Controlling the electronic and optical properties of HfS(2) mono-layers via lanthanide substitutional doping: a DFT+U study

Two dimensional HfS(2) is a material with potential applications in the field of photo-catalysis and advanced solid state devices. Density functional theory with the Hubbard U parameter (DFT+U) 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(2) mono-layer. The calculated electronic band gap for a pristine HfS(2) 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(2) is feasible and experimental realization possible. The introduction of LN dopant atoms in the HfS(2) mono-layer resulted in a significant change of the material properties. We found that the presence of LN dopant atoms in the HfS(2) 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(2) 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(2) mono-layers as photo-catalysts is very different compared with the pristine HfS(2) mono-layer.