|M.Sc Student||Cohn Bar|
|Subject||Surface-Enhanced 2D IR Spectroscopy for Studies of|
Molecular Structure and Ultrafast Dynamics in
|Department||Department of Chemistry||Supervisor||Assistant Professor Lev Chuntonov|
|Full Thesis text|
Two-dimensional infrared (2DIR) spectroscopy is superior method providing crucial information on molecular structure and its ultrafast vibrational dynamics. Typically, 2DIR experiments are conducted in a 1-100um thick samples. Implementation of such method for the study of nano-scale devices is important because confinement of molecules to surfaces, gaps, crevices and other topologies can drastically alter molecular structure and dynamics. However, the dynamic range of the standard detection systems does not allow one to collect the signal from molecules on fabricated nano-devices, as the effective thickness is on the nanometer length scale. Amplifications of 2DIR signal with local fields of plasmonic antenna can permit these types of studies.
This Thesis focuses on implementation of the surface enhanced 2DIR (SE-2DIR), where the signal enhancement is achieved with the help of the gold infrared antennas, which provide the 2DIR signal enhancement of up to 105. However, asymmetric (Fano-type) lineshape arise, which complicates the interpretation of the spectroscopic data. The asymmetric line shapes appear as a result of the interference between different light pathways involving molecules and antennas.
In the field of surface-enhanced vibrational spectroscopy, Fano lineshapes are usually explained by the coupling of local electric fields next to the antenna, to the molecular vibrational transitions. Our results show that for a film thickness beyond only a few nanometers, the near-field interaction is not sufficient to account for the magnitude of the observed signal, which nevertheless has a clear Fano lineshape, suggesting a radiative origin of the molecule-plasmon interaction. Qualitative explanation for the observed effect is given by the extended coupled oscillator model, which considers both near-field and radiative interactions between the plasmonic and molecular transitions.
Overall, we discuss plasmonic antenna, which are tunable to the desired molecular vibrational transition, for the implementation of SE-2DIR spectroscopy. We show that with this technique we can achieve enhancement of the signal much higher as compared to other methods, but at the price of 2D Fano lineshapes.