Donghoon Kim

Silicon nanowire biosensors for detection of cardiac troponin I (cTnI) with high sensitivity

We have demonstrated highly sensitive and label-free detection of cardiac troponin I (cTnI), a biomarker for diagnosis of acute myocardial infarction, using silicon nanowire field-effect transistors. A honeycomb-like structure is utilized for nanowire configuration to offer improved electrical performance and increased sensing area. The fabricated devices show n-type behavior with a relatively high ON-OFF current ratio, small sub-threshold swing and low gate leakage current. Monoclonal antibodies for cTnI were covalently immobilized on the nanowire surface and the attachment of antibodies is clearly visualized by atomic force microscope. The sensitivity with various concentrations of buffer solution was also investigated in order to determine the optimal buffer condition. The devices exhibit highest sensitivity under buffer solutions with low ion concentration. In addition, the detection limit of the sensor is as low as ~5 pg/mL, the lowest reported in the literature to date and nearly an order of magnitude smaller than the suggested threshold limit. The fabricated devices demonstrate a good selectivity for detecting cTnI.

Suspended honeycomb nanowire ISFETs for improved stiction-free performance

This paper reports high performance ion-sensitive field-effect transistors (ISFETs) with a suspended honeycomb nanowire (SHNW) structure. The SHNW can provide a longer, stiction-free channel than that which is possible with a suspended straight nanowire (SSNW) for the realization of gate-all-around biosensors. Devices with SHNWs, SSNWs and conventional nanowires on the substrate have been fabricated using a top-down approach in order to compare their electrical performances. The SHNW devices exhibit excellent electrical characteristics such as lower subthreshold swing, higher transconductance and higher linear drain current. In addition, the SHNW ISFETs show better pH sensitivity than other ISFETs. Based on the results, the SHNW device appears promising for enhancing the intrinsic performance and ensuring the reliable operation of biosensor applications.

Development of high-power density, 4-kV pulse transformers for TWTA

Pulse transformers suitable for high-frequency and high-voltage operations in a traveling-wave tube amplifier (TWTA) had been designed, fabricated, and tested. Two transformers with different operation frequency of 100 and 80 kHz were designed, fabricated, and tested. The transformer with 100-kHz switching frequency had input and output voltages of 250 Vdc and -4.1 kV, respectively. Operating power was 3.06 kW. Achieved power density of the 100-kHz transformers was 22.26 W/cm/sup 3/. The transformer with 80-kHz switching frequency had input and output voltages of 265 Vdc and -4.1 kV, respectively. Normal operating power of the transformer was 1.67 kW. Achieved power density of the 80-kHz transformer was 12.15 W/cm/sup 3/. Three different winding arrangements for the 80-kHz transformer were tested and it was found that the one with a sandwiched primary between secondary windings had the best performance in electrical characteristics. It was possible to reduce the stray capacitance while maintaining low-leakage inductance with the best arrangement. Therefore, the self-resonant frequency was far separated from the operating frequency, and, thus, the new arrangement minimized the self-resonant current that is generally one of source for transformer temperature rise.

A Reconfigurable and Portable Highly Sensitive Biosensor Platform for ISFET and Enzyme-Based Sensors

This paper presents a portable low-cost biosensor platform for ion-sensitive field-effect transistor (ISFET) and enzyme-based sensors. To meet various demands of diagnosis, our portable platform is designed to perform cyclic voltammetry, amperometry, and linear sweep voltammetry for enzyme-based sensor and ISFET sensor. For compatibility with various sensors which require various electrical driving schemes, our system can be easily reconfigured by simple switches. In addition, with disposable printed-circuit-board packages, sensors can be easily replaced. Our platform was tested with various sensors in various measurement methods. In cyclic voltammetry test with a model analyte K3Fe(CN)6, a graph provided by our system has 4.79% relative error at a current peak compared with the data acquired by a 18 times more expensive commercial system. In cyclic voltammetry and amperometry tests with the laccase enzyme-based sensors, our system achieved sensitivities of 341 and 500 μA/mM/cm2, respectively. By running a sensitive SiNW ISFET on our platform in linear sweep voltammetry, we could build a low-cost portable pH sensor system.

Preliminary Study for Designing a Novel Vein-Visualizing Device

Venipuncture is an important health diagnosis process. Although venipuncture is one of the most commonly performed procedures in medical environments, locating the veins of infants, obese, anemic, or colored patients is still an arduous task even for skilled practitioners. To solve this problem, several devices using infrared light have recently become commercially available. However, such devices for venipuncture share a common drawback, especially when visualizing deep veins or veins of a thick part of the body like the cubital fossa. This paper proposes a new vein-visualizing device applying a new penetration method using near-infrared (NIR) light. The light module is attached directly on to the declared area of the skin. Then, NIR beam is rayed from two sides of the light module to the vein with a specific angle. This gives a penetration effect. In addition, through an image processing procedure, the vein structure is enhanced to show it more accurately. Through a phantom study, the most effective penetration angle of the NIR module is decided. Additionally, the feasibility of the device is verified through experiments in vivo. The prototype allows us to visualize the vein patterns of thicker body parts, such as arms.

