Phase-Transition-Driven Regional Distribution of Rare-Earth Ions for Multiplexed Upconversion Emissions.

Phase transition of the polymorphs is critical for controlled synthesis and property modulation of functional materials. Upconversion emissions from an efficient hexagonal sodium rare-earth (RE) fluoride compound, ß-NaREF4, which is generally obtained from the phase transition of the cubic (α-) phase counterpart, are attractive for photonic applications. However, the investigation of the α → ß phase transition of NaREF4 and its effect on the composition and architecture is still preliminary. Herein, we investigated the phase transition with two kinds of α-NaREF4 particles. Instead of a uniform composition, the ß-NaREF4 microcrystals exhibited regionally distributed RE3+ ions, in which the RE3+ with a smaller ionic radius (smaller RE3+) sandwiched the RE3+ with a larger ionic radius (larger RE3+). We unravel that the α-NaREF4 particles transformed to ß-NaREF4 nuclei with no controversial dissolution, and the α → ß phase transition toward NaREF4 microcrystals included nucleation and growth steps. The component-dependent phase transition is corroborated with RE3+ ions from Ho3+ to Lu3+ and multiple sandwiched microcrystals were obtained, in which up to five kinds of RE components were distributed regionally. Moreover, with rational integration of luminescent RE3+ ions, a single particle with multiplexed upconversion emissions in wavelength and lifetime domains is demonstrated, which provides a unique platform for optical multiplexing applications.

Copper(II)-Doped Two-Dimensional Titanium-Based Metal-Organic Frameworks toward Light-Driven CO2 Reduction to Value-Added Products.

Recently, metal-organic framework (MOF)-based photocatalysts for an efficient CO2 reduction reaction have drawn wide attention in multidisciplinary fields and sustainable chemistry. In this work, a series of Cu2+-doped two-dimensional Ti-based MOFs were fabricated by a facile in situ solvothermal method. Cu2+ ions were doped in equal proportions and uniformly dispersed in the crystal structure of the MOF matrix. Interestingly, the doping content of Cu2+ ions and the photocatalytic performance displayed an obvious volcanic relationship, the medium-concentration Cu2+-doped sample (T1-2Cu) held the greatest activity with 100% carbonaceous product (CH4 and CO) formation, and the CH4 production rate was 3.7 µmol g-1 h-1 with 93% electron selectivity. The band structure, local electronic structure, carrier separation kinetics, and CO2 adsorption studies demonstrated that the excellent photocatalytic activity of T1-2Cu benefited from the appropriate amount of Cu2+ ion doping: (1) a doping amount of 2 atom % optimized the conduction band position of the MOF substrate and endowed T1-2Cu with strong reduction potential in thermodynamics, (2) doping Cu2+ ions tuned the local electronic environment around titanium oxide clusters and optimized the generation, separation, and migration processes of photoinduced carriers, and (3) the introduction of Cu2+ ions also provided more accessible active sites and more probabilities for the adsorption and activation of CO2 reactants.

Highly Polarized Upconversion Emissions from Lanthanide-Doped LiYF4 Crystals as Spatial Orientation Indicators.

Polarized emission, an inherent characteristic that correlated with structure and morphology, is very sensitive to orientation. For the upconversion (UC) emission of lanthanides, the mechanism of polarization is rarely discussed, and the highly polarized UC emissions are poorly developed. Herein, with the benefit of the strong anisotropic crystal field, well-resolved emissions from lanthanide-doped LiYF4 crystals were studied, and highly polarized UC emissions from Er3+ and Ho3+ were investigated. With multiple sub-energy level transitions, the UC emissions are classified into two sets, with transition dipoles being either parallel or perpendicular to the c-axis of the LiYF4 crystal. An optical three-dimensional orientation sensor was further investigated, in which the in-plane angle is referenced from the orientation of the transition dipoles. In contrast, the out-of-plane angle can be deduced from the change in the degree of polarization. This research deepens our understanding of the polarized photoluminescence, and it opens up an avenue toward unique UC orientation sensors.

Lanthanide Upconverted Microlasing: Microlasing Spanning Full Visible Spectrum to Near-Infrared under Low Power, CW Pumping.

The miniaturization of lasers holds promise in ultradense data storage and biosensing, but greater pump power is required to reach the lasing thresholds to overcome increased optical losses with reduced resonant cavity sizes. Here, the whispering galley mode (WGM) of Yb3+ /Tm3+ doped upconversion nanoparticles (UCNPs) coupled with microcavities (≈5 µm) is used to achieve ultralow threshold upconverted lasing at 800 nm with excitation fluences as low as 4 W cm-2 . The continuous-wave (CW) upconverted lasing, with a Q factor on the order of 103 , can remain stable for more than 6 h. In addition, ultralow threshold upconverted microlasers spanning the full visible spectrum from Yb3+ /Er3+ , Yb3+ /Ho3+ , and Yb3+ /Tm3+ doped UCNPs are obtained with the same WGM cavity design. These upconverted microlasers working under low power CW 980 nm laser will enable promising applications in biosensing and imaging.