Microwire Arrays Research Articles

Ferromagnetic Microwire Systems as a High-Gradient Magnetic Field Source for Magnetophoresis

The use of amorphous ferromagnetic microwires has potential for applications in biomedicine and biophysics. Microwire arrays create magnetic fields that are characterized by strong spatial gradients. A system of ferromagnetic microwires with magnetizations across the diameter may efficiently accumulate paramagnetic particles along the wire due to forced diffusion. For typical system parameters, the particles concentration at the wire surface increases more than twice for a characteristic time of 30 s. A pair of closely spaced microwires with diametric magnetizations (referred to as a dipole pair) yields a unique potential landscape with a two-dimensional minimum. The possibility of levitation of diamagnetic particles just above the dipole pair array was numerically demonstrated.

Effects of bubbles on the electrochemical behavior of hydrogen-evolving Si microwire arrays oriented against gravity

Silicon microwire-arrays mitigate the effects of gas bubbles on the total overpotential of hydrogen-evolving cathodes oriented against gravity.

Modulation of an intermediate layer between NiCoP and Ni foam substrate in a microwire array electrode for enhancing the hydrogen-evolution reaction.

A hierarchical electrode consisting of a Ni foam substrate, an intermediate layer of a Mo-doped basic cobalt carbonate microwire array, and a surface layer of NiCoP was fabricated for hydrogen evolution. The introduction of the intermediate layer accounted for the low overpotential (45 mV, 10 mA cm-2) and high solar-to-hydrogen conversion efficiency (14.9%) in a PEC water splitting application.

Coaxial semipolar InGaN/GaN microwire array LED with substantially suppressed efficiency droop

GaN nano/micro-structure based light-emitting diodes (LEDs) have drawn much attention owing to their potential applications in display and optoelectronics integration. Here, we have fabricated coaxial semipolar InGaN/GaN multiple quantum wells (MQWs) microwire array LED with superior performances in suppressing efficiency droop. The results show that our as-synthesized microwire has two semipolar planes (1‾101) and (11‾01), and the InGaN/GaN MQWs have superior crystal quality. In addition, the efficiency droop ratio of our device is about 9.7% as the injected current increases from 3 to 100 A/cm2, which is largely declined by 47% compared with that of the conventional polar c-plane LEDs. Meanwhile, the microwire array LED reveals a small wavelength shift (3 nm) as the injected current increases from 3 to 23 A/cm2. The effective advances in the device should be attributed to the weaker quantum-confined stark effect of InGaN/GaN MQWs in semipolar plane. This work proposes a high repeatability method to fabricate microwire array LED for future optoelectronic integrated systems.

Brilliant keV–MeV x-ray emission through weakly unbalanced quasi-static electric and magnetic fields

We proposed a new method to yield brilliant keV–MeV x-ray emission, inspired by Cao et al (2010 Phys. Plasma 17 043103). When a short femtosecond pulse is coupling into a microwire array target, due to the weakly unbalanced quasi-static electric and magnetic fields, the energetic electrons accelerated by the laser will emit a large amount of photons per shot, comparable to that from the laser-wake betatron radiation. It was found that the critical energy of the photons and the total photon yields are strongly dependent on the laser intensity and structure parameters of the target. The peak spectral brightness of the radiation in our scheme was estimated to be 4 × 1019 at the laser intensity 3 × 1020 W cm−2. More important, the photon energies can be extended to hundreds of keV to MeV readily if electron beam accelerated by the laser is in the range of MeV to GeV.

Stretchable and colorless freestanding microwire arrays for transparent solar cells with flexibility

Transparent solar cells (TSCs) are emerging devices that combine the advantages of visible transparency and light-to-electricity conversion. Currently, existing TSCs are based predominantly on organics, dyes, and perovskites; however, the rigidity and color-tinted transparent nature of those devices strongly limit the utility of the resulting TSCs for real-world applications. Here, we demonstrate a flexible, color-neutral, and high-efficiency TSC based on a freestanding form of n-silicon microwires (SiMWs). Flat-tip SiMWs with controllable spacing are fabricated via deep-reactive ion etching and embedded in a freestanding transparent polymer matrix. The light transmittance can be tuned from ~10 to 55% by adjusting the spacing between the microwires. For TSCs, a heterojunction is formed with a p-type polymer in the top portion of the n-type flat-tip SiMWs. Ohmic contact with an indium-doped ZnO film occurs at the bottom, and the side surface has an Al2O3 passivation layer. Furthermore, slanted-tip SiMWs are developed by a novel solvent-assisted wet etching method to manipulate light absorption. Finite-difference time-domain simulation revealed that the reflected light from slanted-tip SiMWs helps light-matter interactions in adjacent microwires. The TSC based on the slanted-tip SiMWs demonstrates 8% efficiency at a visible transparency of 10% with flexibility. This efficiency is the highest among Si-based TSCs and comparable with that of state-of-the-art neutral-color TSCs based on organic–inorganic hybrid perovskite and organics. Moreover, unlike others, the stretchable and transparent platform in this study is promising for future TSCs.

