Marja Malkamäki

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.

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)

Synchrotron radiation investigations of the Sr2MgSi2O7:Eu2+,R3+ persistent luminescence materials

Abstract The electronic and defect energy level structure of polycrystalline Sr2MgSi2O7:Eu2+,R3+ persistent luminescence materials were studied with thermoluminescence and different synchrotron radiation spectroscopies (UV-VUV emission and excitation, X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS)). Special attention was paid on the effect of the R3+ co-dopants on the persistent luminescence properties of the materials. Theoretical calculations using the density functional theory (DFT) were carried out simultaneously with the experimental work. The experimental band gap energy (Eg) value of ca. 7.1 eV agreed very well with the DFT value of 6.7 eV. The variation of the Eg value was attempted to relate with the trap structure as well as with the different properties of the R3+ co-dopants. The trap level energy distribution depended strongly on the R3+ co-dopant except for the shallowest trap energy above the room temperature remaining practically the same, however. The different processes in the mechanism of persistent luminescence from Sr2MgSi2O7:Eu2+,R3+ were assembled and their contributions discussed.

DFT and synchrotron radiation study of Eu 2+ doped BaAl 2 O 4

The structural distortions resulting from the size mismatch between the Eu2+ luminescent centre and the host Ba2+ cation as well as the electronic structure of BaAl2O4:Eu2+(,Dy3+) were studied using density functional theory (DFT) calculations and synchrotron radiation (SR) luminescence spectroscopy. The modified interionic distances as well as differences in the total energies indicate that Eu2+ prefers the smaller of the two possible Ba sites in the BaAl2O4 host. The calculated Eu2+ 4f7 and 4f65d1 ground level energies confirm that the excited electrons can reach easily the conduction band for subsequent trapping. In addition to the green luminescence, a weak blue emission band was observed in BaAl2O4:Eu2+,Dy3+ probably due to the creation of a new Ba2+ site due to the effect of water exposure on the host.

Persistent luminescence fading in Sr 2 MgSi 2 O 7 :Eu 2+ ,R 3+ materials: a thermoluminescence study

The fading of persistent luminescence in Sr2MgSi2O7:Eu2+,R3+ (R: Y, La-Nd, 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.

Luminescence properties of Eu3+ and TiIV/ZrIV doped yttrium oxysulfides (Y2O2S:Eu3+,TiIV/ZrIV)

The red emitting Y2O2S:Eu3+,TiIV (or ZrIV, xEu: 0.01, xTi/Zr: 0.003/0.015/0.03) materials were prepared with a flux method. According to X-ray powder diffraction, the materials have the hexagonal crystal structure. The UV excited (λexc: 250 nm) emission maximum was observed at 628 nm due to the 5D0→7F2 transition of Eu3+. The excitation spectra (λem: 628 nm) consist of broad bands centered at 240 and 320 nm due to the charge transfer transitions O2−→Eu3+ and S2−→Eu3+, respectively. Red persistent luminescence was observed with a maximum at 628 nm, as well. Persistent luminescence was the strongest with the TiIV co-doping though the intensity of persistent luminescence decreased with the increasing amount of both the TiIV and ZrIV co-dopants. The thermoluminescence (TL) glow curves of the Y2O2S:Eu3+,TiIV materials consist of bands at ca. 110 and 200 °C. In Y2O2S:Eu3+,ZrIV, similar bands are observed at lower temperatures viz. at ca. 100 and 180 °C. TL weakens when the amount of co-dopants is increased. The TiIV co-doped materials have stronger TL than the ZrIV co-doped materials. The deconvolution of TL glow curves revealed three distinct traps with depths ranging from 0.6 to 1.0 eV.