Yu-Hao Wu

The emission of a luminescent material is governed not only by the centers it contains but by how they are excited, a distinction that becomes decisive when several emitting phases coexist. Eu- and Dy-doped strontium aluminates, the most widely used persistent-luminescence phosphors, are frequently multiphase, which complicates the assignment of their emission. Here, a mixed-phase Eu,Dy-doped strontium aluminate ceramic, with estimated phase fractions of 71.4% SrAl2O4 and 28.6% Sr4Al14O25 from powder X-ray diffraction, is characterized at the Taiwan Photon Source 23A hard X-ray nanoprobe by X-ray-excited optical luminescence (XEOL) spectroscopy and mapping, X-ray fluorescence (XRF) mapping, and X-ray absorption spectroscopy (XAS). The dominant spectral contributions depend on the excitation channel: under 9670 eV excitation the XEOL is dominated by a 493 nm Eu2+ band characteristic of the minority Sr4Al14O25 phase, whereas ultraviolet (UV)-excited photoluminescence (PL) is dominated by the 519 nm band characteristic of the majority SrAl2O4 host. Because broadband hard X-ray excitation is less site-selective than direct UV excitation, XEOL additionally reveals Eu3+ and Dy3+ lines scarcely visible in PL and sharp Cr3+-related emission near 692–694 nm, and XAS supports these oxidation-state assignments. Energy-dispersive XRF detected trace Mn, whose K-edge absorption profile was most consistent with Mn2+, although no Mn2+ emission was resolved. A weak violet band near 408 nm is tentatively assigned to an F-type color center, although a second-site Eu2+ contribution cannot be excluded. This approach resolves multiple emission features in a multiphase phosphor and demonstrates excitation-dependent spectral weighting consistent with different phase contributions.

Distributions of Eu2+ and Eu3+ do affect the optical properties. However, the substitution of the activator ions hardly perturbed the crystal structure of the host lattice. Without the differences in crystal structure, it is not trivial to develop an effective descriptor to investigate stoichiometry-dependent mechanism to explore the effects of Eu species on valence states. In this study, through x-ray nanodiffraction, x-ray fluorescence, x-ray absorption near edge structure, and x-ray excited optical luminescence, we mapped the valence state distributions of Eu species to calculate the local mixing enthalpy and entropy. The calculated thermodynamics parameters show good agreement with the optical properties. We found that Eu2+ of Eu-doped BaAl2O4 predominantly existed in a divalent state, which results in segregation phenomena of local Eu2+ and Eu3+ ions.

In this study, a combination of X-ray excited optical luminescence (XEOL), time-resolved XEOL (TR-XEOL) and the Hanbury-Brown and Twiss (HB-T) interferometer at the Taiwan Photon Source (TPS) 23A X-ray nanoprobe beamline for exploring quantum materials is demonstrated. On the basis of the excellent spatial resolution rendered using a nano-focused beam, emission distributions of artificial micro-diamonds can be obtained by XEOL maps, and featured emission peaks of a selected local area can be obtained by XEOL spectra. The hybrid bunch mode of the TPS not only provides a sufficiently high peak power density for experiments at each beamline but also permits high-quality temporal domain (∼200 ns) measurements for investigating luminescence dynamics. From TR-XEOL measurements, the decay lifetime of micro-diamonds is determined to be approximately 16 ns. Furthermore, the XEOL spectra of artificial micro-diamonds can be investigated by the HB-T interferometer to identify properties of single-photon sources. The unprecedented strategy of combining XEOL, TR-XEOL and the HB-T interferometer at the X-ray nanoprobe beamline will open new avenues with significant characterization abilities for unraveling the emission mechanisms of single-photon sources for quantum materials.

The cadmium tungstate rods have been given much attention due to their potential for usage in numerous luminescent applications. We have prepared single crystalline Sn-doped Cd_1−xSn_xWO₄ (where x = 0, 1, 3, and 5%) nanorods (NRDs) and characterized them using refined X-ray diffraction and TEM analysis, revealing a monoclinic phase and a crystallite size that decreased from 62 to 38 nm as Sn concentration increased. Precise Sn doping modulation in CdWO₄ NRDs causes surface recombination of electrons and holes, which causes the PL intensity to decrease as the Sn content rises. The chromaticity diagram shows that an increase in the Sn content caused a change in the emission color from sky blue to light green, which was attributed to the increased defect density. The photoluminescence time decay curve of all samples fit well with double-order exponential decay, and the average decay lifetime was found to be 1.11, 0.93, and 1.16 ns for Cd_1−xSn_xWO₄, x = 0, 1, and 5%, respectively. This work provides an understanding of the behavior of Sn-doped CdWO₄ NRDs during electron transitions and the physical nature of emission that could be used in bio-imaging, light sources, displays, and other applications.

The present work extends the examination of selective laser melting (SLM)-fabricated 15-5 PH steel with the 8%-transient-austenite-phase towards fully-reversed strain-controlled low-cycle fatigue (LCF) test. The cyclic-deformation response and microstructural evolution were investigated via in-situ neutron-diffraction measurements. The transient-austenite-phase rapidly transformed into the martensite phase in the initial cyclic-hardening stage, followed by an almost complete martensitic transformation in the cyclic-softening and steady stage. The compressive stress was much greater than the tensile stress at the same strain amplitude. The enhanced martensitic transformation associated with lower dislocation densities under compression predominantly governed such a striking tension-compression asymmetry in the SLM-built 15-5 PH.

This study develops and successfully demonstrates visualization methods for the characterization of europium (Eu)-doped BaAl₂O₄ phosphors using X-ray nanoprobe techniques. X-ray fluorescence (XRF) mapping not only gives information on the elemental distributions but also clearly reveals the valence state distributions of the Eu²⁺ and Eu³⁺ ions. The accuracy of the estimated valence state distributions was examined by performing X-ray absorption spectroscopy (XAS) across the Eu L ₃-edge (6.977 keV). The X-ray excited optical luminescence (XEOL) spectra exhibit different emission lines in the selected local areas. Their corresponding emission distributions can be obtained via XEOL mapping. The emission properties can be understood through correlation analysis. The results demonstrate that the main contribution to the luminescence intensity of the Eu-doped BaAl₂O₄ comes from the Eu²⁺ activator and the emission intensity will not be influenced by the concentration of Eu²⁺ or Eu³⁺ ions. It is anticipated that X-ray nanoprobes will open new avenues with significant characterization ability for unravelling the emission mechanisms of phosphor materials.

Time-resolved X-ray excited optical luminescence (TR-XEOL) was developed successfully for the 23A X-ray nanoprobe beamline located at the Taiwan Photon Source (TPS). The advantages of the TR-XEOL facility include (i) a nano-focused X-ray beam (<60 nm) with excellent spatial resolution and (ii) a streak camera that can simultaneously record the XEOL spectrum and decay time. Three time spans, including normal (30 ps to 2 ns), hybrid (30 ps to 310 ns) and single (30 ps to 1.72 µs) bunch modes, are available at the TPS, which can fulfil different experimental conditions involving samples with various lifetimes. It is anticipated that TR-XEOL at the TPS X-ray nanoprobe could provide great characterization capabilities for investigating the dynamics of photonic materials.