- Persistent luminescence
Commonly referred as phosphorescence, persistent luminescence is the phenomenon encountered in materials which make them glow in the dark after the end of an excitation with UV or visible light.
The mechanism underlying this phenomenon is not fully understood [T. Aitasalo, P. Deren, J. Hölsä, H. Jungner, J.C. Krupa, M. Lastusaari, J. Legendziewicz,J. Niittykoski, and W. Strek. Persistent luminescence phenomena in materials doped withrare earth ions. "J. Solid State Chem.", 171 :114, 2003.] . However, the phenomenon of persistent luminescence must not be mistaken with fluorescence and phosphorescence (see for definitions [ [http://goldbook.iupac.org/F02453.html IUPAC Gold Book - fluorescence ] ] and [ [http://goldbook.iupac.org/P04569.html IUPAC Gold Book - phosphorescence ] ] ). Indeed, in fluorescence, the lifetime of the excited state is in the order of a few ns and in phosphorescence, even if the lifetime of the emission can reach several seconds, the reason of the long emission is due to the desexcitation between two electronic states of differents spin multiplicity. For persistent luminescence, it has been known for a long time that the phenomenon involved energy traps (such as electron or hole trap) in a material [H.W. Leverenz. Luminescent solids (phosphors). "Science", 109 :183–189, 1949. ] which are filled during the excitation. After the end of the excitation, the stored energy is gradually released to emitter centers which emit light usually by a fluorescence-like mechanism.
Examples of use
Persistent luminescence materials are mainly used in safety signs, watch dials, decorative objects and toys. They have also been used as nanoprobes in small animal optical imaging [ Q. le Masne de Chermont, C. Chanéac, J. Seguin, F. Pellé, S. Maitrejean, J.P. Jolivet, D. Gourier, M. Bessodes and D. Scherman. Nanoprobes with near-infrared persistent luminescence for in vivo imaging. "Proc. Natl Acad. Sci. USA" 104 :9266–9271 2007.] .
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