Nanowires

Tight-binding electronic structure calculations for periodic systems are often carried out in non-primitive unit cells, or for imperfect (eg random-alloy) nanostructures, such as nanowires. In the first case bands exist but they are difficult to identify due to the choice of unit cell for the calculation, while in the second case bands only exist in an approximate sense. The Brillouin zone unfolding technique applied to tight-binding calculations provides a powerful tool for extracting the true primitive-cell bands from non-primitive cells. Also, it ...

Arrays of manganese oxide nanowires were electrodeposited into anodized alumina membranes. These nanowire arrays were investigated in terms of their structural and electrochemical properties as cathodes for high energy and high power density Li ion batteries. The nanowire arrays were assembled with a Li counter electrode and non-aqueous electrolyte to form secondary batteries. These cells were capable of multiple charge/discharge cycles, with a cathode specific capacity of approximately 300 mA h/g. Though the cathodes were fabricated without any binder or conductive additives, the electrodes could be discharged at current densities up to 0.1 mA/cm 2 before the onset of any significant polarization.

Self-assembled micro/nanowires of melamine and seveal aromatic acids, including thiocyanuric acid, trimesic acid, terephthalic acid, and 5-sulfoisophthalic acid, are formed through evaporation of water in a solution mixture of acids and melamine. These self assemblies show crystalline structures. Except for microwires of 5-sulfoisophthalic acid and melamine, other microwires are semiconductors. They may be used for sensors, catalyst, optoelectronics, etc. Keywords: Nanowires; Microwires; Self-Assembly; Aromatic Acid; Melamine.

A glimpse on the present status of ion track technology is provided: irradiation, etching and electro-replication of single and multiple ion tracks. The uses of an electrolytic cell for single and counted track studies are sketched. In connection with a data acquisition system such a device enables to determine in real-time: track and bulk etch rate, cone angle, and size. Processing can be terminated at a preset time, current, diameter, or length (of wires). The system can be used to rapidly screen new track materials, etchants, and electro-replication processes.

For the first time, Germanium (Ge) nanowires have been grown on a gold (Au) coated flexible aluminum (Al) foil substrate in high vacuum (1 9 10 -5 mbar) by electron-beam evaporation of germanium using the vaporliquid-solid mechanism at a substrate temperature of 380°C. The grown nanowires have been analyzed for their structural, morphological and chemical properties by employing standard techniques X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy, respectively. X-ray diffraction measurements revealed the formation of cubic Ge phase highly oriented in (111) reflection in plane with the Al foil substrate. The morphological observations by SEM have shown the randomly grown nanowires with an average length and diameter of 600 ± 50 and 100 ± 10 nm, respectively, for a deposition time of 30 min. TEM investigation revealed single crystalline nanowires with free of defects. The wettability studies by contact angle measurement have confirmed the hydrophobic nature of the Ge NWs film surface with contact angle for water 110°± 1°. The growth mechanism of Ge nanowires on Al foil substrate has also been discussed.

Engineering materials at the nanoscale by combining controlled nanomaterial synthesis and directed assembly methods offers the potential to create new electronic and optical devices with improved performance and functionality. Semiconductor nanowires have been of particular interest as a model system for studying new physical phenomena arising from their scaled geometries as well as for applications in high performance vertical transistors,

We find that self-heating effects are not pronounced in silicon nanowire transistors with channel length 10 nm even in the presence of the wrap-around oxide. We observe a maximum current degradation of 6% for V G = V D = 1.0 V in a structure in which the metal gates are far away from the channel. The overall small current degradation is attributed to the significant velocity overshoot effect in these structures. The lattice temperature profile shows moderate temperature rise and velocity of the carriers is slightly deteriorated due to self-heating effects when compared to isothermal simulations. Keywords Analytic bands MC device modeling 1 Motivation for this work As semiconductor devices are scaled into nanometer dimensions [1], a number of issues become important that prevent further transistor scaling. These can be classified into (1) mobility degradation mechanisms (due to increased Coulomb scattering, more pronounced interface roughness due to narrower triangular confinement, line edge roughness, etc.) and (2) statistical effects due to discrete and random nature of the number and position of the impurity atoms

