Convection‐Flow‐Assisted Preparation of a Strong Electron Dopant, Benzyl Viologen, for Surface‐Charge Transfer Doping of Molybdenum Disulfide

Transition metal dichalcogenides (TMDCs) have received attention as atomically thin post‐silicon semiconducting materials. Tuning the carrier concentrations of the TMDCs is important, but their thin structure requires a non‐destructive modulation method. Recently, a surface‐charge transfer doping method was developed based on contacting molecules on TMDCs, and the method succeeded in achieving a large modulation of the electronic structures. The successful dopant is a neutral benzyl viologen (BV0); however, the problem remains of how to effectively prepare the BV0 molecules. A reduction process with NaBH4 in water has been proposed as a preparation method, but the NaBH4 simultaneously reacts vigorously with the water. Here, a simple method is developed, in which the reaction vial is placed on a hotplate and a fragment of air‐stable metal is used instead of NaBH4 to prepare the BV0 dopant molecules. The prepared BV0 molecules show a strong doping ability in terms of achieving a degenerate situation of a TMDC, MoS2. A key finding in this preparation method is that a convection flow in the vial effectively transports the produced BV0 to a collection solvent. This method is simple and safe and facilitates the tuning of the optoelectronic properties of nanomaterials by the easily‐handled dopant molecules. A strong electron dopant molecule, benzyl viologen, is generated with a simple method with air‐stable metals as a reducing agent. The reacted solution shows a strong doping ability for MoS2 to reach a degenerately doped level. The convection flow is found to effectively transport and purify the product dopant from the reactant molecules.