  Raman spectroscopy is a powerful tool to detect, analyze and identify
molecules. It has been a long-history pursuit to push the detection limit of
Raman spectroscopy down to the fundamental single-molecule (SM) level. Due to
the tiny cross section of intrinsic Raman scattering of molecule, some
enhancement mechanisms of light-matter interaction must be implemented to
levitate the Raman scattering intensity by a huge number of ~14-15 orders of
magnitude, to the level comparable with the molecule fluorescence intensity. In
this work we report unambiguous observation of single-molecule Raman
spectroscopy via synergic action of electromagnetic and chemical enhancement
for rhodamine B (RhB) molecule absorbed within the plasmonic nanogap formed by
gold nanoparticle sitting on the two-dimensional (2D) monolayer WS2 and 2 nm
SiO2 coated gold thin film. Raman spectroscopy down to an extremely dilute
value of 10-18 mol/L can still be clearly visible, and the statistical
enhancement factor could reach 16 orders of magnitude compared with the
reference detection sample of silicon plate with a detection limit of 10-2
mol/L. The electromagnetic enhancement comes from local surface plasmon
resonance induced at the nanogap, which could reach ~10-11 orders of magnitude,
while the chemical enhancement comes from monolayer WS2 2D material, which
could reach 4-5 orders of magnitudes. The synergic implementation and action of
these two prestigious Raman scattering enhancement mechanisms in this specially
designed 2D material-plasmon nanogap composite nanoscale system enables
unambiguous experimental observation of single-molecule Raman spectroscopy of
RhB molecule. This route of Raman enhancement devices could open up a new
frontier of single molecule science, allowing detection, identification, and
monitor of single molecules and their spatial-temporal evolution under various
internal and external stimuli.
