In the first part of this lecture, I will discuss the discovery and development of single molecule surface enhanced Raman spectroscopy (SMSERS). Most of my presentation will focus on our efforts to provide a robust existence proof for SMSERS employing a frequency-domain approach with two isotopologues of Rhodamine 6G (R6G) that provide unique vibrational signatures. When an average of one molecule is adsorbed per Ag nanoparticle, only one isotopologue is observed in a dry N2 environment. Isotopologues were subsequently used to study the structure of SMSERS active nanoparticle assemblies by HRTEM, perform single molecule excitation spectroscopy measurements, and analyze the SMSERS of crystal violet (CV). This section will conclude with a demonstration of the first SMSERS study involving nanofabricated surfaces rather than chemically synthesized nanoparticles.
In the second part of this lecture, I will discuss tip-enhanced Raman spectroscopy (TERS). TERS has progressed rapidly in recent years toward the goal of providing spatially resolved vibrational spectroscopy on the nanoscale. Our group has focussed on single molecule tip-enhanced Raman spectroscopy (SMTERS) and low-temperature, ultrahigh vacuum, tip-enhanced Raman spectroscopy (LT-UHV-TERS). The isotopologue proof of single molecule specificity in ambient TERS will be demonstrated. The relative intensity ﬂuctuations observed in SMTERS are not the result of diﬀusion, orientation, or local electromagnetic ﬁeld gradients; but, rather are the result of variations in the excited-state geometry of the single molecule. The use of picosecond pulses to obtain TERS will be discussed. A UHV-TERS instrument has been constructed with atomic resolution of the surface and sub-molecular resolution of the adsorbate. The capabilities of this machine are shown with the copper phthalocyanine (CuPc)/Ag(111) system. The first low temperature (19K) UHV-TERS study has been completed. Low temperature stops the surface diffusion of adsorbates across the solid surface, allowing direct STM imaging. Low temperature (LT)-TER spectra for the R6G/Ag(111) system differ from room temperature (RT)-TER, RT- SER, and LT-SER spectra because the vibrational lines are narrowed and shifted. The high spectral resolution of LT-TERS provides intramolecular insight in that the shifted modes are associated with the ethylamine moiety of R6G. LT-TERS is a promising new approach to unravel the intricacies of adsorbate-substrate interactions that are inaccessible by other means.
Finally, I will discuss the combination of SERS and and femtosecond stimulated Raman spectroscopy (FSRS). Plasmonically enhanced broadband Raman spectra using an ultrafast four wave mixing process, which can simultaneously achieve spectral and temporal resolution below the time-energy uncertainty limit, has been achieved! We can now forsee the day when it will be possible to combine UHV-TERS and surface enhanced FSRS to enable single-molecule spectroscopy with simultaneous nanometer spatial resolution and femtosecond time resolution.
Prof. Christine Payne (404-385-3125)