Verification of a Rapidly Multiplexed Circuit for Scalable Action Potential Recording.

This report presents characterizations of in vivo neural recordings performed with a CMOS multichannel neural recording chip that uses rapid multiplexing directly at the electrodes, without any pre-amplification or buffering. Neural recordings were taken from a 16-channel microwire array implanted in rodent cortex, with comparison to a gold-standard commercial bench-top recording system. We were able to record well-isolated threshold crossings from 10 multiplexed electrodes and typical local field potential waveforms from 16, with strong agreement with the standard system (average SNR = 2.59 and 3.07 respectively). For 10 electrodes, the circuit achieves an effective area per channel of 0.0077 mm2, which is >5x smaller than typical multichannel chips. Extensive characterizations of noise and signal quality are presented and compared to fundamental theory, as well as results from in vivo and in vitro experiments. By demonstrating the validation of rapid multiplexing directly at the electrodes, this report confirms it as a promising approach for reducing circuit area in massively-multichannel neural recording systems, which is crucial for scaling recording site density and achieving large-scale sensing of brain activity with high spatiotemporal resolution.

Room temperature non-balanced electric bridge ethanol gas sensor based on a single ZnO microwire**Project supported by the National Natural Science Foundation of China (Grant Nos. 61574026 and 11405017) and the Liaoning Provincial Natural Science Foundation, China (Grant No. 201602453).

In this paper, ultra-long and large-scaled ZnO microwire arrays are grown by the chemical vapor deposition method, and a single ZnO microwire-based non-balanced electric bridge ethanol gas sensor is fabricated. The experimental results show that the gas sensor has good repeatability, high response rate, short response, and recovery time at room temperature (25 °C). The response rate of the gas sensor exposed to 90-ppm ethanol is about 93%, with a response time and recovery time are 0.3 s and 0.7 s respectively. As a contrast, the traditional resistive gas sensor of a single ZnO microwire shows very small gas response rate. Therefore, ethanol gas sensor based on non-balanced electric bridge can obviously enhance gas sensing characteristics, which provides a feasible method of developing the high performance ZnO-based gas sensor.

A self-sensing microwire/epoxy composite optimized by dual interfaces and periodical structural integrity

The performance of self-sensing composites largely relies on the fillers’ property and filler-matrix interface. Previously we have demonstrated that glass-coated microwires enable self-sensing composites but with limited damage detection. Here, we propose an optimization strategy capitalized on designed dual-interface, i.e. glass shell/metallic core (inner interface) and glass shell/epoxy matrix (outer interface) which could be decoupled to modulate microwave responses under stress stimuli. Moreover, interfacial modification was applied to mediate bonding in the outer interface while preserving the microwire circular magnetic domain structure in the inner interface. The damage detection via microwave testing was exercised on free-standing glass-coated microwire arrays with preset cracks and drilled composites incorporating these arrays. It proved to be conditioned by structural integrity related to the array periodicity, number, and location of cracks and by the degree of stress transfer across the silane-treated interface. The proposed self-sensing composites enabled by properly conditioned dual-interface are promising for real-time monitoring in restricted and safety-critical environments.

Direct laser writing of birefringent photonic crystals for the infrared spectral range

Infrared optical photonic crystals fabricated using direct laser writing, which is based on the two-photon polymerization of suitable monomers, have received substantial interest since the emergence of this process. Two-photon polymerization could be a disruptive technology for the fabrication of all-dielectric photonic crystals in the infrared spectral range, as it allows the synthesis of large scale arrays of uniform structures with arbitrary geometries and arrangements. However, all-dielectric photonic crystals that provide birefringent optical responses in the infrared spectral range have not yet been demonstrated using direct laser writing techniques. Here, the authors explore the form birefringence observed in photonic crystals composed of arrays of subwavelength-sized slanted polymer microwires. The photonic crystals investigated here were fabricated in a single fabrication step using direct laser writing of an infrared transparent photoresist (IP-Dip). A strong contrast of the cross-polarized reflectance of photonic crystals as a function of the in-plane orientation is observed in the mid-infrared spectral range at λ≈6.5μm. This observation is indicative of an anisotropic optical behavior. Finite element based techniques corroborate the experimentally observed responses qualitatively.

