Energy exchange between optically excited Silicon Nanocrystals and Molecular Oxygen
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Energy exchange between optically excited Silicon Nanocrystals and 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 the photosensitizing properties of optically excited Silicon (Si) nanocrystal assemblies that are employed for an efficient generation of singlet oxygen. Spin triplet state excitons confined in Si nanocrystals transfer their energy to molecular oxygen (MO) adsorbed on the nanocrystal surface. This process results in a strong suppression of the photoluminescence (PL) from the Si nanocrystal assembly and in the excitation of MO from the triplet ground state to singlet excited states. The high efficiency of the energy transfer if favored by a broad energy spectrum of photoexcited excitons, a long triplet exciton lifetime and a highly developed surface area of the nanocrystal assembly. Due to the specifics of the coupled system Si nanocrystal – oxygen molecule all relevant physical parameters describing the photosensitization process are accessible experimentally. This includes the role of resonant and phonon-assisted energy transfer, the dynamics of energy transfer, and its mechanism. INTRODUCTION
The transfer of electronic excitation energy between electronically coupled systems is one of the fundamental problems in modern physics of condensed matter [1,2]. It describes the intermediate sequences between the primary event of electronic excitation and the final processes that utilize the energy of electrons. Despite a general knowledge acquired in the field of excitation transfer processes, the high complexity of the systems considered inhibits a detailed understanding of the microscopic processes occurring during the transfer of energy. For this reason more simple configurations are demanded to uncover the mechanism of electronic excitation transfer. Semiconductor nanostructures represent a promising approach to overcome the difficulties in determining the basic parameters of electronic energy transfer processes. The main advantage of these systems is the huge flexibility and degree of freedom in their fabrication. The sizes, shapes and the geometrical arrangements of the nanostructures as well as their chemical composition and the quality of the interfaces are well controllable. These artificial structures combine molecular and solid state properties and allow one a description of the
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physical phenomena by taking advantage of separately developed concepts. To study energy transfer processes in detail, we used microporous silicon (PSi) as a semiconductor nanostructure. PSi consists of a Si nanocrystal skeleton and an interconnected pore network with an average structure size of 2-5 nm [3]. Due to quantum confinement e
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