Tandem intercalation strategy for single-layer nanosheets as an effective alternative to conventional exfoliation processes

Simple and effective generation of transition metal chalcogenides (TMCs) in a single-layer form has been a challenging task. Here we present a tandem molecular intercalation (TMI) as a new exfoliation concept for producing single-layer TMCs from multi-layer colloidal TMC nanostructures in solution phase. TMI requires tandem Lewis base intercalates, where short 'initiator' molecules first intercalate into TMCs to open up the interlayer gap, and the long 'primary' molecules then bring the gap to full width so that a random mixture of intercalates overcomes the interlayer force. Spontaneous exfoliation then yields single-layer TMCs. The TMI process is uniquely advantageous because it works in a simple one-step process under safe and mild conditions (that is, room temperature without sonication or H₂ generation). With the appropriate intercalates, we have successfully generated single-layer nanostructures of group IV (TiS₂, ZrS₂), group V (NbS₂) and VI (WSe₂, MoS₂) TMCs.