Won Jin Choi

Chemically exfoliated transition metal dichalcogenide nanosheet-based wearable thermoelectric generators

To utilize human heat energy as a permanent power source, we demonstrate, for the first time, an intrinsically highly foldable and stretchable thermoelectric generator that is based upon chemically exfoliated 1T-transition metal dichalcogenide (TMDC) nanosheets (NSs) for self-powered wearable electronics. The power factors of WS2 (n-type) and NbSe2 (p-type) NS films were evaluated to be 5–7 μ K−2 m−1 and 26–34 μW K−2 m−1, respectively, near room temperature. With these films, parallel-connected thermoelectric generators that were fabricated were able to constantly produce up to 38 nW of output power at Δ60 K. The thermoelectric device stably sustained its performance, even after 100 bending cycles and after 100 stretching cycles (50% strain). By direct observation, we found that the film is highly stretched by partial tearing and folding but still maintains an electrical percolation pathway. The morphology then is quickly recovered by a plug-in contact between the torn parts as the external strain is released. Finally, we demonstrate the electric power generation from a prototype wearable thermoelectric generator that was woven into a wristband fitted on a real human body.

Three-Dimensional Layer-by-Layer Anode Structure Based on Co3O4 Nanoplates Strongly Tied by Capillary-like Multiwall Carbon Nanotubes for Use in High-Performance Lithium-Ion Batteries

A layer-by-layer (LBL) structure composed of Co3O4 nanoplates and capillary-like three-dimensional (3D) multiwall carbon nanotube (MWCNT) nets was developed as an anode with simultaneous high-rate and long-term cycling performance in a lithium-ion battery. As the current density was increased to 50 A g(-1), the LBL structure exhibited excellent long-term cycling and rate performance. Thus, the Co3O4 nanoplates were in good electrical contact with the capillary-like 3D MWCNT nets under mechanically severe strain during long-term, high-rate cyclic operation.

Binder‐Free and Full Electrical‐Addressing Free‐Standing Nanosheets with Carbon Nanotube Fabrics for Electrochemical Applications

Inorganic functional nanosheets (NSs), which are unilamellar nanoscale building blocks having a thickness on the order of nanometers with lateral dimensions of sub-micrometers, are characterized by their ultra-large surface area per volume, stoichiometrically well-defi ned chemical composition, single crystalline structure with high crystallinity, and unique physicochemical properties. [ 1 , 2 ] Therefore, they have been considered excellent nanomaterials that provide superior performance in various applications such as catalysts, [ 3 ] sensors, [ 4 ] die-sensitized solar cells, [ 5 , 6 ] super-capacitors, [ 7 ] batteries [ 8 ] and fuel cells. [ 9 ]

Programmable Direct-Printing Nanowire Electronic Components

In order for recently developed advanced nanowire (NW) devices(1-5) to be produced on a large scale, high integration of the separately fabricated nanoscale devices into intentionally organized systems is indispensible. We suggest a unique fabrication route for semiconductor NW electronics. This route provides a high yield and a large degree of freedom positioning the device on the substrate. Hence, we can achieve not only a uniform performance of Si NW devices with high fabrication yields, suppressing device-to-device variation, but also programmable integration of the NWs. Here, keeping pace with recent progress of direct-writing circuitry,(6-8) we show the flexibility of our approach through the individual integrating, along with the three predesigned N-shaped sites. On each predesigned site, nine bottom gate p-type Si NW field-effect transistors classified according to their on-current level are programmably integrated.

Structural and Electrical Properties of Solution-Processed Gallium-Doped Indium Oxide Thin-Film Transistors

We fabricated solution-processed gallium-doped indium oxide (GIO) thin-film transistors (TFTs). The electrical property, crystallinity, and transmittance were investigated as a function of gallium content. Varying the gallium/indium ratio is found to have a significant effect on structural and electrical properties of thin films. The shrinkage of the lattice of a GIO film originates from substitution of Ga on In sites in the In2O3 lattice, which was verified by X-ray diffraction (XRD) analysis. By increasing the gallium ratio of the channel material, the GIO film shows an amorphous phase. The optimized GIO film (Ga/In= 0.35) has an electron mobility of 3.59 cm2 V-1 s-1, a threshold voltage of 0.1 V, an on/off current ratio of 8.2×107, and a subthreshold slope of 0.9 V/decade, and is highly transparent (~92%) in the visible region.

