Controlling the electronic and optical properties of HfS mono-layers lanthanide substitutional doping: a DFT+ study.

Two dimensional HfS is a material with potential applications in the field of photo-catalysis and advanced solid state devices. Density functional theory with the Hubbard parameter (DFT+) calculations were carried out to investigate the structural, electronic and optical properties of lanthanide dopant atoms (LN = La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu) in the HfS mono-layer. The calculated electronic band gap for a pristine HfS mono-layer is 1.30 eV with a non-magnetic ground state. The dopant substitutional energies under both Hf-rich and S-rich conditions were evaluated, with the S-rich condition for the dopant atoms being negative. This implies that the incorporation of these LN dopant atoms in the HfS is feasible and experimental realization possible. The introduction of LN dopant atoms in the HfS mono-layer resulted in a significant change of the material properties. We found that the presence of LN dopant atoms in the HfS mono-layer significantly alters its electronic ground states by introducing defect states as well as changes in the overall density of states profile resulting in a metallic ground state for the doped mono-layers. The doped mono-layers are all magnetic with the exception of La and Lu dopant atoms. We found that LN dopant atoms in the HfS mono-layer influence the absorption and reflectivity spectra with the introduction of states in the lower frequency range (<1.30 eV). Furthermore, we showed that the applicability of doped HfS mono-layers as photo-catalysts is very different compared with the pristine HfS mono-layer.