Quantum confinement induced performance enhancement in sub-5-nm lithographic Si nanowire transistors

We demonstrate lithographically fabricated Si nanowire field effect transistors (FETs) with long Si nanowires of tiny cross sectional size (～3-5 nm) exhibiting high performance without employing complementarily doped junctions or high channel doping. These nanowire FETs show high peak hole mobility (as high as over 1200 cm(2)/(V s)), current density, and drive current as well as low drain leakage current and high on/off ratio. Comparison of nanowire FETs with nanobelt FETs shows enhanced performance is a result of significant quantum confinement in these 3-5 nm wires. This study suggests simple (no additional doping) FETs using tiny top-down nanowires can deliver high performance for potential impact on both CMOS scaling and emerging applications such as biosensing.     

A novel method for preparing vertically grown single-crystalline gold nanowires

A surfactant-free, template-less and seed-less method, namely the thermal-assisted photoreduction (TAP) process, has been developed to synthesize vertically grown Au nanowires (30-80?nm in diameter and about 2?μm in length) on the surface of thin film titanium dioxide (TiO(2)), which is locally excited by blackbody radiation. The Au nanowires thus produced are single-crystalline with a preferred [Formula: see text] growth direction. The electrical behavior investigated using a nanomanipulation device indicates that the Au nanowires possess an excellent electrical resistivity of about 3.49 × 10(-8)?Ω?m.     

Stable field emission from arrays of vertically aligned free-standing metallic nanowires

We present a fully elaborated process to grow arrays of metallic nanowires with controlled geometry and density, based on electrochemical filling of nanopores in track-etched templates. Nanowire growth is performed at room temperature, atmospheric pressure and is compatible with low cost fabrication and large surfaces. This technique offers an excellent control of the orientation, shape and nanowires density. It is applied to fabricate field emission arrays with a good control of the emission site density. We have prepared Co, Ni, Cu and Rh nanowires with a height of 3?μm, a diameter of 80?nm and a density of ～10(7)?cm(-2). The electron field emission measurements and total energy distributions show that the as-grown nanowires exhibit a complex behaviour, first with emission activation under high field, followed by unstable emission. A model taking into account the effect of an oxide layer covering the nanowire surface is developed to explain this particular field emission behaviour. Finally, we present an in situ cleaning procedure by ion bombardment that collectively removes this oxide layer, leading to a stable and reproducible emission behaviour. After treatment, the emission current density is ～1?mA?cm(-2) for a 30?V?μm(-1) applied electric field.     

Stable field emission performance of SiC-nanowire-based cathodes

In this paper we report the fabrication and testing of diode-type low-voltage field emission display (FED) devices with SiC-nanowire-based cathodes. The SiC-nanowire FEDs (flat vacuum lamps) were characterized by low emission threshold fields (～2?V?μm(-1)), high current density and stable long-term performance. The analysis of field emission data evidenced that the Schottky effect would have a considerable influence on the field emission from nanowire-based samples, leading to the true values of the field enhancement factor being significantly lower than those derived from Fowler-Nordheim plots.     

Lead-free KNbO(3) ferroelectric nanorod based flexible nanogenerators and capacitors

In spite of high piezoelectricity, only a few one-dimensional ferroelectric nano-materials with perovskite structure have been used for piezoelectric nanogenerator applications. In this paper, we report high output electrical signals, i.e.?an open-circuit voltage of 3.2?V and a closed-circuit current of 67.5?nA (current density 9.3?nA?cm(-2)) at 0.38% strain and 15.2%?s(-1) strain rate, using randomly aligned lead-free KNbO(3) ferroelectric nanorods (～1?μm length) with piezoelectric coefficient (d(33)?～?55?pm?V (-1)). A flexible piezoelectric nanogenerator is mainly composed of KNbO(3)-poly(dimethylsiloxane) (PDMS) composite sandwiched by Au/Cr-coated polymer substrates. We deposit a thin poly(methyl methacrylate) (PMMA) layer between the KNbO(3)-PDMS composite and the Au/Cr electrode to completely prevent dielectric breakdown during electrical poling and to significantly reduce leakage current during excessive straining. The flexible KNbO(3)-PDMS composite device shows a nearly frequency-independent dielectric constant (～3.2) and low dielectric loss (<0.006) for the frequency range of 10(2)-10(5)?Hz. These results imply that short and randomly aligned ferroelectric nanorods can be used for a flexible high output nanogenerator as well as high-k capacitor applications by performing electrical poling and further optimizing the device structure.     

