4f Luminescence Research Articles

Luminescence and scintillation properties of fast Ce,Mg:Lu2YGaxAl5-xO12 ceramic scintillators fabricated from co-precipitated powders

Ce,Mg:Lu2YGaxAl5-xO12 (x = 1.5, 2, 2.5, 3) ceramic scintillators were fabricated by oxygen sintering combined with air annealing using nano-powders synthesized by the co-precipitation method. The Ce3+ 5d1 → 4f (2F5/2, 2F7/2) luminescence band was blue shifted with increasing Ga content due to the decrease in the crystal field splitting of the 5d levels. At room temperature (RT), the decrease of photoluminescence decay time and scintillation decay time was observed with increasing Ga content. At 662 keV γ-rays, the sample Ga2 exhibited high light yield (LY) of 28,700 photons/MeV, fast scintillation decay times of 17.7 ns (17 %) + 45.7 ns (83 %), and good time resolution of ∼220 ps. At RT, the decrease of LY value for the samples Ga2.5 and Ga3 could be attributed to the larger thermal ionization of the excited Ce3+ centers.

The Influence of Halide Ion Substitution on Energy Structure and Luminescence Efficiency in CeBr2I and CeBrI2 Crystals.

This study aims to determine the optimum composition of the CeBr1-xIx compound to achieve the maximum light output. It is based on calculations of the band energy structure of crystals, specifically taking into account the characteristics of the mutual location of local and band 5d states of the Ce3+ ions. The band energy structures for CeBr2I and CeBrI2 crystals were calculated using the projector augmented wave method. The valence band was found to be formed by the hybridized states of 4p Br and 5p I. The 4f states of Ce3+ are located in the energy forbidden band gap. The conduction band is formed by the localized 5d1 states, which are created by the interaction between the 5d states of Ce3+ and the 4f0 hole of the cerium ion. The higher-lying delocalized 5d2 states of Ce3+ correspond to the energy levels of the 5d states of Ce3+ in the field of the halide Cl0 (Br0) hole. The relative location of 5d1 and 5d2 bands determines the intensity of 5d-4f luminescence. The bottom of the conduction band is formed by localized 5d1 states in the CeBr2I crystal. The local character of the bottom of the conduction band in the CeBr2I crystal favors the formation of self-trapped Frenkel excitons. Transitions between the 5d1 and 4f states are responsible for 5d-4f exciton luminescence. In the CeBrI2 crystal, the conduction band is formed by mixing the localized 5d1 and delocalized 5d2 states, which leads to quenching the 5d-4f luminescence and a decrease in the light output despite the decrease in the forbidden band gap. CsBr2I is the optimum composition of the system to achieve the maximum light output.

Luminescence and scintillation properties of Gd3(Al5-xScx)O12:Ce garnet crystals grown by floating zone method

The luminescence and scintillation properties of Ce3+-doped Gd3(Al5-xScx)O12 (x = 1, 1.5, 2, 2.5, 3) garnet crystals grown by the floating zone method were investigated. With increasing Sc content, the Ce3+ 5d1 → 4f luminescence was blue shifted due to a decrease in 5d1-5d2 crystal field splitting (Δ12) of the 5de doublet state. The Sc content showed a strong influence on purity of garnet phase in the Gd3(Al5-xScx)O12:Ce crystals. The presence of secondary phase (perovskite and/or α-Al2O3) results to a poor transparency and a lower light yield (LY) value. The Gd3Al3Sc2O12:Ce crystal exhibited pure garnet phase, highest quantum yield (∼76 %) and transmittance (∼80 %), and highest LY value (6,850 photons/MeV at 662 keV γ rays) among the studied samples. Above RT, a decrease of radioluminescence (RL) yield occurs for all samples due to the thermal ionization process. A decrease of the RL yield at low temperature observed in correlation with the intense thermoluminescence peaks can be explained by the localization of electrons at intrinsic shallow traps.

