PubMed · 16136157
The source did not provide an abstract. Follow the original record for more information.
Explore connections, maps & timelines
Philippe Guyot-Sionnest. 2005. Quantum dots: a new quantum state?. https://doi.org/10.1038/nmat1473
Cite the original work for its findings. Save a collection to share your selection of sources.
KEEP EXPLORING
Colloidal-size particles play an important, yet not very well understood, role in the speciation of phosphate in freshwaters. This study assesses the size distribution of molybdate reactive phosphorous in various 1.2 microm-filtered freshwaters using cross-flow filtration. Concurrently, colloid size was determined by a single particle counter technique, and mineralogical composition by a combination of transmission electronic microscopy coupled with energy dispersive spectroscopy and selected area electron diffraction. The percentage of colloidal molybdate reactive phosphorous varied a great deal in the samples studied: it ranged from 26% down to only 3%. A direct comparison of the percentages of colloidal-P can be misleading. The simultaneous determination of the amount, size distribution and mineralogical composition of the colloids present in the water bodies proved to be essential for interpreting the results. The causes of the observed variation were numerous and were site-dependent.
Three-dimensional (3D) centimeter-sized colloidal crystals can be spontaneously formed simply by dropping a NaOH solution (10 mM, approximately 10 microL) into an aqueous dispersion of dilute charged colloidal silica (particle diameter 110 nm, particle volume fraction phi = 0.023, 3-4 mL). The charge number of the silica particle increases with pH. Upon adding the NaOH solution, first, sub-millimeter-sized polycrystals are formed in the upper part of the sample due to charge-induced crystallization. The local phi value in the crystal region becomes nonuniform. The crystals with a high phi value accumulate at the bottom of the cell and then grow upward as columnar crystals. The crystal widths increase discontinuously with the growth, and in some cases, 3D centimeter-sized crystals are formed. The centimeter-sized crystals are also obtainable by the controlled diffusion of the base from its dilute reservoir. The present findings may prove valuable in the fabrication of large 3D single-crystalline photonic materials.
We investigate the phase behavior of surface-functionalized silica colloids at both the molecular and macroscopic levels. This investigation allows us to relate collective properties such as aggregation, gelation, and aging directly to molecular interfacial behavior. By using surface-specific vibrational spectroscopy, we reveal dramatic changes in the conformation of alkyl chains terminating submicrometer silica particles. In fluid suspension at high temperatures, the interfacial molecules are in a liquid-like state of conformational disorder. As the temperature is lowered, the onset of gelation is identified by macroscopic phenomena, including changes in turbidity, heat release, and diverging viscosity. At the molecular level, the onset of this transition coincides with straightening of the carbon-carbon backbones of the interfacial molecules. In later stages, their intermolecular crystalline packing improves. It is the increased density of this ordered boundary layer that increases the van der Waals attraction between particles, causing the colloidal gas to aggregate. The approach presented here can provide insights into phase transitions that occur through surface modifications in a variety of colloidal systems.