General microscopy characterization

Microscopy characterization

image of a cleft edge of an artificial opal. A triangular arrangement (fcc {111} set of
planes) is observed on the lower part of the image. Square ({100}), rectangular ({110})
and triangular ({111}) orderings appear on the upper part of the image.

It is well know (1), that hard spheres can
equally be close packed into a face centred cubic (fcc), a hexagonal close packed (hcp) or
any mixture thereof. From gravitational energy considerations, all of them are equally
probable. This fact is of crucial importance in PBG technology, since the fcc structure
has been shown to present a full gap, provided an appropriate refractive index contrast is
achieved. In this sense, a sophisticated method was devised in order to force the
colloidal crystallisation in the fcc phase. Only recently, it has been shown both
theoretically, by using computer simulations (2), and experimentally (3) that fcc order is
the most stable phase. In Fig. 1 one can see a low magnification SEM image of internal
facets (observable after cleavage) of an artificial opal made by natural sedimentation as
described above. The triangular order corresponding to a crystalline plane belonging to
the {111} family (lower part of the image) can be seen. Also, square, rectangular and
triangular arrangements of spheres, belonging to {100}, {110} and {111} sets of planes,
can be seen at the cleft edge (upper part of the image).

FIGURE 2. Computer simulated
image of an fcc order (centre image) along with 4 different SEM images of different
crystalline facets. Upper left: external (111), upper right: internal (111), lower left
(110) and lower right (001).

In Fig. 2 one can see a computer
simulation of an fcc order (central image) surrounded by SEM images of actual
crystallographic planes. All these planes can only be present in an fcc stacking thus
demonstrating the statement. The results reported, so far, involve opals in which the
interparticle voids are filled with air. We call those samples "bare opals".
Bare as-grown opals are solid but present a low mechanical stability. To improve it, a
sintering process can be performed.

    Want to see more pictures?

    1.- C. Kittel. 1968. 'Introduction to
    Solid State Physics', Third Edition, John Wiley.

    2.- L.V. Woodcock. 1997. Nature 385,

    3.- H. Míguez, F. Meseguer C. López, A.
    Mifsud, J. S. Moya, and L. Vázquez. 1997. Langmuir. 13, 6009.