Dielectrophoretic Alignment Research Articles

Impact of Static Electric Field on Dielectrophoretic Alignment of Silicon Nanowires

AbstractAmong numerous approaches to assembling nanowires onto electrodes, dielectrophoresis (DEP) is a potential candidate to place the nanowires. However, its yield is still far from perfection, urging fundamental understanding of its dynamics. Here, the impact of a static electric field on dielectrophoretic nanowire assembly on gold electrodes is investigated. Specifically, a 4 peak‐to‐peak alternating voltage with 700 Hz is applied and modulate the offset voltage from 0 to 2V. The highest yield in the alignment of the nanowires at 0.5 V offset voltage is found. With the optical investigation of misaligned nanowires, it is found that rotating wires on top of electrodes, and the analysis of their angular velocity suggest the impact of the induced static charges. The numerical analysis quantifies the length scale of competing two forces, dielectrophoretic force and electric double layer force. This work suggests a quantitative understanding of the interplay between dielectrophoresis and electric double layer, which contributes to the advances in scalable nanowire fabrications.

Optimization of Surfactant Concentration in Carbon Nanotube Solutions for Dielectrophoretic Ceiling Assembly and Alignment: Implications for Transparent Electronics.

Surfactants such as sodium dodecyl sulfate (SDS) are used to improve the dispersity of carbon nanotubes (CNTs) in aqueous solutions. The surfactant concentration in CNT solutions is a critical factor in the dielectrophoretic (DEP) manipulation of CNTs. A high surfactant concentration causes a rapid increase in the solution conductivity, while a low concentration results in undesirably large CNT bundles within the solution. The increase in the solution conductivity causes drag velocity that obstructs the CNT manipulation process due to the electrothermal forces induced by the electric field. The presence of large CNT bundles is undesirable since they degrade the device performance. In this work, mathematical modeling and experimental work were used to optimize the concentration of the SDS surfactant in multiwalled carbon nanotube (MWCNT) solutions. The solutions were characterized using dynamic light scattering (DLS) and ultraviolet–visible spectroscopy (UV–Vis) analysis. We found that the optimum SDS concentration in MWCNT solutions for the successful DEP manipulation of MWCNTs was between 0.1 and 0.01 wt %. A novel DEP configuration was then used to assemble MWCNTs across transparent electrodes. The configuration was based on ceiling deposition, where the electrodes were on top of a droplet. The newly proposed configuration reduced the drag velocity and prevented the assembly of large MWCNT bundles. MWCNTs were successfully assembled and aligned across interdigitated electrodes (IDEs). The assembly of MWCNTs from aqueous solutions across transparent electrodes has potential use in future transparent electronics and sensor devices.

Increase of one-to-one particle encapsulation yield using dielectrophoretic alignment technique with boxcar-type electrodes

Increase of one-to-one particle encapsulation yield using dielectrophoretic alignment technique with boxcar-type electrodes

Dielectrophoretic alignment and electrical characterization of CuO nanowire-based systems

Dielectrophoresis is used to assemble nanowires between metallic electrodes to form scalable functional interconnects. The dielectrophoresis parameters are investigated for semiconductor copper oxide (CuO) nanowires that are desirable for energy conversion and storage, gas sensors and nanoelectromechanical systems. Experimental yields of multiple- and single-nanowire interconnects are explored at dielectrophoresis frequencies from 500 Hz to 500 kHz. The electrical properties of nanowire-electrode physical contact interfaces formed by dielectrophoresis, metal deposition, and dry mechanical transfer are investigated. The electrical transport mechanism in these interconnects is determined to be ohmic conductivity at small electric field, which is overtaken by an order of magnitude higher space charge limited conductivity for electric fields above 105 V/m. The developed method is demonstrated as a promising route to produce nanowire interconnects towards scalable on-chip nanoelectromechanical systems.

