Fundamental Investigations into Single Molecule Surface Enhanced Raman Spectroscopy
After the first claim of single molecule (SM) detection by surface enhanced Raman spectroscopy (SERS) was published in 1997 and years of debate and maturing, SM-SERS can now be considered as an established subfield of SERS. Besides the obvious promising advances in analytical spectroscopy that SM-SERS enables, some more fundamental studies are now also accessible. The main focus of this thesis is to understand certain aspects and tackle some outstanding issues in SM-SERS, both in methods and applications. In the first part of this thesis, we focus on the application of SM-SERS to the study of the homogeneous broadening of molecular vibrations. We show that the homogeneous linewidth of the dye Nile blue as measured on single molecule SERS spectra is much smaller than the inhomogeneous broadening obtained from the average signal. Individual molecules having the central Raman frequency occurring at slightly different positions is therefore the main cause of the inhomogeneous broadening in this system. Furthermore, we show that the homogeneous broadening of the mode of single molecules exhibits a strong temperature dependence from 80K to 300 K. This is suggestive of the vibrational energy exchange model which explicitly relates the temperature dependence of the linewidth of a vibrational mode to its interaction with other modes of the molecule or its environment. The average signal does not show this temperature dependence, this property is indeed washed out by ensemble averaging and its unravelling is made possible by SM-SERS. This study is the first example of direct measurement and study of the homogeneous broadening of a Raman peak. In the second part of this work, we focus on a particular method to prove single molecule sensitivity and demonstrate the single molecule detection of the iconic C₆₀ by SM-SERS using its peculiar spectral properties regarding isotopic substitution. A change in one unit mass in one of the carbon atoms is readily observed as a detectable frequency shift in the Ag(2) mode on the Raman spectrum of one C₆₀. This remarkable result is a direct consequence of the high symmetry of the molecule and is only accessible experimentally by measuring individual molecules. We perform SM-SERS detection of a isotopically enriched C₆₀ and show how the distribution of frequencies for the Ag(2) mode reflects the isotopic spread of the sample. Density Functional Theory (DFT) calculations support the experimental results. This provides the first ever evidence of single molecule detection of C₆₀ via SERS. Finally, we focus on the photostability of dyes excited resonantly in SERS conditions. Photobleaching of the molecule is an issue when doing SERS (and SM-SERS) at resonance. Nile blue is deposited on a highly ordered gold nanolithographic substrate and the time dependence of the SERS signal is monitored. Using a simple two-level system model to describe the mechanisms of photobleaching and express the photobleaching rate, we analyse the SERS intensity decay at different powers. This study is the first to be dedicated to the photobleaching decay rates of molecules on metallic surfaces and to highlight that the decay dynamics contains rates spanning four orders of magnitude. This work can potentially reveal information on the distribution of SERS enhancement factors on the surface.