  We present an ultra-high vacuum scanning tunneling microscopy (STM) study of
structural defects in molybdenum disulfide thin films grown on silicon
substrates by chemical vapor deposition. A distinctive type of grain boundary
periodically arranged inside an isolated triangular domain, along with other
inter-domain grain boundaries of various types, is observed. These periodic
defects, about 50 nm apart and a few nanometers in width, remain hidden in
optical or low-resolution microscopy studies. We report a complex growth
mechanism that produces 2D nucleation and spiral growth features that can
explain the topography in our films.
