Polycrystalline Eu 2+ and Dy 3+ doped barium aluminate materials, BaAl 2 O 4 :Eu 2+ ,Dy 3+ , were... more Polycrystalline Eu 2+ and Dy 3+ doped barium aluminate materials, BaAl 2 O 4 :Eu 2+ ,Dy 3+ , were prepared with solid state reactions at temperatures between 700 and 1500°C. The influence of the thermal treatments on the stability, homogeneity and structure as well as to the UV-excited and persistent luminescence of the materials was investigated by X-ray powder diffraction, SEM imaging and infrared spectroscopies as well as by steady state luminescence spectroscopy and persistent luminescence decay curves, respectively. The IR spectra of the materials prepared at 250, 700, and 1500°C follow the formation of BaAl 2 O 4 composition whereas the X-ray powder diffraction of compounds revealed how the hexagonal structure was obtained. The morphology of the materials at high temperatures indicated important aggregation due to sintering. The luminescence decay of the quite narrow Eu 2+ band at ca. 500 nm shows the presence of persistent luminescence after UV irradiation. The dopant (Eu 2+) and co-dopant (Dy 3+) concentrations affect the crystallinity and luminescence properties of the materials.
A systematic spectroscopic study of the 4 f 7 energy levels of Gd 3ϩ in LiYF 4 in the vacuum-ultr... more A systematic spectroscopic study of the 4 f 7 energy levels of Gd 3ϩ in LiYF 4 in the vacuum-ultraviolet spectral region (50 000-70 000 cm Ϫ1) is reported. Using energy-level calculations, all observed spectral lines could be assigned to free-ion term symbols ͑including term symbols with unusually high L and J, e.g., a 2 Q 23/2 level around 67 000 cm Ϫ1 ͒. From the 6 G J levels around 50 000 cm Ϫ1 quantum cutting ͑or two-photon luminescence, photon-cascade emission͒ is observed: the emission of a red photon due to the 6 G J → 6 P J transition is followed by the emission of an ultraviolet photon due to the 6 P J → 8 S 7/2 transition.
The fading of persistent luminescence in Sr 2 MgSi 2 O 7 :Eu 2+ ,R 3+ (R: Y, LaNd , Sm-Lu) was st... more The fading of persistent luminescence in Sr 2 MgSi 2 O 7 :Eu 2+ ,R 3+ (R: Y, LaNd , Sm-Lu) was studied combining thermoluminescence (TL) and room temperature (persistent) luminescence measurements to gain more information on the mechanism of persistent luminescence. The TL glow curves showed the main trap signal at ca. 80 °C, corresponding to 0.6 eV as the trap depth, with every R co-dopant. The TL measurements carried out with different irradiation times revealed the general order nature of the TL bands. The results obtained from the deconvolutions of the glow curves allowed the prediction of the fading of persistent luminescence with good accuracy, though only when using the Becquerel decay law.
ABSTRACT The existence and effect of different rare earth (R2+/3+/IV) ions in SrAl2O4:Eu2+,R3+ an... more ABSTRACT The existence and effect of different rare earth (R2+/3+/IV) ions in SrAl2O4:Eu2+,R3+ and M2MgSi2O7:Eu2+,R3+ (M: Sr, Ba) persistent luminescence materials was studied with XANES (x-ray absorption near edge structure) measurements at HASYLAB/DESY (Hamburg, Germany) and MAX-lab (Lund, Sweden). The experiments were carried out at 298 K for selected rare earth (co-)dopants (Eu2+; Ce3+, Nd3+, Sm3+, Dy3+ and Yb3+). The co-existence of Eu2+ and Eu3+ was observed in all materials. The co-dopants were always in the trivalent form.
A series of monoclinic strontium aluminates, SrAl 2−x B x O 4 :Eu 2+ (0 ≤ x ≤ 0.3), were prepared... more A series of monoclinic strontium aluminates, SrAl 2−x B x O 4 :Eu 2+ (0 ≤ x ≤ 0.3), were prepared by a solid state reaction. The effect of boron on the preparation, structure and luminescence was studied by thermoanalytical, X-ray powder diffraction (XPD), UV excited as well as thermoluminescence (TL) methods. Boron decreased clearly the reaction threshold temperature but concentration was not found important. Boron decreased also the amount of impurities. The monoclinic structure remained the same irrespective of the boron content. No Al-B solid solution was formed. The luminescence band position, shape, and width were not changed as a function of the boron content. The UV excited luminescence intensity was enhanced but persistent luminescence suppressed when the boron concentration increased. The thermoluminescence peak shifted slightly towards lower temperatures and the TL intensity was suppressed by increasing boron concentration maybe due to the modification of trap concentration and depth.