Gas nanosensor design packages based on tungsten oxide: mesocages, hollow spheres, and nanowires

Achieving proper designs of nanosensors for highly sensitive and selective detection of toxic environmental gases is one of the crucial issues in the field of gas sensor technology, because such designs can lead to the enhancement of gas sensor performance and expansion of their applications. Different geometrical designs of porous tungsten oxide nanostructures, including the mesocages, hollow spheres and nanowires, are synthesized for toxic gas sensor applications. Nanosensor designs with small crystalline size, large specific surface area, and superior physical characteristics enable the highly sensitive and selective detection of low concentration (ppm levels), highly toxic NO(2) among CO, as well as volatile organic compound gases, such as acetone, benzene, and ethanol. The experimental results showed that the sensor response was not only dependent on the specific surface area, but also on the geometries and crystal size of materials. Among the designed nanosensors, the nanowires showed the highest sensitivity, followed by the mesocages and hollow spheres-despite the fact that mesocages had the largest specific surface area of 80.9?m(2)?g( - 1), followed by nanowires (69.4?m(2)?g( - 1)), and hollow spheres (6.5?m(2)?g( - 1)). The nanowire sensors had a moderate specific surface area (69.4?m(2)?g( - 1)) but they exhibited the highest sensitivity because of their small diameter (～5?nm), which approximates the Debye length of WO(3). This led to the depletion of the entire volume of the nanowires upon exposure to NO(2), resulting in an enormous increase in sensor resistance.     

Efficient field emission properties of ZnO (core)/graphite (shell) nanowires

In this work the field emission studies of a new type of field emitter, zinc oxide (ZnO) core/graphitic (g-C) shell nanowires are presented. The nanowires are synthesized by chemical vapor deposition of zinc acetate at 1300 °C Scanning and transmission electron microscopy characterization confirm high aspect ratio and novel core–shell morphology of the nanowires. Raman spectrum of the nanowires mat represents the characteristic Raman modes from g-C shell as well as from the ZnO core. A low turn on field of 2.75 V/μm and a high current density of 1.0 mA/cm² at 4.5 V/μm for ZnO/g-C nanowires ensure the superior field emission behavior compared to the bare ZnO nanowires.     

Fibers of reduced graphene oxide nanoribbons

Reduced graphene oxide nanoribbon fibers were fabricated by using an electrophoretic self-assembly method without the use of any polymer or surfactant. We report electrical and field emission properties of the fibers as a function of reduction degree. In particular, the thermally annealed fiber showed superior field emission performance with a low potential for field emission (0.7?V?μm(-1)) and a giant field emission current density (400?A?cm(-2)). Moreover, the fiber maintains a high current level of 300?A?cm(-2) corresponding to 1?mA during long-term operation.     

Efficient field emission from vertically grown planar ZnO nanowalls on an ITO-glass substrate

Vertically grown planar ZnO nanowalls, with typical dimensions of 40-80?nm thickness and several micrometers wide, were electrodeposited on an indium-tin-oxide (ITO)-glass substrate at 70?°C. X-ray photoelectron spectroscopy (XPS) studies reveal that the nanowalls consist of ZnO covered with a Zn(OH)(2) overlayer. An x-ray diffraction (XRD) study shows that these nanowalls have the wurtzite structure and are highly crystalline. The corresponding Raman and photoluminescence spectra further indicate the presence of oxygen deficiency. These ZnO nanowalls exhibit excellent field emission performance, with not only a considerably lower turn-on field of 3.6?V?μm(-1) (at 0.1?μA?cm(-2)) but also a higher current density of 0.34?mA?cm(-2) at 6.6?V?μm(-1) than most ZnO nanowires and other one-dimensional nanostructures reported to date.     

Flexible cold cathode with ultralow threshold field designed through wet chemical route

A flexible cold cathode based on a uniform array of ZnO nanowires over carbon fabrics was designed via a simple wet chemical route. The structural parameters of the nanowires (i.e. length, diameter) as well as their arrangement over the carbon fibers were tailored by adjusting nutrient solution composition and growth duration. The optimized arrays of ZnO nanowires exhibit excellent electron emission performance with ultralow turn-on as well as threshold fields of 0.27 and 0.56 V μm(-1). This threshold field value is the lowest compared to any of the previous zinc-oxide-based cold cathodes realized through either chemical or vapor phase processes. In addition, the current density can reach an exceptionally high value of ～ 11 mA cm(-2) at an applied electric field of only 0.8 V μm(-1). Flexible electronic devices based on a field emitter cold cathode may thus be realized through chemical processing at low budget but having high efficiency.     

