Kyung Moon Lee

Low temperature wet chemical synthesis of good optical quality vertically aligned crystalline ZnO nanorods

The growth of ZnO nanorods on Au-coated ITO substrates using a low temperature wet chemical process is presented. Electron microscopy and x-ray diffraction observations reveal that the crystalline ZnO nanorods are preferentially oriented along the c axis. Room temperature photoluminescence (PL) measurements reveal a strong band edge emission at 382 nm, a signature of good crystallinity, with a weak and broad orange-red emission, which is typically attributed to the oxygen interstitials, in the range between 520 and 720 nm. Other than the second order feature of the band edge emission at 760 nm, no red or near-infrared bands are observed. The effect of precursor concentration on the morphological, structural and PL properties are studied, and the results are discussed.

Stable and uniform electron emission from nanostructured carbon films

We have systematically studied the electron-emission characteristics of nanostructured carbon films with various amounts of nanoclusters and degrees of nanotube alignment. According to our observation, the dense packing and/or alignment of nanotubes is detrimental to the low-field emission, which we attributed to the electrostatic screening effect. The best emission was observed from the carbon films dominated by nanoclusters; the turn-on field was 1.6 V/μm and the emission-site density was considerably higher than 5×104 site/cm2. The emission from the nanocluster-dominated film was uniform and stable. Raman spectroscopy identified the nanoclusters as crystalline graphite with some structural defects. It is conceivable that the modification of the boning hybridization at the cluster surface resulted in a diamond-like density of states, and that the corresponding small electron affinity was responsible for the excellent emission the from nanoclusters.

Efficient second harmonic generation in ZnO nanorod arrays with broadband ultrashort pulses

Broadband frequency-doubling properties of c-axis oriented zinc oxide (ZnO) nanorod arrays grown by low-temperature chemical bath method on glass substrate were studied. The maximum effective nonlinearity was found to be about 7.5 times higher than that of a type-I beta-barium borate crystal for a pump intensity of 5.5×1010 W/cm2. The angular dependence of second harmonic generation (SHG) was determined experimentally. The measured spectral profile of SHG was found to be in good agreement with theoretical simulations.

Effects of metal buffer layers on the hot filament chemical vapor deposition of nanostructured carbon films

We examined how the addition of different metal buffer layers between the Ni/Fe-alloy-catalyst layer and the silicon substrate affected the growth of nanostructured carbon films; Cr, Ti, Ta, and W were tested as buffer layers. Even when the sputter-deposition of catalytic-metal layers and the hot filament chemical vapor deposition of carbon films were carried out under the identical conditions, different buffer layers resulted in substantially different carbon-film growth. More specifically, carbon-nanoparticle films were produced with the Cr and the W buffer layers, and carbon-nanotube films were produced with the Ti and the Ta buffer layers. X-ray diffraction (XRD) showed a significant and systematic difference between the carbon-nanoparticle and carbon-nanotube films. In the case of the carbon-nanoparticle films deposited with either the Cr or the W buffer layer, the peaks corresponding to the catalytic metal, the carbide phases of the catalytic metal, and the carbide phases of the respective buffer me...

Field electron emission from patterned nanostructured carbon films on sodalime glass substrates

Using the hot filament chemical vapor deposition method, patterned nanostructured carbon films with excellent field emission characteristics have been deposited at the nominal substrate temperatures of 540 °C and 450°C on sodalime glass substrates. These films have demonstrated a good visual emission uniformity and good current stability during the continuous operation for 140 h. Taking advantage of the poor adhesion of the nanostructure carbon without a chrome interlayer and selective growth on the metal catalyst, the formation of the patterned nanostructured carbon films has been carried out via the prepattering of the chrome electrodes and/or metal catalyst using the conventional photolithography process. An example of the patterned emitter with high complexity is presented to demonstrate the feasibility of practical emitter fabrication using nanostructured carbon films.

