Time-Resolved Photoluminescence Spectra of Si Species Encapsulated in Zeolite Supercages

Abstract

Photoluminescence (PL) spectra of Si species encapsulated in zeolite supercages are studied. It is reported that the chained Si species terminated partially with phenyl groups and with some unsaturated bonds are formed in zeolite supercages by the reaction with phenylsilane and they show PL around 4 eV (J. Phys. Chem. 2004, 108, 2501-2508). In the present paper they are reduced with hydrogen to prepare Si chained species terminated and saturated with hydrogen atoms. The PL spectra are deconvoluted to be four components at 1.9, 2.2, 2.6, and 3.7 eV, which can tentatively be assigned to Si nanocrystals and Si quantum wires in addition to defects in SiO2 and uncontrolled organic impurities in zeolite, respectively. At elevated temperatures the Si quantum wires in zeolite pores seem to change the Si nanocrystals with the size larger than that of the zeolite pore diameter. It is the first case in which the PL decay lifetime of oxygen vacancies in zeolite can be detected to be quite short to be about 16 ns. The detected lifetimes of Si quantum wires are significantly very short, about 12 ns. The Si species encapsulated zeolite is solvated with hydrofluoric acid solution to separate the Si quantum wires by dissolving zeolite lattice. The Si quantum wires in the HF solution show intense PL spectra peaked at 2.33 eV and broad UV spectra around 2.8-3.5 eV. They will have different shapes and lengths. The HF solvated zeolite shows still PL spectra characteristic of oxygen vacancies and the absorption edge at 3.6 eV. The result means that zeolite lattice is solvated in HF solution as clusters with a band gap of 3.6 eV and they can still have some oxygen vacancies. Oxygen vacancies situate about 1.0 eV below the zeolite conduction band minimum, and the absorbed energy can be dissipated as PL between the valence band maximum and the oxygen vacancies. It is concluded that the excitation photon energy can be absorbed in zeolite and the Si quantum wires and then the absorbed energies are competitively relaxed in zeolite and the Si quantum wires.