Surface-enhanced Raman scattering of alkyne-conjugated MoS2: a comparative study between metallic and semiconductor phases

Raman enhancement on nonmetallic flat two-dimensional (2D) nanomaterial surfaces has attracted a great deal of attention since the discovery of graphene-enhanced Raman scattering. Molybdenum disulfide (MoS2) is a flat 2D nanomaterial with unique electronic and physical properties that can be applied in surface-enhanced Raman spectroscopy (SERS). Herein, we report a lithium-exfoliated MoS2 (Li-MoS2) has a metallic phase content of about 70%, which is three times higher than the metallic phase content of 20% in thioglycolic acid-exfoliated MoS2 (T-MoS2). Li-MoS2 therefore displays a 2-3 fold increase in the Raman signal for rhodamine 6G (R6G) used as an analyte. Furthermore, the conjugation of a thiol-terminated alkyne with Li-MoS2 also provided a greater SERS signal at 2123 cm(-1) than that of T-MoS2. A defect-rich metallic MoS2 monolayer can therefore be used as the perfect substrate for surface-enhanced Raman scattering, although pristine MoS2 hardly exhibits an SERS effect. This study proved that (1) defect-rich metallic MoS2, (2) dipole-dipole interactions, and (3) the enhanced charge transfer effect of MoS2 monolayers are the three primary and essential parameters for enhancing the Raman signals of analytes on MoS2. Key observations include the fact that some alkyne groups were directly coordinated to the edges of Li-MoS2 defect sites, which shifted the alkyne signal to 2153 cm(-1) in alkyne spectral mapping. More importantly, to quantify the SERS performance of Li-MoS2, SERS imaging of live cells was demonstrated using the unique alkyne signal at 2123 cm(-1).