Dong Jae Bae

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.

Hydrogen adsorption and storage in carbon nanotubes

A comprehensive studies on hydrogen adsorption and storage in carbon nanotubes CNTs have been done both experimentally and theoretically. Hydrogen atoms have been stored electrochemically in CNTs. We find that hydrogens exist as a form of H molecule in an 2 empty space inside CNTs, which was confirmed by Raman spectra. Several adsorption sites inron CNTs are observed during the discharging process. We perform density-functional-based tight-binding calculations to search for adsorption sites and predict maximum hydrogen storage capacity. Our calculations show that the storage capacity of hydrogen, limited by the repulsive forces between H 2 molecules inside nanotubes, increases linearly with tube diameters in single-walled nanotubes, whereas this value is independent of tube . 3 .

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.

Terahertz conductivity of anisotropic single walled carbon nanotube films

Absorption and dispersion of singlewalled carbon nanotube films were measured using an optoelectronic THz beam system for THz time-domain spectroscopy. The anisotropically aligned nanotube films were prepared through simple mechanical squeezing with a bar coater. The angle-dependent absorption and dispersion values were then measured. Results indicate that the index of refraction decreases with increasing frequency (0.1–0.8 THz), whereas the real conductivity increases with increasing frequency. The real conductivity measured is not congruent with the simple Drude model, but it follows a Maxwell–Garnett model, where the nanotubes are embedded in a dielectric host.

X-ray photoemission spectroscopy study of fluorinated single-walled carbon nanotubes

We have investigated the change of atomic and electronic structures of fluorinated single-walled carbon nanotubes (SWCNTs) using x-ray photoemission spectroscopy (XPS), electrical resistivity measurements, and transmission electron microscopy (TEM). The fluorine content increases with increasing reaction temperature up to 300 °C. XPS indicated that the fluorinated SWCNT reveals an ionic-bonding character at low concentration and covalent-bonding character at high concentration. The resistivity increases with reaction temperatures, resulting from the band gap enlargement at high fluorine concentration. It is also observed from TEM that the fluorination at reaction temperature above 250 °C leads to the disintegration of the CNT structures and formation of various phases such as multiwall-like and turbostratic morphologies.

Graphene oxide thin film field effect transistors without reduction

Pristine graphene oxide thin film field effect transistors were fabricated on Si substrates without an additional reduction process. Graphene oxide with an optical band gap of 1.7 eV showed p-type semiconducting behaviour in air and ambipolarity under vacuum. The temperature dependence of conductance confirmed these semiconducting characteristics. I–V characteristics were well fitted to a variable range hopping model with 2 + 3 dimensionality, in good contrast to the 2D fitting in the reduced graphene oxide.

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.

Low pressure synthesis of single-walled carbon nanotubes by arc discharge

High yield of single-walled carbon nanotubes (SWCNTs) has been synthesized by dc arc discharge under low pressure of helium gas with a small amount of a mixture of nickel and iron powders. The highest yield of carbon nanotubes (CNTs) was obtained at low pressure of 100 Torr with 5 wt.% of metal mixture, in good contrast with the previous works. In addition, we introduced sulfur promoter to improve the yield, which gave rise to again the highest yield at low gas pressure. This was attributed to the higher arching temperature at lower gas pressure, facilitating an efficient formation of CNTs.