Luminescence D 6540 Low Temperature Thermoluminescence Properties of Eu 2+ and Ln 3+ Doped CaAl 2... more Luminescence D 6540 Low Temperature Thermoluminescence Properties of Eu 2+ and Ln 3+ Doped CaAl 2 O 4 .-The thermoluminescence properties of lanthanide-doped CaAl 2 O 4 are characterized between 20 and 325 K. La 3+ , Y 3+ , and Lu 3+ ions suppress the thermoluminescence, whereas Pr 3+ , Ho 3+ , and Dy 3+ co-doped materials show an intense but relatively short persistent luminescence at room temperature. Nd 3+ and Tm 3+ ions enhance the thermoluminescence bands close to room temperature and are thus the most suitable co-dopants to induce intense and long persistent luminescence.-(AITASALO, T.;
21 21 Sol-gel preparation, structure and luminescence of Eu doped calcium aluminate (CaAl O :Eu) ... more 21 21 Sol-gel preparation, structure and luminescence of Eu doped calcium aluminate (CaAl O :Eu) was studied. The reaction 2 4 21 threshold temperature was about 500 8C, when using the sol-gel method. Unusual metastable hexagonal CaAl O :Eu (lattice 2 4 parameters a58.74 and c58.08 A), was obtained with the sol-gel method. The broad band UV excited luminescence of the hexagonal 21 6 1 7 21 CaAl O :Eu was observed at the blue region (l 5448 nm) due to transitions from the 4f 5d to the 4f configuration of the Eu 2 4 max ion. The afterglow band position, shape and width were the same as those in the UV-excited spectrum indicating the same luminescent 21 Eu centre. The thermoluminescence curve of the hexagonal modification contained several high temperature peaks, which explain the slow decay of the afterglow.
The UV and dye laser excited optical luminescence spectra at 77 and 300 are reported for the D-Y2... more The UV and dye laser excited optical luminescence spectra at 77 and 300 are reported for the D-Y2Si207:Eu3+ and D-Lu2Si207:Eu3+ systems obtained at high temperature and pressure. The 7/O-s energy level schemes derived from this study are analyzed in terms of a 14 parameter crystal field (cf.) Hamiltonian in a cf. of C2 symmetry. The parameter sets are used to simulate the experimental energy level schemes. The best fit sets of the cf. parameters yield excellent simulation of the experimental cf. splittings of the Fa
Summarv: A detailed analysis of the energy level scheme of the Nd3+ ion with the 4f3 electron con... more Summarv: A detailed analysis of the energy level scheme of the Nd3+ ion with the 4f3 electron configuration is described in the NdOBr matrix. 93 Stark levels were deduced from the optical absorption spectra recorded at the UV, visible and NIR spectral ranges at selected temperatures from 9 to 300 K. The experimental level scheme was simulated with a phenomenological model with 19 parameters accounting simnltaneously for the free ion and crystal field effects. The results of the simulation showed increasing covalence and decreasing crystal field effect from NdOF to NdOBr. Resume: Analvse de scheme de niveanx d'eneraie de Nd3' en NdOBr. L'analyse detaillee de la sequence de niveaux d'energie de l'ion Nd3+ a Btb etudiee dans le compose NdOBr. 93 niveaux Stark ont 6t6 deduits des spectres d'absorption dans l'U.V., le visible et le proche IR, enregistres a des temperatures entre 9 et 300 K. Le sequence des niveaux experimentaux a et6 simulee par un modele phenomenologique de 19 parametres tenant en compte simultanement des effets de l'ion libre et du champ cristallin. Les resultats de la simulation montrent que les effets de covalence s'accroissent, mais le champ cristallin s'at%iblit de NdOF a NdOBr.
... 2009, p. 550 Electronic structure of the SrAl2O4:Eu2+ persistent luminescence material Jorma ... more ... 2009, p. 550 Electronic structure of the SrAl2O4:Eu2+ persistent luminescence material Jorma H ls 1,2, Taneli Laamanen1,3, Mika Lastusaari1,2, Janne Niittykoski1, Pavel Nov k4 (1. Department of Chemistry, University of Turku, FI ... [3] Yamamoto H, Matsuzawa T. Mechanism of ...
