Tailoring trapping defects in PERSISTent luminescence materials – PERSIST

The objectives of the PERSIST project are as follows:
(i) Better adaptation to new LED lighting and solar radiation requirements. We aim if possible to overcome the current properties of persistent luminescence of the reference compound SrAl2O4 of monoclinic variety by its hexagonal polymorph via different cationic substitutions. These will also allow the color of the persistent emission to vary from blue or green to white.
(ii) The PERSIST project relies for glass-ceramics and crystals on a «volume effect« for absorption and emission. If only a few micrometers of the phosphor surface is involved in the case of powdered samples, using transparent materials, the persistence properties will be enhanced. Various materials will be developed within the framework of the project, such as spinels, garnets or mixed aluminates in the form of glass-ceramic and single crystals by the Czochralski method. The processes involved will be characterized by cutting-edge experimental techniques (NMR, EPR, EXAFS, XANES, photoconductivity, thermoluminescence) and band structure calculations with in particular determination of the defect formation energies (intrinsic and extrinsic) and positioning of the levels associated with these defects in the «ideal« electronic structure of a non-faulted material. Current applications of these materials are mainly in the signage of emergency exits, hands and dials for watches and toys. Thanks to a more intense luminescence and good persistence, we could consider new applications.

(i) Elaboration of crystals with persistent luminescence.
Garnet compounds produced by BREVALOR
Identification of the actual compositions compared to the nominal compositions (study carried out at CEMHTI by microprobe and at IRCP by absorption) to be completed in the coming months by X Rietveld and Ga / Al NMR see Y / Gd
(ii) Calculation of the defects for the garnet structure Y3Al5O12 doped Ce3 + / Cr3 +; visualization of Ce3 + and Cr3 + defects in the forbidden band of garnet (IMN Nantes)
(iii) Analysis of the role of Boron as an additive in persistent materials; Boron is added to increase persistence and we have, thanks to very fine analyzes (here the EELS), studied whether the boron is located on the surface of the grains or in solid solution in the compounds. For this, the SrAl14O25: Eu, Dy, B matrix has been characterized.
A «theoretical« analysis of the impact of boron on the electronic structure of Sr4Al14O25 is in progress
(iv) Work on transparent glass-ceramics. The transparent ceramics Sr1 - x / 2Al2 - xSixO4: Eu2 +, Dy3 + (0.1=x=0.5) were produced by crystallization of glass. Compositions with a low SiO2 content (x=0.4) require rapid quenching conditions. Specific design processes such as aerodynamic levitation coupled with laser heating for a glass composition x = 0.5 can be prepared on a large scale by the conventional tempering method. Persistent luminescence ranges from green to light blue. Excitation spectra indicate possible activation in the visible region of persistent luminescence. This is because the persistent luminescence was successfully «charged« using a low power white light source. Thermoluminescence results and band structure calculations indicate the evolution of entrapment with material composition. The variation of the Si content appears to be a promising strategy for obtaining transparent materials.

We will work to define
- Better adaptation to new LED lighting requirements and solar radiation. The garnet crystals show a singular trapping depth by solar excitation compared to selective excitation with LED.
- The volume effect and the surface / volume quality of the samples. If only a few micrometers of the phosphor surface are involved in the case of powdered samples, using transparent materials can we enhance the luminescent phenomena? and in which matrices? We recently obtained a first result for strontium aluminate and garnets with crystals of different thicknesses.
- Optimum materials and doping according to experimental results and simulations of band structures and trap depth in different matrices (ZrO2, SrAl2O4, Sr4Al14O25, (Y / Gd) 3Al2Ga3O12, ZnGa2O4 ...) and advanced experimental techniques (EXAFS, XANES…).
- Charging and discharging capacities for indoor and outdoor applications.
- New «outdoor« applications possible with large transparent materials opening up new applications.
- Structure correlation (X Rietveld, microprobe and Ga / Al NMR see Y / Gd / properties) and mechanism of persistent luminescence of garnet matrices.

