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.     

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.     

Gold nanowires fabricated by immersion plating

The growth mechanism of oriented Au nanowires fabricated by immersion plating was investigated. Both n-type crystal Si (c-Si) and amorphous Si (a-Si) with an electron-beam (E-beam) patterned resist nanotrench were immersed into the plating bath HAuCl(4)/HF. For the Au nanowires fabricated on c-Si, voids, nanograins, and clusters were observed at various plating conditions, time and temperature. The voids were often found in the center of the Au nanowires due to there being fewer nucleation sites on the c-Si surface. However, Au can easily nucleate on the surface of a-Si and form continuous Au nanowires with grain sizes about 10-50?nm. The resistivities of Au nanowires with width 105?nm fabricated on a-Si are about 4.4-6.5?μΩ?cm. After annealing at 200?°C for 30?min in N(2) ambient, the resistivities are lowered to about 3.0-3.9?μΩ?cm, measured in an atomic force microscope (AFM) in contact mode. The grain size of Au is in the range of ～50-100?nm. A scanning electron microscope (SEM) examination and grazing incident x-ray diffraction (GIXRD) analysis were also carried out to study the morphology and crystalline structure of the Au nanowires.     

Synthesis and characterization of cadmium telluride nanowire

CdTe nanowires with controlled composition were cathodically electrodeposited using track-etched polycarbonate membrane as scaffolds and their material and electrical properties were systematically investigated. As-deposited CdTe nanowires show nanocrystalline cubic phase structures with grain sizes of up to 60 nm. The dark-field images of nanowires reveal that the crystallinity of nanowires was greatly improved from nanocrystalline to a few single crystals within nanowires upon annealing at 200?°C for 6?h in a reducing environment (5%?H(2)+95%?N(2)). For electrical characterization, a single CdTe nanowire was assembled across microfabricated gold electrodes using the drop-casting method. In addition to an increase in grain size, the electrical resistivity of an annealed single nanowire (a few 10(5)?Ω?cm) was one order of magnitude greater than in an as-deposited nanowire, indicating that crystallinity of nanowires improved and defects within nanowires were reduced during annealing. By controlling the dopants levels (e.g.?Te content of nanowires), the resistivity of nanowires was varied from 10(4) to 10(0)?Ω?cm. Current-voltage (I-V) characteristics of nanowires indicated the presence of Schottky barriers at both ends of the Au/CdTe interface. Temperature-dependent I-V measurements show that the electron transport mode was determined by a thermally activated component at T>-50?°C and a temperature-independent component below -50?°C. Under optical illumination, the single CdTe nanowire exhibited enhanced conductance.     

Low-resistance ohmic contacts to SiC nanowires and their applications to field-effect transistors

We report on the electrical characterization of two ohmic contacts (Ti/Au and Ni/Au) to unintentionally doped silicon carbide nanowires (SiCNWs) using the modified transmission line model (TLM) method. Our results indicate that subsequently deposited Ni/Au ohmic contacts on SiCNWs had ～40 times lower specific contact resistances (SCRs) of 5.9 × 10(-6) ± 8.8 × 10(-6)?Ω?cm(2) compared to the values of Ti/Au ohmic contacts (2.6 × 10(-4) ± 3.4 × 10(-4)?Ω?cm(2)). We also conducted a comparison study of the electrical characteristics of top-gated SiCNW field-effect transistors (FETs) with two different ohmic contacts as used for ohmic contact studies. The electrical transport measurements on the SiCNW FET with Ni/Au ohmic contacts show much lower resistance contacts to SiC NWs and better FET performances than those for Ti/Au ohmic contact-based FETs.     

Novel fabrication of an SnO(2) nanowire gas sensor with high sensitivity

We fabricated a nanowire-based gas sensor using a simple method of growing SnO(2) nanowires bridging the gap between two pre-patterned Au catalysts, in which the electrical contacts to the nanowires are self-assembled during the synthesis of the nanowires. The gas sensing capability of this network-structured gas sensor was demonstrated using a diluted NO(2). The sensitivity, as a function of temperature, was highest at 200?°C and was determined to be 18 and 180 when the NO(2) concentration was 0.5 and 5?ppm, respectively. Our sensor showed higher sensitivity compared to different types of sensors including SnO(2) powder-based thin films, SnO(2) coating on carbon nanotubes or single/multiple SnO(2) nanobelts. The enhanced sensitivity was attributed to the additional modulation of the sensor resistance due to the potential barrier at nanowire/nanowire junctions as well as the surface depletion region of each nanowire.     

