Sangmoon Park

Defect structure of the high-dielectric-constant perovskite CaCu{sub 3}Ti{sub 4}O{sub 12}

Using transmission electron microscopy (TEM) we studied CaCu{sub 3}Ti{sub 4}O{sub 12}, an intriguing material that exhibits a huge dielectric response, up to kilohertz frequencies, over a wide range of temperature. Neither in single crystals, nor in polycrystalline samples, including sintered bulk and thin films, did we observe the twin domains suggested in the literature. Nevertheless, in the single crystals, we saw a very high density of dislocations with a Burger vector of [110], as well as regions with cation disorder and planar defects with a displacement vector (1/4)[110]. In the polycrystalline samples, we observed many grain boundaries with oxygen deficiency, in comparison with the grain interior. The defect-related structural disorders and inhomogeneity, serving as an internal barrier layer capacitance in a semiconducting matrix, might explain the very large dielectric response of the material. Our TEM study of the structure defects in CaCu{sub 3}Ti{sub 4}O{sub 12} supports a recently proposed morphological model with percolating conducting regions and blocking regions.

EuKNaTaO5: crystal growth, structure and photoluminescence property.

The synthesis, structure determination, and intrinsic luminescent properties of a new tantalate, EuKNaTaO(5), are reported. Single crystals of the title compound were grown out of a reactive KOH/NaOH eutectic melt. Luminescence in many materials is activated by doping a rare earth cation into the solid matrix; we report undoped, all-rare earth EuKNaTaO(5), which exhibits bright orange room temperature photoluminescence.

Near UV excited line and broad band photoluminescence of an anion-ordered oxyfluoride.

A near UV excitable phosphor Sr(2.85)Eu(0.1)Al(0.9)In(0.1)O(4-alpha)F(1-delta) was made by reducing as-made Sr(2.85)Eu(0.1)Al(0.9)In(0.1)O(4)F. This material reveals defect-induced broad-band photoluminescence emissions centered at 600 nm and a line emission at 619 nm due to the(5)D(0) --> (7)F(2) transition of the Eu activator (curve 1). Such combined line and broad-band emitters have the potential when combined with another broad-band emitting phosphor with emissions near 500 nm to create white light by converting near-UV light from a Ga(1-x)In(x)N light-emitting device.

NO2 detection by adsorption induced work function changes in In2O3 thin films

A potentiometric sensor platform has been used for NO2 detection at room temperature by measuring the adsorption induced surface work function changes in In2O3 thin films deposited on Si using a solution based process. The highly resistive films were unsuitable for amperometric detection of NO2; however, significant work function changes of ∼30mV were measured for ∼50s of exposure to 70ppm NO2, and detection down to 600ppb (parts per 109) was possible. A model for the transient response was developed, which satisfactorily fits the experimental data. Acceleration of the desorption transient under ultraviolet illumination has also been investigated.

NOVEL SYNTHESIS AND HIGH PRESSURE BEHAVIOR OF NA0.3COO2X1.3H2O AND RELATED PHASES

We have prepared powder samples of N x CoO 2 .yH 2 O using an alternative synthesis route. Superconductivity was observed in Na 0 . 3 CoO 2 .1.3H 2 O between 4 and 5 K as indicated by the magnetic susceptibility. The bulk compressibilities of Na 0 . 3 CoO 2 .1.3H 2 O, Na 0 . 3 CoO 2 .0.6H 2 O, and Na 0 . 3 CoO 2 were determined using a diamond anvil cell and synchrotron powder diffraction. Chemical changes occurring under pressure when using different pressure-transmitting media are discussed and further transport measurements are advocated.

Crystal growth of two new photoluminescent oxides: Sr3Li6Nb2O11 and Sr3Li6Ta2O11.

