Interwire Spacing Research Articles

On some structural phase transitions in coupled quantum wires at finite temperature

In this paper, we explore some structural phase transitions in GaAs-based coupled electron-electron (e-e) and electron-hole (e-h) quantum wires at finite temperature. To this endeavour, the intra- and inter-wire static-structure factors, pair-correlation functions and static (charge) density susceptibilities are calculated over a wide range of temperature T, particle number density parameter r se and some selected values of inter-wire spacing d. The particle exchange-correlations (xc) are included using the dynamic version of self-consistent mean-field theory of Singwi et al (the qSTLS theory), and the results have been compared with the static STLS model. It has been found that in the e-h system, the inclusion of dynamic nature of xc leads to the formation of Wigner crystal (WC) state in the close proximity of two wires at wave-vector q ∼ 3.5k Fe . However, a charge-density-wave (CDW) instability is observed at q ∼ 2k Fe when the xc are treated statically (k Fe being the electron’s Fermi wave vector). On the other hand, the e-e system shows comparatively small signatures of the WC phase when wires are kept sufficiently far apart, but, a long-wavelength instability is encountered in close vicinity of the wires. Interestingly, the CDW phase is completely missing in the e-e system at the investigated parameters. Expectedly, the quantum phase transitions are predicted to occur in the strongly correlated regime i.e. at sufficiently small T and high r se .

High-Sensitivity Wireless Sensing Using Amorphous Magnetic Microwires

Glass-coated amorphous ferromagnetic microwires subjected to variations in mass loading and parallel wire arrays with 0.5–6-cm interwire spacing were found to deliver exceptional magnetomechanical and wireless giant magnetoimpedance (GMI) responses, in the kilohertz and microwave range, respectively. The microwires allow wireless quantification of microgram mass differences: the magnetomechanical resonance frequency measured in zero applied field demonstrates an approximately linear decrease of 3 Hz/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{g}$ </tex-math></inline-formula> , and a sensitivity response that is ten times greater than that reported for commercial METGLAS-type amorphous magnetic ribbons of comparable length. Microwave giant magnetoimpedance data collected from planar arrays of parallel microwires show either constructive or destructive interference when compared to data obtained from a single microwire. The exceptional responsiveness of the glass-coated amorphous ferromagnetic microwires to mass loading and to geometric arrangement, along with their small diameter and ease of fabrication, highlights their promise for a wide variety of sensor applications, including biosensing, civil infrastructure monitoring, and high-throughput remote detection schemes.

Coulomb drag effect in coupled quantum wire systems: Finite-[formula omitted] and exchange–correlation effects

In this paper, we study the Coulomb drag in GaAs-based coupled electron–electron (e–e) and electron–hole (e–h) quantum wire systems by calculating the drag rate over a wide range of temperature T, particle (electron/hole) number density, and some selected values of inter-wire separation. The exchange–correlations among particles are treated within the self-consistent mean-field approximation of Singwi et al. (the STLS theory) and the inter-wire interaction potential is assumed to be statically screened. We find that the drag rate increases with decrease in particle number density and/or inter-wire separation due to the increased intra- and inter-wire coupling among particles. Further, the drag rate is found to be more for coupled e–h than the e–e quantum wire system due to the stronger e–h correlations on behalf of hole’s larger effective mass. As an important finding, we assert that the drag rate abruptly enhances at the critical system parameters for which the charge-density-wave (CDW) instability has been recently reported to occur in the e–h system. On the other side, the random-phase approximation (RPA) underestimates the drag rate in comparison to the STLS theory and the difference is seen to be more in the particle’s strong coupling domain i.e. at low inter-wire spacing and/or particle number density. In addition, we report for the first time the plasmon dispersion of both the coupled e–h and e–e systems at non-zero T and find that the plasma modes get dispersed into four branches among which two are recognized as optic (in-phase) and remaining two as the acoustic (out-of-phase) plasma modes. All the four plasma energy modes show a consistent blue shift with rise in T, however, the uppermost branches of the optical and acoustic plasma modes seem to repel each other as inter-wire spacing and/or particle number density is lowered. Interestingly, the inclusion of exchange–correlations in the RPA causes a red (blue)-shift in the upper (lower) branch of both the optic and acoustic plasma modes for each of the system leading to an overall enhancement in the Coulomb drag rate.