Influence of seed layers on the vertical growth of ZnO nanowires

We synthesized vertically aligned ZnO nanowires on SiO₂ wafer <100> using the Au, ZnO and Au/ZnO seed layers through the physical vapor deposition process. The growth direction of ZnO nanowire was controlled by using the three different seed layers. From the XRD results, we observed the highest intensity of the (002) peak on the Au/ZnO seed layer among the three seed layers. The SEM images show that all of the ZnO nanowires have an average diameter of about 100 ~ 200 nm and a length of about 5 μm, and the nanowires grown on the Au/ZnO seed layer are oriented the most perpendicularly to the substrate surface. From the PL analysis, we observed that the intensity of broad emissions at 400-600 nm relating the green emission for the ZnO nanowires on the Au/ZnO seed layer was much weaker than that for the ZnO nanowires on the ZnO seed layer. The experiment results indicate that the selection of seed layers is important to grow nanowires vertically for the application of nanoscale devices.     

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.     

High performance horizontal gate-all-around silicon nanowire field-effect transistors

Semiconducting nanowires have been pointed out as one of the most promising building blocks for submicron electrical applications. These nanometer materials open new opportunities in the area of post-planar traditional metal-oxide-semiconductor devices. Herein, we demonstrate a new technique to fabricate horizontally suspended silicon nanowires with gate-all-around field-effect transistors. We present the design, fabrication and electrical measurements of a high performance transistor with high on current density (～150?μA?μm(-1)), high on/off current ratio (10(6)), low threshold voltage (～?-?0.4?V), low subthreshold slope (～100?mV /dec) and high transconductance (g(m)?～?9.5?μS). These high performance characteristics were possible due to the tight electrostatic coupling of the surrounding gate, which significantly reduced the Schottky-barrier effective height, as was confirmed experimentally in this study.     

SiGe nanowire field effect transistors

Si0.5Geo0.5 nanowires have been utilized to fabricate source-drain channels of p-type field effect transistors (p-FETs). These transistors were fabricated using two methods, focused ion beam (FIB) and electron beam lithography (EBL). The electrical analyses of these devices show field effect transistor characteristics. The boron-doped SiGe p-FETs with a high-k (HfO2) insulator and Pt electrodes, made via FIB produced devices with effective hole mobilities of about 50 cm2V(-1)s(-1). Similar transistors with Ti/Au electrodes made via EBL had effective hole mobilities of about 350 cm2V(-1)s(-1).     

On the difficulties in characterizing ZnO nanowires

The electrical properties of single ZnO nanowires grown by vapor phase transport were investigated. While some samples were contacted by Ti/Au electrodes, another set of samples was investigated using a manipulator tip in a low energy electron point-source microscope. The deduced resistivities range from?1 to 10(3)?Ωcm. Additionally, the resistivities of nanowires from multiple publications were brought together and compared to the values obtained from our measurements. The overview of all data shows enormous differences (10(-3)-10(5)?Ωcm) in the measured resistivities. In order to reveal the origin of the discrepancies, the influence of growth parameters, measuring methods, contact resistances, crystal structures and ambient conditions are investigated and discussed in detail.     

Transport properties of hybrid nanoparticle-nanowire systems and their application to gas sensing

Experimental data and theoretical modelling of the I-V characteristics of a gas sensor constructed from a mat of Au nanoparticle-coated GaN nanowires are presented. The principal mechanism for the response of the gas sensor to methane is explained in terms of the formation of a depletion layer within the nanowires due to the presence of the gold nanoparticles. The depth of the depletion layer is modulated by the potential induced by the physisorption of gas molecules onto the Au nanoparticles. A statistical model of the temperature-dependent I-V characteristics of bare and Au nanoparticle-decorated mats of GaN nanowires based on Poisson's equation has been used to determine the depth of the depletion layers of the nanowires. The room-temperature carrier concentration for the GaN nanowires was determined to be approximately 2.2 × 10(17)?cm(-3). The induced potential due to methane physisorption onto the Au nanoparticles that decorate the GaN nanowires was determined to be approximately -37?mV.     

On the thermal annealing conditions for self-synthesis of tungsten carbide nanowires from WC(x) films

The thermal annealing conditions in nitrogen ambient for the self-synthesis of tungsten carbide nanowires from sputter-deposited WC(x) films were investigated. Experimental results show that the temperature window for the growth of nanowires lies in the range of 500-750?°C with the corresponding annealing time interval ranging from 2.5 to 0.25?h. The diameter, length, and density of the grown nanowires are in the range of 10-15?nm, 0.1-0.3?μm, and 210-410?μm(-2), respectively. The degree of carbon depletion in the annealed WC(x) films plays a crucial role in determining both the shape and density of the self-synthesized nanowires. Nanowires synthesized at lower temperatures were seen to be smaller in dimension but higher in density. Material analysis reveals that the phase transition from WC to W(2)C arising from decarburization of the WC(x) film during thermal annealing should be responsible for the self-synthesis of nanowires.     

