  Doping is an effective way to modify the electronic property of
two-dimensional (2D) materials and endow them with new functionalities.
However, wide-range control of the substitutional doping concentration with
large scale uniformity remains challenging in 2D materials. Here we report
in-situ chemical vapor deposition growth of vanadium (V) doped monolayer
molybdenum disulfide (MoS2) with widely tunable doping concentrations ranging
from 0.3 to 13.1 at%. The key to regulate the doping concentration lies in the
use of appropriate V precursors with different doping abilities, which also
generate a large-scale uniform doping effect. Artificial synaptic transistors
were fabricated by using the heavily doped MoS2 as the channel material for the
first time. Synaptic potentiation, depression and repetitive learning processes
are mimicked by the gate-tunable channel conductance change in such transistors
with abundant V atoms to trap/detrap electrons. This work shows a feasible
method to dope monolayer 2D semiconductors and demonstrates their use in
artificial synaptic transistors.
