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Raman spectroscopy is a powerful tool to detect, analyze and identifymolecules. It has been a long-history pursuit to push the detection limit ofRaman spectroscopy down to the fundamental single-molecule (SM) level. Due tothe tiny cross section of intrinsic Raman scattering of molecule, someenhancement mechanisms of light-matter interaction must be implemented tolevitate the Raman scattering intensity by a huge number of ~14-15 orders ofmagnitude, to the level comparable with the molecule fluorescence intensity. Inthis work we report unambiguous observation of single-molecule Ramanspectroscopy via synergic action of electromagnetic and chemical enhancementfor rhodamine B (RhB) molecule absorbed within the plasmonic nanogap formed bygold nanoparticle sitting on the two-dimensional (2D) monolayer WS2 and 2 nmSiO2 coated gold thin film. Raman spectroscopy down to an extremely dilutevalue of 10-18 mol/L can still be clearly visible, and the statisticalenhancement factor could reach 16 orders of magnitude compared with thereference detection sample of silicon plate with a detection limit of 10-2mol/L. The electromagnetic enhancement comes from local surface plasmonresonance induced at the nanogap, which could reach ~10-11 orders of magnitude,while the chemical enhancement comes from monolayer WS2 2D material, whichcould reach 4-5 orders of magnitudes. The synergic implementation and action ofthese two prestigious Raman scattering enhancement mechanisms in this speciallydesigned 2D material-plasmon nanogap composite nanoscale system enablesunambiguous experimental observation of single-molecule Raman spectroscopy ofRhB molecule. This route of Raman enhancement devices could open up a newfrontier of single molecule science, allowing detection, identification, andmonitor of single molecules and their spatial-temporal evolution under variousinternal and external stimuli.

Unambiguous Observation of Single-Molecule Raman Spectroscopy Enabled by Synergic Electromagnetic and Chemical Enhancement