A DFT study of the structural and electronic properties of single and double acceptor dopants in MX2 monolayers (2401.01251v1)

Abstract: Density functional theory calculations are used to systematically investigate the structural and electronic properties of MX$2$ transition metal dichalcogenide monolayers with M = Cr, Mo, W and X = S, Se, Te that are doped with single (V, Nb, Ta) and double (Ti, Zr, Hf) acceptor dopants on the M site with local $D{3h}$ symmetry in the dilute limit. Three impurity levels that arise from intervalley scattering are found above the valence band maxima (VBM): an orbitally doubly degenerate $e'$ level bound to the $K/K'$ VBM and a singly degenerate $a'1$ level bound to the $\Gamma$-point VBM. Replacing S with Se or Te lowers the $\Gamma$ point VBM substantially with respect to the $K/K'$ VBM bringing the $a'_1$ level down with it. The relative positions of the impurity levels that determine the different structural and electronic properties of the impurities in $p$-doped MX$_2$ monolayers can thus be tuned by replacing S with Se or Te. Single acceptors introduce a magnetic moment of 1$\, \mu{\rm B}$ in all MX$_2$ monolayers. Out-of-plane magnetic anisotropy energies as large as 10 meV/dopant atom are found thereby satisfying an essential condition for long-range ferromagnetic ordering in two dimensions. For double acceptors in MS$_2$ monolayers, both holes occupy the high-lying $a'_1$ level with opposite spins so there is no magnetic moment; in MSe$_2$ and MTe$_2$ monolayers the holes occupy the $e'$ level, a Jahn-Teller (JT) distortion wins the competition with exchange splitting resulting in the quenching of the magnetic moments. Even when the JT distortion is disallowed, magnetic double acceptors have a large in-plane magnetic anisotropy energy that is incompatible with long-range magnetic ordering in two dimensions. ....