Kwangsoo No

Principle of ferroelectric domain imaging using atomic force microscope

The contrast mechanisms of domain imaging experiments assisted by atomic force microscope (AFM) have been investigated by model experiments on nonpiezoelectric (silicon oxide) and piezoelectric [Pb(Zr,Ti)O3] thin films. The first step was to identify the electrostatic charge effects between the tip, the cantilever, and the sample surface. The second step was to explore the tip–sample piezoelectric force interaction. The static deflection of the cantilever was measured as a function of dc bias voltage (Vdc) applied to the bottom electrode (n-type Si wafers) for noncontact and contact modes. In addition, a small ac voltage (Vac sin ωt) was applied to the tip to measure the amplitude (Aω) and phase (Φω) of the first harmonic (ω) signal as a function of Vdc. By changing from the noncontact to the contact mode, a repulsive contribution to the static deflection was found in addition to the attractive one and a 180° phase shift in Φω was observed. These results imply that in the contact mode the cantilever buckl...

Self-Powered Cardiac Pacemaker Enabled by Flexible Single Crystalline PMN-PT Piezoelectric Energy Harvester

A flexible single-crystalline PMN-PT piezoelectric energy harvester is demonstrated to achieve a self-powered artificial cardiac pacemaker. The energy-harvesting device generates a short-circuit current of 0.223 mA and an open-circuit voltage of 8.2 V, which are enough not only to meet the standard for charging commercial batteries but also for stimulating the heart without an external power source.

Vertical ZnO nanowires/graphene hybrids for transparent and flexible field emission

We present a transparent and flexible optoelectronic material composed of vertically aligned ZnO NWs grown on reduced graphene/PDMS substrates. Large-area reduced graphene films were prepared on PDMS substrates by chemical exfoliation from natural graphitevia oxidative aqueous dispersion and subsequent thermal reduction. ZnO NWs were hydrothermally grown on the reduced graphene film substrate and maintained their structural uniformity even in highly deformed states. The electrical contact between semiconducting ZnO NWs and the metallic graphene film was straightforwardly measured by electric force microscopy (EFM). It shows a typical metal–semiconductor ohmic contact without a contact barrier. Owing to the mechanical flexibility, transparency, and low contact barrier, the ZnO NWs/graphene hybrids show excellent field emission properties. Low turn-on field values of 2.0 V μm−1, 2.4 V μm−1, and 2.8 V μm−1 were measured for convex, flat, and concave deformations, respectively. Such variation of field emission properties were attributed to the modification of ZnO NWs emitter density upon mechanical deformation.

Biomineralized N-Doped CNT/TiO2 Core/Shell Nanowires for Visible Light Photocatalysis

We report an efficient and environmentally benign biomimetic mineralization of TiO(2) at the graphitic carbon surface, which successfully created an ideal TiO(2)/carbon hybrid structure without any harsh surface treatment or interfacial adhesive layer. The N-doped sites at carbon nanotubes (CNTs) successfully nucleated the high-yield biomimetic deposition of a uniformly thick TiO(2) nanoshell in neutral pH aqueous media at ambient pressure and temperature and generated N-doped CNT (NCNT)/TiO(2) core/shell nanowires. Unlike previously known organic biomineralization templates, such as proteins or peptides, the electroconductive and high-temperature-stable NCNT backbone enabled high-temperature thermal treatment and corresponding crystal structure transformation of TiO(2) nanoshells into the anatase or rutile phase for optimized material properties. The direct contact of the NCNT surface and TiO(2) nanoshell without any adhesive interlayer introduced a new carbon energy level in the TiO(2) band gap and thereby effectively lowered the band gap energy. Consequently, the created core/shell nanowires showed a greatly enhanced visible light photocatalysis. Other interesting synergistic properties such as stimuli-responsive wettabilites were also demonstrated.

Virus-Directed Design of a Flexible BaTiO3 Nanogenerator

Biotemplated synthesis of functional nanomaterials has received increasing attention for applications in energy, catalysis, bioimaging, and other technologies. This approach is justified by the unique abilities of biological systems to guide sophisticated assembly and organization of molecules and materials into distinctive nanoscale morphologies that exhibit physicochemical properties highly desirable for specific purposes. Here, we present a high-performance, flexible nanogenerator using anisotropic BaTiO3 (BTO) nanocrystals synthesized on an M13 viral template through the genetically programmed self-assembly of metal ion precursors. The filamentous viral template realizes the formation of a highly entangled, well-dispersed network of anisotropic BTO nanostructures with high crystallinity and piezoelectricity. Even without the use of additional structural stabilizers, our virus-enabled flexible nanogenerator exhibits a high electrical output up to ∼300 nA and ∼6 V, indicating the importance of nanoscale structures for device performances. This study shows the biotemplating approach as a facile method to design and fabricate nanoscale materials particularly suitable for flexible energy harvesting applications.

