Ionic Liquids at the Interface With the Transition Metal Dichalcogenide MoS2: Effect of the Chemical Structure on Induced Charge Carrier Density.

Transition metal dichalcogenides (TMDs) are emerging layered materials suited for applications in electronics, optoelectronics, and sensing thanks to their semiconductor nature. More recently, electric-field tuning of surface carrier density through carrier accumulation or depletion in TMDs-based field-effect-transistor devices has been attracting a great deal of interest as a new method to control superconductivity in this class of materials. Such results were achieved by means of ionic liquids (ILs) acting as a gating medium for MoS2 crystals. Despite the intensive use of ILs for gating TMDs-based transistors, the impact of the chemical structure of the ions on the induced charge density, a key parameter for controlling the critical temperature (Tc) in superconducting MoS2 crystals, is still lacking. Here we demonstrate that ionic liquid gating of MoS2 channels using [EMIM][TFSI] or [EMIM][BF4] can lead to different carrier densities. [TFSI]-based systems exhibit steeper voltage decay (compared to [BF4]-based systems) meaning the potential drops more rapidly across the electric double layer, indicating stronger local electric fields and more efficient charge accumulation in the MoS2 channel. Our findings represent the first, yet important, step that paves the way towards controlling charge carrier densities on demand in TMDs by chemical design of the anion and cation pair.