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Probing electron transfer processes in Y PO(4):Ce, Sm by combined synchrotron-laser excitation spectroscopy.
MedLine Citation:
PMID:  21393684     Owner:  NLM     Status:  In-Data-Review    
Abstract/OtherAbstract:
Yttrium phosphate co-doped with cerium and samarium acts as a charge storage phosphor, but in highly doped material (0.5% co-doping levels), the proximity of defects leads to the uncontrolled non-radiative loss of stored charge through tunnelling. In order to characterize these defects, their mutual interactions and intra-pair charge transfer routes, experiments have been undertaken in which a laser probe is deployed during luminescence excitation using a synchrotron. Two modes of operation are described; in each case, the laser (2.8 eV) probes only Sm(2 + ) ions, and the detection is set to monitor exclusively Ce(3 + ) 5d-4f emission. Mode 1: the sample is pumped with monochromatic synchrotron photons in the range 4.5-12 eV, and the resultant charge populations probed with the laser 30 s later; this has the effect of sampling electrons trapped at Sm(2 + ) that are in quasi-equilibrium. Here, a clear transition between a sub-bandgap Urbach tail region and excitations above the mobility edge is especially apparent, enabling an accurate value of the conduction band energy of Y PO(4) to be determined, 9.20 eV. Furthermore, the Sm(2 + ) and Ce(3 + ) ground state energies can be positioned within the bandgap (6.8 eV and 3.85 eV above the top of the valence band, respectively). Mode 2: the sample is pumped with monochromatic synchrotron photons in the range 4.5-12 eV and, during this pumping process, the laser probe is activated. This more dynamic process probes direct electron transfer excitation processes between spatially correlated Sm-Ce defect pairs, via their excited states; the laser probe enhances the Ce(3 + ) emission if direct electron transfer from the Ce(3 + ) ground state to the excited states of Sm(2 + ) is being pumped, or quenches the luminescence if the Ce(3 + ) excited states are pumped. The experiments allow for a precise measure of the difference in energy between the Sm(2 + ) and Ce(3 + ) ground states (2.98 eV).
Authors:
N R J Poolton; A J J Bos; G O Jones; P Dorenbos
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Publication Detail:
Type:  Journal Article     Date:  2010-04-15
Journal Detail:
Title:  Journal of physics. Condensed matter : an Institute of Physics journal     Volume:  22     ISSN:  0953-8984     ISO Abbreviation:  J Phys Condens Matter     Publication Date:  2010 May 
Date Detail:
Created Date:  2011-03-11     Completed Date:  -     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  101165248     Medline TA:  J Phys Condens Matter     Country:  England    
Other Details:
Languages:  eng     Pagination:  185403     Citation Subset:  IM    
Affiliation:
Photon Science Institute, Manchester University, Oxford Road, Manchester M13 9PL, UK. Institute of Mathematics and Physics, Aberystwyth University, Aberystwyth SY23 3BZ, UK.
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