We report fabrication of solar cell (n + -p-p + structure) on black silicon substrates consisting of silicon nanowire (SiNW) arrays prepared by Ag induced wet chemical etching process in aqueous HF-AgNO 3 solution. SiNW arrays surface has low reflectivity ( o5%) in the entire spectral range (400-1100 nm) of interest for solar cells. The solar cells were fabricated by conventional cell fabrication protocol. Performance of three types of cells, namely cell with SiNW over the entire front surface, cell with SiNW only in the active device area and control cell (on planar surface), has been compared. It was found that cell based on selectively grown shorter length SiNW arrays has the best cell performance.

We have developed a novel electrochemical route to fabricate highly ordered stoichiometric Ag 2 Se nanowire arrays by electrodeposition from non-aqueous dimethyl sulfoxide (DMSO) solutions. Cyclic voltammetry technique was used to study this cathodic deposition process. X-ray energy dispersion analysis shows that stoichiometric Ag 2 Se nanowires can be obtained in a wide range of deposition condition. X-ray diffraction and electron diffraction patterns demonstrate that the as-deposited nanowires are [0 0 2] oriented orthorhombic b-Ag 2 Se. Furthermore, ternary Ag 2 Se 1 À x Te x nanowires have been electrodeposited in mixed SeCl 4 -TeCl 4 DMSO solutions.

Bunches of ZnO nanowires have been synthesized by hydrothermal process with the assistance of cetyltrimethylammonium bromide. The obtained bunches of ZnO nanowires are hexagonal wurtzite structures, and they exhibit orange visible emission *600 nm. It seems the orange emission *600 nm is due to the presence of Zn(OH) 2 on the surface of ZnO nanowires. On the basis of material information provided by X-ray diffraction, scanning electron microscopy and photoluminescence, a growth mechanism is proposed for the formation of bunches of ZnO nanowires.

We investigate the role of dipolar interactions on the magnetic properties of nanowires aggregates. Micromagnetic simulations show that dipolar interactions between wires are not detrimental to the high coercivity properties of magnetic nanowires composites even in very dense aggregates. This is confirmed by experimental magnetization measurements and Henkel plots which show that the dipolar interactions are small. Indeed, we show

We review recent developments in our group regarding the solution-phase synthesis of one-dimensional nanostructures of metals. The synthetic approaches include solution-liquid-solid growth for nanowires of low-melting-point metals such as Pb; seed-directed growth for Ag nanowires, nanobeams, and nanobelts; kinetically controlled growth for Pt nanorods, nanowires, and multipods; and galvanic replacement for nanotubes of Au, Pt, and Pd. Both characterization and mechanistic studies are presented for each nanostructure. Finally, we highlight the electrical and plasmonic properties of these metal nanostructures and discuss their potential applications in nanoscale devices. (1) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. Alekseeva, A. V.; Bogatyrev, V. A.; Dykman, L. A.; Khlebtsov, B. N.; Trachuk, L. A.; Melnikov, A. G.; Khlebtsov, N.

TiO 2 nanowires with a diameter of ∼150 nm, length of ∼3-4 µm, and aspect ratio of 10:1, were prepared by ultrasonically dispersing electrospun continuous nanofibers in monocarboxylic acids. The resulting pastes were used for making nanowire films on conducting glass substrates with thicknesses in the range of 500 nm to 100 µm, good adhesion, and high nanowire packing. These films were used to fabricate dye-sensitized solar cells using the D131 dye and the iodide/triiodide electrolyte. Transient photocurrent measurements showed a high electron diffusion coefficient in those nanowire films. The measured diffusion coefficient in those TiO 2 nanowires was orders of magnitude higher than that observed in nanoparticles under similar experimental conditions. The charge-transport mechanism in the nanowire sample is discussed in support with the measured open-circuit voltage decay curves.