Exchange-correlation effects in coupled quantum wire systems at finite temperature

We investigate the exchange-correlation effects in coupled quantum wire systems at finite-temperature within the self-consistent mean-field approximation of Singwi et al by assuming the charge carriers to be electrons in one wire and electrons or heavier holes in the other. Numerical results are presented for the intra- and inter-wire static structure factors, pair-correlation functions and the static charge density susceptibility over a wide range of system parameters (viz. temperature T, particle number density and inter-wire spacing) at equal and fixed transverse width of both the wires. We find for the first time that the coupled electron-hole (e-h) quantum wire system may favor a charge-density-wave (CDW) instability at sufficiently low T and carrier density in the close proximity of the wires, where as no such phase transition is observed in the electron-electron (e-e) quantum wire system at any non-zero T. The intra-wire contact pair-correlation functions of both the systems show a non-monotonous behavior with increasing (decreasing) T (carrier number density), and increase consistently with decrease in inter-wire spacing. On the other hand, the corresponding inter-wire contact pair-correlation functions show a non-monotonous T-dependence and consistent increase with decrease in carrier number density and/or inter-wire separation. Results of free exchange-correlation energy for both the e-h and e-e coupled systems are also reported which are found to have a noticeable dependence upon T. To highlight the effect of exchange-correlations, our results have been compared with the predictions of the random-phase approximation (RPA).

Numerical determination of field emission performance of GaN nanowire arrays

Determining the field emission turn-on field of the GaN nanowire array can better adapt to the application of field-assisted photocathode. A numerical simulation of the field emission behavior based on the finite difference method is proposed. Considering the influence of nanowire radius, height, cathode-anode spacing and interwire spacing on the field emission of GaN nanowires. The simulation proves that field enhancement factor of a single GaN nanowire is the most excellent, while emission electrons is relatively small. When GaN nanowires have a larger aspect ratio, it will have a larger enhancement factor and a small required turn-on electric field. Due to an enhanced field shielding, the enhancement factor of the GaN nanowire array reduces as interwire spacing reduces. Cathode-anode spacing hardly influences enhancement factor of GaN nanowire array, while potential distributions are affected by cathode-anode spacing to a certain extent.

Coulomb drag rate in a coupled electron-electron quantum wire system

In this paper, we calculate the Coulomb drag rate in a coupled electron-electron (e-e) quantum wire system over a wide range of temperature T, and some selected values of electron number densityandinter-wire spacing. The intra- and inter-wire exchange-correlations are treated within the self-consistent mean-field approximation of Singwi et al (the so called STLS theory) suitably generalized to the coupled system at finite-T. At a fixed T, the drag rate is found to increase with decrease in inter-wire separation and/or electron number density due to increased intra- and inter-wire coupling among electrons. Further, we note that the magnitude of drag rate is more in the STLS theory as compared to the random-phase approximation (RPA), and the difference consistently increases with decrease in inter-wire spacing.

Fabrication of patterned graphitized carbon wires using low voltage near-field electrospinning, pyrolysis, electrodeposition, and chemical vapor deposition

We herein report a high-resolution nanopatterning method using low voltage electromechanical spinning with a rotating collector to obtain aligned graphitized micro and nanowires for carbon nanomanufacturing. A small wire diameter and a small inter-wire spacing were obtained by controlling the electric field, the spinneret-to-collector distance, the pyrolysis parameters, the linear speed of the spinneret, the rotational speed of the collector. Using a simple scaling analysis, we show how the straightness and the diameter of the wires can be controlled by the electric field and the distance of the spinneret to the collector. A small inter-wire spacing, as predicted by a simple model, was achieved by simultaneously controlling the linear speed of the spinneret and the rotational speed of the collector. Rapid drying of the polymer nanowires enabled the facile fabrication of suspended wires over various structures. Patterned polyacrylonitrile wires were carbonized using standard stabilization and pyrolysis to obtain carbon nanowires. Suspended carbon nanowires with a diameter of <50 nm were obtained. We also established a method for making patterned, highly graphitized structures by using the aforementioned carbon wire structures as a template for chemical vapor deposition of graphite. This patterning technique offers high throughput for nano writing, which outperforms other existing nanopatterning techniques, making it a potential candidate for large-scale carbon nanomanufacturing.

