Dojin Kim

Magnetic properties of epitaxially grown semiconducting Zn1−xCoxO thin films by pulsed laser deposition

We have characterized Zn1−xCoxO (x=0.25) films grown on sapphire (0001) substrates by pulsed laser deposition using various growth conditions to investigate the growth condition dependence of properties of Co-doped ZnO films. The substrate temperature (TS) was varied from 300 to 700 °C and the O2 pressure (PO2) from 10−6 to 10−1 Torr. When TS is relatively low (≲600 °C), homogeneous alloy films with a wurtzite ZnO structure are grown and predominantly paramagnetic, whereas inhomogeneous films of wurtzite ZnO phase mixed with rock-salt CoO and hexagonal Co phases form when TS is relatively high and PO2 is fairly low (≲10−5 Torr). The presence of Co clusters leads to room temperature ferromagnetism in inhomogeneous films. The temperature dependence of the magnetization for the homogeneous Zn1−xCoxO (x=0.25) films shows spin-glass behavior at low temperature and high temperature Curie–Weiss behavior with a large negative value of the Curie–Weiss temperature, indicating strong antiferromagnetic exchange coupl...

Effects of rapid thermal annealing on the ferromagnetic properties of sputtered Zn1−x(Co0.5Fe0.5)xO thin films

We have investigated the effects of rapid thermal annealing under vacuum on the CoFe-doped ZnO [Zn1−x(Co0.5Fe0.5)xO] films grown by reactive magnetron co-sputtering. At least up to x=0.15, the films have the single phase of the same wurtzite structure as pure ZnO. Ferromagnetism was observed for the CoFe-doped ZnO films. We found that rapid thermal annealing leads to a remarkable increase in the spontaneous magnetization of the CoFe-doped ZnO as well as the electron concentration. The annealing also leads to a significant increase in the Curie temperature (TC), resulting in room temperature ferromagnetism with TC>300 K for the CoFe-doped ZnO films.

A high-performance nonenzymatic glucose sensor made of CuO-SWCNT nanocomposites.

Nanocomposites of CuO and single-wall carbon nanotubes (SWCNTs) were synthesized using an arc-discharging graphite rod that contained copper wires. Simultaneous arc discharges produced a CuO-SWCNT composite network. The crystalline structure and morphology of the CuO-SWCNT composite films were investigated using XRD, Raman spectroscopy, FE-SEM and TEM. The electrochemical properties were investigated by cyclic voltammogram and amperometric measurements in a 0.1 M NaOH solution. The CuO content in the CuO-SWCNT nanocomposites was optimized for nonenzymatic glucose detection. The glucose sensing properties of the optimized CuO-SWCNT electrode showed good stability, selectivity, and linear glucose detection that ranged from 0.05 to 1800 μM with a higher sensitivity of 1610 μA cm⁻² mM⁻¹, a quick response time of 1-2 s, and the lowest limit of detection at 50 nM. The sensing performance was better than the pure CuO and SWCNT sensors, and the synergetic effect of the composite sensor was attributed to the high conductivity network of highly porous nanowires. The sensor also showed a good response in a human serum sample, which proves its high potential towards a commercial nonenzymatic glucose sensor.

Enzymatic glucose biosensor based on CeO2 nanorods synthesized by non-isothermal precipitation

Cerium oxide nanorods (CeO(2) NRs) were synthesized without templates through a low cost and simple non-isothermal precipitation method. The structure and morphology of CeO(2) NRs were characterized by X-ray diffraction and transmission electron microscopy. The CeO(2) NRs films, deposited on indium tin oxide (ITO)-coated glass substrates through electrophoretic deposition, were used for the immobilization of glucose oxidase (GOx). Field emission scanning electron microscopy, Fourier transform infrared spectroscopy, cyclic voltammetry, and electrochemical impedance spectroscopy were used to characterize the CeO(2) NRs/ITO and GOx/CeO(2) NRs/ITO electrodes. The GOx/CeO(2) NRs/ITO electrode exhibits a linear range for the detection of glucose from 2 to 26 mM (correlation coefficient: 0.99) at 1-2s response time. Biosensor sensitivity is 0.165 μA mM(-1) cm(-2) with 100 μM detection limit. The anti-interference ability of the biosensor was also examined. The mediator-less application of CeO(2) NRs for glucose sensing was demonstrated.

