Cheol Jin Lee

Field emission from well-aligned zinc oxide nanowires grown at low temperature

Field electron emission from vertically well-aligned zinc oxide (ZnO) nanowires, which were grown by the vapor deposition method at a low temperature of 550 °C, was investigated. The high-purity ZnO nanowires showed a single crystalline wurtzite structure. The turn-on voltage for the ZnO nanowires was found to be about 6.0 V/μm at current density of 0.1 μA/cm2. The emission current density from the ZnO nanowires reached 1 mA/cm2 at a bias field of 11.0 V/μm, which could give sufficient brightness as a field emitter in a flat panel display. Therefore, the well-aligned ZnO nanowires grown at such low temperature can promise the application of a glass-sealed flat panel display in a near future.

Low temperature growth and photoluminescence of well-aligned zinc oxide nanowires

Abstract Well-aligned single-crystalline zinc oxide (ZnO) nanowires with high density were successfully synthesized on nickel monoxide (NiO) catalyzed alumina substrate through a simple metal–vapor deposition method at an extremely low temperature (450 °C). The single-crystalline ZnO nanowires had a hexagonal wurzite structure and diameters of about 55 nm, and lengths up to 2.6 μm. The photoluminescence spectra under excitation 325 nm showed a ultra-violet (UV) emission at 3.26 eV and a green emission at 2.44 eV. The UV emission and green emission bands were attributed to near band-edge transition and radial combination of a singly ionized oxygen vacancy with a photo-induced hole, respectively.

Synthesis of aligned carbon nanotubes using thermal chemical vapor deposition

Aligned carbon nanotubes have been synthesized on transition metal-coated silicon substrates with C2H2 using thermal chemical vapor deposition. It was found that nanotubes can be mostly vertically aligned on a large area of plain Si substrates when the density of metal domains reaches a certain value. Pretreatment of Co–Ni alloy by HF dipping and etching with NH3 gas prior to the synthesis is crucial for vertical alignment. Steric hindrance between nanotubes at an initial stage of growth forces nanotubes to align vertically. Nanotubes are grown by a catalyst-cap growth mechanism. Applications to field emission displays are demonstrated with emission patterns.

Structural study of nitrogen-doping effects in bamboo-shaped multiwalled carbon nanotubes

We have investigated nitrogen doping effects on the structure and crystallinity of bamboo-shaped multiwalled carbon nanotubes (BS-MWNTs) by means of x-ray photoemission spectroscopy (XPS) and transmission electron microscopy. By controlling the NH3/C2H2 flow ratio during the chemical vapor deposition, the nitrogen concentrations of 0.4% to 2.4% were obtained. According to the XPS measurements, the increasing nitrogen concentration gave rise to an increase of the N-sp3 C bonds as well as the deterioration of the crystallinity of the BS-MWNTs. Besides, the N-sp3 C bonds were found to prevail over the N-sp2 C bonds above 5% nitrogen concentration. At higher nitrogen concentrations, the BS-MWNTs showed shorter compartment distances, presumably due to the suppressed surface diffusion of carbon on the catalyst particles.

Growth model of bamboo-shaped carbon nanotubes by thermal chemical vapor deposition

Vertically aligned carbon nanotubes were grown on iron-deposited silicon oxide substrate by thermal chemical vapor deposition of acetylene. The carbon nanotubes have no encapsulated iron particles at the closed tip and a bamboo structure in which the curvature of compartment layers is directed to the tip. A base growth model is suggested for the bamboo-shaped carbon nanotubes grown under our experimental conditions.

Temperature effect on the growth of carbon nanotubes using thermal chemical vapor deposition

Abstract Vertically aligned carbon nanotubes (CNTs) are grown on iron-deposited silicon oxide substrates by thermal chemical vapor deposition (CVD) of acetylene gas at the temperature range 750–950°C. As the growth temperature increases from 750°C to 950°C, the growth rate increases by four times and the average diameter also increases from 30 nm to 130 nm while the density decreases by a factor of about two. The relative amount of crystalline graphitic sheets increases progressively with the growth temperature and a higher degree of crystalline perfection can be achieved at 950°C. This result demonstrates that the growth rate, diameter, density, and crystallinity of CNT can be controlled with the growth temperature.

