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

Abstract

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.