Smooth and conductive DNA-templated Cu₂O nanowires: growth morphology, spectroscopic and electrical characterization

DNA strands have been used as templates for the self-assembly of smooth and conductive cuprous oxide (Cu?O) nanowires of diameter 12-23 nm and whose length is determined by the template (16 μm for λ-DNA). A combination of spectroscopic, diffraction and probe microscopy techniques showed that these nanowires comprise single crystallites of Cu?O bound to the DNA molecules which fused together over time in a process analogous to Ostwald ripening, but driven by the free energy of interaction with the template as well as the surface tension. Electrical characterization of the nanowires by a non-contact method, scanned conductance microscopy and by contact mode conductive AFM showed the wires are electrically conductive. The conductivity estimated from the AFM cross section and the zero-bias conductance in conductive AFM experiments was 2.2-3.3 S cm?1. These Cu?O nanowires are amongst the thinnest reported and show evidence of strong quantum confinement in electronic spectra.     

Programmable assembly of nanoarchitectures using genetically engineered viruses

Biological systems possess inherent molecular recognition and self-assembly capabilities and are attractive templates for constructing complex material structures with molecular precision. Here we report the assembly of various nanoachitectures including nanoparticle arrays, hetero-nanoparticle architectures, and nanowires utilizing highly engineered M13 bacteriophage as templates. The genome of M13 phage can be rationally engineered to produce viral particles with distinct substrate-specific peptides expressed on the filamentous capsid and the ends, providing a generic template for programmable assembly of complex nanostructures. Phage clones with gold-binding motifs on the capsid and streptavidin-binding motifs at one end are created and used to assemble Au and CdSe nanocrytals into ordered one-dimensional arrays and more complex geometries. Initial studies show such nanoparticle arrays can further function as templates to nucleate highly conductive nanowires that are important for addressing/interconnecting individual nanostructures.     

Actin-based metallic nanowires as bio-nanotransporters

The synthesis of conductive nanowires or patterned conductive nanoelements is a challenging goal for the future fabrication of nanoscale circuitry. Similarly, the realization of nanoscale mechanics might introduce a new facet to the area of nanobiotechnology. Here we report on the design of conductive and patterned actin-based gold nanowires, and on the ATP-driven motility of the nano-objects. The polymerization of G-actin labelled with Au nanoparticles, followed by the catalytic enlargement of the nanoparticles, yields gold wires (1-4 microm long and 80-200 nm high) exhibiting high electrical conductivity. The polymerization of the Au nanoparticle/G-actin monomer followed by the polymerization of free G-actin, or alternatively the polymerization of the Au-nanoparticle-labelled G-actin on polymerized F-actin followed by the catalytic enlargement of the particles, yields patterned actin-Au wire-actin or Au wire-actin-Au wire nanostructures, respectively. We demonstrate the ATP-fuelled motility of the actin-Au wire-actin filaments on a myosin interface. These actin-based metallic wires and their nanotransporting funcionality introduce new concepts for developing biological/inorganic hybrid devices.     

Highly Conductive Thin Uniform Gold‐Coated DNA Nanowires

Over the past decades, DNA, the carrier of genetic information, has been used by researchers as a structural template material. Watson‐Crick base pairing enables the formation of complex 2D and 3D structures from DNA through self‐assembly. Various methods have been developed to functionalize these structures for numerous utilities. Metallization of DNA has attracted much attention as a means of forming conductive nanostructures. Nevertheless, most of the metallized DNA wires reported so far suffer from irregularity and lack of end‐to‐end electrical connectivity. An effective technique for formation of thin gold‐coated DNA wires that overcomes these drawbacks is developed and presented here. A conductive atomic force microscopy setup, which is suitable for measuring tens to thousands of nanometer long molecules and wires, is used to characterize these DNA‐based nanowires. The wires reported here are the narrowest gold‐coated DNA wires that display long‐range conductivity. The measurements presented show that the conductivity is limited by defects, and that thicker gold coating reduces the number of defects and increases the conductive length. This preparation method enables the formation of molecular wires with dimensions and uniformity that are much more suitable for DNA‐based molecular electronics.     

