Effect of A-site cations on the broadband-sensitive upconversion of AZrO3:Er3+,Ni2+ (A = Ca, Sr, Ba) phosphors

We have analyzed broadband-sensitive upconversion from 1.1–1.6 μm to 0.98 μm in La(Ga,Sc,In)O3 doped with Er, Ni, and Nb, which could significantly boost the conversion efficiency of crystalline silicon solar cells, in particular, energy transfer from the Ni2+ sensitizers to the Er3+ emitters and back transfer from the Er3+ to the Ni2+. We have compared these processes and the resultant upconversion emission intensities depending on the host material compositions. With increasing the bond length between the Ni2+ and surrounding oxygen ions, the Ni2+ emission band located at around 1.2–1.6 μm red-shifts and hence overlaps more significantly with the Er3+ absorption band ranging from 1.45 μm to 1.6 μm, resulting in more rapid energy transfer from the Ni2+ to the Er3+. However, back energy transfer from the Er3+ to the Ni2+ deteriorates the performance more considerably, because of more significant overlap between the Er3+ emission band and Ni2+ absorption band. This trade-off relationship strongly affects the upconversion emission intensity. The key of the material design for further efficient upconversion is to achieve narrower bands and a larger Stokes shift of the Ni2+ absorption/emission to suppress the back energy transfer while maintaining the efficient energy transfer in the forward direction.

Dynamic exciton-to-dopant energy transfer tuning of Mn2+-Doped perovskite nanocrystals by PBG effect

Spectroscopic properties of Er3+/Yb3+ codoped barium gallogermanate glass demonstrate that an efficient energy transfer from Yb3+ to Er3+ ions occurs while the back energy transfer from Er3+ to Yb3+ ions cannot be ignored. Based on the rate equations of electron transitions, the forward energy transfer Yb3+(F25/2)+Er3+(I415/2)→Yb3+(F27/2)+Er3+(I411/2) and the back transfer Er3+(I411/2)+Yb3+(F27/2)→Er3+(I415/2)+Yb3+(F25/2) coefficients were calculated to be 9.2×10−17 cm3 s−1 and 1.1×10−17 cm3 s−1, respectively. Also the energy transfer upconversion coefficient Yb3+(F25/2)+Er3+(I411/2)→Yb3+(F27/2)+Er3+(F27/2) was determined to be 1.1×10−16 cm3 s−1. The calculated results show that a population inversion between the I413/2 and I415/2 levels can be attained at a low pumping flux of around 1.7×1022 cm−2 s−1, while the inversion threshold between the S43/2 and I415/2 levels is about 30 times higher when pumped by 980 nm laser diode. With increasing pump flux the back energy transfer probability becomes larger and when the pump flux is more than 2.0×1022 cm−2 s−1, the back energy transfer probability dominates the forward energy transfer probability.

Kinetics of radiationless energy transfer from upper excited states

Suppression of back energy transfer is crucial in realizing efficient luminescent lanthanide complexes. However, the only practical method reported to this day is to raise the energy of the triplet excited state so that back energy transfer is energetically unfavorable, which limits the application where the absorbing wavelength of the organic ligands is important. This chapter explores a radically new strategy that focus on the yield of the back energy transfer as opposed to the rate constant of the back energy transfer. By utilizing energy transfer between lanthanide ions, which is a competitive process to back energy transfer, the contribution of back energy transfer can be suppressed. This chapter describes the theoretical background of this concept using kinetic analysis, and then the experimental confirmation of the proposal using [TbnGd9−n(µ-OH)10(Bu)16]NO3 (n = 0, 1, 2, 5, 8, 9). It is revealed that indeed the contribution of the back energy transfer is suppressed in this cluster, implying the potential of lanthanide clusters as functionalized and efficient luminescent material.

Trivalent lanthanide complexes are an important class of luminescent material characterized by their strong absorption of light by the organic ligands and subsequent energy transfer to the lanthanide ion, realizing intense luminescence from the ion. With this mechanism of luminescence, the total quantum yield of a lanthanide complex is the product of the energy transfer efficiency from the ligand to the lanthanide ion and the "intrinsic" quantum yield of the lanthanide ion itself. The "absolute" method in measuring the quantum yield uses an integrating sphere, and this method can be used for measuring both the total and the intrinsic quantum yields. The presence of back energy transfer (the reverse process of energy transfer) adds complication to this by affecting both the dynamics of the excited state of the ligands and the lanthanide ion. Herein, we theoretically derive an equation that shows that in the presence of back energy transfer the intrinsic quantum yield may differ depending on whether it is determined from the measurement through excitation of the ligands or the lanthanide directly. The value measured by direct lanthanide excitation could decrease to 20% or less of the actual value when back energy transfer is prominent. Several previously reported Tb(iii) complexes are within the range to be cautious. This report shows that the "absolute" method for measuring the lanthanide ion-centered quantum yield may not be suitable in the presence of back energy transfer by principle. We also provide a possible workaround in the case that several approximations and assumptions can be made.

