Boosting and Balancing Electron and Hole Mobility in Single- and Bilayer WSe Devices Tailored Molecular Functionalization.

WSe is a layered ambipolar semiconductor enabling hole and electron transport, which renders it a suitable active component for logic circuitry. However, solid-state devices based on single- and bilayer WSe typically exhibit unipolar transport and poor electrical performance when conventional SiO dielectric and Au electrodes are used. Here, we show that silane-containing functional molecules form ordered monolayers on the top of the WSe surface, thereby boosting its electrical performance in single- and bilayer field-effect transistors. In particular, by employing SiO dielectric substrates and top Au electrodes, we measure unipolar mobility as high as μ = 150 cm V s and μ = 17.9 cm V s in WSe single-layer devices when molecular monolayers are chosen. Additionally, by asymmetric double-side functionalization with two different molecules, we provide opposite polarity to the top and bottom layer of bilayer WSe, demonstrating nearly balanced ambipolarity at the bilayer limit. Our results indicate that the controlled functionalization of the two sides of the WSe mono- and bilayer flakes with highly ordered molecular monolayers offers the possibility to simultaneously achieve energy level engineering and defect functionalization, representing a path toward deterministic control over charge transport in 2D materials.