ZnO nanowires have been synthesized by heating Zn powders under nitrogen (N 2 ) gas atmosphere. The influence of the growth temperature on the morphology, structure, and photoluminescence (PL) properties of ZnO nanowires has been investigated. At the higher-temperature growth process, thinner nanowires have been obtained. Interestingly, it is observed that the variation of growth temperature has significantly affected the photoluminescence spectra of the ZnO nanowires, showing an enhancement in the relative intensity of the green to UV emission bands with the increase of the growth temperature. In addition, the oxygen sensing properties of the as-synthesized ZnO nanowires were tested.

We have observed unusual ferromagnetic properties in single-crystalline CoSi nanowire ensemble, in marked contrast to the diamagnetic CoSi in bulk. High-density freestanding CoSi nanowires with B20 crystal structure are synthesized by a vapor-transport-based method. The reaction of cobalt chloride precursor with a Si substrate produces high-aspect-ratio CoSi nanowires. The high-resolution transmission electron microscopy and electron diffraction studies reveal superlattice structure in CoSi nanowires with twice the lattice parameter of simple cubic CoSi lattice. The zero-field-cooled and field-cooled (ZFC-FC) measurements from the nanowire ensemble show freezing of the disordered surface spins at low temperatures. The magnetoresistance (MR) measurements of single nanowire devices show a negative MR whose magnitude gets larger at lower temperatures.

The effects of the solidification parameters, such as growth rate and temperature gradient, on the distance and diameter of Re nanowires have been examined. Both the spacing and diameter increase with decreasing growth rate and temperature gradient, respectively. The ratio of fiber spacing to diameter is 9.1. In addition, it was demonstrated that the temperature gradient influences interface undercooling in the same way as the growth rate and may be used as an additional parameter to control fiber spacing and diameter.

Single-crystals of MnFe2O4 nanorods with an average diameter of 20 nm and length of 250 nm were synthesized by a hydrothermal process at 180 °C after 12 h. High-resolution transmission electron microscopy (HRTEM) and electron diffraction (ED) analysis revealed that the nanorods grow along the [110] axis, which is one of the easy magnetization axes of the material MnFe2O4. It was found that the nanorods exhibited a saturation magnetization (Ms) of 68.02 emu/g, which is much higher than that of quasi-spherical particles of MnFe2O4, prepared by other approaches such as solid-state reaction methods (Ms = 36.7 emu/g), and coprecipitation processes followed by annealing in vacuo at 400 °C (Ms = 24.4 emu/g). The oriented growth along the easy magnetization axis of MnFe2O4 was suggested to be responsible for the improvement of magnetic properties of MnFe2O4 nanorods. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

The preparation of Ce(OH) 3 /CeO 2 nanostructures (NSs) through electrogeneration of base into anodic alumina membranes was studied. The effects of solvent (alcohol and/or water), Ce 3? partner anion nature (chloride or nitrate) and concentration, applied potential or current density in driving the morphology toward nanowires (NWs) and/or nanotubes (NTs) was described. The structural analysis performed by X-Ray Diffraction and Raman Spectroscopy allowed to evidence that the presence of Ce(IV) into the nanostructures strongly depends on the oxygen content in the growing environment.

La 1Àx Ca x MnO 3 (LCMO) nanowires were prepared using a simple sol-gel process and nano-channel alumina (NCA). X-ray diffraction (XRD) and selected-area electron diffraction (SAED) indicate that the nanowires are of monoclinic perovskite structure. Transmission electron microscopy (TEM) shows that the diameters of nanowires are around 30 nm. Furthermore, high-resolution TEM reveals a clear lattice image of {0 2 1} planes in a LCMO nanowire. It can be expected that the method presented in this Letter is also appropriate for the preparation of nanowires of other monocomponent or multi-component oxides. Ó

Molecular imprinted polymers (MIPs), first introduced by Mosbach [1,2] and Wulff, are widely reported materials that can be used as biomimetic molecular recognition elements. They are prepared by polymerization of monomers bearing bonds to a template molecule. Removal of the template leaves

Sustaining Moore's Law requires continual transistor miniaturization. Through silicon innovations and breakthroughs, CMOS transistor scaling and Moore's Law will continue at least through early next decade. By combining silicon innovations with other novel nanotechnologies on the same Si platform, it is expected that Moore's Law will extend well into the next decade. This paper describes the most recent advances made in silicon CMOS transistor technology and discusses the challenges and opportunities presented by the recent emerging nanoelectronic devices such as carbon nanotube field-effect transistors (FET), Si-nanowire FETs and III-V FETs for high-performance, low-power logic applications.

