The wavelength down conversion approach to solid‐state lighting (SSL) uses down conversion materials to produce visible light when excited by near‐UV or blue emission from InGaN LEDs. This review discusses two classes of down conversion materials: phosphors and semiconductor quantum dots (QDs). Strong absorption of the excitation wavelength; high luminous efficacy of radiation, which enables white light with a high color rendering index and a low correlated color temperature; high quantum efficiency; and thermal and chemical stability are some of the criteria for down converters used in SSL. This review addresses the challenges in the development of down converters that satisfy all these criteria. We will discuss the advantages and disadvantages of several phosphor compositions for blue and near‐UV LEDs. The use of core/shell architectures to improve the photoluminescence and moisture resistance of phosphors is presented. QDs are another class of down conversion materials for near‐UV and blue LEDs. Strategies to improve the photostability and reduce the thermal quenching of QDs include strain‐graded core/shell interfaces and alloying. We discuss Cd‐containing II–VI QDs, and Cd‐free III–V and I–III–VI QDs and their potential for SSL applications. Finally, a description of different methods to integrate the phosphors and QDs with the LED is given.

We have investigated the mechanism of nano-YAG:Ce growth in butanediol and glycol solvents. The static autoclave and low synthesis temperature (225 °C) that we employed provided conditions of slow growth in which we were able to observe an intermediate phase, a butanediol-intercalated layered alumina. This phase serves to passivate the surface in nano-YAG:Ce precipitates and thus contributes to increasing the quantum yield of YAG:Ce by diminishing surface effects such as Ce oxidation. While neat 1,4butanediol results in precipitation of the nano-YAG:Ce, a mixture of 1,4-butanediol and diethylene glycol stabilizes a transparent colloid. We attribute this to higher solubility of the layered alumina intermediate in the solvent mixture and, thus, more homogeneous nucleation of the nano-YAG:Ce compared to heterogeneous nucleation in the neat 1,4-butanediol. However, the trade-off is slightly lower quantum yield in the transparent colloid, since the nano-YAG:Ce is not as thoroughly surface-passivated. With the transparent colloid, we were able to encapsulate the nano-YAG:Ce into a transparent epoxy dome that may be utilized in solid-state devices.

Recent synthetic advances have made available very monodisperse zincblende CdSe/CdS quantum dots having near-unity photoluminescence quantum yields. Because of the absence of nonradiative decay pathways, accurate values of the radiative lifetimes can be obtained from time-resolved PL measurements. Radiative lifetimes can also be obtained from the Einstein relations, using the static absorption spectra and the relative thermal populations in the angular momentum sublevels. One of the inputs into these calculations is the shell thickness, and it is useful to be able to determine shell thickness from spectroscopic measurements. We use an empirically corrected effective mass model to produce a "map" of exciton wavelength as a function of core size and shell thickness. These calculations use an elastic continuum model and the known lattice and elastic constants to include the effect of lattice strain on the band gap energy. The map is in agreement with the known CdSe sizing curve and with the shell thicknesses of zincblende core/shell particles obtained from TEM images. If selenium−sulfur diffusion is included and lattice strain is omitted from the calculation then the resulting map is appropriate for wurtzite CdSe/CdS quantum dots synthesized at high temperatures, and this map is very similar to one previously reported (J. Am. Chem. Soc. 2009, 131, 14299). Radiative lifetimes determined from time-resolved measurements are compared to values obtained from the Einstein relations, and found to be in excellent agreement. For a specific core size (2.64 nm diameter, in the present case), radiative lifetimes are found to decrease with increasing shell thickness. This is similar to the size dependence of one-component CdSe quantum dots and in contrast to the size dependence in type-II quantum dots.

The mechanisms of temperature-dependent nonradiative processes, often referred to as thermal quenching, are studied in CdSe, CdSe/ZnSe, and CdTe nanoparticles. These particles exhibit reversible thermal quenching, the extent of which is strongly dependent on the composition of the surface and nature of the surface ligands. Thermal quenching has dynamic (affecting the luminescence lifetimes) and static (affecting the fraction of particles that are bright versus dark) components. The temperature dependence of quantum yields and time-resolved luminescence decays as well as room temperature transient absorption spectroscopy are used to elucidate the thermal quenching mechanisms. Dynamic thermal quenching is due to thermally activated trapping dynamics that occur on the same time scale as the radiative lifetime. This paper focuses on static thermal quenching and several different mechanisms are considered. It is concluded that the dominant mechanism involves thermal promotion of valence band electrons to empty chalcogenide P orbitals on the particle surfaces. This leaves a hole in the valence band, and subsequent photoexcitation produces a positive trion. The trion undergoes relatively rapid nonradiative Auger relaxation, rendering the particle dark. The differences in the extents of thermal quenching between different surface compositions, different types of particles, and different surface ligands can be understood in terms of the density of empty surface chalcogenide orbitals and the valence band energies.

Rare earth tantalate materials are of considerable interest in energy and environmentally related applications including photocatalytic H(2) generation or contaminant decomposition, ion conductivity for batteries and fuel cells, and phosphors for light-emitting diodes (LEDs). These Eu-doped rare earth tantalate pyrochlore nanoparticles, K(1-2x)LnTa(2)O(7-x):Eu(3+) (Ln = Lu, Y, Gd; x = (1)/(3) for Gd, x = 0 for Lu and Y), have quantum yields up to 78% when excited with blue light (464 nm), which is remarkable for nanoparticle forms that can suffer efficiency loss by surface effects or poor crystallinity, and are furthermore quite suitable for LED applications. The Gd analogue with its framework distortions has particularly high quantum yields. The blue excitation peak matches the emission of the GaN LED. The combination of the high quantum yield under blue excitation, low thermal quenching, and chemical stability renders these new materials promising red phosphors for blue-excitation white LEDs for solid-state lighting. In addition, the pyrochlore lattice is very accommodating to dopants and vacancies and will incorporate virtually any metal and coordination environment ranging from four-coordinate to eight-coordinate. Thus, there are virtually unlimited possibilities for tailoring and optimizing photoluminescent properties, as demonstrated by these scoping studies.