Kevin L. Stokes

Simple cubic super crystals containing PbTe nanocubes and their core-shell building blocks.

We report a preparation of high-quality cubic PbTe nanocrystals and their assembly into both square-array, two-dimensional patterns and three-dimensional simple cubic super crystals. The influence of oleylamine in the nanocrystal synthesis and core-shell formation through an anion-exchange mechanism was also studied. The simple cubic super crystals together with two-dimensional assembly patterns containing PbTe nanocubes and their core-shell building blocks were examined using TEM, SEM, AFM, XRD, SAXS, and FTIR. Such super crystals consisting of cubic structural building blocks may allow engineering of more complex materials from which novel properties may emerge.

Ultrafine NiFe2O4 powder fabricated from reverse microemulsion process

NiFe2O4 ultrafine powder with high crystallinity has been prepared through a reverse microemulsion route. The composition in starting solution was optimized, and the resulting NiFe2O4 was formed at temperature of around 550–600 °C, which is much lower than that observed from the solid-state reaction. Magnetic investigation indicates that samples are soft-magnetic materials with low coercivity and with the saturation magnetization close to the bulk value of Ni ferrite.

Microemulsion-processed bismuth nanoparticles

Abstract Bismuth (Bi) is classic semi-metal with a low carrier density, small carrier effective masses, very long mean free path and highly anisotropic Fermi surface. In this work, we present a technique for the synthesis of bismuth nanometer-sized particles using an inverse microemulsion method. To prevent air-oxidation in the characterization stage, poly(vinylpyrrolidone) was used as protecting agent. Our characterizations reveal that single rhombohedral phase of bismuth can be obtained with ultrafine particle with ∼20 nm in size by following this synthetic route. The optical absorption spectrum of the aqueous nanoparticle colloids shows an absorption band at ∼268 nm.

Nanocrystalline bismuth synthesized via an in situ polymerization-microemulsion process

Nanometer-sized bismuth particles were prepared using an inverse microemulsion method. To prevent air-oxidation, an in situ polymerization technique using methyl methacrylate (monomer) and 2-hydroxyethyl methacrylate (co-monomer) with cross-link agent was employed, and polymeric network was formed around the water droplets. Our characterizations reveal that very highly crystalline bismuth particles on the order of 20 nm can be obtained within this polymeric network. Comparison with a similar microemulsion procedure without polymer indicates that the polymeric network protected the bismuth particles against oxidation, especially during post-synthesis annealing. The current investigation demonstrates a feasible route to produce single phase, air-sensitive metal nanocrystallites using this modified microemulsion technique.

CoFe2O4 nanostructures with high coercivity

Nanometer-sized ferrite magnetic materials are the subject of intense research interest due to their potential applications in high-density magnetic information storage. One of the most explored ferrite materials is the cobalt ferrite (CoFe2O4). We have synthesized cobalt ferrite nanowires using cobalt ferrite nanoparticles in a porous anodic alumina template (AAT). The process of embedding ferrimagnetic particles into the pores was assisted by the magnetic field of a permanent magnet placed in vacuum directly under the substrate. Particles synthesized in the template were subsequently annealed at 600°C for 2h in Ar gas forming arrays of cobalt ferrite nanowires inside the AAT. The morphology of the ferrite before and after annealing was observed using a field-emission scanning electron microscope. The crystallographic structure of the nanowires was analyzed using x-ray diffraction and transmission electron microscopy. The magnetization was measured by a superconducting quantum interference device. The co...

Discrete dipole approximation for magneto-optical scattering calculations.

Magneto-optical spectra (Faraday effect) for nanometer-scale particles and collections of particles are calculated using a modification of the discrete dipole approximation (DDA). The approximation is used as a finite-element method for non-spherical particles whose dimensions are on the order of or smaller than the incident light wavelength, lambda. Also, we use the approximation to calculate scattering from arrangements of spherical nanoparticles with diameters << lambda. We propose that for scattering from subwavelength magnetic particles, the specific Faraday rotation should be defined as the difference in optical extinction for left- and right-circularly polarized light. We apply the model to calculations of Faraday rotation from magnetite nanowires as well as a binary (two-component) nanoparticle arrangement. Enhancements in Faraday rotation are predicted for composites containing both noble metal and ferrite nanoparticles.

