  Opals are naturally occurring photonic crystals which can be formed easily
using low-cost self-assembly methods. While the optical behaviour of opals has
received significant attention over the last number of decades, there is
limited information on the effect of crystal thickness on the optical
properties they display. Here, the relationship between volume fraction and
crystal thickness is established with an evaporation-induced self-assembly
(EISA) method of formation. The extent to which thickness can be used to
manipulate the optical properties of the crystals is explored, focusing on the
change in the photonic band gap (PBG). Microscopical structural
characterization and angle-resolved transmission spectroscopy are used to
examine the quality of the photonic crystals formed using different volume
fractions of polystyrene spheres, with thicknesses up to 37 layers grown from
volume fractions of 0.125%. This work provides a direct correlation between
sphere solution volume fraction and crystal thickness, and the associated
optical fingerprint of opal photonic crystals. Maximum thickness is examined,
which is shown to converge to a narrow range over several evaporation rates. We
identify the criteria required to achieve thickness control in relatively fast
evaporation induced self-assembly while maintaining structural quality, and the
change to the spectroscopic signature to the (111) stopband and higher order
(220) reflections, under conditions where a less ordered photonic crystals are
formed.
