  Surface functionalization with organic electron donors (OEDs) is an effective
doping strategy for two-dimensional (2D) materials, which can achieve doping
levels beyond those possible with conventional electric field gating. While the
effectiveness of surface functionalization has been demonstrated in many 2D
systems, the doping efficiencies of OEDs have largely been unmeasured, which is
in stark contrast to their precision syntheses and tailored redox potentials.
Here, using monolayer MoS2 as a model system and an organic reductant based on
4,4-bipyridine (DMAP-OED) as a strong organic dopant, we establish that the
doping efficiency of DMAP-OED to MoS2 is in the range of 0.63 to 1.26 electrons
per molecule. We also achieve the highest doping level to date in monolayer
MoS2 by surface functionalization and demonstrate that DMAP-OED is a stronger
dopant than benzyl viologen, which was the previous best OED dopant. The
measured range of the doping efficiency is in good agreement with the values
predicted from first-principles calculations. Our work provides a basis for the
rational design of OEDs for high-level doping of 2D materials.
