We have developed a new synthetic methodology for preparing ca. 40-300-nm monodisperse silica-cadmium sulfide nanocomposite spheres. This methodology uses water-in-oil microemulsions in which monodisperse silica colloids are produced by the controlled hydrolysis of tetraethyl orthosilicate (TEOS) in water nanodroplets. The resulting pure silica spheres can be grown to between 40 and 80 nm in diameter and can be used as seed particles for production of larger silica colloids upon further reactions with TEOS in the microemulsion. Cadmium sulfide quantum dots are incorporated into the silica colloids during the silica sphere synthesis by the simultaneous coprecipitation of cadmium nitrate and ammonium sulfide in the water nanodroplets. The CdS can be introduced as a homogeneous dispersion of CdS quantum dots (ca. 25 A diameter), as large inclusions, as surface caps, as the central core of a silica particle, or as shells of CdS interleaved between silica shells. These different nanoscale complex morphologies in silica are created by controlling the coprecipitation of CdS. Techniques including TEM, SEM/EDS, X-ray diffraction, and light scattering were used to characterize the elemental analysis as well as particle morphology. In addition, we have prepared doublet and triplet spheres which are connected by welds of CdS. This silica-CdS nanocomposite is a new material with potential utility for nonlinear optics. Further processing makes this material useful for a new class of high surface area catalytic supporting materials.
We have developed a new method to create complex monodisperse silicon dioxide particles which contain voids with unique morphologies. These particles are prepared from monodisperse silica spheres (ca. 100 nm) which contain CdS inclusions organized as large patches on the silica sphere surfaces, as small quantum dots within the spheres, as interior spherical shells, or as the central cores of the silica spheres. We create voids with the identical morphology as the CdS inclusions by etching out the CdS inclusions with strong acid. The silica sphere-CdS composite particles are prepared within a microemulsion reaction medium as described in the accompanying paper (Chang, S.-Y.; et al. J. Am. Chem. Soc., preceding paper in this issue). The etched silica particles have craters on their surface, 2.4-nm diameter spherical voids dispersed within the silica spheres, hollow cores, or hollow shells separating silica shells from silica cores. In addition, we have formed unique ellipsoidal cavities within doublets of silica spheres. These doublets were formed through the attachment of silica spheres by CdS patches on the silica surfaces. A silica shell was subsequently grown around the sphere doublets. Etching away the CdS results in ellipsoidal caverns connecting the two spheres of each doublet. These high surface area materials have geometrically tailorable voids and may prove useful as novel catalyst support media.
The development of optical technologies requires the fabrication of reliable optical switching and limiting devices. Optical switches modulate the transmission or reflection of incident light, while optical limiters serve to limit transmission to prevent the transmitted light intensity from exceeding a defined level. A major application of optical limiters is to protect delicate sensors.
Diffraction line broadening has been adapted to light diffraction to measure ordered domain size. Domain sizes of ~25 gm were observed for Ti02 powders and of ~40 yam for Si02 powders. Scanning electron microscopy was used to confirm the ordered domain size of both Si02 and Ti02.
A novel synthetic methodology has been developed for preparing monodisperse colloidal silica-cadmium sulfide nanocomposite spheres in the 50 – 300 nm size regime. This methodology uses water-in-oil microemulsions as the reaction medium. Monosize silica colloids are first produced by the controlled hydrolysis of tetraethyl orthosilicate in the micro water droplets of the microemulsion. Cadmium sulfide quantum dots are incorporated into the silica colloids during synthesis by the introductions of Cd2+ and S2- microemulsions. Various morphologies of the nanocomposite are fabricated by controlling the heterogeneous coagulation of CdS and SiO2. Unique high surface area silica particles can be prepared when nitric acid etches out the CdS and leaves behind topologically defined voids. The CdS nanocomposites are new materials useful for non-linear optics, while the high surface area silica particles should have novel applications in areas such as catalysis.
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