Novel optical-guiding crystal scintillator composed of an Eu-doped SrI2 core and glass cladding

Scintillators are used not only as single crystals in radiation detection systems but also as arrays, columns, and fibers. In this study, a novel optical-guiding crystal scintillator (OCS) was fabricated using an Eu:SrI2 crystal as the core and borosilicate glass as the cladding. The OCS has an elongated shape and can be flexibly processed like an optical fiber. Furthermore, because inorganic scintillator crystals are used as the core, the OCS has high sensitivity to high-energy radiation, such as γ- and X-ray radiation, unlike plastic scintillators. In this study, by observing the cross section of the fabricated OCS, identifying the crystal structure, and investigating the radioluminescence spectrum under X-ray irradiation, OCS fabrication in accordance with the OCS design specifications was confirmed, and the SrI2 crystal structure and Eu2+ 5d–4f luminescence were maintained. The γ-ray responses, such as light yield and decay time, were also investigated.

Luminescence Thermometry - Crossing the Limits of Operating Range

Temperature is one of the fundamental thermodynamic parameters that affects and regulates the rate and nature of the chemical, physical and biological processes. A precise and accurate measurement of temperature is essential across a broad spectrum of areas, such as nanomedicine, electronics, aerospace, climate, food storage, etc. Therefore, there is a great variety of temperature sensors (which currently account for ≈80% of the worldwide sensor market), and their operating modes can be divided into two main groups: contact and noncontact techniques. However, in some specific cases, only non-contact systems may provide accurate information about temperature and its distribution over an area. These are as follows: harsh environments (for example inside turbines), fast-moving objects (gas jets), intense magnetic fields that prevent the use of metals or materials with free electric charges (NMR therapies) or when the size of the system to measure is smaller than 10 μm (cellular tracking or microelectronic circuits). One promising solution that can break through those limitations is luminescence thermometry (LT). LT is a non-contact and non-invasive spectroscopic technique based on the changes in a phosphor emission when temperature changes (e.g., the intensity of a band, relative intensities, band shapes, excited-state lifetimes, etc.).We shall present our original approach to use intra- and inter- configurational transitions of Pr3+ ion for accurate temperature reading with high relative sensitivity over an outstandingly wide range of temperatures. Studies were focused on Y2SiO5:0.05%Pr and its chemically modified version by a bandgap engineering approach (replacing Si with Ge). The addition of Ge to replace Si was expected to decrease the host bandgap – energetic separation of its valence and conduction bands. Consequently, we expected that the 5d→4f luminescence would be more susceptible to temperature quenching. This, in turn, allowed to control the range of temperatures within which the 5d→4f/4f→4f emission intensity ratio could be utilized for thermometry. Furthermore, possibilities to further improve such sensors will be discussed by widening the operating range of distinct thermometers.This presentation will show that luminescence thermometers working in unprecedented board operating range can be effectively constructed and managed. Further analysis of such an approach will be carefully discussed. This research was supported by the National Science Centre, Poland under grant #2020/37/N/ST5/02507 Figure 1

(Invited) Combining the 5d→4f and 4f→4f Luminescence of Lanthanides for High-Quality Broad-Range Luminescence Thermometry

Accurate and sensitive remote temperature measurement is required in a growing number of existing and emerging applications. As the technology advances and nanotechnology enters still new fields, the size of objects whose temperature needs to be controlled gets smaller and the non-contact temperature reading is the only reasonable approach. Luminescence thermometry is believed to become the technology of choice for such purposes, as it is resistant to disturbances by an electromagnetic field and the sensitivity and accuracy of measurements may be tuned for the specific requirements. Thus, whether it comes to reading temperature at cryo-range, around the room, or at high temperatures, one may design a luminescent thermometer offering high-quality measurements. This is not to say that such designing is easy to do. Even under laboratory conditions, highly-sensitive and accurate luminescent thermometers are not that frequent.Additional problems appear when it comes to measurements over a broad range of temperatures. At first, the basic physics of luminescence processes are not conducive to the design of a luminescence thermometer offering very sensitive and accurate measurements over a broad range of temperature (a few hundred degrees, for example). On the other hand, wide-range luminescence thermometers are of interest for such important fields as aviation and space industries. Fast, accurate, and credible measurements of quickly changing temperatures are of special importance in these areas.In this presentation, we shall report a new approach toward wide-range high-quality luminescence thermometry. Phosphors activated with either Pr3+ or Eu2+ will be examined for that purpose. Both these ions are capable of generating two types of emissions: (i) intra-configurational 5f→4f and inter-configurational 4f→4f ones. We shall prove that such phosphors may offer both broad-range and high-accuracy and high-sensitivity temperature measurements. Furthermore, we will demonstrate that the sensitivity of such sensors may be deliberately tuned to reach its maximum at temperatures where it is most needed. A few examples of such phosphors will be discussed and possible expectations for further development using this approach will be discussed.This research was supported by the Polish National Science Center (NCN) under the grant #UMO-2018/29/B/ST5/00420. Figure 1

