14 February 2003 Two-photon-excitation-induced transient valence switching in Eu-doped Si/SiO2 nanocomposites
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Eu3+-doped crystalline Si-enriched SiO2 nanocomposite thin films were prepared using Ar sputtering deposition on quartz substrates. By conventional laser spectroscopy the material was characterized in either frequency or time domain. The results show that the doped europium ions are present in trivalent state, as Eu3+. They are distributed in SiO2 matrix and on the boundary surface of c-Si nanoparticles. With increased excitation intensity at 532nm, two-photon absorption (TPA) induced new emission was observed. It is characterized by an additional broadband emission with a peak at 560nm and lifetime of ~ 0.8 s. This feature has been identified as the emission from Eu2+ ions. Further measurements reveal that the observed phenomenon originates from charge transfer process giving rise to a photoinduced transient valence switching from Eu3+ to Eu2+ in this particular material. Free carriers were originally created in the conduction band (CB) of c-Si nanoparticles by TPA, then trapped at the surrounding Eu3+ center due to strong Coulomb interaction. Luminescence of the formed divalent Eu2+ is characterized by a broadband d → f transition with fast decay rate. Degenerate four-wave-mixing experiment further revealed that in undoped sample TPA-created charge carriers in CB were trapped in shallow centers of Si nanoparticles. The trapping has an average time period of 500ps, and the carriers were then released for recombination. In Eu3+ -doped sample, however, the average time period of 500ps was no longer observable. It therefore suggests that the strong Coulomb attraction results in immediately capturing of the created charges by positive Eu3+ center.
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Huimin Liu, Huimin Liu, V. S. Vikhnin, V. S. Vikhnin, L. F. Fonseca, L. F. Fonseca, A. Mahfoud, A. Mahfoud, G. A. Nery, G. A. Nery, O. Resto, O. Resto, Z. S. Weisz, Z. S. Weisz, } "Two-photon-excitation-induced transient valence switching in Eu-doped Si/SiO2 nanocomposites", Proc. SPIE 4797, Multiphoton Absorption and Nonlinear Transmission Processes: Materials, Theory, and Applications, (14 February 2003); doi: 10.1117/12.453276; https://doi.org/10.1117/12.453276

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