  Modulating electronic structure of monolayer transition metal dichalcogenides
(TMDCs) is important for many applications and doping is an effective way
towards this goal, yet is challenging to control. Here we report the in-situ
substitutional doping of niobium (Nb) into TMDCs with tunable concentrations
during chemical vapour deposition. Taking monolayer WS2 as an example, doping
Nb into its lattice leads to bandgap changes in the range 1.98 eV to 1.65 eV.
Noteworthy, electrical transport measurements and density functional theory
calculations show that the 4d electron orbitals of the Nb dopants contribute to
the density of states of Nb-doped WS2 around the Fermi level, resulting in an n
to p-type conversion. Nb-doping also reduces the energy barrier of hydrogen
absorption in WS2, leading to an improved electrocatalytic hydrogen evolution
performance. These results highlight the effectiveness of controlled doping in
modulating the electronic structure of TMDCs and their use in electronic
related applications.
