| 1. |
- Carlson, Stefan, et al.
(författare)
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X-ray absorption study of rare earth ions in Sr2MgSi2O7:Eu2+,R3+ persistent luminescence materials
- 2009
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Ingår i: Optical Materials. - : Elsevier BV. - 0925-3467. ; 31:12, s. 1877-1879
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Tidskriftsartikel (refereegranskat)abstract
- The valence of the europium dopant and selected rare earth co-dopants (Ce3+, Dy3+, and Yb3+) in the Sr2MgSi2O7:Eu2+,R3+ persistent luminescence materials were studied by room temperature XANES measurements. The results indicated the co-existence of both divalent and trivalent europium in all the studied materials. The relative amount of Eu3+ was observed to increase upon increasing exposure to X-rays, as expected by the persistent luminescence mechanism. This suggests a simultaneous filling of oxygen vacancies initially created by the reducing preparation conditions. For the Dy and Yb co-dopants, only trivalent species were observed. On the other hand, traces of tetravalent cerium were present in the Eu,Ce co-doped materials. (c) 2009 Elsevier B.V. All rights reserved.
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| 2. |
- Lastusaari, Mika, et al.
(författare)
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The Bologna Stone: history's first persistent luminescent material
- 2012
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Ingår i: European Journal of Mineralogy. - : Schweizerbart. - 1617-4011 .- 0935-1221. ; 24:5, s. 885-890
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Makela, Jaakko, et al.
(författare)
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Comparison of chemical, electronic, and optical properties of Mg-doped AlGaN
- 2016
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 120:50, s. 28591-28597
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Tidskriftsartikel (refereegranskat)abstract
- Hydrogen, carbon, and oxygen are common unintentional impurities of Al(x)Ga(1−x)N crystals. This impurity structure and its interplay with Mg impurities in Al(x)Ga(1−x)N semiconductors are relevant to develop the p-type nitride crystals for various devices (e.g, LEDs, transistors, gas sensors) but are still unclear. Here we have investigated Mg-doped Al0.5Ga0.5N before and after postgrowth annealing with valence-band and core-level photoelectron spectroscopy, photoluminescence, and resistivity measurements. First, it is found that a surface part of the Al0.5Ga0.5N crystal is surprisingly inert with air and stable against air exposure-induced changes. Thus, the relatively surface-sensitive photoelectron spectroscopy measurements reflect in this case also the bulk crystal characteristics. The measurements reveal the presence of deep states up to 1 eV above valence-band maximum before and after the annealing and that oxygen and carbon occupy N lattice sites (i.e., ON and CN). The model where CN-induced acceptor states in the band gap participate in the blue emission (photoluminescence) is supported. Furthermore, the presented Mg 2p core-level spectra demonstrate that part of Mg atoms forms direct bond(s) with oxygen in the bulklike structure of Al0.5Ga0.5N and that the chemical environment of Mg atoms is much richer than was expected previously.
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| 4. |
- Norrbo, Isabella, et al.
(författare)
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Lanthanide and Heavy Metal Free Long White Persistent Luminescence from Ti Doped Li-Hackmanite : A Versatile, Low-Cost Material
- 2017
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Ingår i: Advanced Functional Materials. - : Wiley. - 1616-301X .- 1616-3028. ; 27:17
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Tidskriftsartikel (refereegranskat)abstract
- Persistent luminescence (PeL) materials are used in everyday glow-in-the-dark applications and they show high potential for, e.g., medical imaging, night-vision surveillance, and enhancement of solar cells. However, the best performing materials contain rare earths and/or other heavy metal and expensive elements such as Ga and Ge, increasing the production costs. Here, (Li,Na)(8)Al6Si6O24(Cl,S)(2):Ti, a heavy-metal-and rare-earth-free low-cost material is presented. It can give white PeL that stays 7 h above the 0.3 mcd m(-2) limit and is observable for more than 100 h with a spectrometer. This is a record-long duration for white PeL and visible PeL without rare earths. The material has great potential to be applied in white light emitting devices (LEDs) combined with self-sustained night vision using only a single phosphor. The material also exhibits PeL in aqueous suspensions and is capable of showing easily detectable photoluminescence even in nanomolar concentrations, indicating potential for use as a diagnostic marker. Because it is excitable with sunlight, this material is expected to additionally be well-suited for outdoor applications.