Controlling the growth and field emission properties of silicide nanowire arrays by direct silicification of Ni foil

Nickel silicide nanowire arrays have been achieved by the decomposition of SiH(4) on Ni foil at 650?°C. It is indicated that the nickel silicide nanowires consist of roots with diameter of about 100-200?nm and tips with diameter of about 10-50?nm. A Ni diffusion controlled mechanism is proposed to explain the formation of the nickel silicide nanowires. Field emission measurement shows that the turn-on field of the nickel silicide nanowire arrays is low, at about 3.7?V?μm(-1), and the field enhancement factor is as high as 4280, so the arrays have promising applications as?emitters.     

Selective growth of Ge nanowires by low-temperature thermal evaporation

High-quality single-crystalline Ge nanowires with electrical properties comparable to those of bulk Ge have been synthesized by vapor-liquid-solid growth using Au growth seeds on SiO(2)/Si(100) substrates and evaporation from solid Ge powder in a low-temperature process at crucible temperatures down to 700?°C. High nanowire growth rates at these low source temperatures have been identified as being due to sublimation of GeO from substantial amounts of GeO(2) on the powder. The Ge nanowire synthesis from GeO is highly selective at our substrate temperatures (420-500?°C), i.e., occurs only on Au vapor-liquid-solid growth seeds. For growth of nanowires of 10-20?μm length on Au particles, an upper bound of 0.5?nm Ge deposition was determined in areas of bare SiO(2)/Si substrate without Au nanoparticles.     

Template synthesis and forming electrical contacts to single Au nanowires by focused ion beam techniques

Metallic Au nanowires were electrochemically synthesized in 20?μm thick ion track etched polycarbonate membranes with the nominal pore diameter of 200?nm. Scanning and transmission electron microscopy analysis and x-ray diffraction of samples revealed that the nanowires are dense with a fcc [Formula: see text] texturing. The I-V characteristics of a single Au nanowire were investigated using a four-point microprobe set-up. The Au nanowire was placed in electrical contact with electrodes patterned on planar substrates using a dual-beam focused ion beam technique. The resistivity of the Au nanowires was found to be 2.8 × 10(-4)?Ω?cm.     

Electrical and optical properties of InP nanowire ensemble p⁺-i-n⁺ photodetectors

We report on a comprehensive study of electrical and optical properties of efficient near-infrared p?-i-n? photodetectors based on large ensembles of self-assembled, vertically aligned i-n? InP nanowires monolithically grown on a common p? InP substrate without any buffer layer. The nanowires have a polytype modulated crystal structure of wurtzite and zinc blende. The electrical data display excellent rectifying behavior with an ideality factor of about 2.5 at 300 K. The ideality factor scales with 1/T, which possibly reflects deviations from classical transport models due to the mixed crystal phase of the nanowires. The observed dark leakage current is of the order of merely ～100 fA/nanowire at 1 V reverse bias. The detectors display a linear increase of the photocurrent with reverse bias up to about 10 pA/nanowire at 5 V. From spectrally resolved measurements, we conclude that the photocurrent is primarily generated by funneling photogenerated carriers from the substrate into the NWs. Contributions from direct excitation of the NWs become increasingly important at low temperatures. The photocurrent decreases with temperature with an activation energy of about 50 meV, which we discuss in terms of a temperature-dependent diffusion length in the substrate and perturbed transport through the mixed-phase nanowires.     

Thermoelectric properties of individual single-crystalline PbTe nanowires grown by a vapor transport method

We report the thermoelectric performance of individual PbTe nanowires with sizes ranging from 76 to 436 nm grown from a vapor transport method that synthesizes high-quality, single-crystalline PbTe nanowires. Independent measurements of temperature-dependent Seebeck coefficient (S), thermal conductivity (κ) and electrical conductivity (σ) of individual PbTe nanowires were investigated. By varying the nanowire size, the simultaneous increase and decrease of S (-130 μV K(-1)) and κ (1.2 W m(-1) K(-1)), respectively, are achieved at room temperature. Our results demonstrate the enhanced thermoelectric properties of individual single-crystalline PbTe nanowires, compared to that of bulk PbTe, and can provide guidelines for future work on nanostructured thermoelectrics based on PbTe.     