The smectites represent a versatile class of clay minerals with broad usage in industrial applica... more The smectites represent a versatile class of clay minerals with broad usage in industrial applications, e.g., cosmetics, drug delivery, bioimaging, etc. Synthetic hectorite Na 0.7 (Mg 5.5 Li 0.3)[Si 8 O 20 ](OH) 4 is a distinct material from this class due to its low-cost production method that allows to design its structure to match better the applications. In the current work, we have synthesized for the first time ever nanoclay materials based on the hectorite structure but with the hydroxyl groups (OH −) replaced by Br − or I − , yielding bromohectorite (Br-Hec) and iodohectorite (I-Hec). It was aimed that these materials would be used as phosphors. Thus, OH − replacement was done to avoid luminescence quenching by multiphonon de-excitation. The crystal structure is similar to nanocrystalline fluorohectorite, having the d 001 spacing of 14.30 Å and 3 nm crystallite size along the 00l direction. The synthetic materials studied here show strong potential to act as host lattices for optically active species, possessing mesoporous structure with high specific surface area (385 and 363 m 2 g −1 for Br-Hec and I-Hec, respectively) and good thermal stability up to 800 • C. Both materials also present strong blue-green emission under UV radiation and short persistent luminescence (ca. 5 s). The luminescence features are attributed to Ti 3+ /Ti IV impurities acting as the emitting center in these materials.
The paper discusses the origin of emission quenching and the formation of metal-to-metal charge-t... more The paper discusses the origin of emission quenching and the formation of metal-to-metal charge-transfer states in yttrium orthovanadatearsenates doped with Tb 3+ ions. For that purpose, a series of highly crystalline yttrium orthovanadate-arsenate nanoparticles (type YV x As 1−x O 4 , where x = 2−65 mol %) doped with Tb 3+ ions were prepared using the coprecipitation method. The structural and morphological properties of the particles were studied by means of X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) techniques. The photoluminescence spectral and dynamical properties of YV x As 1−x O 4 doped with 1 mol % Tb 3+ ions were investigated in detail. It was found that the emission quenching is related to changes in the band gap and Tb 3+-to-V 5+ metal-to-metal charge transfer. Further, the Tb 3+-to-As 5+ metal-to-metal charge transfer was identified and rationalized based on different models.
Aluminium oxide thin films were deposited by direct current (d.c.) reactive magnetron sputtering ... more Aluminium oxide thin films were deposited by direct current (d.c.) reactive magnetron sputtering on glass-slide, silicon wafer, stainless-steel and polycarbonate substrates. The thicknesses of the films were between 200nm and 3.0μm. The deposition was controlled by the target voltage to obtain stoichiometric thin films with a high deposition rate. The deposition process was studied in terms of target power, I–U
The up-conversion luminescence where the absorption of two or more low-energy photons results in ... more The up-conversion luminescence where the absorption of two or more low-energy photons results in emission of a higher-energy photon has been highly effective in certain applications such as bioanalytical assays. Currently one of the most efficient up-converting materials is the NaYF 4 :Yb 3+ ,Er 3+. However, some applications (e.g. solar cells) still require improvement of the up-conversion efficiency in order to have actual practical use. Therefore, it is important to enhance the performance of the materials. In this work, the effect of Mn and Cr doping on the up-conversion luminescence of NaYF 4 :Yb 3+ ,Er 3+ was studied. The materials were prepared using the co-precipitation method and the ratios of the doping ions were optimized for best up-conversion intensity. The as-prepared materials showed very similar sizes and morphologies. Cr co-doping was observed to increase the up-conversion luminescence whereas the addition of Mn resulted in quenching. Up-conversion luminescence was also measured with different excitation power densities and the results suggested that Cr co-doping enables the up-conversion in lower power densities than without Cr co-doping. Furthermore, the reasons for the changes in up-conversion luminescence intensity were discussed.
