Tip-induced local strain on Mo S2/graphite detected by inelastic electron tunneling spectroscopy

We report the detection of tip-induced local strain applied to the monolayer MoS2 grown on a graphite substrate by scanning tunneling microscope. Monolayer MoS2 behaves as both mechanical and tunneling barriers that prevent the tip from contacting the graphite while maintaining the tunneling current. Inelastic tunneling electron spectroscopy (IETS) is utilized to probe the phonon modes in graphite. As the tip pushes the sample, IETS reveals a continuous phonon softening in graphite, corroborated by a downward shift of the phonon energy as calculated by density-functional theory. Our results demonstrate a way to apply local mechanical strain and simultaneously detect the induced change in phonon modes by unitizing IETS with two-dimensional materials as a tunneling barrier.