Molecular Electronics uses molecules sandwiched between two metals as a model system to create tailored solutions for applications such as energy harvesting and sensing. Electrons tunnelling across such a junction are a↵ected by the properties of the molecule itself as well as the interaction between metal and molecule. In particular, charge transport is known to couple to molecular vibrations, which can act both to dissipate energy as well as increase conductance. This thesis therefore applies in-situ vibrational spectroscopies, surface-enhanced Raman scattering and vibrational sum frequency generation, to investigate molecular junctions. As a model system, 4-mercaptopyridine sandwiched between a gold surface and an elec- trochemically deposited second metal layer is used. Four aspects are studied in detail in this thesis. Chapter 3 presents a detailed study of surface enhanced Raman spectra of 4-mercaptopyridine on gold. All experimental vibrational modes are assigned and related to the symmetry of the adsorbed molecule with the help of density functional calculations. In particular, the e↵ect of hydrogen bonding on the ring breathing modes of adsorbed mercaptopyridine is revealed for the first time. In chapter 4, surface-enhanced Raman spectroscopy is used to identify a spectroscopic signature of a successfully formed metal-molecule-metal junction after electrochemical deposition of a tran- sition metal layer. Chapter 5 then addresses the use of surface-enhanced Raman spectroscopy to identify charge transfer states of 4-mercaptopyridine by changing bias potential and excitation wavelength. A charge transfer state is found for protonated 4-mercaptopyridine at about 1.7 eV above the Fermi level, while the corresponding state for unprotonated 4-mercaptopyridine must lie at least 0.8 eV higher. Chapter 6 then explores the use of ultrafast vibrational sum frequency generation. The pyridine ring stretching modes are detected and metallisation of the 4-mercaptopyridine layer is seen to decrease the local order of the molecular layer. The influence of the mercaptopyridine charge transfer state can be seen in ultrafast pump - sum frequency probe spectroscopy of the gold substrate. This opens the prospect of investigating coupling between molecular vibrations and charge transfer in these junctions on a timescale of a picosecond or less.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:722059 |
| Date | January 2017 |
| Creators | Wattanavichean, N. |
| Publisher | University of Liverpool |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://livrepository.liverpool.ac.uk/3006908/ |
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