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  Giangrisostomi, E.; Ovsyannikov, R.; Haverkamp, R.; Sorgenfrei, N.L.A.N.; Neppl, S.; Sezen, H.; Johansson, F.O.L.; Svensson, S.; Föhlisch, A.: Inhomogeneity of Cleaved Bulk MoS₂ and Compensation of Its Charge Imbalances by Room-Temperature Hydrogen Treatment. Advanced Materials Interfaces 10 (2023), p. 2300392/1-14

10.1002/admi.202300392
Open Access Version

Abstract:
Synthetic single crystals of bulk molybdenum disulphide cleaved in ultrahigh vacuum are mapped across a large (≈25 mm2) area by X-ray photoelectron spectroscopy, both statically and transiently following above-bandgap excitation by an ultrafast laser. This work finds that: I) A cleaved surface typically displays spatially inhomogeneous properties, manifested by large (≈1 eV) variations in binding energy and band bending and variable degrees of stability of those over time as a result of variable gas uptakes from the residual atmosphere. II) Moderate (350 °C) annealing and exposure to molecular hydrogen can be cycled to switch between smaller and larger surface band bending, the switch being reversible but strongly sample-position dependent. III) Upon exposure to atomic hydrogen, the binding energy of the entire surface levels out to a common (within <0.05 eV) value corresponding to a Fermi level pinned close to mid-bandgap. Such remarkable effect is attributed to the ability of hydrogen atoms to serve as donors and acceptors alike, thus neutralizing local charge imbalances inevitably present at the surface in consequence of intrinsic and/or cleavage-induced defects. With subsequent moderate annealing, the hydrogenated surface preserves a fairly homogenous electronic state which is however characterized by a lower binding energy and little to no band bending.