Dielectrophoretic alignment of Al2O3/PDMS composites: Enhancement of thermal and dielectric properties through structural sedimentation analysis

Thermally conductive composites have emerged as a promising lead towards a more suitable and efficient alternative for thermal management. Their advantage lies in the coupling of different but complementary properties such as thermal conductivity for heat dissipation and flexibility for fitting surface irregularities. However, such materials consisting of ceramic fillers dispersed in a polymeric matrix generally suffer from particle sedimentation that ultimately affects their performance. Therefore, the overall properties of these composites can be further enhanced through structural improvements. Dielectrophoretic alignment has proven to be an effective and practical method to reach better structures that eventually improve the composites’ properties. This paper focuses on the enhancement of thermal and dielectric properties of Al2O3/PDMS composites through dielectrophoretic alignment. A novel method for measuring sedimentation effects is presented with a particular emphasis on the role of dielectrophoresis in counteracting those undesired effects. The influence of the relevant parameters such as the amplitude and frequency of the applied electric field is analyzed in order to draw the best structuring conditions. The thermal conductivity as well as the dielectric response are then compared between structured and unstructured composites. This comparison revealed a significant structural improvement that led to enhanced thermal conductivities while keeping the insulating properties unchanged.

Pd-decorated CNT as sensitive material for applications in hydrogen isotopes sensing - Application as gas sensor

Applicability of multiwall carbon nanotubes (MWCNTs) decorated with palladium nanoparticles as sensitive layer in a resistive microsensor for identification of hydrogen isotopes, Deuterium (2H) and Protium (1H), has been demonstrated. Palladium nanoparticles were anchored on the MWCNTs surface via a chemical process involving micellization, from a precursor chloride solution, in high ultrasonic density field. Pd-MWCNTs are quasi-aligned between the interdigitated gold electrodes of a SiO2 substrate by drop casting and di-electrophoretic alignment in Tetrahydrofuran (THF) and Nafion solution. The morphostructural characterization of the sensitive material has been carried out through SEM, TEM and Raman spectroscopy and its gas sensing properties were evaluated using electrical measurements performed on a series of isotope concentrations (ranging from 0.1% up to 1%, and from 1% to 4%, value to which hydrogen becomes explosive) diluted in argon, to observe the evolution of the sensor sensibility. The two hydrogen isotopes have different behaviors related to the adsorption on the Pd-MWCNT, which is well observed in the resistance change. Therefore, the sensor based on Pd-MWCNTs could be a viable solution to be integrated in systems for hydrogen leakage detection.

Fabrication and Characterization of Double- and Single-Clamped CuO Nanowire Based Nanoelectromechanical Switches.

Electrostatically actuated nanoelectromechanical (NEM) switches hold promise for operation with sharply defined ON/OFF states, high ON/OFF current ratio, low OFF state power consumption, and a compact design. The present challenge for the development of nanoelectromechanical system (NEMS) technology is fabrication of single nanowire based NEM switches. In this work, we demonstrate the first application of CuO nanowires as NEM switch active elements. We develop bottom-up and top-down approaches for NEM switch fabrication, such as CuO nanowire synthesis, lithography, etching, dielectrophoretic alignment of nanowires on electrodes, and nanomanipulations for building devices that are suitable for scalable production. Theoretical modelling finds the device geometry that is necessary for volatile switching. The modelling results are validated by constructing gateless double-clamped and single-clamped devices on-chip that show robust and repeatable switching. The proposed design and fabrication route enable the scalable integration of bottom-up synthesized nanowires in NEMS.