Aluminium nitride nanowire array films for nanomanufacturing applications

The present paper presents a systematic investigation of both catalyst free and catalyst assisted AlN nanowire synthesis by chemical vapour deposition using Al and NH₃ as source materials. Growth runs have mostly been carried out at 1100°C under H₂ as carrier gas. While the catalyst free growth runs resulted in long (～40?μm) and dense AlN nanowire array films, the catalyst assisted growth resulted in short nanowires (3–5?μm). Growth mechanisms have been presented. Raman spectroscopy of the catalyst free grown nanowires has revealed very symmetric and strong phonon modes [e.g. strong E₂ (high)] indicating very good crystal quality of the grown AlN nanowires. In brief, catalyst free growth eliminates catalyst contamination and produces high quality and density of long nanowires, which is very valuable for scale-up manufacturing opportunities of the AlN nanostructures.     

Single-nanowire surface plasmon gratings

We demonstrate single-nanowire plasmonic gratings made by focused-ion-beam milling of single Au nanowires. At the optical communication band, a 290?nm diameter Au nanowire with grating length of 15.6?μm offers evident grating features with a transmission dip up to ～3.3?dB. The grating effects in typical Au nanowires with different grating parameters (e.g.?grating depth, width and length) are also investigated. Our results suggest a novel approach to one-dimensional plasmonic gratings with high compactness and flexibility, which may find applications in low-dimensional wavelength-selective plasmonic circuits and devices.     

Spider-silk-based fabrication of nanogaps and wires

We report on the use of spider fibers as micro- and nanostencils for the fabrication of nanogaps between ultrathin conductive electrodes, and as molds for fabrication of micro- and nanowires by deposition of evaporated gold. Atomic force microscopy (AFM) morphological characterization of the nanogaps is described, together with the measurement of the electrical behavior of both nanogaps and nanowires. Gaps as narrow as 20?nm, comparable to e-beam-fabricated gaps, with electrical resistance higher than 10(13)?Ω have been obtained; while conductive fibers ranging from 350?nm to 1.5?μm in diameter and resistances ranging from 50?MΩ to 100?Ω have been obtained and characterized.     

Enhanced field emission properties of vertically aligned double-walled carbon nanotube arrays

Vertically aligned double-walled carbon nanotube (VA-DWCNT) arrays were synthesized by point-arc microwave plasma chemical vapor deposition on Cr/n-Si and SiO(2)/n-Si substrates. The outer tube diameters of VA-DWCNTs are in the range of 2.5-3.8?nm, and the average interlayer spacing is approximately 0.42?nm. The field emission properties of these VA-DWCNTs were studied. It was found that a VA-DWCNT array grown on a Cr/n-Si substrate had better field emission properties as compared with a VA-DWCNT array grown on a SiO(2)/n-Si substrate and randomly oriented DWCNTs, showing a turn-on field of about 0.85?V?μm(-1) at the emission current density of 0.1?μA?cm(-2) and a threshold field of 1.67?V?μm(-1) at the emission current density of 1.0?mA?cm(-2). The better field emission performance of the VA-DWCNT array was mainly attributed to the vertical alignment of DWCNTs on the Cr/n-Si substrate and the low contact resistance between CNTs and the Cr/n-Si substrate.     

Fabrication of high performance surface enhanced Raman scattering substrates by a solid-state ionics method

Silver nanostructures were prepared by a solid-state ionics method using fast ionic conductor RbAg(4)I(5) films under a direct current electric field (DCEF). The surface morphology of the silver nanostructures grown under different constant current fields was characterized by scanning electron microscopy (SEM). Rhodamine 6G (R6G) aqueous solutions were used as probe molecules to detect the Raman enhancement performance of the silver nanostructure substrates. The effect of external electric field current intensity on the surface morphology of the silver nanostructures during the preparation was studied in detail. The enhancement effect of the silver nanostructure surface enhanced Raman scattering (SERS) substrates with different surface morphologies toward R6G was determined. We found that disordered silver nanowires (DSNW), ordered silver nanowires (OSNW), densely arranged silver nanobamboo arrays (SNBA) and compactly arranged silver nanobud clusters (SNBC) were respectively obtained when the constant current intensity was 3?μA, 5?μA, 8?μA and 12?μA under the same vacuum evaporation plating conditions. The limiting concentrations of R6G for these SERS substrates were found to be 10(-7)?mol?l(-1), 10(-13)?mol?l(-1), 10(-13)?mol?l(-1) and 10(-16)?mol?l(-1), respectively.