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.

Temporal- and Site-Specific Determination of the Origin of the Luminescent Bands in Silicon Nanowires

“…Previous theoretical and experimental works support this hypothesis. Experimental and theoretical efforts have shown that the energy of the luminescence due to the Si nanoparticle increases as the size of the particle decreases once the quantum confinement regime has been reached (<5 nm). , Although the absolute value of the measured and calculated lifetimes vary considerably, the trend is that the lifetimes of Si nanosructures decrease as the energy of the luminescence increases (particle size decreases), − ,,,, which is in agreement with the lifetime trends seen in Figure and Table .…”

“…Previous time-resolved luminescence studies on silicon nanostructures, using conventional sources, have produced lifetimes that ranged from nanoseconds to milliseconds in magnitude, depending on size, morphology, method of preparation, energy, etc. − Although there is considerable variation among the results, the general trend is that as the size of the particle decreasesthereby forming species with a higher energy, more direct bandgapthe lifetime of the emission decreases . In addition, there is the possibility of energy transfer from smaller particles to larger ones via a Forster-type mechanism, which would also tend to shorten the lifetimes of emissions from smaller particles.…”

Silicon quantum dots: surface matters

“…With highly monodispersive SiQDs, more advanced self-assembled structures can be obtained, such as binary superlattices with gold nanoparticles [125]. Complex superstructures can also be achieved via a controlled growth in pre-defined superstructures, such as opals [126,127], zeolites [128,129], or periodic mesoporous silica [130,131]. Embedding SiQDs in chemically resistant polymer hybrids is yet another very interesting technique [132].…”

Optical Spectra and Structure of CdP4 Nanoclusters Fabricated by Incorporation into Zeolite and Laser Ablation

“…However, it is not easy to control the size distribution of small nanoparticles with a countable number of atoms (so-called clusters) using these methods. The matrix method, based on the incorporation of materials into the 3D regular system of voids and channels of zeolite crystals, could be an alternative technique for the fabrication of semiconductor nanoclusters with a controllable size distribution. , Many works have been reported on the nanoclusters incorporated into zeolite pores: semiconductors, − metals, , and polymers. , The subnanometer and nanometer clusters are very interesting as they are intermediates between the molecules and the typical nanocrystals. Usually, the structure of nanoclusters is different from the structure of nanocrystals, which resembles the structure of bulk crystals .…”