Catalyst effect on carbon nanotubes synthesized by thermal chemical vapor deposition

Abstract The catalyst effect on the synthesis of carbon nanotubes (CNTs) using thermal chemical vapor deposition (CVD) was investigated. The respective growth rate of CNTs shows that the performance of catalysts is in the order of nickel ( Ni )> cobalt ( Co )> iron (Fe). The average diameter of CNTs follows the sequence of Fe, Co, and Ni catalysts. The structure of CNTs reveals almost same morphology regardless of catalyst but the crystallinity of CNTs is largely dependent on catalyst. The crystallinity of CNTs synthesized from Fe catalyst is higher than that from Ni or Co catalyst. The results indicate that the growth rate, the diameter, and the crystallinity can be manipulated by the selection of the catalyst.

Low-temperature growth and Raman scattering study of vertically aligned ZnO nanowires on Si substrate

High-density ZnO nanowires (ZnONWs) were aligned onto Au-catalyzed Si substrate through a simple low-temperature physical vapor deposition method. Scanning electron microscope (SEM) observations, x-ray diffraction (XRD) analysis, and photoluminescence spectra showed that the ZnONWs were single-crystalline, with a hexagonal wurzite structure. All of the results inferred from the SEM observations, the XRD rocking curves, and the Raman spectra for the investigated samples confirm that the ZnONWs are well aligned and c-axis oriented. The Raman spectra also indicated that the ZnONWs on Si substrates are under the biaxial compressive stress. Since it takes the advantage of low-cost, easily controlled deposition spot (due to the selective deposition trait of the Au layer), potential for scale-up production, and ability to integrate with Si substrate, this technique has a potential in future for fabricating the ZnONW array-based optoelectronic devices.

Large-area atomically thin MoS2 nanosheets prepared using electrochemical exfoliation

Molybdenum disulfide (MoS2) is an extremely intriguing material because of its unique electrical and optical properties. The preparation of large-area and high-quality MoS2 nanosheets is an important step in a wide range of applications. This study demonstrates that monolayer and few-layer MoS2 nanosheets can be obtained from electrochemical exfoliation of bulk MoS2 crystals. The lateral size of the exfoliated MoS2 nanosheets is in the 5-50 μm range, which is much larger than that of chemically or liquid-phase exfoliated MoS2 nanosheets. The MoS2 nanosheets undergo low levels of oxidation during electrochemical exfoliation. In addition, microscopic and spectroscopic characterizations indicate that the exfoliated MoS2 nanosheets are of high quality and have an intrinsic structure. A back-gate field-effect transistor was fabricated using an exfoliated monolayer MoS2 nanosheet. The on/off current ratio is over 10(6), and the field-effect mobility is approximately 1.2 cm(2) V(-1) s(-1); these values are comparable to the results for micromechanically exfoliated MoS2 nanosheets. The electrochemical exfoliation method is simple and scalable, and it can be applied to exfoliate other transition metal dichalcogenides.

Electrical transport properties of individual gallium nitride nanowires synthesized by chemical-vapor-deposition

We have synthesized high-quality gallium nitride (GaN) nanowires by a chemical-vapor-deposition method and studied the electrical transport properties. The electrical measurements on individual GaN nanowires show a pronounced n-type field effect due to nitrogen vacancies in the whole measured temperature ranges. The n-type gate response and the temperature dependence of the current–voltage characteristics could be understood by the band bending at the interface of the metal electrode and GaN wire. The estimated electron mobility from the gate modulation characteristics is about 2.15 cm2/V s at room temperature, suggesting the diffusive nature of electron transport in the nanowires.