Cationic Effects on Photo- and X-ray Radioluminescence of K3RE(PO4)2:Ce3+/Pr3+ (RE = La, Gd, and Y) Phosphors toward X-ray Detection.

Cationic tuning for lanthanide (Ce3+/Pr3+)-activated inorganic phosphors with stable, efficient, and fast-decay 5d-4f emissions has emerged as an important strategy toward the continuing pursuit of superior scintillators. The in-depth understanding of the cationic effects on photo- and radioluminescence of lanthanides Ce3+ and Pr3+ centers is requisite for the rational cationic tuning. Here, we perform a systematic study on the structure and photo- and X-ray radioluminescence properties of K3RE(PO4)2:Ce3+/Pr3+ (RE = La, Gd, and Y) phosphors to elucidate the underlying cationic effects on their 4f-5d luminescence. By using the Rietveld refinements, low-temperature synchrotron-radiation vacuum ultraviolet-ultraviolet spectra, vibronic coupling analyses, and vacuum-referred binding energy schemes, the origins of lattice parameter evolutions, 5d excitation energies, 5d emission energies, and Stokes shifts as well as good emission thermal stabilities of K3RE(PO4)2:Ce3+ systems are revealed. In addition, the correlations of Pr3+ luminescence to Ce3+ in the same sites are also discussed. Finally, the X-ray excited luminescence manifests that the K3Gd(PO4)2:1%Ce3+ sample possesses a light yield of ∼10,217 photons/MeV, indicating its potentiality toward X-ray detection application. These results deepen the understanding of cationic effects on Ce3+ and Pr3+ 4f-5d luminescence and inspire the inorganic scintillator development.

Experimental and Theoretical Studies of the Site Occupancy and Luminescence of Ce3+ in LiSr4(BO3)3 for Potential X-ray Detecting Applications.

Ce3+-doped LiSr4(BO3)3 phosphors have been prepared by a high-temperature solid-state reaction method, and structural refinement of the host compound has been performed. The excitation and emission spectra in the vacuum ultraviolet-ultraviolet-visible range at cryogenic temperatures reveal that Ce3+ ions preferentially occupy eight-coordinated Sr2+ sites in LiSr4(BO3)3. Such experimental attribution is well corroborated by the calculated 4f-5d transition energies and defect formation energies of Ce3+ ions at two distinct Sr2+ sites in the first-principles framework. In addition, the doping concentration-dependent luminescence and the temperature-dependent luminescence are systematically investigated by luminescence intensity and lifetime measurements, respectively. This shows that concentration quenching does not occur in the investigated doping range, but inhomogeneous broadening exists in the concentrated samples. With the estimated thermal quenching activation energy, the discussions on the thermal quenching mechanisms suggest that the thermal-ionization process of the 5d electron is a dominant channel for thermal quenching of Ce3+ luminescence, despite the fact that thermally activated concentration quenching cannot be excluded for the highly doped samples. Finally, the X-ray excited luminescence measurement demonstrates the promising applications of the phosphors in X-ray detection.

Site Occupancies, VUV-UV-vis Photoluminescence, and X-ray Radioluminescence of Eu2+-Doped RbBaPO4.

RbBaPO4:Eu2+ phosphors have been prepared by a high-temperature solid-state reaction method, and the structure was determined by Rietveld refinement based on powder X-ray diffraction (P-XRD) data. Their VUV-UV-vis photoluminescence properties are systematically investigated with three objectives: (1) based on low-temperature spectra, we clarify the site occupancies of Eu2+, and demonstrate that the doublet emission bands at ∼406 and ∼431 nm originate from Eu2+ in Ba2+ [Eu2+(I)] and Rb+ [Eu2+(II)] sites, respectively; (2) an electron-vibrational interaction (EVI) analysis is conducted to estimate the Huang-Rhys factors, the zero-phonon lines (ZPLs) and the Stokes shifts of Eu2+ in Rb+ and Ba2+ sites; (3) the studies on luminescence decay of Eu2+(I) reveal that dipole-dipole interaction is mainly responsible for the energy transfer from Eu2+(I) to Eu2+(II), and the energy migration between Eu2+(I) is weak. Finally, the X-ray excited luminescence (XEL) spectrum indicates that the light yield of the sample RbBa0.995Eu0.005PO4 is ∼17 700 ph/MeV, showing its potential application in X-ray detecting.

