X-ray Excited Optical Luminescence Research Articles

Tracking coordination environment and optoelectronic structure of Eu3+ and Sm3+ sites via X-ray absorption spectroscopy and X-ray excited optical luminescence

In this work, the photophysical characteristics of the [Eu(tta)3(H2O)2] and [Sm(dbm)3(phen)] complexes, tta: 3-thenoyltrifluoroacetonate, dbm: dibenzoylmethane, and phen: phenanthroline were explored through X-ray excited optical luminescence (XEOL) in correlation with the local electronic/chemical structure evidences from the element-specific X-ray absorption fine structure (XAFS) analysis. The monochromatic hard X-ray photon at a synchrotron light source has added advantageous tunability with desirable penetration depth, avoiding optical photons scattering contrary to the optical LASER to unravel the relationship of the luminescence to the electronic structure and coordination environment of RE3+ (RE: Eu and Sm) site in compounds by tuning the photon energy of monochromatic X-ray to the L-edge of the rare earth. Accordingly, the XEOL spectra of both the rare earth complexes measured under excitations with tunable Eu3+ and Sm3+ L-edge monochromatic X-ray photon energy manifested considerably higher emission intensity from the hypersensitive 5D0→7F2 and 4G5/2→6H9/2 forced electric dipole (FED) transitions of Eu3+ and Sm3+ ions, respectively. This result suggested the non-centrosymmetric RE3+ site (lower symmetry), validating through quantitative XAFS analyses, which confirm the triclinic and monoclinic local structure for the [Eu(tta)3(H2O)2] and [Sm(dbm)3(phen)] complexes, respectively.

Distinguishing xenotime and zircon in ores and estimating the xenotime content for on-site analysis

Estimation of the content of the major minerals containing rare earth element (REE) (e.g., xenotime, monazite, and bastnäsite) to extract metal REEs is a critical task for efficient exploration of mines with REE reserves. X-ray-excited optical luminescence (XEOL) imaging is a promising method for estimating the REE-bearing mineral content on-site. However, distinguishing between xenotime and zircon in ores via XEOL imaging is difficult owing to their similar luminescence colors and intensities. This study reveals that XEOL images of ores before and after annealing at 1300 °C can distinguish xenotime and zircon by investigating images obtained via cathodoluminescence (CL), which is the same phenomenon as XEOL except that it used electron bombardment instead of X-ray irradiation. After annealing, zircon exhibits a luminescence intensity stronger than that of xenotime in the CL images. In these images, zircon corresponds to an area with green luminescence whose CL intensity is drastically enhanced by annealing; in contrast, xenotime corresponds to an area with green luminescence whose CL intensity does not change much. The xenotime content in ores can be estimated from the area corresponding to xenotime in the CL images. The exposure time for CL images, which is comparable to XEOL images, is obtained in 30 s. Therefore, the proposed method can be applied to XEOL imaging and used to on-site prescreen ores before precise quantitative analyses, such as inductively coupled plasma mass spectrometry, electron-probe microanalysis, or scanning electron microscopy based on automated mineralogy, which require a large amount of time; thus, the adoption of the proposed method can lead to a drastic reduction in the time required to explore mines reserving REEs.

Mixing entropy and enthalpy effects on europium ions in Eu-doped BaAl2O4

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.

