Demonstration of the key substrate-dependent charge transfer mechanisms between monolayer MoS2 and molecular dopants

Soohyung Park,Lain-Jong Li,Vincent C Tung,Ali Han,Xiaomin Xu,Berthold Wegner,Areej Aljarb,Patrick Amsalem,Thorsten Schultz,Norbert Koch

doi:10.1038/s42005-019-0212-y

Abstract

Tuning the Fermi level (EF) in two-dimensional transition metal dichalcogenide (TMDC) semiconductors is crucial for optimizing their application in (opto-)electronic devices. Doping by molecular electron acceptors and donors has been suggested as a promising method to achieve EF-adjustment. Here, we demonstrate that the charge transfer (CT) mechanism between TMDC and molecular dopant depends critically on the electrical nature of the substrate as well as its electronic coupling with the TMDC. Using angle-resolved ultraviolet and X-ray photoelectron spectroscopy, we reveal three fundamentally different, substrate-dependent CT mechanisms between the molecular electron acceptor 1,3,4,5,7,8-hexafluoro-tetracyano-naphthoquinodimethane (F6TCNNQ) and a MoS2 monolayer. Our results demonstrate that any substrate that acts as charge reservoir for dopant molecules can prohibit factual doping of a TMDC monolayer. On the other hand, the three different CT mechanisms can be exploited for the design of advanced heterostructures, exhibiting tailored electronic properties in (opto-)electronic devices based on two-dimensional semiconductors.

Highlights

Tuning the Fermi level (EF) in two-dimensional transition metal dichalcogenide (TMDC) semiconductors is crucial for optimizing their application inelectronic devices
To investigate how charge transfer (CT) processes between a TMDC monolayer and the molecular acceptor F6TCNNQ, which has been employed as p-type dopant for organic semiconductors, could be influenced by the choice of substrate, angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) measurements were performed for MoS2/sapphire, MoS2/HOPG, and MoS2/Au with stepwise deposition of F6TCNNQ
Owing to the specific features of the TMDC band structure—related to the high symmetry of the TMDCs—ARUPS spectra feature a high photoemission intensity only along the Γ-K and Γ-M directions of the single-crystal Brillouin zone (BZ), the spectra correspond to a linear superposition of the electronic bands along the aforementioned high symmetry directions[32,33] We note that, while it was shown that the electrical conductivity of sapphire can increase to about 10−9 S/m by creating a high density of defects after annealing to 600 °C and proton bombardment[31], our experimental conditions ensure the insulating character of sapphire

Summary

Introduction

Tuning the Fermi level (EF) in two-dimensional transition metal dichalcogenide (TMDC) semiconductors is crucial for optimizing their application in (opto-)electronic devices. Various methods have been proposed for tuning EF within the band gap of TMDCs, including substitution with dopant atoms[15], stacking with other 2D materials[16], exposure to gases[6,7,8,17], adsorption of alkali metals[18,19], and adsorption of organic molecules[20,21,22,23,24,25,26] In accordance with this approach, deposition of molecular electron acceptors or donors on TMDCs has been suggested as a very effective strategy for controlling the EF position via doping, and high-performance TFTs were realized[22,24,26]. Factual doping of MoS2 proceeds only on the insulating substrates

Methods

Results

Conclusion