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.