Direct hole and delayed electron capture on a picosecond timescale by Eu2+ centers in CaGa2S4 monitored by synchroscan with horizontal blanking

Abstract

A Eu 2þ concentration and temperature dependent energy transfer study from the host lattice to Eu 2þ luminescence centers in Ca (1-x) Eu x Ga 2 S 4 (x ¼ 0.001 to 0.05) was performed with a special streak camera that combines the high timing resolution of a conventional synchroscan operation (<2 ps) with the ability to study long lived states (10 ns À 1 ms) typical for rare earth and transition metal ions. Two transfer mechanisms from the CaGa 2 S 4 host lattice to the Eu 2þ ions were identified. A fast transfer process (<4 ps) is interpreted as sequential hole-electron capture by the Eu 2þ ions, and slower process (>1 ns) is interpreted as the sequential capture of a hole and an electron by Eu 2þ but with the electron first entering an intermediate state trapped near Eu 3þ . Energy transfer via a self-trapped excitonic (STE) state is unlikely because of the absence of an anti-correlation between the STE decay-time and the Eu 2þ rise-time as well as between the STE emission intensity and the Eu 2þ intensity. All rise-time spectra were successfully fitted with a model representing the above transfer mechanisms. V C 2012 American Institute of Physics.