Quasi-one-dimensional transport in conducting polymer nanowires

Recent advances in the synthesis of nanowires based on conducting conjugated polymers are described. The results of recent experimental studies of the electrical properties of polymer nanowires are critically analyzed. The applicability of various theoretical models of tunneling in one-dimensional conductors (variable-range hopping conductivity, Luttinger liquid, Wigner crystal, etc.) to the interpretation of the experimental data on the electronic transport in polymer nanosystems is discussed. A phenomenological model is proposed describing the mechanism of transport in polymer nanowires with allowance for the quasi-one-dimensional nature of molecules of conjugated polymers. The first applications of polymer nanowires in nanoelectronics and prospects for the future are discussed.     

Oxide reduced silicon nanowires

Core crystalline silicon nanowires with a heavily reduced amorphous shell have been successfully synthesised using palladium as a metal catalyst. We present two approaches to reduce the oxidation of the nanowires during the thermal annealing growth. The ratios of the amorphous shell to crystalline core of the nanowires produced, from the two methods, are compared and show a remarkable drop (hence thinner oxide) compared to wires fabricated using currently available techniques. In addition, a focused ion beam was utilised to contact the oxide-reduced nanowires for transport measurements, without first removing the thin oxide shell. The oxygen-reduced core-shell silicon nanowires showed a very low electrical resistivity (4 × 10⁻¹ Ω cm). Our novel approach presents a new alternative to the production of low cost, high yield, highly conducting silicon nanowires offering a wide range of opportunities for semiconductor based technology.     

Electron quantum conductance of bimetallic Pt-Fe nanowires

The magnetic and conducting properties of freestanding Pt-Fe nanowires are investigated by means of molecular dynamics. Our investigation reveals the dependence between a wire’s conductivity and its geometry and magnetic properties. Ab initio calculations show there is no spin polarization of a current in stretched linear Pt-Fe nanowires with antiferromagnetic ordering. Spin polarization of electron transport is observed upon contraction of the wire and is accompanied by its transition into a zig-zag configuration.     

Synthesis of Ordered Ultra-long Manganite Nanowires via Electrospinning Method

We develop a new electrospinning method to prepare ultra-long ordered La_(1-x)Sr_xMnO_3(LSMO) nanowires.The length is up to several centimeters and is only limited by the size of the collector.The well-ordered straight-line structure ensures the transport measurement,which is impossible to be carried out for the random nanowires fabricated by the traditional electrospinning method.Magnetic and transport measurements indicate that the physical properties of the LSMO nanowires depend sensitively on the doping concentration.At the optimum doping,the LSMO wires are ferromagnetic at room temperature with a metal-insulator transition temperature close to room temperature.Magnetic force microscopy studies are also performed to provide a microscopic view of these ultra-long nanowires.     

Electronic transport properties of lead nanowires

Lead nanowire occupies a very important position in an electronic device. In this study, a genetic algorithm(GA)method has been used to simulate the Pb nanowire. The result shows that Pb nanowires are a multishell cylinder. Each shell consists of atomic rows wound up helically side by side. The quantum electron transport properties of these structures are calculated based on the non-equilibrium Green function(NEGF) combined with the density functional theory(DFT),which indicate that electronic transport ability increases gradually with the atomic number increase. In addition, the thickest nanowire shows excellent electron transport performance. It possesses great transmission at the Fermi level due to the strongest delocalization of the electronic state. The results provide valuable information on the relationship between the transport properties of nanowires and their diameter.     

Percolating conduction in finite nanotube networks

The percolating conductance of a new class of nanocomposite thin-film transistors, with channels composed of isotropic ensembles of nanotubes or nanowires, is analyzed as a function of wire/tube density and channel length. The conductance exponents are validated against analytical results for short channel transistors, and against available experimental data for longer channel devices. Our plots of conductance exponents as a function of tube-to-tube coupling strength provide a unified framework to interpret future experiments and should help design better nanocomposite transistors.     

