Deok Hyeon Kim

Tunable blue-green-emitting wurtzite ZnS:Mg nanosheet-assembled hierarchical spheres for near-UV white LEDs

Mg-doped ZnS hierarchical spheres have been synthesized via hydrothermal method using mixed solvents of ethylenediamine and DI water without any surface-active agent. The surface morphology and microstructure studies revealed that the hierarchical spheres were consisted of many well-aligned nanosheets with width 10 nm and length about 50 ~ 100 nm. X-Ray diffraction results show that the ZnS:Mg hierarchical spheres have wurtzite structure with high crystallinity. The absorption edge in the diffuse reflection spectra shifts towards lower wavelength with increasing Mg concentration, indicating an expansion in the bandgap energy that is estimated to be in the range of 3.28 to 3.47 eV. Blue-green photoluminescence with tunable intensity and peak position was observed depending on the Mg content. The Mg2+-activated ZnS phosphor can be good candidates for blue-green components in near-UV white light-emitting diodes.

Growth of Textured CoFe2O4 Thin Films on Platinized Silicon Prepared by a Sol-Gel Method

We fabricated textured polycrystalline CoFe2O4 thin films on Pt(111)/TiO2/SiO2/Si substrate through a sol-gel method. We varied the thickness of the films, by using precursor solutions with different concentrations of 0.1, 0.2, and 0.3 M, and by depositing 5, 8, or 10 layers on the substrate by spin-coating. X-ray diffraction spectra indicated that when the precursor concentration of the solution was higher than 0.1 M, the spin-coated films were preferentially oriented in the direction. Inspection of the surface morphology by scanning electron microscopy revealed that CoFe2O4 thin films prepared with 0.2 M solution and 5-time spin-coatings had smoother surface, as compared to the other conditions. Each coating had an average thickness of about 50 nm. The magnetic properties measured by vibrating sample magnetometer showed magnetic anisotropy, as evidenced from the difference in the in-plane and out-of-plane hysteresis loops, which we attributed to the textured orientation of the CoFe2O4 thin films.

Soft X-ray absorption spectroscopy study of multiferroic Bi-substituted Ba1−xBixTi0.9Fe0.1O3

The electronic structures of multiferroic oxides of Ba1−xBixTi0.9Fe0.1O3 (0 ≤ x ≤ 0.12) have been investigated by employing photoemission spectroscopy and soft X-ray absorption spectroscopy (XAS). The measured Fe and Ti 2p XAS spectra show that Ti ions are in the Ti4+ states for all x and that Fe ions are Fe2+-Fe3+ mixed-valent for x > 0. The valence states of Fe ions are found to be nearly trivalent for x=0 and decrease with increasing x from being nearly trivalent (v(Fe) ~ 3) for x = 0 to v(Fe) ~ 2.6 for x = 0.12. The valence states of both Ti and Ba ions do not change for all x ≤ 0.12. Based on the obtained valence states of Fe ions, the electronic and the magnetic properties of Ba1−xBixTi0.9Fe0.1O3 are explored.