Publications with several partners

1. Persistent luminescence features in hexagonal Sr0. 75Al1. 5Si0. 5O4: Eu2+ phosphor, V Castaing, C Monteiro, AJ Fernández-Carrión, M Allix, B Viana, Optical Components and Materials XVII 11276, 112761O (2020)
2. Hexagonal Sr1- x/2Al2- xSixO4:Eu2+, Dy3+ transparent ceramics with tuneable persistent luminescence properties, V Castaing, C Monteiro, AD Sontakke, K Asami, J Xu, ...Dalton Transactions 49 (46), 16849-16859 (2020)
3. Unravelling the key intrinsic defects in Sr4Al14O25: a first-principles investigation, W. Lafargue-Dit-Hauret et al. en cours d'écriture
4. A theoretical survey on native point defects and dopants in Y3Al5O12 garnet, W. Lafargue-Dit-Hauret et al. en cours d'écriture

Communications
1. Conference Plénière Phosphor Safari Xiamen, Chine novembre 2019Rare earth doped nanocrystals of Cr,RE : ZnGa2O4 for Bioimaging and related applications B. Viana, V. Castaing, C. Chanéa, E. Glais, L. Abiven, S. Tanabe, C. Richard, M. Allix
2. Poster à la conférence SPIE PW San Francisco, janvier 2020, Persistent luminescence features in hexagonal Sr0.75Al1.5Si0.5O4:Eu2+ phosphorV. Castaing, C. Monteiro, A. Fernández-Carrión, M. Allix, B. Viana
3. Persistent luminescence features in hexagonal Sr1-x/2Al2-xSixO4:Eu2+, Dy3+ compounds, V Castaing, C Monteiro, M Allix, B Viana, The European Conference on Lasers and Electro-Optics, ce_p_15 Oral de V. Castaing, Cleo Europe, Munich, Juin 2019
4. Persistent luminescence materials at various sizes and their applications, Poster de B Viana et al. The European Conference on Lasers and Electro-Optics, cl_p_27, Munich, Juin 2019
5. Conférence Orale PRE 2019, Nice septembre 2019
Rare earth doped nanocrystals of Cr,RE:ZnGa2O4 for Bioimaging and related applications, B. Viana, V. Castaing, E. Glais, A. Sontakke, C. Chanéac , J. Xu, S. Tanabe, , M. Allix, C. Genevois, J.F. Carrion, C. Richard

Project summary:
In persistent luminescence materials, energy can be stored by controlled traps/defects under brief irradiation -namely few minutes under UV or natural daylight-. This energy is then released at room temperature for several hours via light emission once the excitation is stopped. The search for innovative and properties-enhanced materials is an active research -mainly in Asia- and has recently led to several new phosphor materials and new applications (such as bio-imaging at the nanoscale). Most of these researches were performed on powder materials. A better understanding of the persistent luminescence mechanism is crucial for the development of practical applications. Moreover, the quest to unravel the mechanism behind the persistent luminescence has entered a new path. In the PERSIST project three complementary research groups (IRCP-Paris, CEMHTI-Orleans and IMN Nantes) and one company (BREVALOR-Swiss) are gathered to propose some breakthroughs in the field. The purpose of the PERSIST project is twofold.
(i) In one part for better adaptation to new lighting requirements, we aim at overtaking the current persistent luminescence properties. For instance by developing cationic substitutions in the hexagonal polymorph of the commercial monoclinic SrAl2O4. Indeed under 460 nm blue LED excitation, the hexagonal polymorph and related compounds obtained with cationic substitutions are expected to glow longer times that the commercial monoclinic. This is expected from the persistent luminescence excitation recently obtained. Furthermore cationic substitutions can also tune the color from blue or green to white. Scientific deep analysis of the trapping and recombination centers will be characterized by combining state of the art of experimental optical and magnetic techniques available in the three research laboratories and band structure calculations.
(ii) The second main objective of the PERSIST project relies on the huge “volume effect” expected from the transparent persistent phosphors to be developed. If only a few micrometers from the phosphor surface are participating to persistent luminescence in powder samples, using transparent or translucent materials will open the path to much enhanced capabilities. PERSIST project will be focused on transparent glass-ceramics with an original way, recently proposed, to obtain crystals embedded in glass, ceramics and single crystals, starting at first by strontium aluminate and relative compounds where first results were recently obtained. Preliminary centimetric large crystals, not optimized, of calcium aluminate were grown for the first time. Other materials will be grown by the Czochralski processes within the project such as garnet spinels and mixed-aluminates.
Within these new materials, charge and discharge capabilities will be tested for indoor and outdoor applications. If the applications at the present time for such materials mostly consist in emergency signalization, dials, displays and medical diagnostics, stronger signal and new designs could open the path to new “outdoor” applications (in building materials, road signs). Finally, it is anticipated that the important enhancement of persistent luminescence expected from transparent materials will also be interesting for new applications in luxury and jewelry.