Heejoon Ahn

Aligned nickel-cobalt hydroxide nanorod arrays for electrochemical pseudocapacitor applications

Nanorod arrays were grown directly on a stainless steel substrate by the chemical bath deposition method. Parallel arrays of nanorods show a specific capacitance of 456 F g−1 with an energy density of 12.8 W h kg−1. This approach provides a one-step, seedless and cost-effective route for fabricating pseudocapacitive materials in 3-D form.

Binary metal hydroxide nanorods and multi-walled carbon nanotube composites for electrochemical energy storage applications

Carbon nanotube and metal oxide/hydroxide hybrids have attracted much interest as electrode materials for electrochemical supercapacitors because of their dual storage mechanism. They can complement or replace batteries in electrical energy storage and harvesting applications, where high power delivery or uptake is needed. Multi-walled carbon nanotube (MWCNT) and nickel–cobalt binary metal hydroxide nanorod hybrids have been developed through the chemical synthesis of binary metal hydroxide on a MWCNT surface. These hybrids show enhanced supercapacitive performance and cycling ability. Growth of a thin film consisting of a coating of binary metal hydroxide, as well as further growth of nanorod structures, is demonstrated using FESEM and TEM, showing that this film is a promising structure for supercapacitor applications. These electrodes yield a significantly high capacitance of 502 F g−1 with a high energy density of 69 W h kg−1 at a scan rate of 5 mV s−1. The film is stable up to 5000 cycles with greater than 80% capacitance retention.

Micron and submicron patterning of polydimethylsiloxane resists on electronic materials by decal transfer lithography and reactive ion-beam etching: Application to the fabrication of high-mobility, thin-film transistors

We describe a technique for fabricating micron and submicron-sized polydimethylsiloxane (PDMS) patterns on electronic material substrates using decal transfer lithography (DTL) in conjunction with reactive ion-beam etching (RIE). We validate the use of this unconventional polymeric system as a suitable resist material for fabricating Si-based microelectronic devices. In this process, an O2∕CF4 gas mixture was used to etch a supporting PDMS thin film that resides atop a closed-form decal polymer to reveal conventional resist structures. These structures provide an effective latent image that, in turn, provides for an extension of soft lithography as a form of multilayer lithography—one yielding submicron structures similar to those obtained from the conventional photochemical methods used to prepare such resists. This combined DTL/RIE patterning procedure was found to be compatible with commercially available planarization layers and provides a direct means for preparing high aspect ratio resist features. ...

Rational design of coaxial structured carbon nanotube-manganese oxide (CNT-MnO2) for energy storage application.

Recently, there has been great research interest in the development of composites (core-shell structures) of carbon nanotubes (CNTs) with metal oxides for improved electrochemical energy storage, photonics, electronics, catalysis, etc. Currently, the synthetic strategies for metal oxides/hydroxides are well established, but the development of core-shell structures by robust, cost-effective chemical methods is still a challenge. The main drawbacks for obtaining such electrodes are the very complex synthesis methods which ultimately result in high production costs. Alternatively, the solution based method offers the advantages of simple and cost effective synthesis, as well as being easy to scale up. Here, we report on the development of multi-walled carbon nanotube-manganese oxide (CNT-MnO2) core-shell structures. These samples were directly utilized for asymmetric supercapacitor (ASC) applications, where the CNT-MnO2 composite was used as the positive electrode and ZIF-8 (zeolitic imidazolate framework, ZIF) derived nanoporous carbon was used as the negative electrode. This unconventional ASC shows a high energy density of 20.44 W h kg(-1) and high power density of 16 kW kg(-1). The results demonstrate that these are efficient electrodes for supercapacitor application.

Cross‐linked Multilayer Polymer‐Clay Nanocomposites and Permeability Properties

Abstract Electrostatically layered aluminosilicate nanocomposites have been prepared by the sequential deposition of poly(allylamine hydrochloride)/poly(acrylic acid)/poly(allylamine hydrochloride)/saponite (PAH/PAA/PAH/saponite)10 on poly(ethylene terephtalate) (PET) film. Exfoliated saponite nanoplatelets were obtained by extensive shaking, sonication, and centrifugation of a water suspension. To minimize permeability and improve the mechanical integrity, cross‐linking of composite films was carried out at different temperatures. The formation of amide linkage induced through heating was observed by Fourier Transform Infrared (FT‐IR) and x‐ray photoelectron spectroscopy (XPS). The cross‐linking of nanocomposites (PAH/PAA/PAH/saponite)10 showed 60% decrease in permeability of oxygen when compared with the pristine PET substrate film. In contrast, water permeability of the nanocomposite membrane was not affected by heating temperature and deposition cycles.

