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.

Automated summary

Scanning probe methods and Raman spectroscopy were used to analyze the electronic and structural properties of monolayer and bilayer 2H-MoS2 on single-layer graphene-coated sapphire substrates. It was found that there was almost no interface coupling between ML MoS2 and graphene, resulting in a bandgap value of 2.23 eV. However, if graphene was involved in hydrogen bonds with the sapphire surface, the bandgap reduced to 1.98 eV due to increased interlayer coupling. Additionally, a in-gap acceptor state about 0.9 eV above the valence band minimum of MoS2 was observed on locally elevated positions, which was attributed to local bending strain in MoS2 nanosheets. These findings provide insights into the impact of the substrate on the topography and band alignment of ML MoS2/SLG heterostructures.