Optical, luminescence and scintillation properties of Mg2+-codoped (Lu,Y)3Al2Ga3O12:Pr garnet crystals: The effect of Y admixture

Mg - codoped Lu3-xYxAl2Ga3O12:Pr (x = 0, 1, 2, and 3) garnet crystals were grown by micro-pulling down method and scintillation characteristics were investigated. An observed red shift of the Pr3+ 5d1 → 4f luminescence with increasing Y content can be attributed to an increase in crystal field splitting of the 5d levels. Temperature dependence of radioluminescence (RL) spectra was measured in the temperature range of 10–350 K, yielding a decrease of RL yield with increasing temperature for all samples. The LuY2Al2Ga3O12:Pr,Mg exhibits light yield of 9100 photons/MeV along with two scintillation decay components of 8.6 ns (9%) and 25 ns (91%). Afterglow and thermoluminescence characteristics are also investigated.

Defects and energy transfer in Sr-=SUB=-9-=/SUB=-Lu(PO-=SUB=-4-=/SUB=-)-=SUB=-7-=/SUB=-, doped with ions Pr-=SUP=-3+-=/SUP=-

Luminescent properties of Sr9Lu(PO4)7 crystalline powders doped with Pr3+ ions upon excitation in the ultraviolet region of the spectrum are considered. in the temperature range 5-320 K, are considered.. In the photoluminescence spectra, an intense glow corresponding to radiative interconfigurational 5d->4f-electronic transitions, weak narrow lines of intra-configuration 4f->4f-transitions, and also the emission of two types of crystal structure defects, are observed, Under pulsed electronic excitation, the temporal characteristics of emission associated with an impurity center and defects and the temperature dependence of the luminescence yield were studied. The processes of energy transfer between defects and an impurity center are discussed. Keywords: 5d->4f luminescence, energy transfer, decay kinetics, ion Pr3+

Scintillation characteristics and temperature quenching of radio- and photoluminescence of Mg2+-codoped (Lu,Gd)3Al2.4Ga2.6O12:Ce garnet crystals

Luminescence and scintillation characteristics of Mg2+-codoped (LuxGd3-x)Al2.4Ga2.6O12:Ce (x = 0.2–0.8) garnet crystals grown by the micro-pulling down method are investigated. The Ce3+ 5d1 → 4f luminescence is blue-shifted with increasing Lu content due to the decrease in crystal field splitting of the 5 d levels. With increasing Lu content, the shortening of scintillation decay time accompanied with a decrease of LY value as measured at room temperature can be explained by thermal ionization of the Ce3+ 5d1 excited state. Co-doping with Mg2+ results in the shortening of scintillation decay time, the reduction of LY value and afterglow intensity. At low temperature the radioluminescence quenching is observed in correlation with large thermoluminescence peaks, which can be attributed to the localization of electrons at intrinsic shallow traps.

How Many Electron Traps are formed in Persistent Phosphors?

Persistent luminescence is caused by a charge carrier detrapping process from the carrier traps filled by excitation light. Thus, the carrier storage capacity is an important factor in determining the persistent luminescence intensity. Here, the electron storage capacity was investigated in the YAGG:Ce3+-Yb3+ transparent ceramic persistent phosphor, in which the Ce3+ ion is the luminescence center and the Yb3+ ion acts as the electron trap. The number density of the Yb2+ electron trapping center was estimated to be approximately 1.6 × 1018 ions cm−3 from the absorption coefficient spectrum of Yb2+:4f–5d photochromic absorption center and the Yb-LIII edge XANES spectrum, which means that approximately 12% of the Yb3+ ions in the sample were changed to the divalent state after charging. Although the maximum energy density of 0.61 J cm−3 was calculated as a storage property of persistent phosphors from the Yb2+ number density and the photon energy of Ce3+:5d1−4f luminescence at 520 nm, the actual energy density which was detected as persistent luminescence was 0.011 J cm−3. It is suggested that the recombination efficiency of the detrapped electrons from the Yb2+ ions with the hole-trapped Ce3+ ions is approximately a few percent.

