Cees Ronda

New Developments in the Field of Luminescent Materials for Lighting and Displays

Owing to their use in fluorescent lamps and many display applications, luminescent materials affect our daily life. Improvement of already very mature as well as development of new materials demanded by a variety of new applications are the focus of research today. The latest advances in the field of electroluminescence enable new displays and light generation concepts that challenge the classical areas of application of luminescent materials.

Luminescence and luminescence quenching in Gd3(Ga,Al)5O12 scintillators doped with Ce3+.

The optical properties of gadolinium gallium aluminum garnet, Gd3(Ga,Al)5O12, doped with Ce(3+) are investigated as a function of the Ga/Al ratio, aimed at an improved understanding of the energy flow and luminescence quenching in these materials. A decrease of both the crystal field strength and band gap with increasing content of Ga(3+) is observed and explained by the geometrical influence of Ga(3+) on the crystal field splitting of the 5d level in line with theoretical work of Muñoz-García et al. ( uñoz-García, A. B.; Seijo, L. Phys. Rev. B 2010, 82, 184118 ). Thermal quenching results in shorter decay times as well as reduced emission intensities for all samples in the temperature range from 100 to 500 K. An activation energy for emission quenching is calculated from the data. The band gap of the host is measured upon Ga substitution and the decrease in band gap is related to Ga(3+) substitution into tetrahedral sites after all octahedral sites are occupied in the garnet material. Based on the change in band gap and crystal field splitting, band diagrams can be constructed explaining the low thermal quenching temperatures in the samples with high Ga content. The highest luminescence intensity is found for Gd3(Ga,Al)5O12 with 40% of Al(3+) replaced by Ga(3+).

Neue Entwicklungen auf dem Gebiet lumineszierender Materialien für Beleuchtungs‐ und Displayanwendungen

In Leuchtstoffrohren und vielen Displayanwendungen begegnen uns lumineszierende Materialien haufig im taglichen Leben. Sowohl die Verbesserung bereits sehr ausgereifter als auch die Entwicklung neuer Materialien fur eine Vielzahl neuer Anwendungen stehen im Zentrum heutiger Forschungsaktivitaten. Die jungsten Fortschritte auf dem Gebiet der Elektrolumineszenz ermoglichen neue Display- und Beleuchtungskonzepte, die Herausforderungen fur die klassischen Anwendungsgebiete von lumineszierenden Materialien sind.

Luminescent materials with quantum efficiency larger than 1, status and prospects

In this paper, the status of research on quantum cutters is reviewed. Three possible mechanisms will be dealt with in detail and compared to each other. None of the mechanisms identified can be applied in combination with a Hg discharge, as all materials require photons of wavelengths shorter than provided by the main low-pressure Hg emission line.

Progress in phosphors and filters for luminescent solar concentrators

Luminescent solar concentrators would allow for high concentration if losses by reabsorption and escape could be minimized. We introduce a phosphor with close-to-optimal luminescent properties and hardly any reabsorption. A problem for use in a luminescent concentrator is the large scattering of this material; we discuss possible solutions for this. Furthermore, the use of broad-band cholesteric filters to prevent escape of luminescent radiation from this phosphor is investigated both experimentally and using simulations. Simulations are also used to predict the ultimate performance of luminescent concentrators.

Anomalous trapped exciton and D-F emission in Sr4Al 14O25:Eu2+

The photoluminescence and time-resolved emission for Eu(2+) in Sr4Al14O25 has been investigated in the temperature range 4 to 500 K. The Eu(2+) emission changes in a peculiar way with temperature. At low temperature two emission bands are observed at 490 and 425 nm, which are attributed to emission from Eu(2+) on the 7- and 10-coordinated sites. Upon raising the temperature, an unexpectedly large blue shift to 400 nm is observed for the 425 nm emission band. To explain these observations, the 400 and 425 nm emission bands are assigned to d-f and trapped exciton emission, for Eu(2+) on the 10-coordinated site. The trapped exciton emission is characterized by a short (0.5 μs) decay time. The temperature dependence of the emission is explained by a configurational coordinate diagram in which the Eu(2+) trapped exciton state is at a slightly lower energy than the lowest energy 4f(6)5d state. Upon raising the temperature, the 4f(6)5d state is thermally populated and emission from this state is observed, and because of the smaller lattice relaxation (smaller Stokes shift), a large blue shift from 425 to 400 nm is observed.

New luminescent materials and filters for luminescent solar concentrators

In a Luminescent Solar Concentrator (LSC), short-wavelength light is converted by a luminescent material into longwavelength light, which is guided towards a photovoltaic cell. In principle, an LSC allows for high concentration, but in practice this is prevented by loss mechanisms like limited sunlight absorption, limited quantum efficiency and high self absorption. To tackle these problems, a suitable luminescent material is needed. Another important loss mechanism is the escape of luminescent radiation into directions that do not stay inside the light guide. To reduce this amount, wavelengthselective filters can be applied that reflect the luminescent radiation back into the light guide while transmitting the incident sunlight. In this paper, we discuss experiments and simulations of new luminescent and filter materials. We will introduce a phosphor with close-to-optimal luminescent properties. A problem for use in an LSC is the large scattering of this material; we will discuss possible solutions for this. Furthermore, we will discuss the use of broad-band cholesteric filters in combination with this phosphor.

Electron traps in Gd 3 Ga 3 Al 2 O 12 :Ce garnets doped with rare-earth ions

The curves of thermally stimulated luminescence of Gd3Ga3Al2O12:Ce3+ ceramics (a nominally pure sample and samples doped with rare-earth ions) are measured in the temperature range of 80–550 K. The depth and the frequency factor of electron traps established by Eu and Yb impurities are determined. An energy-level diagram of rare-earth ions in the bandgap of Gd3Ga3Al2O12 is presented.

Source of rectangular X-ray pulses for studying scintillators

A pulsed X-ray source has been designed and constructed with the following parameters: X-ray tube voltage, 10–50 kV; anode current, up to 2 A; pulse duration (smooth control), 80 ns–2 μs; and leading and trailing front width, 6 ns. The source can operate in both single-pulse and pulse-train mode with a repetition frequency of up to 2 kHz. The X-ray pulse has a rectangular shape with a relative amplitude instability below 0.8%. In combination with appropriate detection system, the proposed X-ray source allows the kinetics of X-ray-induced luminescence growth and decay to be measured in a six-decimal-order dynamic range with respect to time and amplitude. Examples of X-ray-induced luminescence kinetics measured in scintillators are presented.

Luminescence Properties of Very Small Semiconductor Particles

In this chapter, we will treat some basic problems of quantum mechanics. We will describe the properties of a particle in a potential well. In addition, we will briefly touch upon the quantum mechanical description of atoms, taking the simplest ion, the hydrogen atom as example. Then we will treat the properties of electrons in an infinite crystal. Finally we will treat the elementary properties of coupled electrons and holes.