Guicheng Jiang

Strategy for thermometry via Tm 3+ -doped NaYF 4 core-shell nanoparticles

Optical thermometers usually make use of the fluorescence intensity ratio of two thermally coupled energy levels, with the relative sensitivity constrained by the limited energy gap. Here we develop a strategy by using the upconversion (UC) emissions originating from two multiplets with opposite temperature dependences to achieve higher relative temperature sensitivity. We show that the intensity ratio of the two UC emissions, ³F(2,3) and ¹G₄, of Tm³⁺ in β-NaYF₄:20%Yb³⁺, 0.5%Tm³⁺/NaYF₄:1%Pr³⁺ core-shell nanoparticles under 980 nm laser excitation exhibits high relative temperature sensitivity between 350 and 510 K, with a maximum of 1.53%u2009u2009K⁻¹ at 417 K. This demonstrates the validity of the strategy, and that the studied material has the potential for high-performance optical thermometry.

794 nm excited core–shell upconversion nanoparticles for optical temperature sensing

Hexagonal core–shell NaYF4 upconversion nanoparticles (UNCPs) based on Nd3+ sensitization for optical temperature sensing were successfully synthesized by a solvothermal method using oleic acid and octadecene as coordinating solvents. Compared to the conventional Yb3+ sensitized UNCPs, the usage of Nd3+ as the sensitizers can shift the excitation wavelength from 975 nm to 794 nm where the optical absorption of water decreased dramatically, and thus make UNCPs more suitable for biological application. The upconversion (UC) luminescence intensity of the 794 nm-excitation UNCPs is comparable to that of the conventional 975 nm excitation, showing that Nd3+ sensitized UNCPs are efficient. The efficiently successive Nd3+ → Yb3+ → Er3+ energy transfer processes in this UNCP were demonstrated by excitation spectra and time-resolved spectra. The temperature dependence of the fluorescence intensity ratios (FIR) for the two green emissions (525 nm and 545 nm) from the thermally coupled levels of Er3+ was studied in the temperature range from 25 to 60 °C under 808 nm excitation, and the temperature mapping of a device was acquired according to this technique. These indicate that Nd3+ sensitized core–shell UNCPs are promising candidates for application in optical temperature sensors.

Blue upconversion of Tm3+ using Yb3+ as energy transfer bridge under 1532 nm excitation in Er3+, Yb3+, Tm3+ tri-doped CaMoO4

Abstract Nano-sized CaMoO 4 phosphors tri-doped with Er 3+ , Yb 3+ and Tm 3+ ions were successfully synthesized by sol-gel method. Intense blue emission from Tm 3+ ions was observed upon excitation of 1532 nm infrared light in Er-Yb-Tm system, while this blue upconversion could not be achieved with the absence of Yb 3+ ions in Er-Tm co-doped sample. In order to understand this upconversion process, the upconversion spectra in these samples were investigated, and the possible mechanism was proposed based on experimental results. It showed that two different energy transfer from Er 3+ to Tm 3+ existed simultaneously in Er-Yb-Tm system, the one-step direct energy transfer from Er 3+ to Tm 3+ , and the two-step Er 3+ →Yb 3+ →Tm 3+ energy transfer. In particular, the 1 G 4 state of Tm 3+ could only be populated from the 3 H 4 state by cross-relaxation with an excited Yb 3+ ion, producing blue emission of Tm 3+ . In this upconversion process, Yb 3+ ions acted as an energy transfer bridge between Er 3+ and Tm 3+ , which also meant that the upconversion of other rare-earth ions under the excitation of 1532 nm was possible with the presence of Er 3+ and Yb 3+ .

Topochemical transformation of single crystalline SrTiO3 microplatelets from Bi4Ti3O12 precursors and their orientation-dependent surface piezoelectricity

Two-dimensional multifunctional perovskite microcrystals are very important for various applications due to their unique shape-dependent properties. In this work, we reported a facile topochemical approach to synthesize SrTiO3 perovskite microplatelets with remarkably improved characteristics from Aurivillius Bi4Ti3O12 precursors. A stable Bi-containing intermediate phase was not formed during the Aurivillius to perovskite structure transformation, which facilitates the achievement of high phase/composition purity in the product and is easier than many previously reported topochemical reactions for perovskites. Different from the polycrystalline nature of the converted perovskites as reported previously, secondary recrystallization healed the substantial microstructural/crystalline damage induced during structural conversion, retaining the single-crystal memory of Bi4Ti3O12 in the converted SrTiO3 particles. Benefiting from subsequent epitaxial growth and rearrangement via Ostwald ripening, the SrTiO3 microplatelets exhibited more regular shapes and narrower size distributions than the precursors, instead of only closely resembling the precursor morphologies as reported previously. For the first time, local piezoelectricity was detected from the individual SrTiO3 microplatelets using a piezoelectric force microscope. In particular, the piezoelectric response parallel to the [001] direction was much higher than that detected perpendicular to it, possibly being associated with the orientation-dependent surface flexoelectric effect. This work offers a facile in situ topochemical strategy for fabricating other two-dimensional perovskite microcrystals and opens the door to expanding the potential application fields of SrTiO3 microplatelets.