As
a newly emerging approach for surface-enhanced Raman spectroscopy
(SERS), pressure-induced SERS (PI-SERS) has been attracting increasing
interest for its applications in Raman signal enhancement at extreme
conditions. However, how to efficiently realize the PI-SERS enhancement
and elucidate the corresponding mechanism remain open questions. Herein,
we demonstrate the PI-SERS enhancement up to 8.04 GPa using monolayer
molybdenum disulfide (ML-MoS2) as a SERS substrate and
three organic molecules with similar energy levels but different symmetries
as probes. The combined theory and experiment results show that a
pressure-induced increase in the Fermi level of the ML-MoS2 substrate and a decrease in the highest occupied molecular orbital–lowest
unoccupied molecular orbital (HOMO–LUMO) energy gap of probe
molecules lead to a transition from the multiple resonance-related
SERS enhancement to charge transfer (CT)-dominated PI-SERS selective
enhancement, depending on the incident laser energy and the pressure
applied. Such PI-SERS selective enhancement has been discussed in
the framework of CT-induced strengthening of electron–phonon
coupling, as well as a possible match of the structural symmetries
between probe molecules and the substrate. This study provides deep
insights into our understanding of PI-SERS enhancement, and the revealed
mechanism can be extended to other molecules for SERS at extreme conditions.