A simple method for cleaning graphene surfaces with an electrostatic force.

DOI: 10.1002/adma.201303199 Graphene is a 2D conductive nanomaterial composed of a honeycomb-structured array of carbon atoms. [ 1,2 ] It is useful in a variety of fi elds due to its structural and chemical stability and because it possesses unprecedented optical and electrical properties. [ 3–7 ] Graphene can be produced by both bottom-up (chemical vapor deposition: CVD) [ 8–11 ] and top-down (chemical or mechanical exfoliation) approaches. [ 2,12–14 ] CVD techniques can yield layer-controlled graphene sheets over large areas using Ni or Cu catalyst substrates; however, the CVD process is limited to very specifi c growth templates, and graphene is seldom directly grown on insulating substrates. Graphene grown on Cu or Ni catalyst fi lms must be transferred to a target substrate for use in applications. The procedures involved in transferring these fi lms always involve a supporting polymer contact layer because the mechanical strength of the one-atom-thick graphene is insuffi cient to handle the transfer processes. The most commonly used supporting layer materials are poly(methyl methacrylate) (PMMA), polycarbonates (PCs), or conventional photoresistive (PR) layers. [ 8–11,15–19 ] After the transfer step, the supporting layer materials must be removed from the graphene surface. The chemical stripping processes involved in removing the supporting layer are routine because most supporting layer materials readily dissolve in common solvents, such as acetone; however, PMMA layers in direct contact with a graphene layer (1–5 nm from the surface) leave “PMMA-G” contamination materials that cannot be removed using simple chemical means. [ 20–22 ] The residual PMMA-G not only degrades and changes the electrical characteristics of the graphene layer, but it also introduces uncertainty into studies of the intrinsic surface properties of graphene, the development of sensor devices, the fabrication of cell culture substrates for accelerating stem cell differentiation, and the fabrication of atomic-scale honeycomb templates. [ 23–25 ] Many attempts have been made to resolve these issues, and annealing methods, such as H 2 /Ar annealing, [ 20 ] vacuum annealing, [ 21 ] and Joule heating, [ 26 ] have proven to be successful. Some approaches have “scraped the surfaces clean? using atomic force microscopy (AFM) tips. [ 27 ] Hightemperature annealing processes (>250 °C) can introduce defects into graphene surfaces, and process temperatures can be much too high for use with fl exible substrate-based devices. High-temperature annealing normally requires vacuum facilities and/or the application of electrical energy, which increases production costs. Although AFM-derived mechanical cleaning methods avoid some of these complications, their use is limited to laboratory-scale because the methods are extremely lowthroughput. Therefore, an effective cleaning method that does not involve high-temperature heat treatments would greatly benefi t practical applications that rely on transferred graphene. Traditional surface cleaning methods involve rubbing a piece of cloth over the surface. Empirical evidence from everyday life clearly shows that a combination of mechanical abrasion and detergent is much more effective toward cleaning than the simple use of detergents only; however, abrasion methods cannot be used on materials that are mechanically weak. Nanostructured materials, such as graphene or carbon nanotubes, generally have a much larger mechanical strength than conventional steel of the same dimensions; however, the aggregate strength of nanostructured materials is usually not suffi ciently large to withstand macroscopic mechanical stresses. Mechanical abrasion is generally not used to clean the surfaces of nanoscale materials. Electrostatic interactions can remove charged impurities and, therefore, present an alternative approach to direct mechanical cleaning. Rubbing a balloon or a piece of plastic on ones head to induce ones hair to rise is a familiar childhood demonstration of electrostatic interactions, in which stationary or slow-moving electric charges build up in a material. As two interfaces are rubbed against one another, electrons can cross the interface of the materials and generate a charge disparity in each of the materials. The direction of electron transfer depends on the ability of the material to accept a negative charge. For example, as a piece of rubber is rubbed against a cloth, electrons are transferred from the cloth to the rubber. The rubber becomes negatively charged and the cloth becomes positively charged. Because the presence of a static charge creates a non-uniform electrostatic fi eld that tends to attract matter toward it, a material with a large amount of static charge can be used to remove dust from a surface. In this study, extremely fi ne cloth fi bers were used to apply electrostatic-force cleaning (EFC) to a graphene surface by removing the residual PMMA-G layers, yielding an intrinsic graphene surface without causing damage to the structural integrity. The process of removing the PMMA-G layer was monitored on a step-by-step basis using optical microscopy

