An ab-initio study on two-dimensional semiconductor alloys: Monolayer Mo1-xCrxS2

Abstract

The formation energy, structure, electronic and magnetic properties of the monolayer (ML) Mo1-xCrxS2 alloy are calculated based on density functional theory calculations within generalized gradient approximation method. It is found that Cr doping in the ML MoS2 is energetically unfavorable. Its lattice constant exhibits a negative bowing coefficient of −0.022 Å. The Mo–S bond length and the average bond length of the ML Mo1-xCrxS2 show a decline trend while the composition dependence of the Cr–S bond length is not monotonic. It is also found that the ML Mo1-xCrxS2 has a direct bandgap at K for any Cr concentration and its bandgap energy monotonically decreases with increasing Cr concentration. When the Cr concentration is no more than 0.6, the bandgap narrowing is due to the upward shift of K VBM and the downward shift of K CBM. If the Cr concentration goes on increasing, the bandgap narrowing is merely due to the upward movement of the K VBM. Besides, the location of the K CBM for Mo1-xCrxS2 is determined by the coupling interaction between the S-3p and Cr-3d states in the conduction band while the K VBM is dominated by two coupling interactions in the valence band. One is the coupling interaction between the S-3p and Cr-3d states. The other is the coupling interaction between the S-3p and Mo-4d states. In order to study the bandgap energy of the ML Mo1-xCrxS2 alloy quantitatively, a physical model is developed. It is observed that the model can give a good estimation by setting EI,Cr−3d,MoS2=1.68 eV, Cp−d=0.55 eV and b1=0.69 eV. The result of the magnetic properties shows that the magnetism of the ML Mo1-xCrxS2 system is very weak.