In a Different Light: Deciphering Optical and X-ray Sensitization Mechanisms in an Expanded Palette of LaOCI Phosphors
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2018 American Chemical Society. 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.
