High-field magnetization curve of random-anisotropy amorphous magnet: Observation of a crossover and link to structural short-range order

Size-controlled aggregation of cube-shaped EuS nanocrystals with magneto-optic properties in solution phase

Size-controlled formation of colloidal aggregates composed of cube-shaped EuS nanocrystals, EuS NCs, in solution phase are reported and their optical and magneto-optical properties are studied. The average size of the colloidal particles of the EuS NCs-aggregates formed in 1-pentanol, 1-hexanol, and 1-octanol were ca. 800, 500, and 100 nm, respectively. Self-organized cubic-type superlattice structure was evaluated in the colloidal aggregates by means of the small-angle X-ray diffraction measurements, which is similar to those in the macroscopic 3D superlattice structures. The distances between NCs in the EuS NCs-aggregates are dependent on alkyl chain length of the solvent alcohol. Magneto-optic properties of EuS NCs-aggregates and the monomeric EuS nanocrystals in liquid media are characterized with magnetic circular dichroism spectra. The active wavelength of EuS NCs-aggregates is considerably longer than that of the monomeric EuS nanocrystals.

Supracrystals of inorganic nanocrystals: an open challenge for new physical properties

When naturally occurring spherical objects self-organize, the physical properties of the material change. For example, a colorless opal is the result of a disordered aggregate of silica particles. When the silica particles are ordered, however, the opal takes on color, which is determined by the size of the self-assembled particles. In this Account, we describe how these 3D arrangements of nanomaterials can self-organize in 3D arrays called supracrystals; the 3D arrays can fall into the familiar categories of face-centered cubic (fcc), hexagonal compact packing (hcp) crystals, and body-centered (bcc) crystals. The collective properties of these 2D and 3D arrangements are different from the properties of individual nanoparticles and from particles in bulk. Comparison between the approach to saturation of the magnetic curve for supracrystals and disordered aggregates produced from the same batch of nanocrystals is similar to that observed with films or nanoparticles, either highly crystallized or amorphous. We also demonstrate by two various processes and with two types of nanocrystals (silver and cobalt) that when nanocrystals are self-ordered in 3D superlattices, they exhibit a coherent breathing mode vibration of the supracrystal, analogous to a breathing mode vibration of atoms in a nanocrystal. Furthermore, we used 10 nm gamma-Fe(2)O(3) nanocrystals to gain new insight into the scaling law of crack patterns. We found that isotropic and directional crack patterns follow the same universal scaling law over a film height varying by 3 orders of magnitude. These data have led us to propose general analogies between supracrystals of nanocrystals, individual nanocrystals, and the molecules in the bulk phase for certain physical properties based on the ordering of the material. As we continue to study the physical properties of the ordered and disordered arrangements of nanomaterials, we will be able to go further in these analogies. And this exploration leads to new questions: first and foremost, is this behavior general?