Undergraduate Physics Seminar: "Graphene Buffered Strain-Engineering of MoS₂ for Excitonic Device Applications," Jiri Kataman-Kustwan

Abstract: Strain is used to engineer transition metal dichalcogenides (TMDs) such as monolayer MoS₂ (ML-MoS₂), and spatially varying strain is used to funnel excitons in nanoscale strain-engineered solid-state platforms for electrical, optoelectronic, and photonic devices. One method to generate strain is by applying bending of MoS₂ around nanoscale pillars patterned on Au(111), but direct contact with the metal quenches the photoluminescence by interlayer charge transfer. In this work, we model a promising MoS₂/graphene/Au(111) vdW heterostructure, where Au(111) is nanopatterned, using molecular dynamics (LAMMPS) to study strain transfer and strain engineering of MoS₂. Graphene is grown on copper foil using plasma-enhanced chemical vapor deposition (PECVD) by optimizing methane and hydrogen flow rates for large-area, low-defect monolayers. After PECVD, graphene growth is characterized by AFM, SEM, and Raman spectroscopy. The alteration of the local MoS₂ bandgap is observed (in our configuration) with a graphene buffer, with the bandgap scaling linearly with strain. With nanopatterned Au pillars (2–12 Å tall), the local MoS₂ bandgap is locally reduced from 1.89 eV to 1.06 eV (over 40%), creating the well needed to funnel excitons. This graphene buffer can be realized using PECVD synthesis and varied H₂ and CH₄ flow rates on Cu foil. Raman spectroscopy identified H₂ = 4.5 sccm and CH₄ = 0.08 sccm as the best condition tested (D/G ≈ 0.56), although no sample yet shows the Raman signature of monolayer graphene. These results show that the pillar-induced band-gap funnel predicted for MoS₂ on bare Au survives a graphene buffer, and they set targets for growing the buffer layer for future laser-assisted STM studies of strained MoS₂.

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