Tb3+ and Yb3+ co-activated luminescent material that can cut one photon of around 483 nm into two NIR photons of around 1000 nm could be used as a downconversion luminescent convertor in front of crystalline silicon solar cell panels to reduce thermalization loss of the solar cell. The Tb3+ → Yb3+ energy transfer mechanisms in the UV–blue region in Y2O3 phosphor were studied by PL excitation spectra and time-resolved luminescence, from which the charge transfer mechanism and the cooperative transfer mechanism were identified. Tb3+ ions in the 4f75d1 state relax down to the 5D4 level and cooperatively transfer energy to two Yb3+ ions, which is followed by the emission of two photons (λ ∼ 1000 nm). It was found in the (Y0.79Tb0.01Yb0.20)2O3 sample that 37% of the Tb3+ ions at the 5D4 level transfer energy to two neighbouring Yb3+ ions by the cooperative energy transfer mechanism Tb3+ (5D4) → 2Yb3+ (2F5/2). Unfortunately, the high Yb3+ concentration leads to severe concentration quenching that significantly reduces the external quantum efficiency. Moreover, the energy of the Tb3+ 4f75d1 state can also be lost non-radiatively or transferred to the Yb3+ 2F5/2 state via the charge transfer state Tb4+–Yb2+. In conclusion, RE3+ (RE = Ce, Pr, Tb) with strong absorption in the UV region is not an appropriate sensitizer of Tb3+ in Tb3+–Yb3+ codoped downconversion phosphor.

Rare earth (RE 3+ )-doped semiconductors, such as GaN:RE [1] and Si:Er, [2] are technologically important materials in optoelectric devices and have received considerable interest. In most of these applications, efficient energy transfer from host to the RE 3+ is desired. ZnO with a bandgap of 3.37 eV and a bound exciton energy of 60 meV is also an important semiconductor for which UV, [3] blue, [4] and green [5] emissions are widely reported, but intense red emissions are still lacking. (Fig. 1a) at the bottom of the autoclave, which is decomposed into wurtzite ZnO after further annealing at 400°C (Fig. 1b). It is worth noting that the saturated concentration of Eu 3+ in ZnO is relatively low, smaller than 1.0 % (ca. 4.2 × 10 26 cm -3 ). As shown in Fig. 1b Fig. 1s). In addition, no meaningful shift in the X-ray diffraction (XRD) peaks for ZnO were detected, which indicates a low concentration of Eu 3+ in ZnO.Nevertheless, the X-ray photoelectron spectroscopy (XPS) 3d spectrum for Eu provides convincing evidence for Eu 3+ doping in ZnO (see Supporting Information, Fig. 2s). Consequently, in samples for optical measurements, the starting concentration of Eu 3+ was lower than 1.0 % to exclude the disturbance from the Eu 3+ ions outside of ZnO.Hosono et al. [12] proposed that LHZC follows a sheetlike growth feature along the hydrophilic b-and c-axes, whereas the (100) surface is hydrophobic. We describe the mechanism for the nucleation and crystal growth of LHZC microspheres as follows: after homogenous nucleation in solution, nuclea- COMMUNICATION 4510

Hierarchical ZnO microspheres constructed by mesoporous quasi-single-crystalline ZnO nanosheets were fabricated by pyrolysis of the microspheres of layered hydroxide zinc carbonate, Zn5(CO3)2(OH)6, which was the hydrothermal precipitate of zinc nitrate and urea. A growth mechanism of Zn5(CO3)2(OH)6 microspheres was proposed. During the pyrolysis process, single-crystalline Zn5(CO3)2(OH)6 nanosheets were transformed into mesoporous quasi-single-crystalline ZnO nanosheets. When the samples were doped with trivalent rare earth ion, RE3+ (RE = Pr, Sm, Tb, Ho, Tm), no ZnO → RE3+ energy transfer was observed. However, the ZnO:Eu3+ sample showed efficient Eu3+ emissions under UV photon excitation (λ < 365 nm), which is attributed to energy transfer from photon-generated electron−hole pairs to Eu3+ ions in the surface layer of the ZnO nanosheet.