Submicrometer Channels Research Articles

Fabrication of bijels with sub-micron domains via a single-channel flow device

Particle-stabilized bicontinuous emulsions (bijels) are made of two interwoven liquid channel systems. In contrast to emulsion droplets, the liquid-liquid interface of bijels curves both towards the oil and the water phases. Thus, particles with equal wettability for both oil and water are needed to stabilize the interface. In this research paper, we enhance the understanding of nanoparticle functionalization by a surfactant for controlling the particle wettability. To this end, we develop a novel, single-channel, continuous flow method, enabling the rapid synthesis and analysis of bijels formed via solvent transfer induced phase separation (STrIPS). Silica nanoparticles are functionalized with the positively charged surfactant (cetyltrimethylammonium+, CTA+). Zeta-potential and colloidal stability analysis are employed to characterize the CTA+ functionalization. Confocal and electron microscopy are used to visualize the resulting bijel structures. Bijels with nearly uniform, sub-micrometer channels are obtained when the particle functionalization by CTA+ is regulated. To this end, the initial negative zeta-potential of the particles needs to be low enough to prevent excessive CTA+ adsorption. The adsorption is further controlled by adjusting the concentrations of CTA+, salt and glycerol additive. This report shows that the nanoparticle surfactant modification depends on multiple parameters, providing guidance for future bijel synthesis approaches.

Application of an electrokinetic backflow for enhancing pressure-driven charge based separations in sub-micrometer deep channels

In this article, we report significant improvements in the resolving power of pressure-driven charge based separations performed in sub-micrometer deep glass channels upon introducing an electrokinetic backflow in the system. Such improvements are realized as axial electrophoresis aids the pressure-driven separation process in negatively charged glass conduits under these conditions. In addition, the electroosmotic backflow slows down the bulk transport of the background electrolyte subjecting the sample to the separation field for prolonged periods and yields a higher fluid shear across the channel depth further assisting the separation process. Although this increased shear also contributes to additional hydrodynamic dispersion, such contributions are usually small due to fast diffusion across the flow streamlines in sub-micrometer deep channels. In the present work, the pressure-driven flow was generated on-chip by fabricating a polyacrylamide based gel membrane within a chosen access hole upstream of the separation channel. Upon application of an electric field across this structure, the electroosmotic flow generated in the open channel interfacing the membrane was partially blocked producing the needed pressure-gradient. Optimization of the electrical voltage applied to the downstream end of the separation channel then yielded a suitable electrokinetic backflow that significantly improved the resolving power of our separations. For a sample comprising of three 5-TAMRA, SE-labeled amino acids, the noted strategy improved the separation resolution by over an order of magnitude compared to the case when no electrokinetic backflow was present. The band broadening in these separations was also assessed to understand its dependence on the operating conditions.

Self-Aligned Double Injection-Function TFT for Deep Sub-Micrometer Channels’ Length—Application to Solution-Processed Indium Gallium Zinc Oxide

We propose and demonstrate self-aligned Double Injection Function Thin Film Transistor (DIF-TFT) architecture that mitigates short channel effects in 200 nm channel on non-scaled insulator (100 nm SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ). In this conceptual design, a combination of ohmic-like injection contact and a high injection-barrier metal allows maintaining the high ON currents while suppressing drain-induced barrier lowering (DIBL) effects. Using an industrial 2-D device simulator (Sentaurus), we propose two methods to realize the DIF concept. We use one of them to demonstrate, experimentally, a DIF-TFT based on solution-processed indium gallium zinc oxide (IGZO). Using molybdenum as the ohmic contact and platinum as the high injection barrier, we compare three transistors’ source-contacts: ohmic, Schottky, and DIF. The fabricated DIF-TFT exhibits saturation at sub 1 V drain bias with only about a factor of 2 loss in ON current compared to the ohmic contact.

High-speed super-resolution imaging of rotationally symmetric structures using SPEED microscopy and 2D-to-3D transformation.

