Fig. 1.

Crystal structure of undoped and Nb-doped bulk MoS₂. a Illustration of modification in stacking order polytypism of MoS₂ by Nb doping (top and side views). The dashed lines show the corresponding single unit cells of 2H and 3R structures. b Calculated energy difference of 3R phase with respect to 2H phase, with positive (negative) values indicating that the 2H (3R) phase is more stable. Bilayer MoS₂ system is considered for variable Nb doping concentrations (not to the scale). For the MoS₂:Nb, Ⓐ and Ⓑ are obtained with a 6 × 6 supercell by adding 1 and 2 Nb dopants, respectively. Type Ⓒ is calculated by substituting a Nb dopant into a 4 × 4 supercell. So, their Nb doping concentrations correspond to 1.4, 2.8 and 3.1%, respectively. c Convergent beam electron diffraction patterns obtained from the undoped and Nb-doped multilayer MoS₂ at [0001] incidence. Both bulk crystals display mirror symmetries, indicated by yellow and blue lines, for the six-fold (MoS₂, 2H) and three-fold (MoS₂:Nb, 3R), respectively. d Optical reflectance spectrum of bulk MoS₂ and MoS₂:Nb taken at 4.5 K. Two main peaks are assigned as A and B exciton transitions at the K point while the additional feature arising from H-point of the Brillouin zone is only observed in undoped MoS₂. The strong absorption below the A exciton in MoS₂:Nb is evidenced by the absence of the Fabry-Perot oscillation which appears in the Nb-free crystal