Piezoelectric sensors fabricated by depositing solution-grown ZnO nanorods on flexible graphene-derivative electrodes

Zinc oxide nanorods (ZnO-NRs) with high-aspect ratios can significantly enhance the voltage output of mechanically flexible piezoelectric materials. A versatile chemical synthesis process for growing long narrow ZnO-NR from nanoparticle (NP) seeds by regulating the polarity of reaction solvents is introduced in this paper. The efficient nanorod (NR) growth method produces large quantities of high-aspect ratio ZnO-NRs in the reaction solvent. For ultra-small NP seeds (AVG 10.54 nm, SD 3.69), the synthesis process creates NRs with a minimal lateral growth (AVG 13.92 nm, SD 4.77) and significant longitudinal growth (AVG 150.85 nm, SD 64.93). The average aspect ratio of ZnO-NRs in the solution is ∼10.8 (SD 2.48). Once synthesized, the ZnO-NRs are mixed with polydimethylsiloxane (PDMS) to create a thin flexible piezoelectric layer/film. The composite polymer material is spin coated on an inkjet printed graphene/carboxymethyl cellulose (G-CMC) interdigitated electrode (IDE) to form the piezoelectric layer. A dielectrophoretic alignment technique is then used to reposition the NR orientations in the composite prior to final polymer curing. In this study, three different piezoelectric composites are investigated and compared: polyhedral NPs (ZnO-NP/PDMS), non-aligned nanorods (ZnO-NRNA/PDMS), and aligned nanorods (ZnO-NRA/PDMS). Each composite is deposited on a similar IDE and tested for impact loading and low frequency mechanical bending. Under bending, the NP ZnO-NP/PDMS sensor generated 3–4 mV while the non-aligned NR ZnO-NRNA/PDMS sensor produced 70–80 mV. In contrast, the horizontally aligned NR ZnO-NRA/PDMS sensor generated 150–170 mV under the same bending conditions.

Enhancing dielectric and piezoelectric properties of micro-ZnO/PDMS composite-based dielectrophoresis

This work aims to enhance the dielectric and piezoelectric properties of polydimethylsiloxane (PDMS) polymer filled with zinc oxide (ZnO) powders. Both ZnO nanoparticles and microparticles are used to prepare piezoelectric composites and a comparison of their dielectric responses is carried out. The experimental results reveal that a higher particle concentration gives rise to the increased dielectric permittivity of composites, especially when particles are aligned in the poling-direction-based dielectrophoretic process. Hence, the microparticles are chosen instead of the nano ones because the microscale facilitates the fabrication process of the composite, especially with high filler content. It also makes it possible to clearly record the movement of particles under alternating (AC) voltage application using optical microscopy. Significant improvement in the dielectric and piezoelectric behavior of the proposed composite has been successfully achieved via dielectrophoretic alignment of ZnO microparticles. This technique leads to increased connectivity between ZnO/ZnO interfaces, allowing for the creation of continuous aligned piezoelectric particles inside the polymer matrix. As a result, the piezoelectric effect is considerably boosted. Finally, the dielectric constant, as well as the charge coefficient of the ZnO particles, is estimated through theoretical approaches for composites containing particles arranged in chains or randomly dispersed. The model predictions are in good agreement with the experimental results. Furthermore, a finite element model (FEM) is developed using Comsol Multiphysics to evaluate these parameters in a 3D structure, which is then compared to those obtained by the above 2D-analytic models.

Simulation of dielectrophoretic alignment for carbon nanotube on indium tin oxide

Electrode-based dielectrophoresis (DEP) method is used to deposit carbon nanotubes (CNTs) in an organized form. The density and quality of assembled CNTs depend on electrodes geometry and applied electric fields. In this context, we simulated the electric field distribution of dielectrophoresis for CNT suspension on Indium Tin Oxide (ITO) electrodes. The influence of electrode shapes and dimensions were analyzed and discussed by using the COMSOL software. The system quality and alignment angle were also presented at different locations. Rectangular electrodes with DEP parameters of 105 Hz and 20 V were chosen to achieve high-density deposition with approximately 2.5o alignment angle at the electrode center.

