the characteristic size of overlapping sp² domains belonging to different nanosheets. Thus, we propose a microscopic model describing the charge transport as a geometrical phase transition, given by the metal-insulator transition associated with the percolation of quasi-one-dimensional nanofillers with length ξ, showing that the charge transport behavior of the networks is valid for all geometries and defects of the nanosheets, ultimately suggesting a generalized description on vdW and disordered thin films.

A tied Fermi liquid to Luttinger liquid model for nonlinear transport in conducting polymers

Organic conjugated polymers demonstrate great potential in transistors, solar cells and light-emitting diodes, whose performances are fundamentally governed by charge transport. However, the morphology-property relationships and the underpinning charge transport mechanisms remain unclear. Particularly, whether the nonlinear charge transport in conducting polymers is appropriately formulated within non-Fermi liquids is not clear. In this work, via varying crystalline degrees of samples, we carry out systematic investigations on the charge transport nonlinearity in conducting polymers. Possible charge carriers' dimensionality is discussed when varying the molecular chain's crystalline orders. A heterogeneous-resistive-network (HRN) model is proposed based on the tied-link between Fermi liquids (FL) and Luttinger liquids (LL), related to the high-ordered crystalline zones and weak-coupled amorphous regions, respectively. The HRN model is supported by precise electrical and microstructural characterizations, together with theoretic evaluations, which well describes the nonlinear transport behaviors and provides new insights into the microstructure-correlated charge transport in organic solids.

Possible Luttinger liquid behavior of edge transport in monolayer transition metal dichalcogenide crystals

In atomically-thin two-dimensional (2D) semiconductors, the nonuniformity in current flow due to its edge states may alter and even dictate the charge transport properties of the entire device. However, the influence of the edge states on electrical transport in 2D materials has not been sufficiently explored to date. Here, we systematically quantify the edge state contribution to electrical transport in monolayer MoS2/WSe2 field-effect transistors, revealing that the charge transport at low temperature is dominated by the edge conduction with the nonlinear behavior. The metallic edge states are revealed by scanning probe microscopy, scanning Kelvin probe force microscopy and first-principle calculations. Further analyses demonstrate that the edge-state dominated nonlinear transport shows a universal power-law scaling relationship with both temperature and bias voltage, which can be well explained by the 1D Luttinger liquid theory. These findings demonstrate the Luttinger liquid behavior in 2D materials and offer important insights into designing 2D electronics.

Sliding Luttinger liquid with alternating interwire couplings

We study a phase diagram for the sliding Luttinger liquid (SLL) of coupled one-dimensional quantum wires packed in a two-dimensional array in the absence of a magnetic field. We analyse whether the nearest-neighbour inter-wire interactions, stabilise the SLL phase. We construct an analogue of a Su-Schriefer-Heeger (SSH) model (allowing alternating couplings between wires). Calculating the scaling dimensions of the two most relevant perturbations, charge-density wave, and superconducting inter-wire couplings, but excluding the inter-wire single-particle hybridisation, we find a finite stability region for the SLL. It emerges due to the inter-wire forward scattering interaction, and remains stable up to a significant asymmetry between alternating couplings.

Instability of the sliding Luttinger liquid

We revise a phase diagram for the sliding Luttinger liquid (SLL) of coupled one-dimensional quantum wires packed in two- or three-dimensional arrays in the absence of a magnetic field. We analyse whether physically justifiable (reasonable) inter-wire interactions, i.e. either the screened Coulomb or 'Coulomb-blockade' type interactions, stabilise the SLL phase. Calculating the scaling dimensions of the most relevant perturbations (the inter-wire single-particle hybridisation, charge-density wave, and superconducting inter-wire couplings), we find that their combination always destroys the SLL phase for the repulsive intra-wire interaction. However, suppressing the inter-wire tunnelling of repulsive fermions (when the charge-density wave is the only remaining perturbation), one can observe a stability region emerging due to the inter-wire forward scattering interaction.

