Strain, Doping, and Electronic Transport of Large Area Monolayer MoS Exfoliated on Gold and Transferred to an Insulating Substrate.

Gold-assisted mechanical exfoliation currently represents a promising method to separate ultralarge (centimeter scale) transition metal dichalcogenide (TMD) monolayers (1L) with excellent electronic and optical properties from the parent van der Waals (vdW) crystals. The strong interaction between Au and chalcogen atoms is key to achieving this nearly perfect 1L exfoliation yield. On the other hand, it may significantly affect the doping and strain of 1L TMDs in contact with Au. In this paper, we systematically investigated the morphology, strain, doping, and electrical properties of large area 1L MoS exfoliated on ultraflat Au films (0.16-0.21 nm roughness) and finally transferred to an insulating AlO substrate. Raman mapping and correlative analysis of the E' and A' peak positions revealed a moderate tensile strain (ε ≈ 0.2%) and p-type doping ( ≈ -0.25 × 10 cm) of 1L MoS in contact with Au. Nanoscale resolution current mapping and current-voltage (-) measurements by conductive atomic force microscopy (C-AFM) showed direct tunneling across the 1L MoS on Au, with a broad distribution of tunneling barrier values (Φ from 0.7 to 1.7 eV) consistent with p-type doping of MoS. After the final transfer of 1L MoS on AlO/Si, the strain was converted to compressive strain (ε ≈ -0.25%). Furthermore, an n-type doping ( ≈ 0.5 × 10 cm) was deduced by Raman mapping and confirmed by electrical measurements of an AlO/Si back-gated 1L MoS transistor. These results provide a deeper understanding of the Au-assisted exfoliation mechanism and can contribute to its widespread application for the realization of novel devices and artificial vdW heterostructures.