Jae Bok Lee

A Wideband Spiral Antenna for Ingestible Capsule Endoscope Systems: Experimental Results in a Human Phantom and a Pig

This paper presents the design of a wideband spiral antenna for ingestible capsule endoscope systems and a comparison between the experimental results in a human phantom and a pig under general anesthesia. As wireless capsule endoscope systems transmit real-time internal biological image data at a high resolution to external receivers and because they operate in the human body, a small wideband antenna is required. To incorporate these properties, a thick-arm spiral structure is applied to the designed antenna. To make practical and efficient use of antennas inside the human body, which is composed of a high dielectric and lossy material, the resonance characteristics and radiation patterns were evaluated through a measurement setup using a liquid human phantom. The total height of the designed antenna is 5 mm and the diameter is 10 mm. The fractional bandwidth of the fabricated antenna is about 21% with a voltage standing-wave ratio of less than 2, and it has an isotropic radiation pattern. These characteristics are suitable for wideband capsule endoscope systems. Moreover, the received power level was measured using the proposed antenna, a circular polarized receiver antenna, and a pig under general anesthesia. Finally, endoscopic capsule images in the stomach and large intestine were captured using an on-off keying transceiver system.

Effect of PEDOT Nanofibril Networks on the Conductivity, Flexibility, and Coatability of PEDOT:PSS Films.

The use of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) in electrodes and electrical circuits presents a number of challenges that are yet to be overcome, foremost amongst which are its relatively low conductivity, low coatability on hydrophobic substrates, and decreased conductivity at large strains. With this in mind, this study suggests a simple way to simultaneously address all of these issues through the addition of a small amount of a nonionic surfactant (Triton X-100) to commercial PEDOT:PSS solutions. This surfactant is shown to considerably reduce the surface tension of the PEDOT:PSS solution, thus permitting conformal coatings of PEDOT:PSS thin film on a diverse range of hydrophobic substrates. Furthermore, this surfactant induces the formation of PEDOT nanofibrils during coating, which led to the high conductivity values and mechanical stability at large strains (ε=10.3%). Taking advantage of the superior characteristics of these PEDOT:PSS thin films, a highly flexible polymer solar cell was fabricated. The power conversion efficiency of this solar cell (3.14% at zero strain) was preserved at large strains (ε=7.0%).

Detection of graphene domains and defects using liquid crystals

The direct observation of the domain size and defect distribution in a graphene film is important for the development of electronic applications involving graphene. Here we report a promising method for observing graphene domains grown by chemical vapour deposition. The unavoidable development of crack or pinhole defects during the growth and transfer processes is visualized using a liquid crystal layer. Liquid crystal molecules align anisotropically with respect to the graphene domains and exhibit distinct birefringence properties that can be used to image the graphene domains. This approach is useful for visualizing the crack distributions and their generation process in graphene films under external strain. This type of simple observation method provides an effective route to evaluating the quality and reliability of graphene sheets for use in various electronic devices.

Drying‐Mediated Self‐Assembled Growth of Transition Metal Dichalcogenide Wires and their Heterostructures