Scintillation yield and temperature dependence of radioluminescence of (Lu,Gd)3Al5O12:Ce garnet crystals

The scintillation characteristics of Lu3-xGdxAl5O12:Ce (x = 1, 1.25, 1.75, 2, and 2.25) garnet crystals were investigated. With increasing Gd content, the Ce3+ 5d1 → 4f luminescence band was red-shifted due to an increase in the crystal field splitting of the 5d levels. In addition, both the light yield value and contribution of fast scintillation component increased. We attribute this effect to the reduced electron trapping effects of shallow traps buried in the bottom of the conduction band, the lower-energy shift of which is due to the increasing Gd content. A decrease of scintillation yield at the lowest temperature region observed for all samples, showing large thermoluminescence peaks in the same temperature range, can be caused by the localization of electrons at shallow traps. Afterglow signals observed at 10 K for all samples after X-rays irradiation can be explained by quantum tunneling of electrons from nearby traps towards the holes captured at the Ce3+ recombination centers.

The key roles of 4f-level splitting and vibronic coupling in the high-efficiency luminescence of Ce3+ ion in LuAG transparent ceramic phosphors

Ce3+ ion-activated lutetium aluminum garnet (LuAG:Ce) polycrystalline transparent ceramics (TCs) were prepared with high-temperature vacuum-sintering technology. The quantum efficiency of the characteristic Ce3+ 5d→4f luminescence of the prepared LuAG:Ce TC phosphor with 0.50at.% Ce sample was measured to be an almost unit. More interestingly, although the acquired low-temperature luminescence spectra still seemingly consist of double broad bands with considerable overlapping, the five major emission lines due to the multiple splittings of 4f shell of Ce3+ are at least unveiled on the basis of theoretical calculations with the semiclassical time-dependent approach of Heller. The pure electronic transition energy and electron-phonon coupling information of each individual emission line are thus determined. The study leads to some important understandings: (i) The 5d→4f transitions of Ce3+ ion are extremely efficient even in the complex polycrystalline environment; (ii) The radiative transitions are much complicated than the commonly understood picture; (iii) As the radiation energy increases, the vibronic coupling characterized by Huang-Rhys factor becomes weaker; Meanwhile, the contribution to the total luminescence tends to increase; (iv) The involved lattice vibration mode could be the low frequency mode of ∼260 cm−1.

Luminescence and scintillation characteristics of Cs3ScCl6:Ce crystals

In this study, we fabricated Cs3ScCl6 crystals doped with different concentrations of Ce and investigated their luminescence properties. In both the photoluminescence (PL) and scintillation spectra, emission bands attributable to Ce3+ 5d–4f transition were observed. The PL temporal profiles revealed decay time constants of ~35 ns, which are typical values for Ce3+ 5d–4f transition. In the scintillation temporal profiles, two decay components were observed. The fast and slow components could be attributed to Ce3+ 5d–4f luminescence and self-trapped excitons (STEs) or a slow energy transfer from STEs to Ce3+, respectively. The maximum light yield was 9200 photons/MeV for a Ce concentration of 0.5 mol%.

Pressure-induced variation of persistent luminescence characteristics in Y3Al5-xGaxO12:Ce3+-M3+ (M = Yb, and Cr) phosphors: opposite trend of trap depth for 4f and 3d metal ions.