Fabrication of a nanostructure thermal property measurement platform

Measurements of the electrical and thermal transport properties of one-dimensional nanostructures (e.g.?nanotubes and nanowires) are typically obtained without detailed knowledge of the specimen's atomic-scale structure or defects. To address this deficiency, we have developed a microfabricated, chip-based characterization platform that enables both transmission electron microscopy (TEM) of the atomic structure and defects as well as measurement of the thermal transport properties of individual nanostructures. The platform features a suspended heater line that physically contacts the center of a suspended nanostructure/nanowire that was placed using in?situ scanning electron microscope nanomanipulators. Suspension of the nanostructure across a through-hole enables TEM characterization of the atomic and defect structure (dislocations, stacking faults, etc) of the test sample. This paper explains, in detail, the processing steps involved in creating this thermal property measurement platform. As a model study, we report the use of this platform to measure the thermal conductivity and defect structure of a GaN nanowire.     

Sn-doped hematite nanostructures for photoelectrochemical water splitting

We report on the synthesis and characterization of Sn-doped hematite nanowires and nanocorals as well as their implementation as photoanodes for photoelectrochemical water splitting. The hematite nanowires were prepared on a fluorine-doped tin oxide (FTO) substrate by a hydrothermal method, followed by high temperature sintering in air to incorporate Sn, diffused from the FTO substrate, as a dopant. Sn-doped hematite nanocorals were prepared by the same method, by adding tin(IV) chloride as the Sn precursor. X-ray photoelectron spectroscopy analysis confirms Sn(4+) substitution at Fe(3+) sites in hematite, and Sn-dopant levels increase with sintering temperature. Sn dopant serves as an electron donor and increases the carrier density of hematite nanostructures. The hematite nanowires sintered at 800 °C yielded a pronounced photocurrent density of 1.24 mA/cm(2) at 1.23 V vs RHE, which is the highest value observed for hematite nanowires. In comparison to nanowires, Sn-doped hematite nanocorals exhibit smaller feature sizes and increased surface areas. Significantly, they showed a remarkable photocurrent density of 1.86 mA/cm(2) at 1.23 V vs RHE, which is approximately 1.5 times higher than that of the nanowires. Ultrafast spectroscopy studies revealed that there is significant electron-hole recombination within the first few picoseconds, while Sn doping and the change of surface morphology have no major effect on the ultrafast dynamics of the charge carriers on the picosecond time scales. The enhanced photoactivity in Sn-doped hematite nanostructures should be due to the improved electrical conductivity and increased surface area.     

Contactless Electrical and Structural Characterization of Semiconductor Nanowires with Axially Modulated Doping Profiles

Efficient characterization of semiconductor nanowires having complex dopant profiles or heterostructures is critical to fully understand these materials and the devices built from them. Existing electrical characterization techniques are slow and laborious, particularly for multisegment nanowires, and impede the statistical understanding of highly variable samples. Here, it is shown that electro‐orientation spectroscopy (EOS)—a high‐throughput, noncontact method for statistically characterizing the electrical properties of entire nanowire ensembles—can determine the conductivity and dimensions of two distinct segments in individual Si nanowires with axially encoded dopant profiles. This analysis combines experimental measurements and computational simulations to determine the electrical conductivity of the nominally undoped segment of two‐segment Si nanowires, as well as the ratio of the segment lengths. The efficacy of this approach is demonstrated by comparing results generated by EOS with conventional four‐point‐probe measurements. This work provides new insights into the control and variability of semiconductor nanowires for electronic applications and is a critical first step toward the high‐throughput interrogation of complete nanowire‐based devices.     

