Electronic Energy Transfer from Excitons Confined in Silicon Nanocrystals to Molecular Oxygen
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Electronic Energy Transfer from Excitons Confined in Silicon Nanocrystals to Molecular Oxygen E. Gross, D. Kovalev, N. Künzner, J. Diener, and F. Koch Technische Universität München, Physik Department E16, 85747 Garching, Germany V.Yu. Timoshenko Faculty of Physics, Moscow State M.V. Lomonosov University, 119992 Moscow, Russia Minoru Fujii, Department of Electrical and Electronics Engineering, Faculty of Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan ABSTRACT We report on efficient electronic energy transfer from excitons confined in silicon (Si) nanocrystals to molecular oxygen (MO). The remarkable photosensitizing properties of Si nanocrystal assemblies result from a broad energy spectrum of photoexcited excitons, a long triplet exciton lifetime and a highly developed surface area. Quenching of photoluminescence (PL) of Si nanocrystals by MO physisorbed on their surface is found to be most efficient when the energy of excitons coincides with the triplet-singlet splitting energy of oxygen molecules. Spectroscopic analysis of the quenched PL spectrum evidences that energy transfer is accompanied by multi-phonon emission. From time-resolved measurements the characteristic time of energy transfer is found to be in the range of microseconds. The dependence of PL quenching efficiency on the surface termination of nanocrystals is consistent with short-range resonant electron exchange mechanism of energy transfer. The energy transfer to oxygen molecules in the gaseous phase at elevated temperatures is demonstrated. INTRODUCTION Energy transfer is of broad interest in various scientific research fields, since it describes the intermediate process between the primary event of electronic excitation and the final processes that utilize the energy of electrons. Therefore, the analysis of the mechanism and of the efficiency are indispensable in the studies of the interaction between light and matter. It is well known that the 3Σ ground state (superscript denotes the spin multiplicity) of MO has spin triplet multiplicity, while the lowest excited states (1∆, 1Σ) have spin singlet nature [1]. Thus, the direct photoexcitation of the 1∆ and 1Σ state, lying 0.98 eV and 1.63 eV above the ground state [2], respectively, is forbidden by spin selection rules (inset of figure 1). The electronic excitation of MO requires photosensitizers as intermediate light-absorbing substances, e.g. dye molecules in solution, which subsequently transfer the energy to oxygen molecules. However, thermal and collisional broadening of the energy levels involved in the energy transfer process inhibit the clarification of its mechanism in detail by means of experimental techniques. Semiconductors nanostructures combine the unique properties of “artificial atoms” and the cooperative effects of condensed matter and are viewed as promising systems to uncover the insight of energy transfer processes. Microporous Si (PSi) consists of a Si nanocrystal skeleton
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