  The valley of transition metal dichalcogenides provides an additional
platform to manipulate spin due to its unique selection rule. Normally,
intralayer optical transitions in magnetic field show a Zeeman splitting with g
factor of about $-4$. Here we report remarkable valley Zeeman effect exhibited
by splitting of excitonic emission in a bilayer WS$_{2}$, with a value of g
factor as large as $-16.5$. The observed large g factor results from the
interlayer recombination, as the conduction band and valence band are modified
in opposite directions by magnetic field in different layers. The interlayer
recombination is due to the defect induced inversion symmetry breaking, which
is theoretically not accessible in ideal bilayer WS$_{2}$ with inversion
symmetry. Large g factor of interlayer emission offers potential benefits for
future optical spin control and detection.
