The applications of lanthanide-doped upconversion nanocrystals in biological imaging, photonics, photovoltaics and therapeutics have fuelled a growing demand for rational control over the emission profiles of the nanocrystals. A common strategy for tuning upconversion luminescence is to control the doping concentration of lanthanide ions. However, the phenomenon of concentration quenching of the excited state at high doping levels poses a significant constraint. Thus, the lanthanide ions have to be stringently kept at relatively low concentrations to minimize luminescence quenching. Here we describe a new class of upconversion nanocrystals adopting an orthorhombic crystallographic structure in which the lanthanide ions are distributed in arrays of tetrad clusters. Importantly, this unique arrangement enables the preservation of excitation energy within the sublattice domain and effectively minimizes the migration of excitation energy to defects, even in stoichiometric compounds with a high Yb(3+) content (calculated as 98 mol%). This allows us to generate an unusual four-photon-promoted violet upconversion emission from Er(3+) with an intensity that is more than eight times higher than previously reported. Our results highlight that the approach to enhancing upconversion through energy clustering at the sublattice level may provide new opportunities for light-triggered biological reactions and photodynamic therapy.

Precise detection of low-dose X- and γ-radiations remains a challenge and is particularly important for studying biological effects under low-dose ionizing radiation, safety control in medical radiation treatment, survey of environmental radiation background, and monitoring cosmic radiations. We report here a photoluminescent uranium organic framework, whose photoluminescence intensity can be accurately correlated with the exposure dose of X- or γ-radiations. This allows for precise and instant detection of ionizing radiations down to the level of 10 Gy, representing a significant improvement on the detection limit of approximately two orders of magnitude, compared to other chemical dosimeters reported up to now. The electron paramagnetic resonance analysis suggests that with the exposure to radiations, the carbonyl double bonds break affording oxo-radicals that can be stabilized within the conjugated uranium oxalate-carboxylate sheet. This gives rise to a substantially enhanced equatorial bonding of the uranyl(VI) ions as elucidated by the single-crystal structure of the γ-ray irradiated material, and subsequently leads to a very effective photoluminescence quenching through phonon-assisted relaxation. The quenched sample can be easily recovered by heating, enabling recycled detection for multiple runs.

Four new sodium uranyl borates, R-Na[(UO 2 ) 2 B 10 O 15 (OH) 5 )] (NaUBO-1), β-Na[(UO 2 ) 2 B 10 O 15 -(OH) 5 ] (NaUBO-2), Na[(UO 2 ) 2 B 10 O 15 (OH) 5 ] 3 3H 2 O (NaUBO-3), and Na[(UO 2 )B 6 O 10 (OH)] 3 2H 2 O (NaUBO-4), and four new thallium uranyl borates, R-Tl 2 [(UO 2 ) 2 B 11 O 18 (OH) 3 ] (TlUBO-1), β-Tl 2 [(UO 2 ) 2 B 11 O 18 (OH) 3 ] (TlUBO-2), Tl[(UO 2 ) 2 B 10 O 16 (OH) 3 ] (TlUBO-3), and Tl 2 [(UO 2 ) 2 B 11 O 19 -(OH)] (TlUBO-4), have been prepared via the reaction of sodium nitrate or thallium nitrate, uranyl nitrate, and excess boric acid at 190 °C. These compounds share a common structural motif consisting of a linear uranyl, UO 2 2þ, cation surrounded by BO 3 triangles and BO 4 tetrahedra to create a UO 8 hexagonal bipyramidal environment around uranium. The borate anions bridge between uranyl units to create sheets. Additional BO 3 triangles extend from the polyborate layers and are directed approximately perpendicular to the sheets. In some compounds, these units can link the layers together to yield three-dimensional networks with large pores to house the Na þ or Tl þ cations and water molecules. The structures are all noncentrosymmetric and are either polar or chiral. While the uranyl borate layers are noncentrosymmetric in and of themselves, there is also twisting of the interlayer BO 3 groups to reduce the interlayer spacing, producing helical features in some structures. Na[(UO 2 )B 6 O 10 (OH)] 3 2H 2 O and β-Tl 2 [(UO 2 ) 2 B 11 O 18 (OH) 3 ], which can be obtained as pure phases, display second-harmonic generation of 532 nm light from 1064 nm light.

Photon upconversion in rare earth activated phosphors involves multiple mechanisms of electronic transitions. Stepwise optical excitation, energy transfer, and various nonlinear and collective light-matter interaction processes act together to convert low-energy photons into short-wavelength light emission. Upconversion luminescence from nanomaterials exhibits additional size and surface dependencies. A fundamental understanding of the overall performance of an upconversion system requires basic theories on the spectroscopic properties of solids containing rare earth ions. This review article surveys the recent progress in the theoretical interpretations of the spectroscopic characteristics and luminescence dynamics of photon upconversion in rare earth activated phosphors. The primary aspects of upconversion processes, including energy level splitting, transition probability, line broadening, non-radiative relaxation and energy transfer, are covered with an emphasis on interpreting experimental observations. Theoretical models and methods for analyzing nano-phenomena in upconversion are introduced with detailed discussions on recently reported experimental results.