Applying and detecting tip-induced local strain on monolayer MoS₂/graphite with scanning tunneling microscopy and inelastic electron tunneling spectroscopy

Strain engineering of two-dimensional (2D) materials gained great attention because of their superior mechanical properties and applicability to novel devices such as wearable electronics. The extremely small size of modern electronic devices necessitates a method to investigate strain effects at the nanoscale in these materials. Here we utilize scanning tunneling microscopy (STM) and inelastic electron tunneling spectroscopy (IETS) to apply and detect the local strain on monolayer MoS₂ grown on a graphite substrate. Monolayer MoS₂ behaves as a mechanical and tunneling barrier, and by controlling the distance between the tip and the sample, STM can apply local strain and simultaneously detect the change in phonon modes by IETS. IETS revealed that the phonon energy of graphite decreased by the tip-induced strain. Density functional theory calculation of phonon density of the states also showed the phonon-softening by strain, which substantiated our observation.