Why do the edges of a thin film appear dark when illuminated by monochromatic light?
Answers
there may be several processing reasons, such as nonuniform deposition of your thin film, for example. The physical reason for bleaching is diffraction and interference - the thin film acts as a “photon trapper”, and slight changes in thickness are especially pronounced and can fringe about the edges of your sample. If you deposit thin films using plasma, you may be a victim of potential differences across the sample, thus messing up your uniformity. If you are using a thermal evaporator, check if your substrate is perpendicular and doesn't wave (Google silicon wafer waviness for reference).
Basically, thin film dep is an art and Ive personally seen the effect you mention not only there, but also ie. In printed thin film samples. It always boils down to process control, substrate prep and properly calibrated growth sensors.