Multisegment nanowire sensors for the detection of DNA molecules

We describe a novel application for detecting specific single strand DNA sequences using multisegment nanowires via a straightforward surface functionalization method. Nanowires comprising CdTe-Au-CdTe segments are fabricated using electrochemical deposition, and electrical characterization indicates a p-type behavior for the multisegment nanostructures, in a back-to-back Schottky diode configuration. Such nanostructures modified with thiol-terminated probe DNA fragments could function as high fidelity sensors for biomolecules at very low concentration. The gold segment is utilized for functionalization and binding of single strand DNA (ssDNA) fragments while the CdTe segments at both ends serve to modulate the equilibrium Fermi level of the heterojunction device upon hybridization of the complementary DNA fragments (cDNA) to the ssDNA over the Au segment. Employing such multisegment nanowires could lead to the fabrication more sophisticated and high multispecificity biosensors via selective functionalization of individual segments for biowarfare sensing and medical diagnostics applications.     

Effect of silver nanowires on electrical conductance of system composed of silver particles

Silver nanowires were prepared by polyol process and doped to the system containing conventional micro-scaled sphere or flake silver particles. It is found that the conductance of doped system is much better than the undoped system with the same quantity silver. A plausible mechanism is described as that these nanowires are favor to act as bridges to establish perfect linkage among particles, and the chance of contact and contact area become more than the cases without wires. This development will be meaningful for the preparation of advanced electrically conductive adhesive (ECA) with high conductivity and good adhesive strength.     

Electrical and rheological percolation of polymer nanocomposites prepared with functionalized copper nanowires

The morphological, electrical and rheological characterization of polystyrene nanocomposites containing copper nanowires (CuNWs) functionalized with 1-octanethiol is presented. Characterization by SEM and TEM shows that surface functionalization of the nanowires resulted in significant dispersion of CuNWs in the PS matrix. The electrical characterization of the nanocomposites indicates that functionalized CuNWs start to form electrically conductive networks at lower concentrations (0.25?vol% Cu) than using unfunctionalized CuNWs (0.5?vol% Cu). The organic coating on the nanowires prevents significant changes in the electrical resistivity in the vicinity of the percolation threshold. Percolated nanocomposites showed electrical resistivity in the range of 10(6)-10(7)?Ω?cm. The transition from liquid-like to solid-like behavior (rheological percolation) of the nanocomposites was studied using dynamic rheology at 200?°C. Unfunctionalized CuNWs result in electrical and rheological percolation at similar concentrations. Functionalized CuNWs show rheological percolation at higher concentration (1.0-2.0?vol%) than that required for electrical percolation. This is attributed to the decrease in the interfacial tension between nanowires and polymer chains and its effect on the viscoelastic behavior of the combined polymer-nanowire networks.     

High quantum efficiency of band-edge emission from ZnO nanowires

External quantum efficiency (EQE) of photoluminescence as high as 20% from isolated ZnO nanowires were measured at room temperature. The EQE was found to be highly dependent on photoexcitation density, which underscores the importance of uniform optical excitation during the EQE measurement. An integrating sphere coupled to a microscopic imaging system was used in this work, which enabled the EQE measurement on isolated ZnO nanowires. The EQE values obtained here are significantly higher than those reported for ZnO materials in forms of bulk, thin films or powders. Additional insight on the radiative extraction factor of one-dimensional nanostructures was gained by measuring the internal quantum efficiency of individual nanowires. Such quantitative EQE measurements provide a sensitive, noninvasive method to characterize the optical properties of low-dimensional nanostructures and allow tuning of synthesis parameters for optimization of nanoscale materials.     

