Jung Inn Sohn

Engineering of efficiency limiting free carriers and an interfacial energy barrier for an enhancing piezoelectric generation

The energy harvesting efficiency is of tremendous importance for the realization of a high output-power nanogenerator serving as the basis for self-powered electronics. Here we report that the device performance of a sound-driven piezoelectric energy nanogenerator (SPENG) is remarkably improved by controlling both the carrier density and the interfacial energy in a semiconducting ZnO nanowire (NW), thereby achieving its intrinsic efficiency limits. A SPENG with carrier-controlled ZnO NWs exhibits excellent energy harvesting characteristics with an average power density of 0.9 mW cm−3, as well as a near 50 fold increase in both output voltage and current compared to those of a conventional ZnO NW. In addition, we demonstrate for the first time that an optimized SPENG is large enough and very suitable to drive electrophoretic ink displays based on voltage-drive systems. This fundamental progress makes it possible to fabricate high performance nanogenerators for viable industrial applications in portable/wearable personal electronics such as electronic papers and smart identity cards.

Ink-jet printed ZnO nanowire field effect transistors

Semiconducting nanowires provide routes for realizing high-performance electronic devices, but for many applications of such devices low-cost manufacturing techniques are needed. The authors demonstrate here top-gated zinc oxide nanowire field effect transistors (NW-FETs) fabricated by ink-jet printing. High resolution submicrometer gold gaps between source and drain electrodes were defined by a self-aligned ink-jet printing technique, and the nanowires were deposited from solution onto these electrode arrays and gated from the top using a spin-coated poly(methyl methacrylate) gate dielectric. The typical NW-FETs exhibited a mobility of 2–4cm2∕Vs, a current on/off ratio of 104, and a transconductance of 20.5nS. The process provides a pathway for fabrication of NW-FETs by low-cost, large-area solution processing and direct printing techniques.

Influence of surface structure on the phonon-assisted emission process in the ZnO nanowires grown on homoepitaxial films

We carried out temperature-dependent photoluminescence measurements to investigate the influence of surface roughness on the phonon-assisted emission of the surface-tailored ZnO nanowires (NWs). For the rough ZnO NWs, the observation of strong defect emission with vibration peaks by the exciton-phonon coupling reflects the presence of a high density of surface defects, resulting in a rapid shift to lower energy region of free exciton emission and a strong contribution of the first order phonon-assisted free exciton in ultraviolet emission. This investigation indicates that the surface defects associated with roughness have a significant influence on the phonon-assisted exciton emission.

Enhancement of piezoelectricity via electrostatic effects on a textile platform

We have shown the enhanced piezoelectricity by electrostatic effects on a textile based platform. The electrostatic and piezoelectric effects were hybridized by integrating piezoelectric ZnO nanowires and a charged dielectric film on a wearable textile substrate. The hybrid textile nanogenerator produced an output voltage of 8 V and an output current of 2.5 μA. Using a simple AC–DC converter circuit, we operated the green organic light-emitting diode and a liquid crystal display panel using a 100 dB sonic wave.

Stress-induced domain dynamics and phase transitions in epitaxially grown VO₂ nanowires.

We demonstrate that surface stresses in epitaxially grown VO₂ nanowires (NWs) have a strong effect on the appearance and stability of intermediate insulating M₂ phases, as well as the spatial distribution of insulating and metallic domains during structural phase transitions. During the transition from an insulating M1 phase to a metallic R phase, the coexistence of insulating M₁ and M₂ phases with the absence of a metallic R phase was observed at atmospheric pressure. In addition, we show that, for a VO₂ NW without the presence of an epitaxial interface, surface stresses dominantly lead to spatially inhomogeneous phase transitions between insulating and metallic phases. In contrast, for a VO₂ NW with the presence of an epitaxial interface, the strong epitaxial interface interaction leads to additional stresses resulting in uniformly alternating insulating and metallic domains along the NW length.

The influence of surface chemical dynamics on electrical and optical properties of ZnO nanowire field effect transistors

We demonstrate the effect of surface chemical dynamics on carrier transport and recombination processes of electron-hole pairs in ZnO nanowire field effect transistors. We have found that the electrical conductance decreases and the threshold voltages shift in a positive gate voltage direction, as electrical characteristics are measured repeatedly. We associate this with the enhancement of oxygen adsorption by capturing electrons from the induced current during the probing. This results in an overall depletion of electrons and thus causes the positive shift in threshold voltages associated with the origin and width of characteristic hysteresis loops. In addition, the surface environment dependence of the photo-response related to a recombination process in ZnO nanowires is discussed in terms of the surface chemical reaction and band bending.

Controlled Assembly for Well-Defined 3D Bioarchitecture Using Two Active Enzymes

This paper reports that a bioarchitecture with two different active enzymes can be fabricated conveniently on a prepatterned surface by highly selective surface-templated layer-by-layer (LBL) assembly by coupling a bilayer of avidin/biotin-lactate oxidase (biotin-LOD) with a bilayer of avidin/biotin-horseradish peroxidase (biotin-HRP). The two different active enzymes can be utilized as excellent building blocks for the fabrication of well-defined 3D nanostructures with precise control of the position and height on the surface. In addition, the LBL assembled bienzyme structures are highly functional, and bioarchitectures based on LOD and HRP-mediated coupling reaction can be employed in a number of viable biosensing applications.

Density control of ZnO nanowires grown using Au-PMMA nanoparticles and their growth behavior

Au nanoparticles stabilized by poly(methyl methacrylate) (PMMA) were used as a catalyst to grow vertically aligned ZnO nanowires (NWs). The density of ZnO NWs with very uniform diameter was controlled by changing the concentration of Au-PMMA nanoparticles (NPs). The density was in direct proportion to the concentration of Au-PMMA NPs. Furthermore, the growth process of ZnO NWs using Au-PMMA NPs was systematically investigated through comparison with that using Au thin film as a catalyst. Au-PMMA NPs induced polyhedral-shaped bases of ZnO NWs separated from each other, while Au thin film formed a continuous network of bases of ZnO NWs. This approach provides a facile and cost-effective catalyst density control method, allowing us to grow high-quality vertically aligned ZnO NWs suitable for many viable applications.