Electrochemical capacitors have recently been attracting significant interest because they can instantaneously provide a higher power density than batteries and a higher energy density than conventional dielectric capacitors. Such outstanding properties make them excellent candidates for hybrid electric vehicles, computers, mobile electric devices, and other applications. [1,2] Generally, an electrochemical capacitor may be operated based on the electrochemical double-layer capacitance (EDLC) formed along an electrode/electrolyte interface or a pseudocapacitance resulted from a fast reversible faradic process of redox-active materials (e.g., metal oxides or conductive polymers). For an EDLC-based capacitor, the rapid charge/ discharge process provides the capacitor with a high power density, yet the energy density is limited by its effective doublelayer area. To date, a large number of largesurface-area materials, such as activated carbon, templated carbon, and carbon nanotubes (CNTs), have been extensively studied. Activated carbons, with surface areas from 1 000-2 500 m 2 g À1 , are the most commonly used materials and may provide a capacitance up to 320 F g À1 at low potential scanning rate. However, the capacitance may drop dramatically at high scanning rates because of their tortuous pore structure and high microporosity. [3] The templated carbons, on the other hand, exhibit uniform pore geometry and larger pore size; however, they have not shown any significant improvement in either energy or power performance. [4] For comparison, multiwalled CNTs [5] show capacitances up to 135 F g À1 and single-wall CNTs [6] show capacitances up to 180 F g À1 , which are still low for an actual device application. Compared with the EDLC-based capacitors, pseudocapacitors based on transition metal oxides or conducting polymers, such as RuO 2 , [7] MnO 2 , [8] NiO, [9] Co 3 O 4 , [10] V 2 O 5 , [11] and polyaniline, [12] may provide much higher specific capacitances, up to 1 000 F g À1 of the active material. Nonetheless, their actual applications are still limited by high cost, low operation voltage, or poor rate capability, mostly because of inefficient mass transport or of slow faradic redox kinetics. To design a better electrochemical capacitor with both high energy and power density, a common strategy is to construct a hybrid capacitor that integrates both electric double-layer capacitance and pseudocapacitance within a single electrode. For example, Sato et al. loaded ruthenium oxide onto activated carbon, resulting in a capacity of 308 F g À1 at 7.1 wt% ruthenium loading and a low scanning rate of 2 mV s À1. [13] Dong et al. reported a composite of MnO 2 and templated carbon with a capacitance of 156 F g À1 at 20 wt% MnO 2 loading and a scanning rate of 50 mV s À1 , which is about two times of that of the constituent carbon. [14] Kim et al. dispersed ruthenium oxide nanoparticles on carboxylated CNTs and obtained a total capacitance of 304 Fg À1 at a RuO 2 loading of 50 wt%. [15] Similarly, composites prepared by electrodepositing MnO 2 on vertically aligned CNT arrays exhibit

We provide an overview of the ReaxFF reactive force field method and discuss each calculation performed to determine the energy of a system at each iteration step. The concept of bond order is examined as well as how it is used to determine bonding, angle, and torsion energies. The calculation of the Coulomb and van der Waals non-bonded interactions is also discussed. In addition to a description of the ReaxFF method, we also investigate the thermal decomposition dynamics of a zinc-oxide nanowire with and without the presence of water. Our results indicate that the presence of water significantly weakens the ZnO surface bonding and leads to an acceleration of the failure of the material.

Fluid convection and mixing induced by bubble-propelled tubular microengines are characterized using passive microsphere tracers. Enhanced transport of the passive tracers by bubble-propelled micromotors, indicated by their mean squared displacement (MSD), is dramatically larger than that observed in the presence of catalytic nanowires and Janus particle motors. Bubble generation is shown to play a dominant role in the effective fluid transport observed in the presence of tubular microengines. These findings further support the potential of using bubble-propelled microengines for mixing reagents and accelerating reaction rates. The study offers useful insights toward understanding the role of the motion of multiple micromotors, bubble generation, and additional factors (e.g., motor density and fuel concentration) upon the observed motor-induced fluid transport.

