Tula Jutarosaga

Nitrogen – Doped SnO2 Thin Films Prepared by Direct Current Magnetron Sputtering

Nitrogen - doped tin oxide (N-doped SnO2) thin films were prepared on unheated glass substrate by dc magnetron sputtering of a Sn target in gas mixtures of O2 and N2. The N2 flow rates were varied from 0 to 15 SCCM with the same working pressure of 1×10-2 Torr. The as-deposited films were annealed in vacuum at 400 °C for 1 h. The films structure, electrical properties and optical properties were characterized by X-ray diffraction (XRD), 4-point probe and Hall effect measurement and portable fiber optic UV-vis spectrometer, respectively. The observed XRD patterns of films showed preferred (101) orientation of the SnO2 tetragonal structure. The average crystalline size of the (101) diffraction peak decreased from 5.10 to 4.07 nm with N2 flow rate increased. Hall measurement indicated that resistivity increased and carrier concentrations decreased as N2 flow rate increased. The carrier concentrations decreased because N atoms substituted oxygen atom in SnO2 lattice. The N atoms may forms acceptor level in SnO2 band gap resulting in hole generation. The electron concentration from intrinsic defect were neutralized with the hole concentration. The carrier concentration decreased from 3.42×1017 cm-3 for un-doped SnO2 to the order of 1014 cm-3. The average percent transmittance of un-doped SnO2 of about 77.5% in visible range (400-700 nm) decreased to 60% with increasing N2 flow rate. The optical band gap decreased from 3.64 eV for un-doped SnO2 to 3.45 eV for N-doped SnO2 films.

Effect of Curvature-Induced Superlattice Structures on Energy Band Structures of Helically Coiled Carbon Nanotubes

We studied electronic properties of a two-dimensional electron gas confined on the surface of helical curved tube, specifically, curvature induced by the geometry of helically coiled carbon nanotubes. The curvature generates two types of effective couplings that are (i) Penney-Kronig potential and (ii) a periodic spin-orbit potential. The energy-momentum dispersion relations are theoretically calculated using new variables to explain the electron spiraling around the circular helix line in counterclockwise and clockwise directions. Then we have to set up the formalism for dealing with a periodic potential as Bloch’s theorem. For calculation, we rewrite the Dirac equation in curved space-time in the form of a block matrix equation. Our numerical results demonstrate that there are pseudo-Landau quantization levels and the motion of an electron in circumference direction coupling with the arc length motion coupling. In addition, the one-dimensional dispersion relation lines present characteristics of time reversal symmetry and inversion symmetry. Finally, we also investigated the differential energy of band structures for the case where the periodic spin-orbit potential increased with respect to the increase of curvature. We found that the non-linearly increase of the differential energy was due to the periodic spin-orbit potential, corresponding to Zeeman effect.

Curvature effect on polarization of light emitted from chiral carbon nanotubes

We investigate that effect of the curvature on induced hybridization and modification of emission profiles for each chirals index single-wall carbon nanotubes (SWCNTs). According to the Schwinger two particle pair state method, we provide an analytical expression by calculating polar of spot intensity as a function of the polar angle. The emission profiles for indirect transition have an asymmetric shape as a function of the electron wave vector of the axis direction kt and depend on the chiral index. Here we show polarization-dependent, given analytically by expanding the matrix element into the scalar product of the light polarization vector and the dipole vector. These scalar products having a maximum value depend on the summation of phase factors of spinors of electrons in the conduction band Φc and valence band Φv. In the case of direct transition, dipole vector tube axis is maximum at the phase summation of Φc and Φv is 0 or 2π. In contrast, the maximum dipole vector circumference is obtained at the phase summation of π for the case of indirect transition. We can predict a strong emission peak and emission profiles which can be used to identify optical transitions in an individual SWCNT with different chiral indices experimentally.

Highly Sensitive H2 Sensors Based on Pd- and PdO-Decorated TiO2 Thin Films at Low-Temperature Operation

Abstract In this work, the low-temperature H2-sensing properties of palladium (Pd) and palladium oxide (PdO) nanoparticles decorated titanium dioxide (TiO2) thin film were studied. The TiO2 thin films were prepared by the dc reactive magnetron sputtering. The Pd and PdO nanoparticles were sputtered on the top surface of TiO2 surface in order to enhance the sensitivity to the H2 gas. Morphologies, crystal structures, and chemical element of the examiner samples were investigated by the field-emission scanning electron microscopy (FE-SEM), grazing-incident X-ray diffraction (GIXRD), and X-ray photoelectron spectroscopy (XPS), respectively. The effects of the Pd and PdO nanoparticles on H2-sensing performance of TiO2 were investigated over a low concentration range of 150-3,000 ppm H2 at 50-250°C-operating temperatures. This result exhibited that the PdO decorated on TiO2 surface showed very high response to H2 at a low operating temperature of 150°C.