Strain, doping and electronic transport of large area monolayer MoS2 exfoliated on gold and transferred to an insulating substrate

Abstract

Gold-assisted mechanical exfoliation currently represents a promising method to separate ultra-large (cm-scale) transition metal dichalcogenides (TMDs) 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 the key to achieve this nearly perfect 1L exfoliation yield. On the other hand, it may affect significantly 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 $Mo{S}_2$ exfoliated on ultra-flat $Au$ films ($0.16-0.21nm$ roughness) and finally transferred to an insulating $A{l}_2{O}_3$ substrate. Raman mapping and correlative analysis of the $E^{\prime }$ and $A{1}^{\prime }$ peaks positions revealed a moderate tensile strain ($0.2\%$) and p-type doping ($n=-0.25\times 10^{13}c{m}^{-2}$) of 1L $Mo{S}_2$ in contact with $Au$. Nanoscale resolution current mapping and current-voltage (I-V) measurements by conductive atomic force microscopy (C-AFM) showed direct tunnelling across the 1L $Mo{S}_2$ on $Au$, with a broad distribution of tunnelling barrier values (from 0.7 to 1.7 eV) consistent with the p-type doping of $Mo{S}_2$. After the final transfer of $1L-Mo{S}_2$ on $A{l}_2{O}_3/Si$, the strain was converted to compressive ($-0.25\%$). Furthermore, an n-type doping ($n=0.5\times 10^{13}c{m}^{-2}$) was deduced by Raman mapping and confirmed by electrical measurements of an $A{l}_2{O}_3/Si$ back-gated 1L $Mo{S}_2$ transistor. These results provide a deeper understanding of the $Au$-assisted exfoliation mechanisms and can contribute to its widespread applications for the realization of novel devices and artificial vdW heterostructures