Highly-tolerant distributed Bragg reflectors (DBRs) based on the same materials consisting of nanoporous/dense titanium dioxide (TiO2) film pair structures with wide-angle and broadband highly-reflective properties at visible wavelengths are reported. For a high refractive index contrast, the two dense and nanoporous TiO2 film stacks are alternatingly deposited on silicon (Si) substrates by a oblique angle deposition (OAD) method at two vapor flux angles (θα) of 0 and 80° for high and low refractive indices, respectively. For the TiO2 DBRs at a center wavelength (λ(c)) of 540 nm, the maximum level in reflectance (R) band is increased with increasing the number of pairs, exhibiting high R values of > 90% for 5 pairs, and the normalized stop bandwidth (∆λ/λ(c)) of ~17.8% is obtained. At λ(c) = 540 nm, the patterned TiO2 DBR with 5 pairs shows an uniform relative reflectivity over a whole surface of 3 inch-sized Si wafer and a large-scalable fabrication capability with any features. The angle-dependent reflectance characteristics of TiO2 DBR at λ(c) = 540 nm are also studied at incident angles (θ(inc)) of 20-70° for p-, s-, and non-polarized lights in the wavelength region of 350-750 nm, yielding high R values of > 70.4% at θ(inc) values of 20-70° for non-polarized light. By adjusting the λ(c)/4 thicknesses of nanoporous and dense films, for λ(c) = 450, 540, and 680 nm, tunable broadband TiO2 DBRs with high R values of > 90% at wavelengths of 400-800 nm are realized.
Color-tunable CaWO4:xDy3+ phosphors were prepared via a simple conventional solid-state reaction and their luminescent properties were investigated as a function of Dy3+ ion concentration under 258 and 353 nm excitations. A gradual enhancement in the emission intensity was observed with the increment of Dy3+ ion concentration, reaching its maximum value when x = 0.05. The main reason for the concentration quenching of Dy3+ emission in CaWO4 host lattice is due to the electric dipole-dipole interaction. The cathodoluminescence spectra, which were measured at different accelerating voltages and filament currents, were in consistent with the photoluminescence spectrum excited at 258 nm. Additionally, the emission color of CaWO4:xDy3+ phosphors can be suitably tuned from blue to green, and finally to yellow by the modulation of excitation wavelength and Dy3+ ion concentration. Ultimately, these color-tunable phosphors may have potential applications in the fields of miniature color displays.
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