Seong Chu Lim

High-Capacitance Supercapacitor Using a Nanocomposite Electrode of Single-Walled Carbon Nanotube and Polypyrrole

A nanocomposite electrode of single-walled carbon nanotube (SWNT) and polypyrrole (Ppy) is fabricated to improve the specific capacitance of the supercapacitor. The individual nanotubes and nanoparticles are uniformly coated with Ppy by in situ chemical polymerization of pyrrole. To characterize the SWNT-Ppy nanocomposite electrodes, a charge-discharge cycling test for measuring specific capacitance, cyclic voltammetry, and an ac impedance test are executed. The SWNT-Ppy nanocomposite electrode shows much higher specific capacitance than pure Ppy and as-grown SWNT electrodes, due to the uniformly coated Ppy on the SWNTs. The effects of the conducting agent added in the nanocomposite electrodes on specific capacitance and internal resistance of supercapacitors are also investigated.

Probing graphene grain boundaries with optical microscopy

Grain boundaries in graphene are formed by the joining of islands during the initial growth stage, and these boundaries govern transport properties and related device performance. Although information on the atomic rearrangement at graphene grain boundaries can be obtained using transmission electron microscopy and scanning tunnelling microscopy, large-scale information regarding the distribution of graphene grain boundaries is not easily accessible. Here we use optical microscopy to observe the grain boundaries of large-area graphene (grown on copper foil) directly, without transfer of the graphene. This imaging technique was realized by selectively oxidizing the underlying copper foil through graphene grain boundaries functionalized with O and OH radicals generated by ultraviolet irradiation under moisture-rich ambient conditions: selective diffusion of oxygen radicals through OH-functionalized defect sites was demonstrated by density functional calculations. The sheet resistance of large-area graphene decreased as the graphene grain sizes increased, but no strong correlation with the grain size of the copper was revealed, in contrast to a previous report. Furthermore, the influence of graphene grain boundaries on crack propagation (initialized by bending) and termination was clearly visualized using our technique. Our approach can be used as a simple protocol for evaluating the grain boundaries of other two-dimensional layered structures, such as boron nitride and exfoliated clays.

High yield purification of multiwalled carbon nanotubes by selective oxidation during thermal annealing

Multiwalled carbon nanotubes were synthesized by electric arc discharge method in helium ambient with the pressure of 400 Torr and then purified by thermal annealing. During the annealing in air, the quartz tube in which the raw samples were placed was rotated in order to expose evenly the nanotubes and the carbonaceous particles to the air. The carbonaceous particles were presumably etched away by the selective oxidation with faster etching rate than nanotubes. This gives rise to very high yield of about 40%. It was found from Raman scattering measurements that the ratio of the intensity of G-line peak (1583 cm−1) to that of D-line peak (1285 cm−1) increased drastically by this purification process. Our density-functional tight-binding calculations clearly show that the desorption energy barrier of a C–O pair from the nanotube edge is 2.48 eV, higher than 0.3∼2.1 eV from an amorphous carbon, confirming the current approach of purification by the selective oxidation.

Contact resistance between metal and carbon nanotube interconnects: Effect of work function and wettability

The contact resistance of 14 different electrode metals with the work function between 3.9 and 5.7 eV has been investigated for carbon nanotube (CNT) interconnects. We observed that the contact resistance was mainly influenced by the two following parameters: the wettability and the work function difference of electrode metal to CNT. Ti, Cr, and Fe with good wettability showed lower resistance than other metals. Furthermore, no dependence of the contact resistance on the work function difference has been observed. However, the contact resistance of Au, Pd, and Pt with poor wettability increased as the work function difference became larger.

Effect of surface morphology of Ni thin film on the growth of aligned carbon nanotubes by microwave plasma-enhanced chemical vapor deposition

Aligned carbon nanotubes were synthesized on Ni-coated Si substrates using microwave plasma-enhanced chemical vapor deposition. The surface morphology of Ni thin films was varied with the rf power density during the rf magnetron sputtering process. It was found that the growth of carbon nanotubes was strongly influenced by the surface morphology of Ni thin film. Pure carbon nanotubes were synthesized on Ni thin film with uniformly distributed grain sizes, whereas large amounts of carbonaceous particles were produced in addition to the nanotubes, when the nanotubes were grown on Ni thin film with widely distributed grain sizes. With decreasing Ni-grain size, the diameter of nanotubes decreased and the length increased. High-resolution transmission electron microscope images clearly demonstrated the nanotubes to be multiwalled, and the graphitized structures were confirmed from the Raman spectra. Efficient field emission was observed from the diode structure with the nanotube tips.