Electrical Contact Tunable Direct Printing Route for a ZnO Nanowire Schottky Diode

Although writing was the first human process for communication, it may now become the main process in the electronics industry, because in the industry the programmability as an inherent property is a necessary requirement for next-generation electronics. As an effort to open the era of writing electronics, here we show the feasibility of the direct printing of a high-performance inorganic single crystalline semiconductor nanowire (NW) Schottky diode (SD), including Schottky and Ohmic contacts in series, using premetallization and wrapping with metallic nanofoil. To verify the feasibility of our process, SDs made of Al-premetalized ZnO NWs and plain ZnO NWs were compared with each other. Even with cold direct printing, the Al-premetalized ZnO NW SD showed higher performance, specifically 1.52 in the ideality factor and 1.58 x 10(5) in its rectification ratio.

Low-Temperature, Aqueous-Solution-Processed Zinc Tin Oxide Thin Film Transistor

We fabricate solution-processed zinc tin oxide (ZTO) thin-film transistors (TFTs). The solution used is prepared by precipitating metal hydroxide using NaOH and dissolving it using NH4OH. The X-ray diffraction (XRD) data of the spin-coated ZTO film demonstrates an amorphous phase, and the atomic force microscopy (AFM) image shows a smooth surface. The device performance of solution-processed TFTs was analyzed as a function of annealing temperature. The fabricated TFTs were operated in the enhancement mode, and exhibited a carrier mobility of 3.03 cm2 V-1 s-1, a threshold voltage of 10.2 V, an on/off current ratio of 1.23×107, a subthreshold slope of 0.78 V/decade, and high transparency (with ~90% transmittance) at a low annealing temperature of 300 °C.

Role of Alkaline-Earth Metal in Solution-Processed Indium Oxide Based Thin-Film Transistors

We fabricated alkaline-earth metal doped indium oxide thin-film transistors (TFTs) using a solution process. To analyze the effects of Mg, Ca, and Sr on the solution-processed indium oxide TFTs, thermogravimetric analysis, X-ray diffraction, atomic force microscopy analysis, and X-ray photoelectron spectroscopy were performed. The main difference in electrical performance results from the dehydroxylation energy of alkaline-earth metal, each ion radius and optical band gap. The optimized Mg-, Ca-, and Sr-doped indium oxide TFTs show mobilities of 2.03, 1.64, and 1.08 cm2 V-1 s-1, respectively, and an on-off current ratio of about ~105.

Enhancing gas sensing properties of graphene by using a nanoporous substrate

Substrate engineering is shown to be a viable approach for improving the use of graphene thin films for gas sensor applications. The performance of two-terminal devices fabricated on smooth SiO2 and nanoporous anodized aluminum oxide (AAO) substrates are compared. Raman studies indicated that both types of samples exhibit similarly low point-defect densities, but the mobility values of the SiO2-supported films were found to be three times larger than those on porous AAO substrates. However, the AAO-supported graphene devices exhibit a 3-fold enhanced sensitivity to both NO2 and NH3 gases when compared to the devices supported on SiO2. We attribute this sensitivity enhancement to the inhomogeneous electrostatic potential landscape that results from the porous nature of the AAO substrate, as well as extended defects made of wrinkles or folds originated from AAO. This substrate design strategy could be extended to other semiconductor-based sensor devices.