Although two-dimensional monolayer transition metal dichalcogenides reveal numerous unique features that are inaccessible in bulk materials, their intrinsic properties are often obscured by environmental effects. Among them, work function, which is the energy required to extract an electron from a material to vacuum, is one critical parameter in electronic/optoelectronic devices.

In this study, we systematically measure the electronic structure of monolayered MoS₂ – work function, energy band gap, conduction band and valence band structure by in-situ photoemission spectroscopy (PES), inverse photoemission spectroscopy (IPES), reflective electron energy loss spectroscopy (REELS) and Kelvin Probe (KP) under various ambient condition (air, ultra-high vacuum, oxygen and nitrogen gases). The energy band gap by REELS of monolayer MoS₂ on SiO₂ is 1.7 eV. It shows a increase as compared with the optical band gap of 1.2 eV of Bulk MoS₂ [1]. And also, the valence band offset and conduction band offset of mono layer MoS₂ are shifted higher binding energy side of 0.5 eV. A work function measured by in-situ KP of 4.04 eV in vacuum was converted to 4.47 eV with O₂ exposure, which is comparable with a large variation in graphene.

The homojunction diode by partially passivating a transistor reveals an ideal junction with an ideality factor of almost one and perfect electrical reversibility. The estimated depletion width obtained from photocurrent mapping was ~200 nm, which is much narrower than bulk semiconductors.

References

[1] Y. Zhang, Nature nanotech. 9 111 (2014).