Detlef Uwe Wiechert

Narrow-band red-emitting Sr[LiAl3N4]:Eu2+ as a next-generation LED-phosphor material

To facilitate the next generation of high-power white-light-emitting diodes (white LEDs), the discovery of more efficient red-emitting phosphor materials is essential. In this regard, the hardly explored compound class of nitridoaluminates affords a new material with superior luminescence properties. Doped with Eu(2+), Sr[LiAl3N4] emerged as a new high-performance narrow-band red-emitting phosphor material, which can efficiently be excited by GaN-based blue LEDs. Owing to the highly efficient red emission at λ(max) ~ 650 nm with a full-width at half-maximum of ~1,180 cm(-1) (~50 nm) that shows only very low thermal quenching (>95% relative to the quantum efficiency at 200 °C), a prototype phosphor-converted LED (pc-LED), employing Sr[LiAl3N4]:Eu(2+) as the red-emitting component, already shows an increase of 14% in luminous efficacy compared with a commercially available high colour rendering index (CRI) LED, together with an excellent colour rendition (R(a)8 = 91, R9 = 57). Therefore, we predict great potential for industrial applications in high-power white pc-LEDs.

Optical characterization of diamond

Abstract The state-of-the-art of major optical techniques to characterize diamond and related carbon-based materials is reviewed. The focus is on Raman spectroscopy, transmission spectroscopy, luminescence spectroscopy and ellipsometry.

Stress in optical waveguides. 2: Fibers

The stress properties of GeO(2)- and F-doped optical fibers drawn in different conditions have been investigated. The results are in excellent agreement with calculated data based on a generalized theoretical model. For constant drawing forces the influence on the stress profiles was found to be independent of drawing speed and temperature. The total observed stress is the sum of preform stress and drawing-induced stress.

Optically Functional Zeolites: Evaluation of UV and VUV Stimulated Photoluminescence Properties of Ce3+- and Tb3+-doped Zeolite X

The optical properties of Tb3+/Ce3+ doped zeolites are elucidated with emphasis on ultraviolet (UV) and vacuum ultraviolet (VUV) excitation and luminescence. Ce3+ sensitized Tb3+ emission with quantum yields of 85 % may be obtained at 330 nm excitation. Low absorptivity at 254 nm due to low Ce3+ concentrations or low Ce3+/Tb3+ ratios, which are required for the suppression of UV components, restricts their applicability as phosphors for Hg-based discharges, e.g., in conventional fluorescent lamps. Near band edge excitation at 172 nm revealed an immediate quantum yield of 50 % enabled by a zeolite Ce3+ (5d1) Tb3+ (4f75d1) energy transfer channel, which may be exploited for the down-conversion of the Xe2 excimer emission.

Raman and X-ray studies of polycrystalline CVD diamond films

Abstract Differently prepared microwave plasma-deposited diamond films with a broad spectrum of morphological and Raman spectroscopic features were investigated by scanning electron microscopy (SEM), Raman spectroscopy and X-ray diffraction (XRD). XRD indicates the presence of graphitic polytypes in most samples, independent of growth conditions or morphology. X-ray texture analysis reveals pronounced fibre textures not only for well-faceted deposits, but also for smooth, fine grain films. Information about internal strain, stacking faults and average crystallite sizes is obtained from diffraction peak shifts and widths. Crystallite sizes deduced from XRD profiles are found to be orders of magnitudes smaller than the grain sizes observed by SEM. An analysis of the diamond Raman peak position, width and shape suggests that Raman peak broadening is dominated by phonon lifetime reduction. Raman and X-ray diffraction data reveal a reciprocal relationship between the width of the diamond Raman line and the crystallite size.