Lanthanide (Ln(III)) complexes form an important class of highly efficient luminescent materials showing characteristic line emission after efficient light absorption by the surrounding ligands. The efficiency is however lowered by back energy transfer from Ln(III) ion to the ligands, especially at higher temperatures. Here we report a new strategy to reduce back energy transfer losses. Nonanuclear lanthanide clusters containing terbium and gadolinium ions, TbnGd9−n clusters ([TbnGd9−n(μ-OH)10(butylsalicylate)16]+NO3−, n = 0, 1, 2, 5, 8, 9), were synthesized to investigate the effect of energy transfer between Tb(III) ions on back energy transfer. The photophysical properties of TbnGd9−n clusters were studied by steady-state and time-resolved spectroscopic techniques and revealed a longer emission lifetime with increasing number of Tb(III) ions in TbnGd9−n clusters. A kinetic analysis of temperature dependence of the emission lifetime show that the energy transfer between Tb(III) ions competes with back energy transfer. The experimental results are in agreement with a theoretical rate equation model that confirms the role of energy transfer between Tb(III) ions in reducing back energy transfer losses. The results provide a new strategy in molecular design for improving the luminescence efficiency in lanthanide complexes which is important for potential applications as luminescent materials.

Enhanced 1.8 μm emission in Yb3+/Tm3+ co-doped tellurite glass: Effects of Yb3+↔Tm3+ energy transfer and back transfer

We report on the structural, morphological, and luminescent properties of Y2O3:Yb3+ (2%)–Er3+ (1%) nanofibers synthesized by a hydrothermal method as a function of the solvent composition ethanol/water. The average length and diameter of the nanofibers ranges from 1.1 to 2.3 μm, and from 50 to 110 nm, respectively. A cubic crystalline structure was obtained, and no effect of the solvent was observed. However, the increment of OHs, because of the increment of water, modifies the quality of the nanofibers. Such impurities improve the emission bands under 940 and 490 nm excitation, especially the red band, via multiphonon relaxation. Relaxation dynamics is explained, based on direct and back energy transfer, multiphonon relaxation, and cross-relaxation. The direct energy transfer coefficients (Cb2, Cb4, and Cb5) calculated by the proposed theoretical model point out the fact that upconverted emissions are notably favored by the increment of OHs. The energy backtransfer (C5b) and cross-relaxation (C51) coefficients depend only on the ion concentration and not on the OH content.

Optical transitions and frequency upconversions of Ho 3+ and Ho 3+/Yb 3+ ions in Al(NO 3) 3–SiO 2 sol–gel glasses

Enhanced upconversion of sub20 nm core/shell/shell nanophosphors for temperature and rhodamine B sensing

Down- and up-conversion emissions in Er3+–Yb3+ codoped TeO2–ZnO–ZnF2 glasses

The enhanced phosphorescence from Alq3 fluorescent materials by phosphor sensitization

The N-terminal domain of human apolipoprotein E3 (apoE3) adopts an elongated, globular four helix bundle conformation in the lipid-free state. Upon lipid binding, the protein is thought to undergo a significant conformational change that is essential for manifestation of its low density lipoprotein receptor recognition properties. We have used fluorescence resonance energy transfer (FRET) to characterize helix repositioning which accompanies lipid interaction of this protein. ApoE3(1-183) possesses a single cysteine at position 112 and four tryptophan residues (positions 20, 26, 34, and 39). Modification of Cys112 with the chromophore, N-iodoacetyl-N'-(5-sulfo-1-naphthyl)etheylenediamine (AEDANS) was specific and did not alter the secondary structure content of the protein. The efficiency of energy transfer from donor Trp residues to the AEDANS moiety was 49% in buffer, consistent with close proximity of the chromophores. Guanidine HCl titration experiments induced characteristic changes in the efficiency of energy transfer, indicating that FRET data faithfully reports on the conformational status of the protein. Interaction of AEDANS-apoE3(1-183) with dimyristoylphosphatidylcholine to form disk particles, or with detergent micelles, resulted in large decreases in the efficiency of energy transfer. Distance calculations based on the FRET measurements revealed that lipid binding increases the average distance between the four Trp donors and the AEDANS acceptor from 23 A to 44 A. The results obtained demonstrate the utility of FRET to investigate conformational adaptations of exchangeable apolipoproteins and are consistent with the hypothesis that, upon lipid binding, apoE3(1-183) undergoes conformational opening, repositioning helix 1 and 3 to adopt a receptor-active conformation.

Here we report a new strategy to tune both excitation and emission peaks of upconversion nanoparticles (UCNPs) into the first infrared biowindow (NIR‐I, 650–900 nm) with high NIR‐I‐to‐NIR‐I upconversion efficiency. By introducing the sensitizer Nd3+, activator Er3+, energy migrator Yb3+ and energy manipulator Mn2+ into specific region to construct proposed energy migration processes in the designed core–shell–shell nanoarchitecture, back energy transfer (BET) from activator to sensitizer or migrator can be greatly blocked and the NIR‐to‐red upconversion emission can be efficiently promoted. Consequently, BET‐induced photon quenching and the undesired green‐emitting radiative transition are entirely eliminated, leading to high‐efficiency single‐band red upconversion emission upon 808 nm NIR‐I laser excitation. Our findings provide insights into fundamental lanthanide interactions and advance the development of UCNPs for bioapplications with techniques that overturn traditional limitations.