Luminescence and Energy Transfer between Ce3+ and Pr3+ in BaY2Si3O10 under VUV-vis and X-ray Excitation.

A detailed investigation on photoluminescence properties and energy transfer (ET) dynamics of Ce3+, Pr3+-doped BaY2Si3O10 is provided along with the potential X-ray excited luminescence application. The luminescence properties of Pr3+ are studied in VUV-UV-vis spectral range at low temperature, and the spectral profiles of Pr3+3P0 and 1D2 emission lines are determined using time-resolved emission spectra. Upon 230 nm excitation, the electron population from Pr3+ 4f5d state to its 4f2 excited state is discussed in detail. As Pr3+ concentration rises, Pr3+3P0 and 1D2 luminescence possess different concentration-related properties. The incorporation of Ce3+ in the codoped sample produces the strong Ce3+ luminescence under 230 nm excitation, which is the combined result of Pr3+ 4f5d → Ce3+ 5d ET and Ce3+ intrinsic excitation. On the other hand, the increasingly strong ET of Ce3+ 5d → Pr3+ 4f2 results in the decrease of Ce3+ emission intensity and the gradual deviation of Ce3+ luminescence decay from the single exponential in the system. By employing the Inokuti-Hirayama model, the dipole-dipole interaction is confirmed as the predominant multipolar effect in controlling this ET process, and the value of C DA is determined to be 9.97 × 10-47 m6·s-1. Finally, the relatively low scintillation light yield of Ce3+-doped BaY2Si3O10 material impedes its application potential in the scintillator field, and the cosubstitution of Pr3+ results in the observable decline of scintillation performance.

The Effect of Sr2+ on Luminescence of Ce3+-Doped (Ca,Sr)2Al2SiO7.

A series of Ce3+-doped (Ca,Sr)2Al2SiO7 phosphors with different Ce3+ and Ca2+/Sr2+ concentrations were prepared by a high temperature solid-state reaction technique. To get insight into the structure-luminescence relationship, the impact of incorporation of Sr2+ on structure of (Ca,Sr)2Al2SiO7 was first investigated via Rietveld refinement of high quality X-ray diffraction (XRD) data, and then the VUV-UV excitation and UV-vis emission spectra of (Ca,Sr)2Al2SiO7:Ce3+ were collected at low temperature. The results reveal that the crystal structure evolution of (Ca,Sr)2Al2SiO7:Ce3+ has influences on band gaps and Ce3+ luminescence properties including 4f-5di (i = 1-5) transition energies, radiative lifetime, emission intensity, quantum efficiency, and thermal stability. Moreover, the influence of Sr2+ content on the energy of Eu3+-O2- charge-transfer states (CTS) in (Ca,Sr)2Al2SiO7:Eu3+ was studied in order to construct vacuum referred binding energy (VRBE) schemes with the aim to further understand the luminescence properties of (Ca,Sr)2Al2SiO7:Ce3+. Finally, X-ray excited luminescence (XEL) spectra were measured to evaluate the possibility of (Ca,Sr)2Al2SiO7:Ce3+ as a scintillation material.

The in vivo activation of persistent nanophosphors for optical imaging of vascularization, tumours and grafted cells.

Optical imaging for biological applications requires more sensitive tools. Near-infrared persistent luminescence nanoparticles enable highly sensitive in vivo optical detection and complete avoidance of tissue autofluorescence. However, the actual generation of persistent luminescence nanoparticles necessitates ex vivo activation before systemic administration, which prevents long-term imaging in living animals. Here, we introduce a new generation of optical nanoprobes, based on chromium-doped zinc gallate, whose persistent luminescence can be activated in vivo through living tissues using highly penetrating low-energy red photons. Surface functionalization of this photonic probe can be adjusted to favour multiple biomedical applications such as tumour targeting. Notably, we show that cells can endocytose these nanoparticles in vitro and that, after intravenous injection, we can track labelled cells in vivo and follow their biodistribution by a simple whole animal optical detection, opening new perspectives for cell therapy research and for a variety of diagnosis applications.

