Atomic force microscopy studies for investigating the smectic structures of colloidal crystals of β-FeOOH

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 th...