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 EFTERS = 1.0 x 10(13) because only one molecule at a time is measured. Furthermore, EFTERS can be partitioned into an electromagnetic contribution of 10(6) and a molecule-localized resonance Raman contribution of 10(7).