Electronic structure effects on stability and quantum conductance in 2D gold nanowires

In this study, we have investigated the stability and conductivity of unsupported, two-dimensional infinite gold nanowires using ab initio density functional theory (DFT). Two-dimensional ribbon-like nanowires with 1–5 rows of gold atoms in the non-periodic direction and with different possible structures have been considered. The nanowires with >2 rows of atoms exhibit dimerization, similar to finite wires, along the non-periodic direction. Our results show that in these zero thickness nanowires, the parallelogram motif is the most stable. A comparison between parallelogram- and rectangular-shaped nanowires of increasing width indicates that zero thickness (111) oriented wires have a higher stability over (100). A detailed analysis of the electronic structure, reveals that the (111) oriented structures show increased delocalization of s and p electrons in addition to a stronger delocalization of the d electrons and hence are the most stable. The density of states show that the nanowires are metallic and conducting except for the double zigzag structure, which is semiconducting. Conductance calculations show transmission for a wide range of energies in all the stable nanowires with more than two rows of atoms. The conductance channels are not purely s and have strong contributions from the d levels, and weak contributions from the p levels.     

The effect of the electron-phonon coupling on the thermal conductivity of silicon nanowires

The thermal conductivity of free-standing silicon nanowires (SiNWs) with diameters from 1-3?nm has been studied by using the one-dimensional Boltzmann's transport equation. Our model explicitly accounts for the Umklapp scattering process and electron-phonon coupling effects in the calculation of the phonon scattering rates. The role of the electron-phonon coupling in the heat transport is relatively small for large silicon nanowires. It is found that the effect of the electron-phonon coupling on the thermal conduction is enhanced as the diameter of the silicon nanowires decreases. Electrons in the conduction band scatter low-energy phonons effectively where surface modes dominate, resulting in a smaller thermal conductivity. Neglecting the electron-phonon coupling leads to overestimation of the thermal transport for ultra-thin SiNWs. The detailed study of the phonon density of states from the surface atoms and central atoms shows a better understanding of the nontrivial size dependence of the heat transport in silicon nanowire.     

Charge carrier transport in conducting polymers on the metal side of the metal-insulator transition: A review

The advances achieved in the field of the synthesis of conducting polymer films and the results of experimental investigations of the mechanism of charge carrier transport in heavily doped conducting polymer films on the metal side of the metal-insulator transition have been considered. The samples studied belong to the last generation of conducting polymers with a low degree of structural disorder, which makes it possible to dope samples to a highly conducting state characterized by an increased stability. Apart from the study of the morphology, structure, and doping conditions, the mechanism of low-temperature transport in these polymers has been investigated in detail. The results of investigations of the transport at low temperatures (T < 1 K) have been described, and a phenomenological model of charge carrier transfer in these systems has been proposed. The polymers under investigation have been widely used as injecting and accumulating layers in light-emitting organic diodes and solar cells.     

Semiconductor nanowires for novel one-dimensional devices

Low-dimensional semiconductors offer interesting physical phenomena but also the possibility to realize novel types of devices based on, for instance, 1D structures. By using traditional top-down fabrication methods the performance of devices is often limited by the quality of the processed device structures. In many cases damage makes ultra-small devices unusable. In this work we present a recently developed method for bottom-up fabrication of epitaxially nucleated semiconductor nanowires based on metallic nanoparticle-induced formation of self-assembled nanowires. Further development of the vapor–liquid–solid growth method have made it possible to control not only the dimension and position of nanowires but also to control heterostructures formed inside the nanowires. Based on these techniques we have realized a series of transport devices such as resonant tunneling and single-electron transistors but also optically active single quantum dots positioned inside nanowires displaying sharp emission characteristics due to excitons.     

Experimental evidence and physical understanding of ZnO vapor-liquid-solid nanowire growth

Metal-catalyst-assisted thermal chemical vapor transport is one of most popular techniques for ZnO nanowires preparation, and the vapor-liquid-solid (VLS) process is recognized to be responsible for ZnO nanowires growth upon metal-catalyst-assisted thermal chemical vapor transport. However, there have been very few investigations to provide substantial experimental evidence for supporting ZnO VLS nanowires growth upon metal-catalyst-assisted thermal chemical vapor transport, so far. Herein, we report a study of ZnO nanowires growth using metal-catalyst-assisted thermal chemical vapor transport based on laser ablation, and we provide solid experimental evidence for the VLS process of ZnO nanowires.     