Praseodymium doped ZrO 2 materials were prepared via sol-gel route and structurally characterized... more Praseodymium doped ZrO 2 materials were prepared via sol-gel route and structurally characterized by X ray powder diffraction (XPD) technique as well as Rietveld refinements. The addition of the Gd 3 þ codopant gradually changes the zirconia structure from monoclinic to tetragonal, and then to cubic. Intensification of the Pr 3 þ luminescence was observed with the increasing Gd 3 þ co-dopant concentration. Emission spectra of the Zr 0.99 À x Gd x Pr 0.01 O 2 materials show an initial strengthening of the red emission of Pr 3 þ (1 D 2-3 H 4 transition) with increasing Gd 3 þ co-doping. However, the luminescence is quenched at the highest Gd 3 þ concentration-possibly due to strongly increased concentration of the charge compensation defects. The valence change (Pr IV-Pr 3 þ) is supported by the XANES results on the L III edge of Pr. Although predominantly in the Pr 3 þ form irrespective of the Gd 3 þ concentration, the contribution from Pr IV is clearly visible at low (or zero) Gd 3 þ concentrations leading to the loss of Pr 3 þ and to poor luminescence output. Though enhancing the emission intensity of Pr 3 þ , the defect clusters engender short Pr 3 þ-Pr 3 þ distances enhancing the cross-relaxation process coupling the 3 P 0-1 D 2 relaxation with the 3 H 4-3 H 6 excitation. This process leads to the high red/blue-green emission ratio by quenching the 3 P 0-3 H 4 transition in blue-green. Eventually, the increased Gd 3 þ co-doping dilutes the Pr 3 þ ions and, the cross-relaxation process becomes non-operational; the quenching of the 3 P 0-3 H 4 transition is reversed.
A series of potassium lanthanum praseodymium ternary chlorides, K 2 La 1Àx Pr x Cl 5 , (0.02 6 x ... more A series of potassium lanthanum praseodymium ternary chlorides, K 2 La 1Àx Pr x Cl 5 , (0.02 6 x 6 0.15) single crystals was grown with Bridgman techniques. The structures of the mixed crystals were found isotypic with that of K 2 PrCl 5 and K 2 LaCl 5. The high resolution absorption, luminescence and excitation spectra were recorded at 4, 77 and 293 K. Broad band emission of Pr 3+ reabsorbed by the 3 H 4 ! 3 P J , 1 D 2 transitions was observed simultaneously with the emission from the 3 P 0 level after excitation in the UV region. The 4f 1 5d 1 level scheme derived from the absorption, excitation spectra and from VUV synchrotron excitation was partly established. The 4f 2 energy level scheme of the Pr 3+ ion in K 2 PrCl 5 consisting of 86 (out of 91) Stark components determined from the absorption and emission spectra was simulated by using a phenomenological crystal field model. The model including eight free ion and nine crystal field parameters (according to the C 2v symmetry) yielded a rms deviation of 17 cm À1 between the experimental and calculated energy level schemes. Magnetic susceptibility measurements were carried out between 2 and 298 K. Good agreement was obtained between the experimental and calculated paramagnetic susceptibilities as a function of temperature by using the van Vleck formalism and the energies and wave functions derived from the spectral data.
The charge compensation effects induced by aliovalent doping were studied in the monoclinic CdSiO... more The charge compensation effects induced by aliovalent doping were studied in the monoclinic CdSiO 3. An interstitial oxide ion may be feasible in this host to provide the extra negative charge required by the R 3+ doping. The oxide was studied in CdSiO 3 using density functional theory (DFT) calculations and synchrotron radiation (SR) luminescence spectroscopy. The crystal structure of this host was significantly modified by the interstitial oxide. The experimental band gap energy (E g) was perfectly reproduced by the calculations and intrinsic electron traps were revealed. Defect levels were found also in the interstitial oxide containing host, however their role has to be studied further.
The luminescence of LiYF 4 and YF 3 doped with Ho 3 was investigated using synchrotron radiation.... more The luminescence of LiYF 4 and YF 3 doped with Ho 3 was investigated using synchrotron radiation. In the excitation spectra transitions to previously unidenti®ed 4f 10 levels were observed thus extending the energy level diagram of Ho 3 into the (vacuum) ultraviolet region of the electromagnetic spectrum. Some high energy emissions were observed and could be attributed to spin forbidden 4f 9 5d 3 4f 10 emissions in YF 3 and LiYF 4. For YF 3 doped with Ho 3 emission from the 3 P1 2 level (situated at 63 000 cm À1) was observed. Furthermore, several photon cascade emission processes were observed in which one VUV photon is converted into two or three UV/Vis photons, albeit with a low eciency.
The present status and future progress of the mechanisms of persistent luminescence are criticall... more 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 andperhaps 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.
