Probing Interaction between Individual Submonolayer Nanoislands and Bulk MoS₂ Using Ambient TERS

Tip-enhanced Raman spectroscopy (TERS) has shown that detecting single molecules with a high spatial resolution is possible in ultrahigh vacuum (UHV) at low temperature with plasmonic metallic substrates. It is still challenging to probe interactions of molecules with semiconductors, which is important in biosensing, photovoltaics, and many other applications. Here we demonstrate that in ambient conditions it is possible to obtain Raman signals from submonolayer molecular islands on bulk MoS₂ using TERS. Analysis of relative Raman signal intensity ratio and Raman spectral peak position from spatial TERS mapping showed differences in the adsorbate–adsorbate and adsorbate–substrate interactions on Au and MoS₂ substrates. The Raman transition which involves the vibration of the metal center of the CuPc molecule experienced a change in the relative Raman signal intensity ratio due to the differences in the molecule–substrate charge transfer interaction. In comparison to the other vibrational modes, the vibrational modes of the surface charge transfer interacting moieties involving the metal center experienced the smallest shift in the Raman spectral peak position on both Au and MoS₂ substrates. Further, the distributions of the peak position and relative intensity were narrower at the center of the island with respect to the isolated molecules due to the adsorbate–adsorbate interaction. This study shows the contribution of charge transfer between molecules and MoS₂ in the TERS spectra