Increasing Wire Length Research Articles

Investigation of cold-wire spatial and temporal resolution issues in thermal turbulent boundary layers

In this study, we use simulations and experiments to investigate spatial and temporal attenuation of cold-wire sensors. A direct numerical simulation (DNS) of a heated smooth-wall turbulent channel flow is used to simulate the sensor spatial filtering effect in the spanwise direction, with results confirming that the increasing wire length does not affect the mean temperature, but significantly attenuates the variance of temperature fluctuations, especially in the vicinity of the wall. With increasing viscous-scaled wire length, the peaks of the one- and two-dimensional energy spectrograms of temperature fluctuations are increasingly attenuated, but in a manner that is approximately predictable based on the sensor length. The temporal resolution of the cold-wire is analysed through experimental measurements in a wind-tunnel thermal boundary layer. The wire length-to-diameter aspect ratio (lw/dw) is varied by changing the wire diameter and keeping the wire length constant, such that the viscous-scaled wire length remains constant. The wire aspect ratio has no effect on the mean temperature profile (similar to the observations for viscous-scaled wire length). When the wire aspect ratio lw/dw approaches 1000, we observe no additional temporal attenuation of the temperature fluctuations beyond that attributed to the spatial resolution of the finite sensor size. However, as lw/dw is reduced below this value (<1000), we observe increasing temporal attenuation. The attenuation of temperature fluctuations due to insufficient wire aspect ratios is seen at almost all wall-normal distances and is broadband, affecting all energy scales in the boundary layer. In contrast, the spatial attenuation only influences small-scale energy on the order of the probe scale, which predominates in the near-wall region. For a given wire diameter of the cold-wire sensor, temperature fluctuations are better resolved by maintaining lw/dw≈1000 and sacrificing spatial resolution by increasing the viscous-scaled wire length. Smaller diameter wires must be employed to alleviate this compromise.

Channel modeling of wireless 3D-chip based on ray-tracing

The resource constraints and accuracy requirements for Internet of Things (IoT) memory chips need three-dimensional (3D) monolithic integrated circuits, of which the increasing stack layers (currently more than 176) also cause excessive energy consumption and increasing wire length. In this paper, we propose and describe the channel modeling for wireless 3D chips based on ray-tracing. However, due to the reflection and refraction characteristics in each layer, the complex and diverse wireless paths in 3D chips add great difficulty to the channel characterization. To facilitate the modeling in massive layer wireless 3D chips based on ray-tracing, both boundary-less and boundary-constrained wireless 3D chips models are proposed based on ray-tracing, of which the channel gain can be obtained by a computational efficient approximate algorithm. These wireless 3D models with approximation algorithm can well characterize the wireless 3D chip channel in aspect of complete reflection and refraction characteristics, and avoid massive wired connections, high power consumption of cross-layer communication and high-complexity of 3D chips channel characterization. Numerical results show that: (1) The difference rate between the two models is lower than 0.001% (signal transmit through 20 layers); (2) the channel gain decreases sharply if refract time increases; and (3) the approximate algorithm can achieve an acceptable accuracy (error rate lower than 0.1%).

Cu2O/TiO2 Nanowire Assemblies as Photocathodes for Solar Hydrogen Evolution: Influence of Diameter, Length and NumberDensity of Wires

Abstract The performance of free-standing parallel-aligned nanowire arrays and interconnected networks of single-crystalline cuprous oxide (Cu2O) coated with titanium oxide (TiO2) as photocathodes for solar energy harvesting was analyzed. The nanostructures were synthesized by electrodeposition in polymer membranes prepared by ion-track technology. To enhance the photoelectrochemical stability of the nanowires in aqueous solution, they were conformally coated with a 10 nm thick TiO2 layer by atomic layer deposition. The diameter, size, geometry and number density of the parallel nanowires were systematically varied. The generated photocurrents show a clear increase as a function of wire diameter and wire number. In turn, the photocurrent does not get larger with increasing wire length. Highly interconnected networks of nanowires under 45° from various directions enabled further increase of wire density number and exhibited higher photocurrent densities compared to parallel arrays.

