Changwook Kim

Experimental and theoretical studies on the structure of N-doped carbon nanotubes: Possibility of intercalated molecular N2

The concentration distribution and electronic structure of N atoms doped in multiwalled banboo-like carbon nanotubes (CNTs) are examined by photon energy-dependent x-ray photoelectron spectroscopy and x-ray absorption near edge structure. The inner part of the nanotube wall has a higher N concentration and contains molecular N2 presumably intercalated between the graphite layers. These results are supported by the self-consistent charge-density-functional-based tight-binding calculation of double-walled CNTs, showing that the intercalation of N2 is energetically possible and the graphite-like N structure conformer becomes more stable when the inner wall is more heavily doped.

Effect of electric field on the electronic structures of carbon nanotubes

We have investigated the electronic structures of a capped single-walled carbon nanotube under the applied electric field using density functional calculations. The capped tube withstands field strengths up to 2 V/A. When the electric field is applied along the tube axis, charges are transferred from the occupied levels localized at the top pentagon of the cap, and not from the highest occupied level localized at the side pentagon, to the unoccupied levels. We find that the charge densities at the top of the armchair cap show two- or five-lobed patterns depending on the field strength, whereas those of the zigzag cap show a three-lobed pattern. The interpretation for the images of the field emission microscope is also discussed.

First-principles study on secondary electron emission of MgO surface

We theoretically investigate secondary-electron-emission properties of MgO when noble gases are incident on the surface. We consider both potential and kinetic emission mechanisms. For the potential emission through Auger neutralization, densities of states and vacuum level are obtained from the first-principles calculations. It is found that secondary-emission coefficients decrease in the following sequence of surface directions; (111)-OH>(100)>(110), a tendency that is in agreement with experimental observations. For a surface model including F center, the secondary-emission coefficient substantially increases for Kr and Xe. To investigate the kinetic emission mechanism by an energetic ion impinging on MgO surfaces, first-principles molecular dynamics simulations are performed. Dynamic up-shifts of antibonding states between ions and oxygen atoms are found to lead to the secondary-electron emission at kinetic energies as low as 30 eV. Various collision conditions are compared based on the temporal interval during which excited states stay within the conduction band.We theoretically investigate secondary-electron-emission properties of MgO when noble gases are incident on the surface. We consider both potential and kinetic emission mechanisms. For the potential emission through Auger neutralization, densities of states and vacuum level are obtained from the first-principles calculations. It is found that secondary-emission coefficients decrease in the following sequence of surface directions; (111)-OH>(100)>(110), a tendency that is in agreement with experimental observations. For a surface model including F center, the secondary-emission coefficient substantially increases for Kr and Xe. To investigate the kinetic emission mechanism by an energetic ion impinging on MgO surfaces, first-principles molecular dynamics simulations are performed. Dynamic up-shifts of antibonding states between ions and oxygen atoms are found to lead to the secondary-electron emission at kinetic energies as low as 30 eV. Various collision conditions are compared based on the temporal inter...

Molecular dynamics study on low-energy sputtering properties of MgO surfaces

In an effort to understand microscopic processes occurring between MgO protective layers and impinging plasma ions in a discharge cell of plasma-display panel, sputtering properties of MgO(100) surface by He, Ne, and Xe atoms are studied with molecular dynamics simulations. Interatomic potentials between constituent atoms are fitted to first-principles data sets for representative configurations. Various incident directions of ions are considered with kinetic energies under 100eV. It is found that sputtering yields for the Ne atom are largest among tested noble gases. The angle dependence of sputtering yields indicates that (111)-oriented MgO films are much more vulnerable to ion attacks than (100)-oriented layers. A surface model including the monolayer step is also studied and it is found that the yields increase substantially for grazing-angle incidence.

Field Emission and Lifetime Characteristics of Titanium-coated Carbon Nanotubes

We investigated the effect of titanium (Ti)-coated carbon nanotubes (CNTs) theoretically and experimentally. We found that adsorption of single Ti atom lowers the work function of CNTs by density functional calculations. Also, Ti-coated CNTs showed the largest increase in local density of states around Fermi level under electric fields. Coating of Ti metal on CNTs was carried out by electroless plating method. Ti-coated CNTs were mixed with conductive pastes, and then screen-printed. The measurement of field emission carried out using a diode structure showed that the electron emission of Ti-coated CNT films uniformly had a field of 3.7 V/mum at a current density of 100 mu/cm2 (1/500 duty), as compare to bare CNT films showed a field of 5.5 V/mum. Furthermore, the lifetime of our CNT samples was about a few times as much as that of the pristine ones. Therefore, Ti-coated CNTs improved the characteristics of CNT-based field emission emitters

Growth of Hexagonal Gallium Nitride Films On The (111) Surfaces of Silicon with Zinc Oxide Buffer Layers

The growth of gallium nitride films on sapphire substrates has not been straightforward because of the large lattice mismatch between gallium nitride and sapphire. Zinc oxide is structurally the closest material to gallium nitride and therefore is finding use as the substrate for gallium nitride. Single crystal wafers of zinc oxide are hard to obtain and very expensive. However, a thin layer of zinc oxide on a suitable substrate might solve this problem. In this work, highly c -axis oriented zinc oxide buffer layers were grown on Si(lll) substrates at temperatures 410–540 °C by chemical vapor deposition of bis(2,2,6,6-tetramethyl–3,5-heptanedionato)zinc, Zn(tmhd) 2 , and the hexagonal GaN films were subsequently deposited on them at 500 °C using the single precursor tris(diethyl -μ-amido-gallium), [(C 2 H 5 ) 2 GaNH 2 ] 3 . The compound Zn(tmhd) 2 was found to require oxygen for the deposition of zinc oxide. In the case of gallium nitride, low pressure chemical vapor deposition of tris(diethyl-μ-amido-gallium) worked reasonably well with or without a carrier gas. The buffer layers and the GaN films were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM), and reflection high energy elctron diffraction (RHEED).