Cetyltrimethyl ammonium bromide cationic (CTAB) surfactant was used as template for the synthesis of nanoclusters of ZnS composed of nanowires, by hydrothermal method. The structural and morphological studies were performed by using X-ray diffraction (XRD), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM) techniques. The synthesized ZnS nanoclusters are composed of nanowires and high yield on the substrate was observed. The ZnS nanocrystalline consists of hexagonal phase and polycrystalline in nature. The chemical composition of ZnS nanoclusters composed of nanowires was studied by X-ray photo electron microscopy (XPS). This investigation has shown that the ZnS nanoclusters are composed of Zn and S atoms.

Nanostructured polyanilines (PANIs) with a variety of morphologies were synthesized in acidic aqueous solutions with an added ionic liquid, 1-butyl-3-methylimidazolium chloride. The influence of various reaction conditions, i.e., the molar ratio of aniline to ionic liquid and initial aniline concentration, on the morphology and properties of the synthesized PANIs were investigated. The morphologies of the PANIs, as studied by scanning electron microscopy (SEM), range from nanowires to complex three-dimensional structures and were influenced particularly by the molar ratio of aniline to ionic liquid. The chemical structure and molecular-weight characteristics, as determined by Fourier-transform infrared spectroscopy (FTIR), UV-Vis spectroscopy and size-exclusion chromatography (SEC), were not affected by the reaction conditions, indicating that the aniline polymerization mechanism did not change in the presence of the ionic liquid, which acted as a soft template. The conductivities of the prepared PANI samples, as measured by impedance spectroscopy, were of the order of 10 −2 S/cm.

Optical reflection, transmission, and absorption in arrays of GaAs and GaAs/AlGaAs core-shell nanowires are studied using transfer matrix and photonic bandgap formalisms, analyzing the effects of size, geometry, height, packing density, and polarization. Energy dependence of the spectra demonstrates optical modes in the dielectric, similar to guided resonant modes, and also the air bands. Simulation of polarization dependence verifies higher absorption with the electric field along the wire axis. Higher absorption at much lower volume compared to thin film, combined with excellent charge transport, make core-shell nanowire arrays excellent candidates for optoelectronics applications.

We have measured the resistances (and resistivities) of Ag and Cu nanowires of diameters ranging from 15 nm to 200 nm in the temperature range 4.2 K-300 K with the specific aim to assess the applicability of the Bloch-Grüneisen formula for electron phonon resistivity in these nanowires. The wires were grown within polymeric templates by electrodeposition. We find that in all the samples the resistance reaches a residual value at T=4.2 K and the temperature dependence of resistance can be fitted to the Bloch-Grüneisen formula in the entire temperature range with a well defined transport Debye temperature (ΘR). The value of Debye temperature obtained from the fits lie within 8% of the bulk value for Ag wires of diameter 15 nm while for Cu nanowires of the same diameter the Debye temperature is significantly lesser than the bulk value. The electron-phonon coupling constants (measured by α el−ph or αR) in the nanowires were found to have the same value as that of the bulk. The resistivities of the wires were seen to increase as the wire diameter was decreased. This increase in the resistivity of the wires may be attributed to surface scattering of conduction electrons. The specularity p was estimated to be about 0.5. The observed results allow us to obtain the resistivities exactly from the resistance and gives us a method of obtaining the exact numbers of wires within the measured array (grown within the template).

In the recent past, investigations have focused increasingly on confined systems with micrometer and, in particular, nanometer dimensions. Due to their restricted size, these structures exhibit novel physical and chemical properties, and have opened up a large new field of basic research as well as possible applications. Electrochemical deposition is becoming an increasingly attractive method for the synthesis of new materials and nanostructures.

