Global minimum structures and electronic properties variation of hydrogen adatoms on/in H-phase MX2 (M = Mo, W; X = S, Se, Te)

Although hydrogen is almost always present in the growth environments of two-dimensional (2D) transition metal dichalcogenides (TMDs), there is a lack of global structure search of hydrogen atoms on different TMDs as well as the corresponding electronic properties variation and the underlying mechanisms for the different electronic properties. A hydrogen atom in/on monolayer (ML) H-phase MX2 (M = Mo, W; X = S, Se, Te) can be an interstitial atom in MX2 (HInt) or as an adatom on MX2 either vertically on (HVert) or tilted above (HTilt) an X atom. In this paper, a global scanning of adsorption structures of hydrogen atom on/in MX2's shows that the most stable structure with MoS2 and WS2 is the HTilt structure due to the repulsion between the lone-pair electrons of sulfur atoms and the hydrogen-sulfur bonding electrons, while with MoSe2, WSe2, and MoTe2, the HInt structure becomes favored due to the larger lattice constant of these TMDs. Both HTilt and HInt structures introduce fully spin-polarized levels close to the conduction band but have different origins of spin polarization of M atoms, with M atoms gaining and losing electrons in HTilt and HInt structures, respectively. The special case is hydrogen in MoTe2, which does not exhibit spin polarization due to electron delocalization. Moreover, diffusion barrier analyses for hydrogen adatoms with MoS2 indicate that the transition from HTilt to HInt is a relatively slow process at room temperature. In this paper, we broaden the understanding of TMDs with hydrogen atoms and clarify several interaction mechanisms of hydrogen with different MX2's. These insights are of vital importance for the various applications of TMDs such as in (opto)electronics, spintronics, and catalysis.