Polycrystalline Eu 2+ and Dy 3+ doped barium aluminate materials, BaAl 2 O 4 :Eu 2+ ,Dy 3+ , were... more Polycrystalline Eu 2+ and Dy 3+ doped barium aluminate materials, BaAl 2 O 4 :Eu 2+ ,Dy 3+ , were prepared with solid state reactions at temperatures between 700 and 1500°C. The influence of the thermal treatments on the stability, homogeneity and structure as well as to the UV-excited and persistent luminescence of the materials was investigated by X-ray powder diffraction, SEM imaging and infrared spectroscopies as well as by steady state luminescence spectroscopy and persistent luminescence decay curves, respectively. The IR spectra of the materials prepared at 250, 700, and 1500°C follow the formation of BaAl 2 O 4 composition whereas the X-ray powder diffraction of compounds revealed how the hexagonal structure was obtained. The morphology of the materials at high temperatures indicated important aggregation due to sintering. The luminescence decay of the quite narrow Eu 2+ band at ca. 500 nm shows the presence of persistent luminescence after UV irradiation. The dopant (Eu 2+) and co-dopant (Dy 3+) concentrations affect the crystallinity and luminescence properties of the materials.
A systematic spectroscopic study of the 4 f 7 energy levels of Gd 3ϩ in LiYF 4 in the vacuum-ultr... more A systematic spectroscopic study of the 4 f 7 energy levels of Gd 3ϩ in LiYF 4 in the vacuum-ultraviolet spectral region (50 000-70 000 cm Ϫ1) is reported. Using energy-level calculations, all observed spectral lines could be assigned to free-ion term symbols ͑including term symbols with unusually high L and J, e.g., a 2 Q 23/2 level around 67 000 cm Ϫ1 ͒. From the 6 G J levels around 50 000 cm Ϫ1 quantum cutting ͑or two-photon luminescence, photon-cascade emission͒ is observed: the emission of a red photon due to the 6 G J → 6 P J transition is followed by the emission of an ultraviolet photon due to the 6 P J → 8 S 7/2 transition.
The fading of persistent luminescence in Sr 2 MgSi 2 O 7 :Eu 2+ ,R 3+ (R: Y, LaNd , Sm-Lu) was st... more The fading of persistent luminescence in Sr 2 MgSi 2 O 7 :Eu 2+ ,R 3+ (R: Y, LaNd , Sm-Lu) was studied combining thermoluminescence (TL) and room temperature (persistent) luminescence measurements to gain more information on the mechanism of persistent luminescence. The TL glow curves showed the main trap signal at ca. 80 °C, corresponding to 0.6 eV as the trap depth, with every R co-dopant. The TL measurements carried out with different irradiation times revealed the general order nature of the TL bands. The results obtained from the deconvolutions of the glow curves allowed the prediction of the fading of persistent luminescence with good accuracy, though only when using the Becquerel decay law.
ABSTRACT The existence and effect of different rare earth (R2+/3+/IV) ions in SrAl2O4:Eu2+,R3+ an... more ABSTRACT The existence and effect of different rare earth (R2+/3+/IV) ions in SrAl2O4:Eu2+,R3+ and M2MgSi2O7:Eu2+,R3+ (M: Sr, Ba) persistent luminescence materials was studied with XANES (x-ray absorption near edge structure) measurements at HASYLAB/DESY (Hamburg, Germany) and MAX-lab (Lund, Sweden). The experiments were carried out at 298 K for selected rare earth (co-)dopants (Eu2+; Ce3+, Nd3+, Sm3+, Dy3+ and Yb3+). The co-existence of Eu2+ and Eu3+ was observed in all materials. The co-dopants were always in the trivalent form.
A series of monoclinic strontium aluminates, SrAl 2−x B x O 4 :Eu 2+ (0 ≤ x ≤ 0.3), were prepared... more A series of monoclinic strontium aluminates, SrAl 2−x B x O 4 :Eu 2+ (0 ≤ x ≤ 0.3), were prepared by a solid state reaction. The effect of boron on the preparation, structure and luminescence was studied by thermoanalytical, X-ray powder diffraction (XPD), UV excited as well as thermoluminescence (TL) methods. Boron decreased clearly the reaction threshold temperature but concentration was not found important. Boron decreased also the amount of impurities. The monoclinic structure remained the same irrespective of the boron content. No Al-B solid solution was formed. The luminescence band position, shape, and width were not changed as a function of the boron content. The UV excited luminescence intensity was enhanced but persistent luminescence suppressed when the boron concentration increased. The thermoluminescence peak shifted slightly towards lower temperatures and the TL intensity was suppressed by increasing boron concentration maybe due to the modification of trap concentration and depth.
