  Two-dimensional (2D) transition metal dichalcogenides (TMDCs) with unique
electrical properties are fascinating materials used for future electronics.
However, the strong Fermi level pinning effect at the interface of TMDCs and
metal electrodes always leads to high contact resistance, which seriously
hinders their application in 2D electronics. One effective way to overcome this
is to use metallic TMDCs or transferred metal electrodes as van der Waals (vdW)
contacts. Alternatively, using highly conductive doped TMDCs will have a
profound impact on the contact engineering of 2D electronics. Here, a novel
chemical vapor deposition using mixed molten salts is established for
vapor-liquid-solid growth of high-quality rhenium (Re) and vanadium (V)-doped
TMDC monolayers with high controllability and reproducibility. A tunable
semiconductor to metal transition is observed in the Re and V-doped TMDCs.
Electrical conductivity increases up to a factor of 108 in the degenerate
V-doped WS2 and WSe2. Using V-doped WSe2 as vdW contact, the on-state current
and on/off ratio of WSe2-based field-effect transistors have been substantially
improved (from ~10-8 to 10-5 A; ~104 to 108), compared to metal contacts.
Future studies on lateral contacts and interconnects using doped TMDCs will
pave the way for 2D integrated circuits and flexible electronics.
