Making clean electrical contacts on 2D transition metal dichalcogenides

2D semiconductors, particularly transition metal dichalcogenides (TMDs), have emerged as highly promising for new electronic technologies. However, a key challenge in fabricating devices out of 2D semiconductors is the need for ultra-clean contacts with resistances approaching the quantum limit. The lack of high-quality, low-contact-resistance P-type and N-type contacts on 2D TMDs has limited progress towards the next generation of low-power devices, such as the tunnel field-effect transistors. In this Expert Recommendation, we summarize strategies and provide guidance for making clean van der Waals contacts on monolayered TMD semiconductors. We also discuss the physics of contacts in 2D semiconductors and prospects for achieving quantum conductance.