Low-temperature VUV photoluminescence and thermoluminescence of UV excited afterglow phosphor Sr3AlxSi1-xO5:Ce(3+),Ln(3+) (Ln = Er, Nd, Sm, Dy and Tm).

Low-temperature (10 K) photoluminescence excitation and emission spectra of undoped Sr3SiO5 as well as Ce(3+) and Eu(3+) single doped Sr3SiO5 have been investigated. They show the host exciton band and the O(2-) to Eu(3+) charge transfer band at 5.98 eV (207 nm) and 3.87 eV (320 nm) respectively. Low-temperature thermoluminescence measurements are reported for Ce(3+) and lanthanide (Er, Nd, Sm, Dy, Er and Tm) co-doped Sr3AlxSi1-xO5. The results show that Ce(3+) is the recombination centre and Nd, Sm, Dy and Tm work as electron traps with trap depths of 0.95 eV, 1.89 eV, 1.02 eV, and 1.19 eV, respectively. Thermoluminescence excitation spectra of Sr2.98Al0.02Si0.98O5:0.01Ce(3+),0.01Dy(3+) show that the traps can be charged by 260 nm UV excitation.

Luminescence and X-ray absorption studies on 0.5% Ce(3+) doped BaCa2MgSi2O8 phosphor.

0.5% Ce(3+) doped BaCa2MgSi2O8 phosphor was prepared by a conventional solid state reaction method. Luminescence spectra as well as fluorescence decay were monitored in the VUV-UV range. Ce(3+) emissions are assigned to cerium ions on a Ba(2+) site, and the five 4f-5d excitation bands of Ce(3+) were determined at low temperature. The light yield is estimated to be around 10,600 ph MeV(-1) under X-ray excitation. X-ray absorption near-edge structure (XANES) was explored to study the energy transfer efficiency to optical centers from each element in the phosphor; the results show that the contributions to luminescence are not identical for each element.

Avoiding concentration quenching and self-absorption in Cs4EuX6 (X = Br, I) by Sm2+ doping.

The benefits of doping Cs4EuBr6 and Cs4EuI6 with Sm2+ are studied for near-infrared scintillator applications. It is shown that undoped Cs4EuI6 suffers from a high probability of self-absorption, which is almost completely absent in Cs4EuI6:2% Sm. Sm2+ doping is also used to gain insight in the migration rate of Eu2+ excitations in Cs4EuBr6 and Cs4EuI6, which shows that concentration quenching is weak, but still significant in the undoped compounds. Both self-absorption and concentration quenching are linked to the spectral overlap of the Eu2+ excitation and emission spectra which were studied between 10 K and 300 K. The scintillation characteristics of Cs4EuI6:2% Sm is compared to that of the undoped samples. An improvement of energy resolution from 11% to 7.5% is found upon doping Cs4EuI6 with 2% Sm and the scintillation decay time shortens from 4.8 s to 3.5 s in samples of around 3 mm in size.

Scintillation and Optical Characterization of CsCu2I3 Single Crystals from 10 to 400 K.

