Inhomogeneous defect distribution of triangular WS₂ monolayer revealed by surface-enhanced and tip-enhanced Raman and photoluminescence spectroscopy

Confocal optical microscopy and tip-enhanced optical microscopy are applied to characterize the defect distributions in chemical vapor deposition-grown WS₂ monolayer triangles qualitatively and quantitatively. The presence of defects in individual monolayer WS₂ triangles is revealed with diffraction-limited spatial resolution in their photoluminescence (PL) images, from which the inhomogeneous defect density distribution is calculated, showing an inverse relationship to the PL intensity. The defect-related surface-enhanced Raman spectroscopy (SERS) effect is investigated by depositing a thin copper phthalocyanine layer (5 nm) as the probe molecule on the monolayer WS₂ triangles surface. Higher SERS enhancement effects are observed at the defect-rich areas. Furthermore, tip-enhanced optical measurements are performed, which can reveal morphologically defected areas invisible in the confocal optical measurements. Furthermore, the area with high defect density appears brighter than the low-defected area in the tip-enhanced optical measurements, which are different from the observation in the confocal optical measurements. The underlying reasons are attributed to the near-field enhancement of the defect exciton emission induced by the optically excited tip and to an improved coupling efficiency between the tip-generated near-field with the altered dipole moment orientation at the local defect.