RESUMEN
Order within sub-monolayers of nanoparticles, fabricated by electrophoretic deposition, was assessed during nanoparticle deposition in a liquid suspension and after the films had dried by grazing-incidence small-angle X-ray scattering. Experiments were performed in a custom-made, liquid-phase cell. The results indicated that ordering occurred during the drying event.
Asunto(s)
Compuestos Férricos/química , Nanopartículas/química , Electroforesis , Dispersión de Radiación , Rayos XRESUMEN
The degree of order within nanoparticle monolayers deposited through electrophoretic deposition on lithographically patterned and unpatterned substrates was analyzed using four complementary measures of order: Voronoi-cell edge-fraction entropy, local bond-orientation order parameter, translational order parameter, and anisotropy order parameter. From these measures of order, we determined that the pattern had an influence on some aspects of the ordering within the nanoparticle monolayer but had no effect on others. The Voronoi-cell edge-fraction entropy did not measurably change due to the pattern, indicating that the pattern has no effect on the number of defects present. The translational order parameter also had no change due to the pattern. The local bond-orientation order parameter had a measurable change, indicating the pattern increased the bond ordering slightly. Also, the anisotropy order parameter developed herein indicated an increase in order. The direction of the increased order corresponded with the direction of the anisotropy designed on the patterned substrate, strongly suggesting that the pattern drives the particles to become more ordered.
Asunto(s)
Técnicas Electroquímicas/métodos , Nanopartículas/química , Nanotecnología/métodos , Anisotropía , EntropíaRESUMEN
Monolayers of iron oxide nanoparticles of two different sizes, 9.6 nm and 16.5 nm, were fabricated through electrophoretic deposition. The arrangements of nanoparticles within the films were analyzed using the technique of Voronoi tessellations. These analyses indicated that the films possessed equivalent degrees of ordering, and that the films were uniform over centimeter length scales. Precise measurements of the interparticle spacing were obtained, and the magnitudes of magnetic dipole interactions were calculated. The dipole-dipole interaction among the larger nanoparticles was 14 times larger than that of the smaller nanoparticles, indicating that magnetic coupling interactions could not have been the lone source of ordering in the system.
RESUMEN
The controlled electrophoretic deposition of monolayers and ultrathin films of 4.0 nm TiO(2) nanocrystals from stable, nonpolar solvent-based suspensions is reported. Stable suspensions were prepared in hexane, and the electrophoretic mobility of the nanocrystals was enhanced by a combination of a liquid-liquid extraction followed by mechanical surfactant removal by high-speed centrifugation. The controlled evolution of the density of TiO(2) nanocrystal monolayers was studied by transmission electron microscopy and optical transmittance spectroscopy. Ultrathin films were assembled while maintaining monolayer-by-monolayer growth and uniform density of the film. A time-dependent, equivalent circuit model has been proposed to characterize the electrophoretic current that was recorded during our experiments. Further, we demonstrate that the proposed model, coupled with the mobility, provides a means to estimate the deposition rate and, hence, the time necessary to fabricate a submonolayer, a monolayer, and multilayers of nanocrystals.
RESUMEN
Iron-oxide nanoparticle monolayers and multilayers were assembled using dc electrophoretic deposition. The rate of deposition and the total particle deposition were controlled by varying the concentration of nanoparticles and the deposition time, respectively. Using scanning electron microscopy, we performed a time-resolved study that demonstrated the growth of the monolayer from a single isolated nanoparticle to a nearly complete layer. We observed tight, hexagonal packing of the nanoparticles indicating strong particle-particle interaction. Multilayer growth was assessed using scanning electron microscopy and atomic force microscopy, revealing a monolayer-by-monolayer growth process.
Asunto(s)
Compuestos Férricos/química , Nanopartículas del Metal/química , Electrodos , Galvanoplastia , Microscopía de Fuerza AtómicaRESUMEN
This report describes methods to produce large-area films of graphene oxide from aqueous suspensions using electrophoretic deposition. By selecting the appropriate suspension pH and deposition voltage, films of the negatively charged graphene oxide sheets can be produced with either a smooth "rug" microstructure on the anode or a porous "brick" microstructure on the cathode. Cathodic deposition occurs in the low pH suspension with the application of a relatively high voltage, which facilitates a gradual change in the colloids' charge from negative to positive as they adsorb protons released by the electrolysis of water. The shift in the colloids' charge also gives rise to the brick microstructure, as the concurrent decrease in electrostatic repulsion between graphene oxide sheets results in the formation of multilayered aggregates (the "bricks"). Measurements of water contact angle revealed the brick films (79°) to be more hydrophobic than the rug films (41°), a difference we attribute primarily to the distinct microstructures. Finally, we describe a sacrificial layer technique to make these graphene oxide films free-standing, which would enable them to be placed on arbitrary substrates.