Mika Lastusaari

Persistent luminescence of Eu2+ doped alkaline earth aluminates, MAl2O4:Eu2+

Abstract The polycrystalline Eu2+ and RE3+ (rare earth) doped alkaline earth aluminates MAl2O4:Eu2+,RE3+ (M=Ca and Sr) were prepared by a solid state reaction starting from the corresponding oxides (Al, RE) and carbonates (Ca, Sr). The UV-excited photoluminescence, persistent luminescence (afterglow) and thermoluminescence of these materials was studied and compared. The two spectra (band position and width) were very similar indicating that the luminescent centre is an Eu2+ ion in both cases. A slight blue shift was observed in the afterglow emission in some cases. The RE3+ co-doping had no effect on the position and shape of the persistent luminescence band but the Nd3+ and Dy3+ ions enhanced the most the afterglow and thermoluminescence of the CaAl2O4:Eu2+ and SrAl2O4:Eu2+ materials, respectively. Easily reducible rare earth ions such as Sm3+ and Yb3+ suppressed both the afterglow and thermoluminescence of MAl2O4:Eu2+. Based on these observations, the mechanism of RE3+ co-doping is discussed.

Persistent luminescence mechanisms: human imagination at work

The present status and future progress of the mechanisms of persistent luminescence are critically treated with the present knowledge. The advantages to be achieved by a further need as well as the pitfalls of the excessive use of imagination are shown. As usual, in the beginning of the present era of persistent luminescence since the mid 1990s, the imagination played a more important role than the sparse solid experimental data and the chemical common sense and knowledge was largely ignored. Since some five years, the mechanistic studies seem to have reached the maturity and – perhaps deceivingly – it seems that there are only details to be solved. However, the development of red emitting nanocrystalline materials poses a challenge also to the more fundamental studies and interpretation. The questions still luring in the darkness include the problems how the increased surface area affects the defect structure and how the “persistent energy transfer” really works. There is still some light to be thrown onto these matters starting with agreeing on the terminology: the term phosphorescence should be abandoned altogether. The long lifetime of persistent luminescence is due to trapping of excitation energy, not to the forbidden nature of the luminescent transition. However, the technically well-suited term “afterglow” should be retained for harmful, short persistent luminescence.

Crystal field strength in C-type cubic rare earth oxides

Abstract Luminescence of the Eu 3+ -doped cubic C-type rare earth (RE) oxides, RE 2 O 3 :Eu 3+ (RE=Eu, Gd, Lu, Y, In and Sc) powder samples at 77 and 298 K was investigated under UV, argon ion and dye laser excitation. Intense transitions from the 5 D 0 to 7 F 0–4 levels of the ground 7 F multiplet were observed between 575 and 720 nm. The complete lifting of the 7 F J level degeneracy as well as the absence of transition selection rules imposed by the group theory are consistent with the C 2 point symmetry of the RE 3+ site. Only a few lines originating from Eu 3+ ions in the high symmetry S 6 site were observed. The crystal field (c.f.) analysis conducted on the basis of the 18–21 7 F 0–4 c.f. levels yielded satisfactory results despite the high number (14) of the B k q and S k q parameters. The strength of the c.f. effect increases in the RE series with decreasing ionic radius of the RE 3+ host cation. This results from the increased electrostatic effect of the host lattice on the Eu 3+ ion.

Persistent luminescence behavior of materials doped with Eu 2+ and Tb 3+

In this work, the persistent luminescence mechanisms of Tb3+ (in CdSiO3) and Eu2+ (in BaAl2O4) based on solid experimental data are compared. The photoluminescence spectroscopy shows the different nature of the inter- and intraconfigurational transitions for Eu2+ and Tb3+, respectively. The electron is the charge carrier in both mechanisms, implying the presence of electron acceptor defects. The preliminary structural analysis shows a free space in CdSiO3 able to accommodate interstitial oxide ions needed by charge compensation during the initial preparation. The subsequent annealing removes this oxide leaving behind an electron trap. Despite the low band gap energy for CdSiO3, determined with synchrotron radiation UV-VUV excitation spectroscopy of Tb3+, the persistent luminescence from Tb3+ is observed only with UV irradiation. The need of high excitation energy is due to the position of 7F6 level deep below the bottom of the conduction band, as determined with the 4f8→4f75d1 and the ligand-to-metal charge-transfer transitions. Finally, the persistent luminescence mechanisms are constructed and, despite the differences, the mechanisms for Tb3+ and Eu2+ proved to be rather similar. This similarity confirms the solidity of the interpretation of experimental data for the Eu2+ doped persistent luminescence materials and encourages the use of similar models for other persistent luminescence materials.

Quantitative multianalyte microarray immunoassay utilizing upconverting phosphor technology.

A quantitative multianalyte immunoassay utilizing luminescent upconverting single-crystal nanoparticles as reporters on an antibody array-in-well platform was demonstrated. Upconverting nanoparticles are inorganic rare earth doped materials that have the unique feature of converting low energy infrared radiation into higher energy visible light. Autofluorescence, commonly limiting the sensitivity of fluorescence-based assays, can be completely eliminated with photon upconversion technology because the phenomenon does not occur in biological materials. Biotinylated antibodies for three analytes (prostate specific antigen, thyroid stimulating hormone, and luteinizing hormone) were printed in an array format onto the bottom of streptavidin-coated microtiter wells. Analyte dilutions were added to the wells, and the analytes were detected with antibody-coated upconverting nanoparticles. Binding of the upconverting nanoparticles was imaged with an anti-Stokes photoluminescence microwell imager, and the standard curves for each analyte were quantified from the selected spot areas of the images. Single analyte and reference assays were also carried out to compare with the results of the multianalyte assay. Multiplexing did not have an effect on the assay performance. This study demonstrates the feasibility of upconverting single-crystal nanoparticles for imaging-based detection of quantitative multianalyte assays.

