Two-dimensional electronic transport and surface electron accumulation in MoS₂

Because the surface-to-volume ratio of quasi-two-dimensional materials is extremely high, understanding their surface characteristics is crucial for practically controlling their intrinsic properties and fabricating p-type and n-type layered semiconductors. Van der Waals crystals are expected to have an inert surface because of the absence of dangling bonds. However, here we show that the surface of high-quality synthesized molybdenum disulfide (MoS₂) is a major n-doping source. The surface electron concentration of MoS₂ is nearly four orders of magnitude higher than that of its inner bulk. Substantial thickness-dependent conductivity in MoS₂ nanoflakes was observed. The transfer length method suggested the current transport in MoS₂ following a two-dimensional behavior rather than the conventional three-dimensional mode. Scanning tunneling microscopy and angle-resolved photoemission spectroscopy measurements confirmed the presence of surface electron accumulation in this layered material. Notably, the in situ-cleaved surface exhibited a nearly intrinsic state without electron accumulation.