High resolution study of domain nucleation and growth during polarization switching in Pb(Zr,Ti)O3 ferroelectric thin film capacitors

The domain nucleation and growth during polarization switching in Pb(Zr,Ti)O3 (PZT) ferroelectric thin film capacitors with Pt top (TE) and bottom electrodes (BE) were studied by means of atomic force microscopy (AFM). The experimental configuration used in this study differs from that conventionally used (AFM tip/PZT/BE) where the AFM tip acts as a positionable TE. A small ac voltage was applied between the electrodes with a step by step increasing dc bias voltage. The induced piezoelectric vibration was detected by the AFM tip, its amplitude and phase determined with the lock-in amplifier. The phase difference between the applied ac voltage and the piezoelectric signal as a function of the x-y position was nearly locked at 0 or 180, representing film regions with parallel (in-phase) and antiparallel (antiphase) polarization direction, respectively. The polarization reversal was induced by application of a step by step increasing dc bias field opposite to the polarization of the prepoled sample. At each ...

Effects of oxygen partial pressure on the microstructure and electrical properties of indium tin oxide film prepared by d.c. magnetron sputtering

We prepared ITO films using d.c. magnetron sputtering and investigated effects of oxygen partial pressure on the microstructure and the electrical properties of the films. The ITO films deposited at low oxygen partial pressure showed resistivity of 2 × 10−4Ω cm and optical transmittance of about 90%. The resistivity increased as the oxygen partial pressure increased. The preferred orientation of the film was changed as the oxygen partial pressure was changed. The films deposited at high oxygen partial pressure consisted of relatively large grains, but those deposited at low oxygen partial pressure consisted of two distinctive features: aggregate of small grains and that of long grains. Possible cause of the observations was speculated using the oxygen vacancy concentration.

Low-temperature growth of carbon nanotubes by thermal chemical vapor deposition using Pd, Cr, and Pt as co-catalyst

Abstract Palladium (Pd), chromium (Cr), and platinum (Pt) are used as co-catalysts to decrease the growth temperature of carbon nanotubes to 500–550°C. Pd is found to be the most efficient co-catalyst for the growth of carbon nanotubes on cobalt-nickel catalytic particles deposited on a silicon oxide substrate by thermal chemical vapor deposition using C 2 H 2 . High-resolution transmission electron microscopy reveals the bamboo-shaped carbon nanotubes grown at 500°C using Pd, while the curled carbon nanofibers are grown at 550°C using Cr.

Quantitative analysis of the bit size dependence on the pulse width and pulse voltage in ferroelectric memory devices using atomic force microscopy

The Bit formation using atomic force microscopy (AFM) was studied on 270-nm-thick 〈111〉 preferentially oriented Pb(Zr0.4Ti0.6)O3 (PZT) films prepared by the sol-gel process. To minimize the cantilever-sample capacitive force interaction, the experiment was carried out at or near the sample edge. Bit formation was investigated by calculating the electric field in AFM-tip/PZT film/bottom electrode configuration. It was found both experimentally and theoretically that the bit size is linearly dependent on the pulse voltage and the logarithmic value of the pulse width. The linear dependence of the bit size on the logarithmic value of pulse width was explained from the relationship between the switching time and electric field. It was found that the minimum bit size of a fully penetrating domain equals the film thickness.

AC frequency characteristics of coplanar impedance sensors as design parameters

Glass-based microchannel chips were fabricated using photolithographic technology, and Pt thin-film microelectrodes, as coplanar impedance sensors, were integrated on them. Longitudinal design parameters, such as interelectrode spacing and electrode width, of coplanar impedance sensors were changed to determine AC frequency characteristics as design parameters. Through developing total impedance equations and modeling equivalent circuits, the dominant components in each frequency region were illustrated for coplanar impedance sensors and the measured results were compared with fitted values. As the ionic concentration increased, the value of the frequency-independent region decreased and cut-off frequencies increased. As the interelectrode spacing increased, cut-off frequencies decreased and total impedance increased. However, the width of each frequency-independent region was similar. As the electrode area increased, f(low) decreased but f(high) was fixed. We think that the decrease in R(Sol) dominated over the influence of other components, which resulted in heightening f(low) and f(high). The interelectrode spacing is a more significant parameter than the electrode area in the frequency characteristics of coplanar sensors. The deviation of experimentally obtained results from theoretically predicted values may result from the fringing effect of coplanar electrode structure and parasitic capacitance due to dielectric substrates. We suggest the guidelines of dominant components for sensing as design parameters.