Role of Defects in Evaporated Silver Films on the Nucleation of Sulfide ``Patches''

Abstract

The role of defects in the formation of silver sulfide tarnish patches on silver was studied by transmission electron microscopy. The patches were formed on thin (001) single‐crystal films of pure silver at room temperature by exposure to the atmosphere. The observations showed that (1) stacking faults (2) partial dislocations and (3) hole edges were the main sites for nucleation. The patches were roughly square in shape and were of two sorts, one with sides parallel to 〈110〉 Ag directions nucleating at type (1) and (2) sites, the second with sides parallel to 〈100〉 Ag directions nucleating at type (3) sites. Both sorts of patches were probably single crystal oriented overgrowths. Their orientations could not be identified. The patch density was typically about 3×10⁹ cm⁻². Positive identification of the patches as monoclinic beta Ag₂S was only possible in a duplicate experiment in which randomly nucleated patches of irregular shape were obtained. These patches were highly oriented, predominantly with (001) β‐Ag₂S parallel to (001) Ag.