Nanoscale Mapping of Relativistic Carrier Mobility and Photoconduction Properties in Graphene/MoS₂ Heterostructure

In this paper, relativistic carrier mobilities and photoconduction properties in graphene/MoS₂ heterostructure (GM-HS) are mapped via photoconductive scanning noise microscopy with a conducting nanoprobe. Graphene on MoS₂ (HS) regions shows two-times reduced sheet conductance (G_Sh) without a significant increase in trap densities (N_T) compared to that in graphene on SiO₂ (Gr) regions, due to the electron-doping in graphene by underlying MoS₂. Interestingly, both regions exhibit G_Sh independent of N_T, which indicates that the relativistic nature of graphene is preserved in the overall GM-HS. The mobility map is obtained from conductivity maps at different gate biases, showing a rather low mobility in HS regions compared to that in Gr regions. Notably, the overall film exhibits a sharp increase in mobility at low carrier densities, as previously reported in graphene having relativistic carriers. Under the illumination with photon energies above the MoS₂ bandgap, HS regions exhibit pronounced negative photoresponses originating from the injection of photo-excited electrons from MoS₂ to graphene. Unexpectedly, under the illumination with photon energies below the MoS₂ bandgap, HS regions exhibit marginally negative photoresponses, possibly due to additional carriers de-trapped from trapping sites in MoS₂. Interestingly, photoconductance is independent of phototraps, indicating that the relativistic photoconduction is preserved in GM-HS.