Patterned polydiacetylene-embedded polystyrene nanofibers based on electrohydrodynamic jet printing

AbstractElectrohydrodynamic (EHD) jet printing is a direct-writing technique which ejects ink through a fine nozzle using an electric field, which has the advantages of high-resolution, rapid printing speed and a wide range of ink selectivity. In this article, the EHD jet printing system is utilized to print patterns of polystyrene (PS) nanofibers. The effect of parameters such as ink concentration, working distance, applied voltage, and stage speed on the diameter of the printed nanofibers was investigated. The EHD jet printing technology is further utilized to print various patterns of polydiacetylene (PDA)-embedded PS nanofibers. The EHD jet printing based nanofiber printing is advantageous over conventional electrospining based approaches in terms of patterned PDA images. In addition, an advanced EHD jet printing system which is adopted for aligned nanofiber printing will expand the application of nanofibers from bio and chemical sensors to tissue engineering and electronics.

Fabrication of microstructured silicon (µs-Si) from a bulk Si wafer and its use in the printing of high-performance thin-film transistors on plastic substrates

In this paper, we report a new fabrication route to generate microstructured, single-crystalline silicon (μs-Si) ribbons using (110) silicon. Two different methods were explored for producing these printable structures. This work also introduces a second-process innovation in the fabrication of microstructured semiconductor objects for printed large-area circuits, namely the direct integration of a high-quality, thermally grown silicon dioxide (SiO2) layer for use as a gate dielectric in top-gate metal-oxide-silicon field effect transistors. We also demonstrate and characterize a soft, conformable lamination process that considerably enhances the mechanical stability of devices printed on plastic, allowing bending radii as small as 0.8 cm. These structures enable a reduction of the bending strains localized at the device interface. These improvements were fully characterized by finite element simulations of the strain distribution present in a descriptive model of the multilayer laminated circuit.

Interfacially treated dye-sensitized solar cell with in situ photopolymerized iodine doped polythiophene.

A thin film of iodine doped polythiophene was grown photoelectrochemically around the dye-sensitized TiO(2) nanoparticles in a Grätzel cell, and the effect of iodine doping level on the cell performance was investigated using X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, and photovoltage decay. At an optimum doping level, the cell demonstrated the enhanced energy conversion efficiency by 27.52% compared to the cell without polythiophene.

Thermal conductivities of epoxy composites comprising fibrous carbon and particulate silicon carbide fillers

Thermal conductivities of epoxy composites consisting of carbon fiber (CF) and particulate silicon carbide (SiC) fillers were investigated. Composites composed of both fillers were found to have a higher packing density according to a void volume, than composites composed of either single filler on its own. The thermal conductivities were measured using a laser flash method. The CF-epoxy composite exhibited a higher thermal conductivity than SiC-epoxy composite for a filler loading of 80 wt%. The thermal conductivity of a mixed-filler composite containing 30% CF and 50% SiC by weight was found to be 10.6 W/mK, which is twice the value of that of a CF-epoxy composite, six times greater than that of a SiC-epoxy composite, and approximately 48 times greater than that of unmodified epoxy resin. This increased thermal conductivity is due to the fibrous and particulate morphologies of the fillers, which bring about an increase in the number of contact points throughout reducing void volume and increasing dispersibility of carbon fiber, thus resulting in an improved heat transfer path.

Molecular Assembly by Sequential Ionic Adsorption of Nanocrystalline TiO2 and a Conjugated Polymer

Abstract Cationic nanocrystalline TiO2 particles have been synthesized for which the size and composition of the nanoparticles were analyzed by a transmission emission microscopy and energy dispersive x‐ray spectrometer (EDXS). Multilayered films have been fabricated by sequential adsorption of TiO2 nanoparticles and poly(3‐thiophene acetic acid) (PTAA). Each layer of the nanoparticles and PTAA in the thin film has also been characterized by x‐ray photoelectron spectroscopy, atomic force microscopy, and UV‐visible spectroscopy. These types of multilayered nanocomposite films may find applications in the fabrication of efficient light harvesting photovoltaic cells. #Dedicated to the memory of Professor Sukant K. Tripathy (deceased).