Tunable Axion Plasma Haloscopes.

We propose a new strategy for searching for dark matter axions using tunable cryogenic plasmas. Unlike current experiments, which repair the mismatch between axion and photon masses by breaking translational invariance (cavity and dielectric haloscopes), a plasma haloscope enables resonant conversion by matching the axion mass to a plasma frequency. A key advantage is that the plasma frequency is unrelated to the physical size of the device, allowing large conversion volumes. We identify wire metamaterials as a promising candidate plasma, wherein the plasma frequency can be tuned by varying the interwire spacing. For realistic experimental sizes, we estimate competitive sensitivity for axion masses of 35-400 μeV, at least.

A systematic investigation of radiative recombination in GaN nanowires: The influence of nanowire geometry and environmental conditions

Nanowires intrinsically exhibit a large surface area, which makes them sensitive to physical and chemical interactions with their environment. Here, we investigate the surface recombination at m-plane side walls of selective area-grown GaN nanowires on Si (111) subjected to different environmental conditions. In contrast to the stable photoluminescence observed from c-plane surfaces of films, nanowires exhibit a distinct time-dependent photoluminescence quenching by over 90% within the time scale of seconds in the presence of air or dissociated liquids. This quenching is most pronounced for 50 nm diameter nanowires with interwire spacings larger than 500 nm due to internal electric field and external light field distributions. Ion- and pH-sensitive measurements, in combination with an externally applied voltage, allow the assignment of this effect to anions from the surroundings to accumulate at the nonpolar GaN side walls of the UV-exposed GaN nanowires. The decay times of the luminescence signal follow the dynamics of valence band holes, which deplete GaN surface states and positively charge the nanowire surfaces. This, in turn, induces the buildup of a capacitive anion shell around the nanowires, leading to an enhanced nonradiative surface recombination of photo-generated charge carriers from the GaN nanowire. In the absence of UV light, a recovery of the photoluminescence signal within tens of minutes indicates the dissolution of the anionic shell via charge balancing. The impact of light-induced electronic and ionic charge redistribution on photocarrier recombination represents an important mechanism of function for GaN nanowire-based devices, ranging from sensors to photocatalysts.

Domain wall structure and interactions in 50 nm wide Cobalt nanowires

Arrays of cobalt nanowires with widths of 50 nm, thickness of 5 and 20 nm and periodicity of 70 nm were fabricated by pattern transfer from a self-assembled block copolymer film. Transverse domain walls (DWs) were imaged by magnetic force microscopy, indicating repulsive interactions between DWs of the same sign in the 20 nm thick wires. Micromagnetic simulations were used to identify the interactions in the six distinct cases of a pair of transverse DWs in adjacent wires, considering all the possible combinations of head-to-head and tail-to-tail DWs and the orientation of the core magnetization. The boundary between repulsive and attractive DW interactions is mapped out for wires as a function of thickness, width and interwire spacing.

Highly-ordered silicon nanowire arrays for photoelectrochemical hydrogen evolution: an investigation on the effect of wire diameter, length and inter-wire spacing

The influence of structural parameters of highly-ordered silicon nanowire arrays on their solar-driven HER performance is systematically investigated.

Coulomb interaction driven instabilities of sliding Luttinger liquids

We study systems made of periodic arrays of one dimensional quantum wires, coupled by Coulomb interaction. Using bosonization an interacting metallic fixed point is obtained, which is shown to be a higher dimensional analogue of the Tomonaga-Luttinger liquid, or a sliding Luttinger liquid. This non-Fermi liquid metallic state, however, is unstable in the presence of weak interwire backscatterings, which favor charge density wave states and suppress pairing. Depending on the effective strength of the Coulomb repulsion and the size of interwire spacing various charge density wave states are stabilized, including Wigner crystal states. Our method allows for the determination of the specific ordering patterns, and corresponding energy and temperature scales.