Mechanism of X-ray excited optical luminescence in NaGdF4-based nanoparticles

X-ray irradiating rare-earth nanoparticles could emit fluorescence (∼50 keV) and luminescence (∼eV) photons simultaneously. Utilizing this phenomenon, it is expected to achieve online multi-modal imaging and integrated radiation and photodynamic therapy for deep tumor. In this scenario, ALnF4: RE3+ (A = alkali metal; Ln = lanthanides acting as matrix; RE3+: rare-earth element used for luminescence center ion) nanoparticles are often selected due to their stable chemical and optical properties. In the application of imaging and therapy, the quantitative calculation of luminescence yield is meaningfully significant for nanoparticle design and to improve the imaging quality and therapeutic effect. However, there is a lack of theoretical calculation on the X-ray excited optical luminescence (XEOL) yield. In this paper, we propose a method to calculate the XEOL yield of ALnF4: RE3+ through theoretical derivation. Taking NaGdF4: Tb3+ as an example, the luminescence mechanism under X-ray irradiation is analyzed and the semi-quantitative formula of luminescence yield is obtained, which is based on the interaction theory of X-ray with materials and the energy transfer principle between different lanthanides. Then, the quantitative XEOL yield formula is gotten by fitting experimental results and the correctness of this formula is verified by the correlation coefficient between fitting results and experimental results. Our results could provide theoretical guidance for the design of NaGdF4-based nanoparticles to manipulate XEOL spectrum and increase XEOL yield. It also laid the foundation for the subsequent development of effective drugs or probes for X-ray driving integrated diagnosis and treatment of deep tumors in vivo.

Combination of XEOL, TR-XEOL and HB-T interferometer at the TPS 23A X-ray nanoprobe for exploring quantum materials.

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.

Investigation of X-ray excited optical luminescence properties of Er, Nd-doped YVO4 phosphors using a hard X-ray beam

Scintillators based on YVO4:Er3+ and YVO4:Er3+, Nd3+, synthesized through glucose-assisted sol-gel method and calcined under different temperatures were studied via X-ray excited optical luminescence (XEOL). The samples' crystalline structure was studied via X-ray powder diffraction and the crystallite size estimated by Scherrer's method. The results indicated the crystal purity phase in all samples, indexed as YVO4, and crystallite size in nanometric order, which is in accordance with Transmission Electron Microscopy results that indicate the formation of nanoparticles with spherical shape tendency. The spectroscopic studies taken by XEOL demonstrated typical Er3+ and Nd3+ transitions were dominant on the emission spectra. An unusual host emission was also identified c.a. 340 and 600 nm and ascribed as an intrinsic host vanadate emission related with the material optical band gap (c.a. 3.5 eV) and/or associated with VO43− to rare earth ions energy transfer. The scintillators kept the emission stability even exposed to ∼1010 photon/sec, which demonstrated their radiation hardness, which is an important feature to the application of YVO4 based-materials as scintillators. Independently of the calcination temperature, glucose-assisted sol-gel method was reliable to produce YVO4-based scintillators nanopowders with stable optical properties with potential to be applied in biological imaging and/or in theranostics.

X-ray excited optical luminescence from lanthanide ions in double phosphate and double silicate crystalline hosts

X-ray Excited Optical Luminescence (XEOL) from rare earth doped whitlockite phosphates and silicate garnets [Ca9La(PO4)7 and Ca9Lu(PO4)7, doped with 1 mol% Ce3+; Ca3Sc2Si3O12, doped with 0.5 mol% Pr3+, 1 mol% Eu3+ and 1 mol% Tb3+], was obtained upon X-ray excitation with a tunable synchrotron light source. Excitation at photon energies across the M5,4-edge of the dopants reveals additional information not normally obtainable with conventional UV/visible or an X-ray line source (Al Kα). It is found that despite the low concentration, photoluminescence yields excited with photon energy at the M5,4-edge of the rare-earth dopant can positively and negatively affect the overall energy transfer to the optical center. The implication of these observations is discussed. The application of XEOL in the investigation of X-ray scintillators is illustrated.

Tuning scintillation property of CsLu2F7: Pr crystals for X-ray imaging

Lanthanide doped fluoride scintillators possess a diverse range of applications in the fields of computed tomography and medical imaging. In this study, CsLu2F7: Pr crystals have been synthesized by a simple hydrothermal method to realize X-ray excited optical luminescence (XEOL). Our results reveal that the reaction temperature and amount of cetyltrimethylammonium bromide (CTAB) can significantly change the morphology of the crystals; the suitable hydrothermal reaction conditions, the amount of CTAB, and the doping concentration of Pr3+ ions are favorable for the enhancement of the XEOL intensity; and the crystals exhibit good irradiation stability under prolonged X-ray excitation. In addition, we used the crystal to prepare scintillation films to realize XEOL imaging. This research provides crucial information to support an in-depth investigation into the XEOL performances of CsLu2F7 doped with lanthanide ions.