Luminescence D 6540 Low Temperature Thermoluminescence Properties of Eu 2+ and Ln 3+ Doped CaAl 2... more Luminescence D 6540 Low Temperature Thermoluminescence Properties of Eu 2+ and Ln 3+ Doped CaAl 2 O 4 .-The thermoluminescence properties of lanthanide-doped CaAl 2 O 4 are characterized between 20 and 325 K. La 3+ , Y 3+ , and Lu 3+ ions suppress the thermoluminescence, whereas Pr 3+ , Ho 3+ , and Dy 3+ co-doped materials show an intense but relatively short persistent luminescence at room temperature. Nd 3+ and Tm 3+ ions enhance the thermoluminescence bands close to room temperature and are thus the most suitable co-dopants to induce intense and long persistent luminescence.-(AITASALO, T.;
21 21 Sol-gel preparation, structure and luminescence of Eu doped calcium aluminate (CaAl O :Eu) ... more 21 21 Sol-gel preparation, structure and luminescence of Eu doped calcium aluminate (CaAl O :Eu) was studied. The reaction 2 4 21 threshold temperature was about 500 8C, when using the sol-gel method. Unusual metastable hexagonal CaAl O :Eu (lattice 2 4 parameters a58.74 and c58.08 A), was obtained with the sol-gel method. The broad band UV excited luminescence of the hexagonal 21 6 1 7 21 CaAl O :Eu was observed at the blue region (l 5448 nm) due to transitions from the 4f 5d to the 4f configuration of the Eu 2 4 max ion. The afterglow band position, shape and width were the same as those in the UV-excited spectrum indicating the same luminescent 21 Eu centre. The thermoluminescence curve of the hexagonal modification contained several high temperature peaks, which explain the slow decay of the afterglow.
The UV and dye laser excited optical luminescence spectra at 77 and 300 are reported for the D-Y2... more The UV and dye laser excited optical luminescence spectra at 77 and 300 are reported for the D-Y2Si207:Eu3+ and D-Lu2Si207:Eu3+ systems obtained at high temperature and pressure. The 7/O-s energy level schemes derived from this study are analyzed in terms of a 14 parameter crystal field (cf.) Hamiltonian in a cf. of C2 symmetry. The parameter sets are used to simulate the experimental energy level schemes. The best fit sets of the cf. parameters yield excellent simulation of the experimental cf. splittings of the Fa
Summarv: A detailed analysis of the energy level scheme of the Nd3+ ion with the 4f3 electron con... more Summarv: A detailed analysis of the energy level scheme of the Nd3+ ion with the 4f3 electron configuration is described in the NdOBr matrix. 93 Stark levels were deduced from the optical absorption spectra recorded at the UV, visible and NIR spectral ranges at selected temperatures from 9 to 300 K. The experimental level scheme was simulated with a phenomenological model with 19 parameters accounting simnltaneously for the free ion and crystal field effects. The results of the simulation showed increasing covalence and decreasing crystal field effect from NdOF to NdOBr. Resume: Analvse de scheme de niveanx d'eneraie de Nd3' en NdOBr. L'analyse detaillee de la sequence de niveaux d'energie de l'ion Nd3+ a Btb etudiee dans le compose NdOBr. 93 niveaux Stark ont 6t6 deduits des spectres d'absorption dans l'U.V., le visible et le proche IR, enregistres a des temperatures entre 9 et 300 K. Le sequence des niveaux experimentaux a et6 simulee par un modele phenomenologique de 19 parametres tenant en compte simultanement des effets de l'ion libre et du champ cristallin. Les resultats de la simulation montrent que les effets de covalence s'accroissent, mais le champ cristallin s'at%iblit de NdOF a NdOBr.
... 2009, p. 550 Electronic structure of the SrAl2O4:Eu2+ persistent luminescence material Jorma ... more ... 2009, p. 550 Electronic structure of the SrAl2O4:Eu2+ persistent luminescence material Jorma H ls 1,2, Taneli Laamanen1,3, Mika Lastusaari1,2, Janne Niittykoski1, Pavel Nov k4 (1. Department of Chemistry, University of Turku, FI ... [3] Yamamoto H, Matsuzawa T. Mechanism of ...