Fabrication of Dielectrically Sensitive Carbon Nanotube/Polyethylene Nanohybrid Shish-Kebab Structure for Potential Tissue Engineering Applications

Carbon nanotube (CNT)/polymer nanohybrid shish-kebab (NHSK) structure is a unique approach to functionalize CNTs and mimic the hierarchical architecture of native collagen fibrils in the extracellular matrix (ECM), owing to their favorable geometric similarity. This study thus aimed at combining the unique NHSK architecture of CNTs with their favorable dielectric properties to fabricate a collagen-like, dielectrically sensitive and biocompatible material. The NHSK structure was fabricated with CNTs and polyethylene (PE) by means of a solution crystallization technique. Configuration of PE in terms of molecular weight and structure was found to be one of the important factors that determined the morphology of the NHSK. Moreover, the material was exposed to dielectrophoresis (DEP) to guide the deposition of CNT/PE NHSK into the directional pattern. The PE-decorated CNTs exhibited enhanced dielectrophoretic sensitivity and controllable structural alignment, while the neat CNTs did not. Human osteoblast-like MG-63 cells were cultured on the scaffolds to examine their biocompatibility and results indicated that the nanotopography of the kebab crystals on the CNTs facilitated cell adhesion and proliferation while the DEP-patterned NHSK structure further boosted cell attachment and viability, thus suggesting that the CNT/polymer nanohybrid shish kebab structure has potential applications in tissue engineering.

Flexible Piezoelectric Touch Sensor by Alignment of Lead‐Free Alkaline Niobate Microcubes in PDMS

A highly sensitive, lead‐free, and flexible piezoelectric touch sensor is reported based on composite films of alkaline niobate K0.485Na0.485Li0.03NbO3 (KNLN) powders aligned in a polydimethylsiloxane (PDMS) matrix. KNLN powder is fabricated by solid‐state sintering and consists of microcubes. The particles are dispersed in uncured PDMS and oriented by application of an oscillating dielectrophoretic alignment field. The dielectric constant of the composite film is almost independent of the microstructure, while upon alignment the piezoelectric charge coefficient increases more than tenfold up to 17 pC N−1. A quantitative analysis shows that the origin is a reduction of the interparticle distance to under 1.0 µm in the aligned bicontinuous KNLN chains. The temperature stable piezoelectric voltage coefficient exhibits a maximum value of 220 mV m N−1, at a volume fraction of only 10%. This state‐of‐the‐art value outperforms bulk piezoelectric ceramics and composites with randomly dispersed particles, and is comparable to the values reported for the piezoelectric polymers polyvinylidenefluoride and its random copolymer with trifluoroethylene. Optimized composite films are incorporated in flexible piezoelectric touch sensors. The high sensitivity is analyzed and discussed. As the fabrication technology is straightforward and easy to implement, applications are foreseen in flexible electronics such as wireless sensor networks and biodiagnostics.

In-situ poling and structurization of piezoelectric particulate composites.

Composites of lead zirconate titanate particles in an epoxy matrix are prepared in the form of 0–3 and quasi 1–3 with different ceramic volume contents from 10% to 50%. Two different processing routes are tested. Firstly a conventional dielectrophoretic structuring is used to induce a chain-like particle configuration, followed by curing the matrix and poling at a high temperature and under a high voltage. Secondly a simultaneous combination of dielectrophoresis and poling is applied at room temperature while the polymer is in the liquid state followed by subsequent curing. This new processing route is practiced in an uncured thermoset system while the polymer matrix still possess a relatively high electrical conductivity. Composites with different degrees of alignment are produced by altering the magnitude of the applied electric field. A significant improvement in piezoelectric properties of quasi 1–3 composites can be achieved by a combination of dielectrophoretic alignment of the ceramic particles and poling process. It has been observed that the degree of structuring as well as the functional properties of the in-situ structured and poled composites enhance significantly compared to those of the conventionally manufactured structured composites. Improving the alignment quality enhances the piezoelectric properties of the particulate composites.

