  Controlling the threshold voltage (Vth) of a field-effect transistor is
important for realizing robust logic circuits. Here, we report a facile
approach to achieve bidirectional Vth tuning of molybdenum disulfide (MoS2)
field-effect transistors. By increasing and decreasing the amount of sulfur
vacancies in the MoS2 surface, the Vth of MoS2 transistors can be left- and
right-shifted, respectively. Transistors fabricated on perfect MoS2 flakes are
found to exhibit two-fold enhancement in mobility and a very positive Vth. More
importantly, our elegant hydrogen treatment is able to tune the large Vth to a
small value without any performance degradation simply by reducing the atomic
ratio of S:Mo slightly; in other words, creating a certain amount of sulfur
vacancies in the MoS2 surface, which generate defect states in the band gap of
MoS2 that mediate conduction of a MoS2 transistor in the subthreshold regime.
First-principles calculations further indicate that the edge and width of
defect band can be tuned according to the vacancy density. This work not only
demonstrates for the first time the ease in tuning the Vth of MoS2 transistors,
but also offers a process technology solution that is critical for further
development of MoS2 as a mainstream electronic material.
