Boosting and Balancing Electron and Hole Mobility in Single- and Bi-Layer WSe2 Devices via Tailored Molecular Functionalization.

WSe2 is a layered ambipolar semiconductor enabling hole and electron transport, which renders it a suita-ble active component for logic circuitry. However, solid-state devices based on single- and bi-layer WSe2 typically exhibit unipolar transport and poor electrical performances when conventional SiO2 dielectric and Au electrodes are used. Here, we show that silane-containing functional molecules form ordered mono-layers on the top of the WSe2 surface, thereby boosting its electrical performance in single- and bi-layer field-effect transistors. In particular, by employing SiO2 dielectric substrates and top Au electrodes, we measure unipolar mobility as high as µh = 150 cm2V-1s-1 and µe = 17.9 cm2V-1s-1 in WSe2 single-layer devices when ad hoc molecular monolayers are chosen. Additionally, by asymmetric double-side function-alization with two different molecules, we provide opposite polarity to the top and bottom layer of bi-layer WSe2, demonstrating nearly balanced ambipolarity at the bi-layer limit. Our results indicate that the con-trolled functionalization of the two sides of WSe2 mono- and bi-layer flakes with highly ordered molecular monolayers offers the possibility to simultaneously achieve energy level engineering and defect functional-ization, representing a path towards the deterministic control over charge transport in 2D materials.