Patterned growth and field emission properties of vertically aligned carbon nanotubes

Abstract Vertically aligned carbon nanotubes were grown selectively on patterned Ni thin films by microwave plasma-enhanced chemical vapor deposition and their field emission properties were investigated using a diode-structure. Ni thin films patterned with a form of dot-arrays were prepared using a shadow mask having an array of holes. The nanotubes were found to be well-graphitized with multiwalled structures. The measurements of field emission properties revealed that the carbon nanotube tips emitted high current density at low macroscopic electric field. The Fowler–Nordheim (F–N) plot clearly showed two characteristic regions where the current saturates at the high electric field region. It was found that the saturation behavior was caused by the adsorbates-enhanced field emission mechanism. Eliminating the adsorbates resulted in no saturation behavior, increasing turn-on field, decreasing current, and increasing field enhancement factor. Using ZnS/Cu,Al phosphor, very bright and uniform emission patterns were obtained.

Laser thinning for monolayer graphene formation: heat sink and interference effect.

Despite the availability of large-area graphene synthesized by chemical vapor deposition (CVD), the control of a uniform monolayer graphene remained challenging. Here, we report a method of acquiring monolayer graphene by laser irradiation. The accumulation of heat on graphene by absorbing light, followed by oxidative burning of upper graphene layers, which strongly relies on the wavelength of light and optical parameters of the substrate, was in situ measured by the G-band shift in Raman spectroscopy. The substrate plays a crucial role as a heat sink for the bottom monolayer graphene, resulting in no burning or etching. Oscillatory thinning behavior dependent on the substrate oxide thickness was evaluated by adopting a simple Fresnels equation. This paves the way for future research in utilizing monolayer graphene for high-speed electronic devices.

LARGE-AREA GRAPHENE-BASED FLEXIBLE TRANSPARENT CONDUCTING FILMS

A simple approach by direct synthesis of few-layer graphene sheets and transferring them onto flexible substrate is demonstrated. The hydrogen effect on the D band intensity of Raman spectra for few-layer graphene sheets synthesized by chemical vapor deposition (CVD) on Ni-evaporated silicon substrate is investigated by optimizing the mixing ratio of C2H4/H2. While the Ni etchant is used to melt away the squeezed Ni layers between graphene sheet and silicon substrate, the graphene sheet is transferred onto polyethylene terephthalate film by a fishing method. It is found that the condition of graphene transfer strongly relies on the cooling rate of the film during CVD synthesis. The sheet resistance of the film decreases as the film thickness increases. A sheet resistance of 233 Ω/sq is obtained at a transmittance of 62%.

Dual-catalyst growth of vertically aligned carbon nanotubes at low temperature in thermal chemical vapor deposition

Abstract Vertically aligned carbon nanotubes (CNTs) were successfully synthesized at 550 °C on Ni-coated Si substrate placed parallel to Pd plate as a dual catalyst and a tungsten wire filament. The CNT length increased with increasing gas flow rates even at such a low growth temperature, suggesting that the hydrocarbon gases were sufficiently decomposed in the presence of tungsten wire filament and dual Pd catalyst. The difference of growth behavior from that at high temperature was explained by introducing the Ni–C solid solution, which enables the carbon atoms to diffuse more easily even at low temperature.

FIELD EMISSION AND APPLICATION OF CARBON NANOTUBES

The mechanism of field emission from a metal surface was well explained based on the quantum mechanics in early 20th century. Since then, various materials have been studied for field emitters. However, so far, we have been using only limited materials as a field emitter and an application in some area requires further scientific understandings and technological advancements. In this paper, we review the current status of researches in field emission and emission phenomena of carbon nanotubes (CNTs). This may include current saturation induced by gas adsorbates, screening effects, high current emission, degradation of emitter, and field enhancement factor. We also introduce the present status in the development of various CNT-based field emission devices and discuss their performances. In this part, various potential applications such as field emission display, ionization gauge, X-ray gun, and lamp will be presented.