Abstract
AbstractThe rare earth Er3+ and Yb3+ codoped system is the most attractive for showcasing energy transfer upconversion. This system can generate green and red emissions from Er3+ under infrared excitation of the sensitizer Yb3+. It is well known that the red-emitting state can be populated from the upper green-emitting state. The contribution of multiphonon relaxation to this population is generally considered important at low excitation densities. Here, we demonstrate for the first time the importance of a previously proposed but neglected mechanism described as a cross relaxation energy transfer from Er3+ to Yb3+, followed by an energy back transfer within the same Er3+–Yb3+ pair. A luminescence spectroscopy study of cubic Y2O3:Er3+, Yb3+ indicates that this mechanism can be more efficient than multiphonon relaxation, and it can even make a major contribution to the red upconversion. The study also revealed that the energy transfers involved in this mechanism take place only in the nearest Er3+–Yb3+ pairs, and thus, it is fast and efficient at low excitation densities. Our results enable a better understanding of upconversion processes and properties in the Er3+–Yb3+ system.
Highlights
Infrared to visible upconversion luminescence has been extensively studied for its fundamental value[1,2,3] and its various potential applications in upconversion lasers,[4] bioimaging,[5] etc
We find the CRB can be more efficient than Multiphonon relaxation (MPR) and can even make a major contribution to the red upconversion luminescence (UCL)
A similar result upon Er31 4F7/2 excitation with 488 nm was observed in Y2O3:Er31, Yb31 nanocrystals,[11] and an interaction between two excited Er31 ions (4F7/2R4F9/2 and 4F9/2r4I11/2)[7] was considered the main excitation mechanism of the red-emitting 4F9/2 state
Summary
Infrared to visible upconversion luminescence has been extensively studied for its fundamental value[1,2,3] and its various potential applications in upconversion lasers,[4] bioimaging,[5] etc. The red upconversion benefits from several possible excitation mechanisms.[6,7] Multiphonon relaxation (MPR) from the upper 4S3/2 state and ETU from the lower intermediate 4I13/2 state are generally considered dominant at low infrared excitation densities because other mechanisms involving three photon processes[6,7] become important only at high infrared excitation densities,[8] which is not the topic of this work. The MPR is not the only mechanism for populating the 4F9/2 from the 4S3/2; a non-MPR mechanism was proposed earlier,[8] but it has not been considered important since This mechanism involves two sequential energy transfers between Er31 and Yb31. The Yb31induced green emission quenching of Er31 by the CR has been widely recognized.[9,10] the population of the 4F9/2 by the CRB from the 4S3/2 has rarely been studied or valued in both photoluminescence (PL) and upconversion luminescence (UCL)
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