Sub-diffraction nanoscale Raman imaging of the interface in a 2D semiconductor heterostructure*

Transition metal dichalcogenide (TMD) semiconductors are solids composed of single sheets of atomic layers that are weakly bonded together via the van der Waals interaction. A single layer yields a 2D semiconductor with a direct band gap and strong light-matter interactions. This work studies the interface in single-layer 2D MoS₂/WS₂ lateral heterostructures with resonant and non-resonant tip-enhanced Raman scattering (TERS) imaging and spectroscopy at spatial resolutions better than 50 nm. In conventional confocal Raman spectroscopy, a spatial resolution on this scale is not possible because of the diffraction limit. With the sub-diffraction spatial resolution and the vibrational fingerprinting ability of Raman spectroscopy, TERS allows us to probe the composition, size, and heterogeneity of the 2D system on length scales that are most relevant for nanoscale optoelectronic technologies. We use TERS to reveal that the alloyed transition region varies in size from 50-600 nm within a single crystallite. TERS nanoscale imaging of the transition region allows for tracking of vibrational modes as they evolve across the MoS₂/Mo_xW_1-xS₂/WS₂ system. This work demonstrates the capabilities of TERS in characterizing monolayer lateral heterostructures on the nanoscale.