Efficient Luminescent Materials Research Articles

Strongly Luminescent Poly(ethylene glycol)-2,2′-bipyridine Lanthanide Ion Complexes

A highly efficient luminescent material is produced from a 2,2′-bipyridine-lanthinide-ion coordination complex. The authors describe how this coordination complex can be stabilized by ethylene glycol oligomers. The material, obtained by simple mixing of the components, emits blue, green, and red photoluminescence by single near-UV excitation. Additionally, the complex can be mixed with silica through a sol-gel process, producing transparent soft solids that preserve all photophysical characteristics of the original liquid mixture. These materials may find applications in electroluminescent displays, lasers, or high-density optical storage.

Porous silicon: a route towards a Si-based photonics?

By partial anodic dissolution of Si, it is possible to form an efficient room-temperature luminescent material named porous silicon (p-Si). In this paper the properties of p-Si are reviewed with special emphasis on the growth issue and on its application to light emitting diodes. The physical processes at the base of the luminescence are also discussed. It is suggested that the p-Si luminescence results from radiative recombination of excitons quantum confined in Si nano-crystals.

The luminescence of Sb3+ in LaOCl

Antimony-activated lanthanum oxychloride is an efficient luminescent material with two emissions: a blue one due to isolated Sb3+ in LaOCl, and a green one due to clusters of Sb3+ in an oxychloride phase inside the grains. The Stokes shift of the emission is very large.

Luminescence in one-dimensional chains of closed-shell transitionmetal oxocomplexes

The luminescence of undoped and doped Ln 3MO 7 (Ln = Y, La, Gd; M = Nb, Ta) and Ba 2WO 5 is reported and discussed. Dopants are Eu 3+ and Nb 5+ (for Ta 5+). The crystal structures are characterized by chains consisting of closed-shell transition-metal oxocomplexes sharing corners. The luminescence results show that the excited state in these chains is delocalized. Energy transfer to the dopants is strongly counteracted by defect sites in the crystal structure which trap the excitation energy before it reaches the intentionally added activator. Therefore, these types of crystal structures do not offer potential as the basis for efficient luminescent materials. The luminescence of La 3TaO 7 is especially remarkable, because it seems to show the shortest-wavelength emission (≈ 280 nm) known for tantalates and a small Stokes shift (≈ 8000 cm −1).

New scintillators for X-ray computed tomography

The conversion of X-ray radiation to visible light can be performed by luminescent materials. This effect finds an attractive application in X-ray computed tomography (XCT). The centre of the XCT system with a solid state detector arrangement is the luminescent material coupled to a photosensitive diode. The requirements for efficient luminescent materials are: high X-ray absorption, high light output, fast decay time, and low afterglow. The dominant single crystalline scintillators which meet these specifications, at least in part, are CsI:T1, CdWO 4. and ZnWO 4. Recently, polycrystalline scintillators such as rare earth-doped gadolinium oxysulphide, Gd 2O 2 S:RE, and yttrium-gadolinium oxide, (Y,Gd) 2O 3:RE have been created. These scintillators exhibit high chemical stability. Homogeneous incorporation of rare earth (RE) activators as well as other chemical additives is achieved by controlled ceramic processing.

Luminescence processes in the crystalline and glass modifications of LnBGeO 5-type compositions

The luminescence properties of the Eu 3+ ion in the crystalline and glass modifications of LnBGeO 5-type compositions ( Ln = La, Gd, Y, and CaBPO 5) are investigated. The efficiency of the luminescence upon broad band excitation is higher in the crystals than it is in the glasses. Whereas crystalline GdBGeO 5:Eu is a very efficient luminescent material, crystalline LaBGeO 5:Eu and CaBPO 5:Eu show only medium efficiency, and the glass modifications are of a low to very low efficiency. The influence of the charge and the size of the ion for which Eu 3+ is substituted is discussed. The influence of these cations on the efficiency of the luminescence of the Eu 3+ ion is well known for crystalline compositions, but is observed for the first time in the glass modifications, where they play the role of network modifiers. The undoped compositions show a blue luminescence. Further we observed Eu 2+ emission in the europium-doped CaBPO 5 glass. It shows inhomogeneous broadening, so that not only broad band but also a small amount of sharp line emission could be observed.

Luminescence properties of benzoic acid-terbium complexes

A series of highly efficient luminescence materials, benzoic acid-terbium complexes, were investigated by absorption, infrared, fluorescence and time-resolved fluorescence spectroscopy. Terbium ion and benzoic acid form a ground state complex as indicated by absorption and emission spectroscopy. The luminescence property of terbium-acid complexes varies dramarically with the structure, bonding strength and intramolecular energy transfer efficiency of the complex. The influences of the properties and pH of the solvent are discussed as well.