Radial heterostructure and interface effects on thermoelectric transport properties of Bi/Sn and Bi/Sb core/shell nanowires

The thermoelectric transport properties of Bi/Sn and Bi/Sb core/shell (C/S) nanowires grown by the method of on-film formation of nanowires were systematically investigated. The electrical conductivity and Seebeck coefficient of nanowires with different diameters were measured as a function of the temperature. The contribution of Sn and Sb shells to the total transport in the C/S nanowires was determined using analytical fitting based on the parallel combination of the conductive system model. The carrier-interface boundary scattering at the C/S interface was quantitatively evaluated as the sheet resistance. In addition, the effect of hole doping on the transport properties was also observed in the Bi/Sn C/S nanowires.     

Combination of Fe-Mn based Li-rich cathode materials and conducting-polymer polypyrrole nanowires with high rate capability

The polypyrrole (PPy) nanowires are conducting 1D materials, which can significantly improve the electrical conductivity of the composites. A novel Li_1.26Fe_0.22Mn_0.52O₂ (LFMO) @ PPy nanowire composites were synthesized by simply ultrasonic dispersing LFMO and PPy nanowires in aqueous ethanol. The structure and morphology of pristine LFMO and LFMO@PPy are investigated by XRD, SEM, and TEM. The elemental mapping and FTIR results demonstrate the conductive network of PPy nanowires exists in the composites and the LFMO particles uniformly distribute on the PPy nanowires. LFMO combined with PPy nanowires exhibits better rate capability, higher capacity, coulombic efficiency, and cycleability than the pristine. The rate performance of composites with 10 wt% PPy nanowires shows the discharge capacities of 132.2 mAh/g and 98 mAh/g at 1C and 3C rate after 50 cycles, respectively. Electrochemical impedance spectroscopy test suggests that the conductive PPy nanowires can significantly decrease the charge-transfer resistance of LFMO. The composite with 10 wt% PPy nanowires shows a discharge capacity retention of 71% after 50 cycles at 1C, while the pristine sample only has 50% capacity retention.     

Template synthesis of M/M2S (M = Ag,Cu) hetero-nanowires by electrochemical technique

Metal chalcogenide-based mixed ionic–electronic conductors such as Ag₂S and Cu₂S can be specifically architected for application in nanoelectronic devices. We present a template-confined synthesis of metal chalcogenide (e.g., Ag₂S, Cu₂S) nanowires for mixed conductor-based nanoelectronics. First, the metal nanowire array was electroplated into pores of a porous alumina membrane. Anodic polarization was then used to transform the metal into the metal sulfide in aqueous hydrosulfide (HS⁻) solutions. The as-synthesized mixed conductors' hetero-nanowire array was characterized by X-ray diffraction and electron microscopy. Electronic transport measurements show non-linear and reproducible electrical switching characteristics. The high and low resistance states can be reversibly changed by altering the polarity of the applied voltage between the bottom and top electrodes. The electrical-switching behavior is attributed to electric-field-induced accumulation and dissolution of metallic conducting pathways inside the mixed conductors' nanowires.     

Control of ZnO nanowire arrays by nanosphere lithography (NSL) on laser-produced ZnO substrates

Nanosphere lithography (NSL) is a successful technique for fabricating highly ordered arrays of ZnO nanowires typically on sapphire and GaN substrates. In this work, we investigate the use of thin ZnO films deposited on Si by pulsed laser deposition (PLD) as the substrate. This has a number of advantages over the alternatives above, including cost and potential scalability of production and it removes any issue of inadvertent n-type doping of nanowires by diffusion from the substrate. We demonstrate ordered arrays of ZnO nanowires, on ZnO-coated substrates by PLD, using a conventional NSL technique with gold as the catalyst. The nanowires were produced by vapor phase transport (VPT) growth in a tube furnace system and grew only on the areas pre-patterned by Au. We have also investigated the growth of ZnO nanowires using ZnO catalyst points deposited by PLD through an NSL mask on a bare silicon substrate.     