Changing the electronic structure of materials by pressure and the accompanying changes in optical properties have attracted scientific interest. We have reported that the energy position of the conduction band (CB) bottom and the crystal field splitting of the Ce3+:5d excited level in Y3Al5-xGaxO12:Ce3+ are changed by applying pressure, which results in the red shifting of the Ce3+:5d → 4f luminescence and the increase of the quenching temperature. We also reported dramatic improvement of the persistent luminescence performance by either Cr3+ or Yb3+ codoping into the Y3Al5-xGaxO12:Ce3+ phosphors. The different trap depths formed by Cr3+ and Yb3+ affect the initial persistent luminescence intensity and the persistent luminescence duration. In this study, the effect of pressure on the persistent luminescence performance was investigated. For the Y3AlGa4O12:Ce3+-Yb3+ phosphor, the slope of persistent luminescence decay curve becomes more gentle with increasing pressure, while for the Y3AlGa4O12:Ce3+-Cr3+ phosphor the slope becomes steeper. These results indicate that the trap depth of Yb3+ becomes deeper and that of Cr3+ becomes shallower with increasing pressure. Based on the pressure-dependence of the luminescence quenching and the trap depth change estimated from the decay slopes, the relative electronic energies of the CB bottom and the Yb2+ (4f14) or Cr2+ (3d4) levels are discussed. The CB bottom energy is increased relative to the ground 1S0 state of Yb2+ with increasing pressure, which results in deepening of the electron trap depth of the Yb2+ state. The opposite tendency of the Cr3+ codoped sample was described by a decreasing tendency of the energy gap between the CB bottom and the Cr2+:eg level, the relative energy level of which is increased by the increase of the crystal field with increasing pressure in the garnet host material, where the electron-trapping Cr2+ ions take the high spin state (t32ge1g) rather than the low-spin state (t42g).

Luminescence of 5d–4f transitions of Pr3+ in natural fluorite CaF2, anhydrite CaSO4 and apatite Ca5(PO4)3F

Natural fluorite, anhydrite and apatite under deep UV excitation exhibits several narrow emission bands in the spectral range of 225–300 nm with the strongest peaks at 230–240 nm and 260–290 nm and with half-widths of 20–30 nm. They are characterized by narrow excitation band peaking at 220 nm and very short decay times of 20–50 ns. Those spectral-kinetic parameters enable their interpretation as 4f5d–4f transitions in Pr3+ luminescence center. Such kind of luminescence has been studied extensively over the past several decades in synthetic phosphors, including those which are analogous to natural compounds, but it is the first time 5d–4f luminescence transitions of Pr3+ is being proved in minerals.

Catalytic Decomposition of Hypochlorite Anions by Ceria Nanoparticles Visualized by Spectroscopic Techniques

Hypochlorous acid (HOCl) and hypochlorite anions (−OCl) are nonselective oxidants produced by activated neutrophils and monocytes and play an important role in the formation of immune response of a living organism. At the same time, because of the strong oxidative properties of hypochlorites, their concentrations in cells need to be under strict control. In this paper, cerium oxide nanoparticles (nanoceria) were used for spectroscopically controlled catalytic decomposition of hypochlorite anions. The processes of scavenging of hypochlorite anions by ceria nanoparticles and regeneration of nanoceria were monitored using the changes in hypochlorite absorption and 5d → 4f luminescence of Ce3+ ions during oxidation–reduction cycles. The high oxygen deficiency of ceria nanoparticles provides sufficient mobility of oxygen ions, leading to the involvement of both surface and bulk Ce3+ ions into oxidation, whereas the recovery of initial Ce3+/Ce4+ ratio is terminated by the slow diffusion of oxygen out of ceria nanoparticles.

Luminescence of Pr3+ Impurity Centers and Defects in Sr9Sc (PO4)7:Pr3+

The spectroscopic characteristics and luminescence decay kinetics of Sr9Sc(PO4)7:Pr3+(1%) were investigated using synchrotron radiation of ultraviolet (UV), vacuum ultraviolet (VUV) and X-ray ranges, as well as pulsed cathodoluminescence (PCL) methods at temperatures of 10, 90, and 295 K. The luminescence spectra contain three groups of bands and lines. In the UV range, the observed bands correspond to interconfigurational 5d → 4f transitions in Pr3+ ions. In the visible spectrum, the wide luminescence band of defects dominates, as well as narrow lines associated with intraconfigurational 4f → 4f transitions in Pr3+ ions. When excited by an electron beam (5 Hz), the main component with a decay time τ ∼ 17 ns dominates in the decay kinetics of the 5d → 4f luminescence. The decay kinetics of 5d → 4f luminescence upon excitation with high-frequency (τ ∼ 8 MHz) synchrotron radiation of the X-ray range contains a fast component (τ ∼ 15 ns) against the background of the slow component of the μs-range. An effective nonradiative energy transfer is observed from impurity centers to defects, as well as from the host to Pr3+ ions and defects.