Various super-resolution imaging techniques have been developed to break the diffraction-limited resolution of light microscopy. However, it still remains challenging to obtain three-dimensional (3D) super-resolution information of structures and dynamic processes in live cells at high speed. We recently developed high-speed single-point edge-excitation sub-diffraction (SPEED) microscopy and its two-dimensional (2D)-to-3D transformation algorithm to provide an effective approach to achieving 3D sub-diffraction-limit information in subcellular structures and organelles that have rotational symmetry. In contrast to most other 3D super-resolution microscopy or 3D particle-tracking microscopy approaches, SPEED microscopy does not depend on complex optical components and can be implemented onto a standard inverted epifluorescence microscope. SPEED microscopy is specifically designed to obtain 2D spatial locations of individual immobile or moving fluorescent molecules inside sub-micrometer biological channels or cavities at high spatiotemporal resolution. After data collection, post-localization 2D-to-3D transformation is applied to obtain 3D super-resolution structural and dynamic information. The complete protocol, including cell culture and sample preparation (6-7 d), SPEED imaging (4-5 h), data analysis and validation through simulation (5-13 h), takes ~9 d to complete.

Suppressing off-state current via molecular orientation in submicrometer polymer field-effect transistors

Field-effect transistors based on conjugated polymers have reached mobilities comparable to those of polysilicon rivals and recently demonstrated self-healing capability, indicating their great potential for use in future plastic electronics. For practical applications, the operating speed of polymer transistors must be increased, which requires not only high mobility but also short channel length. Decreasing channel length is an important engineering issue for high-frequency operation and high-density integration. However, shortening channel length has proved challenging because of the poor compatibility of conjugated polymer transistors with conventional lithography; i.e., fragile polymer semiconductor films may be damaged during lithographic processes. The underlying device physics, particularly of those with short channels, remain poorly understood. Here, we fabricated polymer transistors with channel lengths below 1000 nm. A dry self-patterning process lowered the gate leakage current to less than 10 –12 A, enabling quantitative analysis of devices with very short channels. We found that a high Schottky barrier with a sizable space-charge region (>1 μm) was responsible for the high off-state current because the space-charge zones at the source and drain joined when the channel was short, lowering the energy barrier to carrier diffusion in the off state. Such diffusion was suppressed simply by controlling the molecular alignment so that the deep traps distributed at the domain boundaries limited the carrier diffusion and thus suppressed the off-state current. • Polymer transistors with submicrometer channels are fabricated by a new method. • Some of the devices suffer from high off-state current. • A quantitative analysis of the Schottky barrier is performed. • The high off current can be suppressed by appropriate molecular orientation.

Simple Polymethylglutarimide Microfluidic Channels With Hydrogel-Assisted Fluid Exchange.

We present an experimental protocol for fabricating enclosed microfluidic channels using polymethylglutarimide (PMGI). PMGI is optically transparent, biocompatible, and can be used to readily fabricate micrometer-scale lateral and vertical dimension channels using conventional photolithography. The low auto-fluorescence intensity of PMGI facilitates imaging of analytes without interference. The hydrophilicity of PMGI allows fluid exchange in micrometer-scale channels using a hydrogel as an interface without an external pump. As a demonstration, we assemble fluorescently-labeled lipid bilayers in PMGI microfluidic channels and show that PMGI has negligible auto-fluorescence intensity compared to the lipid bilayer. PMGI channels together with hydrogel-assisted fluidic exchange provides a simple approach to fabricate micrometer and sub-micrometer scale fluidic channels for optofluidics, molecular biology, and other medical diagnostic and sensing applications.

High Aspect Ratio Sub-Micrometer Channels Using Wet Etching: Application to the Dynamics of Red Blood Cell Transiting through Biomimetic Splenic Slits.