Programming shape and tailoring transport: advancing hygromorphic bilayers with aligned nanofibers

Natural systems utilize nanofiber architectures to guide water transport, tune mechanical properties, and actuate in response to their environment. In order to harness these properties, a hygromorphic bilayer composite comprised of a self-assembled fiber network and an aligned electrospun fiber network was fabricated. Molecular gel self-assembly was utilized to increase hydrophobicity and strength in one layer, while aligned electrospun poly(vinyl alcohol) (PVA) nanofibers increased the rate of hydration and facilitated tunable actuation in the other. Interfacing these two fiber networks in a poly(ethylene oxide-co-epichlorohydrin) (EO-EPI) matrix led to hydration-driven actuation with tunable curvature. Specifically, variations in fiber alignment were achieved by cutting at 0, 90, and 45 degree angles in relation to the length edge of the composite. Along with the ability to program the natural curvature, the utilization of aligned nanofibers increased water transport compared to random nanofiber systems, resulting in a reduction in response time from 20+ minutes to 2-3 minutes.

Supra-Nanoparticle Functional Assemblies through Programmable Stacking

The quest for the by-design assembly of material and devices from nanoscale inorganic components is well recognized. Conventional self-assembly is often limited in its ability to control material morphology and structure simultaneously. Here, we report a general method of assembling nanoparticles in a linear "pillar" morphology with regulated internal configurations. Our approach is inspired by supramolecular systems, where intermolecular stacking guides the assembly process to form diverse linear morphologies. Programmable stacking interactions were realized through incorporation of DNA coded recognition between the designed planar nanoparticle clusters. This resulted in the formation of multilayered pillar architectures with a well-defined internal nanoparticle organization. By controlling the number, position, size, and composition of the nanoparticles in each layer, a broad range of nanoparticle pillars were assembled and characterized in detail. In addition, we demonstrated the utility of this stacking assembly strategy for investigating plasmonic and electrical transport properties.

Apparent Power Law Scaling of Variable Range Hopping Conduction in Carbonized Polymer Nanofibers

We induce dramatic changes in the structure of conducting polymer nanofibers by carbonization at 800 °C and compare charge transport properties between carbonized and pristine nanofibers. Despite the profound structural differences, both types of systems display power law dependence of current with voltage and temperature, and all measurements can be scaled into a single universal curve. We analyze our experimental data in the framework of variable range hopping and argue that this mechanism can explain transport properties of pristine polymer nanofibers as well.

One-Dimensional Liquid ^{4}He: Dynamical Properties beyond Luttinger-Liquid Theory

We compute the zero-temperature dynamical structure factor of one-dimensional liquid ^{4}He by means of state-of-the-art quantum Monte Carlo and analytic continuation techniques. By increasing the density, the dynamical structure factor reveals a transition from a highly compressible critical liquid to a quasisolid regime. In the low-energy limit, the dynamical structure factor can be described by the quantum hydrodynamic Luttinger-liquid theory, with a Luttinger parameter spanning all possible values by increasing the density. At higher energies, our approach provides quantitative results beyond the Luttinger-liquid theory. In particular, as the density increases, the interplay between dimensionality and interaction makes the dynamical structure factor manifest a pseudo-particle-hole continuum typical of fermionic systems. At the low-energy boundary of such a region and moderate densities, we find consistency, within statistical uncertainties, with predictions of a power-law structure by the recently developed nonlinear Luttinger-liquid theory. In the quasisolid regime, we observe a novel behavior at intermediate momenta, which can be described by new analytical relations that we derive for the hard-rods model.