Effect of silver nanostructures on the resistivity of electrically conductive adhesives composed of silver flakes

Silver flakes are the most widely applied conductive fillers in electrically conductive adhesives (ECAs) because of their high conductivity and stable chemical properties. It is expected that there are advanced ECAs with both high electrical conductance and good adhesive strength. The high filler loadings can improve the conductance of ECAs, whereas the adhesive strength is decreased. Silver nanostructures are incorporated for the purpose of electrical conductance and adhesive strength improvement of ECAs. A simple method has enabled the synthesis of silver nanostructures by reducing silver nitrate with ethylene glycol in the presence of poly(N-vinylpyrrolidone). They are added to ECAs by dispersing them in ethanol while it is used as the diluent to adjust the volatility of ECAs, preventing them from the aggregation. This proposed process offers the possibility to effectively use silver nanostructures for improving the conductivity of ECAs at the low content of conductive fillers while good adhesive strength may be obtained.     

Electrical nanowelding and bottom-up nano-construction together using nanoscale solder

A new bottom-up nanowelding technique enabling the welding of complex 3D nanoarchitectures assembled from individual building blocks using nanovolumes of metal solder is reported in this work. The building blocks of gold nanowires, (Co72Pt28/Pt)n multilayer nanowires, and nanosolder Sn99Au1 alloy nanowires were successfully fabricated by a template technique. Individual metallic nanowires dispersed on Si/SiO2(100 nm) wafers were manipulated and assembled together. Conductive nanostructures were then welded together by the new electrical nanowelding technique using nanovolumes of similar or dissimilar nanosolder. At the weld sites, nanoscale volumes of a chosen metal are deposited using nanosolder of a sacrificial nanowire, which ensures that the nanoobjects to be bonded retain their structural integrity. The whole nanowelding process is clean, controllable and reliable, and ensures both mechanically strong and electrically conductive contacts. The quality check of nanoweld achieve a resistance as low as 20 omega by using Sn99Au1 alloy solder. This technique should provide a promising way to conquer the challenge of the integration obstacle for bottom-up nanotechnology.     

Selective synthesis of copper nanoplates and nanowires via a surfactant-assisted hydrothermal process

A facile solution-phase process has been demonstrated for the selective preparation of single-crystalline Cu nanoplates and nanowires by reducing Cu⁺ with ascorbic acid (VC) in the presence of cetyltrimethylammonium bromide (CTAB) or cetyltrimethylammonium chloride (CTAC). To study the formation process of nanoplates and nanowires, samples obtained at various stages of the growth process were studied by TEM and XRD. The possible mechanism was discussed to elucidate the formation of different morphologies of Cu nanostructures. UV–vis spectra of the Cu nanoplates and nanowires were recorded to investigate their optical properties, which indicated that the as-prepared Cu nanostructures exhibited morphology-dependent optical property.     

Controllable hydrothermal synthesis of Cu2S nanowires on the copper substrate

In this paper, a simple solution-based method has been applied to fabricate metal chalcogenide nanostructures. Abundant Cu₂S nanowires on Cu substrates are successfully prepared through the in-situ hydrothermal reaction between sulfur powder and Cu foil. It is observed that the addition of hydrazine and cetyltrimethylammonium bromide plays an important role in the growth of Cu₂S nanowires. A rolling-up mechanism of metal chalcogenide film is used to illustrate the growth of these nanostructures. UV-vis spectrum of Cu₂S nanowires reveals obvious absorption below the wavelength of 900 nm. The calculated band gap of Cu₂S nanowires (1.5 eV) shows obvious blue shift because of the quantum size effect.     

Synthesis and characterization of nanowire-based anisotropic conductors

We investigated the potential of commercially available porous templates to be used for the fabrication of functional anisotropic conductors. A galvanostatic deposition technique was used to fabricate arrays consisting of 200 nm diameter nanowires inside the pores of polycarbonate membranes. A tape lift-off procedure allowed the complete removal of any residual metal from both sides of the polymer membrane to form an anisotropic conductive film. The 10 microm thick film has roughly 3 x 10(8) nanowires per cm2, and it showed near zero electrical resistance perpendicular to the surface while appearing completely open to circuits between any points on the surface. The preparation of the film, characterization using SEM, AFM, and resistance measurements are presented. The 1D conductivity of these membranes may have many potential applications for microelectronic interconnects for packaging technologies.     