Nanoparticles delivering drugs, disseminating cancer cells, and red blood cells (RBCs) during splenic filtration must deform and pass through the sub-micrometer and high aspect ratio interstices between the endothelial cells lining blood vessels. The dynamics of passage of particles/cells through these slit-like interstices remain poorly understood because the in vitro reproduction of slits with physiological dimensions in devices compatible with optical microscopy observations requires expensive technologies. Here, novel microfluidic PDMS devices containing high aspect ratio slits with sub-micrometer width are molded on silicon masters using a simple, inexpensive, and highly flexible method combining standard UV lithography and anisotropic wet etching. These devices enabled revealing novel modes of deformations of healthy and diseased RBCs squeezing through splenic-like slits (0.6-2 × 5-10 × 1.6-11 µm3 ) under physiological interstitial pressures. At the slit exit, the cytoskeleton of spherocytic RBCs seemed to be detached from the lipid membrane whereas RBCs from healthy donors and patients with sickle cell disease exhibited peculiar tips at their front. These tips disappeared much slower in patients' cells, allowing estimating a threefold increase in RBC cytoplasmic viscosity in sickle cell disease. Measurements of time and rate of RBC sequestration in the slits allowed quantifying the massive trapping of spherocytic RBCs.

High efficiency hydrodynamic chromatography in micro- and sub-micrometer deep channels using an on-chip pressure-generation unit

In this article, we report the development of a microchip based hydrodynamic chromatographic device that has an integrated pressure-generation unit to drive the mobile phase during the separation process. The pressure-gradient in this system was realized through introduction of a mismatch in electroosmotic flow rate at the junction of two glass channel segments with different depths as has been previously reported by our research group. A fraction of the resulting pressure-driven flow was then guided into an analysis channel to enable the size-based particle separations. The reported device allowed the realization of hydrodynamic chromatography in both micro- and sub-micrometer deep analysis channels as experiments showed the noted approach to yield pressure-driven velocities in our system that were nearly unaffected upon scaling down the depth of the entire fluidic network. Separations with plate numbers in the range of 500–2000 plates/mm were realized in a 3 cm long analysis column for a mixture of 25, 100 and 250 nm diameter polystyrene beads with analysis times as short as 12 s. The noted separation efficiency exceeds those previously reported in the literature for hydrodynamic chromatographic analysis performed on microchips likely due to a faster mass transfer across the channel cross-section as well as narrower band injections in our assays.

High-resolution direct-writing of metallic electrodes on flexible substrates for high performance organic field effect transistors

We report on organic field-effect transistors (OFETs) with sub-micrometer channels fabricated on plastic substrates with fully direct-written electrical contacts. In order to pattern source and drain electrodes with high resolution and reliability, we adopted a combination of two digital, direct writing techniques: ink-jet printing and femtosecond laser ablation. First silver lines are deposited by inkjet printing and sintered at low temperature and then sub-micrometer channels are produced by highly selective femtosecond laser ablation, strongly improving the lateral patterning resolution achievable with inkjet printing only. These direct-written electrodes are adopted in top gate OFETs, based on high-mobility holes and electrons transporting semiconductors, with field-effect mobilities up to 0.2cm2/Vs. Arrays of tens of devices have been fabricated with high process yield and good uniformity, demonstrating the robustness of the proposed direct-writing approach for the patterning of downscaled electrodes for high performance OFETs, compatibly with cost-effective manufacturing of large-area circuits.

Downscaling of n-channel organic field-effect transistors with inkjet-printed electrodes

In this contribution we demonstrate for the first time a downscaled n-channel organic field-effect transistors based on N,N′-dialkylsubstituted-(1,7&1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide) with inkjet printed electrodes. First we demonstrate that the use of a high boiling point solvent is critical to achieve extended crystalline domains in spin-coated thin films and thus high electron mobility >0.1 cm 2 V −1 s −1 in top-gate devices. Then inkjet-printing is employed to realize sub-micrometer scale channels by dewetting of silver nanoparticles off a first patterned gold contact. By employing a 50 nm crosslinked fluoropolymer gate dielectric, ∼200 nm long channel transistors can achieve good current saturation when operated <5 V with good bias stress stability.

