Charge transfer from Eu to trivalent lanthanide co-dopants: Systematic behavior across the series.

Electron transfer processes between lanthanide activators are crucial for the functional behavior and performance of luminescent materials. Here, a multiconfigurational ab initio study reveals how direct metal-to-metal charge transfer (MMCT) between the Eu luminescence activator and a Ln co-dopant (Ln = Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb) systematically dictates the luminescence and optical properties of CaF. The combination of the structures and energies of the electronic manifolds, the vibrational force constants, and the structural properties of the donor and acceptor in the host determines the predictions of five different behaviors of CaF:Eu, Ln co-doped materials after MMCT absorption: formation of stable traps, MMCT emission, emission quenching, Ln emission, and Eu emission.

Broadband infrared LEDs based on europium-to-terbium charge transfer luminescence.

Efficient broadband infrared (IR) light-emitting diodes (LEDs) are needed for emerging applications that exploit near-IR spectroscopy, ranging from hand-held electronics to medicine. Here we report broadband IR luminescence, cooperatively originating from Eu and Tb dopants in CaS. This peculiar emission overlaps with the red Eu emission, ranges up to 1200 nm (full-width-at-half-maximum of 195 nm) and is efficiently excited with visible light.

Fine-Tuning the Cr R-Line by Controlling Pauli Antisymmetry Strength.

Fine-tuning of the Cr zero-phonon R-line fluorescence (E-A) has recently been achieved by varying the composition of Cr-doped solid solution garnets. Chemical substitutions of Y with Lu and Tb produce R-line shifts and multiplets that cannot be associated with direct or indirect changes in the Cr-O covalency alone and raise important questions on the nature of the interactions between the CrO active center and its environment, hence on the impact of local disorder. In this Letter we show that Pauli antisymmetry interactions with next-nearest neighbor ions, which are commonly omitted from theoretical analyses, induce a nephelauxetic effect on the CrO active center that shifts the R-line.

Direct Evidence of Intervalence Charge-Transfer States of Eu-Doped Luminescent Materials.

Direct evidence is given for the existence of intervalence charge transfer (IVCT) states of Eu/Eu pairs in Eu-doped CaF, SrF, and BaF. They are detected in diffuse reflectance spectra. In doped materials, IVCT states, in which an electron transfer occurs between two metal sites differing only in oxidation state, are rather difficult to observe because the absorption bands are extremely broad and flat, their intensity is low, and no emission follows the IVCT absorptions.

Evidence That the Anomalous Emission from CaF:Yb Is Not Described by the Impurity Trapped Exciton Model.

Yb-substituted CaF exhibits an anomalous red-shifted luminescence after UV excitation, attributed to the relaxation of impurity trapped excitons (ITE). CaF:Yb is the archetype system for this model, in which the Yb ions can be excited into a long-lived (ms) exciton state. Upon de-excitation, the emission intensity should be proportional to the Yb concentration, but that could not be checked when this model was first proposed.