Kirkendall approach to the fabrication of ultra-thin ZnO nanotubes with high resistive sensitivity to humidity

Ultra-thin ZnO nanotubes with an inner diameter of ～3?nm and an outer diameter of ～13?nm have been prepared via Kirkendall effect for the first time. The synthetic process was started from ultra-thin Zn nanowires, which were subjected to direct oxidation to form Zn-ZnO core-shell tubular nanostructures with continuous expansion of Kirkendall voids, and finally evolve into the ZnO nanotubes. The ultra-thin ZnO nanotubes have shown unusually high sensitivity to humidity in a resistive sensor mode, which can be attributed to not only the high specific surface areas, but more importantly, the overlapped electric double layers in the nanoscale channels with a significantly enhanced proton conductivity. This new type of proton conductive nanomaterials has important implications in the development of membranes in modern fuel cells among others.     

Highly conductive coaxial SnO(2)-In(2)O(3) heterostructured nanowires for Li ion battery electrodes

Novel SnO(2)-In(2)O(3) heterostructured nanowires were produced via a thermal evaporation method, and their possible nucleation/growth mechanism is proposed. We found that the electronic conductivity of the individual SnO(2)-In(2)O(3) nanowires was 2 orders of magnitude better than that of the pure SnO(2) nanowires, due to the formation of Sn-doped In(2)O(3) caused by the incorporation of Sn into the In(2)O(3) lattice during the nucleation and growth of the In(2)O(3) shell nanostructures. This provides the SnO(2)-In(2)O(3) nanowires with an outstanding lithium storage capacity, making them suitable for promising Li ion battery electrodes.     

A combined nonequilibrium Green’s function/density-functional theory study of electrical conducting properties of artificial DNA duplexes

DNA duplexes have attracted much attention as a primary candidate for nanowires possessing self-organizing capability. To employ DNA duplexes as nanowires, however, a major drawback must be overcome; the guanine bases undergo oxidative degradation in a hole transport through DNA duplexes, which is likely caused by the presence of adjoining adenine bases that do not effectively mediate the charge transport through DNA duplexes. To overcome the drawback, several artificial nucleobases based on adenine have been designed and tested, confirming that the artificial nucleobase-containing DNA duplexes do not suffer from such an oxidative damage and exhibit high efficiency in hole transport through the DNA duplexes. In the present study, we examine the electrical conducting properties of these artificial DNA duplexes by use of nonequilibrium Green’s function and density-functional theory methods. The results explicate the origin of the experimentally observed high conductivity through the DNA duplexes containing the artificial DNA bases. We also put forth a computer-aided design of novel artificial DNA bases with low ionization energies, and examine the electrical conductivity of the DNA duplexes containing the designer nucleobases for potential use as highly conductive nanowires.     

Nanostructures of Sn and their enhanced, shape-dependent superconducting properties

A noncatalytic and template-free vapor transport process was developed to make possible simultaneous growth of single-crystalline tin nanowires, nanosquares, nanodisks, and polycrystalline nanoparticles. The formation of such a rich variety of morphologies in a single growth experiment can be attributed to variations in the growth rate among different crystallographic planes when employing the vapor-solid growth mechanism. Structural characterization with high-resolution transmission electron microscopy reveals a preferential growth direction of [100] in Sn nanowires, nanosquares, and nanodisks. Shape-dependent superconducting properties are observed. These four types of Sn nanostructures all show typical diamagnetic behavior in magnetization measurements, with the three anisotropically shaped nanostructures (nanowires, nanosquares, and nanodisks) showing one order of magnitude enhancement in the working magnetic field ranges for superconductivity, compared to bulk Sn and Sn nanoparticles. The magnetic field range is broadest for nanowires, followed by nanodisks, nanosquares, and nanoparticles.