Effective Temperature and Universal Conductivity Scaling in Organic Semiconductors

We investigate the scalability of the temperature- and electric field-dependence of the conductivity of disordered organic semiconductors to ‘universal’ curves by two different but commonly employed methods; by so-called universal scaling and by using the effective temperature concept. Experimentally both scaling methods were found to be equally applicable to the out-of-plane charge transport in PEDOT:PSS thin films of various compositions. Both methods are shown to be equivalent in terms of functional dependence and to have identical limiting behavior. The experimentally observed scaling behavior can be reproduced by a numerical nearest-neighbor hopping model, accounting for the Coulomb interaction, the high charge carrier concentration and the energetic disorder. The underlying physics can be captured in a simple empirical model, describing the effective temperature of the charge carrier distribution as the outcome of a heat balance between Joule heating and (effective) temperature-dependent energy loss to the lattice.

Coulomb Blockade in a Two-Dimensional Conductive Polymer Monolayer

Electronic transport was investigated in poly(3-hexylthiophene-2,5-diyl) monolayers. At low temperatures, nonlinear behavior was observed in the current-voltage characteristics, and a nonzero threshold voltage appeared that increased with decreasing temperature. The current-voltage characteristics could be best fitted using a power law. These results suggest that the nonlinear conductivity can be explained using a Coulomb blockade (CB) mechanism. A model is proposed in which an isotropic extended charge state exists, as predicted by quantum calculations, and percolative charge transport occurs within an array of small conductive islands. Using quantitatively evaluated capacitance values for the islands, this model was found to be capable of explaining the observed experimental data. It is, therefore, suggested that percolative charge transport based on the CB effect is a significant factor giving rise to nonlinear conductivity in organic materials.

Significant electronic thermal transport in the conducting polymer poly(3,4-ethylenedioxythiophene)

Suspended microdevices are employed to measure the in-plane electrical conductivity, thermal conductivity, and Seebeck coefficient of suspended poly(3,4-ethylenedioxythiophene) (PEDOT) thin films. The measured thermal conductivity is higher than previously reported for PEDOT and generally increases with the electrical conductivity. The increase exceeds that predicted by the Wiedemann-Franz law for metals and can be explained by significant electronic thermal transport in PEDOT.

Electrical transport properties of an isolated CdS microrope composed of twisted nanowires

CdS is one of the important II-VI group semiconductors. In this paper, the electrical transport behavior of an individual CdS microrope composed of twisted nanowires is studied. It is found that the current-voltage (I-V) characteristics show two distinct power law regions from 360 down to 60 K. Space-charge-limited current (SCLC) theory is used to explain these temperature- and electric-field-dependent I-V curves. The I-V data can be well fitted by this theory above 100 K, and the corresponding carrier mobility, trap energy, and trap concentration are also obtained. However, the I-V data exhibit some features of the Coulomb blockade effect below 80 K.

Hierarchically structured microfibers of "single stack" perylene bisimide and quaterthiophene nanowires

Organic nanowires and microfibers are excellent model systems for charge transport in organic semiconductors under nanoscopic confinement and may be relevant for future nanoelectronic devices. For this purpose, however, the preparation of well-ordered organic nanowires with uniform lateral dimensions remains a challenge to achieve. Here, we used the self-assembly of oligopeptide-substituted perylene bisimides and quaterthiophenes to obtain well-ordered nanofibrils. The individual nanofibrils were investigated by spectroscopic and imaging methods, and the preparation of hierarchically structured microfibers of aligned nanofibrils allowed for a comprehensive structural characterization on all length scales with molecular level precision. Thus, we showed that the molecular chirality resulted in supramolecular helicity, which supposedly serves to suppress lateral aggregation. We also proved that, as a result, the individual nanofibrils comprised a single stack of the π-conjugated molecules at their core. Moreover, the conformational flexibility between the hydrogen-bonded oligopeptides and the π-π stacked chromophores gave rise to synergistically enhanced strong π-π interactions and hydrogen-bonding. The result is a remarkably tight π-π stacking inside the nanofibrils, irrespective of the electronic nature of the employed chromophores, which may render them suitable nanowire models to investigate one-dimensional charge transport along defined π-π stacks of p-type or n-type semiconductors.

