Dae Joon Kang

Synthesis and characterization of CuO nanowires by a simple wet chemical method

We report a successful synthesis of copper oxide nanowires with an average diameter of 90 nm and lengths of several micrometers by using a simple and inexpensive wet chemical method. The CuO nanowires prepared via this method are advantageous for industrial applications which require mass production and low thermal budget technique. It is found that the concentration and the quantity of precursors are the critical factors for obtaining the desired one-dimensional morphology. Field emission scanning electron microscopy images indicate the influence of thioglycerol on the dispersity of the prepared CuO nanowires possibly due to the stabilization effect of the surface caused by the organic molecule thioglycerol. The Fourier transform infrared spectrum analysis, energy dispersive X-ray analysis, X-ray diffraction analysis, and X-ray photoemission spectrum analysis confirm clearly the formation of a pure phase high-quality CuO with monoclinic crystal structure.

Reversibly Light-Modulated Dirac Point of Graphene Functionalized with Spiropyran

Graphene has been functionalized with spiropyran (SP), a well-known photochromic molecule. It has been realized with pyrene-modified SP, which has been adsorbed on graphene by π-π interaction between pyrene and graphene. The field-effect transistor (FET) with SP-functionalized graphene exhibited n-doping effect and interesting optoelectronic behaviors. The Dirac point of graphene in the FET could be controlled by light modulation because spiropyran can be reversibly switched between two different conformations, a neutral form (colorless SP) and a charge-separated form (purple colored merocyanine, MC), on UV and visible light irradiation. The MC form is produced during UV light irradiation, inducing the shift of the Dirac point of graphene toward negative gate voltage. The reverse process back to the neutral SP form occurred under visible light irradiation or in darkness, inducing a shift of the Dirac point toward positive gate voltage. The change of the Dirac point by UV and visible light was reproducibly repeated. SP molecules also improved the conductance change in the FET device. Furthermore, dynamics on conversion from MC to SP on graphene was different from that in solution and solid samples with SP-grafted polymer or that on gold nanoparticles.

MoO3-MWCNT nanocomposite photocatalyst with control of light-harvesting under visible light and natural sunlight irradiation

One of the most challenging tasks in solar energy utilization is the quest for inexpensive and stable photocatalysts which can efficiently utilize solar irradiation or interior lighting. Here we report new porous MoO3-MWCNT nanocomposites with excellent chemical stability which can efficiently decompose organic contaminants in aqueous solution under light visible and natural sunlight irradiation. The precise control of light harvesting and photocatalytic efficiency was achieved by controlling the quantity of MWCNTs in the nanocomposites. In comparison to other photocatalysts, porous photocatalysts are chemically and thermally stable and provide excellent electron transfer during photocatalytic reactions. Overall, this work shows that the synthesized MoO3-MWCNT nanocomposites can be efficiently used to improve the disinfection and decontamination of water under light visible and natural sunlight irradiation.

Realization of high performance ferroelectric-gate FET nonvolatile memory using p-type Si nanowire channel

We successfully fabricated ferroelectric-gate nonvolatile memory devices using a p-type Si nanowire coated with Omega-shaped gate organic ferroelectric poly(vinylidene fluoride-trifluoroethylene). Our FEFET memory devices exhibits excellent memory characteristics with a large modulation in channel conductance between ON and OFF states exceeding 105, a long retention time of over 5×104 sec and a high endurance of over 105 programming cycles while maintaining an ION/IOFF ratio higher than 103. This result offers a viable way to fabricate a high performance, high density nonvolatile memory device using a low temperature fabrication processing technique, which makes it suitable for flexible electronics.

Field emission study of strain controlled ZnO nanowire arrays via a hydrothermal technique

We report a novel approach to the rational fabrication of high quality strain controlled wurtzite ZnO nanowire arrays on conductive silicon and ITO substrates by a facile hydrothermal method. The quality of the ZnO nanowire arrays is dramatically improved by hanging the substrate above from the bottom of the Teflon lined autoclave. For comparison, the field emission device performance of the ZnO nanowire arrays grown under different experimental processes is investigated. The low turn-on voltage (1.9 V/μm) from field emission measurement was observed from ZnO nanowire arrays grown by substrate hanging growth procedure.

Field emission study of Te nanowire arrays grown on conducting silicon substrate

In this communication, synthesis of high quality and length controlled Te nanowire arrays directly grown on silicon substrate with and without the coating of seed layer by a facile hydrothermal method was reported. The most preferential orientation of tellurium NWs was found along [001] through micro structural investigation. Field emission properties of as grown and annealed under Ar: H2 reduction atmosphere are studied meticulously. The emission current density of vacuum annealed Te nanowire arrays was found to be 25μA/cm2 at 3.0 V/μm.