X-ray excited optical luminescence at Carnaúba, the Sirius X-ray nanoprobe beamline

This work presents the developments and initial results obtained using X-ray excited optical luminescence (XEOL) at the Carnaúba, the nanoprobe beamline from Sirius, the Brazilian 4th generation synchrotron source. XEOL is a highly important tool to study luminescence mechanisms and identify optical channels in various types of materials under X-ray irradiation. Using this capability combined with a nanoprobe, one can probe optical channels, for example, in heterogenous materials, and directly correlate them with chemical composition, oxidizing state of absorbers, structure and/or morphology in simultaneous studies. Carnaúba is composed by two experimental station and at the Tarumã station, the first entering in operation, it has been possible to investigate XEOL in the integrated and emission modes, and 2D images in hyperspectral mapping with submicrometric resolution. Herein, it is showed some examples of studies combining excitation energy, emission wavelength and emission intensity in 3D spectra, 2D imaging, and XEOL emission on the study of very sensitive samples that can be easily damaged by the probe. This work also highlights some of the upcoming developments that include the time-resolved and in situ experiments, this last one using microfluidic cells, and the improvement of the covered emission wavelength up to 1700 nm, which will open new opportunities for studying optical materials.

3D and Multimodal X-Ray Microscopy Reveals the Impact of Voids in CIGS Solar Cells.

Small voids in the absorber layer of thin-film solar cells are generally suspected to impair photovoltaic performance. They have been studied on Cu(In,Ga)Se2 cells with conventional laboratory techniques, albeit limited to surface characterization and often affected by sample-preparation artifacts. Here, synchrotron imaging is performed on a fully operational as-deposited solar cell containing a few tens of voids. By measuring operando current and X-ray excited optical luminescence, the local electrical and optical performance in the proximity of the voids are estimated, and via ptychographic tomography, the depth in the absorber of the voids is quantified. Besides, the complex network of material-deficit structures between the absorber and the top electrode is highlighted. Despite certain local impairments, the massive presence of voids in the absorber suggests they only have a limited detrimental impact on performance.

Hidden Hexavalent Chromium Ions with Subtle Structural Evolution in Near-Infrared Phosphors.

Cr-doped inorganic materials are pivotal in developing near-infrared optical materials; however, multivalent Cr ions and their respective distribution in the materials remain ambiguous. Herein, a series of Li(Sc1-xInx)O2:Cr phosphors containing both Cr3+/Cr6+ ions are prepared. High-resolution synchrotron X-ray diffraction (XRD) reveals two similar phases in Li(Sc1-xInx)O2. Raman spectra further confirm distinct scattering patterns for the two end-member compositions, corroborating the findings from the synchrotron XRD analysis. Cr K-edge X-ray absorption near-edge structure and extended X-ray absorption fine structure demonstrate that most Cr ions in the as-prepared samples are Cr6+, while Cr3+ becomes dominant after washing with water. Moreover, the source and distribution of Cr3+ and Cr6+ ions in the as-prepared and washed samples are revealed through X-ray fluorescence and X-ray excited optical luminescence techniques, which indicate that Cr6+ ions aggregate within the sample, while Cr3+ ions are evenly distributed. Photoluminescence, decay curves, and line shape analyses are implemented to resolve the electron-lattice interactions, and the corresponding mechanisms are provided to explain the asymmetry between photoluminescence and photoluminescence excitation spectra. Overall, this study provides valuable insights into the distribution of low-concentration multivalence ions in solid-state materials and offers a deeper understanding of the approaches to precisely resolve the subtle changes in the crystal structure.