Label-Free Virus Capture and Release by a Microfluidic Device Integrated with Porous Silicon Nanowire Forest.

Viral diseases are perpetual threats to human and animal health. Detection and characterization of viral pathogens require accurate, sensitive, and rapid diagnostic assays. For field and clinical samples, the sample preparation procedures limit the ultimate performance and utility of the overall virus diagnostic protocols. This study presents the development of a microfluidic device embedded with porous silicon nanowire (pSiNW) forest for label-free size-based point-of-care virus capture in a continuous curved flow design. The pSiNW forests with specific interwire spacing are synthesized in situ on both bottom and sidewalls of the microchannels in a batch process. With the enhancement effect of Dean flow, this study demonstrates that about 50% H5N2 avian influenza viruses are physically trapped without device clogging. A unique feature of the device is that captured viruses can be released by inducing self-degradation of the pSiNWs in physiological aqueous environment. About 60% of captured viruses can be released within 24 h for virus culture, subsequent molecular diagnosis, and other virus characterization and analyses. This device performs viable, unbiased, and label-free virus isolation and release. It has great potentials for virus discovery, virus isolation and culture, functional studies of virus pathogenicity, transmission, drug screening, and vaccine development.

Microwires enabled metacomposites towards microwave applications

The work describes the microwave behavior of polymer composites containing parallel Fe-based and continuous/short-cut Co-based microwire arrays. A magnetic field-tunable metacomposite feature has been identified in the hybrid microwires composite containing 3mm spaced Co-based wires and confirmed by the presence of transmission windows in the frequency band of 1–3.5GHz. The magnetically tuned redshift-blueshift in the transmission window is due to the competing dynamic interactions between the different wires and the ferromagnetic resonance of the Fe-based microwires. When the Co-based inter-wire spacing is increased to 10mm, dual-band transmission windows in the 1.5–3.5GHz and 9–17GHz bandwidths were observed. These transmission windows are likely induced by the ferromagnetic resonance of Fe-based wires and the long range dipolar resonance arising between Fe–Co wire couples. The hybridization of parallel Fe-based and short Co-based wires in the composites leads to a significant enhancement of the transmission window in the 1–6GHz band due to the band-gap nature of the Co-based wires. The hybrid metacomposites containing microwires seem attractive in radio frequency identification application.

Novel insight toward engineering of arrayed Cu@Sn nanoelectrodes: Rational microstructure refinement and its remarkable “harvesting effect” on lithium storage capability

The impact of morphological and compositional evolutions of arrayed Cu@Sn nanoelectrodes on their lithium storage capability is investigated in this work. Strikingly, it is found that the diameter of Cu nanowire-core, inter-wire spacing, as well as Sn/Cu ratio of such nanohybrids could be directly adjusted through a synchronous electrochemical dissolution/deposition strategy in a single bath. In line with expectation, nanoarchitecture tailoring of the array-type electrode brings in expanded space available and enhanced interfacial adhesion, which can not only effectively enable high loading of conformal Sn nanolayers on per unit footprint area but also help in suppressing capacity fading, and thus harvesting outstanding areal capacity (∼1.46 mAh cm−2 at 0.2 mA cm−2) and rate capability. It is envisioned that such work will shed light on ways to facilely tune the characteristic parameters of a series of Cu-cored hybrid nanowire arrays with desired functionalities for nanodevice applications.

In Situ Electrochemical Porosimetry of Vertically-Aligned Carbon Nanotube Carpets through Impedance Spectroscopy