Bundled tungsten oxide nanowires under thermal processing

Ultra-thin W(18)O(49) nanowires were initially obtained by a simple solvothermal method using tungsten chloride and cyclohexanol as precursors. Thermal processing of the resulting bundled nanowires has been carried out in air in a tube furnace. The morphology and phase transformation behavior of the as-synthesized nanowires as a function of annealing temperature have been characterized by x-ray diffraction and electron microscopy. The nanostructured bundles underwent a series of morphological evolution with increased annealing temperature, becoming straighter, larger in diameter, and smaller in aspect ratio, eventually becoming irregular particles with size up to 5?μm. At 500?°C, the monoclinic W(18)O(49) was completely transformed to monoclinic WO(3) phase, which remains stable at high processing temperature. After thermal processing at 400?°C and 450?°C, the specific surface areas of the resulting nanowires dropped to 110?m(2)?g(-1) and 66?m(2)?g(-1) respectively, compared with that of 151?m(2)?g(-1) for the as-prepared sample. This study may shed light on the understanding of the geometrical and structural evolution occurring in nanowires whose working environment may involve severe temperature variations.     

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.     

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.     

Electrical properties of individual CoPt/Pt multilayer nanowires characterized by in situ SEM nanomanipulators

Here we report for the first time accurate and comprehensive measurements of electrical properties of individual CoPt/Pt multilayer nanowires both with periodic and non-periodic layer structures. A remarkably high failure current density of 1.69 × 10(12) A m(-2) for the periodic MNW and a similar 1.76 × 10(12) A m(-2) for the non-homogeneous MNW has been measured. The resistance of both types of multilayer nanowire structures are well fitted by a series resistance model, determining the separate resistance contribution of the component layers and magnetic/nonmagnetic interfaces for a single multilayer nanowire. The field-dependent interface resistance of both samples is calculated, 13.2 Ω for periodic layer structures and 4.84 Ω for non-periodic layer structures. The clear physical picture of the resistance distribution within individual multilayer nanowires is then determined. The accurate electrical testing of magnetic multilayer nanowires provides basic and necessary electrical parameters for their usage as building blocks or interconnects in nanoelectronics and nanosensors.     

Synthesis of vertical arrays of ultra long ZnO nanowires on noncrystalline substrates

Vertically aligned arrays of ultralong ZnO nanowires were synthesized on SiO₂ substrates with carbothermal vapor phase transport method with Au seeding layer. High density of vertically aligned ZnO nanowires with lengths from a few to ∼300 μm could be grown by controlling growth conditions. Supply of high concentration of Zn vapor and control of the ratio between Zn vapor and oxygen are found to have the most significant effects on the growth of long ZnO nanowires in the vapor-solid growth mechanism. The nanowires are of high crystalline quality as confirmed by various structural, compositional, and luminescent measurements. Luminescent and electrical properties of ZnO nanowires with different growth conditions were also investigated.     

Toward local growth of individual nanowires on three-dimensional microstructures by using a minimally invasive catalyst templating method

We present a novel minimally invasive postprocessing method for catalyst templating based on focused charged particle beam structuring, which enables a localized vapor-liquid-solid (VLS) growth of individual nanowires on prefabricated three-dimensional micro- and nanostructures. Gas-assisted focused electron beam induced deposition (FEBID) was used to deposit a SiO(x) surface layer of about 10 × 10 μm(2) on top of a silicon atomic force microscopy cantilever. Gallium focused ion beam (FIB) milling was used to make a hole through the SiO(x) layer into the underlying silicon. The hole was locally filled with a gold catalyst via FEBID using either Me(2)Au(tfac) or Me(2)Au(acac) as precursor. Subsequent chemical vapor deposition (CVD)-induced VLS growth using a mixture of SiH(4) and Ar resulted in individual high quality crystalline nanowires. The process, its yield, and the resulting angular distribution/crystal orientation of the silicon nanowires are discussed. The presented combined FIB/FEBID/CVD-VLS process is currently the only proven method that enables the growth of individual monocrystalline Si nanowires on prestructured substrates and devices.     

Synthesis of single-crystal manganese dioxide nanowires by a soft chemical process

α-?and β-MnO(2) single-crystal nanowires have been successfully synthesized through a soft chemical process, which involves no catalysts or templates. The diameters of the nanowires are about?10-40?nm and the lengths up to several tens of micrometres. Experimental results showed that MnO(2) polymorphic forms can be synthesized by selected-control reaction temperatures and their crystal phases in the nanowires were confirmed by XRD and TEM measurement. Further, the formation process of nanowires is discussed.