B.K. Moon

Luminescence characteristics of ZnGa2O4 thin film phosphors grown by pulsed laser deposition

Abstract ZnGa 2 O 4 thin film phosphors have been deposited using a pulsed laser deposition technique on Si(100) substrates at a substrate temperature of 550 °C with various oxygen pressures and post-annealing temperatures. The structural characterization was carried out on a series of ZnGa 2 O 4 films grown at an oxygen pressure range of 50–300 mTorr, and subsequently post-annealed at 600 and 700 °C. The optimum oxygen pressure for luminescent characteristics was about 100 mTorr. The luminescent spectra show a broad band extending from 350 to 600 nm peaking at 460 nm. A post-annealing treatment of ZnGa 2 O 4 thin films led to the different shape of luminescent intensity and grain size. β-Ga 2 O 3 phase appeared at an annealing temperature of 600 °C and increased with increasing annealing temperature. We have also investigated the effect of the ligand field strength on the resultant energy levels for ZnGa 2 O 4 thin film phosphor. The annealing process at different temperatures may lead to different dominant emissions.

Dielectric and ionic conduction properties in LiYF4 single crystals

Abstract A LiYF 4 single crystal was grown by the Czochralski technique in an Ar-gas atmosphere. The complex dielectric constants and complex impedances are measured in the range of 100 Hz–10 MHz and up to 200°C for a c -plate of a LiYF 4 single crystal. The LiYF 4 reveals a strong low frequency dielectric dispersion below 300 kHz, from room temperature to 200°C. It is believed that the strong dielectric dispersion is related to the ionic hopping conduction, which arises mainly from the jumping of Li + -ions between neighboring sites, with an activation energy of Δ=0.49 eV. The conduction mechanism seems to be the non-interacting Debye type and the relaxation times satisfy the Arrhenius relation.

Influence of Bi Excess on Electrical Properties of Bi3.25 + x La0.75Ti3O12+δ Thin Films

The Bi-excess Bi3.25+ x La0.75Ti3O12+δ (x = 0, 0.05, 0.10, 0.15, 0.20, BLT) thin films were prepared by pulsed laser deposition. Single phases of BLT thin films was confirmed by X-ray diffraction. For the small Bi-excess BLT films, ferroelectric properties are improved. One of them, 10 mol% Bi-excess BLT films exhibits high remanent polarization, low leakage current density, and improved switching polarization. Influence of Bi excess on ferroelectric and electrical properties are investigated by ferroelectric P-E hysteresis loops, transient current, and leakage current.

Investigation of 4f7 levels of Eu2+ ions in KMgF3 crystal by two-photon excitation spectroscopy

Intraconfigurational 4f 7 -4f 7 transitions of Eu 2+ in KMgF 3 are investigated by two-photon excitation (TPE) spectroscopy. The emissions due to the 6 P 7/2 → 8 S 7/2 and 6 I 7/2 → 8 S 7/2 transitions are obtained by the two-photon excitation into the 6 D 9/2 at 10 K. It is found that the TPE spectrum of the 6 I 7/2 → 8 S 7/2 emission is different from that of the 6 P 7/2 → 8 S 7/2 emission. The result is discussed in terms of the relative position between the lowest crystal-field component of the 6 I 7/2 level and the lowest 4f 6 5d level of Eu 2+ at the cubic and non-cubic sites in KMgF 3 .

Synthesis and photoluminescence of Eu3 + in Y1−xGdxAlO3 nanophosphors

Europium-doped Y1−xGdxAlO3 perovskite (YAP) nanophosphors were synthesized by a solvothermal method. A YAP phase without impurity was formed from phosphors sintered at 1100 ° C. All the phosphors except YAlO3 can be estimated as mixed cubic and orthorhombic YAP phase. With the increase in Gd, the crystallinity was changed from simple cubic to q mixed phase of cubic and orthorhombic. Due to a 5D0→7F2 forced electric dipole transition of Eu3+, the nanophosphors with two mixed phases emitted radiation of red light with a 614 nm wavelength, whose brightness was dramatically increased by a factor 8 in comparison to that of the cubic perovskite phase. It is ascribed to the enhancement of non-symmetric environment for the Eu3+ in the nanophosphors with two mixed phases. The intensity ratios (IED/MD) between the structural hypersensitive force electric dipole transition, 5D0→7F2, and the intensity of the magnetic dipole transition, 5D0→7F1, are 0.4 for cubic phase and 1.4 for mixed phase.