Optimization of a zinc oxide urchin-like structure for high-performance gas sensing

Zinc oxide (ZnO) hollow hemisphere (HS) and urchin-like (UL) structures were fabricated and examined for application to a gas sensor. Films of hollow ZnO-HS arrays floating over substrates were synthesized via Zn sputtering onto the template of a polystyrene sphere array followed by oxidation. Growing ZnO nanorods upon HS surfaces via a hydrothermal method formed hollow ZnO–UL structures. The thicknesses of the HS films and the lengths of nanorods in the UL structures were varied to obtain the maximum response to NO gas. Both sensor structures showed a sensing of tens of parts per billion of levels of NO concentrations with good response and gas selectivity. The highest response was realized through the thinness and the open porosity of the structures. The surface depletion determined the sensor response signal for the sensor geometry with the highest response.

CHARACTERIZATION OF FE-CATALYZED CARBON NANOTUBES GROWN BY THERMAL CHEMICAL VAPOR DEPOSITION

The growth of carbon nanotubes (CNTs) on Fe catalytic films by thermal chemical vapor deposition has been investigated. Fe thin films deposited on Si substrates agglomerate to nanoparticles after heat treatment. The Fe nanoparticles are easily oxidized to Fe2O3 after the synthesis, but the nanoparticles during the CNT synthesis are shown to be in the liquid iron phase due to the reducing force of NH3. This is revealed by cross-sectional transmission electron microscopy examinations of the nanoballs and the Fe inclusions along with relevant chemical analyses. The CNT growth mode and mechanism with a Fe catalyst was very similar to that with a Ni catalyst in thermal chemical vapor deposition method.

Porous Au-embedded WO3 Nanowire Structure for Efficient Detection of CH4 and H2S

We developed a facile method to fabricate highly porous Au-embedded WO3 nanowire structures for efficient sensing of CH4 and H2S gases. Highly porous single-wall carbon nanotubes were used as template to fabricate WO3 nanowire structures with high porosity. Gold nanoparticles were decorated on the tungsten nanowires by dipping in HAuCl4 solution, followed by oxidation. The surface morphology, structure, and electrical properties of the fabricated WO3 and Au-embedded WO3 nanowire structures were examined by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and current–voltage measurements. Formation of a nanowire structure resulted in significant enhancement in sensing response to H2S and CH4 gases. Furthermore, Au embedment into the WO3 nanowire structures remarkably improved the performance of the sensors. The increase in response performance of sensors and adsorption–desorption kinetic processes on the sensing layers were discussed in relation with the role of Au embedment.

Synthesis and characterization of red phosphor (Y, Gd)BO3:Eu by the coprecipitation method

The synthesis and luminescent properties of (Y,Gd)BO 3 :Eu phosphor were investigated. A coprecipitation method was designed for preparing Eu doped (Y,Gd)BO 3 . This method features a low-temperature formation of single phase with semi-spherical shape compared with the conventional method. Due to the homogeneous distribution of activator, the emission intensity of the (Y,Gd)BO 3 :Eu phosphor prepared by the coprecipitation method is higher than the commercially available red phosphor.

Realization of an open space ensemble for nanowires: a strategy for the maximum response in resistive sensors

Here, we used NO as a test gas to propose a strategy for a nanowire gas sensor with the maximum response—the lowest detection limits. The apparatus uses an open space ensemble structure of nanowires with diameters at near total-depletion. For this purpose, a series of open space nanowire structures of WO3 was fabricated with diameters varying from 35 to 82 nm, and a corresponding conduction nanowire sensor model was proposed. The nanowire structures revealed the highest response and a lowest detection limit of 30 ppb. Furthermore, the sensor response was maximum with nanowires of ∼40 nm, which is the diameter corresponding to total depletion conditions; the response was decreased at smaller diameters. The sensor model successfully explained the ultimate lower limits of the size effect in the nanowire sensors. To realize optimum sensor performance with the practical ensemble type nano-structures, an open space morphology is critical to remove the effect of gas diffusion throughout the structure.

Electrochromic properties of porous WO3–TiO2 core–shell nanowires

A highly porous TiO2–WO3 core–shell nanowire structure for an electrochromic (EC) coating was fabricated by sputter deposition of titanium and tungsten on a porous single-walled carbon nanotube template. This process was followed by thermal oxidation. The morphology and crystalline quality of composite materials were investigated by scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. The electrochemical and EC properties were also examined and compared with thin TiO2–WO3 composite films as well as individual WO3 and TiO2 nanowire structures. The highly porous composite nanowire structure showed a highly enhanced proton intercalation capacity with good reversible electrochemical cycling of intercalation–deintercalation. The nanostructure also showed significantly improved EC contrast and coloration efficiency. This enhancement was observed with high chemical stability of the material. We proposed an atomic model of proton insertion into oxygen vacancies to explain the EC property and gas sensing simultaneously.