An investigation of surficial conduction heat loss in perfectly aligned micro-wire array

Recently, geometrical effects in micro-/nanomaterials have gained great attention of researchers in diverse scientific and engineering fields because unique physical properties can be achieved not only by the micro-/nanosize effects of materials but also by adjustment of the geometrical parameters. In this work, we investigated heat-transfer in a perfectly aligned microwire array structure. When the structure has a very narrow gap (g) between the wires, which is less than twice the mean free path of an ambient medium (g < 2λmedium), there is a significant suppression of conduction heat-loss due to the reduction of the “molecular diffusion,” resulting in the improved power-efficiency in joule-heating characteristics. To clearly understand the suppressed conduction heat-loss with regard to the gap reduction, we performed analytical calculation and finite-element-method (FEM) simulation. To experimentally investigate the phenomenon, we develop a setup that is a combination of a microstructure having various gaps and an air-pressure controllable chamber. Through the experiment, we verified that a remarkable improvement of joule-heating operation can be achieved with the structure having very small gaps (g < 2λmedium) due to the suppression of conduction heat-loss.

Multistep nucleation and growth mechanisms of organic crystals from amorphous solid states

Molecular self-assembly into crystallised films or wires on surfaces produces a big family of motifs exhibiting unique optoelectronic properties. However, little attention has been paid to the fundamental mechanism of molecular crystallisation. Here we report a biomimetic design of phosphonate engineered, amphiphilic organic semiconductors capable of self–assembly, which enables us to use real-time in-situ scanning probe microscopy to monitor the growth trajectories of such organic semiconducting films as they nucleate and crystallise from amorphous solid states. The single-crystal film grows through an evolutionary selection approach in a two-dimensional geometry, with five distinct steps: droplet flattening, film coalescence, spinodal decomposition, Ostwald ripening, and self-reorganised layer growth. These sophisticated processes afford ultralong high-density microwire arrays with high mobilities, thus promoting deep understanding of the mechanism as well as offering important insights into the design and development of functional high-performance organic optoelectronic materials and devices through molecular and crystal engineering.

Optimization of All Figure-of-Merits in Well-Aligned GaN Microwire Array Based Schottky UV Photodetectors by Si Doping

GaN nano/microwires, due to their large surface-to-volume ratio and the reduced dimensionality, have been widely used in high-performance ultraviolet (UV) photodetectors (PDs). However, there has been a fundamental trade-off between the photocurrent gain and the speeds of PDs, which have limited their practical applications. In this work, highly ordered GaN microwire array based Schottky UV PDs have been fabricated, and optimizing Si doping concentrations can entirely improve the performances of the devices. The results show that output photocurrent increases monotonously as increasing doping concentration, while the dark current experiences a decrease followed by an increase, resulting in an optimized value under a certain doping concentration. At doping concentration of 1 × 1018 cm–3 with 2.2 mW cm–2 illuminating intensity, the sensitivity, responsivity, and detectivity of the devices are greatly enhanced by 1.84 × 104%, 163%, and 2103%, respectively. Meanwhile, the time-dependent response in Si-doped d...

Enhanced Stability and Efficiency for Photoelectrochemical Iodide Oxidation by Methyl Termination and Electrochemical Pt Deposition on n-Type Si Microwire Arrays

Arrays of Si microwires doped n-type (n-Si) and surface-functionalized with methyl groups have been used, with or without deposition of Pt electrocatalysts, to photoelectrochemically oxidize I–(aq) to I3–(aq) in 7.6 M HI(aq). Under conditions of iodide oxidation, methyl-terminated n-Si microwire arrays exhibited stable short-circuit photocurrents over a time scale of days, albeit with low energy-conversion efficiencies. In contrast, electrochemical deposition of Pt onto methyl-terminated n-Si microwire arrays consistently yielded energy-conversion efficiencies of ∼2% for iodide oxidation, with an open-circuit photovoltage of ∼400 mV and a short-circuit photocurrent density of ∼10 mA cm–2 under 100 mW cm–2 of simulated air mass 1.5G solar illumination. Platinized electrodes were stable for >200 h of continuous operation, with no discernible loss of Si or Pt. Pt deposited using electron-beam evaporation also resulted in stable photoanodic operation of the methyl-terminated n-Si microwire arrays but yielded substantially lower photovoltages than when Pt was deposited electrochemically.

Shape-Controlled Silicon Microwire Arrays from Au–Ag-Catalyzed Metal-Assisted Chemical Etching for Radial Junction Solar Cells

Metal-assisted chemical etching (MaCE) has been extensively studied as a cost-effective way to produce silicon nano/micro structure arrays. However, it is hard to keep the resultant morphologies of...

Scattering of Microwaves by a Passive Array Antenna Based on Amorphous Ferromagnetic Microwires for Wireless Sensors with Biomedical Applications.

