  The nature of the interface in lateral heterostructures of 2D monolayer
semiconductors including its composition, size, and heterogeneity critically
impacts the functionalities it engenders on the 2D system for next-generation
optoelectronics. Here, we use tip-enhanced Raman scattering (TERS) to
characterize the interface in a single-layer MoS2/WS2 lateral heterostructure
with a spatial resolution of 50 nm. Resonant and non-resonant TERS
spectroscopies reveal that the interface is alloyed with a size that varies
over an order of magnitude-from 50-600 nm-within a single crystallite.
Nanoscale imaging of the continuous interfacial evolution of the resonant and
non-resonant Raman spectra enables the deconvolution of defect-activation,
resonant enhancement, and material composition for several vibrational modes in
single-layer MoS2, MoxW1-xS2, and WS2. The results demonstrate the capabilities
of nanoscale TERS spectroscopy to elucidate macroscopic structure-property
relationships in 2D materials and to characterize lateral interfaces of 2D
systems on length scales that are imperative for devices.