Enhanced Switchable Ferroelectric Photovoltaic Effects in Hexagonal Ferrite Thin Films via Strain Engineering

Ferroelectric photovoltaics (FPVs) are being extensively investigated by virtue of switchable photovoltaic responses and anomalously high photovoltages of ∼104 V. However, FPVs suffer from extremely low photocurrents due to their wide band gaps (Eg). Here, we present a promising FPV based on hexagonal YbFeO3 (h-YbFO) thin-film heterostructure by exploiting its narrow Eg. More importantly, we demonstrate enhanced FPV effects by suitably exploiting the substrate-induced film strain in these h-YbFO-based photovoltaics. A compressive-strained h-YbFO/Pt/MgO heterojunction device shows ∼3 times enhanced photovoltaic efficiency than that of a tensile-strained h-YbFO/Pt/Al2O3 device. We have shown that the enhanced photovoltaic efficiency mainly stems from the enhanced photon absorption over a wide range of the photon energy, coupled with the enhanced polarization under a compressive strain. Density functional theory studies indicate that the compressive strain reduces Eg substantially and enhances the strength of d-d transitions. This study will set a new standard for determining substrates toward thin-film photovoltaics and optoelectronic devices.

Electron−hole separation in ferroelectric oxides for efficient photovoltaic responses

Significance Photovoltaics (PVs) benefitting from ferroelectric polarizations can overcome critical limitations of conventional type PVs. In this class, Bi2FeCrO6 is known to be the best-performing material; however, a fundamental understanding of the origin is lacking, which has limited further performance improvements. Here, we carried out a theoretical investigation of the electronic structure of this material. As a result, electron−hole (e-h) pairs are observed to separate upon photoexcitation, which can be a dominant underlying mechanism for the exceptional PV responses. Based on this understanding, we further suggest five novel materials that can offer a combination of strong e-h separations and visible-light absorptions. We expect the community of ferroelectric PVs to immediately benefit from the features of the new suggested materials. Despite their potential to exceed the theoretical Shockley−Queisser limit, ferroelectric photovoltaics (FPVs) have performed inefficiently due to their extremely low photocurrents. Incorporating Bi2FeCrO6 (BFCO) as the light absorber in FPVs has recently led to impressively high and record photocurrents [Nechache R, et al. (2015) Nat Photonics 9:61–67], which has revived the FPV field. However, our understanding of this remarkable phenomenon is far from satisfactory. Here, we use first-principles calculations to determine that such excellent performance mainly lies in the efficient separation of electron−hole (e-h) pairs. We show that photoexcited electrons and holes in BFCO are spatially separated on the Fe and Cr sites, respectively. This separation is much more pronounced in disordered BFCO phases, which adequately explains the observed exceptional PV responses. We further establish a design strategy to discover next-generation FPV materials. By exploring 44 additional Bi-based double-perovskite oxides, we suggest five active-layer materials that offer a combination of strong e-h separations and visible-light absorptions for FPV applications. Our work indicates that charge separation is the most important issue to be addressed for FPVs to compete with conventional devices.

Improving DMMP (Salin simulant) sensing characteristics of TFQ functionalized graphene chemiresistive sensors

In order to detect DMMP (dimethyl methyl-phosphonate; sarin simulant), tetrafluorohydroquinone (TFQ) functionalized graphene chemiresistive sensors are successfully developed. The graphene sensors show significantly enhanced sensitivity with various DMMP concentrations. In addition, for the real-time DMMP detection, a method using the first derivative of current sensitivity will be presented.

Effect of surface functional groups on the pH sensitivity in ion-sensitive field effect transistors

For ion-sensitive devices with SiO2 sensing membrane, the pH sensitivity response has been analyzed when additional functional groups are attached on the SiO2 membrane. A modified Site-Binding model (SBM) and Gouy-Chapman-Stern (GCS) model are used for the theoretical analysis. In the presence of both silanol and amine groups, the highest pH sensitivity can be achieved at a functional group ratio of 1:3 or 3:1. The differential capacitance and intrinsic buffer capacity were also compared in order to understand their effects on pH sensitivity.

Effects of buffer concentration on sensing performances of ion-sensitive field-effect transistors wth si-nanowires

We have experimentally investigated the effect of buffer-dilution on sensing characteristics of the Si-nanowire (Si-NW) ion-sensitive field-effect transistors (ISFETs). Phosphate-buffered saline (PBS) with various buffer concentrations was prepared. The result showed that the sensitivity increases as the buffer concentration decreases for bio-molecule detection, while the pH sensitivity of the Si-NW FETs is insensitive to the buffer solutions. The Debye length of the buffer solution can be a crucial factor to detect biomolecules using FET sensors. For the buffer solution with high ionic strength, the Debye length becomes shorter than the distance between the sensing membrane and the target-molecules so that the charges of target-molecules are screened out. For the pH sensing, however, small hydrogen ions can be bound close to the channel surface and thus little dependence on the buffer concentration.