Electronic structure of the SrAl2O4:Eu2+ persistent luminescence material

Abstract The electronic structure of the strontium aluminate (SrAl2O4:Eu2+) materials was studied with a combined experimental and theoretical approach. The UV-VUV synchrotron radiation was applied in the experimental study while the electronic structure of the non-optimized and optimized crystal structure were investigated theoretically by using the density functional theory. The structure of the valence and conduction bands as well as the band gap energy of the material together with the position of the Eu2+ 4f7 8S7/2 ground state were calculated. The calculated band gap energy (6.4 eV) agreed well with the experimental value of 6.6 eV. The valence band consisted mainly of oxygen states whereas the bottom of the conduction band of strontium states. In agreement with the experimental results, the calculated 4f7 8S7/2 ground state of Eu2+ lies in the energy gap of the host. The position of the 4f7 ground state depended on the Coulomb repulsion strength. The position of the 4f7 ground state with respect to the valence and conduction bands was discussed using theoretical and experimental evidence available.

Crystal field energy levels of Eu3+ and Yb3+ in the C2 and S6 sites of the cubic C-type R2 O3

In the cubic C-type rare-earth (R) sesquioxides, C-R2 O3, the trivalent R ions (R3+) occupy two different crystallographic sites with S6 and C2 symmetries. The R ions in the C2 lattice site have been studied intensively whereas the properties of the R3+ ions in the S6 site are largely unknown or the data are contradictory. Based on the spectroscopic data reinterpreted by a phenomenological crystal field (cf) analysis a new interpretation was obtained for the energy level scheme of the Eu3+ ions in the S6 site of C-R2 O3. The cf parameters obtained were then used to predict the energy level scheme of the Yb3+ ion in the same host lattices. In the prediction, the evolution of the cf parameters along the R series studied earlier in the R oxide and garnet systems was used. The relationship between the cf strength parameter and the overall splitting of the 2S+1 L J levels as well as the relationship between the barycentres of the free ion levels were used, also, to reinterpret the energy level scheme of the Yb3+ ion in the C2 site of C-Y2 O3.

Influence of titanium and lutetium on the persistent luminescence of ZrO 2

Non-doped as well as titanium and lutetium doped zirconia (ZrO2) materials were synthesized via the sol-gel method and structurally characterized with X-ray powder diffraction. The addition of Ti in the zirconia lattice does not change the crystalline structure whilst the Lu doping introduces a small fraction of the tetragonal phase. The UV excitation results in a bright white-blue luminescence at ca. 500 nm for all the materials which emission could be assigned to the Ti3+eg → t2g transition. The persistent luminescence originates from the same Ti3+ center. The thermoluminescence data shows a well-defined though rather similar defect structures for all the zirconia materials. The kinetics of persistent luminescence was probed with the isothermal decay curve analyses which indicated significant retrapping. The short duration of persistent luminescence was attributed to the quasi-continuum distribution of the traps and to the possibility of shallow traps even below the room temperature.

Defect to R3+ energy transfer: colour tuning of persistent luminescence in CdSiO3

Luminescence from trivalent rare earth (R3+: La3+–Lu3+, excluding Pm3+) ions was studied in the CdSiO3 host. The positions of the R2+/3+ energy levels in the band structure of CdSiO3 suggest that the doping of CdSiO3 with R2+ ions is difficult if not impossible. Red, pink, blue, green and close to white persistent luminescence colours were obtained by doping with Pr3+, Sm3+, Gd3+, Tb3+ and Dy3+, respectively. The efficiency of the defect to R3+ energy transfer determines if persistent luminescence arises from the 4f–4f, defect or a combination of these two emissions. In contrast to what is observed for Pr3+ and Tb3+, the defect to R3+ energy transfer did not give efficient persistent luminescence for Sm3+ and Dy3+, probably due to high energy losses and/or back transfer from the rare earth to defects. In line with the experimental observations, the in situ synchrotron radiation XANES spectra indicated the presence of only the trivalent Pr3+ and Tb3+ species thus excluding the direct R3+ → RIV oxidation during the charging process of persistent luminescence. Finally, based on the band gap energy, R2+/3+ energy level positions, trap energies, and other optical and structural properties, the mechanism of persistent luminescence was developed for Pr3+ doped CdSiO3. For practical applications, the CdSiO3:R3+ system offers an excellent possibility for colour tuning of persistent luminescence by changing only the R3+ dopant instead of altering the host as is the case with the Eu2+ doped materials. Eventually, this will avoid the waste of both intellectual and financial resources.

The Bologna Stone: history's first persistent luminescent material

In 1603, the Italian shoemaker Vincenzo Cascariolo found that a stone (baryte) from the outskirts of Bologna emitted light in the dark without any external excitation source. However, the calcination of the baryte was needed prior to this observation. The stone later named as the Bologna Stone was among the first luminescent materials and the first documented material to show persistent luminescence. The mechanism behind the persistent emission in this material has remained a mystery ever since. In this work, the Bologna Stone (BaS) was prepared from the natural baryte (Bologna, Italy) used by Cascariolo. Its properties, e. g. impurities (dopants) and their valences, luminescence, persistent luminescence and trap structure, were compared to those of the pure BaS materials doped with different (transition) metals (Cu, Ag, Pb) known to yield strong luminescence. The work was carried out by using different methods (XANES, TL, VUV-UV-vis luminescence, TGA-DTA, XPD). A plausible mechanism for the persistent luminescence from the Bologna Stone with Cu+ as the emitting species was constructed based on the results obtained. The puzzle of the Bologna Stone can thus be considered as resolved after some 400 years of studies. (Less)