DOI: 10.1002/adma.201501475 Figure 1 a depicts the dip coating process for the production of certain classes of TMD structures. First, a piece of SiO 2 / Si or quartz wafer is immersed into an aqueous (NH 4 ) 2 MoS 4 (or (NH 4 ) 2 WS 4 ) solution (0.23% w/v (g mL −1 ) in DI water) and removed at different speeds to control the drying velocity. As the water evaporates, small nuclei are formed at the solution–substrate interface, and these nuclei initiate the growth of various solid (NH 4 ) 2 MoS 4 , or (NH 4 ) 2 WS 4 morphologies as intermediate states of MoS 2 and WS 2 . The self-assembled TMD morphologies are strongly dependent on the evaporation rate of the solvent and the diffusion rate of the TMD-precursors in solution. Figure 1 b,c presents optical images of the selfassembled (NH 4 ) 2 MoS 4 structures formed at different evaporation speeds and pH conditions. At very fast evaporation speeds (approximately 320 nl s −1 ) induced by high temperature (80 °C), a uniform thin fi lm of (NH 4 ) 2 MoS 4 was formed after the complete removal of the solvent (condition A, Figure 1 b) because there was insuffi cient time to induce the formation of nucleation seeds. In contrast, at evaporation speeds that were two orders of magnitude slower (0.67–1.72 nl s −1 ), dendritic structures were formed (condition B, Figure 1 c). The dendrite structures have many worm-like stems with a large number small side branches; self-assemblies of these seeds randomly formed on the substrate. In addition to these two nonoriented phases, the spontaneous formation of well-oriented wire patterns was observed when the acidity of solution increased from pH 6.41 to pH 5.02 at the same evaporation speed as condition B (condition C, Figure 1 d). Interestingly, these long wires, which grew from seeds formed in parallel arrays at the initial solution/substrate/air contact line, displayed uniform spacing between the wires without serious distortion of the arrays. The formation of aligned wires can be explained by the “fi ngering instability” phenomenon. [ 18,19 ] When the solvent begins evaporating at the solution/substrate contact line, fi ngering instability induced by the regulation of the evaporation speed, which affects the internal fl ow of the solution, leads to the periodic formation of nucleation-seed arrays. In turn, this results in the spontaneous growth of regular, unidirectional wire arrays from the seeds during the drying process (Figure 1 d). The critical dependence of the directional growth mechanism on the pH variation is not yet clear. However, we believe that the acidity of the reaction media can effectively suppress the gradation of the precursor diffusion, which is the driving force in the generation of the dendrite side-arms and the lateral expansion of the selfassembly, thereby preserving the main stream and, as a result, enhancing the formation of aligned wire arrays. [ 20 ]

Graphene-Based Heat Spreader for Flexible Electronic Devices

Graphene known for its superb physical properties, such as high transparency and thermal conductivity, is proposed as a solution to the problem of thermal management of the electronic devices, requiring transparency and cooling. It is shown that graphene heat spreader layer drives the heat out of the device more efficiently as compared with the commercially used metal thin films for integrated circuit cooling. An application of graphene heat spreader is proposed and tested in chip-on-film packaging. Graphene performance is compared with a gold layer with a similar transparency experimentally and theoretically as a proof of the efficient thermal management capability of graphene.

Vertical field effect tunneling transistor based on graphene-ultrathin Si nanomembrane heterostructures

Graphene-based heterostructured vertical transistors have attracted a great deal of research interest. Herein we propose a Si-based technology platform for creating graphene/ultrathin semiconductor/metal (GSM) junctions, which can be applied to large-scale and low-power electronics compatible with a variety of substrates. We fabricated graphene/Si nanomembrane (NM)/metal vertical heterostructures by using a dry transfer technique to transfer Si NMs onto chemical vapor deposition-grown graphene layers. The resulting van der Waals interfaces between graphene and p-Si NMs exhibited nearly ideal Schottky barrier behavior. Due to the low density of states of graphene, the graphene/Si NM Schottky barrier height can be modulated by modulating the band profile in the channel region, yielding well-defined current modulation. We obtained a maximum current on/off ratio (Ion/Ioff) of up to ~103, with a current density of 102 A cm−2. We also observed significant dependence of Schottky barrier height Δb on the thickness of the Si NMs. We confirmed that the transport in these devices is dominated by the effects of the graphene/Si NM Schottky barrier.

A new type of the matching structure of a H-plane T-junction for a high power system

This paper presents a H-plane T-junction which is designed for the feed networks of waveguide antenna array. The purpose of the T-junction is to divide power and phase equally without any reflection. To do so, we usually add an iris or a post in the junction area so that this iris or post behaves as an inductor and makes each port of the tee-junction matched [1]-[4]. By changing its shape and the location, it can divide power equally without reflection.