Thickness-Dependent Enhancement of Electronic Mobility of MoS₂ Transistors via Surface Functionalization

The thickness dependence of the chemical and physical properties is one fundamental characteristic shared by many two-dimensional layered transition-metal dichalcogenides, including molybdenum disulfide (MoS₂). Recently, in order to expand the scope of applications of MoS₂, surface functionalization has been employed to engineer its chemical and electrical properties for the purposes of drug delivery, photothermal therapy, gas sensing, and biosensing. Here, we report a facile method for controlled functionalization of MoS₂ field-effect transistors of a wide range of thicknesses with α-lipoic acid (LA). Atomic force microscopy (AFM), Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) show evidence of chemical bonding. After functionalization, a significant increase of on current is observed in the MoS₂ transistors, caused by the enhancement of electronic mobility. The maximum increase of mobility can reach ∼100% for monolayer devices. The thickness dependence of the mobility enhancement is analyzed, and a theoretical model based on vacancy filling and charge impurity scattering is proposed to reveal the microscopic origin. These results provide new opportunities of controlling the electronic properties of MoS₂ by surface functionalization.