Han Gil Na

Significant enhancement of the sensing characteristics of In2O3 nanowires by functionalization with Pt nanoparticles.

We report a significant enhancement in the gas sensing properties of In(2)O(3) nanowires by functionalizing their surfaces with Pt nanoparticles. For Pt-functionalization, In(2)O(3)-Pt core-shell nanowires are synthesized by the sputtering deposition of Pt layers on bare In(2)O(3) nanowires. Next, continuous Pt shell layers are transformed into Pt nanoparticles of cubic phase by heat treatment. In an O(2) gas sensing test, the Pt-functionalized In(2)O(3) nanowires reveal exceptionally higher sensitivity and faster response than bare In(2)O(3) nanowires.

Promotion of acceptor formation in SnO2 nanowires by e-beam bombardment and impacts to sensor application.

We have realized a p-type-like conduction in initially n-type SnO2 nanowires grown using a vapor-liquid-solid method. The transition was achieved by irradiating n-type SnO2 nanowires with a high-energy electron beam, without intentional chemical doping. The nanowires were irradiated at doses of 50 and 150 kGy, and were then used to fabricate NO2 gas sensors, which exhibited n-type and p-type conductivities, respectively. The tuneability of the conduction behavior is assumed to be governed by the formation of tin vacancies (under high-energy electron beam irradiation), because it is the only possible acceptor, excluding all possible defects via density functional theory (DFT) calculations. The effect of external electric fields on the defect stability was studied using DFT calculations. The measured NO2 sensing dynamics, including response and recovery times, were well represented by the electron-hole compensation mechanism from standard electron-hole gas equilibrium statistics. This study elucidates the charge-transport characteristics of bipolar semiconductors that underlie surface chemical reactions. The principles derived will guide the development of future SnO2-based electronic and electrochemical devices.

Microwave-Assisted Synthesis of Graphene–SnO2 Nanocomposites and Their Applications in Gas Sensors

We obtained extremely high and selective sensitivity to NO2 gas by fabricating graphene-SnO2 nanocomposites using a commercial microwave oven. Structural characterization revealed that the products corresponded to agglomerated structures of graphene and SnO2 particles, with small secondary SnOx (x ≤ 2) nanoparticles deposited on the surfaces. The overall oxygen atomic ratio was decreased with the appearance of an SnOx (x < 2) phase. By the microwave treatment of graphene-SnO2 nanocomposites, with the graphene promoting efficient transport of the microwave energy, evaporation and redeposition of SnOx nanoparticles were facilitated. The graphene-SnO2 nanocomposites exhibited a high sensor response of 24.7 for 1 ppm of NO2 gas, at an optimized temperature of 150 °C. The graphene-SnO2 nanocomposites were selectively sensitive to NO2 gas, in comparison with SO2, NH3, and ethanol gases. We suggest that the generation of SnOx nanoparticles and the SnOx phase in the matrix results in the formation of SnO2/SnO2 homojunctions, SnO2/SnOx (x < 2) heterojunctions, and SnO2/graphene heterojunctions, which are responsible for the excellent sensitivity of the graphene-SnO2 nanocomposites to NO2 gas. In addition, the generation of surface Sn interstitial defects is also partly responsible for the excellent NO2 sensing performance observed in this study.

Fabrication and Characteristics of β-Bi2O3 Nanowires Prepared by Heating a Mixture of In and Bi Powders

We studied the characteristics of tetragonal β-Bi 2 O 3 nanowires synthesized by heating a mixture of Bi and In powders. Sufficient growth time and In powder ratio are required for the generation of nanowires. Based on our observations, we propose a Au-catalyzed vapor-liquid-solid process as the dominant growth mechanism, in which In powder affects the morphology of nanowires. Photoluminescence (PL) analysis indicates that the nanowires exhibit emission bands centered at 1.58, 2.40, and 2.76 eV, regardless of the measurement temperature. The overall PL intensity tends to decrease as the measurement temperature increases.

Temperature-controlled synthesis of In2Ge2O7 nanowires and their photoluminescence properties

By controlling the heating temperature of a mixture of In and Ge powders, we have obtained monoclinic In2Ge2O7 nanowires at 600?700??C, whereas we have produced cubic In2O3 nanowires at 900??C. The In2Ge2O7 nanowires grown at 600??C were terminated by Au-containing nanoparticles, giving evidence that the vapour?liquid?solid model is the major growth mechanism. With the growth process at 700?900??C being dominated by a vapour?solid process, we have discussed the temperature-induced change in growth mechanisms. Photoluminescence measurements at 10?300?K revealed a broad visible emission centred at around 2.5?eV.

