Metal-insulator transition in monolayer MoS₂ via contactless chemical doping

Much effort has been made to modify the properties of transition metal dichalcogenide layers via their environment as a route to new functionalization. However it remains a challenge to induce large electronic changes without chemically altering the layer or compromising its two-dimensionality. Here a non-invasive technique is used to shift the chemical potential of monolayer MoS₂ through p- and n-type doping of graphene (Gr) which remains a well-decoupled 2D substrate. With the intercalation of oxygen (O) under Gr a nearly rigid Fermi level shift of 0.45 eV in MoS₂ is demonstrated whereas the intercalation of europium (Eu) induces a metal–insulator transition in MoS₂ accompanied by a giant band gap reduction of 0.67 eV. Additionally the effect of the substrate charge on 1D states within MoS₂ mirror-twin boundaries (MTBs) is explored. It is found that the 1D nature of the MTB states is not compromised even when MoS₂ is made metallic. Furthermore with the periodicity of the 1D states dependent on substrate-induced charging and depletion the boundaries serve as chemical potential sensors functional up to room temperature.