Co-based amorphous microwires presenting the giant magnetoimpedance effect are proposed as sensing elements for high sensitivity biosensors. In this work we report an experimental method for contactless detection of stress, temperature, and liquid concentration with application in medical sensors using the giant magnetoimpedance effect on microwires in the GHz range. The method is based on the scattering of electromagnetic microwaves by FeCoSiB amorphous metallic microwires. A modulation of the scattering parameter is achieved by applying a magnetic bias field that tunes the magnetic permeability of the ferromagnetic microwires. We demonstrate that the OFF/ON switching of the bias activates or cancels the amorphous ferromagnetic microwires (AFMW) antenna behavior. We show the advantages of measuring the performing time dependent frequency sweeps. In this case, the AC-bias modulation of the scattering coefficient versus frequency may be clearly appreciated. Furthermore, this modulation is enhanced by using arrays of microwires with an increasing number of individual microwires according to the antenna radiation theory. Transmission spectra show significant changes in the range of 3 dB for a relatively weak magnetic field of 15 Oe. A demonstration of the possibilities of the method for biomedical applications is shown by means of wireless temperature detector from 0 to 100 °C.

Design of Wavy Ag Microwire Array for Mechanically Stable, Multimodal Vibrational Haptic Interface

AbstractA vibrotactile interface is an actuator device to convey haptic information intuitively from electronics to users. For the next‐generation of user‐friendly interface applications, the vibrotactile actuator is required to be vibration intensity/frequency controllable, mechanically stable, transparent, and have large scalability. Previously, although these requirements are satisfied via several approaches using a random network film of Ag wires or a mixture with conductive polymers, the random‐network‐based materials only have limited control on material density and uniformity, which in turn hinders precise control over vibration intensity and device transparency. Here, a new approach to assemble large‐scale Ag microwire arrays is demonstrated by involving an evaporative assembly method and is presented to overcome the current limitations. In particular, the 1D wavy structure derived from fractal designs promotes vibration intensity and cycling due to greater areal coverage and improved mechanical stability. Furthermore, by taking advantage of the precisely aligned microwires array, tunable multimode vibration frequencies are obtained by generating two different voltage frequencies. The large‐scale wavy Ag microwire array with precise spatial controllability will be directly adaptable as a user‐friendly interface in electronic applications like wearable devices, computer interfaces, and flexible mobile phones.

Microwave programmable response of Co-based microwire polymer composites through wire microstructure and arrangement optimization

Traditional approaches to realize microwave tunability in microwire polymer composites which mainly rely on topological factors, magnetic field/stress stimuli, and hybridization are burdensome and restricted to rather narrow band frequencies. This work presents a facile strategy based on a single component tunable medium to program the transmission response over wide frequency bands. Structural modification of one type of microwire through suitable current annealing and arrangement of the annealed wires in multiple combinations were sufficient to distinctly red-shift the transmission dip frequency of the composites. Such one wire control-strategy endorsed a programmable multivariable system grounded on the variations in both the overall array conductivity or “effective” area determined by the wires arrangement and the relaxation time dictated by the annealing degree of microwires. These results can be used to prescribe transmission frequency bands of desired features via diverse microwire arrays and microwave performance from a single component to composite system design.

Dielectric Effect: Dielectric Selection for Solution‐Processed High‐Mobility TIPS‐Pentacene Microwire Field‐Effect Transistors (Adv. Mater. Interfaces 13/2019)

By selecting a preferred solvent to ensure good wettability of 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-pentacene) solution on both hydrophobic and hydrophilic dielectrics, the highly ordered TIPS-pentacene microwire arrays are successfully obtained. The highest mobility of TIPS-pentacene organic field-effect transistors (OFETs) on divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB)/SiO2 is ascribed to the matching of both the polar and dispersive components of surface energy between dielectric and semiconductor. More details can be found in article number 1801984 by Yanhong Tong, Qingxin Tang, and co-workers.

Parallel, minimally-invasive implantation of ultra-flexible neural electrode arrays

Objective. Implanted microelectrodes provide a unique means to directly interface with the nervous system but have been limited by the lack of stable functionality. There is growing evidence suggesting that substantially reducing the mechanical rigidity of neural electrodes promotes tissue compatibility and improves their recording stability in both the short- and long-term. However, the miniaturized dimensions and ultraflexibility desired for mitigating tissue responses preclude the probe’s self-supported penetration into the brain tissue. Approach. Here we demonstrate the high-throughput implantation of multi-shank ultraflexible neural electrode arrays with surgical footprints as small as 200 µm2 in a mouse model. This is achieved by using arrays of tungsten microwires as shuttle devices, and bio-dissolvable adhesive polyethylene glycol (PEG) to temporarily attach a shank onto each microwire. Main results. We show the ability to simultaneously deliver electrode arrays in designed patterns, to adjust the implantation locations of the shanks by need, to target different brain structures, and to control the surgical injury by reducing the microwire diameters to cellular scale. Significance. These results provide a facile implantation method to apply ultraflexible neural probes in scalable neural recording.