Garnets have the general formula of A3B2C3O12 and form a wide range of inorganic compounds, occurring both naturally (gemstones) and synthetically. Their physical and chemical properties are closely related to the structure and composition. In particular, Ce3+-doped garnet phosphors have a long history and are widely applied, ranging from flying spot cameras, lasers and phosphors in fluorescent tubes to more recent applications in white light LEDs, as afterglow materials and scintillators for medical imaging. Garnet phosphors are unique in their tunability of the luminescence properties through variations in the {A}, [B] and (C) cation sublattice. The flexibility in phosphor composition and the tunable luminescence properties rely on design and synthesis strategies for new garnet compositions with tailor-made luminescence properties. It is the aim of this review to discuss the variation in luminescence properties of Ce3+-doped garnet materials in relation to the applications. This review will provide insight into the relation between crystal chemistry and luminescence for the important class of Ce3+-doped garnet phosphors. It will summarize previous research on the structural design and optical properties of garnet phosphors and also discuss future research opportunities in this field.

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Ce3+-Doped garnet phosphors: composition modification, luminescence properties and applications.

Light emitting diodes (LEDs) are on the verge of a breakthrough in general lighting, due to their rapidly improving efficiency. Currently, white LEDs with high color rendering are mainly based on wavelength conversion by one or more phosphor materials. This Review first describes how to quantify the quality of a light source, discussing the color rendering index (CRI) and alternative color quality indices. Then, six main criteria are identified and discussed, which should be fulfilled by a phosphor candidate to be considered for actual application in LEDs. These criteria deal with the shape and position of the emission and the excitation spectra, the thermal quenching behavior, the quantum efficiency, the chemical and thermal stability and finally with the occurrence of saturation effects. Based on these criteria, the most common dopant ions (broad-band emitting Eu 2+ , Ce 3+ and Mn 2+ , line-emitting rare earth ions,...) and host compounds (garnets, sulfides, (oxy)nitrides,...) are evaluated. Although many phosphor materials have been proposed in literature in recent years, the number of phosphors effectively fulfilling all six requirements is relatively small.

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Selecting Conversion Phosphors for White Light-Emitting Diodes

Since the mid-1990s, phosphors have played a key role in emerging solid-state white-lighting technologies that are based on combining a III-nitride-based near-UV or blue solid-state light source with downconversion to longer wavelengths. Almost all widely used phosphors comprise a crystalline oxide, nitride, or oxynitride host that is appropriately doped with either Ce3+ or Eu2+. These ions, with [Xe] 4fn5d0 configurations (n = 1 for Ce3+ and 7 for Eu2+) have proximal excited states that are [Xe] 4fn−15d1. Optical excitation into these states and concomitant reemission can be tuned into the appropriate regions of the visible spectrum by the crystal these ions are hosted in. In this article, we review the current needs and key aspects of the conversion process. We describe some currently used families of phosphors and consider why they are suitable for solid-state lighting. Finally, we describe some empirical rules for new and improved host materials.

Phosphors for Solid-State White Lighting

Nanoparticulate functional materials offer manifold perspectives for the increasing miniaturization and complexity of technical developments. Nanoparticles also make a major contribution to utilization of materials that is sparing of natural resources. Besides these obvious aspects, however, the importance of nanoparticles is due to their fundamentally novel properties and functions. These include photonic crystals and efficient luminophors, single particles and thin films for electronic storage media and switching elements, magnetic fluids and highly selective catalysts, a wide variety of possibilities for surface treatments, novel materials and concepts for energy conversion and storage, contrast agents for molecular biology and medical diagnosis, and fundamentally novel forms and structures of materials, such as nanocontainers and supercrystals. Creating high-quality nanoparticles requires that numerous parameters, involving the particle core and surface, colloidal properties, and particle deposition, are taken into consideration during synthesis of the material. An appropriate characterization and evaluation of the properties requires the incorporation of a wide range of expertise from widely differing areas. These circumstances are what challenges and appeals to the nanoscientist.

Nanoparticulate functional materials.

In this review, the major achievements and research and development (R&D) trends from the last decade in the field of single crystal scintillator materials are described. Two material families are included, namely, those of halide and oxide compounds. In most cases, the host crystals are doped with Ce3+, Pr3+ or Eu2+ rare earth ions. Their spin- and parity-allowed 5d–4f transitions enable a rapid scintillation response, on the order of tens to hundreds of nanoseconds. Technological recipes, extended characterization by means of optical and magnetic spectroscopies, and theoretical studies are described. The latter provide further support to experimental results and provide a better understanding of the host electronic band structure, energy levels of specific defects, and the emission centers themselves. Applications in medical imaging and dosimetry, security measures, high-energy physics and the high-tech industry, in which X(γ)-rays or particle beams are used and monitored, are recognized as the main driving factor for R&D activities in this field.

