The MoS2-Graphene-Sapphire Heterostructure: Influence of Substrate Properties on the MoS2 Band Structure

Van der Waals MoS2/graphene heterostructuresare promisingcandidates for advanced electronics and optoelectronics beyond graphene.Herein, scanning probe methods and Raman spectroscopy were appliedfor analysis of the electronic and structural properties of monolayer(ML) and bilayer 2H-MoS2 deposited on single-layer graphene(SLG)-coated sapphire (S) substrates by means of an industrially scalablemetal organic chemical vapor deposition process. The SLG/S substrateshows two regions with distinctly different morphology and variedinterfacial coupling between SLG and S. ML MoS2 nanosheetsgrown on the almost free-standing graphene show no detectable interfacecoupling to the substrate, and a value of 2.23 eV for the MoS2 quasiparticle bandgap is determined. However, if the grapheneis involved in hydrogen bonds to the hydroxylated sapphire surface,an increased MoS2/graphene interlayer coupling results,marked by a shift of the conduction band edge toward Fermi energyand a reduction of the ML MoS2 quasiparticle bandgap to1.98 eV. The surface topography reveals a buckle structure of ML MoS2 in conformity with SLG that is used to determine the dependenceof the ML MoS2 bandgap on the interfacial spacing of thisheterostructure. In addition, an in-gap acceptor state about 0.9 eVabove the valence band minimum of MoS2 has been observedon locally elevated positions on both SLG/S regions, which is attributedto local bending strain in the grown MoS2 nanosheets. Thesefundamental insights reveal the impact of the underlying substrateon the topography and the band alignment of the ML MoS2/SLG heterostructure and provide the possibility for engineeringthe quasiparticle bandgap of ML MoS2/SLG grown on controlledsubstrates that may impact the performance of electronic and optoelectronicdevices therewith.