Dielectrophoretic Alignment of Functional Nanowires for Chemical Sensing Application

Dielectrophoresis has received much attention owing to its capability to manipulate the polarizable nanowires at the specific locations on a substrate. Bottom-up integration of batch-synthesized nanowires with CMOS circuitry represents significant step toward the multifunctional miniaturized chip with the chemical diversity. We investigate the spatially-confined dielectrophoretic nanowires alignment on the parallel electrode array, covered by the dielectric layer, where they tend to repel each other because of the local electrostatic interaction of the polarized charges between them. Furthermore, the local capacitive coupling between the assembled nanowires and biased electrodes determines the final configuration of the nanowires relative to the electrodes. By using this combination of long- and short-range forces, it was observed that the uniformly-spaced nanowire array can be readily obtained over large area (~5×5 mm2) and even expandable to the entire wafer. Theoretical calculation and experimental data show that we can achieve the spacing-controlled nanowire array at the desired locations with the excellent end-to-end registration. Its combination with the periodic array of micron-scale recessed photoresist well produced the ultrahigh density nanowire device array, ~106 devices/cm2, on a substrate with sub-micron registration accuracy and high assembly yield exceeding ~90%. Post-assembly process including the electrodeposition provides a mechanically robust and low-resistant contact to the assembled nanowires while it also removes the misaligned nanowires. Device isolation using a dry etch completes the individual single nanowire device made of the metallic and semiconducting materials. They turned out to be all functional, achieving ~90% yield of single nanowire device fabrication on the desired sites, sparsely distributed within a chip. This scheme is also applicable to the integration of individual semiconducting nanowires with the desired locations on the complex circuitry which typically includes the metal interconnects, discrete devices, and topographical profiles.

Online optical and dielectric monitoring of anisotropic epoxy/BaTiO3 composite formation tailored by dielectrophoresis

Anisotropic composites can be obtained by applying an alternating (ac) electric field, forming particle chains in its direction. The control of the particle chains will directly impact the final properties of the composite. Nevertheless, up to now the monitoring of the particle chain formation has only been made by direct optical or post-curing observations. A new technique for the monitoring of the particle dielectrophoretic alignment is proposed, based on the online measurement of the dielectric permittivity. Epoxy/barium titanate (BaTiO3) composites, in the range of 0.25 vol% to 20 vol% of BaTiO3 microparticles, are cured while an ac field (600 Vrms mm−1) is applied. The ac current magnitude and the phase shift angle are measured to determine the dielectric properties of the composite. The same experiment is achieved under optical microscope observation for 0.25 vol% to correlate the changes of the composite dielectric properties to the particle chain formation. As a result, the permittivity variations can be correlated to the particle chains formation and to their growth.

NH3 sensing with self-assembled ZnO-nanowire μHP sensors in isothermal and temperature-pulsed mode

Dielectrophoretic alignment is found to be a simple and efficient method to deposit the solution prepared ZnO nanowires onto micro hot plate substrates. Due to the strong surface effects, positive temperature coefficient for resistance was encountered with ZnO nanowires in the high temperature range (>250°C). The response to ammonia (NH3) was evaluated in isothermal and temperature-pulsed operation mode; the relative higher response observed in the latter case demonstrates that the use of this methodology is a good strategy to improve the performance of metal oxide sensors based on nanomaterials. Here, we evaluate the response to NH3 and qualitatively describe the sensing mechanism in temperature-pulsed mode, highlighting the main differences compared to the standard isothermal methodology.

Direct Transfer Printing with Metal Oxide Layers for Fabricating Flexible Nanowire Devices

A direct printing method for fabricating devices by using metal oxide transfer layers instead of conventional transfer media such as polydimethylsiloxane is presented. Metal oxides are not damaged by organic solvents; therefore, electrodes with gaps less than 2 μm can be defined on a metal oxide transfer layer through photolithography. In order to determine a suitable metal oxide for use as transfer layer, the surface energies of various metal oxides are measured, and Au layers deposited on these oxides are transferred onto polyvinylphenol (PVP). To verify the feasibility of our approach, Au source–drain electrodes on transfer layers and Si nanowires (NWs) addressed by the dielectrophoretic (DEP) alignment process are transferred onto rigid and flexible PVP‐coated substrates. Based on transfer test and DEP process, Al2O3 is determined to be the best transfer layer. Finally, Si NWs field effect transistors (FETs) are fabricated on a rigid Si substrate and a flexible polyimide film. As the channel length decreases from 3.442 to 1.767 μm, the mobility of FET on the Si substrate increases from 127.61 ± 37.64 to 181.60 ± 23.73 cm2 V−1 s−1. Furthermore, the flexible Si NWs FETs fabricated through this process show enhanced electrical properties with an increasing number of bending cycles.

