Single Molecule Tip-Enhanced Raman Spectroscopy with Silver Tips

We present single molecule tip-enhanced resonance Raman spectra from brilliant cresyl blue (BCB) submonolayers adsorbed on a planar Au surface with Ag tips. A gap of 1 nm between a Ag tip and the Au substrate was employed to create a highly enhanced electric field and to generate Raman scattering from an area of ∼100 nm². Three lines of evidence are presented to prove the single molecule sensitivity of our experiments:  (1) Extremely diluted samples were used. Estimations show that at most a few molecules were excited by the Ag tip. (2) Spectroscopic fluctuations, including intensity fluctuations, frequency shifts, and line shape changes were observed. A histogram analysis of the intensity fluctuations of two different BCB coverages was carried out. The results clearly show the features of single molecule behavior. (3) Discrete signal losses also were observed. This is because of photochemical processes involving single molecules. Besides BCB, which shows a strong resonant absorption at 633 nm (the wavelength of the excitation laser), a self-assembled monolayer of benzenethiol, which does not strongly absorb at 633 nm, was studied. Good quality spectra were recorded with a short exposure time (10 s) and time-dependent spectral changes were also observed.