Many treatments of energy transfer (ET) phenomena in current literature employ incorrect arguments and formulae and are not quantitative enough. This is unfortunate because we witness important breakthroughs from ET experiments in nanoscience. This review aims to clarify basic principles by focusing upon Förster-Dexter electric dipole-electric dipole (ED-ED) ET. The roles of ET in upconversion, downconversion and the antenna effect are described and the clichés and simple formulae to be avoided in ET studies are highlighted with alternative treatments provided.

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The electronic structure of alkaline-earth silicon nitride MSiN2 (M = Sr, Ba) was calculated using the CASTEP code. BaSiN2 is calculated to be an intermediate band gap semiconductor with a direct energy gap of about 2.9 eV, while SrSiN2 is an intermediate band gap semiconductor with an indirect energy gap of about 3.0 eV. As expected, the calculated optical band gaps of MSiN2 (M = Ba, Sr) are lower compared to the experimentally determined values (about 4.1 eV for BaSiN2 and 4.2 eV for SrSiN2). In addition, the luminescence properties of Eu2+ and Ce3+ in MSiN2 (M = Sr, Ba) have been studied. Ba1−x Eu x SiN2 (0 < x ≤ 0.1) shows a broad emission band in the wavelength range of 500–750 nm with maxima from about 600 to 630 nm with increaseing Eu2+ concentration, while Sr1−x Eu x SiN2 (0 < x ≤ 0.1) shows a broad emission band in the wavelength range of 550–850 nm with maxima from 670 to 685 nm with increasing Eu2+ concentration. The high absorption and strong excitation bands of M1−x Eu x SiN2 (0 < x ≤ 0.1; M = Sr, Ba) in the wavelength range of 300–530 m are very favorable properties for application as light-emitting-diode conversion phosphors. Ce3+- and Li+- codoped MSiN2 (M = Sr, Ba) exhibits a broad emission band in the wavelength range of 400–700 nm with a peak center at about 485 nm for BaSiN2 and about 535 nm for SrSiN2. A comparison is made between the luminescence properties of Eu2+ and Ce3+ in the Sr versus Ba compounds. The long-wavelength excitation and emission of Eu2+ and Ce3+ ions in the host of MSiN2 (M = Sr, Ba) are attributed to the effect of a high covalency and a large crystal field splitting on the 5d bands of Eu2+ and Ce3+ in the nitrogen coordination environment.

An optical temperature sensor and optical heater based on Yb3+/Tm3+ co-doped Ba5Gd8Zn4O21 phosphors.

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A red upconverted emission is reported for yttria codoped with Eu 3+ and Yb 3+ upon excitation by a 970 nm laser diode. The concentration dependence and temperature dependence down to 10 K of the upconverted emission have been investigated. Two alternative models are presented to simulate the emission intensity-temperature dependence.

Cr 3+ -activated persistent luminescent phosphors with a spinel structure are emerging materials in bio-imaging applications for their distinctive features of deep biotissue penetration and rechargeable nearinfrared persistent emission. To realize the long-term and multicycle imaging purpose, reactivation using in situ external light with deep biotissue penetration is an alternative strategy, apart from the efforts on trap modulations for the host lattice. However, recharging with high-energy ultraviolet /visible photons will result in low activation efficiency because of the absorption by biological tissues. Here, we report a low-energy photonrechargeable near-infrared persistent material, with rechargeable efficiency 400 times higher than that of the ZnGa 2 O 4 :Cr 3+ reference material. The crystal field strength and band gap energies can be tailored by control of cation occupancy, contributing the red shift of both the persistent luminescence excitation and emission spectra of the optimized complex spinel samples. The persistent emission red shift is of interest for improved deep tissue penetration for bioimaging, whereas the persistent excitation red shift facilitates the activation of persistent luminescence by low-energy radiation. Furthermore, it demonstrates that increasing the rate of cation site inversion in the spinel can lead to higher storage capacity of charge trapping.