Crack networks developed in thin films of silica-based photonic crystals

Submicrometer-sized silica spheres were synthesized by hydrolyzing and condensing tetraethyl orthosilicate in ethyl alcohol. If the resultant silica spheres were sticky, densely distributed cracks were developed in the silica-based thin films when sticky silica spheres were cast onto silicon wafers and then dried in oven at about 40°C. If the resultant silica spheres were non-sticky, crack-free photonic crystals could be obtained when non-sticky silica spheres were cast onto glass substrates and then slowly dried in air. Characterized with the scanning electron microscopy and computer simulation, the characteristics of the cracks were analyzed and their fractal dimensions were calculated to be in the range of 2.4-2.7. Our results demonstrated that crack-free silica-based photonic crystals could be obtained by minimizing residual stresses in the thin films. The formation mechanisms of the cracks were discussed for the case of our silicabased thin films.