Comparative KPFM measurements on doped monolayer MoS₂

Tuning the electrical properties of monolayer MoS₂ through doping allows better control and flexibility over future device design. Here we present electrical measurements on sulfur-rich and oxygen-rich MoS₂ flakes using kelvin probe force microscopy (KPFM) and a field effect transistor sample geometry, allowing us to resolve spatial variations. Preliminary KPFM measurements show that oxygen-rich MoS₂ has a larger work function than sulfur-rich MoS₂ and that the work function of monolayer MoS₂ differs from that of multilayer MoS₂. The trend in work functions between differently doped MoS₂ flakes observed via KPFM is consistent with the trend in threshold voltages extracted from bulk current-voltage measurements on the flakes as well as the trend in work functions calculated via density functional theory. The sensitivity of the shift in work function of the MoS₂ flakes from an applied gate voltage suggests that local density of states measurements can be extracted with KPFM to experimentally examine the energy levels of the differently doped monolayer MoS₂ flakes.