  Semiconducting 2D crystals are currently receiving significant attention due
to their great potential to be an ultra-thin body for efficient electrostatic
modulation which enables to overcome the limitations of silicon technology.
Here we report that, as a key building block for 2D semiconductor devices,
vertical p-n junctions are fabricated in ultrathin MoS2 by introducing AuCl3
and benzyl viologen dopants. Unlike usual unipolar MoS2, the MoS2 p-n junctions
show (i) ambipolar carrier transport, (ii) current rectification via modulation
of potential barrier in films thicker than 8 nm, and (iii) reversed current
rectification via tunneling in films thinner than 8 nm. The ultimate thinness
of the vertical p-n homogeneous junctions in MoS2 is experimentally found to be
3 nm, and the chemical doping depth is found to be 1.5 nm. The ultrathin MoS2
p-n junctions present a significant potential of the 2D crystals for flexible,
transparent, high-efficiency electronic and optoelectronic applications.
