  We present a method for synthesizing large area epitaxial single-layer
MoS$_2$ on the Au(111) surface in ultrahigh vacuum. Using scanning tunneling
microscopy and low energy electron diffraction, the evolution of the growth is
followed from nanoscale single-layer MoS$_2$ islands to a continuous MoS$_2$
layer. An exceptionally good control over the MoS$_2$ coverage is maintained
using an approach based on cycles of Mo evaporation and sulfurization to first
nucleate the MoS$_2$ nano-islands and then gradually increase their size.
During this growth process the native herringbone reconstruction of Au(111) is
lifted as shown by low energy electron diffraction measurements. Within these
MoS$_2$ islands, we identify domains rotated by 60$^{\circ}$ that lead to
atomically sharp line defects at domain boundaries. As the MoS$_2$ coverage
approaches the limit of a complete single-layer, the formation of bilayer
MoS$_2$ islands is initiated. Angle-resolved photoemission spectroscopy
measurements of both single and bilayer MoS$_2$ samples show a dramatic change
in their band structure around the center of the Brillouin zone. Brief exposure
to air after removing the MoS$_2$ layer from vacuum is not found to affect its
quality.
