  We report the first observation of gate-controlled field emission current
from a tungsten diselenide (WSe2) monolayer, synthesized by chemical-vapour
deposition on SiO2/Si substrate. Ni contacted WSe2 monolayer back-gated
transistors, under high vacuum, exhibit n-type conduction and drain-bias
dependent transfer characteristics, which are attributed to oxygen/water
desorption and drain induced Schottky barrier lowering, respectively. The
gate-tuned n-type conduction enables field emission, i.e. the extraction of
electrons by quantum tunnelling, even from the flat part of the WSe2
monolayers. Electron emission occurs under an electric field ~100 V {\mu}m^(-1)
and exhibit good time stability. Remarkably, the field emission current can be
modulated by the back-gate voltage. The first field-emission vertical
transistor based on WSe2 monolayer is thus demonstrated and can pave the way to
further optimize new WSe2 based devices for use in vacuum electronics.