Growth of Titanium Dioxide Nanorods in 3D-Confined Spaces

Three-dimensional (3D) nanowire (NW) networks are promising architectures for effectively translating the extraordinary properties of one-dimensional objects into a 3D space. However, to uniformly grow NWs in a 3D confined space is a serious challenge due to the coupling between crystal growth and precursor concentration that is often dictated by the mass flow characteristic of vapor or liquid phase reactants within the high-aspect ratio submicrometer channels in current strategies. We report a pulsed chemical vapor deposition (CVD) process that successfully addressed this issue and grew TiO(2) nanorods uniformly covering the entire inner surface of highly confined nanochannels. We propose a mechanism for the anisotropic growth of anatase TiO(2) based on the surface-reaction-limited CVD process. This strategy would lead to the realization of NW-based 3D nanoarchitectures from various functional materials for the applications of sensors, solar cells, catalysts, energy storage systems, and so forth.

D-2-1 ナノインプリントリソグラフィとUVリソグラフィの融合によるバイオアッセイ用サブマイクロ灌流デバイスの作製(D-2 設計・プロセス,口頭発表:マイクロナノメカトロニクス)

D-2-1 ナノインプリントリソグラフィとUVリソグラフィの融合によるバイオアッセイ用サブマイクロ灌流デバイスの作製(D-2 設計・プロセス,口頭発表:マイクロナノメカトロニクス)

High-power and high-speed organic three-dimensional transistors with submicrometer channels

Three-dimensional organic field-effect transistors with high current density and high switching speed are developed with multiple submicrometer channels arranged perpendicularly to substrates. The short channel length is defined by the height of a multicolumnar structure without an electron-beam-lithography process. For devices using dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene, extremely high current density exceeding 10 A/cm2 and fast switching within 0.2 μs are realized with an on-off ratio of 105. The unprecedented performance is beyond general requirements to control organic light-emitting diodes, so that even more extensive applications to higher-speed active-matrices and display-driving circuits can be realized with organic semiconductors.

High-power Organic Field-effect Transistors Using a Three-dimensional Structure

AbstractThree-dimensional organic field-effect transistors with multiple sub-micrometer channels are developed to exhibit high current density and high switching speed. The sub-micrometer channels are arranged perpendicularly to substrates and are defined by the height of a multi-columnar structure fabricated without using electron-beam-lithography technique. For devices with dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene, extremely high current density exceeding 10 A/cm2 and fast switching within 200 ns are realized with an on-off ratio of 105. The unprecedented performance is beyond general requirements to control organic light-emitting diodes, so that even more extensive applications to higher-speed active-matrices and display-driving circuits can be realized with organic semiconductors.

Modeling of the Gas Dynamic Processes During the Ddeposition of Nanolayers on the Surface of Submicrometer Channels of Porous Solids

Numerical investigation of the gas dynamic processes that occur during film deposition by means of CVD on the surface of the submicron channels of cylindrical shape was carried out. For this purpose, a combination of Monte-Carlo procedure and molecular dynamics was applied. Molecules were simulated with solid spheres with a given gas kinetic diameter. The interaction between molecules was described using Lennard-Jones potential, while the interaction between the molecules and the walls was described with the repulsive branch of Lennard-Jones potential. Adsorption and energy exchange with the surface of channels were described with the help of Monte-Carlo procedure. A hit of a molecule into a channel through the inlet section was described with Monte-Carlo procedure, too. This allowed us to investigate the effect of pressure and temperature of the gas phase, and surface temperature, as well as the effect of the geometric parameters of the inlet sections on the distribution of the thickness of the formed film along the channel axis.

Modeling of the Gas Dynamic Processes During the Deposition of Nanolayers on the Surface of Submicrometer Channels of Porous Solids

not Available.

