Co-precipitation synthesis and photoluminescence properties of BaTiF6:Mn4+: an efficient red phosphor for warm white LEDs

BaTiF6:Mn4+: a fluoride-based red phosphor with a sharp emission line and broad absorption in the blue region used for warm WLEDs.

TWO-STEP ELECTRON BEAM EXCITATION OF Al2O3:Cr BY GALLIUM NITRIDE RECOMBINATION RADIATION

We report on a two-step excitation process of a GaN/A 2 O 3: Cr specimen by an external electron beam of 30 keV. The epitaxial growth of GaN on the (0001) faces of ruby is described, as well as the experimental technique applied in the excitation and recording of the combined luminescence spectrum. It is known that the spinel and ruby are frequently used as substrate materials for the epitaxial deposition of GaN. At the same time is ruby one of the most useful layer materials with a red emission line λ = 694 nm. Due to its dielectric character, the pumping of ruby is performed usually subjecting it to the light of an intense flash lamp. Electron beam excitation, on the other hand, which could be more powerful, is impaired with electrical insulator material. This discrepancy can be removed by a two-step excitation, where at first the semiconducting GaN-layer, epitaxially grown on a (0001)Al 2 O 3: Cr (ruby) crystal is excited to radiation emission by an external electron beam. The following internal absolption process, by the ruby crystal, of the GaN luminescence radiation provides for the characteristic 2 E to 4 A 2-transition at 1.786 eV. We found a strong and sharp emission line of that photon energy, which additionally displays polarization, typical for the anisotropic uniaxial ruby crystal. Such an indirect electron-beam excitation of ruby allows to generate almost monochromatic red light, and might point into a direction of interesting practical applications.

Fluorescence from Coated Oxide Nanoparticles

ABSTRACTIn many cases, coated nanoparticles behave like isolated ones. Using the microwave plasma process, it is possible to produce oxide nanoparticles with ceramic or polymer coating. Coating the particles has the additional advantage that by proper selection of the coating it is possible to suspend the particles in distilled water without using any colloid stabilizer. From quantum dots made of sulfides or selenides, it is well known from literature that fluorescence depends strongly on the coating of the kernels. In the case of CdSe, the kernels are coated with CdS. Within this study, similar phenomena are found with coated oxide nanoparticles having sizes of ca. 6 nm exhibiting a very narrow particle size distribution. The coating consists of a second ceramic phase or a polymer one, each one influencing fluorescence differently. Obviously, the type of coating is a tool to modify fluorescence. This behavior is demonstrated on kernels made of Al2O3, ZrO2, HfO2, ZnO etc. PMMA, PTFE, or Al2O3were used as coating material. In most cases, the fluorescence spectra showed broad bands. In some cases, such as ZnO, additionally, a sharp emission line in the UV appears. It is interesting to note that coatings made of fluorine containing polymer materials did not lead to fluorescence intensities comparable with PMMA coatings. The observed spectra are equivalent whether the powder is in aqueous suspensions or dry on a quartz glass carrier. The experimental results in this study indicate that the combination of non-fluorescent oxide core with a non-fluorescent polymer coating may lead to a nanocomposite with strong fluorescence. This is a phenomenon not described in literature until now.

Observation of mass loss in R CrB during the visual light minimum

R CrB at minimum light shows three type of spectra: 1) an absorption line spectrum similar to that observed at maximum light; 2) a sharp emission line spectrum mainly due to singly ionised metals which is displaced to the blue by 3 to 10 kms-1 with reference to the absorption spectrum observed at maximum. This is supposed to be a kind of permanent feature which exists even at the time of light maximum; 3) a broad emission line spectrum consisting of mainly λ3889 of Hel, H and K lines of Call and the D lines of Nal. The widths of the broad emission features indicate expansion velocity of 200 kms-1, much greater than the escape velocity (Gaposchkin 1963, Rao 1975).