Katsumi Nagasaka

CdS microcrystal-doped silica glass prepared by the sol-gel process

Abstract Glass of composition of 4.4CdO·95.6SiO2 was prepared by the sol-gel process from Si(OC2H5)4 and Cd(CH3COO)2·2H2O. CdS crystals were precipitated by exposing the glass of H2S gas, which were identified with hexagonal wurtzite CdS crystals. From the X-ray diffraction analyses and transmission electron micrographs, these crystals were 2 to 6 nm in diameter. In the optical absorption spectra, the absorption edge exhibited a blue shift compared to that of the bulk CdS and its energy shift was reciprocally proportional to the square of the crystal size. Thus the quantum size effect could be found for the glass containing CdS prepared by the sol-gel process.

Microcrystalline PbS doped silica glasses prepared by the sol-gel process

Abstract The sol-gel process has been applied to the preparation of small size PbS crystal doped silica glasses. Silica glasses containing 1.8 and 4.5 wt% PbO were prepared by hydrolysis of Si(OC 2 H 5 ) 4 and Pb(CH 3 COO) 2 ·3H 2 O and subsequent reaction with H 2 S gas to form fine, cubic PbS microcrystal doped glasses. The microcrystal size (2 to 10 nm in diameter) was dependent on H 2 S gas reaction temperature. The absorption edge exhibited a blue shift of ∼ 2.0 eV from that of the bulk PbS crystal. Thus the quantum size effect was found for glasses containing PbS prepared by the sol-gel process.

Sol-gel processing of small-sized CdSe crystal-doped silica glasses

The sol-gel process has been applied to the preparation of small-sized CdSe-doped silica glasses with a significant quantum-sized effect. Gels synthesized by the hydrolysis of complex solution of Si(OC2H5)4, Cd(CH3COO)2 · 2H2O and Se were heated above 300°C in hydrogen-nitrogen atmosphere to form hexagonal CdSe microcrystals. The sizes of CdSe crystals, determined from the line broadening of X-ray diffraction pattern, increased from 3 to 10 nm diameter with increasing temperature of the heat treatment. The optical absorption spectra are red shifted as the crystal size increases, and the gap energy is reciprocally proportional to the square of the crystal size. This result is interpreted in terms of quantum size effect of small crystals and the effective reduced mass is estimated to be 0.36m0 which is about three times larger than that calculated from the theoretical prediction.

Preparation and quantum size effect of CuBr microcrystal doped glasses by the sol-gel process

Abstract CuBr microcrystal doped glasses were prepared by the sol-gel process. Gels synthesized through hydrolysis of a complex solution of Si(OC2H5)4, Al(OC4H9)3 and CuBr were heated to 900°C in a nitrogen atmosphere to form cubic CuBr microcrystals with diameters ranging from 5 to 8 nm. In the optical absorption spectra of samples heated above 700°C, peaks were observed at ca. 3.05 and 3.20 eV at 77 K. These peaks are attributed to the Z1,2 and Z3 excitons, respectively, in CuBr microcrystals and are shifted to the higher energy side as the sizes of the CuBr crystals decrease. This phenomenon is interpreted in terms of a quantum confinement effect of the excitons in the CuBr microcrystals.

Toughened glass-ceramics containing ZrO2 and Al2O3 prepared by the sol-gel process from metal alkoxides

Abstract Glasses of compositions 5ZrO 2 ·5SiO 2 (ZS), 5ZrO 2 ·Al 2 O 3 ·4SiO 2 (ZAS) and 5 5ZrO 2 ·0.5Al 2 O 3 ·0.5Na 2 O·4SiO 2 (ZANS) were prepared by the sol-gel process from metal alkoxides and sintered to make glass-ceramics. Tetragonal ZrO 2 was precipitated by heat treatment at 900 to 1300°C. The activation energy for tetragonal ZrO 2 crystal growth was extremely high in Al 2 O 3 containing glasses. ZAS and ZS were sintered to the near theoretical densities above 1200°C, at which the predominant phase was tetragonal ZrO 2 . On the other hand, for ZANS, high densification was not attained owing to the large pores enclosed by the glass phase. Strength and fracture toughness increased with the densification and the crystal growth of tetragonal ZrO 2 , reaching 450 MPa and 9 MN/m 1.5 , respectively.

Preparation of semiconducting sulfides microcrystalline-doped silica glasses by the sol-gel process

The sol-gel process has been applied successfully to the preparation of small-particle-size ZnS, CdS or PbS-doped silica glasses with a significant quantum size effect. Gels prepared through the hydrolysis of complex solutions of Si(0C2H5)4 and acetate of Zn, Cd or Pb were heated at 500 to 900°C, then reacted with H2S gas to form fine microcrystals doped glasses. From X-ray diffraction analyses and transmission electron mlcrographs, these crystals were cubic ZnS, hexagonal CdS and cubic PbS crystal, respectively, and their sizes were 2 to 8 nm in diameter. In the optical absorption spectra, the absorption edge exhibited a blue shift compared with those of the bulk sulfides crystals. Size dependence of energy shift was discussed in relation to size quantization of electron-hole in microcrystals. The nonlinearity was estimated to be 1.5 x i0O esu for 2% CdS doped glass.

Effect of water and catalyst on hydrolysis-condensation of ZrOSi alkoxides☆

Abstract The hydrolysis-condensation reaction of ZrOSi alkoxide complexes was studied using small angle X-ray scattering measurements.ZrOSi alkoxide complexes containing up to 30 mol% ZrO 2 were synthesized by reacting Zr(OC 3 H 7 ) 4 with Si(OC 2 H 5 ) 4 hydrolyzed with 1 mol H 2 O per 1 mol Si(OC 2 H 5 ) 4 and HCl catalyst. The synthesized ZrOSi alkoxides comprised chainlike polymers, in which the Zr atom was bonded with two siloxane polymers. The mass fractal dimension increased proportionally to an increase in the H 2 O/metal alkoxide molar ratio, indicating the alkoxides form aggregates of three-dimensional branched polymers. The Guinier increased to 80 A with an increase in both water and Zr(OC 3 H 7 ) 4 . No difference of condensation mechanism was observed from the acidity or basicity of the catalyst.

Oxidation of small-sized CdS crystals in sol-gel-derived glasses

Abstract Small-sized CdS crystals were prepared in SiO2 and Na2O ZrO2 SiO2 glasses by the sol-gel process and the oxidation of CdS crystals was investigated. Gels synthesized by the hydrolysis of a complex of metal alkoxides and Cd(CH3COO)2·2H2O were heated at 500°C and allowed to react with H2S gas to form hexagonal CdS microcrystals. CdS crystals in SiO2 glass were sensitive to oxidation by heating in air. CdS crystals in Na2O ZrO2 SiO2 glass were smaller than those in SiO2 glass and stable against oxidation in air. The effect of glass composition on the oxidation of CdS crystals was apparent in the emission spectra of glasses soaked in methyl viologen.

Microcrystalline-CdTe-doped silica glasses: sol-gel preparation and quantum confinement effect

Microcrystalline CdTe-doped glasses were prepared by heating the gels in H2-N2 atmosphere. When heated in H2-N2 gas, Te ions were reduced into Te2-, which reacted with Cd2+, ions to form CdTe crystals. The size of CdTe crystals was about 3.5 nm in radius, which was controlled by changing the conditions of gel synthesis. The optical absorption edge energies were reciprocally proportional to the square of the crystal radius.