Recent R&D Trends in Inorganic Single-Crystal Scintillator Materials for Radiation Detection

Ce3+-doped YAG nanoparticles have been synthesized at relatively low temperatures by the co-precipitation and polymer-assisted sol–gel methods and characterized by Fourier transform infrared spectroscopy (FT-IR), x-ray diffraction (XRD) and x-ray absorption near-edge structure (XANES) measurements. Calcination of the product was carried out at temperatures between 800 and 1100 °C. Products calcined at the lower temperature contained a greater proportion of Ce4+, and this contributed in part to the lower intensity of the characteristic yellow photoluminescence. The synchrotron radiation excitation spectrum of this yellow emission band exhibits bands due to Ce3+ absorption as well as the host band gap. The origin of a luminescence band at about 300 nm obtained under band-gap excitation is discussed. Previous assignments of the 5d electronic energies in YAG:Ce3+ are critically assessed, and in the light of the new energy level and transition intensity calculations a revised assignment is put forward for the 5d energy levels.

Soft synthesis and vacuum ultraviolet spectra of YAG:Ce3+ nanocrystals: reassignment of Ce3+ energy levels

The magnetic properties of the Gd12O18 cluster cut from the bulk Gd2O3 crystal are investigated using the spin-polarized density functional theory within the broken-symmetry approach. Our work reveals that in the ground state of the cluster the antiferromagnetic coupling between adjacent Gd (4f7) spins is preferred energetically. This result is in contrast to a recent prediction made by Pedersen and Ojamae (Pedersen, H.; Ojamae, L. Nano Lett. 2006, 6, 2004) but is consistent with recent experimental observations. The optimized structures of the cluster in the lowest-energy broken-symmetry state and the highest-spin ferromagnetic state are almost identical. The latter state is 71.5 cm−1 higher in energy than the former one, giving a value of about −0.24 cm−1 for the magnetic coupling constant, which is comparable to that estimated from experiments on the bulk crystal. The relative energies of various 4f7 spin patterns of the cluster are calculated, and certain characteristics of the cluster in the lowest-e...

Density functional theory study of magnetic coupling in the Gd12O18 cluster.

We present here an ab initio calculation of the quenching rate of erbium luminescence by the ligand C–H vibrations in erbium(III) tris(8-quinolinolate) within the framework of energy-transfer theory. The C–H vibrational transition matrix elements are evaluated on the basis of DFT calculations of the quinolate ligand at the B3LYP/6-311++G(2d, 2p) level of theory. The luminescence lifetime calculated by considering the dipole–dipole and quadrupole–dipole interaction mechanisms is in good agreement with the experimental value. Based on the theoretical results for the complex, the dependence of the luminescence quenching rates on the distance between the erbium ion and the C–H bond is generalized and discussed, and comparisons with available experimental results are made.

Infrared luminescence quenching in erbium(III) tris(8-quinolinolate): an ab initio approach

Density functional theory calculations of crystal-field energy levels for Y^3+ in the Cs_2NaYF_6 crystal

The processes of 4 I13/2 → 4 I15/2 luminescence quenching by OH vibrations are investigated for LaF3:Er 3+ nanoparticles dissolved in solution. The energy transfer (ET) rates, involving the 4 I13/2 → 4 I15/2 transition of Er 3+ and the first overtone absorption of OH, are estimated. In order to calculate the relevant OH transition matrix elements, a model for the OH vibration in solution is developed with the use of a Morse potential. Various multipole‐multipole ET mechanisms are considered and their dependences on the distance between Er 3+ and OH are studied. Based on these ET mechanisms, the ET rates from an Er 3+ in the nanoparticle to all the OH in solution are estimated and compared with respect to changes in location of the ion, size of the nanoparticle and OH concentration in solution. The effective 4 I13/2 → 4 I15/2 luminescence decay times that are contributed by all the Er 3+ in the nanoparticle are then calculated with different Er 3+ concentrations. The calculations satisfactorily account for experimental observations.

Lixin Ning

Papers

Soft synthesis and vacuum ultraviolet spectra of YAG:Ce3+ nanocrystals: reassignment of Ce3+ energy levels

Density functional theory study of magnetic coupling in the Gd12O18 cluster.

Infrared luminescence quenching in erbium(III) tris(8-quinolinolate): an ab initio approach

Density functional theory calculations of crystal-field energy levels for Y^3+ in the Cs_2NaYF_6 crystal

Theoretical study of 4I13/2 → 4I15/2 luminescence quenching by OH for LaF3:Er3+ nanoparticles in solution