Hydrogen peroxide sensing using Ce3+ luminescence of cerium oxide (CeO2-x) nanoparticles

In the paper the new application of nanoceria (CeO2-x nanoparticles) for detection and quantification of hydrogen peroxide (HP) content in water solutions is shown. Sensing of hydrogen peroxide is based on the reversible shift of Ce3+↔ Ce4+ balance in ceria nanoparticles as a result of nanoceria-oxidant interaction. Intensity of 5 d→4f luminescence of Ce3+ ions being a measure of Ce3+/Ce4+ ratio in nanoceria depends linearly on the hydrogen peroxide concentration in water solutions and, so, can be used for reliable quantification of HP content. The nanoceria-based sensor is reversible and applicable for long-term monitoring of oxidant level in biological systems.

Pr3+ Luminescence to Improve the Range, Accuracy and Sensitivity of Temperature Measurement

For years, various contact techniques were used to measure temperature. Development of novel and evolving technologies raised the interest in elaboration of non-contact thermometry. Nowadays, this is radiation pyrometry which is the method of choice for such purposes. Yet, it has important limitations – sensitivity to stray light or reflected radiation as well as sensitivity to flame interferences. Also accuracy of temperature measurement, which is not better than ~2 K, is not always satisfactory. Today technologies do not fulfill the needs of temperature sensing in microelectronics, microoptics, photonics, microfluidics, nanomedicine and they require alternative solutions. Among promising approaches, overcoming the limitations of other techniques, luminescent methods are of great interest. While the idea is not new [1], a real breakthrough using this technique was not yet achieved. Combining a high sensitivity and accuracy as well as broad range of temperature usability within one temperature sensor appeared challenging [2,3]. In this presentation we shall prove that Pr3+ luminescence characteristics may offer a lot in this field. While in some compositions this ion generates only 4f®4f luminescence due to relaxation of the higher 3P0 and lower 1D2 levels, in some others also 5d®4f luminescence may take place. We explored the possibility to use all these three luminescent components to sense the temperature accurately and in a broad range. The main argument for that is that all the three emissions are characterized by different mechanisms of their thermal quenching. Furthermore, temperature strongly affects the non-radiative relaxation between the three levels. Our presentation will show first results of the approach presented above using data for Sr2GeO4:Pr and will prove a validity and potential of this new attitude. Figure 1 presents profound changes in emission spectra of Sr2GeO4:Pr in the range of 17-600 K. [1] P. Neubert, US2085508 A, 1937. [2] L. D. Carlos, F. Palacio, Thermometry at the Nanoscale: Techniques and Selected Applications, Royal Society of Chemistry, Oxfordshire 2016. [3] C. D. S. Brites, P. P. Lima, N. J. O. Silva, A. Millán, V. S. Amaral, F. Palacio, L. D. Carlos, Nanoscale 2012, 4, 4799. Figure 1

Investigation of luminescence quenching and persistent luminescence in Ce3+ doped (Gd,Y)3(Al,Ga)5O12 garnet using vacuum referred binding energy diagram

Optical properties of Ce3+:5d−4f luminescence in garnet compounds are strongly related to the energy locations of the excited 5d levels and the host conduction band (CB). By constructing the vacuum referred binding energy (VRBE) diagram including information on these energy location for GdyY3-yAl5-xGaxO12 garnet (GYAGG) from the measured spectroscopic data, the luminescence quenching and persistent luminescence properties of GYAGG:Ce3+(-Cr3+) were discussed. The quantum yield (QY) of Ce3+:5d−4f in GYAGG:Ce3+ is clearly related to the energy gap, ΔE5d1−CB, between the lowest 5d level and the bottom of conduction band in the constructed VRBE diagram. This result indicates that the luminescence quenching of GYAGG:Ce3+ is caused by the thermal ionization. For GYAGG:Ce3+-Cr3+ persistent phosphors, the depth of the electron trap formed by Cr3+ was remarkably changed from 0.27 to 0.69 eV by varying the Gd and Ga contents. The estimated VRBE value of Cr2+ in GYAGG can be used as effective value to predict the electron trap depth by Cr3+ in other oxide compounds as well as garnet hosts.