(Invited) Non-Equilibrium Treatment of Molecular Dopants for 2D Materials

Abstract

2D materials gather attention due to their unique and quantum-confined electronic structures applicable to optoelectronics and a platform for physics. One representative 2D material is transition metal dichalcogenides (TMDCs), which consist of a three-atomic-layered structure with a thickness of only ~0.7 nm. Representative TMDCs are MoS2 and WSe2, and both materials are semiconducting materials with the direct-band gap at the monolayer limit. To apply these unique materials for atomically thin electronics and quantum devices, controlling the carrier concentration is important. In this talk, I will present our recent results to control the carrier behaviors of TMDCs using molecules, with a particular focus on the molecules in non-equilibrium environments, starting with the preparation process of a dopant molecule in one (Ref.1). We found a method to prepare a strong electron dopant, benzyl viologen (BV), in a non-equilibrium manner. In the previous works, the BV molecule is synthesized using a strong reducing agent, sodium borohydride (NaBH4), in water. However, the method is inefficient because NaBH4 reacts vigorously with water at the same time. We replaced NaBH4 with indium metal and found a synthetic method to prepare BV dopant molecules under a non-equilibrium convection flow environment. The convection flow continuously synthesizes BV dopant molecules to a high concentration; furthermore, the method eliminates the purification process to use the dopant solution. Additionally, I will show a non-equilibrium dopant molecular assembly on TMDCs (Ref. 2 and 3). BV molecules usually interact immediately with the TMDC surface and assemble uniformly. Because of the hydrophobic character of the BV molecules, it is difficult to apply photoresists to make local carrier modulation at submicron scales. We overcome the issue by using the self-pattern formation process of the BV molecular films on the surface of TMDCs. By applying hydrophobic/hydrophilic repulsive interaction, the BV film destabilizes and shows pattern formation at scales of about ~200 nm periodicity. Our analysis found that the patterns induce pn and metal/semiconductor junctions on TMDCs. This non-equilibrium process would be useful to prepare electronic device structures via the spontaneous process, which is uniformly distributed at large scale. Keywords: Molecular dopant, Transition metal dichalcogenides, TMDCs, convection flow, molecular assembly, spontaneous pattern formation