The length and hydrogenation effects on electronic transport properties of carbon-based molecular wires

Abstract It is known that the conductance of a wire decreases linearly with the increase of its length in the macro circuit. But things are different in mesoscopic field, especially in molecular dimensional circuits due to the quantum interferences in molecular size. In this work, first principles is used to investigate how wire length and hydrogenation degree influence the conductance of molecular wires. For comparison, two types of carbon-based molecular wires, single carbon atomic wire and phenyl ring wire with single hydrogenation or dihydrogenation, are chosen and coupled to two zigzag graphene nanoribbon electrodes, respectively. We show that the conductance of zigzag carbon atomic wire with single hydrogenation exhibits an evident even-odd oscillatory behavior with the increase of wire length, while that of phenyl ring wire starts to increase before decreasing. However, when the above types of wires are edged with dihydrogenation, their conductance decreases exponentially with the increasing wire length. In addition, fine current rectification is found in asymmetric phenyl wires induced by hydrogenation degree, which can be used as diode in future molecular devices. Analyses via orbital hybridization, transmission path, HOMO-LUMO position and so on reveal the microscopic mechanism of the above interesting results.

Exponential protection of zero modes in Majorana islands.

Majorana zero modes are quasiparticle excitations in condensed matter systems that have been proposed as building blocks of fault-tolerant quantum computers. They are expected to exhibit non-Abelian particle statistics, in contrast to the usual statistics of fermions and bosons, enabling quantum operations to be performed by braiding isolated modes around one another. Quantum braiding operations are topologically protected insofar as these modes are pinned near zero energy, with the departure from zero expected to be exponentially small as the modes become spatially separated. Following theoretical proposals, several experiments have identified signatures of Majorana modes in nanowires with proximity-induced superconductivity and atomic chains, with small amounts of mode splitting potentially explained by hybridization of Majorana modes. Here, we use Coulomb-blockade spectroscopy in an InAs nanowire segment with epitaxial aluminium, which forms a proximity-induced superconducting Coulomb island (a 'Majorana island') that is isolated from normal-metal leads by tunnel barriers, to measure the splitting of near-zero-energy Majorana modes. We observe exponential suppression of energy splitting with increasing wire length. For short devices of a few hundred nanometres, sub-gap state energies oscillate as the magnetic field is varied, as is expected for hybridized Majorana modes. Splitting decreases by a factor of about ten for each half a micrometre of increased wire length. For devices longer than about one micrometre, transport in strong magnetic fields occurs through a zero-energy state that is energetically isolated from a continuum, yielding uniformly spaced Coulomb-blockade conductance peaks, consistent with teleportation via Majorana modes. Our results help to explain the trivial-to-topological transition in finite systems and to quantify the scaling of topological protection with end-mode separation.

Tuning the magnetic properties of high aligned strontium ferrite nanowires formed in alumina template

Ordered hexagonal array of strontium ferrite nanowires were fabricated by dip coating in alumina templates and the influence of diameter and length on the magnetic properties of high aligned nanowire arrays were studied. The diameter of nanowires, synthesized in high aspect ratios, was changed from 30 to 60 nm while maintaining the same center to center distance between them. The samples were characterized by X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM) equipped with energy dispersive X-ray spectrum (EDS). Magnetic properties of nanowires were measured by a SQUID; the hysteresis loops were obtained with the applied magnetic field parallel and perpendicular to the wire axis. Magnetic properties of nanowire arrays were also investigated as a function of temperature. The Results showed that both coercivity and squareness increased with increasing wire length, while for constant center-to-center distance, coercivity decreased with increasing the diameter. Parallel anisotropy was achieved for 30 ± 4 nm diameter nanowire array having 15 ± 0.01 μm length. A curling mode of magnetization was suggested in strontium nanowires.

FeCo nanowires with enhanced heating powers and controllable dimensions for magnetic hyperthermia

A detailed study of the magnetic properties and heating capacities of electrodeposited FeCo nanowires with varying lengths (2–40 μm) and diameters (100 and 300 nm) is reported. We find that specific absorption rate (SAR) increases rapidly with increasing wire length up to 10 μm, followed by a gradual increase for larger lengths. Magnetic and hyperthermia measurements have revealed the important effect of dipolar interactions between the nanowires on their magnetic and inductive heating responses. Both calorimetric and AC magnetometry methods consistently show that the physical movement contribution of the nanowires to the SAR is small, and that for applied fields exceeding the coercive field, the nanowires tend to align parallel to the field, thus enhancing the SAR. Maximum SAR values of ∼1500 W/g have been achieved for the largest wires at H = 300 Oe and f = 310 kHz.