Growth and evaluation of nanostructured carbon films for triode field emitter application

To identify the deposition conditions that can minimize the nanotube density in the nanostructured carbon films without compromising the emission properties, we carried out a systematic investigation of the effect of deposition conditions on the emission properties and the structure of the nanostructured carbon films. Catalyst-layer thickness, methane concentration, deposition time, deposition pressure, and substrate temperature were the main deposition parameters we investigated. Within the parameter range for nanoparticle-dominant growth, substrate temperature and deposition time were the two factors that had the largest effect on the variation of the turn-on field. However, catalyst-layer thickness and methane concentration turned out to be the factors allowing the minimization of nanotube density with rather small concomitant variations of the turn-on field. A 50 h test showed that the emission stability of a nanoparticle film was better than that of a multiwall nanotube film with comparable emission ...

Fully Solution-Processed Transparent Conducting Oxide-Free Counter Electrodes for Dye-Sensitized Solar Cells: Spray-Coated Single-Wall Carbon Nanotube Thin Films Loaded with Chemically-Reduced Platinum Nanoparticles

We report fully solution-processed fabrication of transparent conducting oxide-free counter electrodes (CEs) for dye-sensitized solar cells (DSSCs) by combining spray-coating of single-wall carbon nanotubes (SWCNTs) and chemical reduction of chloroplatinic acid precursor to platinum nanoparticles (Pt NPs) with formic acid. The power conversion efficiency of a semitransparent DSSC with such SWCNT-based CE loaded with Pt NPs is comparable to that of a control device with a conventional CE. Quantification of Pt loading shows that network morphology of entangled SWCNTs is efficient in forming and retaining chemically reduced Pt NPs. Moreover, electron microscopy and electrochemical impedance spectroscopy results show that mainly Pt NPs, which are tens of nanometers in diameter and reside at the surface of SWCNT CEs, contribute to electrocatalytic activity for triiodide reduction, to which we attribute strong correlation between power conversion efficiency of DSSCs and time constant deduced from equivalent-circuit analysis of impedance spectra.

Variations in structure and emission characteristics of nanostructured carbon films prepared by the hot-filament chemical-vapor-deposition method due to the addition of ammonia in the source

We investigated the structural variation and the concomitant change in the emission characteristics of hot-filament chemical-vapor-deposition-grown nanostructured carbon films induced by the addition of ammonia to source gases. At low ammonia concentrations the film morphology was dominated by carbon nanoparticles encapsulating metal and/or carbide cores. As the ammonia concentration was increased nanotube density increased, and in the medium ammonia-concentration range nanotubes dominated the film morphology. However, at 80% of ammonia concentration the number of nanotubes decreased substantially and nanoparticles became the dominant species again. The diameter, length, and shape of nanotubes also showed systematic ammonia-concentration- dependent variations, while the nanoparticle size did not change significantly. Another important variation was found in the quality of carbon layers constituting the carbon nanoparticles and nanotubes. As more and more ammonia was added to the source gas, large structur...

Non-Contact Local Conductance Mapping of Individual Graphene Oxide Sheets during the Reduction Process

We used electrostatic force microscopy (EFM) to investigate local conducting states of atomically thin individual graphene oxide (GO) sheets and monitor the spatial evolution of their conducting properties during the reduction process. Because of the thinness of the GO sheets and finite carrier density, the electric field is partially screened in the reduced GO, which is manifested in the EFM phase signals. We found inhomogeneous oxidation states in as-prepared GO sheets and followed the evolution of reduction process in the individual GO sheets during both thermal and chemical reduction. We also compared the EFM measurement results with simultaneous IV characteristics to assess correlations between two measurements.

Variations in structure and emission characteristics of nanostructured carbon films prepared by HFCVD method due to the addition of ammonia in source gases

We investigated the structural variation and the concomitant change in the emission characteristics of HFCVD-grown nanostructured carbon films induced by the addition of ammonia to source gases; nanostructured films were the mixture of nanotubes and nanoclusters. As the concentration of ammonia was increased, large structural variation, including the increase of disorder in the atomic bonding and the decrease in size of graphitic sheets making up nanotubes and nanoparticles, was observed. Moreover, changes in the emission properties corresponding to the structural degradation was observed.