ABSTRACT
New materials made of two- and three-dimensional superlattices of nanoparticles exhibit unique collective properties arising from the ordering of the nanoparticles. Here, dodecanethiol-coated silver nanocrystals self-assembled in 2D were subjected to oxygen plasma using the reactive ion etching process. The careful investigation by transmission electronic microscopy (TEM) of the same areas before and after exposure allows one to distinguish two behaviors under plasma treatment, which are determined by the level of ordering of the 2D organizations. Higher ordered self-assemblies remain unchanged, while less ordered organizations coalesce into larger nanocrystals with spheroidal shapes that could be single crystals. Electron energy-loss spectroscopy (EELS) measurements show that there is no oxidation of the silver either on the large coalesced nanocrystals or on the stable self-assemblies. A new intrinsic property, immunity of highly self-ordered nanocrystals provided by the ordering, is reported, and a mechanism of the coalescence is proposed.
ABSTRACT
Silver nanodisks, having two different sizes, and spherical particles are synthesized by soft chemistry. By using centrifugation, nanodisks are mainly selected. The experimental absorption spectra of these nanodisks with different sizes are compared to those simulated using the discrete dipole approximation method. For small nanodisk sizes, the nanodisk shape is neglected and the simulated spectra closest to the experiments are obtained by assuming a spheroidal particle. Conversely, for larger nanodisks, the precise geometries represented by snip and aspect ratio are needed for good agreement between experiments and simulations.
ABSTRACT
Solid mesostructures made of cylinders are produced by the slow evaporation of cobalt nanocrystals dispersed in hexane and subjected to an applied field perpendicular to the substrate. Varying the initial nanocrystal concentration is found to be an efficient method for changing the pattern size. The experimental structures and the theoretical predictions based on the minimization of the total free energy are in good agreement. A comparison of experiment with theory allowed us to conclude that the mesostructures form as a result of a liquid-gas phase transition during the evaporation process. Within the theoretical model and the experimental data, it is concluded that the phase ratio of the magnetic to the total volume and the height of the cylinders govern the pattern geometry. In contrast, because of the saturation of the magnetization curve, the mesostructures are not influenced by the field strength.
ABSTRACT
Self-organization of nanocrystals depends on the type of nanomaterial and the coating. With silver nanocrystals, the self-organization is partially perturbed by the latter. With cobalt nanocrystals, the size distribution and thus the self-organization is controlled by the amount of reducing agent added during the chemical reaction and not by the micellar solution. It is possible to make "supra" crystals of cobalt nanoparticles in a face centered cubic (fcc) structure. By applying, during the evaporation process, an external magnetic field to ferrite nanocrystals dispersed in solution, nanocrystal organizations markedly change with their coating. Hence tubes are obtained with nanocrystals coated with citrate ions whereas thick films are produced when the coating is replaced by dodecanoic acid. Collective magnetic properties due to the organization in tubes are observed with a behavior similar to that observed with nanowires. The use of nanocrystals as a mask to produce various patterns on silicon is described.
ABSTRACT
When a thin layer of magnetic fluid confined with an immiscible nonmagnetic liquid is subjected to a perpendicular field, the formation of hexagonal and labyrinthine patterns is observed experimentally. To develop a coherent theoretical description of this phenomenon, the free energy functionals of both types of magnetic structures are derived. Both energy functionals have the same form, which explains that the theoretical results found in this paper for hexagonal and labyrinthlike striped patterns are analogous. The size of the patterns is determined by minimizing the free energy. The influence of the method for computing the magnetic energy on the theoretical results is studied. An accurate computation of the magnetic energy proves important in predicting the experimental pattern size as a function of external field and of layer height. How the results change, when a constant magnetization is assumed during the pattern formation is also investigated. The transition between hexagonal and striped structures is studied by a comparison of their free energies. The ratio of the magnetic to the nonmagnetic liquid is found to be an important factor for the relative stability of the patterns. In agreement with experiments, striped structures are observed at large phase ratios, whereas at small phase ratios hexagonal patterns predominate.