The smectites represent a versatile class of clay minerals with broad usage in industrial applica... more The smectites represent a versatile class of clay minerals with broad usage in industrial applications, e.g., cosmetics, drug delivery, bioimaging, etc. Synthetic hectorite Na 0.7 (Mg 5.5 Li 0.3)[Si 8 O 20 ](OH) 4 is a distinct material from this class due to its low-cost production method that allows to design its structure to match better the applications. In the current work, we have synthesized for the first time ever nanoclay materials based on the hectorite structure but with the hydroxyl groups (OH −) replaced by Br − or I − , yielding bromohectorite (Br-Hec) and iodohectorite (I-Hec). It was aimed that these materials would be used as phosphors. Thus, OH − replacement was done to avoid luminescence quenching by multiphonon de-excitation. The crystal structure is similar to nanocrystalline fluorohectorite, having the d 001 spacing of 14.30 Å and 3 nm crystallite size along the 00l direction. The synthetic materials studied here show strong potential to act as host lattices for optically active species, possessing mesoporous structure with high specific surface area (385 and 363 m 2 g −1 for Br-Hec and I-Hec, respectively) and good thermal stability up to 800 • C. Both materials also present strong blue-green emission under UV radiation and short persistent luminescence (ca. 5 s). The luminescence features are attributed to Ti 3+ /Ti IV impurities acting as the emitting center in these materials.
The paper discusses the origin of emission quenching and the formation of metal-to-metal charge-t... more The paper discusses the origin of emission quenching and the formation of metal-to-metal charge-transfer states in yttrium orthovanadatearsenates doped with Tb 3+ ions. For that purpose, a series of highly crystalline yttrium orthovanadate-arsenate nanoparticles (type YV x As 1−x O 4 , where x = 2−65 mol %) doped with Tb 3+ ions were prepared using the coprecipitation method. The structural and morphological properties of the particles were studied by means of X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) techniques. The photoluminescence spectral and dynamical properties of YV x As 1−x O 4 doped with 1 mol % Tb 3+ ions were investigated in detail. It was found that the emission quenching is related to changes in the band gap and Tb 3+-to-V 5+ metal-to-metal charge transfer. Further, the Tb 3+-to-As 5+ metal-to-metal charge transfer was identified and rationalized based on different models.
Aluminium oxide thin films were deposited by direct current (d.c.) reactive magnetron sputtering ... more Aluminium oxide thin films were deposited by direct current (d.c.) reactive magnetron sputtering on glass-slide, silicon wafer, stainless-steel and polycarbonate substrates. The thicknesses of the films were between 200nm and 3.0μm. The deposition was controlled by the target voltage to obtain stoichiometric thin films with a high deposition rate. The deposition process was studied in terms of target power, I–U
The up-conversion luminescence where the absorption of two or more low-energy photons results in ... more The up-conversion luminescence where the absorption of two or more low-energy photons results in emission of a higher-energy photon has been highly effective in certain applications such as bioanalytical assays. Currently one of the most efficient up-converting materials is the NaYF 4 :Yb 3+ ,Er 3+. However, some applications (e.g. solar cells) still require improvement of the up-conversion efficiency in order to have actual practical use. Therefore, it is important to enhance the performance of the materials. In this work, the effect of Mn and Cr doping on the up-conversion luminescence of NaYF 4 :Yb 3+ ,Er 3+ was studied. The materials were prepared using the co-precipitation method and the ratios of the doping ions were optimized for best up-conversion intensity. The as-prepared materials showed very similar sizes and morphologies. Cr co-doping was observed to increase the up-conversion luminescence whereas the addition of Mn resulted in quenching. Up-conversion luminescence was also measured with different excitation power densities and the results suggested that Cr co-doping enables the up-conversion in lower power densities than without Cr co-doping. Furthermore, the reasons for the changes in up-conversion luminescence intensity were discussed.
