Atomic Force Microscopy Studies for Investigating the Smectic Structures of Colloidal Crystals of β-FeOOH

Abstract

Smectic, or multilayer, structures in dry sol sediments of β-FeOOH (ferric oxyhydroxide) have been investigated using an atomic force microscope (AFM). The AFM technique has provided three-dimensional information of the smectic structures in a nondestructive manner. The shading in the AFM images of the dry sol surfaces evidently shows that in the iridescent (red, orange, yellow-green, green, and blue) regions, the β-FeOOH crystals are standing upright at a tilt (with respect to the plane of the smectic layer) and form approximately a square lattice. In contrast, in the noniridescent (gray and brown) regions, the crystals lie parallel or randomly oriented to each other. By determining the tilt angle of the crystals in the layer and the refractive index of the layer, we have found the thickness of the layer to be close to the wavelength of visible light in the layer. This supports an earlier periodic-spacing model that attempts to explain the origin of the iridescent colors. The AFM images also show that the individual crystal surface is homogeneous crystallographically, thus disputing an earlier bundle-structure model of individual β-FeOOH crystals. Furthermore, the AFM images show that the crystals have stepped surfaces, tapered ends, and either convex or concave ends, thus indicating nonuniform crystal growth. The dry sol surfaces of β-FeOOH have been further investigated with a scanning electron microscope (SEM) on a larger scale than is used with AFM. In the SEM images, we have found the following: (1) The in-layer structure of the smectics is, at least locally, square. Although defects destroy the two-dimensional crystalline order, a longer-ranged bond-orientational order still exists. (2) The smectic structures exhibit bending of the directors (which is usually observed in nematics) and overall layer undulation. We have also observed the coexistence of several smectic domains, each of which has a distinct orientation, and mono- and several-folded layers confined in the sols. The smectic structures of dry β-FeOOH sols have been found to be obtained not only from suspensions with attractive-force interparticle interactions but also from those with repulsive-force interactions.

Drying Patterns of Dispersions and Solutions

“…133 Maeda studied the crystal structure of collagen. 138 121 These results support the finding that both the interpolymer and polymersubstrate attractive interactions are strong for COL31. It should be recalled here that the broad ring at the outside edge disappeared and the very small inner broad rings formed instead for NaHLA.…”

Millimeter‐Scale Assembly of CdSe Nanorods into Smectic Superstructures by Solvent Drying Kinetics

“…Capillary solvent flow is known to carry a significant portion, or in many cases all, of the material contained in a drying droplet to the coffee ring. [25][26][27][28][29][30][31] We observed in all experiments that a significant number of NRs occupy the regions between the coffee ring and the center of the droplet. Though the extent of smectic superstructure formation is reduced in these regions, we do observe a variety of revealing assembly trends.…”

“…[25,26] This behavior results in a so-called coffee-ring region and has been demonstrated for many materials and at a range of size scales. [27][28][29][30][31] However, most cases have involved micrometer-scale and larger particles. In the nanometer-scale regime, examples of capillary-flow-mediated assembly include disordered Ag nanoparticle arrays, [32] aligned nanotubes [33,34] and diblock copolymers, [35] and zigzag DNA assemblies.…”

Controlled organization of ZnO building blocks into complex nanostructures

“…In addition to the control of size and shape, the controlled organization of the single nanocrystals that act as building blocks into complex nanostructures represents another challenge for nanomaterials research, which should be given extra attention. Different 1-, 2-, and 3-dimensional nanostructures have been fabricated by several self-assembly processes based on different driving mechanisms [6][7][8][9][10][11][12]. However, compared with this great success in the spatial orientation of nanocrystals, little attention has been devoted to the controlled organization of primary building units into curved nanostructures.…”