Abstract
We investigate the Fermi-level (EF) dependence of electron-phonon coupling (EPC) in stacked graphene and graphene/transition metal dichalcogenides (TMD) heterostructures. The samples include single layer graphene (SLG), misoriented bilayer graphene (BLG), single layer graphene/molybdenum disulfide (SLG/MoS2), and single layer graphene/tungsten diselenide (SLG/WSe2). All materials were prepared by chemical vapor deposition and graphene transformation. We employ an ion-gel gate on top on graphene as a gate-electrode to tune EF. To reveal EPC, we use Raman spectroscopy to study the 2D or G peak shifts of graphene as a function of EF. The Raman spectra are measured at room temperature, with the 633-nm line of a He-Ne laser as exciting radiation. EPC can be characterized by the changes of G peak position with EF. We find the changes of G-peak position in BLG are similar to the case in SLG, suggesting the weak induced EPC between two stacked graphene. Intriguingly, the G- peak position changes dramatically in SLG/MoS2 and SLG/WSe2. In contrast, the characteristic Raman peaks of MoS2 and WSe2 exhibit weak EF dependence. Data analysis suggests that the EPC strength is enhanced in SLG/MoS2 and SLG/WSe2 samples, and is increased with the increase of the carrier density. The increase of the EPC may be related to possible charge transfer in between the stacked layers. Our results provide useful information for the mechanisms of EPC for TMD heterostructures, and have strong implications in the device applications.