Tunability of electronic states in ultrathin gold nanowires

The electronic state in ultrathin gold nanowires is tuned by careful engineering of the device architecture via a chemical methodology. The electrons are localized to an insulating state (showing variable range hopping transport) by simply bringing them close to the substrate, while the insertion of an interlayer leads to a Tomonaga Luttinger liquid state.

Electronic transport in conducting polymer nanowire array devices

We report on the temperature dependent conductivity and current-voltage (I-V) properties of novel polyaniline nanowire array devices. Below 60 K, I-V measurements show a transition to non-linear behaviour, leading to the onset at 30 K of a threshold voltage, for potentials below which little current flows. By considering an intrinsic morphology of small conducting regions separated by tunnel junctions, we show that charging of the conducting regions leads to Coulomb blockade effects that can account for this behaviour.

Apparent power-law behavior of conductance in disordered quasi-one-dimensional systems

The dependence of hopping conductance on temperature and voltage for an ensemble of modestly long one-dimensional wires is studied numerically using the shortest-path algorithm. In a wide range of parameters this dependence can be approximated by a power law rather than the usual stretched-exponential form. The relation to recent experiments and prior analytical theory is discussed.

Bias dependent crossover from variable range hopping to power law characteristics in the resistivity of polymer nanowires

The electronic transport properties of ultra-low doped conducting polymer nanowires exhibit characteristics of a pinned one-dimensional Wigner crystal (1D WC) due to the long range electron-electron interaction at low temperature (<30 K). These wires also show characteristics of three-dimensional variable range hopping (3D VRH) at higher temperature. Here we report a resistivity study of these nanowires as a function of the bias around and above 30 K, to show that a crossover takes place from 3D VRH to power law behavior as the bias voltage or current is increased from a low to a relatively high value. The experimental results for this temperature range show several similarities to the theoretically predicted properties of disordered Lüttinger liquid, though at lower temperature the characteristics of the 1D WC are obtained for these nanowires.

Entanglement-free fibrils of aligned polyacetylene films that produce single nanofibers

An aligned polyacetylene (PA) film was synthesized in a macroscopically-aligned nematic liquid crystal (N-LC) solvent using a gravity-flow method. Long and single nanofibers of less than 100 nm in radius were successfully prepared by ultrasonication of the aligned PA film immersed in ethanol. The usual PA film was synthesized in an isotropic solvent, such as toluene, only yielding short and non-dispersed fibers after the ultrasonication due to the entangled fibril morphology. Entanglement-free fibrils of the aligned PA were well-separated into single fibrils through ultrasonication, even without a surfactant.

Nonlinear transport in semiconducting polymers at high carrier densities

Conducting and semiconducting polymers are important materials in the development of printed, flexible, large-area electronics such as flat-panel displays and photovoltaic cells. There has been rapid progress in developing conjugated polymers with high transport mobility required for high-performance field-effect transistors (FETs), beginning with mobilities around 10(-4) cm(2) V(-1) s(-1) to a recent report of 1 cm(2) V(-1) s(-1) for poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT). Here, the electrical properties of PBTTT are studied at high charge densities both as the semiconductor layer in FETs and in electrochemically doped films to determine the transport mechanism. We show that data obtained using a wide range of parameters (temperature, gate-induced carrier density, source-drain voltage and doping level) scale onto the universal curve predicted for transport in the Luttinger liquid description of the one-dimensional 'metal'.

Directional growth of metallic and polymeric nanowires

This work delineates the mechanism by which directional nanowire growth occurs in the directed electrochemical nanowire assembly (DENA) technique for growing nanowires on micro-electrode arrays. Indium, polythiophene, and polypyrrole nanowires are the subjects of this study. This technique allows the user to specify the growth path without the use of a mechanical template. Nanowire growth from a user-selected electrode to within +/- 3 microm of the straight line path to a second electrode lying within a approximately 140 degrees angular range and a approximately 100 microm radius of the selected electrode is demonstrated. Theory for one-dimensional electrochemical diffusion in the inter-electrode region reveals that screening of the applied voltage is incomplete, allowing a long range voltage component to extend from the biased to the grounded electrode. Numerical analysis of two-dimensional multi-electrode arrays shows that a linear ridge of electric field maxima bridges the gap between selected electrodes but decays in all other directions. The presence of this anisotropic, long range voltage defines the wire growth path and suppresses the inherent tip splitting tendency of amorphous polymeric materials. This technology allows polythiophene and polypyrrole to be grown as wires rather than fractal aggregates or films, establishing DENA as an on-chip approach to both crystalline metallic and amorphous polymeric nanowire growth.

