Tip-Enhanced Raman Spectroscopy of Monolayer MoS2 on Au(111)

Employing gap-mode plasmons in a scanning tunneling microscope junction, we studied tip-enhanced Raman spectroscopy (TERS) of MoS2 on Au(111). We observed a mode denoted as "m", positioned to the right of A(1)(') by a difference of approximately 6 cm(-1). In the MoS2 region that is not in direct contact with the Au(111) substrate, the m peak still exists, indicating that the m mode originates from MoS2. The m mode could originate from non-Gamma-point phonon modes in the ZO branch or LA(K) + TA(K). Additionally, our investigation reveals that the TERS intensity reaches its maximum by continuing to approach MoS2 after the tip contacts MoS2. The tip displacement required to reach the maximum TERS intensity depends on the tip conditions. Our results provide guidance for obtaining the optimal TERS intensity for MoS2 and have potential application to other two-dimensional transition metal dichalcogenides.