Well-aligned ZnO nanowire arrays with a long length of more than 40 μm were prepared successfully by using the polyethylenimine (PEI)-assisted preheating hydrothermal method (PAPHT). Several important synthetic parameters such as PEI content, growth time, preheating time and zinc salt concentration were found to determine the growth of ultralong ZnO nanowire arrays, including length, diameter, density and alignment degree. The photoluminescence (PL) spectrum of as-grown ultralong ZnO nanowire arrays revealed a UV emission and a yellow emission, which was attributed to the absorbed hydroxyl group based on the peak shift after annealing in various atmospheres. The performance of dye-sensitized solar cells (DSSCs) increased with increasing length of ZnO nanowire arrays, which was mainly ascribed to the aggrandized photocurrent and reduced recombination loss according to electrochemical impedance spectroscopy (EIS). A maximum efficiency of 1.3% for a cell with a short-circuit current density (J sc ) = 4.26 mA cm 2 , open-circuit voltage (V oc ) = 0.69 V and (fill factor) FF = 0.42 was achieved with a length of 40 μm.

We report on chemically prepared silver nanowires (diameters around 100 nm) sustaining surface plasmon modes with wavelengths shortened to about half the value of the exciting light. As we find by scattered light spectroscopy and near-field optical microscopy, the nonradiating character of these modes together with minimized damping due to the well developed wire crystal structure gives rise to large values of surface plasmon propagation length and nanowire end face reflectivity of about 10 m and 25%, respectively. We demonstrate that these properties allow us to apply the nanowires as efficient surface plasmon Fabry-Perot resonators.

The synthesis and properties of catalyst-free III-V nanowires with MBE is reviewed. The two main methods are Selective Area Epitaxy and gallium-assisted synthesis. The growth mechanisms are reviewed, along with the design possibilities of each technique. Finally, the excellent structure and ultra-high purity are presented by Raman and Photoluminescence spectroscopy.

We present experimental evidence that the room temperature thermal conductivity (κ) of individual multiwall carbon nanotubes and boron-nitride nanotubes does not obey Fourier's law as do ordinary thermal conductors. By varying the length (L) of the nanotube, we find that κ diverges as L 0.6∼0.9 . Our results show that Fourier's law is violated despite the fact that the ballistic phonon condition is not satisfied and large isotopic disorder is present.

New experimental evidence concerning the formation of La(OH) 3 nanowires on anodic alumina membranes by cathodic polarization in 0.05 M lanthanum nitrate solution is provided to further support the conclusions previously reached in our work (Bocchetta et al 2007 Electrochem. Commun. 9 683-8) and recently criticized by González-Rovira et al (Nanotechnology 2008 19 495305). Some unconvincing aspects of the paper of González-Rovira et al, according to which the same electrochemical process should lead to the formation of hydroxycarbonate nanotubes, are also discussed.

Using the first-principles plane-wave pseudopotential method within density functional theory, we have systematically investigated structural, electronic, and mechanical properties of M 2 Y 6 X 6 , Y 6 X 6 ͑X =Se,Te,S; Y = Mo, Cr, W; and M =Li,Na͒ nanowires and bulk phase of M 2 Y 6 X 6 . We found that not only Mo 6 X 6 , but also transition metal and chalcogen atoms lying in the same columns of Mo and Se can form stable nanowires consisting of staggered triangles of Y 3 X 3 . We have shown that all wires have nonmagnetic ground states in their equilibrium geometry. Furthermore, these structures can be either a metal or semiconductor depending on the type of chalcogen element. All Y 6 X 6 wires with X = Te atom are semiconductors. Mechanical stability, elastic stiffness constants, breaking point, and breaking force of these wires have been calculated in order to investigate the strength of these wires. Ab initio molecular dynamic simulations performed at 500 K suggest that overall structure remains unchanged at high temperature. Adsorption of H, O, and transition metal atoms like Cr and Ti on Mo 6 Se 6 have been investigated for possible functionalization. All these elements interact with Mo 6 Se 6 wire forming strong chemisorption bonds, and a permanent magnetic moment is induced upon the adsorption of Cr or Ti atoms. Molybdenum selenide-type nanowires can be alternative for carbon nanotubes, since the crystalline ropes consisting of one type of ͑M 2 ͒Y 6 X 6 structures can be decomposed into individual nanowires by using solvents, and an individual nanowire by itself is either a metal or semiconductor and can be functionalized.