High-mobility organic single crystal transistors with submicrometer channels

We demonstrate high-performance electric-field effects in submicrometer-channel organic transistors with rubrene single crystals. Platinum source and drain electrodes are embedded in silicon dioxide gate insulators to reduce thickness of the dielectrics and to minimize the short-channel effect. The miniaturized devices exhibit typical output characteristics with Ohmic linear region, well-defined current saturation, and on-off ratio of 106. Mobility values are in the range of 0.1–0.3cm2∕Vs, which is comparable to those of the best submicrometer organic transistors. Anisotropy in the mobility is detected, indicating that bandlike transport is responsible for the high transistor performance of the short-channel devices.

Parametrical studies of electroosmotic transport characteristics in submicrometer channels

Spatially two-dimensional nonequilibrium mathematical model describing electroosmotic flow through a submicrometer channel with an electric charge fixed on the channel walls is presented. This system is governed by the hydrodynamic, electrostatic, and mass transport phenomena. The model is based on the coupled mass balances, Poisson, Navier–Stokes, and Nernst–Planck equations. Nonslip boundary conditions are employed. The effect of an imposed electric field on the system behavior is studied by means of a numerical analysis of the model equations. We have obtained the following findings. If the channel width is comparable to the thickness of the electric double layer, the system behaves as an ion-exchange membrane and the dependence of the electric current passing through the channel on the applied voltage is strongly nonlinear. In the case of negatively (positively) charged walls, a narrow region of very low conductivity (so-called ionic gate) is formed in the free electrolyte near the channel entry facing the anode (cathode) side. For a wide channel, the electric current is proportional to the applied voltage and the velocity of electrokinetic flow is linearly proportional to the electric field strength. Complex hydrodynamics (eddy formation and existence of ionic gates) is the most interesting characteristics of the studied system. Hence, current–voltage and velocity–voltage curves and the corresponding spatial distributions of the model variables at selected points are studied and described in detail.

Use of a Self-Assembling Organogel as a Reverse Template in the Preparation of Imprinted Porous Polymer Films

The concept of reverse templating of an organogel to form imprinted porous divinylbenzene polymer films with submicrometer channels is demonstrated. The organogel comprising a 1:1 molar ratio of two organogelators, that is, bis(2-ethylhexyl) sodium sulfosuccinate and 4-chlorophenol, was formed in divinylbenzene. The gel was cast as a thin film before UV polymerization of the solvent, and the organogelators were later removed by simple washing with water and isooctane. The integrity of the fiber bundles of the organogel was preserved during polymerization, and an exact hollow replica was obtained after the organogelators were leached away. It is easily possible to imprint gel fiber bundle structures into polymeric films through this technique. The gel can also be formed on macroporous substrates to yield supported thin porous polymeric films. With the incorporation of functional nanoparticles in AOT inverse micelles and hence the organogel, nanoparticle-containing porous polymer films exhibiting luminescence or magnetic properties are envisioned.

Crystallization of lysozyme in pores of etched heavy-ion tracks

Intensive studies of protein crystallization are provoked by the need to accumulate structural information and to open up novel potential applications in science and technology. This work focuses on the crystallization of hen egg white lysozyme (HEWL) in micrometer-size templates of etched swift-ion tracks produced in foils of polycarbonate, polyimide, and mica. HEWL deposition was accomplished by the salting with sodium chloride method at 20 and 6 °C. The crystals overgrowing the pore orifices showed no orientation when crystallized at room temperature. At 6 °C, they presented a regular habit and were uniaxially oriented on a polycarbonate substrate. Biaxial orientation was observed on polyimide membranes, and epitaxial orientation dominated on mica. However, no crystallization was observed on the mica membrane covered with a thin gold layer. The epitaxial growth of HEWL was particularly pronounced in the case of ordered substrates and low crystallization temperature, i.e. at higher supersaturation. By computer simulation it was possible to determine the surface lattice planes and thus provide the preferential crystal-growth orientation. Crystallization of lysozyme and other proteins in ion-track templates of submicrometer channels should be of high practical interest.