Virgīnija Vītola (ISSP UL) at scientific workshop of Doctoral School “Functional Materials and Nanotechnologies” at ISSP UL, November 29, at 13:00.

SrAl2O4 doped with Eu and Dy is a material that exhibits long and intense afterglow when excited by visible or ultraviolet light. Therefore, the material is widely used - luminescent emergency exit signs, which do not require electricity, road marks glowing in the dark, which would make remote roads much safer, road signs, indication marks, various biological markers.

Although SrAl2O4:Eu,Dy has been known for some time, there is still ambiguity about its afterglow mechanism. To clarify the ambiguity, doped and undoped SrAl2O4 material was studied at temperature range of 10-400K. Emission spectra under different types of excitations, luminescence decay kinetics operating with thermally stimulated luminescence measurements were investigated to determine the nature and trap depth of charge carrier trap centres. 

As a result of the work, improvements to existing long lasting luminescence mechanisms including electron tunnelling from trapping centre to Eu3+, as well as proof of Dy4+ existence in strontium aluminate were proposed. Luminescence of undoped material consisting of transition metal and intrinsic defect luminescence has also been observed.

The results of this work bring new insights and improvements to existing long lasting luminescence mechanisms in Eu doped strontium aluminate. They also open up some possibilities for wider application of persistent phosphorescents.

Thesis to be defended:

1. Excited Eu2+ luminescence centres in SrAl2O4:Eu,Dy material can be formed by electron tunnelling from trap to Eu2+. Thus, an improvement is proposed to the mechanisms currently available in scientific publications for long afterglow in SrAl2O4:Eu,Dy material.

2. Under X-ray excitation, Eu2+ and Dy3+ act as hole trap centres and Eu3+ as well as Dy4+ are accumulated. Electron trapping centres are positioned relative to Eu3+ and Dy4+ so that the probability of electron tunnelling is significant.

3. Undoped material luminescence has been observed, consisting of trace impurity and intrinsic defects luminescence.

The experimental results and scientific evidence of thesis are summarized in publications:

1. Liepina V., Millers D., Smits K. Tunnelling luminescence in long lasting afterglow of SrAl2O4:Eu,Dy, Journal of Luminescence, 185 (2017)

2. Liepina V., Millers D., Smits K., Zolotarjovs A. X-ray excited luminescence of SrAl2O4:Eu,Dy at low temperatures, Journal of Physics and Chemistry of Solids, 115 (2018), 381-385.

3. Vitola V., Millers D., Smits K., Bite I., Zolotarjovs A. The search for defects in undoped SrAl2O4 material, Optical Materials (2018)