Thermoluminescence spectroscopy of Eu2+ and Mn2+ doped BaMgAl10O17

Abstract Thermoluminescence (TL) studies of Eu2+ and Mn2+ doped BaMgAl10O17 (BAM) are reported and discussed. The TL spectra that are measured after irradiation with ultraviolet (120– 350 nm ) show a series of TL peaks between 100 and 300 K . The TL spectra are similar for BAM with the two dopants, which suggest that the shallow traps are typical for the BAM host lattice. Using the Hoogstraaten analysis trap depths between 0.1 and 0.2 eV are determined. A model is proposed based on thermally activated recombination in local TL centres (not via the conduction band). Further support for this model is obtained from the observation that the TL signal is strongest for excitation around the band edge of BAM (∼180 nm ) . Upon heating the samples in air all low temperature TL peaks decrease in intensity. In addition a new peak appears in the TL spectrum, which is connected with a deeper trap (0.3 eV ) and also a partial oxidation of Eu2+ to Eu3+ is observed. The luminescence efficiency is lower and the UV induced degradation is faster after annealing in air. These results indicate that the shallow traps are related to oxygen vacancies. The shallow traps do not have a negative influence on performance (efficiency and degradation) of BAM as a lighting phosphor. The luminescence efficiency and stability are strongly influenced by the formation of Eu3+ and a deeper trap during annealing in air. Subsequent annealing in a reducing atmosphere restores the original properties.

Optimised co-activated willemite phosphors for application in plasma display panels

AbstractProperties, relevant for PDP application, of Mn2 ‘ -activated Zn2SiO4 phosphors non-co-activatedand co-activatedwith Ba2 ‘ ,Cd2 ‘ ,Fe2 ‘ ,Fe3 ‘ ,Al3 ‘ /Li ‘ or Gd3 ‘ /Li ‘ were investigated. For Zn(1.95)Mn0.05SiO4, co-activation withBa2 ‘ and Gd3 ‘ reduces the q10{value at 170 nm excitation from 17 to 10 ms with e

CVD diamond: a novel high γ-coating for plasma display panels?

ciencies still 86% relative tonon-co-activatedZn2SiO4 :Mn2 ‘ .Theq10{ valueincreasesforco-activationwithFe2 ‘ orhighconcentrationofCd2 ‘ .Decay properties were also studied as a function of excitation wavelength. ( 2000 Elsevier Science B.V. All rightsreserved. Keywords: Zinc silicate; Plasma display panels; Mn2 ‘ luminescence; Co-activators 1. IntroductionPhosphors for PDPs need to emit at least 90% oftheir luminescence within the refresh rate of the displayafter excitation by the emission of a Xe/Ne gas dis-charge at 170 nm. The commercial PDP phosphorZn2SiO4:Mn2 ‘ (P1) has a 10% value q10{of 15 mswhichis too long for TV application. In the past, severalattempts were made to improve on the P1 reference. Itwas shown [1,2] that the q10{value can be reduced byincreasingthe Mn2

High emission current density microwave-plasma-grown carbon nanotube arrays by postdepositional radio-frequency oxygen plasma treatment

Minimization of the firing voltage of plasma display panels requires electrode coatings with high ion-induced secondary electron emission coefficients (γ-coefficients). This paper discusses the methodology of Paschen-curve-based γ-measurements and compares measured γ-coefficients of MgO, the standard material for this application, with data measured for CVD diamond of various thicknesses, a-C:H and ta-C films grown on glass. Ne, Ar and Xe discharges are investigated. The negative electron affinity (NEA) of H-terminated CVD diamond is shown to lead to high γ-values, stable operation and low plasma firing voltages that rival or even surpass those of the best quality MgO(111) single crystals with the advantage of diamond being less sensitive to panel processing conditions. Replacing the H-termination of diamond by positive electron affinity (PEA) O-termination results in a dramatic increase of the firing voltage, thus demonstrating the importance of the electron affinity for ion-induced secondary electron emission. Diamond is found to be particularly interesting for high Xe panel fillings. The γ-coefficients of DLC- and ta-C layers are significantly lower and show considerable instability and degradation over time. The advantages and drawbacks of diamond as a PDP cell coating are discussed.

Blue emitting BaMgAl10O17:Eu with a blue body color

Highly stable field emission current densities of more than 6A∕cm2 along with scalable total field emission currents of ∼300μA per 70μm diameter carbon nanotube (CNT)-covered electron emitter dot are reported. Microwave-plasma chemical vapor deposition, along with a novel catalyst sandwich structure and postdepositional radio-frequency (rf) oxygen plasma treatment lead to well-structured vertically aligned CNTs with excellent and scalable emission properties. Scanning electron and transmission electron microscope investigations reveal that postdepositional treatment reduces not only the number but modifies the structure of the CNTs. Well-structured microwave-plasma-grown nanotubes become amorphous during rf oxygen plasma treatment and the measured work functions of CNTs change from 4.6eVto4.0eV before and after treatment, respectively. Our experiments outline a novel fabrication route for structured CNT arrays with improved and scalable field emission characteristics.