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

Abstract

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.

Halide Replacement with Complete Preservation of Crystal Lattice in Mixed‐Anion Lanthanide Oxyhalides

“…Figure S6C plots La N 4,5 -edge (4d!4f)X ANES spectra measured for Tb-alloyed LaOI nanocrystals.Apreedge 1 S 0 -3 D 1 transition is observed centered at 101.7 eV and a 1 S 0 -1 P 1 transition is observed centered at 118.2 eV.T he former is symmetry forbidden but observed as ar esult of spin-orbit-coupling-induced relaxation of selection rules.A s ar esult of strong 4d-4fo rbital overlap,t he latter allowed transition exhibits an anomalously high absorption crosssection;t he giant resonance,w hich is asymmetrically broadened as aresult of the short lifetime of the excited state. [23,36,38] Following absorption of X-rays at the atomic absorption edge, the generated electron-hole pairs are thermalized and transfer energy to luminescent Tb 3+ dopants,a ctivating radiative recombination channels.S ubsequently,e xcited Tb 3+ ions relax through emission of characteristic blue and green photons which can be detected in XEOL. As ap hoton-in photon-out method, XEOL enables element-selective excitation of ahost lattice.Bysimultaneously measuring XANES and photoluminescence spectra at each excitation energy up to and across the atomic absorption edge,2Dmaps of XEOL intensity across the visible-NIR spectrum are acquired as af unction of X-ray excitation energy [39,40] providing insight into energy transfer mechanisms and relaxation pathways accessible within as pecific structure (a more detailed Angewandte Chemie discussion of XEOL is provided in the Supporting Information).…”

“…Above the post-edge region, with increasing excitation energy, alarger number of visible photons are generated corresponding to the efficient activation of the Tb 3+ centers giving rise to intense blue-green luminescence.H owever,a tt he giant resonance,t he observed emission intensity is diminished as the excitation only yields asingular electron-hole pair and the 4d shell decays through an on-radiative Auger emission process. [3,23] Figure S8 in the Supporting Information shows powder XRD data for LaOCl, LaOBr, and LaOI nanocrystals incorporating Tb 3+ on La-sites prepared by metathesis reactions of Tb x La 1Àx OI nanocrystals as per Equation (3). Figure S8B shows amonotonic shift of the (002) reflections to higher 2q values reflecting the closer interplanar separation of the (LaO) n n+ blocks [32] in going from the LaOI to LaOF with retention of the matlockite crystal structure.B ased on ICP-MS measurements,the Tb:X ratio remains almost unchanged at 0.08 upon topotactic reaction suggesting preservation of the cation sublattice.While the green coloration characteristic of the Tb 3+ chromophore is retained, Figure S9 in the Supporting Information shows PL excitation and emission data as well as aCommission International de IEclairage (CIE) chromaticity plot for the topochemically transformed oxyhalide nanocrystals illustrating ablue shift of the excitation band and red shift of the 5 D 4 !…”

“…[19][20][21][22] Thec ompositional and structural diversity of this family of compounds provides am eans of color tunability in REOX-based UV,X -ray,a nd electron-excited phosphors. [23,24] In this article,w ed escribe the synthesis of LaOI nanocrystals through al ow-temperature,n on-hydrolytic condensation reaction, and illustrate their topotactic transformation into other lanthanide oxyhalides by metathesis reactions with ammonium salts.L uminescence signatures are embedded by the incorporation of Tb 3+ ions, [25] on the lanthanum lattice. Theclosely spaced energy levels of Tb 3+ provides ameans of sensitively probing coupling of the electronic states to the phonons across the set of oxyhalides.A ss uch, Tb 3+ with numerous emissive states that are accessible through phononassisted excitation relaxation pathways serves as as ensitive reporter for interrogating excitation channels and relaxation pathways across the entire LaOX series using X-ray-excited optical luminescence (XEOL) measurements.…”

Halide Replacement with Complete Preservation of Crystal Lattice in Mixed‐Anion Lanthanide Oxyhalides

“…Figure S6C plots La N 4,5 -edge (4d!4f)X ANES spectra measured for Tb-alloyed LaOI nanocrystals.Apreedge 1 S 0 -3 D 1 transition is observed centered at 101.7 eV and a 1 S 0 -1 P 1 transition is observed centered at 118.2 eV.T he former is symmetry forbidden but observed as ar esult of spin-orbit-coupling-induced relaxation of selection rules.A s ar esult of strong 4d-4fo rbital overlap,t he latter allowed transition exhibits an anomalously high absorption crosssection;t he giant resonance,w hich is asymmetrically broadened as aresult of the short lifetime of the excited state. [23,36,38] Following absorption of X-rays at the atomic absorption edge, the generated electron-hole pairs are thermalized and transfer energy to luminescent Tb 3+ dopants,a ctivating radiative recombination channels.S ubsequently,e xcited Tb 3+ ions relax through emission of characteristic blue and green photons which can be detected in XEOL. As ap hoton-in photon-out method, XEOL enables element-selective excitation of ahost lattice.Bysimultaneously measuring XANES and photoluminescence spectra at each excitation energy up to and across the atomic absorption edge,2Dmaps of XEOL intensity across the visible-NIR spectrum are acquired as af unction of X-ray excitation energy [39,40] providing insight into energy transfer mechanisms and relaxation pathways accessible within as pecific structure (a more detailed…”

“…REOX compounds have found applications as X‐ray phosphors, anion conductors, and heterogeneous catalysts by dint of their low phonon cut‐off energies and rich defect chemistry [19–22] . The compositional and structural diversity of this family of compounds provides a means of color tunability in REOX‐based UV, X‐ray, and electron‐excited phosphors [23, 24] …”