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| 5. |
- Norrbo, Isabella, et al.
(författare)
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Mechanisms of Tenebrescence and Persistent Luminescence in Synthetic Hackmanite Na8Al6Si6O24(Cl,S)(2)
- 2016
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 8:18, s. 11592-11602
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Tidskriftsartikel (refereegranskat)abstract
- Synthetic hackmanites, Na8Al6Si6O24(Cl,S)(2), showing efficient purple tenebrescence and blue/white persistent luminescence were studied using different spectroscopic techniques to obtain a quantified view on the storage and release of optical energy in these materials. The persistent luminescence emitter was identified as impurity Ti3+ originating from the precursor materials used in the synthesis, and the energy storage for persistent luminescence was postulated to take place in oxygen vacancies within the aluminosilicate framework. Tenebrescence, on the other hand, was observed to function within the Na-4(Cl,S) entities located in the cavities of the aluminosilicate framework. The mechanism of persistent luminescence and tenebrescence in hackmanite is presented for the first time.
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| 6. |
- Norrbo, Isabella, et al.
(författare)
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Solar UV index and UV dose determination with photochromic hackmanites : from the assessment of the fundamental properties to the device
- 2018
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Ingår i: Materials Horizons. - : Royal Society of Chemistry (RSC). - 2051-6347 .- 2051-6355. ; 5:3, s. 569-576
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Tidskriftsartikel (refereegranskat)abstract
- Extended exposure to sunlight or artificial UV sources is a major cause of serious skin and eye diseases such as cancer. There is thus a great need for convenient materials for the easy monitoring of UV doses. While organic photochromic molecules are tunable for responses under different wavelengths of UV radiation, they suffer from rather poor durability because the color changes involve drastic changes in molecular structure. Inorganic materials, on the other hand, are durable, but they have lacked tunability. Here, by combining computational and empirical data, we confirm the mechanism of coloration in the hackmanites, nature-based materials, and introduce a new technique called thermotenebrescence. With knowledge of the mechanism, we show that we can control and thus tune the energy of electronic states of synthetic hackmanites (Na,M)(8)Al6Si6O24(Cl,S)(2) so that their body color is sensitive to the solar UV index as well as UVA, UVB or UVC radiation levels. Finally, we demonstrate that it is possible to use images taken with an inexpensive cell phone to quantify the radiation dose or UV index. The hackmanite materials thus show great potential for use in portable healthcare both in everyday life and in laboratories.
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| 7. |
- Pihlgren, Laura, et al.
(författare)
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On the mechanism of persistent up-conversion luminescence in the ZrO2:Yb3+,Er3+ nanomaterials
- 2014
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Ingår i: Optical Materials. - : Elsevier BV. - 0925-3467. ; 36:10, s. 1698-1704
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Tidskriftsartikel (refereegranskat)abstract
- The up-converting ZrO2:Yb3+,E3+ nanomaterials were prepared with the combustion and sal-gel methods. The structure of the materials was cubic except for the minor monoclinic and tetragonal impurities associated with the use of the sal-gel method. The XANES results revealed only the trivalent form for both the Yb and Er dopants. The distance distributions calculated from EXAFS confirm that Er3+ and Yb3+ occupy the Zr-IV site in the structure. The nanomaterials show red (650-700 nm) and very weak green (520-560 nm) up-conversion luminescence due to the F-4(9/2) -> I-4(15/2) and (H-2(11/2),S-4(3/2)) -> I-4(15/2) transitions of Er3+, respectively. The nanomaterials obtained with the combustion synthesis yield the most intense luminescence though showing significant afterglow, as well. The thermoluminescence measurements revealed the existence of traps with depths between 0.68 and 1.03 eV well matching to room temperature persistent emission. Finally, the mechanism for the persistent up-conversion luminescence was introduced based on the experimental results and discussed. (C) 2014 Elsevier B.V. All rights reserved.
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