Praseodymium doped ZrO 2 materials were prepared via sol-gel route and structurally characterized... more Praseodymium doped ZrO 2 materials were prepared via sol-gel route and structurally characterized by X ray powder diffraction (XPD) technique as well as Rietveld refinements. The addition of the Gd 3 þ codopant gradually changes the zirconia structure from monoclinic to tetragonal, and then to cubic. Intensification of the Pr 3 þ luminescence was observed with the increasing Gd 3 þ co-dopant concentration. Emission spectra of the Zr 0.99 À x Gd x Pr 0.01 O 2 materials show an initial strengthening of the red emission of Pr 3 þ (1 D 2-3 H 4 transition) with increasing Gd 3 þ co-doping. However, the luminescence is quenched at the highest Gd 3 þ concentration-possibly due to strongly increased concentration of the charge compensation defects. The valence change (Pr IV-Pr 3 þ) is supported by the XANES results on the L III edge of Pr. Although predominantly in the Pr 3 þ form irrespective of the Gd 3 þ concentration, the contribution from Pr IV is clearly visible at low (or zero) Gd 3 þ concentrations leading to the loss of Pr 3 þ and to poor luminescence output. Though enhancing the emission intensity of Pr 3 þ , the defect clusters engender short Pr 3 þ-Pr 3 þ distances enhancing the cross-relaxation process coupling the 3 P 0-1 D 2 relaxation with the 3 H 4-3 H 6 excitation. This process leads to the high red/blue-green emission ratio by quenching the 3 P 0-3 H 4 transition in blue-green. Eventually, the increased Gd 3 þ co-doping dilutes the Pr 3 þ ions and, the cross-relaxation process becomes non-operational; the quenching of the 3 P 0-3 H 4 transition is reversed.
A series of potassium lanthanum praseodymium ternary chlorides, K 2 La 1Àx Pr x Cl 5 , (0.02 6 x ... more A series of potassium lanthanum praseodymium ternary chlorides, K 2 La 1Àx Pr x Cl 5 , (0.02 6 x 6 0.15) single crystals was grown with Bridgman techniques. The structures of the mixed crystals were found isotypic with that of K 2 PrCl 5 and K 2 LaCl 5. The high resolution absorption, luminescence and excitation spectra were recorded at 4, 77 and 293 K. Broad band emission of Pr 3+ reabsorbed by the 3 H 4 ! 3 P J , 1 D 2 transitions was observed simultaneously with the emission from the 3 P 0 level after excitation in the UV region. The 4f 1 5d 1 level scheme derived from the absorption, excitation spectra and from VUV synchrotron excitation was partly established. The 4f 2 energy level scheme of the Pr 3+ ion in K 2 PrCl 5 consisting of 86 (out of 91) Stark components determined from the absorption and emission spectra was simulated by using a phenomenological crystal field model. The model including eight free ion and nine crystal field parameters (according to the C 2v symmetry) yielded a rms deviation of 17 cm À1 between the experimental and calculated energy level schemes. Magnetic susceptibility measurements were carried out between 2 and 298 K. Good agreement was obtained between the experimental and calculated paramagnetic susceptibilities as a function of temperature by using the van Vleck formalism and the energies and wave functions derived from the spectral data.
The charge compensation effects induced by aliovalent doping were studied in the monoclinic CdSiO... more The charge compensation effects induced by aliovalent doping were studied in the monoclinic CdSiO 3. An interstitial oxide ion may be feasible in this host to provide the extra negative charge required by the R 3+ doping. The oxide was studied in CdSiO 3 using density functional theory (DFT) calculations and synchrotron radiation (SR) luminescence spectroscopy. The crystal structure of this host was significantly modified by the interstitial oxide. The experimental band gap energy (E g) was perfectly reproduced by the calculations and intrinsic electron traps were revealed. Defect levels were found also in the interstitial oxide containing host, however their role has to be studied further.
The luminescence of LiYF 4 and YF 3 doped with Ho 3 was investigated using synchrotron radiation.... more The luminescence of LiYF 4 and YF 3 doped with Ho 3 was investigated using synchrotron radiation. In the excitation spectra transitions to previously unidenti®ed 4f 10 levels were observed thus extending the energy level diagram of Ho 3 into the (vacuum) ultraviolet region of the electromagnetic spectrum. Some high energy emissions were observed and could be attributed to spin forbidden 4f 9 5d 3 4f 10 emissions in YF 3 and LiYF 4. For YF 3 doped with Ho 3 emission from the 3 P1 2 level (situated at 63 000 cm À1) was observed. Furthermore, several photon cascade emission processes were observed in which one VUV photon is converted into two or three UV/Vis photons, albeit with a low eciency.
The present status and future progress of the mechanisms of persistent luminescence are criticall... more 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 andperhaps 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.
Uploads
Papers by Jorma Hölsä