In this paper, the upconversion luminescent properties of Y2O3:Er3+(1%)/Yb3+(4%) nanoparticles with different sizes (13−55 nm) and its corresponding bulk material as a function of excitation power were studied under 978-nm excitation. Red (4F9/2 → 4I15/2), green (2H11/2, 4S3/2 → 4I15/2), and blue (2H9/2 → 4I15/2) transitions were observed. The results indicated that the relative intensity of the blue as well as the red to the green increased gradually with decreasing particle size. As a function of excitation power, the slope in the ln−ln plot for the red emission changed between 2.0 and 1.0 and gradually decreased with increasing particle size, which was attributed to competition between linear decay and upconversion processes for the depletion of the intermediate excited states. As the particle size decreased to 13 nm, a three-photon populating process occurred for the green emission. As the excitation power varied in different paths, gradually increasing or gradually decreasing, a hysteresis loop appeared in the power dependence of emission intensity, which was mainly caused by a local thermal effect induced by laser irradiation. The intensity ratio of 2H11/2 → 4I15/2 to 4S3/2 → 4I15/2 (R HS ) varied complicatedly with excitation power, which was theoretically explained considering the thermal distribution and relaxation processes. Two novel cross-relaxation paths were proposed on the basis of the variation of R HS under excitation at different wavelengths, 488 and 978 nm.

We consider a multilevel hydrogen atom in interaction with the quantum electromagnetic field and separately calculate the contributions of the vacuum fluctuation and radiation reaction to the rate of change of the mean atomic energy of the atom for uniform acceleration. It is found that the acceleration disturbs the vacuum fluctuations in such a way that the delicate balance between the contributions of vacuum fluctuation and radiation reaction that exists for inertial atoms is broken, so that the transitions to higher-lying states from ground state are possible even in vacuum. In contrast to the case of an atom interacting with a scalar field, the contributions of both electromagnetic vacuum fluctuations and radiation reaction to the spontaneous emission rate are affected by the acceleration, and furthermore the contribution of the vacuum fluctuations contains a non-thermal acceleration-dependent correction, which is possibly observable.

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This work reports a simple hydrothermal route using citric acid as a "shape modifier" for the controlled synthesis of luminescent TbPO 4 :Eu nanocrystals. The size and morphology of products change remarkably when the proportion of citric acid involved in the reaction increases. The multiple roles that citric acid plays during the controlled synthesis are discussed to try to understand the crystallization and growth dynamics of TbPO 4 crystals. The photoluminescence properties of TbPO 4 :Eu are investigated. The excitation spectra and the variation of the 5 D 4 lifetime values as a function of the Eu 3+ concentration points out the occurrence of Tb 3+ -to-Eu 3+ energy transfer, resulting in a maximum absolute emission quantum yield of 0.14. The possibility to tune the size, the shape, and the optical properties of the nanocrystals reported in this work might be useful for applications in optoelectronics or biolabeling. Moreover, this simple approach might also be applied for the synthesis of other luminescent phosphates.

MAX phase materials are emerging as attractive engineering materials in applications where the material is exposed to severe thermal and mechanical conditions in an oxidative environment. The Ti2AlC MAX phase possesses attractive thermomechanical properties even beyond a temperature of 1000 K. An attractive feature of this material is its capacity for the autonomous healing of cracks when operating at high temperatures. Coupling a specialized thermomechanical setup to a synchrotron X-ray tomographic microscopy endstation at the TOMCAT beamline, we captured the temporal evolution of local crack opening and healing during multiple cracking and autonomous repair cycles at a temperature of 1500 K. For the first time, the rate and position dependence of crack repair in pristine Ti2AlC material and in previously healed cracks has been quantified. Our results demonstrate that healed cracks can have sufficient mechanical integrity to make subsequent cracks form elsewhere upon reloading after healing.

Ln 3+ doped (Yb 3+ ,Tm 3+ codoped and Yb 3+ ,Er 3+ ,Tm 3+ tridoped) NaYF 4 /poly(vinyl pyrrolidone)(PVP) (M w ≈ 1 300 000) composite fibers with an average diameter of 300-800 nm were prepared by electrospinning and characterized by X-ray diffraction, field emission scanning electron micrography, and Fourier transform infrared spectra. Their upconversion (UC) luminescence properties were studied in contrast to the corresponding Ln 3+ doped NaYF 4 nanoparticles (15-20 nm) under 980-nm excitation. The results demonstrate that in the Yb 3+ ,Tm 3+ codoped composite fibers the blue emission of 1 G 4 -3 H 6 is dominantly strong, while in the nanoparticles the red emission of 3 F 2,3 -3 H 6 contributes considerably to the increase of the excitation power. This indicates that the color purity of blue is improved greatly by the modification of PVP. In the tridoped Yb 3+ ,Er 3+ ,Tm 3+ composite fibers, white light with more stable color balance (blue 1 G 4 -3 H 6 of Tm 3+ , green 2 H 11/2 / 4 S 3/2 -4 I 15/2 , and red 4 F 9/2 -4 I 15/2 of Er 3+ ) was obtained. The improved UC properties in the composite fibers are attributed to the suppressed local thermal effect. The energy transfer and UC populating processes are discussed.

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