Enhanced Gas Sensing Characteristics of Ag2O-Functionalized Networked In2O3 Nanowires

We have fabricated Ag2O-functionalized In2O3 nanowires, in which the NO2 gas sensing properties are enhanced. To achieve the functionalization, the core In2O3 nanowires were sputter-deposited with the Ag shell layer, which turned out to be composed of cubic Ag particles. Subsequent thermal annealing changed the Ag nanoparticles to cubic nanoparticles with a cubic Ag2O phase. In spite of shell-coating and subsequent annealing, scanning electron microscopy images revealed that the products consisted of one-dimensional nanowires. In a NO2 gas sensing test, the sensitivity of the Ag2O-functionalized sensor was lower than that of the nonfunctionalized sensor, presumably owing to the significant volume of the depletion region in the Ag2O–In2O3 interface. However, the Ag2O-functionalized In2O3 nanowires exhibited exceptionally fast response and recovery compared with bare In2O3 nanowires. We suggest that not only the catalytic effect but also the spillover effect of Ag2O nanoparticles is mainly responsible for the observed enhancement of sensing capabilities in terms of response/recovery time.

Fabrication of Bi-doped In2O3-ITO nanocomposites and their photoluminescence properties

For the first time, Bi-doped In2O3-indium tin oxide (ITO) nanocomposites were prepared on Si substrates with the assistance of a Au catalyst through the simple gas-phase transport of a mixture of Bi, In, and Sn powders. The square-shaped Bi-doped In2O3-ITO nanostructures were straight, a few hundreds of nanometres in width, and below a few tens of micrometres in length. Electron microscopy, x-ray diffraction, and energy-dispersive x-ray spectroscopy analyses indicated that the Bi-doped In2O3-ITO nanorods were single crystals with a basis of cubic In2O3 structures. The photoluminescence spectra revealed that the Bi-doped In2O3-ITO nanorods had a strong orange emission band centred at approximately 626 nm without any shoulder bands. The enhancement of orange emission might be due to the oxygen deficiencies of structural defects in the nanorods.

Emission of SiOx Nanowires Resulting from Pressure Changes in a Closed System Comparable to a Tungsten Electron Gun at a High Voltage

SiOx nanowire emission with changes in the vapor pressure of Sn metal results in a screw motion of SiOx nanowires in a closed system consisting of Sn and SiOx. A convoluted flouncing-off process is elucidated here, in which the elementary particles are not electrons, but nanowires. In order to understand the unprecedented synthetic phenomena, the system that emits SiOx nanowires is compared with tungsten electron gun by comparing the component, functionalization, and theoretical background of both the techniques. The results so obtained, which are the first of its kind, open the way not only to produce three-dimensional micro- and nano-structures from one-dimensional nanostructures without requiring any additional artificial manipulation process but also to focus them onto a desired small region for elaborate deposition.

Comparison of structural and optical properties of TeO2 nanostructures synthesized using various substrate conditions

Several TeO2 low-dimensional nanostructures were prepared by thermal evaporation using four substrate conditions: (1) a bare substrate, (2) a scratched substrate, (3) a Au-catalyst-assisted substrate, and (4) a multi-walled carbon nanotube (MWCNT)-assisted substrate. Scanning electron microscopy and transmission electron microscopy analysis reveals that the morphologies of the nanostructures synthesized using these methods gradually changed from nanoparticles to ultra-thin nanowires with single tetragonal-type TeO2. Photoluminescence (PL) spectra reveal that the PL intensities of the TeO2 nanomaterials obtained using methods (1) and (2) are slightly increased, whereas the intensities of the TeO2 nanostructures obtained using methods (3) and (4) differ significantly depending on the initial substrate conditions. The emission peak is also blue-shifted from ~440 nm to ~430 nm for the scratched surface condition due to an excitonic transition. The increase in the blue emission for the MWCNT-assisted condition is attributed to the degree and type of excitons and defects in the TeO2 nanostructures.

Growth Mechanism and Luminescent Properties of Amorphous SiOx Structures via Phase Equilibrium in Binary System.

Balloon whisk-like and flower-like SiOx tubes with well-dispersed Sn and joining countless SiOx loops together induce intense luminescence characteristics in substrate materials. Our synthetic technique called “direct substrate growth” is based on pre-contamination of the surroundings without the intended catalyst and source powders. The kind of supporting material and pressure of the inlet gases determine a series of differently functionalized tube loops, i.e., the number, length, thickness, and cylindrical profile. SiOx tube loops commonly twist and split to best suppress the total energy. Photoluminescence and confocal laser measurements based on quantum confinement effect of the embedded Sn nanoparticles in the SiOx tube found substantially intense emissions throughout the visible range. These new concepts related to the synthetic approach, pre-pollution, transitional morphology, and permeable nanoparticles should facilitate progress in nanoscience with regard to tuning the dimensions of micro-/nanostructure preparations and the functionalization of customized applications.