Phase Stability and Raman/IR Signatures of Ni-Doped MoS2 from Density Functional Theory Studies

Ni-doped MoS2 has useful tribological, optoelectronic, and catalytic properties. Experiment and theory on doped MoS2 have focused on monolayers or finite particles: theoretical studies of bulk Ni-doped MoS2 are lacking and the mechanisms by which Ni alters bulk properties are largely unsettled. We use density functional theory calculations to determine the structure, mechanical properties, electronic properties, and formation energies of bulk Ni-doped 2H-MoS2 as a function of doping concentration. We find four metastable structures: Mo or S substitution and tetrahedral (t-) or octahedral (o-) intercalation. We compute phase diagrams as a function of chemical potential to guide experimental synthesis. Convex hull analysis shows that t-intercalation (favored over o-intercalation, with doping formation energy ∼10 meV per Ni) is stable against phase segregation and other compounds containing Ni, Mo, and S. Intercalation forms strong interlayer covalent bonds and does not increase the c-parameter. Ni-doping creates new states in the electronic density of states in MoS2 and shifts the Fermi level. We calculate infrared and Raman spectra and find new peaks and shifts in existing peaks that are unique to each dopant site, and therefore may be used to identify the site experimentally, which has thus far been challenging.