Spin-dependent electron transport in a Rashba quantum wire with rough edges

We investigate theoretically the spin-dependent electron transport in a Rashba quantum wire with rough edges. The charge and spin conductances are calculated as function of the electron energy or the wire length by adopting the spinresolved lattice Green function method. For a single disordered Rashba wire, it is found that the charge conductance quantization is destroyed by the edge disorder. However, a nonzero spin conductance can be generated and its amplitude can be manipulated by the wire length, which is attributed to the broken structure symmetries and the spin-dependent quantum interference induced by the rough boundaries. For a large ensemble of disordered Rashba wires, the average charge conductance decreases monotonically, however, the average spin conductance increases to a maximum value and then decreases, with increasing wire length. Further study shows that the influence of the rough edges on the charge and spin conductances can be eliminated by applying a perpendicular magnetic field to the wire. In addition, a very large magnitude of the spin conductance can be achieved when the electron energy lies between the two thresholds of each pair of subbands. These findings may not only benefit to further apprehend the transport properties of the Rashba low-dimensional systems but also provide some theoretical instructions to the application of spintronics devices.

Electronic conductance via atomic wires: a phase field matching theory approach

A model is presented for the quantum transport of electrons, across finite atomic wire nanojunctions between electric leads, at zero bias limit. In order to derive the appropriate transmission and reflection spectra, familiar in the Landauer-B\"{u}ttiker formalism, we develop the algebraic phase field matching theory (PFMT). In particular, we apply our model calculations to determine the electronic conductance for freely suspended monatomic linear sodium wires (MLNaW) between leads of the same element, and for the diatomic copper-cobalt wires (DLCuCoW) between copper leads on a Cu(111) substrate. Calculations for the MLNaW system confirm the correctness and functionality of our PFMT approach. We present novel transmission spectra for this system, and show that its transport properties exhibit the conductance oscillations for the odd- and even-number wires in agreement with previously reported first-principle results. The numerical calculations for the DLCuCoW wire nanojunctions are motivated by the stability of these systems at low temperatures. Our results for the transmission spectra yield for this system, at its Fermi energy, a monotonic exponential decay of the conductance with increasing wire length of the Cu-Co pairs. This is a cumulative effect which is discussed in detail in the present work, and may prove useful for applications in nanocircuits. Furthermore, our PFMT formalism can be considered as a compact and efficient tool for the study of the electronic quantum transport for a wide range of nanomaterial wire systems. It provides a trade-off in computational efficiency and predictive capability as compared to slower first-principle based methods, and has the potential to treat the conductance properties of more complex molecular nanojunctions.

Gold Nanowire Scattering and Absorption Measurements

ABSTRACTLithographically prepared gold nanodots and nanowires were placed onto a thermal sensor film to measure heat absorption. These identical wires are also subjected to dark field scattering measurements allowing for a comparison between absorption and scattering at the excitation wavelength. An increasing liner trend is found to exist between nanowires of increasing aspect ratio. The nanostructures also exhibit a decreasing temperature change with increasing wire length with a constant laser flux of 1.3 x 1010 W/m2.

Characterization of optical absorption and photovoltaic properties of silicon wire solar cells with different aspect ratio

The characteristics of optical absorption and photovoltaic properties of silicon(Si) wire solar cells with different aspect ratio have been studied. Vertically aligned Si wire arrays were fabricated by electrochemical etching and p–n junction was prepared by spin-on-dopant (SOD) diffusion method. To demonstrate the effect of wire length, it was controlled with varying electrochemical etching time. We also investigated the optical properties of wire arrays with increasing wire length. 50-nm-thick Al-doped zinc oxide (AZO) film was used as a surface passivation and electrical contact layer that conformally deposited on the radial p–n junction Si solar cell by atomic layer deposition (ALD) technique. We have observed efficiency enhancement with increasing wire length that increases the light trapping within the Si wire arrays, while the large surface area results in increased surface recombination. From the cell efficiency data, we identify that the surface recombination effect becomes more significant in the large surface area solar cells. To overcome this issue, we demonstrate the effect of a highly conformal AZO film grown by ALD to serve as surface passivation layer and top contact electrode. The conversion efficiency of the best sample is 7.1% and short circuit current ( J sc) of 25.3 mA/cm 2 under 100 mW/cm 2 of air mass (AM) 1.5G illumination, when the Si wire length is 8 μm with AZO coating. These results present that optimization of the wire length and surface passivation is necessary for silicon based solar cells prepared by electrochemical etching to enhance the solar cell energy conversion efficiency.

