Hee Suk Roh

RbBaPO4:Eu2+: a new alternative blue-emitting phosphor for UV-based white light-emitting diodes

A novel blue-emitting phosphor on a phosphate-based host matrix, RbBaPO4:Eu2+ (RBP), was synthesized by a solid-state reaction. This phosphor could be excited efficiently by ultra violet-visible light in the 220–420 nm range to exhibit a highly efficient emission peak in the range of 420–440 nm. To investigate the possibility of commercial use, it was compared with the commercial blue phosphors of BaMgAl10O17:Eu2+ (BAM) and Sr3MgSi2O8:Eu2+ (SMS) to assess the photoluminescence properties. Compared to commercial phosphors, RBP displayed enhanced photoluminescence properties, in this case a high quantum efficiency and excellent thermal stability under near-Ultra Violet (n-UV) excitation. Furthermore, bright white light-emitting diodes (LEDs) were fabricated by integrating the RBP phosphor, along with commercial green/red phosphors, into n-UV LEDs. These results strongly suggest that the newly discovered RBP phosphor can be commercially utilized in UV-based white LEDs.

Size-controlled synthesis of monodispersed mesoporous α-Alumina spheres by a template-free forced hydrolysis method

We demonstrated the formation of monodispersed spherical aluminum hydrous oxide precursors with tunable sizes by controlling the variables of a forced hydrolysis method. The particle sizes of aluminum hydrous oxide precursors were strongly dependent on the molar ratio of the Al(3+) reactants (sulfates and nitrates). In addition, the systematic phase and morphological evolutions from aluminum hydrous oxide to γ-alumina (Al(2)O(3)) and finally to α-Al(2)O(3) through thermal dehydrogenation were characterized by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). After annealing the amorphous aluminum hydrous oxide in air at 900 °C and 1100 °C for 1 h, we observed complete conversion to phase-pure γ- and α-Al(2)O(3), respectively, while maintaining monodispersity (125 nm, 195 nm, 320 nm, and 430 nm diameters were observed). Furthermore, both γ- and α-Al(2)O(3) were found to be mesoporous in structure, providing enhanced specific surface areas of 102 and 76 m(2) g(-1), respectively, based on the Brunauer-Emmett-Teller (BET) measurement.

Template-free synthesis of monodispersed Y3Al5O12:Ce3+ nanosphere phosphor

Monodispersed Ce3+-doped Y3Al5O12 (YAG:Ce3+) nanospheres were synthesized through forced hydrolysis using a urea method, followed by thermal calcination processes, and their luminescence properties were examined. The crystallization of the YAG:Ce3+ phase and morphological evolutions after subsequent heat treatment were characterized by X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). Energy dispersive spectroscopy (EDS) analysis revealed that the amorphous aluminum oxide layer played an important role in preventing necking between the particles during heat treatment. As a result, stand-alone YAG:Ce3+ nanospheres with an amorphous aluminum oxide layer shell were synthesized while maintaining monodispersity with an average particle diameter of about 33 nm. These nanospheres had a dense structure and smooth surface with relatively good crystallinity after annealing at 1075 °C. They absorbed light efficiently in the visible region of 400–500 nm, and showed a single broadband emission peak at 536 nm with a luminescence quantum efficiency (QE) of 33% and relatively good photostability.

γ-Al2O3 nanospheres-directed synthesis of monodispersed BaAl2O4:Eu2+ nanosphere phosphors

Monodispersed BaAl2O4:Eu2+ nanospheres with 180 nm size were synthesized through forced hydrolysis using γ-Al2O3 nanospheres as a template followed by a subsequent heat treatment. The incorporation of a barium precursor onto an individual γ-Al2O3 template nanosphere was optimized by controlling the reaction time. The photoluminescence properties of the BaAl2O4:Eu2+ nanospheres were comparable to those of the bulk counterpart prepared at 1300 °C through a conventional solid-state reaction method.