  Low-resistance ohmic contacts are a challenge for electronic devices based on
two-dimensional materials. We show that an atomically precise junction between
a two-dimensional semiconductor and a metallic contact can lead to a
semiconductor-to-metal transition in the two-dimensional material--a finding
which points the way to a possible method of achieving low-resistance
junctions. Specifically, single-layer WS$_2$ undergoes a semiconductor-to-metal
transition when epitaxially grown on Ag(111), while it remains a direct band
gap semiconductor on Au(111). The metallicity of the single layer on Ag(111) is
established by lineshape analysis of core level photoemission spectra.
Angle-resolved photoemission spectroscopy locates the metallic states near the
Q point of the WS$_2$ Brillouin zone. Density functional theory calculations
show that the metallic states arise from hybridization between Ag bulk bands
and the local conduction band minimum of WS$_2$ near the Q point.