From synthesis to fabrication: Engineering thin translucent films with green persistent luminescent nanoparticles

Green-emitting ZnGa2O4:Mn2+ persistent luminescent nanoparticles were synthesized via a solvothermal method followed by a microwave-assisted sintering at 1150 °C. The obtained cubic-like particles averaged 62 ± 16 nm by Transmission Electron Microscopy (TEM) and presented afterglow for up to 2 h after a 5 min excitation in the UV. By Electron Paramagnetic Resonance (EPR), it was observed that the Mn2+ dopant replaces uniquely Zn2+ sites during synthesis. The obtained particles were dispersed with hydroxypropyl methylcellulose (HPMC) in water to create thin films by drop-casting. The films with different concentrations of nanoparticles (1 g/m2, 10 g/m2 and 100 g/m2) had average visible transmittances between 20% and 24%, and presented persistent luminescence after UV excitation, with longer duration by increasing nanoparticle concentration. By synchrotron X-ray Fluorescence (XRF) nanomapping of these films it is possible to see clusters of up to 6 μm of the nanoparticles in the film due to water pockets during film-casting. The X-ray Excited Optical Luminescence (XEOL) showed only Mn2+ emission and the XEOL-XRF mapping proved the integrity of the nanoparticles after film fabrication. By combining the solvothermal method, microwave-assisted sintering, and drop casting, this study establishes a promising pathway for the development of advanced flexible and translucent persistent luminescent composites.

Europium(III) diphenylphosphinate as a potential low-dimensional nanostructured coordination polymer scintillator

Rare earth diphenylphosphinates – [RE(dpp)3]n – are low-dimensional nanostructured luminescent materials with considerable thermal and chemical stability, obtained in powder form, consisting of highly insoluble nanowire/nanorod-shaped particles. As a potential candidate for applications as nanocrystalline scintillator devices, [Eu(dpp)3]n has been synthesized by wet route precipitation methodologies. X-ray excited optical luminescence (XEOL) measurements revealed that the samples show intense emission under X-ray excitation, presenting structural and electronic stability against continuous and prolonged exposure to ionizing radiation without changing the chemical environment in which the trivalent europium ion is inserted. Indeed, the self-structuring of [RE(dpp)3]n in a polymeric matrix of high thermal stability allowed [Eu(dpp)3]n to be a low-dimensional nanostructured luminescent coordination polymer resistant to radiation damage.

Simultaneous X-ray ptychography, X-ray fluorescence and X-ray excited optical luminescence experiments in metal halide perovskites: correlating morphology, chemical heterogeneity and optoelectronic properties

Simultaneous X-ray ptychography, X-ray fluorescence and X-ray excited optical luminescence experiments in metal halide perovskites: correlating morphology, chemical heterogeneity and optoelectronic properties

The influence of hydrothermal synthesis temperature on the electronic structure and luminescence property of Cr-doped ZnGa2O4 nanoparticles

Cr-doped ZnGa2O4 (CZGO) is a near-infrared-emitting material with long-lasting persistent luminescence. This unique property makes it a great candidate in optical imaging and sensing for various biomedical applications. Nanosized CZGO can be synthesized via a hydrothermal approach, but the reaction temperatures reported in existing literature vary from 120 °C to 220 °C, approaching the highest temperature allowed for a standard autoclave vessel. The reason for choosing a particular hydrothermal synthesis temperature has rarely been discussed, and it is unclear whether the resulting CZGO possess the same optical properties. This work compares CZGO nanoparticles synthesized at temperatures from the lowest reported 120 °C and to as high as 220 °C. We find that although all the synthesized CZGO are light emitting, they exhibit different luminescence intensities and respond differently to the change of excitation energy. X-ray absorption fine structure (XAFS) analysis is employed to investigate the local chemical environment around Zn, Ga and Cr, respectively. We found synthesis temperature strongly influences the Ga species formed in these particles. The energy transfer mechanism is further elucidated using X-ray excited optical luminescence (XEOL) in combination with element-specific XAFS. Two energy transfer paths are identified, which explains the different excitation energy dependencies of the observed photoluminescence.