Structure, stability, and quantum conductivity of small diameter silicon nanowires

Structures and energetics of various types of silicon nanowires have been investigated using both quantum and classical molecular dynamics simulations to determine the most stable forms. The tetrahedral type nanowires have been found to be the most stable and, surprisingly, the polycrystalline forms of nanowires, while having the smallest surface to bulk ratio, are found to be the least stable. We also show that the cagelike nanowires have greater thermal stability than the tetrahedral nanowires. Furthermore, their electrical conducting properties are found to be better than those of tetrahedral nanowires, suggesting useful molecular electronic applications.     

Tuning dimensionality by nanowire adsorption on layered materials

The dimensionality of electronic states determines a number of physical phenomena such as phase transitions, transport, or superconductivity. Employing scanning tunneling microscopy combined with angle-resolved photoemission spectroscopy we demonstrate how the dimensionality of electronic states can be continuously tuned from three to two dimensions. This is achieved by adsorption of nanowires on surfaces of layered crystals without changing the chemical composition of the material. Exemplary results for Rb nanowires on TiTe2 are discussed with the help of electronic structure calculations.     

Hybrid solar cells with conducting polymers and vertically aligned silicon nanowire arrays: The effect of silicon conductivity

Organic/inorganic hybrid solar cells, based on vertically aligned n-type silicon nanowires (n-Si NWs) and p-type conducting polymers (PEDOT:PSS), were investigated as a function of Si conductivity. The n-Si NWs were easily prepared from the n-Si wafer by employing a silver nanodot-mediated micro-electrochemical redox reaction. This investigation shows that the photocurrent-to-voltage characteristics of the n-Si NW/PEDOT:PSS cells clearly exhibit a stable rectifying diode behavior. The increase in current density and fill factor using high conductive silicon is attributed to an improved charge transport towards the electrodes achieved by lowering the device's series resistance. Our results also show that the surface area of the nanowire that can form heterojunction domains significantly influences the device performance.     

Electronic transport behavior of diameter-graded Ag nanowires

Ag nanowires with a graded diameter in anodic aluminum oxide (AAO) membranes were fabricated by the direct-current electrodeposition. The Ag nanowires have a graded-change in diameter from 8 to 32 nm, which is matched with the graded-change of the AAO pore diameter. Electronic transport measurements show that there is a transport behavior similar to that of a metal-semiconductor junction along the axial direction in the diameter-graded Ag nanowires. Such a novel homogeneous nanojunction will be of great fundamental and practical significance.     

Magnetic field effects on total and partial conductance histograms in Cu and Ni nanowires

Conductance histograms have become a powerful tool for studying transport properties of metallic nanowires. However, the individual conductance curves display a very rich structure that might be concealed by the statistical procedure of finding preferred conductance values by building conductance occurrence histograms using consecutive nanocontact breakage experiments. This is particularly true when it comes to discerning 1/2G₀=e²/hquantization in magnetic nanowires. The effect of disorder, added to possible magnetic sources of scattering, and different magnetic states of different nanowires, might hide its appearance as histogram peaks. This work analyzes and compares Ni and Cu nanowire experimental histograms at room temperature (RT). Those obtained with no curve selection criteria are basically unaffected by the presence of a magnetic field. A selection of particular sets of conductance curves shows that conductance quantization could occur in steps of e²/h and 2e²/h in Ni as well as in Cu in the presence or absence of a magnetic field. Sorting out curves in sets that present conductance plateaus at half integer and integer values, and compiling statistics on the number of such curves that appear, depending on the applied magnetic field, results in differences between the behaviour of Cu and Ni. While for Cu, the magnetic field keeps the ratio of curves that present plateaus at 1/2G₀with respect those presenting G₀ plateaus unchanged; for Ni, the number of curves which exhibit plateaus at just G₀ almost disappears with the applied field. This experimental fact might indicate that the magnetic field removes spin degeneracy in these magnetic nanowires. PACS 72.25.Ba; 73.40.Jn; 73.63.Rt; 75.75.+a