Transport properties of partially equilibrated quantum wires

We study the effect of thermal equilibration on the transport properties of a weakly interacting one-dimensional electron system. Although equilibration is severely suppressed due to phase-space restrictions and conservation laws, it can lead to intriguing signatures in partially equilibrated quantum wires. We consider an ideal homogeneous quantum wire. We find a finite temperature correction to the quantized conductance, which for a short wire scales with its length, but saturates to a length-independent value once the wire becomes exponentially long. We also discuss thermoelectric properties of long quantum wires. We show that the uniform quantum wire is a perfect thermoelectric refrigerator, approaching Carnot efficiency with increasing wire length.

On spatial resolution issues related to time-averaged quantities using hot-wire anemometry

The effect of spatial resolution on streamwise velocity measurements with single hot-wires is targeted in the present study, where efforts have been made to distinguish between spatial resolution and Reynolds number effects. The basis for measurements to accurately determine the mean velocity and higher order moments is that the probability density distribution is measured correctly. It is well known that the turbulence intensity is increasingly attenuated with increasing wire length. Here, it is also shown (probably for the first time) that besides the probability density distribution and hence the higher order moments, even the mean velocity is affected, albeit to subtle extent, but with important consequences in studies of concurrent wall-bounded turbulence.

Heteroepitaxial integration of metallic nanowires: transition from coherent to defectiveinterfaces via molecular dynamics

We use molecular dynamics to characterize the epitaxial integration of axially heterogeneousface-centered cubic (fcc) metallic nanowires. In order to isolate the effect of lattice mismatch we focus onPt/Pt* wireswhere Pt* differs from Pt only in its lattice parameter. We characterize the critical lattice mismatchbeyond which coherent interfaces are unable to withstand the interfacial stress and defectsare introduced. We studied wires of various radii, lengths, and orientations (, and ). We find that one-dimensional structures can withstand a very large lattice mismatchwith defect-free interfaces (over 10% for wires with small radius), much larger than possible in planar geometries (thin films). Oursimulations show that the critical lattice mismatch increases with decreasingradius; this increase is very rapid for radii below about 3 nm. Surprisingly, in thissmall radius regime the critical mismatch increases with increasing wire length;this behavior is the opposite of what occurs in thin films and what we observefor wires with larger radii. We find that the critical mismatch shows a strongdependence on the orientation of the wires associated with the resolved shear stress onavailable slip systems caused by the shear present along the intersection between theinterfacial plane and the free surface, where plastic deformation nucleates. We alsoperformed a detailed characterization of the dislocations that are introducedto release the interfacial strain as a function of geometry and orientation; ouranalysis reveals a novel interfacial relaxation mechanism in wires involving interfacial dislocations of screw character.

Embedded Green-function calculation of the conductance of oxygen-incorporated Au and Ag monatomic wires

We investigate the electronic structure and ballistic conductance of O-incorporated noble-metal atomic wires by a first-principles calculation using the embedded Green-function technique and the full-potential linearized augmented plane wave method. We consider straight monatomic wires with a single O atom inserted between two metal atoms. It will be shown that the conductance of the Au $6s$ (Ag $5s$) channel is reduced significantly by electron scattering via the inserted O atom. On the other hand, the O $2{p}_{x,y}$ $(\ensuremath{\pi})$ states, which form a resonant peak near the Fermi level ${E}_{F}$, provide two additional conduction channels in a narrow energy range corresponding to the resonant peak. Since the ${d}_{xz,yz}$ components of neighboring metal atoms, which hybridize with the O $2{p}_{x,y}$ orbitals and mediate coupling between them and the energy continuum in the leads, decay exponentially with distance from the O atom, the width of the resonance as well as the conductance associated with these two states decreases with increasing wire length. For longer wires, the wire undergoes a transition to a spin-polarized state with the O $2{p}_{x,y}$ resonance splitting into a fully occupied majority-spin peak and a partially occupied minority-spin peak. The latter contributes to a partially spin-polarized conductance at ${E}_{F}$.