Luminescence and energy transfer in the columbite structure

The luminescence properties of undoped and Eu 3+- and Dy 3+- doped CdNb 2O 6 and CaNb 2O 6 are reported and discussed in connection with the luminescence of niobates and tungstates with wolframite and columbite structure. It is shown that these structures lead in principle to highly efficient luminescent materials. However, disorder between the metal ions reduces the efficiency considerably. In CdNb 2O 6 the niobate excitation energy is more mobile than in CaNb 2O 6. This has important consequences for the rare-earth activated materials. At 300 K the amount of rare-earth emission from the CdNb 2O 6-based samples is high. This is due to energy migration in the host lattice and efficient transfer to the activator ( R c ≅ 7 A ̊ ).

On the luminescence of Ti 4+ In Mg 5SnB 2O 10 and Mg 3ZrB 2O 8

The following new compounds are reported: Mg 5TiB 2O 10 (ludwigite structure), Mg 5SnB 2O 10 (orthopinakiolite structure) and Mg 3ZrB 2O 8 (warwickite structure). The luminescences of the systems Mg 5(Sn,Tl)B 2O 10 and Mg 3(Zr,Ti)B 2O 8 were measured. The results are discussed in terms of delocalization in clusters of titanate octahedra. A general scheme for the luminescence of Ti 4+-activated compositions is given. The composition Mg 5SnB 2O 10: Ti is a very efficient luminescent material for titanium concentrations below some 20 mole percent.

Ba 2WO 3F 4, a new fluorotungstate with high luminescence efficiency

Ba 2WO 3F 4 is an efficient luminescent material under UV and X-ray excitation. The luminescence properties show a similarity with those of the tungstates, especially MgWO 4.

Ba 2WO 3F 4, a new fluorotungstate with high luminescence efficiency

Ba2WO3F4 is an efficient luminescent material under UV and X-ray excitation. The luminescence properties show a similarity with those of the tungstates, especially MgWO4.

Designing of lanthanum-tantalate-based luminophores

Luminescent spectral characteristics of polycrystal oxides of lanthanum, neodymium and europium tantalates are considered. Diagrams of Nd 3+ and Eu 3+ low energy levels in the above crystals are presented. The aspects of synthesizing efficient tantalate-based luminescent materials have been investigated. A class of new concentrated lanthanoid geptatantalates luminophores is described.

How far are luminescence properties predictable?

Our knowledge of the luminescence of isolators has increased considerably during the past decade. As a consequence it has become possible to understand the luminescence of technically important phosphors and even to predict efficient luminescent materials. We first illustrate how emission spectra of a given activator can be varied by changing the host lattice. Second we consider the factors influencing the luminescent efficiency. Using a single-configurational coordinate model, we have performed calculations on a model system. These illustrate which factors are important for efficiency. Some clear results are reported. Finally, we discuss the mobility of excitation energy. In some phosphors the excitation is highly mobile even at low temperatures. As a consequence emission originates from centers which trap the excitation energy. In other phosphors this mobility is low, and at low temperatures emission occurs from the regular luminescence centers. At higher temperatures the excitation energy can become mobile because of thermal activation.

Luminescence Excitation by Protons and Electrons, Applied to Apollo Lunar Samples

A proton accelerator, modified for work on lunar samples, is described; 60 keV protons at about 1 μA cm−2 are used. Luminescence emission spectra in the visible and near IR regions are shown for lunar samples from Apollo 11 and 12, and preliminary results are given for Apollo 14 samples. Lunar samples are compared with terrestrial and meteoritic materials. Plagioclase is found to be the most efficient luminescent material present in the lunar samples, and the activator for its dominant green peak is found to be Mn2+.Apparatus is described for taking colour photographs of the luminescence emission from rock chips under 6 keV electron excitation. Most of the lunar samples investigated show only plagioclase emission, but one breccia shows a wide variety of colours from different luminescent constituents.

Some New Aspects of Germanate and Fluoride Phosphors*

By crystallizing with excess magnesium oxide the photo-luminescent efficiency of manganese-activated magnesium orthogermanate has been increased 500 percent. The emission spectra of these materials extend from 6100A to 6700A, and exhibit a unique fine structure consisting of four bands about 50A wide and having peaks approximately 90A apart. Small changes in composition leave the positions of the emission peaks invariant but greatly change the relative intensities of the peaks. This fine structure offers a valuable key to determining the optical transitions involved in the luminescence of manganese-activated phosphors. Manganese-activated zinc-magnesium fluoride phosphors exhibit under cathode-ray excitation the longest trulyexponential afterglow of any efficient known luminescent material. Crystallization of these phosphors under 100 atmospheres of hydrogen fluoride does not greatly alter their luminescent characteristics. Fluoride phosphors, particularly manganese-activated zinc fluoride, possess the unique feature among luminescent materials of being capable of evaporation and condensation in a vacuum to form thin uniform transparent layers without unduly affecting the luminescent efficiency.