Spectroscopic Measurements of Doping and Strain at Two-Dimensional Material Interfaces

Abstract

Measurements of charge doping and mechanical strain in graphene have shown a strong dependence on the graphene source, transfer method, and substrate choice. Here I report Raman spectroscopy measurements of chemical vapor deposition (CVD) grown graphene transferred to a number of relevant substrates and freestanding, as well as commercially available graphene, in order to study the variation in charge doping and strain between these samples. I utilize a vector decomposition model to simultaneously determine the charge doping and strain levels based on the positions of the Raman G and 2D peaks. I observe variations in charge doping between different synthesis conditions but find direct transfer methods to maintain doping homogeneity. I observe a large distribution in strain both within and between transfers and observe a surprisingly low slope (Δ?2?Δ??) suggesting that the Grüneisen parameter for CVD graphene differs from exfoliated graphene. At suspended interfaces with gold and chromium, I observe very similar levels of doping and strain despite large differences in the metals’ work functions, suggesting that the graphene/metal interface is more influenced by growth and transfer conditions than the choice of metal. Second-harmonic generation (SHG) microscopy has previously been used to study uniaxial and biaxial strain in monolayer two-dimensional materials. Here I report SHG microscopy measurements of chemical vapor deposition (CVD) grown MoS2 on Si/SiO2 and c-cut sapphire, as well as in-plane WSe2/NbS2 in-plane heterostructures. I utilize a three-point polarization-resolved measurement to determine the position dependent biaxial strain, uniaxial strain, and the local direction of the principal strain axis. Biaxial strain is found to be inhomogeneous in MoS2 on both substrates, ranging from -0.25 % compressive strain to 0.64 % tensile strain. Local uniaxial strains are similarly inhomogeneous, but only tensile strains are measured, ranging from 0.17 to 0.60 %. The direction of the principal strain axis is observed to be highly uniform between isolated crystals, suggesting that CVD synthesis results in an oriented uniaxial strain across the growth substrate. Two-photon photoemission experiments were calibrated using several well-defined surfaces, and proof of concept time-resolved two-photon photoemission experiments were carried out on rubrene single crystal samples. These results demonstrate the feasibility of TR-2PPE experiments using this experimental apparatus. Possibilities for improvements in the experimental methods are identified.