Currently only Eu2+-based scintillators have approached the light yield needed to improve the 2% energy resolution at 662 keV of LaBr3:Ce3+,Sr2+. Their major limitation, however, is the significant self-absorption due to Eu2+. CsCu2I3 is an interesting new small band gap scintillator. It is nonhygroscopic and nontoxic, melts congruently, and has an extremely low afterglow, a density of 5.01 g/cm3, and an effective atomic number of 50.6. It shows self-trapped exciton emission at room temperature. The large Stokes shift of this emission ensures that this material is not sensitive to self-absorption, tackling one of the major problems of Eu2+-based scintillators. An avalanche photo diode, whose optimal detection efficiency matches the 570 nm mean emission wavelength of CsCu2I3, was used to measure pulse height spectra. From the latter, a light yield of 36 000 photons/MeV and energy resolution of 4.82% were obtained. The scintillation proportionality of CsCu2I3 was found to be on par with that of SrI2:Eu2+. Based on temperature-dependent emission and decay measurements, it was demonstrated that CsCu2I3 emission is already about 50% quenched at room temperature. Using temperature-dependent pulse height measurements, it is shown that the light yield can be increased up to 60 000 photons/MeV by cooling to 200 K, experimentally demonstrating the scintillation potential of CsCu2I3. Below this temperature, the light yield starts to decrease, which can be linked to the unusually large increase in the band gap energy of CsCu2I3.

VUV-UV-vis photoluminescence, X-ray radioluminescence and energy transfer dynamics of Ce3+ and Eu2+ in Sr2MgSi2O7.

Ce3+ and Eu2+ doped and Ce3+-Eu2+ co-doped Sr2MgSi2O7 phosphors are prepared via a high-temperature solid-state reaction technique. The synchrotron radiation vacuum ultraviolet-ultraviolet (VUV-UV) excitation and ultraviolet-visible (UV-vis) emission spectra of diluted Ce3+ and Eu2+ doped Sr2MgSi2O7 samples are measured at cryogenic temperatures. The electron-vibrational interaction (EVI) between Ce3+ and its surroundings is analyzed. The dependencies of the 4f-5d transitions of Ce3+ on the structure of the host compounds Sr2MgSi2O7, Ba2MgSi2O7 and BaMg2Si2O7 are discussed in detail. Then the thermal quenching channel is proposed based on the measurements of temperature dependent luminescence intensities and decay times of Ce3+ and Eu2+ in Sr2MgSi2O7, and the Ce3+ → Eu2+ energy transfer mechanism is understood by three luminescence dynamic models. In addition, Sr2MgSi2O7:Ce3+/Eu2+ samples are evaluated for the possibilities of X-ray detection applications using X-ray excited luminescence (XEL) spectroscopy, and it was found that they are not suitable.

Temperature dependent scintillation properties and mechanisms of (PEA)2PbBr4 single crystals.

In this work the scintillation properties of PEA2PbBr4 are studied as function of temperature, accessing the potential use of these materials for low temperature applications. The scintillation properties and mechanism have been studied using a combination of temperature dependent photoluminescence emission and excitation, X-ray excited emission and decay measurements. At room temperature the X-ray excited emission is dominated by the 442 nm emission with a lifetime of 35.2 ns. Under UV-Vis photon excitation an additional emission peak is observed at 412 nm. At 10 K, both X-ray and UV-Vis photon excited emission spectra show a narrow emission peak at 412 nm and a broad emission band centred around 525 nm with a lifetime of 1.53 ns (24%) and 154 ns (76%) respectively. The exact nature of the observed emission peaks is not known. For this reason two potential mechanisms explaining the difference between UV-Vis photon and X-ray excitation and their temperature dependent emissions are explored. The total spectral intensity decreases to 72% of the intensity at room temperature at 10 K. It is suggested that the observed negative thermal quenching behaviour results from a combination of more self absorption and a higher degree of self trapped exciton formation under X-ray excitation. Based on the observed fast decay component at 10 K and light yield of 9400 photons per MeV at room temperature, showing only a 28% decrease at 10 K, could make this material potentially interesting for low temperature and fast timing applications.

Structure, luminescence of Eu2+ and Eu3+ in CaMgSi2O6 and their co-existence for the excitation-wavelength/temperature driven colour evolution.

