Christopher M. Sorensen

Light Scattering by Fractal Aggregates: A Review

This paper presents a review of scattering and absorption of light by fractal aggregates. The aggregates are typically diffusion limited cluster aggregates (DLCA) with fractal dimensions of D

Structural phase behavior in II-VI semiconductor nanoparticles

Cubic structured CdS, CdSe, and CdTe, II–VI semiconductor nanoparticles have been synthesized using aqueous solution precipitation at room temperature. The ‘‘as‐prepared’’ particles have a size of about 30 A. Thermal annealing causes (a) an increase in particle size; (b) a structural transition from the cubic to the bulk, hexagonal structure for CdS and CdSe; and (c) no structural transition for CdTe. The unexpected cubic phase for small particles of CdS and CdSe may be due to either metastability or an equilibrium surface effect. The latter would imply a strong correlation between structure and the size and surface properties, and that there is a minimum size for hexagonal phase stability.Cubic structured CdS, CdSe, and CdTe, II–VI semiconductor nanoparticles have been synthesized using aqueous solution precipitation at room temperature. The ‘‘as‐prepared’’ particles have a size of about 30 A. Thermal annealing causes (a) an increase in particle size; (b) a structural transition from the cubic to the bulk, hexagonal structure for CdS and CdSe; and (c) no structural transition for CdTe. The unexpected cubic phase for small particles of CdS and CdSe may be due to either metastability or an equilibrium surface effect. The latter would imply a strong correlation between structure and the size and surface properties, and that there is a minimum size for hexagonal phase stability.

The Mobility of Fractal Aggregates: A Review

A review of the experimental and theoretical literature describing the mobility of fractal aggregates over the previous three decades is presented. Aggregates are those formed via both diffusion and reaction limited cluster-cluster aggregation processes, DLCA and RLCA, which form aggregates with fractal dimensions and prefactors of ca. 1.78 and 1.3 and 2.1 and 0.94, respectively. Emphasis is placed on DLCA aggregates. The entire Knudsen number range from continuum to free molecular is reviewed. The review finds a simple and general consensus description of mobility for the entire range.

Light-scattering measurements of monomer size, monomers per aggregate, and fractal dimension for soot aggregates in flames

A new method for the in situ optical determination of the soot-cluster monomer particle radius a, the number of monomers per cluster N, and the fractal dimension D is presented. The method makes use of a comparison of the volume-equivalent sphere radius determined from scattering-extinction measurements RSe and the radius of gyration Rg, which is determined from the optical structure factor. The combination of these data with the measured turbidity permits for a novel measurement of D. The parameters a and N are obtained from a graphical network-analysis scheme that compares R(se) and Rg. Corrections for cluster polydispersity are presented. The effects of uncertainty in various input parameters and assumptions are discussed. The method is illustrated by an application to data obtained from a premixed methane-oxygen flame, and reasonable values of a, N, andD are obtained.

Biocidal Activity of Nanocrystalline Silver Powders and Particles

While the activity of the SMAD powders is lower than that of pure silver nitrate, it has the ability to kill bacteria very effectively and over long periods of time.

The density of supercooled water. II. Bulk samples cooled to the homogeneous nucleation limit

We have measured the density of supercooled water (H2O) in the range −33.41≤T≤−5.23 °C. Our samples were held in glass capillary tubing with an approximate inside diameter of 0.3 mm=300 μ. These samples were prepared by Mossop’s method and could be cooled to their homogeneous nucleation limit before freezing. We compare our density data to other measurements using capillaries and demonstrate what appears to be an excess density in smaller capillaries which is inversely proportional to the capillary inside diameter. The origins of this excess density are unknown, but we show its effect is insignificant on our measurement. The thermal expansivities derived from our data are fit to a power law in temperature relative to a singular temperature. These results are inconclusive due to a poor knowledge of the background expansivity.

Digestive Ripening, Nanophase Segregation and Superlattice Formation in Gold Nanocrystal Colloids

A novel digestive ripening process is shown to narrow the particle size distribution from a highly polydisperse dodecanethiol ligated gold colloid. Unlike the Ostwald ripening process, the digestion occurs through transferring materials from large particles to small particles. Temperature-induced size segregation can further select the particle sizes. By using these two methods, highly ordered superlattices using nanocrystals as building blocks can be synthesized directly from a polydisperse colloid.

Preparation of manganese ferrite fine particles from aqueous solution

Fine manganese ferrite particles have been prepared by a coprecipitation method and subsequent digestion process (below 100°C). Manganous salts mixed with either ferric or ferrous salts were coprecipitated with sodium hydroxide. Particles produced from ferric salts were MnFe2O4 with relatively smaller sizes (5 to 25 nm) while ferrous salts created MnxFe3−xO4 (0.2 < × < 0.7) with bigger sizes up to 180 nm. In either case, the particle size appeared to be a unique function of the ratio of metal ion concentration to hydroxide ion concentration when the digestion conditions were fixed. For the system with ferric salts, the undigested samples were polycrystals with crystallite sizes of about 2 nm. Digestion, which could be described as an Ostwald ripening process, did not change the crystalline structure but increased both the crystallite size and the particle size. A basic solution was essential for an effective digestion process in this system. The system with ferrous salts, on the contrary, needed an acidic solution to create a single ferrite phase. Digestion changed both the crystalline structure and the particle size of the precipitated precursors. This process involved a dissolution and renucleation/growth mechanism. Cation and anion effects on the particle size and the evolution during digestion were also studied.

Exchange anisotropy in oxide passivated Co fine particles

Oxide passivated Co particles have been prepared by vapor deposition in the size range of 50–350 A. A strong exchange anisotropy was observed due to the core‐shell structure of the Co particles, where the core consists of metallic Co and the shell of Co oxides. The exchange anisotropy of the particles was studied using shifted hysteresis loops after the sample was field cooled (FC). The shift in the FC loop exhibited a maximum at a particle size of 80 A (shift=10.7 kOe) and is related to the amount of surface oxidation. The shift in the loops disappeared around 150 K in all the samples irrespective of their particle size, and this is attributed to the superparamagnetic behavior of the antiferromagnetic oxide shell.

Magnetic properties of nanophase cobalt particles synthesized in inversed micelles

Cobalt particles were prepared with the microemulsion method in the binary system of DDAB (didodecyldimethylammonium bromide)/toluene by reduction of CoCl2 with NaBH4. The average particle size of the as‐prepared samples could be varied from 1.8 to 4.4 nm by controlling the concentration of CoCl2 in the solution of DDAB in toluene. TEM studies showed that the particles were quite uniform and well isolated. The particle sizes determined from magnetic data were consistent with those measured by TEM. The coercivity of the particles at 10 K increased from 640 to 1250 Oe as particle size increased from 1.8 to 4.4 nm. The blocking temperature of the particles increased from 19 to 50 K for the same size range. The saturation magnetization σs at 2 K increased with decreasing particle size. The value of σs of the particles with average size of 1.8 nm was about 200 emu/g, which is 20% higher than the bulk value. This implies that the magnetic moment per atom is enhanced in the nanoparticle system.