Taehoon Kim

Bioinspired, Highly Stretchable, and Conductive Dry Adhesives Based on 1D-2D Hybrid Carbon Nanocomposites for All-in-One ECG Electrodes.

Here we propose a concept of conductive dry adhesives (CDA) combining a gecko-inspired hierarchical structure and an elastomeric carbon nanocomposite. To complement the poor electrical percolation of 1D carbon nanotube (CNT) networks in an elastomeric matrix at a low filler content (∼1 wt %), a higher dimensional carbon material (i.e., carbon black, nanographite, and graphene nanopowder) is added into the mixture as an aid filler. The co-doped graphene and CNT in the composite show the lowest volume resistance (∼100 ohm·cm) at an optimized filler ratio (1:9, total filler content: 1 wt %) through a synergetic effect in electrical percolation. With an optimized conductive elastomer, gecko-inspired high-aspect-ratio (>3) microstructures over a large area (∼4 in.(2)) are successfully replicated from intaglio-patterned molds without collapse. The resultant CDA pad shows a high normal adhesion force (∼1.3 N/cm(2)) even on rough human skin and an excellent cycling property for repeatable use over 30 times without degradation of adhesion force, which cannot be achieved by commercial wet adhesives. The body-attachable CDA can be used as a metal-free, all-in-one component for measuring biosignals under daily activity conditions (i.e., underwater, movements) because of its superior conformality and water-repellent characteristic.

DNA-decorated carbon-nanotube-based chemical sensors on complementary metal oxide semiconductor circuitry.

We present integration of single-stranded DNA (ss-DNA)-decorated single-walled carbon nanotubes (SWNTs) onto complementary metal oxide semiconductor (CMOS) circuitry as nanoscale chemical sensors. SWNTs were assembled onto CMOS circuitry via a low voltage dielectrophoretic (DEP) process. Besides, bare SWNTs are reported to be sensitive to various chemicals, and functionalization of SWNTs with biomolecular complexes further enhances the sensing specificity and sensitivity. After decorating ss-DNA on SWNTs, we have found that the sensing response of the gas sensor was enhanced (up to approximately 300% and approximately 250% for methanol vapor and isopropanol alcohol vapor, respectively) compared with bare SWNTs. The SWNTs coupled with ss-DNA and their integration on CMOS circuitry demonstrates a step towards realizing ultra-sensitive electronic nose applications.

Topological transitions in carbon nanotube networks via nanoscale confinement.

Efforts aimed at large-scale integration of nanoelectronic devices that exploit the superior electronic and mechanical properties of single-walled carbon nanotubes (SWCNTs) remain limited by the difficulties associated with manipulation and packaging of individual SWNTs. Alternative approaches based on ultrathin carbon nanotube networks (CNNs) have enjoyed success of late with the realization of several scalable device applications. However, precise control over the network electronic transport is challenging due to (i) an often uncontrollable interplay between network coverage and its detailed topology and (ii) the inherent electrical heterogeneity of the constituent SWNTs. In this article, we use template-assisted fluidic assembly of SWCNT networks to explore the effect of geometric confinement on the network topology. Heterogeneous SWCNT networks dip-coated onto submicrometer wide ultrathin polymer channels become increasingly aligned with decreasing channel width and thickness. Experimental-scale coarse-grained computations of interacting SWCNTs show that the effect is a reflection of a topology that is no longer dependent on the network density, which in turn emerges as a robust knob that can induce semiconductor-to-metallic transitions in the network response. Our study demonstrates the effectiveness of directed assembly on channels with varying degrees of confinement as a simple tool to tailor the conductance of the otherwise heterogeneous network, opening up the possibility of robust large-scale CNN-based devices.

Flexible Near-Field Nanopatterning with Ultrathin, Conformal Phase Masks on Nonplanar Substrates for Biomimetic Hierarchical Photonic Structures

Multilevel hierarchical platforms that combine nano- and microstructures have been intensively explored to mimic superior properties found in nature. However, unless directly replicated from biological samples, desirable multiscale structures have been challenging to efficiently produce to date. Departing from conventional wafer-based technology, new and efficient techniques suitable for fabricating bioinspired structures are highly desired to produce three-dimensional architectures even on nonplanar substrates. Here, we report a facile approach to realize functional nanostructures on uneven microstructured platforms via scalable optical fabrication techniques. The ultrathin form (∼3 μm) of a phase grating composed of poly(vinyl alcohol) makes the material physically flexible and enables full-conformal contact with rough surfaces. The near-field optical effect can be identically generated on highly curved surfaces as a result of superior conformality. Densely packed nanodots with submicron periodicity are uniformly formed on microlens arrays with a radius of curvature that is as low as ∼28 μm. Increasing the size of the gratings causes the production area to be successfully expanded by up to 16 in(2). The nano-on-micro structures mimicking real compound eyes are transferred to flexible and stretchable substrates by sequential imprinting, facilitating multifunctional optical films applicable to antireflective diffusers for large-area sheet-illumination displays.

