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Christopher M Sorensen

Publications and source records attributed to Christopher M Sorensen.

11 recordsLinked to original sources
PubMed2006

Low-temperature metallic alloying of copper and silver nanoparticles with gold nanoparticles through digestive ripening.

We describe a remarkable and simple alloying procedure in which noble metal intermetallic nanoparticles are produced in gram quantities via digestive ripening. This process involves mixing of separately prepared colloids of pure Au and pure Ag or Cu particles and then heating in the presence of an alkanethiol under reflux. The result after 1 h is alloy nanoparticles. Particles synthesized according to this procedure were characterized by UV-vis spectroscopy, EDX analysis, and high-resolution electron microscopy, the results of which confirm the formation of alloy particles. The particles of 5.6+/-0.5 nm diameter for Au/Ag and 4.8+/-1.0 nm diameter for Cu/Au undergo facile self-assembly to form 3-D superlattice ordering. It appears that during this digestive ripening process, the organic ligands display an extraordinary chemistry in which atom transfer between atomically pure copper, silver, and gold metal nanoparticles yields monodisperse alloy nanoparticles.

Alloys↗
PubMed2005

Multiple-scattering effects on static light-scattering optical structure factor measurements.

We show that the extent and effect of multiple scattering on angularly resolved light-scattering intensity measurements, the optical structure factor, can be quantitatively described by a single parameter, the average number of scattering events along the scattering volume. This quantity is easily measured or calculated and hence provides a useful experimental indicator of multiple scattering, which is a hindrance to accurate structure factor measurements.

Journal Article↗
PubMed2005

Patterns in Mie scattering: evolution when normalized by the Rayleigh cross section.

An alternative to using the traditional scattering angle theta to describe light scattering from a uniform dielectric sphere is the dimensionless parameter qR, where R is the radius of the sphere, q = 2k sin(theta/2), and k is the wavenumber of the incident light. Simple patterns appear in the scattered intensity if qR is used in place of theta. These patterns are characterized by the envelopes approximating the scattered intensity distributions and are quantified by the phase-shift parameter rho = 2kR/m - 1/, where m is the real refractive index of the sphere. Here we find new patterns in these envelopes when the scattered intensity is normalized to the Rayleigh differential cross section. Mie scattering is found to be similar to Rayleigh scattering when p < 1 and follows simple patterns for p > 1, which evolve predictably as a function of p. These patterns allow us to present a unifying picture of the evolution of Mie scattering for changes in kR and m.

Journal Article↗
PubMed2005

Reversible transformations of gold nanoparticle morphology.

Herein is reported a metamorphosis taking place in a gold nanosized system. The observed phenomenon of shape and size transformations was found to be completely reversible. Unlike most procedures in the literature where shape and size control occur in the synthetic step by adding growth- and shape-controlling agents such as surfactants or polymers, in this system postsynthetic changes in shape and size can be carried out simply by changing the ratio of reactive, competing reagents, more specifically, alkylthiols versus tetralkylammonium salts. Interestingly, the transfer of gold metal occurs (large prismatic particles to small particles and vice versa) under the influence of reagents that do not cause such interactions with bulk gold. All intermediate steps of the morphology change were observed using HRTEM and electron diffraction. The processes of breaking down and "welding back" solid metal nanoparticles occur under mild conditions and are remarkable examples of the unique chemical properties of nanomaterials. The described process is expected to be relevant to other nanoscale systems where similar structural circumstances could occur.

Letter↗
PubMed2005

Quantitative information about the hydrogen bond strength in dilute aqueous solutions of methanol from the temperature dependence of the Raman spectra of the decoupled OD stretch.

We have obtained quantitative information about the hydrogen bond strength in pure water and in dilute aqueous solutions of methanol by analyzing the temperature dependence of Raman spectra of the decoupled OD stretch from 21 to 160 degrees C with the hydrogen bond energy dispersion method. A minimum at 2440 cm(-1) assigned to strong icelike hydrogen bonds and a maximum at 2650 cm(-1) due to maximally (but not completely) broken hydrogen bonds result in all cases. The energy of the minimum decreases upon addition of methanol due to formation of stronger water-methanol hydrogen bonds, whereas the energy of the maximum increases because water hydrogen atoms in the vicinity of the methyl group might participate in "more broken" hydrogen bonds than in bulk water.