Continuous-flow system and monitoring tools for the dielectrophoretic integration of nanowires in light sensor arrays

Although nanowires (NWs) may improve the performance of many optoelectronic devices such as light emitters and photodetectors, the mass commercialization of these devices is limited by the difficult task of finding reliable and reproducible methods to integrate the NWs on foreign substrates. This work shows the fabrication of zinc oxide NWs photodetectors on conventional glass using transparent conductive electrodes to effectively integrate the NWs at specific locations by dielectrophoresis (DEP). The paper describes the careful preparation of NW dispersions by sedimentation and the dielectrophoretic alignment of NWs in a home-made system. This system includes an impedance technique for the assessment of the alignment quality in real time. Following this procedure, ultraviolet photodetectors based on the electrical contacts formed by the DEP process on the transparent electrodes are fabricated. This cost-effective mean of contacting NWs enables front-and back-illumination operation modes, the latter eliminating shadowing effects caused by the deposition of metals. The electro-optical characterization of the devices shows uniform responsivities in the order of 106 A W−1 below 390 nm under both modes, as well as, time responses of a few seconds.

Parallel integration of aligned carbon strings in polymer composite: Dielectrophoretic preparation, finite element simulation, and electrical characterization

We report on dielectrophoretic alignment of carbon black particles into radially arranged string‐like assemblies in oligourethane mixtures followed by photopolymerization. Using finite element modelling and optical microscopy we find significant difference in field distributions depending on the substrate beneath aligned films. On glass, the field is concentrated between the electrode tips leading to string‐like assemblies between the tips. On oxide covered silicon, the field is distributed along the electrode circumference leading to more distributed fractal assemblies. Using comprehensive dc resistance measurements and ac‐impedance spectroscopy we show that the resistance of the strings varies from 120 kΩ to 5 MΩ with a mean of 1.03 MΩ. Strings on oxide covered silicon show significantly higher resistances than the strings on glass. We also demonstrate the effect of aging and an increase in resistance through elongating aligned strings. POLYM. COMPOS., 36:1866–1874, 2015. © 2014 Society of Plastics Engineers

Spontaneous Assembly of Carbon-Based Chains in Polymer Matrixes through Surface Charge Templates

Stable chains of carbon-based nanoparticles were formed directly in polymer matrixes through an electrode-free approach. Spontaneous surface charges were generated pyroelectrically onto functionalized ferroelectric crystals, enabling the formation of electric field gradients that triggered the dipole-dipole interactions responsible for the alignment of the particles, while embedded in the polymer solution. The phenomenon is similar to the dielectrophoretic alignment of carbon nanotubes reported in the literature. However, here the electric fields are generated spontaneously by a simple heat treatment that, simultaneously, aligns the particles and provides the energy necessary for curing the host polymer. The result is a polymer sheet reinforced with well-aligned chains of carbon-based particles, avoiding the invasive implementation of appropriate electrodes and circuits. Because polymers with anisotropic features are of great interest for enhancing the thermal and/or the electrical conductivity, the electrode-free nature of this technique would improve the scaling down and the versatility of those interconnections that find applications in many fields, such as electronics, sensors, and biomedicine. Theoretical simulations of the interactions between the particles and the charge templates were implemented and appear in good agreement with the experimental results. The chain formation was characterized by controlling different parameters, including surface charge configuration, particle concentration, and polymer viscosity, thus demonstrating the reliability of the technique. Moreover, micro-Raman spectroscopy and scanning electron microscopy were used for a thorough inspection of the assembled chains.