Large-Scale Nanorods Nanomanufacturing by Electric-Field-Directed Assembly for Nanoscale Device Applications

A fast and highly scalable room-temperature nanomanufacturing process for fabricating metallic nanorods from nanoparticles for applications such as interconnects and sensors is presented. Metallic nanoparticles are precisely assembled into prefabricated vias by applying a controlled dynamic electric field between the electrodes at the bottom of the vias and a counter electrode placed far away from the vias. The nanoscale vias are fabricated employing conventional electron-beam nanolithography. The dimension of the fabricated nanorods is controlled by the size of the vias and assembly parameters such as the amplitude and frequency of the applied electric field. The mechanism of the assembly process is discussed by examining the effect of the applied voltages on the assembly process to provide a fundamental understanding for scaling down to nanoscale dimensions. This room-temperature aqueous fabrication process is environmentally friendly and can be used to make nanorods using different types of metallic particles.

Effect of Different Deposition Mediums on the Adhesion and Removal of Particles

The purpose of this study is to investigate the effect of the different deposition mediums on the adhesion and removal of particles. Polystyrene latex (PSL) particles (50 μm) are deposited on thermal oxide and silicon nitride coated silicon wafers using different suspension mediums: air, isopropyl alcohol (IPA), and deionized water and then removed in a dry environment. The results show that PSL particles deposited on oxide are easier to remove than those on nitride due to a higher van der Waals force in all deposition mediums. In addition, dry particles deposited in air are much easier to remove than those desposited in a liquid medium. When particles are deposited from a liquid suspension, a liquid meniscus is formed between the particle and the substrate, resulting in a capillary force. The capillary force induces a plastic deformation for soft particles such as PSL, which increases the contact area between the particle and the substrate, making them more difficult to remove. The liquid meniscus evaporates shortly after it is exposed to either a dry air environment or vacuum; however, the plastic deformation of particles would take place mainly due to the initial adhesion force in addition to the short time exposure of the capillary force.

3-D perpendicular assembly of SWNTs for CMOS interconnects

Due to their superior electrical properties such as high current density and ballistic transport, carbon nanotubes (CNT) are considered as a potential candidate for future very large scale integration (VLSI) interconnects. However, direct incorporation of CNTs into a complimentary metal oxide semiconductor (CMOS) architecture by the conventional chemical vapor deposition (CVD) growth method is problematic because it requires high temperatures that might damage insulators and doped semiconductors in the underlying CMOS circuits. In this paper, we present a directed assembly method to assemble aligned CNTs into pre-patterned vias perpendicular to the substrate. A dynamic electric field with a static offset is applied to provide the force needed for directing the SWNT assembly. It is also shown that by adjusting assembly parameters the density of the assembled CNTs can be significantly enhanced. This highly scalable directed assembly method is conducted at room temperature and pressure and is accomplished in a few minutes. I-V characterization of the assembled CNTs was conducted using a Zyvex nanomanipulator in a scanning electron microscope (SEM) and the measured value of the resistance was 270 kΩs.

Shockwave-induced deformation of organic particles during laser shockwave cleaning

Although the laser shockwave cleaning process offers a promising alternative to conventional dry-cleaning processes for nanoscale particle removal, its difficulty in removing organic particles has been an unexplained problem. This work elucidates the physics underlying the ineffectiveness of removing organic particles using laser shock cleaning utilizing polystyrene latex particles on silicon substrates. It is found that the shockwave pressure is high enough to deform the particles, increasing the contact radius and consequently the particle adhesion force. The particle deformation has been verified by high-angle scanning electron microscopy. The Maugis-Pollock theory has been applied to predict the contact radius, showing good agreement with the experiment.

Laser shock removal of nanoparticles from Si capping layer of EUV mask

As the device design rule rapidly decreases below 50 nm, EUV (Extreme Ultra Violet) lithography i s being developed for the next generation lithography technology. In EUV lithography, reflective masks should be used instead of conventional transparent mask. However, EUV mask has no pellicle in order to maintain high reflectivity. EUV masks have to be cleaned at every exposure level to remove the residues or air-bone particles. Conventional RCA wet cleaning including H202 can attack and damage the metal absorber layer. In this paper, a new dry laser shock cleaning has been applied to remove nanoparticles from Si capping layer for EUV reticles. 4 and I I nm Si capping layer was deposited on 40 pairs of SiMo multi layer. As a contaminant, the fluorescent PSL (polystyrene latex) particles of 63nm diameter were deposited on Si capping layers by using an air spray method. An optical microscope (Nikon Optiphot 200D) with fluorescent filter cube and Xenon arc-lamp was used to inspect and measure the particles on substrate before and after cleaning. A Q-switched Nd:YAG laser (Fig. 1 ) with a fundamental wavelength of 1064nm was used to remove the particles from Si capping layer. Both of laser shock wave and UV laser (266 nm) were generated from the original laser source. Because the thickness of Si capping layers was very thin, the layers could be damaged from the laser shock wave and UV irradiation. In order to avoid the damages of Si capping layer, UV irradiation energy was decreased from 110 mJ to 8 mJ and the gap distance between laser beam spot and wafer surface was adjusted to 10.5 as shown in Figures 2 and 3. The removal efficiency of 63nm fluorescent PSL particles was about 50% when UV was exposed to particles as shown in Fig. 4. However, the removal efficiency increased slightly when the number of UV irradiation multiplied. Laser shock cleaning was immediately performed to the Si capping layers after U V laser cleaning. Greater than 95% PSL particle removal efficiencies was measured on 4 and l l n m Si capping layer without any surface damages.