Vertically aligned carbon nanotube (VA-CNT) electrodes for electrochemical systems have been utilized in supercapacitors [1], composite supercaps [2], and a platform for lithium air batteries [3]. Developing fundamental understanding of the interplay between pore volume, carbon nanowire spacing, and device performance can help unlock the potential of aligned CNTs for highly efficient energy storage. Previous studies have shown that densification of CNTs can lead to increased volumetric capacitance while maintaining gravimetric capacity [2]. However, to the best of our knowledge, the effects of increased proximity between nanowires on the resistance to ion transport through the interwire spacing has not been examined closely. This is because there are significant challenges to characterizing nanowire geometry using typical methods such as TEM, SEM [4], and BET. In this investigation, we apply impedance spectroscopy to measure the VA-CNT porosimetry, in situ, thereby preserving the carpet and accurately calculating the electrochemically active geometry. To link impedance data with geometry, we utilize a transmission line model developed by De Levie [5] and further expanded by Song et al. [6] where pores are modeled as having an ion transport resistance that depends on geometric factors. We begin by modeling the voids between VA-CNTs as inverse cylinders, with the nanowire spacing as the diameter and forest height as the length of the nanotube. By assuming the diameter of the nanowire spacings in the VA-CNT array have a log normal distribution we can characterize the relationship between distribution shape and CNT density, giving us full geometric understanding of the system. Experimental VA-CNT carpet samples were grown on 1x1 cm2 substrates to heights of ~1mm. Carpets were mechanically densified using bi-axial compression. Volume fractions (Vf) of carpets are defined as the growth substrate area, divided by the final planar area, following compression. We examined the porosimetry of carpets with volume fractions of 1% (as grown) to 20% (Figure 1a-d). VA-CNT carpets were bonded to a non-corrosive current collector using a gold-gold thermocompression, outlined in [7]. In order to ensure proper boundary conditions, the electrode was fully sealed with electromasking tape, with only a window cut to expose the CNT carpet. Cylic voltammetry tests were performed to determine the appropriate voltage window for studying purely capacitive effects, which was 0-0.5 V vs Ag/AgCl as shown in Figure 1e. Impedance spectroscopy from 1 kHz to 100 mHz was conducted on the cell. Figure 1f shows there is a characteristic 45° angle at high frequencies (characteristic of transmission line model), and a < 90° slope at low frequencies (characteristic of materials with a distribution of pore sizes [6]). Applying the transmission line model to impedance data and assuming a lognormal distribution for the pores show that the 1% Vf has a mean penetration depth of 0.87 and sigma 1.91. The 5% Vf has a mean of 0.62 and sigma 0.68. The decreasing sigma with increasing volume fraction indicates that densification leads to reduced variation in nanowire spacing. The fitting results were extended to determine the electrode ionic resistance, total pore volume and average interspacing, based on measurements of the mass, length, and outer diameter of the CNTs. These experimental studies allow us to determine the role of geometry on electrode properties. Through impedance spectroscopy and simple geometric modeling for porous materials we can determine the distribution of CNT spacing, which can be used to predict packing and performance of VA-CNT carpets. [1] M . Hughes, G. Z. Chen, M. S. P. Shaffer, D. J. Fray, and A. H. Windle, Chemistry of Materials, 14, 1610 (2002). [2] N. Lachman, H. Xu, Y. Zhou, M. Ghaffari, M. Lin, D. Bhattacharyya, A. Ugur, K. K. Gleason, Q. M. Zhang, and B. L. Wardle, Advanced Materials Interfaces, 1, (2014). [3] B. M. Gallant, R. R. Mitchell, D. G. Kwabi, J. Zhou, L. Zuin, C. V. Thompson, and Y. Shao-Horn, The Journal of Physical Chemistry C, 116, 20800-20805 (2012). [4] I. Y. Stein and B. L. Wardle, Physical Chemistry Chemical Physics, 15, 4033 - 4040 (2013). [5] R. De Levie, Electrochimica Acta, 8, 751-780 (1963). [6] H.-K. Song, J.-H. Sung, Y.-H. Jung, K.-H. Lee, L. H. Dao, M.-H. Kim, and H.-N. Kim, Journal of the electrochemical society, 151, E102-E109 (2004). [7] R. Enright, R. Mitchell, H. Mutha, C. Lv, M. Christiansen, C. V. Thompson, and E. N. Wang, in MRS Proceedings, 1407, (2012) Figure 1

Nanowire-imposed geometrical control in studies of actomyosin motor function.