Hydrogen Bonding↗
PubMed2005

Synthesis of spherical silver nanoparticles by digestive ripening, stabilization with various agents, and their 3-D and 2-D superlattice formation.

Capped nanoparticles of silver were synthesized via the solvated metal atom dispersion (SMAD) technique followed by a digestive ripening procedure producing gram quantities of monodisperse spherical nanoparticles. This shows for the first time that a digestive ripening protocol is possible for an element other than gold. The particle size and optical spectra were found to be dependent on the capping agent used. Particles capped with dodecane thiol had a mean diameter of 6.6+/-1 nm, while trioctyl phosphine capped particles were 6.0+/-2 nm determined via TEM microscopy. These particles were found to organize into two- and three-dimensional superlattices with a well defined geometry through self-assembly in a liquid solution, that was dictated by the ligand used resulting in a triangular or circular lattice.

Journal Article↗
PubMed2003

Calorimetric study of magnetic fluids under a magnetic field.

In our past study, it was found that the strong magneto-optical effect of a magnetic fluid (MF) under a magnetic field is due to the second-order phase transition from colloidal particles' monodispersed phase to the particles' anisotropically agglomerated microclusters phase. These results, however, contradicted the Landau criterion. In the present study, in order to clarify whether the transition is of first order or second order, we performed differential scanning calorimetry (DSC) experiments on three different MFs to look for the existence of a phase transition latent heat. The DSC measurement was performed in the temperature range 22-150 degrees C with zero magnetic field and under approximately 10 kA/m magnetic field, respectively. No phase transition heat was observed within an experimental error of 0.03 kJ/kg for all the samples whether or not the field was applied.

Journal Article↗
PubMed2002

Experimental test of the Rayleigh-Debye-Gans theory for light scattering by fractal aggregates.

Calibrated measurements of optical (lambda = 488-nm) scattering cross sections, in the form of Rayleigh ratios, are presented for two fractal aggregate aerosols. The aggregates have radii of gyration of approximately 280 nm, fractal dimensions of approximately 1.75, and monomer sizes of approximately 20 nm with approximately 150 monomers per cluster. One aerosol was composed of vitreous SiO2 with a refractive index of 1.46, the other of anatase TiO2 with a refractive index of 2.61. We found good agreement with the Rayleigh-Debye-Gans prediction for the scattering cross section of fractal aggregates.

Journal Article↗
PubMed2002

Gram-scale synthesis of monodisperse gold colloids by the solvated metal atom dispersion method and digestive ripening and their organization into two- and three-dimensional structures.

We describe a synthetic procedure for preparation of large quantities of monodisperse thiol-stabilized gold colloids in toluene solution. The method is based on the solvated metal atom dispersion technique (SMAD), which is very suitable for preparation of large amounts of metal colloidal solutions, as well as of metal sulfide, metal oxide, and other types of dispersed compounds in different solvents. A combination of two different solvents like acetone and toluene is used for the preparation of the gold colloids. The necessity of initially carrying out the SMAD reaction in acetone comes from its high degree of solvation of gold particles. Acetone acts as a preliminary stabilizing agent. After its removal from the system, the particles are stabilized by dodecanethiol molecules, which enable their very good dispersion in toluene solution. A digestive ripening procedure is carried out with the gold-toluene colloid, and for this purpose pure toluene as solvent is necessary. This has a dramatic effect on the narrowing of particle size distribution and almost monodisperse colloids are obtained (some discussion of the probable mechanism of this remarkable digestive ripening step is given). These colloidal solutions have a great tendency to organize in two- and three-dimensional structures (nanocrystal superlattices, NCSs). We believe that this procedure provides a real opportunity to synthesize large amounts of gold nanocrystals as well as NCSs.

Journal Article↗
PubMed2002

Patterns in the ripple structure of Mie scattering.

We study the ripple structure in the scattered intensity predicted by Mie scattering theory in the angular behavior of the scattered intensity for homogeneous, dielectric spheres. We find that for small values of the phase shift parameter rho = 2kR absolute value(m - 1), where k = 2 pi/lambda, R is the sphere radius, and m is the relative refractive index, the ripples are periodic with spacing equal to pi when plotted versus the dimensionless qR, where q = 2k sin(theta/2) and theta is the scattering angle. However, as rho increases, this outcome switches to nonuniform spacing of approximately pi cos(theta/2). The latter spacing is equivalent to a uniform spacing of pi/kR when plotted versus theta.

Journal Article↗