Conductance of quantum wires: A numerical study of effects of an impurity and interactions

We use the non-equilibrium Green's function formalism along with a self-consistent Hartree-Fock approximation to numerically study the effects of a single impurity and interactions between the electrons (with and without spin) on the conductance of a quantum wire. We study how the conductance varies with the wire length, the temperature, and the strength of the impurity and interactions. The dependence of the conductance on the wire length and temperature is found to be in rough agreement with the results obtained from a renormalization group analysis based on the Hartree-Fock approximation. For the spin-1/2 model with a repulsive on-site interaction or the spinless model with an attractive nearest neighbor interaction, we find that the conductance increases with increasing wire length or decreasing temperature. This can be qualitatively explained using the Born approximation in scattering theory. For a strong impurity, the conductance is significantly different for a repulsive and an attractive impurity; this is due to the existence of a bound state in the latter case. In general, the large density deviations at short distances have an appreciable effect on the conductance which is not captured by the renormalization group analysis.

Low temperature growth and dimension- dependent photoluminescence efficiency of semiconductor nanowires

Low temperature growth and dimension dependent photoluminescence (PL) efficiency of semiconductor nanowires were investigated with CdS as a model system. The CdS nanowires were prepared with a simple, low temperature metal-organic chemical vapor deposition (MOCVD) process via the vapor–liquid–solid (VLS) mechanism. The low growth temperature of 360 °C was made possible with a newly developed single-source precursor of CdS and by using sputtered Au as the catalyst for the VLS growth. The length and diameter of the nanowires were adjusted by reaction time and sputtering conditions of Au, respectively. Nanowires of up to several μm in length and 20 to 200 nm in diameter were obtained. The PL quantum yield of the nanowires was found to decrease with increasing wire length, but to increase with decreasing wire diameter. This dimension-dependent PL efficiency of one-dimensional nanostructure, unlikely resulting from the quantum size confinement effect, appears to be a new observation that carries application significance.

Detection of Ferromagnetic Nanowires Using GMR Sensors

The detection of ferromagnetic Ni nanowires with integrated giant magnetoresistance sensors is investigated for possible use in magnetic biosensing. Single nanowires with magnetic moments at least as small as 1.5/spl times/10/sup -10/ emu are readily detectable in a zero-applied field. The signal is a linear function of the number of nanowires, making the sensor suitable for use as a particle counter. With increasing wire length, the magnitude of the sensor signal increases at first and then appears to saturate. The signal shows significant dependence on the angle of the wire with respect to the sensing axis and changes sign when the wire's moment is antiparallel to the sensing direction.

Quantum Electron Transport in Disordered Wires with Symplectic Symmetry

The conductance of disordered wires with symplectic symmetry is studied by the supersymmetric field theory. Special attention is focused on the case where the number of conducting channels is odd. Such a situation can be realized in metallic carbon nanotubes. The average dimensionless conductance g is obtained using Zirnbauer's super-Fourier analysis. It is shown that with increasing wire length, g approaches to 1 in the odd-channel case, while g vanishes in the ordinary even-channel case. It should be emphasized that the so-called Zirnbauer's zero mode, which has been believed to be unphysical, is essential for describing the anomalous behavior in the odd-channel case.

Shot noise in nanoscale conductors from first principles

We describe a field-theoretic approach to calculate quantum shot noise in nanoscale conductors from first principles. Our starting point is the second-quantization field operator to calculate shot noise in terms of single quasi-particle wavefunctions obtained self-consistently within density functional theory. The approach is valid in both linear and nonlinear response and is particularly suitable in studying shot noise in atomic-scale conductors. As an example we study shot noise in Si atomic wires between metal electrodes. We find that shot noise is strongly nonlinear as a function of bias and it is enhanced for one- and two-Si wires due to the large contribution from the metal electrodes. For longer wires it shows an oscillatory behavior for even and odd number of atoms with opposite trend with respect to the conductance, indicating that current fluctuations persist with increasing wire length.