High Energy Spectroscopy and Resonance Effects

Bonnelle, C.; Spector, Nissan

doi:10.1007/978-90-481-2879-2_3

Cited by 2 publications

(4 citation statements)

References 115 publications

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“…Figure presents XANES and XEOL spectra measured for the oleylamine-capped La 1– x – y Tb x Eu y OCl nanocrystals. In Figure A, the XANES spectra show element-specific (4d → 4f) forbidden 1 S 0 → 3 D 1 and allowed 1 S 0 → 3 P 1 absorptions characteristic of trivalent lanthanum at 101.7 and 117.5 eV, respectively, similar to spectra observed in Figure A for nanocrystals grown using TOPO as the passivating ligand. ,, The XEOL spectra acquired using X-ray excitation at the center of the giant resonance (at 117.5 eV) are plotted in Figure B; spectra acquired for the multiply doped solid-solution nanocrystals are contrasted with spectra measured for La 0.941 Eu 0.059 OCl and La 0.911 Tb 0.089 OCl nanocrystals under the same conditions. Intriguingly, the characteristic 5 D 4 → 7 F J green emission and 5 D 3 → 7 F J blue emission bands of Tb 3+ are dominant for all of the multiply alloyed samples, even for materials where the ratio of divalent europium atoms to trivalent terbium atoms approaches 4:1.…”

Section: Resultssupporting

confidence: 54%

“…117.5 eV (this feature is about 10 eV above the ionization threshold, reflecting strong electron correlation in these systems) . The broad line shape of this absorption feature is indicative of a short lifetime . Given the relatively low concentrations of Eu 3+ and Tb 3+ , absorption features corresponding to the giant resonances of the luminescent centers are not observed in the XANES spectra.…”

Section: Resultsmentioning

confidence: 96%

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In a Different Light: Deciphering Optical and X-ray Sensitization Mechanisms in an Expanded Palette of LaOCl Phosphors

et al. 2018

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The conversion and numerical amplification of X-ray photons to visible light is at the heart of numerous technological applications spanning the range from X-ray detectors and scintillators to radiographic medical imaging devices. The need for increased sensitivity and spatial resolution to reduce radiation exposure and provide better differentiation of specimens presenting an X-ray contrast has been a strong driving force in the search for novel X-ray phosphors. However, the current palette of X-ray phosphors is rather sparse. The development of color tunable phosphors necessitates the incorporation of multiple dopants, which in turn interact through complex sensitization mechanisms that are poorly understood for high-energy excitation. In this work, we describe the stabilization of multiply alloyed LaOCl nanocrystals incorporating Tb3+ cations in conjunction with either divalent or trivalent europium ions, yielding phosphors emitting in the blue–green and green–red regions of the electromagnetic spectrum, respectively. The choice of the coordinating ligand (tri-n-octylphosphine oxide versus oleylamine) dictates the oxidation state of the incorporated Eu-ions. Pronounced differences are observed in sensitization mechanisms upon optical and X-ray excitation thereby considerably modifying the perceived color of the X-ray phosphors as compared to stimulation by ultraviolet illumination. Upon UV illumination, strong Tb3+ → Eu3+ sensitization is observed for LaOCl nanocrystals incorporating trivalent europium ions; however, upon X-ray excitation, this sensitization pathway is instead supplanted by independent La3+ → Eu3+ and La3+ → Tb3+ sensitization routes. In contrast, strong Eu2+ → Tb3+ sensitization is observed for LaOCl nanocrystals incorporating divalent europium ions upon both optical and X-ray excitation (La3+ → Eu2+ → Tb3+ and La3+ → Tb3+ pathways are also observed in X-ray excited optical luminescence spectra). The increased efficacy of the Eu2+ → Tb3+ as compared with Tb3+→ Eu3+ sensitization pathway derives from the parity allowed nature and orders of magnitude higher absorption cross-section for the excitation of the divalent Eu-ion. A strong suppression of luminescence intensity is observed upon excitation at the giant resonance absorption for all of the observed emission bands and corresponds to a change in mechanism from the creation of multiple thermal electron–hole pairs to a nonradiative Coster–Kronig process dominated by Auger photoionization.

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Section: Resultssupporting

confidence: 54%

Section: Resultsmentioning

confidence: 96%

In a Different Light: Deciphering Optical and X-ray Sensitization Mechanisms in an Expanded Palette of LaOCl Phosphors

et al. 2018

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“…Interestingly, the 1 S 0 → 3 D 1 transition observed in the pre-edge region is forbidden by LS selection rules but is nevertheless observed as a result of strong spinorbit coupling. 53 Because of the low concentration of Eu 3+ and Eu 2+ in the matrix, no discernible giant resonance feature is observed for these species.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…10 eV above the ionization threshold, which reflects the strong electron correlation of the 4f electrons localized within a narrow band (this effect is further amplified in later lanthanides). − The pronounced enhancement of the absorption cross-section of the 1 S 0 → 1 P 1 giant resonance and its broad line shape indicate a much shorter lifetime of the excitation as compared to the excitation to the triplet final state. Interestingly, the 1 S 0 → 3 D 1 transition observed in the pre-edge region is forbidden by LS selection rules but is nevertheless observed as a result of strong spinorbit coupling . Because of the low concentration of Eu 3+ and Eu 2+ in the matrix, no discernible giant resonance feature is observed for these species.…”

Section: Resultsmentioning

confidence: 99%

Ligand-Mediated Control of Dopant Oxidation State and X-ray Excited Optical Luminescence in Eu-Doped LaOCl

et al. 2018

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The development of an expanded palette of X-ray phosphors is a critical imperative for applications in medical imaging, radiation detection, and scientific instrumentation. The rational design of X-ray phosphors has been stymied by the absence of fundamental understanding of activation channels, sensitization mechanisms, and recombination pathways induced upon high-energy excitation of luminescent centers. In this article, we describe the preparation of Eu-doped LaOCl nanocrystals based on the condensation of molecular precursors. The synthetic route allows for control of the oxidation state of the incorporated Eu-atoms based on ligand-induced oxidation or reduction of the Eu-precursors. Nanocrystals exhibiting blue and red X-ray excited optical luminescence are developed by tuning the oxidation state of europium ions incorporated within the LaOCl nanocrystal matrix. Pronounced modulation of the intensity of the optical luminescence is evidenced at and near the giant resonance absorption of the host matrix as a result of distinctly divergent recombination channels. Resonant excitation results in recombination via Auger electron ionization and relaxation of a single electron-hole pair, whereas excitation away from the giant resonance results in thermalization of "hot" electron-hole pairs, while launching cascades of energy transfer, excitation, and radiative recombination events at the Eu-luminescent centers. Mechanistic elucidation and the development of a generalizable synthetic route starting from molecular precursors paves the way to an expanded palette of X-ray phosphors.

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High Energy Spectroscopy and Resonance Effects

Cited by 2 publications

References 115 publications

In a Different Light: Deciphering Optical and X-ray Sensitization Mechanisms in an Expanded Palette of LaOCl Phosphors

In a Different Light: Deciphering Optical and X-ray Sensitization Mechanisms in an Expanded Palette of LaOCl Phosphors

Ligand-Mediated Control of Dopant Oxidation State and X-ray Excited Optical Luminescence in Eu-Doped LaOCl

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