MoSSe/MoSeS Janus crystals: nanoscale defects and composition misconceptions revealed through cross-correlated SPM and TERS imaging

In the course of our study, we characterized MoSSe and MoSeS Janus materials derived from MoS2 and MoSe2 correspondingly. Preliminary Raman characterization of the as-synthesized crystal performed in a single point with the single excitation wavelength (532nm) showed weak Raman peaks of the precursors, what was misinterpreted as incomplete conversion. SPM and TERS imaging revealed that the precursor monolayers featured multiple nanoscale multi-layer islands. These islands have been identified in the Janus crystals transferred to Au or Ag via the SPM imaging and their composition was confirmed by TERS imaging. The morphology of the Janus crystals derived from MoS2 and MoSe2 was also fundamentally different. In the course of conversion of MoSe2 to MoSeS accumulated tensile strain led to physical breakage of the crystals. TERS showed that the gaps between the domains in MoSeS monolayers seen in SPM images were physical cracks. Conversely, compressive strain appearing in MoSSe results in the formation of wrinkles that after the transfer to Au or Ag look like cracks in MoSeS, but in reality there is no physical breakage in these crystals.