Single crystals of Sr3Li6M2O11 (M = Nb, Ta) were grown out of a high-temperature Sr(OH)2/LiOH/KOH flux. The single crystal X-ray diffraction data were indexed to the orthorhombic Pmma system, with a = 10.5834(15) A, b = 8.3103(13) A, c = 5.8277(8) A, V = 512.55(13) A(3), and Z = 2 for Sr3Li6Nb2O11 and a = 10.5936(6) A, b = 8.3452(5) A, c = 5.8271(4) A, V = 515.15(6) A(3), and Z = 2 for Sr3Li6Ta2O11. The crystal structure consists of sheets of interconnected SrO8 polyhedra that are separated by M-O layers and an intervening LiO(x) polyhedral framework, representing a new structural type. The M-O layers exhibit a rare occurrence of both five- and six-coordinated M(5+) ions in the same structure. The oxides, upon excitation at 250 nm, exhibit violet emission at room temperature.

Predominant green emission of Ce3+–Tb3+ activated Y7O6F9 phosphors

Ce3+–Tb3+-doped Y7O6F9 vernier phosphors composed of Y7(1−m−n)Ce7mTb7nO6F9 (m = 0.01–0.1, n = 0–0.15) were prepared using a flux-assisted solid-state reaction. The X-ray diffraction patterns of the resultant phosphors were examined to index the peak positions. The photoluminescence (PL) excitation and emission spectra of the Tb3+-activated yttrium-oxyfluoride phosphors were clearly monitored with critical emission quenching as a function of Tb3+ content in Y7(1−n)Tb7nO6F9. After doping the Y7O6F9 structure with Ce3+ and Tb3+ activators, significantly enhanced green emission were observed in the PL spectra under near-ultraviolet (NUV) excitation. The dependence of the luminescent intensity of the Tb3+ co-doped (n = 0, 0.01, 0.05, 0.1, 0.15) host lattices on Ce3+ content (m = 0.01, 0.05) was also investigated. Co-doping Tb3+ into the Ce3+-doped host structure enabled strong energy transfer from Ce3+ to Tb3+; this energy transfer mechanism is discussed. The intense green emission light was compared with that from a commercial green phosphor at room temperature.

Spectral-converting study of Ba((9-3)(m+n)/2)Er(m)Yb(n)Y2Si6O24 (m = 0.005 - 0.2, n = 0 - 0.3) orthosilicate phosphors.

Optical materials composed of Ba((9-3)(m+n)/2)Er(m)Yb(n)Y2Si6O24 (m = 0.005-0.2, n = 0-0.3) were prepared using a solid-state reaction. The X-ray diffraction patterns of the obtained phosphors were examined to index the peak positions. The photoluminescence (PL) excitation and emission spectra of the Er(3+)-activated phosphors and the critical emission quenching as a function of Er(3+) content in the Ba(9-3m/2)Er(m)Y2Si6O24 structure were monitored. The spectral conversion properties of Er(3+) and Er(3+)-Yb(3+) ions doped in Ba9Y2Si6O24 phosphors were elucidated under diode-laser irradiation at 980 nm. Up-conversion emission spectra and the dependence of the emission intensity on pump power for the Ba8.55Er0.1Yb0.2Y2Si6O24 phosphor were investigated. The desired up-conversion of the emitted light, which passed through the green, yellow, orange and red regions of the spectrum, was achieved through the use of appropriate Er(3+) and/or Yb(3+) concentrations in the host structure and 980 nm excitation light. The up-conversion mechanism in the phosphors is described by an energy-level schematic.

Investigation of functionalization layers for NO 2 detection

Functionalization layers are investigated for NO2 detection using a highly sensitive and versatile potentiometric sensor platform for gas and vapor sensing applications. Work function changes were measured for different functionalization layers made of ln2O3 and SnO2 thin films and ZnO nanowires deposited on a Si substrate. A sensitivity of ~0.36 mV/sec for surface work function change was found out for 70 ppm NO2 concentration for isolated clusters of ZnO nanowires indicating that this technique is applicable even for nano-clusters of sensing materials where amperometric detection is impossible due to material discontinuity. A surface work function change of ~10 mV was observed in sub-ppm regime (600 ppb NO2) for highly insulating ln2O3 (resistivity >1 TOmega / ). Graphene layers and nanocrystalline graphite (NG) were compared with the transition oxide based functionalization layer for sensing sub-ppm NO2 (600 ppb) sensing. It was observed that NG shows much higher sensitivity and lower response times.