Recently, molecular motor gliding assays with actin and myosin from muscle have been realized on semiconductor nanowires coated with Al2O3. This opens for unique nanotechnological applications and novel fundamental studies of actomyosin motor function. Here, we provide a comparison of myosin-driven actin filament motility on Al2O3 to both nitrocellulose and trimethylchlorosilane derivatized surfaces. We also show that actomyosin motility on the less than 200 nm wide tips of arrays of Al2O3-coated nanowires can be used to control the number, and density, of myosin-actin attachment points. Results obtained using nanowire arrays with different inter-wire spacing are consistent with the idea that the actin filament sliding velocity is determined both by the total number and the average density of attached myosin heads along the actin filament. Further, the results are consistent with buckling of long myosin-free segments of the filaments as a factor underlying reduced velocity. On the other hand, the findings do not support a mechanistic role in decreasing velocity, of increased nearest neighbor distance between available myosin heads. Our results open up for more advanced studies that may use nanowire-based structures for fundamental investigations of molecular motors, including the possibility to create a nanowire-templated bottom-up assembly of 3D, muscle-like structures.

High performance binder-free Sn coated carbon nanotube array anode

We report a binder-free Li-ion battery anode based on a tin (Sn) coated vertical-aligned carbon nanotube (VACNT) array. In this anode, the VACNT array, acting as a robust core matrix to support high theoretical capacity active material Sn, offers large inter-wire spacing for Sn expansion during the lithiation/de-lithiation processes and forms direct conductive pathways for electrons transporting. A remarkable enhancement in the capacity retentions abilities of this anode compared with pure Sn film anode has been achieved. A reversible capacity as high as 900mAhg−1 over 400 cycles at 0.2C and nearly 400mAhg−1 over 1000 cycles at 0.5C has been demonstrated respectively.

Optimization of Delay and Energy in On-Chip Buses using Bus Encoding Technique

In very deep sub-micron (VDSM) fault-tolerant busses, crosstalk noise and logic faults caused due to shrinking wiresize and reduced inter-wire spacing are major factors affecting the performance of onchip interconnects, such as high power consumption and increased delay. In this paper we propose a bus optimization technique which reduce the energy and power-delay using Hamming Single Error Correcting Code. In this coding scheme we implement Fibonacci representation of optimal (7,4) Hamming Code which is more efficient than Single Error correction (9,4) Hamming Code. Also the proposed scheme eliminates crosstalk classes among the interconnects wires, there by reducing delay and energy consumption. The proposed techniques achieves an efficiency of 11% in energy consumption and a reduction of delay with respect to the existing techniques.

Three-Dimensionally Constrained Actomyosin Motility on Oxide Coated Semiconductor Nanowires

In vitro motility assays with surface-adsorbed myosin motor fragments have given important insights into the molecular physiology and pathology of striated muscle contraction and inspired nanotechnological applications e.g. lab-on-a-chip devices. However, to date neither precise localized control of the motor density nor introduction of nanoscale three-dimensional geometrical constraints has been possible. This hampers studies of cooperative phenomena and realization of three-dimensional (3D) transport systems. Here, we take critical steps to overcome these limitations, using aluminium oxide coated gallium phosphide nanowires as scaffolds for heavy meromyosin (HMM) adsorption. The wires (diameter: 100-200 nm; height < 5 μm) were either positioned vertically in regular arrays or horizontally on surfaces after etching the GaP core to give hollow nanowires. Upon ATP addition, actin filaments were propelled by HMM on top of the arrays, with filaments spanning inter-wire distances up to 1 μm. The filaments also moved up and down vertical nanowires as detected using sub-wavelength light guiding properties of the nanowires. Motility on top of nanowire arrays holds potential for studies of cooperative phenomena e.g. local enhancement of myosin binding along actin filaments upon actomyosin interactions. Here we tested whether the low velocity for long filaments seen at uniformly low motor densities on flat surfaces may be attributed to loss of cooperative enhancement of myosin head binding locally to actin close to an existing actomyosin cross-bridge. Velocity data showing 1/3 the velocity with 1 μm compared to 300 nm inter-wire spacing, in both cases with ∼90 HMM molecules per 150 nm wide wire tip (giving high local motor density), argue against this idea. We further demonstrate HMM propelled transport through hollow nanowires of 80 nm inner diameter. Uses of hollow nanowires in fundamental studies of actomyosin and in nanotechnological applications will be considered.