  Single-layer transition metal dichalcogenide (TMD) WSe2 has recently
attracted a lot of attention because it is a 2D semiconductor with a direct
band-gap. Due to low doping levels it is intrinsic and shows ambipolar
transport. This opens up the possibility to realize devices with the Fermi
level located in valence band, where the spin/valley coupling is strong and
leads to new and interesting physics. As a consequence of its intrinsically low
doping, large Schottky barriers form between WSe2 and metal contacts, which
impede the injection of charges at low temperatures. Here, we report on the
study of single-layer WSe2 transistors with a polymer electrolyte gate
(PEO:LiClO4). Polymer electrolytes allow the charge carrier densities to be
modulated to very high values, allowing the observation of both the electron-
and the hole-doped regimes. Moreover, our ohmic contacts formed at low
temperatures allow us to study the temperature dependence of electron and hole
mobilities. At high electron densities, a re-entrant insulating regime is also
observed, a feature which is absent at high hole densities.
