Unravelling Work Function Contributions and Their Engineering in 2H-MoS₂ Single Crystal Discovered by Molecular Probe Interaction

In this study, a comprehensive investigation of the role of sulfur vacancies on the electronic structure and surface reactivity of molybdenum disulfide is presented. A 2H-MoS₂ single crystal was annealed at two different temperatures, namely, 300 and 500 °C in vacuum, in order to generate sulfur vacancies in a controlled manner. The detailed characterization of the electronic structure by means of X-ray and ultraviolet photoelectron spectroscopy clearly evidences the formation of a strong surface dipole as well as surface band bending due to the excess of negative charge on the Mo centers, as a consequence of the generated sulfur vacancies. After thermal treatment, a mercaptoundecylphosphonic acid molecule, which consists of a thiol (S–H) tail group and a phosphonic acid group on the other end, was covalently attached on the surface through wet chemical functionalization in order to refill the sulfur vacancies. As a consequence of the vacancy refilling, the surface band bending is reversed and the surface dipole is remarkably decreased, being close to the initial value of the pristine surface.