Probing the emission properties of color centers in MgAl2O4 wafers using hard X-ray nanoprobes

In this study, we used hard X-ray-excited optical luminescence (XEOL) to investigate the emission properties of MgAl2O4 wafers with a crystal orientation of (100), (110), or (111). MgAl2O4(110) exhibited the lowest XEOL emission intensity because the (110) orientation is thermochemical unstable and therefore unfavorable for generating oxygen vacancies. By fitting XEOL spectra, we identified 13 and 6 clear emission peaks corresponding to the color centers in MgAl2O4 wafers and the 2Eg→4A2g radiative transition of Cr ion impurities, respectively. Using an X-ray nanoprobe, we obtained an X-ray fluorescence spectroscopy (XRF) map of the elemental distribution of Cr with 100 nm spatial resolution. X-ray absorption spectroscopy (XAS) revealed that the valence state of the Cr ion impurities was Cr3+. We thus also clearly demonstrate the advantages of combining XEOL, XRF, and XAS in the X-ray nanoprobe beamline by obtaining emission spectra using XEOL and using XRF and XAS to analyze elemental distributions and valence states to further elucidate the emissions of samples.

Optimization of SiO2 reflective layer thickness for improving the performance of structured CsI scintillation screen based on oxidized Si micropore array template in X-ray imaging.

Structured scintillation screen based on oxidized Si micropore array template can effectively improve the spatial resolution of X-ray imaging. The purpose of this study is to investigate the effect of SiO2 layer thickness on the light guide and X-ray imaging performance of CsI scintillation screen when the structural period is as small as microns. Cylindrical micropores with a period of 4.3 µm, an average diameter of 3.3 µm and a depth of about 40 µm were prepared in Si wafers. SiO2 layer was formed on the pore walls after thermal oxidation. Increasing SiO2 layer thickness would be beneficial to the propagation of scintillation light along the cylindrical channels. What was not previously anticipated was that the pore size gradually shrank as the SiO2 layer thickened. The pore shrinkage would reduce the filling rate of CsI in the templates and thus would reduce the production of scintillation light. The structured CsI scintillation screens with different SiO2 layer thicknesses were fabricated by filling CsI scintillator into the oxidized silicon micropore array template. The morphology, crystallinity, X-ray excited optical luminescence, and X-ray imaging performance of the screens were studied. The results show that the spatial resolutions of X-ray images measured using the structured CsI scintillation screens with different SiO2 layer thicknesses are close to each other, and they are all about 110 lp/mm. However, the X-ray excited optical luminescence of the screen and detective quantum efficiency of X-ray imaging vary with the thickness of the SiO2 layer. The optimal thickness is about 350 nm.

Tuning Oxygen Vacancy in CaZnOS through Cation Substitution for Substantially Enhanced Multimode Luminescence of Mn2+ and Tb3+

AbstractCurrent luminescence materials are abundantly available for visualization fields on a theoretical level, while the lack of luminescent intensity has largely hindered their practical application. Here, the substantial enhancement of the multimode luminescence is demonstrated including mechanoluminescence (ML), photoluminescence (PL), and X‐ray excited optical luminescence (XEOL) for Mn2+ and Tb3+ in zinc calcium oxysulfide (CaZnOS) by increasing the concentration of oxygen vacancies (OVs). Through a comprehensive structure analysis, it is found that the CaZnOS lattice has a tolerance of 60% of Sr2+ with respect to Ca2+ in the methodology. The experimental characterizations verified that the lattice distortion of CaZnOS originating from Sr2+ incorporation can efficiently promote the elimination of oxygen elements and simultaneously produce more OVs in the matrix, which can effectively fortify the trapped electrons and ultimately promote stronger luminescence. The ML, PL, and XEOL achieve around two to six times’ enhancement after Sr2+ incorporation. The findings provide insight into mechanisms underlying the luminescence behavior change of phosphors after cation substitution and may have wide implications for the practical application of intrinsically defective phosphors.