Luminescent materials with controllable colour evolution features are demanded for the development of multi-level anti-counterfeiting technologies. Here we report the structural and luminescence properties of CaMgSi2O6:Ln (Ln = Eu2+, Eu3+, Eu2+/3+) samples in detail and reveal their excitation-wavelength/temperature driven colour evolution characteristics. By tuning either the excitation-wavelength (276, 304, 343, 394 nm) or temperature (in the 330-505 K range), the designed samples with co-existing Eu2+/Eu3+ ions can achieve diverse and controllable colour evolution from red, to pink, purple and blue. This shows their potential application in anti-counterfeiting with the help of sophisticated pattern design. In addition, the underlying mechanism of the Stokes shift of the Eu2+ emission and valence stability of both Eu2+/Eu3+ ions in CaMgSi2O6 are also studied in depth. These results are valuable for designing colour-controllable luminescent materials based on the co-existence of the Eu2+/Eu3+ ions for anti-counterfeiting applications.

Luminescence dynamics in Tb(3+)-doped CaWO(4) and CaMoO(4) crystals.

Single crystals of CaWO(4) and CaMoO(4) doped with Tb(3+) have been grown by the flux growth method. Their luminescence properties have been investigated in the 10-600 K temperature range under different experimental conditions. In spite of very similar spectra at low temperature upon excitation at 365 nm, the crystals show a very different behavior as the temperature is raised or the excitation wavelength is changed. These differences have been accounted for on the basis of models that take into consideration the position of the energy levels of the rare earth relative to the bandgap of the host material.

Lanthanide 4f-level location in AVO(4):Ln(3+) (A = La, Gd, Lu) crystals.

The spectral properties of LaVO(4), GdVO(4) and LuVO(4) crystals doped with Ce(3+), Pr(3+), Eu(3+) or Tb(3+) have been investigated in order to determine the position of the energy levels relative to the valence and conduction bands of the hosts along the trivalent and divalent lanthanide series. Pr(3+) and Tb(3+) ground state levels are positioned based on the electron transfer energy from those states to the conduction band, the so-called intervalence charge transfer (IVCT). This approach is compared with an alternative model that is based on electron transfer from the valence band to a lanthanide.

Impacts of 5d electron binding energy and electron-phonon coupling on luminescence of Ce3+ in Li6Y(BO3)3.

In this work, the crystal structure and electronic structure as well as the synchrotron radiation vacuum ultraviolet-ultraviolet-visible (VUV-UV-vis) luminescence properties of Li6Y(BO3)3 (LYBO):Ce3+ phosphors were investigated in detail. The Rietveld refinement and DFT calculation reveal the P21/c monoclinic crystal phase and the direct band gap of the LYBO compound, respectively. Only one kind of Ce3+ 4f-5d transition is resolved in terms of the low temperature VUV-UV excitation, UV-vis emission spectra and luminescence decay curves. Furthermore, by constructing the vacuum referred binding energy (VRBE) scheme and applying the frequency-degenerate vibrational model, the impacts of 5d electron binding energy and electron-phonon coupling on luminescence of Ce3+ in LYBO are analysed. The results show that the Ce3+ emission in LYBO possesses a moderate intrinsic thermal stability. With the increase in concentration, the thermal stability of the emission gets worse due to the possible thermally-activated concentration quenching. In addition, the simulation of Ce3+ emission profile at low temperature reveals that the 4f-5d electronic transitions of Ce3+ ions can be treated to couple with one frequency-degenerate vibrational mode having the effective phonon energy of ∼257 cm-1 with the corresponding Huang-Rhys parameter of ∼6, which indicates a strong electron-phonon interaction of Ce3+ luminescence in the Li6Y(BO3)3 host. Finally, the X-ray excited luminescence spectrum of the LYBO:5%Ce3+ phosphor is measured to check the potential scintillator applications.

Insight into Eu redox and Pr3+ 5d emission in KSrPO4 by VRBE scheme construction.

A series of Ln-doped KSrPO4 (Ln = Ce3+, Eu3+, Eu2+, Pr3+) phosphors are prepared through a high-temperature solid-state method. The KSrPO4 compound is confirmed to possess a ß-K2SO4 structure with the Pnma group by Rietveld refinement, and the temperature-dependent lattice parameters are investigated with the powder X-ray diffraction results at different temperatures. Ce3+ and Eu3+ ions are introduced to probe the crystal field strength (CFS) and the lanthanide site symmetry by using VUV-UV-vis spectroscopy. The temperature-dependent luminescence properties of KSrPO4: Ce3+/Eu2+ exhibit an excellent thermal stability of Ce3+/Eu2+ luminescence. Based on the VUV-UV-vis spectra of Ce3+ and Eu3+ doped KSrPO4, the vacuum referred binding energy (VRBE) scheme is constructed to understand the redox properties of Eu, the 5d energy levels of Pr3+ and the thermal quenching characteristics of Ce3+ and Eu2+ luminescence.