Tailored polymer-based nanofibers and nanotubes by means of different infiltration methods into alumina nanopores

Template synthesis is one of the most effective methods for the preparation of one-dimensional polymer-based nanostructures (1DPNs). Both hollow nanotubes and solid nanorods or nanofibers with tailored dimensions can be obtained by simply templating a porous material with suitable pore size. The mechanism of polymer infiltration into the pores is also very important in order to obtain the desired one-dimensional nanostructures and to control their final morphology. In this study, several infiltration methods were explored with the aim to obtain different 1DPNs. It was shown that, with these infiltration methods, it is possible to obtain nanofibers and nanotubes of any diameter and length composed of polymers with a wide chemical nature (poly(methyl methacrylate), poly(vinyl chloride), poly(vinyl alcohol), poly(vinylidene fluoride), etc.), or even composed of nanoparticulate composites. Finally, the selection of infiltration method for desired nanostructure is discussed.

Electrochemical deposition of polypyrrole nanolayers on discontinuous ultrathin gold films

Ultrathin layers of polypyrrole (PPy) were electrochemically grown between microelectrodes on a Si/SiO(2) substrate. Conducting nanolayers of PPy are directly grown onto ultrathin discontinuous gold (Au) film between the microelectrodes, with thicknesses in the range 10-100 nm. The system therefore forms a novel (PPy/Au) nanocomposite conductor. Atomic force microscopy (AFM) imaging and conductivity measurements indicate that at all thicknesses a relatively uniform film is formed but with significant roughness that reflects the roughness of the metallic island layer. In PPy/Au films with thickness ∼10 nm, the small barriers around the gold islands dominate the conduction, and as the film thickness increases to 100 nm the intrinsic conductivity of highly doped PPy dominates the charge transport.

Highly Twisted Helical Polyacetylene with Morphology Free From the Bundle of Fibrils Synthesized in Chiral Nematic Liquid Crystal Reaction Field

We synthesized novel axially chiral binaphthyl derivatives with highly twisting powers by substituting phenylcyclohexyl (PCH) mesogenic moieties into 2,2' positions or 2,2',6,6' positions of binaphthyl rings. The di- and tetrasubstituted binaphthyl derivatives, abbreviated as D-1 and D-2, respectively, were adopted as chiral dopants to induce chiral nematic liquid crystals (N*-LCs) available for synthesis of helical polyacetylene. The helical twisting power (betaM) of D-2 was 449 microm(-1), which was ca. 2.6 times larger than that of D-1 (171 microm(-1)). We prepared two kinds of induced N*-LCs with 5 microm and 270 nm in helical pitch by adding the chiral dopants D-1 and D-2 into the host N-LCs, respectively. The helical polyacetylene synthesized in the N*-LC containing D-2 exhibited highly screwed fibrils, but not a bundle of fibrils. This result is in quite contrast to the usual fibril morphology, where the screwed fibrils are gathered to form the bundle of fibrils, as observed in the helical polyacetylene synthesized in the N*-LC containing a chiral dopant with moderate helical twisting power, such as D-1. It is of keen interest that the helical pitch (270 nm) of the N*-LC including D-2 is much smaller than the diameter (ca. 1 microm) of the bundle of fibrils, which should depress the formation of the bundle of fibrils. The morphology free from the bundle of fibrils might enable us to evaluate more precisely intrinsic electromagnetic properties of a single screwed fibril of helical polyacetylene.