Yoonho Ahn

Highly precise nanofiber web-based dry electrodes for vital signal monitoring

A great variety of dry electrodes have been developed over many decades in order to overcome the drawbacks of conventional Ag/AgCl gel electrodes, but their applications are still restricted due to the low accuracy of the dry electrodes. To improve their accuracy, highly elastic thermoplastic polyurethane (TPU) and poly(styrene-b-butadiene-b-styrene) (SBS) were electrospun into nanofiber webs containing Ag nanoparticles. Then, these webs were silver plated using electroless plating to prepare silver plated nanofiber web (AgNFw)-based dry electrodes that allowed for the detection of high fidelity biopotential signals. The electrode properties of AgNFw dry electrodes were investigated thoroughly using agar phantom systems. Human subject tests were also carried out to determine the real performance of AgNFw dry electrodes on a human body in terms of biopotential recording and electrical impedance tomography (EIT) measurements. The experimental results revealed that AgNFw dry electrodes can exhibit performances comparable to Ag/AgCl gel electrodes, indicating the newly designed AgNFw dry electrodes have superior accuracy to most existing dry electrodes.

Magnetic force microscopy of conducting nanodots in NiO thin films

We report a nanoscale magnetic conducting filament in a resistive random access memory (RRAM) device by the direct investigation of conducting nanobits in NiO thin films using magnetic force microscopy. The conducting nanobit in a NiO RRAM capacitor formed by CAFM and KFM exhibited a typical bistable resistive switching characteristic. The magnetizations of the conducting nanobit were measured as a function of the set-reset switching cycle and as the switching cycles were increased, a strong ferromagnetic signal was observed. The metallic Ni formation in the nanoscale magnetic conducting filament could be a possible reason for the origin of the magnetism.

Ferroelectric properties of highly a-oriented polycrystalline Bi2WO6 thin films grown on glass substrates

Polycrystalline Bi2WO6 (BWO) thin films were deposited on Pt/Ta/glass substrates by pulsed laser deposition (PLD). In this study, we comparatively investigate the influence of oxygen partial pressure on structural and ferroelectric properties of the BWO films. In comparison with the BWO films deposited at oxygen partial pressure of 100 and 300 mTorr, the BWO film deposited at 300 mTorr exhibits a highly a-oriented crystalline structure. The highly a-oriented polycrystalline BWO thin film shows good ferroelectric properties with a remnant polarization of about 21.5μC/cm2. The piezoresponse force microscope study reveals that the highly a-oriented BWO thin film possesses larger ferroelectric domain patterns due to smaller domain wall energy.

Mn doping effect on the ferroelectric domain structure of BaTiO3 thin films

We comparatively investigated the ferroelectric domain structure of BaTiO3 (BTO) and Mn-doped BTO epitaxial thin films grown on Pt/MgO substrates by pulsed laser deposition. The advantages of Mn doping included enhanced ferroelectric polarization, low leakage current, and ferromagnetism. Piezoresponse force microscopy revealed that the increased domain wall energy associated with Mn doping led to an increase in the ferroelectric domain structure.

Enhanced Multiferroic Properties in Epitaxial Yb-Doped BiFeO3 Thin Films

AbstractWe report the enhanced multiferroic properties of a ytterbium (Yb)- doped BiFeO3 thin film (Bi0.85Yb0.15FeO3) deposited on a (001) SrRuO3/(100) SrTiO3 substrate by pulsed laser deposition. The crystal structure, surface morphology, ferroelectric domain structure, and the electrical and magnetic behavior of the epitaxial Bi0.85Yb0.15FeO3 film, 100 nm in thickness, were investigated. The results were compared with those of an undoped BiFeO3 thin film. The x-ray diffraction patterns showed that both films have tetragonal-like crystal structures. Atomic force microscopy showed that the Bi0.85Yb0.15FeO3 and BiFeO3 films have flat and clear surface steps, characteristic of a layer-by-layer growth mechanism. Furthermore, the strip-like ferroelectric domain structures were clearly observed in piezoelectric force microscopy. The Bi0.85Yb0.15FeO3 films had significantly higher remanent polarizations of approximately 73 μC/cm2 and lower leakage currents compared to the BiFeO3 thin film. Ferromagnetic enhancement was also observed in the Bi0.85Yb0.15FeO3 film at room temperature.

Ionic cellulose-stabilized gold nanoparticles and their application in the catalytic reduction of 4-nitrophenol

A novel strategy for the synthesis of highly stable gold nanoparticles (GNPs) was designed by reducing HAuCl4 with NaBH4 in an aqueous solution of water-soluble ionic cellulose composed of dimethylimidazolium cations and phosphite-bound cellulose anions. NMR and UV-Vis analysis along with the measurement of the zeta potential suggest that the exceptionally high stability of GNPs originates from the strong interaction of GNPs with the phosphite groups of the ionic cellulose. The thus prepared GNPs exhibit excellent catalytic activity for the reduction of 4-nitrophenol to 4-aminophenol, a model hydrogenation reaction.

Ferroelectric properties of lead-free polycrystalline CaBi2Nb2O9 thin films on glass substrates

CaBi2Nb2O9 (CBNO) thin film, a lead-free ferroelectric material, was prepared on a Pt/Ta/glass substrate via pulsed laser deposition. The Ta film was deposited on the glass substrate for a buffer layer. A (115) preferred orientation of the polycrystalline CBNO thin film was verified via X-ray diffraction measurements. The CBNO thin film on a glass substrate exhibited good ferroelectric properties with a remnant polarization of 4.8 μC/cm2 (2Pr ∼9.6 μC/cm2), although it had lower polarization than the epitaxially c-oriented CBNO thin film reported previously. A mosaic-like ferroelectric domain structure was observed via piezoresponse force microscopy. Significantly, the polycrystalline CBNO thin film showed much faster switching behavior within about 100 ns than that of the epitaxially c-oriented CBNO thin film.

Ferroelectric properties and piezoresponse force micoroscopy study of Bi3TaTiO9 thin films

We investigate the ferroelectric properties and crystal structures of Bi3TaTiO9 (BTTO) thin films deposited on single-crystal Nb-doped (100) SrTiO3 substrates via pulsed laser deposition. The BTTO films exhibited either a (001)-epitaxial crystalline structure or a mixed a- and c-oriented polycrystalline structure depending on the substrate temperature. The ferroelectric polarization and piezoelectric coefficient of the mixed a- and c-oriented film were larger than those of the (001)-epitaxial film because its polar axis was perpendicular to the c-axis. Vertical and lateral piezoresponse force microscopy studies indicate that the ferroelectric domains of the (001)-epitaxial film were all parallel to the in-plane orientation, whereas the mixed a- and c-oriented film comprised both square grains with in-plane-oriented ferroelectric domains and longish grains with ferroelectric domains out of orientation with the plane.

Multiferroic YCrO3 thin films grown on glass substrate: Resistive switching characteristics

Polycrystalline YCrO3 thin films were deposited on (111) Pt/Ta/glass substrates by pulsed laser deposition. The YCrO3 thin films exhibited good ferroelectric properties with remnant polarization of about 5 µC/cm2. Large leakage current was observed by I-V curve and ferroelectric hysteresis loop. The YCrO3 resistive random access memory (RRAM) capacitor showed unipolar switching behaviors with SET and RESET voltages higher than those of general NiO RRAM capacitors.