Chia-Yen Li

Control of Particle Size Distribution of CdS Quantum Dots in Gel Matrix

A novel sol-gel process has been developed to prepare nano-sized CdS quantum dots to improve the nonlinear optical properties. A bifunctional ligand, 3-aminopropyl triethoxysilane H2N(CH2)3Si(OC2H5)3, was used to disperse the Cd2+ ions in the gel solution. The CdO and CdS particles were observed by transmission electron microscope (TEM). The size of CdS microcrystallites with concentrations up to 13 wt.% in SiO2 gel matrix was found to be in the range of 2–4 nm with a very sharp size distribution. A well-defined absorption edge was observed in the absorption spectrum.

ORMOSILS as matrices in inorganic-organic nanocomposites for various optical applications

Organically modified silicates (Ormosils) have been used as the matrix material in the preparation of several different kinds of nanocomposites for optical applications via the sol-gel method. Polydimethylsiloxane (PDMS) and dimethyl-phenylmethylsiloxane copolymer have been used as the organic components to modify SiO2 network in the Ormosils. A broad range of optically active materials such as semiconductors, metals, oxides, organics, and ions have been incorporated into Ormosils. The preparation process for the photo-active material doped Ormosils is described. The dependency of Ormosil properties on processing parameters and the effects of organic modifications on the silica network are investigated. Some examples of using Ormosils derived from tetraethoxysilane and PDMS as matrices to prepare photo- active composite materials are reported. Ormosils proved to be a promising new class of materials for various optical applications.

Improving CdS quantum-dot materials by the sol-gel method

In this paper we discuss our approaches and results for the preparation of improved semiconductor quantum dot materials by the sol-gel method. By taking advantage of the low- temperature sol-gel route, the nanostructure and optical properties of CdS quantum dots were greatly improved for device applications. Pore-free sodium borosilicate (NBS) glasses and organically modified silicates (ORMOSILs) were used as matrices for the CdS nanocrystallites. Results from both degenerate four-wave mixing and pump-probe techniques indicated large third-order nonlinear responses of the order of 10-6 esu from CdS-doped NBS glasses. By using potassium ion exchange, the first CdS-doped channel waveguides were fabricated in NBS glasses. Propagation of femtosecond laser pulses through the waveguide was investigated. Pulse breakup effect that may be the result of soliton formation was observed in these waveguides. Polydimethylsiloxane (PDMS) was uses as the organic component to modify the silica gel. CdS-doped ORMOSILs derived from the PDMS- TEOS system exhibits improved mechanical property and film-forming ability compared to purely inorganic gels. Both CdS-doped PDMS-TEOS ORMOSILs and NBS glasses were much more resistant to photodarkening than the glasses made by melting. The use of the bifunctional ligand 3-aminopropyltriethoxysilane (APTES) significantly reduced the average particle size and narrowed the size distribution of CdS quantum dots in silica gels and densified NBS glasses.

Preparation of quantum dots by the sol-gel process

The preparation of semiconductor dots with high number density and narrow particle size distribution, which are essential for high nonlinearity susceptibility, has been studied on CdS dispersed soda-borosilicate glasses by the sol-gel process using tetramethoxysilane (TMOS), triethylborate (B(OEt)3), sodium acetate (NaOAc) and cadmium acetate (Cd(OAc)2) as the starting materials. Both number density and particle size distribution of the CdS crystallites were dependent upon the treatment conditions in the conversion of Cd(OAc)2 to CdO and CdO to CdS inside the porous gel. The optimum heat-treatment temperature for the conversion of Cd(OAc)2 to CdO was around 400 degree(s)C, and that for the conversion of CdO and CdS by exposing the gel to H2S was around 120 degree(s)C. Glass containing about 10 wt% CdS crystallites 4 - 7 nm in diameter was obtained by sintering the gel at 590 degree(s)C under the above treatment conditions.

Integrated optics devices by ultraviolet light imprinting in sol-gel silica glass

We describe a simple low temperature method to produce integrated optics devices in a photosensitive, hybrid, organically modified sol-gel silica glass. In particular, we report on fabrication and characterization of slab and channel waveguides, waveguides with grating and a directional coupler. The fabrication process is appealing for its simplicity, entailing few steps and utilizing elementary photodefinition to give robust, mechanically rigid devices.

Photoluminescence excitation spectroscopy study of CdS nanocrystals in ormosils

Abstract Photoluminescence excitation spectroscopy (PLE) has been used to characterize the size distribution of cadmium sulfide nanocrystals embedded in an Ormosil matrix. PLE spectroscopy is shown to be an excellent tool for determining average particle size and distribution for these samples. Through the use of current quantum confinement models, average size and size distribution information is obtained from PLE data. These data are shown to agree with x-ray diffraction analysis, absorption, photoluminescence, and TEM measurements within the experimental limits.

CdS-doped ormosils as nonlinear optical materials

Organically modified silicates (Ormosils) have been applied as matrices in the preparation of CdS-doped glassy nanocomposites via a low temperature route. EDX spectra show that a CdS concentration up to about 20 wt.% has been obtained. The CdS microcrystallites of hexagonal wurtzite structure with average particle size in the range of 20 to 120 angstroms were formed within Ormosils matrices. The quantum confinement effects were clearly observed in samples with smaller particle sizes from absorption and photoluminescence excitation spectra. High- quality CdS-doped films with enough thickness have been prepared by spin-coating for waveguide device application. The Ormosils used are thermally stable up to 360 degree(s)C which is higher than the processing temperature for normal integrated circuits. The nonlinear optical properties of these nanocomposites measured by degenerated four wave mixing technique on picosecond time scales using a Nd+3:YAG laser at 532 nm are described.

Preparation of Metal-Cluster-Ormosil Nanocomposites

A series of metal-cluster-organically modified silicate, or Ormosil, nanocomposites were prepared by the sol-gel method. Each of the metals (Ag, Au, Cu, and Pt), was incorporated individually into the Ormosil matrices at various weight percents by reduction of the appropriate metal salt. The preparation for these metal-cluster doped-Ormosils is described. Ormosil proved to be an ideal matrix material, offering excellent transparency and good cracking resistance. The presence of the metal clusters inside the Ormosil matrices was detected by transmission electron microscope and verified by X-ray diffraction. Optical absorption. measurements of these nanocomposites are also presented.

Sol-Gel Reaction Mechanisms of Ormosils and Effects of Ultrasonic Irradiation

Sol-gel reaction mechanisms of organically modified silicates (Ormosils) were investigated by liquid state 29 Si-NMR (nuclear magnetic resonance) spectroscopy. Samples were prepared from tetraethoxysilane (TEOS), polydimethylsiloxane (PDMS), water, isopropanol, tetrahydrofuran (THF) and hydrochloric acid (HCl). After a new proposed site, D(Q), was assigned from the spectrum of the solution of dimethyldiethoxysilane (DMDES)/TEOS system, the bonds between PDMS and hydrolyzed TEOS were confirmed by 29 Si-NMR spectroscopy. Hydrolysis and self-condensation of TEOS were found to occur prior to the copolymerization with PDMS. The effect of ultrasonic irradiation on the gelation process was also investigated. Ultrasound enhanced the over all reaction rate and the decreasing amount of solvent brought further increase in the reaction rate.