Single-Molecule Tip-Enhanced Raman Spectroscopy

An existence proof for single-molecule tip-enhanced Raman spectroscopy (SMTERS) is given using the frequency domain approach involving the two isotopologues of Rhodamine 6G (R6G) that were previously employed for single-molecule surface-enhanced Raman spectroscopy (SMSERS). A combination of experimental and theoretical studies provides a detailed view of the isotopic response of R6G– d 0 and R6G– d 4 in the 600 – 800 cm –1 region. The single-molecule nature of the TERS experiment is confirmed through two lines of evidence. First, the vibrational signature of only one isotopologue at a time was observed from multiple TER spectra. Second, the spectral wandering of the 610 cm –1 mode of R6G– d 0 was less than ±4 cm –1, which in turn is less than the 10 cm –1 isotopic shift so that no confusion in assignment resulted. As a consequence, the total TERS enhancement factor can now be accurately established as EF TERS = 1.0 × 10 13 because only one molecule at a time is measured. Furthermore, EF TERS can be partitioned into an electromagnetic contribution of 10 6 and a molecule-localized resonance Raman contribution of 10 7 .