Solvothermal synthesis of rhombic shape GdF<sub>3</sub>:Tb<sup>3+</sup> nanoparticles for biomedical applications

Purpose of the study. In this work, we have investigated the mechanism of structure formation of GdF3:Tb3+(15 %) nanocrystals synthesized by solvothermal synthesis in the temperature range from RT to 200 °C with a step of 50 °C.Materials and methods. Nanocrystals of GdF3:Tb3+(15 %) were synthesized by the solvothermal method using a high-pressure reactor (autoclave) designed for temperatures up to 250 °C. The structure, size and morphology were determined by transmission electron microscopy (TEM), the type of crystal lattice and the size of crystallites of nanoparticles were studied by X-ray diffraction (XRD), hydrodynamic size of nanoparticles, particle size distribution, ζ-potential, agglomeration of nanoparticles in colloidal solutions were determined by dynamic light scattering (DLS), the chemical composition of the nanocrystals surface was studied by Fourier-t ransform infra-red spectroscopy (FT-IR), the nanoparticles ability to absorb UV radiation was analyzed by UV-visible spectroscopy (UV-vis) and X-ray excited optical luminescence (XEOL).Results. With an increase in the temperature of the synthesis reaction, a structural change in the crystallites phase occurs from hexagonal to orthorhombic. At low temperatures, agglomerated particles consisting of hexagonal nanocrystals are formed, while at a temperature higher than the boiling point of the solvent, monodisperse rhombic- shaped nanoparticles with orthorhombic phase are formed. At mild temperatures, agglomerated particles with different morphology and with mixed hexagonal and orthorhombic phases are formed. Based on the analysis of X-ray spectrum, it was found that the size of GdF3:Tb3+(15 %) nanocrystals varies from 10 to 50 nm for different synthesis temperature conditions (T = RT, 50 °C, 100 °C, 150 °C, 200 °C). The hydrodynamic size of nanoparticles decreases with increasing synthesis temperature. All GdF3:Tb3+(15 %) nanocrystals obtained at different temperatures are transparent to visible light and absorb UV radiation. Absorption in the UV region increases with an increase in the size of particle crystallites. Upon X-ray irradiation of the colloidal GdF3:Tb3+(15 %) solution, X-ray excited optical luminescence spectra showed emission peaks at 490 nm, 543 nm, 585 nm and 620 nm.Conclusion. The mechanism of structure formation of rhombic‑ shaped GdF3:Tb3+(15 %) nanocrystals has been investigated. These monodisperse rhombic- shaped nanoparticles can be used for X-ray induced photodynamic therapy (X-PDT) of superficial, solid and deep-seated tumors.

Development of a red persistent luminescent composite: Electrospun nanofiber polymer coating prevents emission quenching by water

Sulfide persistent luminescent materials are known for their efficient luminescence, although a limitation in application of these materials is their susceptibility to hydrolysis. In this work, the electrospinning technique was used to encapsulate polycrystalline SrS:Eu2+,Sm3+ within PVDF-co-HFP nanofibers, after which thermostimulated luminescence could be observed with the naked eye following treatment with boiling water, confirming that the storage capacity of the powder was maintained after encapsulation. The composite material presented high levels of flexibility, hydrophobicity, and thermal and chemical stability. The structural characteristics of the synthesized SrS:Eu2+,Sm3+ were elucidated using X-ray diffraction (XRD), infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The morphologies of the SrS:Eu2+,Sm3+ grains and the electrospun nanofibers were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray fluorescence (XRF) mapping, and scanning transmission X-ray microscopy (STXM). Mechanical properties including elastic modulus and deformation degree were evaluated by PeakForce quantitative nanomechanical mapping (PF-QNM). The luminescent properties were investigated using photoluminescence spectroscopy (PLS), diffuse reflectance spectroscopy (DRS), and X-ray excited optical luminescence (XEOL), which showed that the photonic functionality of the material in the nanofibers was preserved, even after severe treatment with boiling water.