Charge Carrier Trapping Processes in RE2O2S (RE = La, Gd, Y, and Lu).

Two different charge carrier trapping processes have been investigated in RE2O2S:Ln3+ (RE = La, Gd, Y, and Lu; Ln = Ce, Pr, and Tb) and RE2O2S:M (M = Ti4+ and Eu3+). Cerium, praseodymium and terbium act as recombination centers and hole trapping centers while host intrinsic defects provide the electron trap. The captured electrons released from the intrinsic defects recombine at Ce4+, Pr4+, or Tb4+ via the conduction band. On the other hand, Ti4+ and Eu3+ act as recombination centers and electron trapping centers while host intrinsic defects act as hole trapping centers. For these codopants we find evidence that recombination is by means of hole release instead of electron release. The released holes recombine with the trapped electrons on Ti3+ or Eu2+ and yield broad Ti4+ yellow-red charge transfer (CT) emission or characteristic Eu3+ 4f-4f emission. We will conclude that the afterglow in Y2O2S:Ti4+, Eu3+ is due to hole release instead of more common electron release.

The electronic level structure of lanthanide impurities in REPO4, REBO3, REAlO3, and RE2O3 (RE = La, Gd, Y, Lu, Sc) compounds.

The vacuum referred binding energies of electrons in divalent and trivalent lanthanide impurity states and host band states in the rare earth (RE = La, Gd, Y, Lu, Sc) orthophosphates REPO4, orthoborates REBO3, aluminum perovskites REAlO3, and sesqui-oxides RE2O3 have been determined by combining the recently developed chemical shift model with spectroscopic data from the archival literature. The main trends in impurity and host band level locations with changing type of RE, which determines the site size, and with changing P, B, Al, or RE cation, which determines the strength of bonding with the oxygen ligands, are identified. Sc(3+)-based compounds are characterized by a relatively low energy for the conduction band bottom, or equivalently a high electron affinity, which is attributed to a relatively strong electron bonding in the 3d-shell of Sc(2+).

Experimental comparison of high-density scintillators for EMCCD-based gamma ray imaging.

Detection of x-rays and gamma rays with high spatial resolution can be achieved with scintillators that are optically coupled to electron-multiplying charge-coupled devices (EMCCDs). These can be operated at typical frame rates of 50 Hz with low noise. In such a set-up, scintillation light within each frame is integrated after which the frame is analyzed for the presence of scintillation events. This method allows for the use of scintillator materials with relatively long decay times of a few milliseconds, not previously considered for use in photon-counting gamma cameras, opening up an unexplored range of dense scintillators. In this paper, we test CdWO4 and transparent polycrystalline ceramics of Lu2O3:Eu and (Gd,Lu)2O3:Eu as alternatives to currently used CsI:Tl in order to improve the performance of EMCCD-based gamma cameras. The tested scintillators were selected for their significantly larger cross-sections at 140 keV ((99m)Tc) compared to CsI:Tl combined with moderate to good light yield. A performance comparison based on gamma camera spatial and energy resolution was done with all tested scintillators having equal (66%) interaction probability at 140 keV. CdWO4, Lu2O3:Eu and (Gd,Lu)2O3:Eu all result in a significantly improved spatial resolution over CsI:Tl, albeit at the cost of reduced energy resolution. Lu2O3:Eu transparent ceramic gives the best spatial resolution: 65 µm full-width-at-half-maximum (FWHM) compared to 147 µm FWHM for CsI:Tl. In conclusion, these 'slow' dense scintillators open up new possibilities for improving the spatial resolution of EMCCD-based scintillation cameras.