  Two-dimensional materials are an emerging class of new materials with a wide
range of electrical and optical properties and potential applications.
Single-layer structures of semiconducting transition metal dichalcogenides are
gaining increasing attention for use in field-effect transistors. Here, we
report a photoluminescence switching effect based on single-layer WSe2
transistors. Dual gates are used to tune the photoluminescence intensity. In
particular, a side-gate is utilized to control the location of ions within a
solid polymer electrolyte to form an electric double layer at the interface of
electrolyte and WSe2 and induce a vertical electric field. Additionally, a
back-gate is used to apply a 2nd vertical electric field. An on-off ratio of
the light emission up to 90 was observed under constant pump light intensity.
In addition, a blue shift of the photoluminescence line up to 36 meV was
observed. We attribute this blue shift to the decrease of exciton binding
energy due to the change of nonlinear in-plane dielectric constant and use it
to determine the 3rd order off-diagonal susceptibility \c{hi}^((3)
)=3.50*10^(-19)m2/V2.
