Crystals of contorted hexabenzocoronene (HBC), a semiconducting small molecule, form when the amorphous film is thermally annealed at 240 degrees Celsius. The resulting crystals assume various morphologies in films of different thickness.
This photograph captures a range of film thicknesses in one optical micrograph - from 0nm to 130nm thick - and shows how crystal morphology changes from rods in thin films to spherulites in thicker ones, with a secondary morphology that appears in the thickest films. The colors are the result of thin film interference effects. The underlying substrate is purple, and the thinnest portion of the film is blue, which then fades through the rainbow as the HBC film thickness grows. The thickness gradient that led to this photograph was created by a shadowing effect from a small magnet during thermal sublimation of HBC to form the film.