Nanocrystalline SnO 2 particles and twofold-coordinated sn defect centers in sol-gel-derived SnO 2 -SiO 2 glasses
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Semiconductive nanocrystals of stannic oxide (SnO2) were precipitated in silica (SiO2) glasses synthesized via a sol-gel route. Kayanuma’s equation, which describes the quantum confinement of an electron–hole pair in a semiconductive particle, well explained the absorption-edge shift due to the SnO2 nanocrystals in the optical absorption spectra. The adequate anneal of the SnO2–SiO2 glass ceramics in H2 gas led to the decomposition of the SnO2 nanocrystals and concurrently the production of twofold-coordinated tin atoms (Sn02) that provided a violet photoluminescence. The thermal behavior was studied with the x-ray diffraction measurement and photoluminescence and photoluminescence excitation spectroscopy.
I. INTRODUCTION
Since vitreous silicon dioxide (SiO2) materials are of technological importance in optoelectronic communication, much work is devoted to elucidation of a variety of their optical properties.1– 4 In recent years, Sakurai reported a 3.1 eV emission of a SiO2 glass produced by modified chemical vapor deposition (MCVD) method,1 and Glinka et al. observed a fine-structured luminescence in a green region, which arose from hydrogen-related species on a surface of SiO2 nanoparticles.2 An ultraviolet (UV) radiation-induced absorption at 4.8 eV was observed in high performance silica evaluated for F2 laser-based semiconductor lithography.3 For compatibility with the current technology in the semiconductor industry, extensive research on Si-based emitting materials also continued in which photoluminescence (PL) properties of Si-, Ge-, and Sn-implanted SiO2 layers were studied.4 This paper presents a variety of PL properties of SnO2–SiO2 glasses synthesized by a sol-gel method. The precipitation of stannic oxide (SnO2) nanocrystals in SiO2 glasses offered us a broad, orange emission under a photoexcitation by nitrogen gas (N2) laser. The obtained glasses were transparent and homogeneous up to 20 mol% SnO2. On the basis of Kayanuma’s equation, which describes the quantum confinement of an electronhole pair in a semiconductive particle, the observed red shift of the absorption edge with increasing SnO2 content was explained. In was also shown that a violet PL could be extracted from a reduction process of the SnO2–SiO2
a)
e-mail: [email protected] J. Mater. Res., Vol. 17, No. 6, Jun 2002
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glass ceramics. The PL and photoluminescence excitation (PLE) studies revealed that the violet PL originated from twofold-coordinated tin (Sn) atoms. Since a high concentration of SnO2 and annealing in a stronger reduction atmosphere lead to the formation of nanometer-sized particles of Sn metal and, as a result, a blue-opaque coloration, we mainly deal with 5–10 mol% of the SnO2 content in the nominal composition. II. EXPERIMENTS A. Sample preparation
The starting materials were tetraethoxysilane (TEOS; Colcoat) and SnCl2 ⭈ 2H2O (Kanto Chem. Co., Tokyo, Japan) as received. A traditional sol-gel synthesis by the use of these materials, which is described below,
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