Work function variation of MoS₂ atomic layers grown with chemical vapor deposition: The effects of thickness and the adsorption of water/oxygen molecules

The electrical properties of two-dimensional atomic sheets exhibit remarkable dependences on layer thickness and surface chemistry. Here, we investigated the variation of the work function properties of MoS₂ films prepared with chemical vapor deposition (CVD) on SiO₂ substrates with the number of film layers. Wafer-scale CVD MoS₂ films with 2, 4, and 12 layers were fabricated on SiO₂, and their properties were evaluated by using Raman and photoluminescence spectroscopies. In accordance with our X-ray photoelectron spectroscopy results, our Kelvin probe force microscopy investigation found that the surface potential of the MoS₂ films increases by ∼0.15V when the number of layers is increased from 2 to 12. Photoemission spectroscopy (PES) with in-situ annealing under ultra high vacuum conditions was used to directly demonstrate that this work function shift is associated with the screening effects of oxygen or water molecules adsorbed on the film surface. After annealing, it was found with PES that the surface potential decreases by ∼0.2V upon the removal of the adsorbed layers, which confirms that adsorbed species have a role in the variation in the work function.