Synthesis and magnetic properties of CoPt–poly(methylmethacrylate) nanostructured composite material

We have prepared nanometer-sized CoPt particles dispersed in a poly(methyl methacrylate) (PMMA) matrix, as a novel nanostructured magnetic plastic, through a soft chemical processing route. In this work, CoPt nanoparticles were successfully synthesized from a solution phase reduction system in the presence of capping ligands and stabilizing agents at high temperature. The CoPt nanoparticles were annealed at 400 °C for 3 h, and were subsequently re-dispersed in methylmethacrylate (monomer). The polymerization was induced by a UV source and the hardness of final product was adjusted by varying the amount of monomeric cross-link agent. Annealed bare CoPt nanoparticles as a “core” material and CoPt–PMMA composite material were characterized by using energy dispersive spectroscopy, transmission electron microscopy, and x-ray diffraction, indicating that we are able to prepare CoPt nanoparticles with <10 nm in diameter (after annealing) by employing this high temperature colloidal processing method. Magnetic in...

Nanoneedles of maghemite iron oxide prepared from a wet chemical route

Nanometer-sized maghemite iron oxide ({gamma}-Fe{sub 2}O{sub 3}) particles were produced first by synthesis of a precursor, {gamma}-FeO(OH), in a surfactant-less microemulsion and subsequent heat treatment of the {gamma}-FeO(OH). The precursors and {gamma}-Fe{sub 2}O{sub 3} powder were characterized using thermogravimetric analysis (TGA), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Susceptometer from Quantum Design (SQUID) measurements. TGA and XRD analysis indicated the formation of single cubic phase when the samples were heat-treated at 240 deg. C. TEM reveals that the {gamma}-Fe{sub 2}O{sub 3} particles are needle-shaped with an aspect ratio of {approx}20; typically 5-10 nm wide and over 150 nm long. It was found that these microemulsion-derived {gamma}-Fe{sub 2}O{sub 3} nanoneedles possess an intrinsic coercivity of 28 Oe at 300 K and 950 Oe at 2 K.

Self-assembly of FePt nanoparticles into nanorings

The application of nanoparticles as quantum dots in nanoelectronics demands their arrangement in ordered arrays. Shape controlled self-assembly is a challenge due to the difficulties of obtaining proper self-assembling parameters, such as solvent concentration, organic ligands, and nanoparticle size. In this article, hard magnetic FePt nanoparticles were synthesized using a combination approach of reduction and thermal decomposition. The nanoparticles are about 4.5 nm and appeared as truncated octahedral enclosed by the {100} and {111} crystal facets of fcc structure. The nanoparticles are of hexagonal close packing and orient randomly in the self-assembly nanoarrays. By diluting the solution for large-area self-assembly, monolayer, submonolayer, and multilayer nanorings of FePt nanoparticles were formed. The nanoring formation is determined by hydrodynamics, surface effects, and interaction between the FePt nanoparticles and substrates.

Magneto-optical spectra of closely spaced magnetite nanoparticles

The Faraday rotation spectrum of composites containing magnetite nanoparticles is found to be dependent on the interparticle spacing of the constituent nanoparticles. The composite materials are prepared by combining chemically synthesized Fe3O4 (magnetite) nanoparticles (8-nm diameter) and poly(methylmethacrylate). Composites are made containing a range of nanoparticle concentrations. The peak of the main spectral feature depends on nanoparticle concentration; this peak is observed to shift from approximately 470 nm for (dilute composites) to 540 nm (concentrated). We present a theory based on the discrete-dipole approximation which accounts for optical coupling between magnetite particles. Qualitative correlations between theoretical calculations and experimental data suggest that the shifts in spectral peak position depend on both interparticle distance and geometrical configuration.